DDibaryons: Molecular versus Compact Hexaquarks
H. Clement and T. SkorodkoPhysikalisches Institut der Universit¨at T¨ubingenandKepler Center for Astro and Particle Physics, University of T¨ubingen,Auf der Morgenstelle 14, D-72076 T¨ubingen, Germanyemal: [email protected] 26, 2020
Abstract
Hexaquarks constitute a natural extension of complex quark systems like also tetra- and pen-taquarks do. To this end the current status of d ∗ (2380) in both experiment and theory is reviewed.Recent high-precision measurements in the nucleon-nucleon channel and analyses thereof have es-tablished d ∗ (2380) as an indisputable resonance in the long-sought dibaryon channel. Importantfeatures of this I ( J P ) = 0(3 + ) state are its narrow width and its deep binding relative to the∆(1232)∆(1232) threshold. Its decay branchings favor theoretical calculations predicting a com-pact hexaquark nature of this state. We review the current status of experimental and theoreticalstudies on d ∗ (2380) as well as new physics aspects it may bring in the future. In addition, wereview the situation at the ∆(1232) N and N ∗ (1440) N thresholds, where evidence for a number ofresonances of presumably molecular nature have been found — similar to the situation in charmedand beauty sectors. Finally we briefly discuss the situation of dibaryon searches in the flavoredquark sectors. a r X i v : . [ nu c l - t h ] O c t ontents Single-Pion Production — Early Results on Dibaryonic States near the ∆(1232) N Threshold
Two-Pion Production — Observation of the Deeply Bound ∆(1232)∆(1232)
State d ∗ (2380) 42.2.1 pp -induced two-pion production . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.3 pn -induced double-pionic fusion: observation of a narrow resonance . . . . . . . . . . . 42.3.1 The d ∗ → ∆∆ decay vertex: ABC effect . . . . . . . . . . . . . . . . . . . . . . . 62.3.2 d ∗ resonance structure in non-fusing isoscalar two-pion production . . . . . . . . 82.3.3 d ∗ (2380) – a resonance pole in np scattering . . . . . . . . . . . . . . . . . . . . 82.3.4 hadronic decay branchings of d ∗ (2380) . . . . . . . . . . . . . . . . . . . . . . . . 10 d ∗ (2380)
104 Status of Theoretical Work on d ∗ (2380) the width issue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144.2 hexaquark versus molecular structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 ∆(1232) N , N ∗ (1440) N and ∆(1232)∆(1232) Regions. 15 N threshold region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155.2 N*(1440)N region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165.3 ∆∆ region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
The recent observations of exotic multi-quark states in form of tetra- and pentaquark systems in charmedand beauty meson and baryon sectors, respectively, demonstrate that there exist more complex con-figurations in nature than just quark-antiquark and three-quark systems — as already suggested byGell-Mann in his pioneering presentation of the quark model [1]. A natural extension of complexity isthe quest for hexaquark systems, which provides the transition to two-baryon, i.e. dibaryon systems.Generally dibaryons are solely defined by their baryon quantum number B = 2. In this sense weknow since 1932, when the deuteron was discovered [2] that at least a single one does exist. Due to itsvery small binding energy of only 2.2 MeV the deuteron constitutes a large extended hadronic moleculewith a charge radius of 2.1 fm [3, 4]. I.e. , the proton and the neutron inside the deuteron are on average4 fm apart from each other and do not overlap.All the time since then it has been questioned, whether there are more states in the two-baryonsystem than just the deuteron groundstate with I ( J P ) = 0(1 + ). Follow-up nucleon-nucleon ( N N )scattering experiments revealed this state in the S partial wave to be the only bound state in the N N I ( J P ) = 1(0 + ) state in the S partial wave, was foundto be already slightly unbound.With the recognition of quarks being the basic building blocks of hadrons, the idea of dibaryonsbeing not just hadronic molecules but rather clusters (”sixpacks”) of quarks sitting in a common quarkbag stimulated the dibaryon search enormously. A manyfold of quark models predicting a huge numberof dibaryon states initiated a rush of experimental searches for such objects. Unfortunately, practicallynone of the many claims for experimental evidences survived rigorous experimental checks. For a reviewof the history of dibaryon predictions and searches see, e.g. Ref. [5].This situation has changed about 10 years ago, when the CELSIUS/WASA [6] and the WASA-at-COSY collaborations [7, 8] started to report their experimental results obtained in a series of experimentson two-pion production in
N N collisions and in neutron-proton scattering. In all relevant two-pionchannels the Lorentzian energy dependence of a narrow isoscalar resonance – named d ∗ (2380) — wasobserved. By measurement of polarized proton-neutron scattering and its inclusion in the phase-shiftanalysis a circular counter-clockwise move in the D partial wave was revealed establishing a pole with I ( J P ) = 0(3 + ) at around 2380 MeV [9, 10]. From these investigations the branching ratios of d ∗ (2380)were determined for all its hadronic decays [11].In the following chapters 2 and 3 a short review is given starting from the first solid observationof d ∗ (2380) in the double-pionic fusion measurements at CELSIUS/WASA [6] and WASA-at-COSY [7]until its current status in hadronic and electromagnetic excitation and decay processes. In chapter 4the status of theoretical work on d ∗ (2380) is reviewed with emphasis on the width issue and the keyquestion, whether it constitutes a compact hexaquark or a dilute molecular system.Chapter 5 deals with resonance structures at ∆(1232) N and N ∗ (1440) N thresholds pointing todibaryonic states of molecular character — in analogy to the situation for tetra- and pentaquark systemsin charm and beauty sectors. Finally, the current dibaryon situation in flavored quark sectors is shortlytouched in chapter 6. Resonances in single- and two-baryon systems decay preferentially by emission of one or several pions.Hence pion production in
N N collisions gives access to the physics of resonances both in baryon anddibaryon systems. The latter are of particular interest here. The oldest prediction of six-quark objectsdecaying by pion emission dates back to Dyson and Xuong [12], who — based on SU(6) symmetryconsiderations — predicted the existence of six non-strange dibaryon states.Since there existed no detailed data base on pion production in
N N collisions, a systematic study –in particular of two-pion production – started in the nineties at CELSIUS and was continued later-on atCOSY using the hermetic WASA detector. All CELSIUS/WASA and WASA-at-COSY measurementson single- and multiple-pion production reported here were carried out exclusively and kinematicallycomplete — in most cases kinematically over-constrained, in order to improve the momentum resolutionby kinematic fits and in order to provide data free of background.
Single-Pion Production — Early Results on Dibaryonic States near the ∆(1232) N Threshold
The search for resonances in the system of two baryons dates back to the fifties, when first measurementsof the πd → pp reaction at Dubna [13, 14, 15] indicated a resonance-like structure near the ∆ N thresholdconnected to the D partial wave in the N N system. Later-on high quality data on total and differentialcross sections and polarization observables for pp and πd elastic scattering as well as πd → pp and3 p → πd reactions revealed a pronounced looping of the D N N partial wave in the Argand diagramrepresenting a pole of a resonance with I ( J P ) = 1(2 + ), mass m ≈ ≈ M eV [16, 17].Though the clear looping is in favor of a true s-channel resonance, the close neighborhood of its massto that of the ∆ N threshold and the compatibility of its width with that of ∆(1232) casted doubtson its s-channel nature. It has been argued that the observed features could be merely a thresholdphenomenon and the observed looping just a reflection of the usual ∆ excitation process in the presenceof the other nucleon, which due to the threshold condition has to be at rest relative to the active one.The situation about this resonance structure has been discussed controversial in many papers, see, e.g. [5]. In a number of publications Hoshizaki demonstrated that this resonance structure constitutesa true S-matrix pole rather than a threshold cusp or a virtual ∆ N state[18, 19]. Similar conclusionswere reached by Ueda et al. [20].The resonance structure seen in the D N N -partial wave is the by far most pronounced one seen in
N N scattering and πd (cid:42)(cid:41) N N reactions. But also in other partial waves a resonant behavior had beennoted in the region of the ∆ N threshold, though by far not as spectacular. In partial wave solutionsof the SAID analysis group, e.g. , also the P − F , F and F − H N N -partial waves exhibit a clearlooping in the Argand diagram [16, 17, 21].The fact that all these states near the ∆ N threshold exhibit a width close to that of the ∆(1232)is not too surprising, since the available phase space of a ∆ N state for a fall-apart decay into itscomponents N and ∆ is tiny close to the ∆ N threshold and hence the only sizeable decay contributionarises from the decay of the component ∆. We will return to the discussion of states near thresholds inchapter 5. In the next chapters first the situation about the hitherto only example of a deeply bound(relative to the ∆∆ threshold) dibaryon state, the d ∗ (2380), will be reviewed. Two-Pion Production — Observation of the Deeply Bound ∆(1232)∆(1232)
State d ∗ (2380) pp -induced two-pion production The two-pion production program at CELSIUS started out in 1993 with exclusive and kinematicallycomplete high-statistics measurements of pp -induced two-pion production from the threshold region upto the GeV region.As a result of these systematic studies it was found that isovector induced two-pion production upto √ s ≈ t -channel meson exchange leading to theexcitation of the N ∗ (1440) Roper resonance and the excitation of the ∆(1232)∆(1232) system. Whereasthe first process dominates at lower beam energies close to threshold, the latter dominates at energiesabove 1 GeV, i.e. √ s > pp → dπ + π . Measurementsof its differential cross sections in the region √ s ≈ t -channel∆∆ calculations. And the energy dependence of its total cross section exhibits a broad resonance-likestructure with a width of about 2Γ ∆ in accord with the t -channel ∆∆ calculations [8, 22] — see toppanel of Fig. 1. pn -induced double-pionic fusion: observation of a narrow resonance When pn -induced two-pion production was looked at, the situation changed strikingly. The measure-ments were carried out with either a deuteron beam or deuterium target by taking advantage of thequasi-free process, e.g. , by looking on the process pd → dπ π + p spectator within the pd → dpπ π reaction. Since all these measurements were exclusively and kinematically complete (in most cases even4 [GeV]s [ m b ] σ ) π + π d → (pp σ I = 1 ∆∆ [GeV]s2.2 2.4 2.6 2.8 3 [ m b ] σ ) π π d → (pn σ I = 0d* [GeV]s2.2 2.4 2.6 2.8 3 [ m b ] σ ) π + π d → (pn σ I = 0 + 1d* ∆∆ Figure 1: Total cross section of the double-pionic fusion to deuterium and its isospin decomposition.Top panel: the isovector part of the pn → dπ + π − reaction given by half the cross section of the pp → dπ + π reaction. Solid dots show WASA-at-COSY [8] results, open symbols previous results[22, 23, 24]. The dashed curve represents a t -channel ∆∆ calculation fitted in height to the data [22].Middle panel: The isoscalar part of the pn → dπ + π − reaction given by twice the cross section ofthe pn → dπ π reaction. The CELSIUS/WASA results [6] are shown by open triangles. The othersymbols refer to WASA-at-COSY measurements [7, 8]. The dotted line denotes the d ∗ resonance curvewith momentum dependent widths [28], mass m = 2370 MeV and total width Γ = 70 MeV. Bottompanel: the isospin-mixed reaction pn → dπ + π − . Solid dots represent WASA-at-COSY measurements,open symbols previous bubble-chamber measurements at DESY (circles) [25], Dubna (squares) [26] andGatchina (triangles) [27]. The dashed line represents the t -channel ∆∆ excitation. From Ref. [8].5ver-constrained allowing kinematic fitting with improving thus resolution and purity of the collectedevents), the effective total energy of the pn → dπ π subprocess was known on a event-by-event basis.That way the energy dependence of the quasi-free process could be measured over an appropriate energyrange with a single beam energy setting.For the dπ π channel there were no previous measurements at all, since a hermetic detector likeWASA with its capability to detect both charged and uncharged particles over a solid angle of nearly4 π was not available at other installations for hadron research.By use of isospin relations [23, 27] and isospin recoupling in case of an intermediate ∆∆ system [5],the cross section of the t -channel ∆∆ process in the dπ π channel can be determined to be only 1/5of that in the dπ + π channel, i.e. , about 0.04 mb at √ s ≈ t -channel ∆∆ processpeaks. Due to this low cross section of conventional processes, this reaction channel is predestinatedfor the observation of unconventional isoscalar processes, so to speak the ”golden” channel.The measurements for this channel [6, 7, 8] displayed in the middle panel of Fig. 1 as well as inFig. 2, indeed revealed the cross section around 2.5 GeV to be of this magnitude. However, the bigsurprise was that at lower energies a much larger cross section was observed exhibiting a pronouncednarrow resonance-like structure, which can be very well fitted by a Breit-Wigner ansatz with momentumdependent widths [28], mass m = 2370 MeV and total width Γ = 70 MeV – see dotted line in the middlepanel of Fig. 1.The measurement of the deuteron angular distribution displayed in Fig. 2 led to a J = 3 assignmentfor the resonance structure [7]. Together with the isoscalar character of the pn → dπ π reaction thisgives the isospin-spin-parity combination I ( J P ) = 0(3 + ). Due to its isoscalar character and the baryonnumber B =2 the resonance structure is formally compatible with an excited state of the deuteron,hence its denotation as d ∗ .The Dalitz plot and its mass-squared projections are shown in Fig. 2. Together with the N π angulardistribution [7] they suggest a ∆∆ configuration in relative s -wave as an intermediate configuration,which according to the observed mass of 2370 MeV must be bound by about 90 MeV relative to thenominal ∆∆ threshold mass of 2464 MeV [7].In measurements of the pn → dπ + π − reaction (bottom panel of Fig. 1) and the isospin decomposition[8, 23, 27] of its cross section according to the relation σ ( pn → dπ + π − ) = 2 σ ( pn → dπ π ) + 12 σ ( pp → dπ + π ) . (1)it has been demonstrated that the resonance structure shows up only in the isoscalar part of thedouble-pionic fusion to the deuteron, but not in its isovector part, i.e. it has a definite isospin I = 0. The d ∗ → ∆∆ decay vertex: ABC effect The pronounced low-mass enhancement observed in the Dalitz plot and its projection onto the ππ -invariant mass-squared as displayed Fig. 2 is very remarkable. In fact, such low-mass enhancementshad been noticed already before in double-pionic fusion experiments. Actually, they laid the trace forthe discovery of d ∗ at WASA [5, 29].In 1960 Abashian, Booth and Crowe [30] noticed an enhancement in the He missing mass spectrumof the inclusively measured pd → HeX reaction. This enhancement occured just in the kinematicregion corresponding to the emission of two pions with low ππ -invariant mass. Follow-up measurementsrevealed this enhancement to occur in the double-pionic fusion reactions pn → dππ , pd → He ππ and dd → He ππ , but not in the fusion to H, where an isovector pion pair is emitted.In all the years since then no conclusive explanation could be presented for the observed low-massenhancement in spite of many theoretical attempts. Hence it was just abbreviated as ”ABC” effect in the6 [GeV]s2.2 2.4 2.6 [ m b ] σ h41 Entries 265Mean 2.399RMS 0.06098 h412
Entries 1497Mean 2.393RMS 0.05733 (cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:0)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1) pn d π π d* d threshold ππ ∆ ) c.m.d Θ cos( 1 0.5 0 0.5 1 b ] µ ) [ c . m . d Θ / d c o s ( σ d = 2.38 GeVs Θ c.m.d ] [GeV ππ M0.1 0.2 0.3 ] [ G e V π d M s = 2.38 GeV [GeV] π d2 M4 4.5 5 [ m b / G e V ] π d / d M σ d π d2 M4 4.5 5 [ m b / G e V ] π d / d M σ d π d2 M4 4.5 5 [ m b / G e V ] π d / d M σ d M π d2 d π ] M [GeV ] [GeV] ππ M0.1 0.2 0.3 ] [ m b / G e V ππ / d M σ d ππ M0.1 0.2 0.3 ] [ m b / G e V ππ / d M σ d ππ M0.1 0.2 0.3 ] [ m b / G e V ππ / d M σ d ππ M0.1 0.2 0.3 ] [ m b / G e V ππ / d M σ d ππ M0.1 0.2 0.3 ] [ m b / G e V ππ / d M σ d ππ M0.1 0.2 0.3 ] [ m b / G e V ππ / d M σ d ππ M0.1 0.2 0.3 ] [ m b / G e V ππ / d M σ d M ππ ππ M [GeV ] Figure 2: Measurements of the ”golden” reaction channel pn → dπ π with WASA at COSY. Top: totalcross section exhibiting the pronounced resonance structure. The blue open symbols show the data ofRef. [7]. They have been normalized in absolute scale to the data of Ref. [8], which are plotted by redstars. The black shaded area represents an estimate of systematic uncertainties. The solid curve showsa calculation of the d ∗ (2380) resonance with momentum-dependent widths according to Ref. [28] andincluding t -channel Roper and ∆∆ excitations as background reactions. The filled circles represent thedifference between this calculation and the data in the low-energy tail of d ∗ (2380). Middle: deuteronangular distribution (left) and Dalitz plot (right) at the peak energy of √ s = 2.38 GeV. Open andsolid circles refer to measurements with the spectator proton in the target and in the beam (reversedkinematics), respectively. The dashed curve gives a Legendre fit with L max = 6 corresponding to J = 3.Bottom: Dalitz plot projections yielding the distributions of the squares of the dπ - (left) and π π -(right) invariant masses. The low-mass enhancement in the latter spectrum denotes the so-called ABCeffect. The solid lines represent a calculation of the pn → d ∗ (2380) → ∆ + ∆ → dπ π process. FromRefs. [7, 31, 32]. 7iterature using the initials of the authors Abashian, Booth and Crowe, who noticed this enhancementfirst.The WASA measurements of the complete double-pionic fusion to the deuteron comprising all threereactions pp → dπ + π , pn → dπ π and pn → dπ + π − deciphered now this effect to be stringentlycorrelated with the appearance of the isoscalar resonance structure d ∗ [7, 8, 22] in double-pionic fusionprocesses. There the ABC effect just reflects the vertex function of the decay vertex d ∗ → ∆∆ andshows up in the ππ invariant mass spectrum only, if the nucleons in the final state fuse to a boundsystem [28]. Subsequent WASA experiments showed that also in the double-pionic fusions to He and He the dibaryon resonance d ∗ is formed, though it appears much broadened there due to collisiondamping with the surrounding nucleons [5, 33, 34]. d ∗ resonance structure in non-fusing isoscalar two-pion production Recently also the non-fusion two-pion production reactions pn → ppπ π − [35], pn → pnπ π [36], pn → pnπ + π − [37] have been investigated. All these channels are isospin-mixed, i.e. contain bothisoscalar and isovector contributions. Hence the d ∗ signal appears just on top of a substantial and —due to its four-body character — steeply rising background of conventional processes. Nevertheless itstill shows up clearly in the energy dependence of the total cross sections for these reaction channels.By use of the isospin-decomposition of N N -induced two-pion production [5, 23, 27] the expectedsize of the d ∗ contribution in these channels can be easily estimated. A more detailed treatment takesinto account also the different phase-space situation, when the deuteron is replaced by the unbound pn system in these reactions [38, 39].In summary, all N N -induced two-pion production channels are in accordance with the appearanceof an I ( J P ) = 0(3 + ) dibaryon resonance at 2.37 GeV with a width of 70 MeV. Even in the channels,which are only partially isoscalar, the d ∗ contribution is still the dominating process. The conventional t -channel processes there underpredict the data in the d ∗ energy region by factors two to four [35, 36, 37]. d ∗ (2380) – a resonance pole in np scattering If the resonance structure d ∗ observed in two-pion production indeed is a true s -channel resonance,then it has to show up in principle also in the entrance channel, i.e. in the np scattering channel.There it has to produce a pole in the partial waves corresponding to I ( J P ) = 0(3 + ), i.e. in the coupledpartial-waves D - G .The expected resonance contribution to the elastic np scattering can be calculated from the know-ledge of the resonance contributions to the various two-pion production channels — under the assump-tion that there is no decay into the isoscalar single-pion production channel, which is forbidden to firstorder in case of an intermediate ∆∆ formation. In Ref. [11] this resonance contribution has beenestimated to be about 170 µ b, a value, which is tiny compared to the value of nearly 40 mb for thetotal np cross section.The analyzing power angular distribution of the elastic scattering is a particularly suitable observableto sense such a small contribution of d ∗ (2380), since it is composed just of interference terms in thepartial waves and hence sensitive to even small terms in the coupled D − G partial waves. In theangular distribution of the analyzing power the contribution of a resonance with J = 3 is given by theangular dependence of the associated Legendre polynomial P . Therefore the resonance contributionis expected to be largest at 90 ◦ , which is also the angle, where the differential cross section is smallest.For the sensitivity of other observables to the d ∗ resonance see Ref. [10].In the region of interest for the d ∗ issue there existed no analyzing power data from previous measure-ments. Precise measurements at SACLAY ended just below the d ∗ region [41, 42]. Hence correspondinganalyzing power measurements extending over practically the full angular range were undertaken with8 nergy [GeV]2.2 2.4 2.6 2.8 A na l ys i ng P o w e r = 83 deg cmn Θ + I(J )=0(3 ) p dibaryon [deg] * Θ y A = 2.377 GeVs pole Figure 3: Analyzing power data for elastic pn scattering in the d ∗ (2380) energy region and their partial-wave analysis [9, 10, 40]. Top: energy dependence of the analyzing power in the vicinity of Θ cmn = 90 ◦ ,where the effect of the d ∗ (2380) resonance is expected to be largest. The solid circles denote WASAresults, the open symbols previous data [41, 42, 43, 44, 45, 46, 47, 48]. The solid line gives the previousSAID partial-wave solution, the dashed line the new SAID solution after including the WASA data.Middle: Angular distribution of the analyzing power A y at the resonance energy. The curves have thesame meaning as in the top figure. Bottom: Argand diagram of the new SAID solution for the D partial wave with a pole at 2380 MeV. The thick solid circle denotes the pole position. From Ref. [5].9ASA at COSY — again in the quasi-free mode. By use of inverse kinematics a polarized deuteronbeam was directed onto the hydrogen pellet target [9, 40].The WASA data are shown in Fig. 3 together with previous measurements [41, 42, 43, 44, 45, 46,47, 48]. The top panel displays the energy dependence of the analyzing power near a center-of-massscattering angle of 90 ◦ , where the d ∗ resonance effect is expected to be largest. A pronounced narrowresonance-like structure is observed in the data at the d ∗ energy position. Accordingly, the measuredangular distribution of the analyzing power, displayed in the middle panel of Fig. 3, deviates from theconventionally expected distribution largest in the 90 ◦ region. In both panels the solid line shows thesolution SP07 from the SAID partial-wave analysis prior to the WASA measurements [49].The subsequent partial-wave analysis by the SAID group including now the WASA data is given bythe dashed and dotted lines, respectively in both top and middle panels. This partial-wave solution,denoted AD14, finds a pole in the coupled D − G partial waves at the position (2380 ± − i (40 ± D partial wave.It exhibits a pronounced looping of this partial wave in agreement with a resonant behavior. The polesin D and G partial waves have been reproduced in a theoretical study of nucleon-nucleon scatteringwithin the constituent quark models of the Nanjing group [50].Very recently also data for the differential cross sections of the pn scattering in the region of d ∗ (2380)could be extracted from the WASA data base. It turned out that the new experimental data are perfectlydescribed by the AD14 partial-wave solution, which is a remarkable success putting further confidenceinto the uniqueness and predictive power of this solution [51].With this result the resonance structure observed in two-pion production has been established as agenuine s -channel resonance in the proton-neutron system. Due to its isoscalar character the notation d ∗ (2380) has been chosen in analogy to the denotation for isoscalar excitations of the nucleon. hadronic decay branchings of d ∗ (2380)From the various two-pion production measurements as well as from the np scattering experiments andtheir analysis the branching ratios given in Table 1 have been extracted for the hadronic decays of d ∗ (2380) [5, 11]. The decay into the not measured nnπ + π channel has been taken to be identical tothat into the ppπ o π − channel by isospin symmetry.The observed d ∗ decay branchings into the diverse two-pion channels are consistent with expectationsfrom isospin decomposition [11] as well as explicit QCD model calculations [52].In a dedicated WASA search for the hypothetical decay d ∗ (2380) → ( N N π ) I =0 no signal from d ∗ (2380) could be sensed in the experimental isoscalar single-pion production cross section, but anupper limit of 5% at 90% C.L. could be derived for such a branching [53, 54]. Note that in Ref. [53] theupper limit was given too high by a factor of two [54]. This result disfavors strongly models predictinga molecular structure for d ∗ (2380) [57, 58, 55, 56], but is in full accord with a compact hexaquark-∆∆structure [59].It should be pointed out that the successful reproduction of all hadronic decay branchings of d ∗ (2380)and its total width by the theoretical calculations also means that all experimentally observed crosssections in the various hadronic channels are understood theoretically in a quantitative manner. d ∗ (2380) The electromagnetic decay channels are very interesting, since they offer the possibility to excite theresonance also by photo- and electro-production, respectively. The latter, in particular, offers thepossibility to measure that way the transition form-factor, which could give experimental access to thesize of d ∗ (2380) and thus further clarify the question about the structure of d ∗ (2380). From the γ decay10able 1: Branching ratios in percent of the d ∗ (2380) decay into pn , N N π , N N ππ and dγ channels. The experimental results [11, 53, 54, 68, 69, 70] are compared to resultsfrom a theoretical calculation [52, 59] starting from the theoretical d ∗ wave function andincluding isospin breaking effects. They are also compared to values expected from pureisospin recoupling of the various N N ππ channels. In the latter case the branching into the dπ π channel is normalized to the data.decay channel experiment theory [52, 59] N N ππ isospin recoupling dπ π ± dπ + π − ± npπ π ± npπ + π − ± ppπ π − ± nnπ + π ± N N π ) I =0 < C.L. ) 0.9 – np ± dγ ≈ (cid:80) ( total ) 103 ± pn → d ∗ (2380) → ∆∆ → dπ γ and pn → d ∗ (2380) → ∆∆ → dγγ are smaller than the hadronic decays by two and four ordersof magnitude, respectively. For the dγ decay channel the situation is presumably similar.Possibly an indication of d ∗ (2380) photo-excitation has been observed already in the seventies inthe photo-absorption reaction γd → pn by measuring the polarization of the ejected protons. Afterobservation [60, 61] of an anomalous structure in the proton polarization from deuteron photodisin-tegration Kamae and Fujita [62] suggested the possible existence of a deeply bound ∆∆ system with I ( J P ) = 0(3 + ) at √ s = 2.38 GeV with a width of 160 MeV. Subsequent analyses based on an increasedbasis of polarization measurements yielded the possible existence of at least two dibaryon resonanceswith either I ( J P ) = 0(3 + ) or 0(1 + ) at √ s = 2.38 GeV and I ( J P ) = 1(3 − ) or 1(2 − ) at √ s = 2.26 GeVwith widths of 200 MeV and above [63, 64].New measurements of the γd → pn reaction with polarized photons at MAMI — measuring alsofor the first time the polarization of the emerging neutrons — are consistent with an excitation of d ∗ (2380) in this process [65, 66]. At the photon energy corresponding to the d ∗ (2380) mass both thepreviously measured proton polarization [63, 64, 67] and the newly measured neutron polarization peakat a scattering angle of 90 degrees in the center-of-mass system with reaching a polarization of P y =-1.This extreme value means that the final pn system must be in a spin triplet state as required for a d ∗ (2380) → pn decay. The measured [66] energy dependence of P y at 90 degrees is in agreement witha Lorentzian energy dependence having the width of d ∗ (2380).Very recently also first data for the γd → dπ π reaction appeared reaching in energy down to the d ∗ (2380) region. The measurements conducted at ELPH, Japan, only reach down to the high-energyside of the d ∗ (2380) region [68, 69]. Measurements performed at MAMI reach even below the d ∗ region[70]. Both measurements have to fight heavily with background contributions at the lowest energies.However, both measurements find a surplus of cross section in the d ∗ region in comparison to thestate-of-the-art calculations of Fix and Arenh¨ovel [71, 72], which describe the data very well above11 s =2.40 GeV. A d ∗ cross section of about 20 nbarn provides a good description of both data sets inthe d ∗ region. This is just four orders of magnitude smaller than the cross section in the correspondinghadronic channel providing an electromagnetic branching of 10 − for the d ∗ (2380) → dγ transition— in agreement with the estimate given above. First attempts to understand the photo-absorptionprocess γd → d ∗ (2380) theoretically [73, 74] provide cross sections, which are still too low by an orderof magnitude. d ∗ (2380) There have been a huge number of dibaryon predictions in the past. The oldest one dates back to1964, when based on SU(6) symmetry considerations Dyson and Xuong [12] predicted six non-strangedibaryon states D IJ , where the indices denote isospin I and spin J of the particular state. Theyidentified the two lowest-lying states D and D with the deuteron groundstate and the virtual S state, respectively, the latter being known from low-energy N N scattering and final-state interaction.Identifying in addition the third state D with the at that time already debated resonance-like structureat the N ∆ threshold having I ( J P ) = 1(2 + ), they fixed all parameters in their mass formula. As a resultthey predicted a state D with just the quantum numbers of d ∗ (2380) at a mass of 2350 MeV, which isremarkably close to the now observed d ∗ mass. The remaining predicted states are D and D . Dueto their isospins I = 2 and 3 they are N N -decoupled. According to Dyson and Xuong they should havemasses very similar to those of D and D , respectively.On the one hand it appears very remarkable, how well the prediction of Dyson and Xuong works.On the other hand this may not be too surprising, because we know since long that the mass formulasfor baryons and mesons derived from symmetry breaking considerations also do remarkably well, if afew phenomenological parameters are adjusted to experimental results.Oka and Yazaki were the first to apply the nonrelativistic quark model to the problem of the nuclearforce. They demonstrated that the interplay between the Pauli principle and the spin-spin interactionbetween quarks leads to a strong short-range repulsion between nucleons, but to an attractive force fora ∆∆ system with I ( J P ) = 0(3 + ) [75].Terry Goldman, Fan Wang and collaborators [76] pointed out later that a ∆∆ configuration with theparticular quantum numbers of d ∗ (2380) must have an attractive interaction due to its special color-spinstructure, so that any model based on confinement and effective one-gluon exchange must predict theexistence of such a state — the ”inevitable dibaryon” as they called it. In their quark-delocalizationand color-screening model (QDCSM) they initially predicted a binding energy of 350 MeV relative tothe nominal ∆∆ threshold, but approached the experimental value in more recent work [77, 78, 50].In fact, many groups calculated meanwhile such a state at actually similar mass based either onquark-gluon [77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 97] or hadronic interactions [55, 56, 62].Already the early bag-model calculations of the Nijmegen theory group [85, 86] including the work ofMulders and Thomas [87] and also Saito [88] predicted d ∗ (2380) at about the correct mass. However,in these calculations also numerous other unflavored dibaryon states were predicted, which have notbeen observed (at least so far) or which have been observed at a significantly different mass like, e.g. ,the D state.Another correct real prediction, i.e. a prediction before the experimental observation of d ∗ (2380), isthe one by the IHEP theory group led by Z. Y. Zhang, who studied this state in the chiral SU(3) quarkmodel within the resonating group method [79]. This work and follow-up investigations of this group[80, 81, 82, 83] include the concept of ”hidden color”. Hidden-color six-quark states are a rigorous first-principle prediction of SU(3) color gauge theory. Six quark color-triplets 3 c combine to five differentcolor-singlets in QCD and will significantly decay to ∆∆ as shown in Refs. [89, 90]. Problems relatedto the application of hidden color in multi-quark systems have been discussed by Fan Wang et al. [91].12hey point out, while the ∆∆ and hidden-color configurations are orthogonal for large separationsbetween the two quark clusters, they loose their orthogonality, when they start to overlap at smallseparations. Another point of caution has been noted by F. Huang and W.L. Wang in Ref. [92]. In thiswork they study the masses of octet and decuplet baryon ground states, the deuteron binding energyas well as the N N scattering phase shifts (for J ≤
6) below the pion-production threshold within achiral SU(3) quark model. They demonstrate that all these can be well described, if the consistencyrequirement for the single-baryon wave functions to satisfy the minima of the Hamiltonian are strictlyimposed in the determination of the model parameters. In earlier quark-model calculations usually thenucleon is set to be at the minimum of the Hamiltonian by a particular choice of the model parameters.In consequence, the ∆, which is of different size, is not stable against its size parameter in the wavefunction, i.e. its wavefunction is not the real solution of the Hamiltonian. Hence one needs to bevery careful when extending the model from the study of the NN interaction to other baryon-baryonsystems and one may need to introduce additional channels like the hidden-color channel to lower theenergy of the ∆∆ system. These channels might not be the physical ones, but are partially needed tochange the internal wave function of the single ∆. Therefore one has to be cautious in explaining theconfiguration structure of the coupled ∆∆-hidden-color system. Hence the IHEP result of 2/3 hidden-color components in d ∗ (2380) has to be taken with some caution with regard to its interpretation of theconfiguration of d ∗ (2380). An improved calculation for d ∗ (2380) with a consistent treatment of the ∆∆system is in progress [93].Recently also a diquark model has been proposed for d ∗ (2380) [94]. In this work it is assumedthat d ∗ (2380) is composed of three vector diquarks and its mass is calculated by use of an effectiveHamiltonian approach. Surprisingly, in this rough and simple model both mass and width (see nextsubsection) come out in good agreement with the experimental data. In a subsequent paper [95] Gal andKarliner questioned the applicability of diquark models in the light-quark sector by demonstrating thatthe use of the effective Hamiltonian with parameters given in Ref. [94] leads to masses for deuteron-and virtual-like states, which are 200-250 MeV above the physical deuteron and the virtual S state.However, as pointed out in a reply, the latter two states interpreted as three-axial-vector-diquark statesreside in spin-flavor multiplets different from the one of d ∗ (2380) and need a Hamiltonian with moreinteractions included [93].As a historic side remark we note that a diquark model for a deuteron-like object had been proposedalready some time ago [96], where three scalar diquarks were coupled in relative P -wave to an isoscalar J P = 0 − object, the so-called ”demon deuteron” possessing a highly suppressed decay. In this contextthe data for np → dπ + π − , which were available at that time and which indicated a peak in the totalcross section around √ s ≈ d ∗ (2380) was found instead.Meanwhile also a QCD sum rule study has found this state at the right mass [97], whereas anotherQCD-based work without any inclusion of hadron degrees of freedom can construct such a state as acompact object only at much higher masses [98].Most recently first lattice QCD calculations for d ∗ (2380) were presented by the HAL QCD collab-oration [99, 100] finding evidence for a bound ∆∆ system with the quantum numbers of d ∗ (2380). Inthese calculations the pion mass is still unrealistically large, because ∆(1232) has to be assumed to be astable particle, in order to make such calculations feasible at present. Therefore the lattice results wererecently extrapolated down to the real pion mass by methods based on Effective Field Theory with theresult that indeed such a bound state is likely to exist [101].Gal and Garcilazo also obtained this state at the proper mass in recent relativistic Faddeev calcula-tions based on hadronic interactions within a baryon-baryon-pion system [55, 56] and assuming a decay d ∗ (2380) → D π → ∆ N π . Such a decay was also investigated by Kukulin and Platonova [57, 58].13 .1 the width issue More demanding than the mass value appears to be the reproduction of the small decay width of d ∗ (2380). As worked out in a paper together with Stanley Brodsky [102], the small width points to anunconventional origin, possibly indicating a genuine six-quark nature. With the dominant decay being d ∗ (2380) → ∆∆ one would naively expect a reduction of the decay width from Γ ∆∆ =240 MeV to 160MeV for a ∆∆ system bound by 90 MeV – using the known momentum dependence of the width of the∆ resonance. This is twice the observed width. On the other hand, if d ∗ (2380) is a genuine six-quarkstate, we need to understand its large coupling to ∆∆. This can be explained, if one assumes that d ∗ (2380) is dominated by ”hidden-color” configurations.So far there have been five predictions for the decay width based on Faddeev calculations [55, 56],quark-model calculations [52, 78, 82, 83, 77, 103, 104, 94] or some general considerations [106]. A widthof 160 MeV as discussed above is also obtained initially in the quark-model calculations of Fan Wang etal. [77]. By accounting in addition for correlations in a more detailed treatment they finally arrive at110 MeV [78]. A similar width is obtained in the Faddeev calculations [55, 56]. For a resonance mass of2383 MeV they obtain a width of 94 MeV for the decay into all experimentally observed N N ππ decaychannels. Adding the decay width into the pn channel, which they cannot account for in their model,leads finally to a width of 104 MeV.The quark-model calculations of the IHEP group, which include hidden-color configurations, asdiscussed in Refs. [89, 90, 102], arrive at the experimentally observed width [52, 82, 83, 103, 104, 105].In these calculations the d ∗ (2380) hexaquark of size 0.8 fm contains about 67% hidden-color components,which cannot decay easily and hence reduce the width to the experimental value.Also the diquark model of Shi, Huang and Wang [94] reproduces the observed narrow width. Herethe width is naturally explained by the large tunneling suppression of a quark between a pair of diquarks.Again, Gal and Karliner [95] question this result arguing that in the calculation of the decay an isospin-spin recoupling factor of 1/9 has been overlooked, which would reduce the width to less than 10 MeV.However, in a reply Shi and Huang point out that such a recoupling factor appears only in uncorrelatedquark models, but not in the diquark model [93].For completeness we mention here also the recent work of Niskanen [106], who considers the energybalance in ∆ N and ∆∆ systems. He arrives at the surprising conclusion that both these systemsshould have widths, which are substantially smaller than the width of the free ∆ at the correspondingmass. This conclusion is not only counterintuitive as he also notes, but also in sharp contrast to theexperimental results. E.g. , for d ∗ (2380) he obtains a width of about 40 MeV and for D a width ofabout 75 MeV, i.e. in both cases much smaller than observed experimentally. Such Fermi motionconsiderations have been taken up recently also by Gal [107] for the discussion of the expected size ofthe ∆∆ configuration of d ∗ (2380). In Ref. [108] it is demonstrated that such considerations lead toconflicts with the observed mass distributions. What is observed in these spectra are ∆s of mass 1190MeV with a width of 80 MeV — as expected from the mass-width relation of a free ∆. This is in linewith the expectation that during the decay process d ∗ (2380) → ∆∆ the distance between the two ∆increases continuously eliminating thus the Fermi motion finally and putting mass and width of the ∆sback to their asymptotic values. hexaquark versus molecular structure If the scenario of the models, which correctly reproduce the experimental width, is true, then theunusually small decay width of d ∗ (2380) signals indeed an exotic character of this state and points toa compact hexaquark nature of this object as discussed by the IHEP group [82, 103, 104, 105]. In fact,the IHEP calculations as well as the quark-model calculations of the Nanjing group [91] give a valueas small as 0.8 - 0.9 fm for the root-mean-square radius of d ∗ (2380), i.e. , as small as the nucleon. Also14atest lattice QCD calculations provide values in the same range [99]. Actually, such values appear tobe not unreasonable, if one uses just the uncertainty-relation formula [109] R ≈ ¯ hc/ (cid:113) µ ∆∆ B ∆∆ ≈ . f m (2)for an order-of-magnitude estimate of the size of a ∆∆ system bound by B ∆∆ = 80 MeV and areduced mass µ ∆∆ = m d ∗ (2380) .In contrast, the Faddeev calculations of Gal and Garcilazo give a molecular-like D − π structurewith radius of about 2 fm [107, 110], i.e. as large as the deuteron. Unfortunately, as demonstratedrecently [5, 107], the d ∗ decay branchings into the various N N ππ channels based on isospin couplingturn out to be identical for the routes d ∗ → ∆∆ → N N ππ and d ∗ → D π → N N ππ , respectively,and hence do not discriminate between these two scenarios. Fortunately, however, there is a way outby looking at a possible decay into the single-pion channel. In leading order such a decay is forbiddenfor d ∗ → ∆∆ → ( N N π ) I =0 . The consideration of higher order terms gives a branching of less than1% [59]. The situation is much different for an intermediate D π configuration, since D → N N hasa branching of 16 - 18% [10, 21]. Hence the d ∗ → N N π decay ought to have the same branching inthis scenario. However, exactly this has been excluded by the dedicated WASA single-pion productionexperiment [53, 54]. In consequence of this experimental result Avraham Gal proposed a mixed scenario,where the main component of d ∗ (2380) consists of a compact core surrounded by a dilute cloud of D − π structure [107].Summarizing, in the present discussion about the nature of d ∗ (2380) it is no longer the questionabout its existence itself, but about its structure. Is it a dilute molecular-like object or is it a compacthexaquark object? The measured decay properties of d ∗ (2380) clearly favor the latter. ∆(1232) N , N ∗ (1440) N and ∆(1232)∆(1232) Regions. ∆ N threshold region Stimulated by the success in establishing d ∗ (2380) as the first narrow dibaryon resonance of non-trivialnature, new experiments have been started recently to search for other possible dibaryon resonances.With the ANKE detector at COSY the pp → ppπ reaction was studied with polarized protons over alarge energy range √ s = 2040 - 2360 MeV and under the kinematical condition that the emitted protonpair is in the relative S state [111]. Thus these measurements are complementary to the ones of the pp → dπ + reaction, where the nucleons bound in the deuteron are in relative S state — aside fromthe small D -wave admixture in the deuteron.In the partial wave analysis of their data the ANKE collaboration finds the P → S s and P → S d transitions to be resonant peaking at 2201(5) and 2197(8) MeV, respectively, with widths of 91(12) and130(21) MeV, respectively. The resonance parameters point to ∆ N threshold states with I ( J P ) = 1(0 − )and 1(2 − ), respectively, where the two constituents N and ∆ are in relative P wave. The particularsignature of the P → S s and P → S d transitions is that they constitute proton spinflip transitions,which cause concave shaped pion angular distribution in contrast to the conventional convex shapedones. This peculiar behavior was noted already before in PROMICE/WASA [112] and COSY-TOF[113] measurements of the pp → ppπ reaction at energies near the pion production threshold providingthus first hints for a resonant behavior of these partial waves. The masses of these p -wave resonancesare slightly above the nominal ∆ N mass. This is understood to be due to the additional orbital motion[111].For the I ( J P ) = 1(2 − ) resonance corresponding to the P N N -partial wave a pole had been foundalready before in SAID partial-wave analyses of data on pp elastic scattering and the pp (cid:42)(cid:41) dπ + reaction1516, 21]. In these analyses also evidence for poles in F and F − H partial waves have been foundnear the ∆ N threshold, though these evidences appear much less pronounced than for the above cases.These poles would correspond to states with I ( J P ) = 1(3 − ) and 1(4 − ).Kukulin and Platonova have demonstrated recently that by accounting for the isovector P -waveresonances as well as the dominant isovector 2 + resonance the pp (cid:42)(cid:41) dπ + cross section and polarizationobservables can be described quantitatively for the first time with form-factor cut-off parameters, whichare consistent to those obtained in elastic scattering descriptions [114].Not coupled to the N N channel, but in the region of the ∆ N threshold, there is supposed to beanother state with quantum numbers mirrored to those of the I ( J P ) = 1(2 + ) state. This state with I ( J P )) = 2(1 + ) — first predicted by Dyson and Xuong in 1964 [12] and denoted by D – is decoupledfrom the elastic N N channel due to its isospin I = 2. Hence, it only can be produced in
N N -initiatedreactions associatedly, e.g., by the pp → D π − → ppπ + π − reaction. Though the total cross section ofthis two-pion production channel runs smoothly over the ∆ N threshold region, it was noted recentlythat its slope there is not in accord with isospin relations between this channel and the pp → ppπ π channel. Whereas the latter cannot contain a D resonance excitation due to Bose symmetry, the firstcan do so. And indeed, a detailed analysis of WASA-at-COSY pp → ppπ + π − data revealed pronounceddifferences in invariant mass spectra and angular distributions associated with π + or π − . These cannotbe understood by conventional t -channel reaction mechanism, however, quantitatively described by thepresence of D with m = 2140(10) MeV and Γ = 110(10) MeV [148, 149]. Aside from the predictionof Dyson and Xuong also Gal and Garcilazo obtain this state with about the same mass and width[56], whereas the Nanjing group does not get enough binding in their calculation for the formation of abound state [150]. N*(1440)N region
In contrast to ∆ N resonances, which can couple solely to isovector N N channels, N ∗ N resonances canconnect both to isoscalar and isovector N N channels. So the most likely configurations, where N ∗ and N are in relative S -waves, can couple to S and S N N -partial waves possessing the quantumnumbers I ( J P ) = 1(0 + ) and 0(1 + ), respectively.In fact, evidence for the existence of such states has been found just recently in N N -initiated single-and double-pion production. In a study dedicated initially to the search for a decay d ∗ (2380) → N N π — see section 2.3.4 — the isoscalar part of the single-pion production was measured in the d ∗ resonanceregion covering also the N ∗ (1440) N excitation region [53]. As a result, the isoscalar total cross section isobserved to increase monotonically from the N N π threshold up to √ s ≈ N ∗ excitation process mediated by t -channel meson exchange. However, one wouldexpect in such a case that the cross section keeps rising as the beam energy is further increased. Instead,the measurements beyond 2.32 GeV exhibit a decreasing cross section forming thus a bell-shaped energyexcitation function for the isoscalar total cross section. Since the simultaneously measured isoscalar N π -invariant-mass distribution is in accord with an excitation of the Roper resonance N ∗ (1440) [53],the observation has to be interpreted as evidence for a N ∗ N resonance [116, 117]. We deal here witha state below the nominal N ∗ N mass of m N ∗ + m N = 2.38 GeV. Therefore N ∗ and N have to be inrelative S -wave and the quantum numbers of this resonance must be I ( J P ) = 0(1 + ). I.e. , it is fed bythe S partial wave in the N N -system.Since the Roper resonance decays also by emission of two pions, this N ∗ N structure could possiblybe seen in isoscalar two-pion production, too. This is particularly true for the pn → dπ π reaction,where the background of conventional processes is lowest and where also d ∗ (2380) is observed best. Asseen in Figs. 1 and 2 there is, indeed, a small surplus of cross section in the region of √ s ≈ d ∗ (2380) resonance, which could berelated to the isoscalar N ∗ N state. 16sospin decomposition of data on various pp -induced two-pion production channels had revealedalready some time ago that the Roper N ∗ (1440) excitation process exhibits a bump-like structurethere, too, peaking in the region of the N ∗ N mass [115]. Since the initial pp -system is of isovectorcharacter, the observed structure must correspond to a N ∗ N state with I ( J P ) = 1(0 + ) formed by the S partial wave in the initial pp channel.Both resonance structures peak around 2320 MeV and have a width of Γ ≈
150 MeV. These valuesconform with the pole parameters of 1370 - i 88 MeV for the Roper resonance, however, not with itsBreit-Wigner values of m ≈ ≈
350 MeV [118]. If the latter mass value is taken for thenominal N ∗ N mass, then the two N ∗ N structures appear to be bound by about 70 MeV, which couldexplain that the observed width is smaller than typical for a Roper excitation. In fact, the formation of a N ∗ N resonance state would also explain the observation that in nucleon-accompanied Roper excitationslike, e.g., in hadronic J/ Ψ → ¯ N N π decay [119] and αN scattering [120, 121], this excitation is alwaysseen with values close to its pole parameters, but not as expected with its Breit-Wigner values. ∆∆ region In addition to the peak for the excitation of the d ∗ (2380) resonance at √ s = 2.37 GeV there appear twofurther peaks at 2.47 and 2.63 GeV in the total cross section of the γd → dπ π reaction, as measuredrecently both at ELPH [69] and at MAMI [70]. Whereas conventionally these two bumps are explainedto belong to electromagnetic excitations of the nucleon in the so-called second and third resonanceregion [71, 72], the collaboration at ELPH demonstrates that the measured angular distributions arenot in accord with such a quasifree reaction process, but rather with a process for the formation ofisoscalar dibaryon resonances with masses m = 2469(2) and 2632(3) MeV and widths of Γ = 120(3) and132(5) MeV, respectively [69]. No spin-parity assignments are given, but the dπ -invariant mass spectrasuggest a decay of these putative resonances via D , the isovector 2 + state near the ∆ N threshold.Whereas the peak at 2.63 GeV is beyond the energy range measured at WASA in the pn → dπ π reaction, the peak at 2.47 GeV is still within this range. Since the peak cross section at 2.47 GeV isroughly double as high as that for d ∗ (2380) at 2.37 GeV, one would naively expect a similar situationalso in the hadronic excitation process measured by WASA. But nothing spectacular is seen around2.5 GeV in the WASA measurements. The observed small cross section in this region — see Fig. 2 —is well understood by the conventional t -channel ∆∆ process as indicated in Fig. 1. A possible wayout could be the conception that similar to the situation with the Roper resonance also the higher-lying nucleon excitations undergo a kind of molecular binding with the neighboring nucleon at theirthreshold. Since in N N -induced reactions the excitation of the hit nucleon into states of the secondand third resonance region has a much smaller cross section [122] than the conventional ∆∆ process,it could be understandable, why WASA does not observe the peak at 2.47 GeV seen in γ -induced π π production, where the ∆∆ process is not possible.Five out of six dibaryonic states predicted by Dyson and Xuong [12] in 1964 have been foundmeanwhile with masses even close to the predicted ones — if the interpretation of the WASA data asevidence for D is correct, see section 5.3. Therefore it appears very intriguing to investigate, whetheralso the sixth one exists, perhaps also close to its predicted mass value. This N N -decoupled state D with I ( J P ) = 3(0 + ), i.e. with quantum numbers mirrored to those of D = d ∗ (2380) and of ∆∆ nature,too, is particularly difficult to find, since one needs at least two associatedly produced pions, in orderto produce it in N N -initiated reactions.An attempt to search for it in WASA data for the pp → ppπ + π + π − π − reaction was undertakenrecently [151]. No stringent signal of such a state was observed in these data and only upper limitsfor its production cross section could be derived, because the theoretical description of conventionalprocesses for four-pion production is not well known so far. However, it could be shown that the upperlimit is at maximum for the combination m = 2.38 GeV and Γ = 100 MeV. I.e. , if this state really17xists, then this mass-width combination is most likely.Since this mass is compatible with the d ∗ (2380) mass, it would agree perfectly with the prediction ofDyson and Xuong, who get equal masses for both these states. Also other theoretical studies [56, 80, 81]find D to lie in this mass region. Despite of numerous experimental attempts no single dibaryon candidate could be established firmly sofar in the flavored quark sector. Most of experiments were carried out in the strange sector, in particularsearching for the so-called H -dibaryon, a bound ΛΛ state predicted 1977 by Jaffe [123]. For a recentreview see, e.g. Ref. [5]. According to very recent lattice QCD simulations close to the physical point( m π = 146 MeV, m K = 525 MeV) performed by the HAL QCD collaboration, there is no bound orresonant H -dibaryon around the ΛΛ threshold [124]. However, a possible H resonance close to the Ξ N threshold cannot yet be excluded by these calculations.The dibaryon search in the strange sector has received a new push, after the lattice QCD calcula-tions by the HAL QCD collaboration keep predicting slightly bound ΩΩ and Ω − p systems [125, 126].The latter result is also in accord with quark model calculations by the Nanjing group [127]. Firstmeasurements of the Ω − p correlation function by the STAR experiment at RHIC give first hints thatthe scattering length is indeed positive in favor of a bound state in this system [128].An established unusual structure found in the strange sector is a narrow spike at the Σ N threshold,conventionally interpreted as a cusp effect [5, 129, 130, 131]. But also a possible Λ N state has beendiscussed, for a recent review on this subject, see, e.g. , Ref. [132].At JPARC experiments are going on to search for a bound ppK − system. Recent results are in favorof the existence of such a system [133], however, a definite confirmation has to be still awaited.Lately particular attention was paid to the charm and beauty sector, where tetra- and pentaquarksystem were observed recently. This finding suggests that in these sectors the attraction is againlarge enough to form dibaryons. Such expectation has been supported meanwhile by numerous modelcalculations of increasing sophistication. E.g. , Fr¨omel et al. [134] started out with well-established phe-nomenological nucleon-nucleon potentials applying quark-model scaling factors for scaling the strengthsof the different interaction components and obtained first indications of deuteron-like bound states be-tween nucleons and singly- as well as doubly-charmed hyperons. On the other hand a quark-modelinvestigation of doubly-heavy dibaryons does not find any bound or metastable state there [135, 136].Another quark-model study finds four sharp resonance states near the Σ c N and Σ ∗ c N thresholds [137].A recent lattice QCD study based on the HAL QCD method [138] comes to the conclusion that theattraction in the Λ c N system is not strong enough to form a bound system.Within the one-boson-exchange model Zhu et al. have undertaken a systematic investigation ofsingle- and doubly-heavy baryon-baryon combinations [139, 140, 141, 142, 143, 144]. They find severalcandidates of loosely bound molecular states in the Ξ cc N system [142], for loosely bound deuteron-likestates in the Ξ c Ξ c and Ξ (cid:48) c Ξ (cid:48) c [139] as well as in Λ c Λ c and Λ b Λ b [140] systems. They also get bindingsolutions for the Ξ cc Ξ cc system [141] and a pair of spin-3/2 singly-charmed baryons with the strikingresult that molecular states of Ω ∗ c Ω ∗ c might be even stable [143]. Possible molecular states composedof doubly charmed baryons are also investigated and good candidates have been found. But it is alsodemonstrated that coupled-channel effects can be important for the question, whether there is a bindingsolution or not [144].Unfortunately there are no experimental results yet. But with such many promising candidates itwill be very interesting to watch future experiments in this area of charm and beauty.18 Summary
Following a trace laid by the ABC effect the WASA measurements of the two-pion production in np collisions revealed the existence of a narrow dibaryon resonance, first observed in the pn → dπ π reaction. As it turned out, this has been the golden channel for the dibaryon discovery, since thebackground due to conventional processes is smallest in this channel. There was also no chance todiscover it by previous experiments in this channel, since there existed no other installation, whichwould have been able to take reliable data for this reaction channel at the energies of interest.By subsequent WASA measurements of all possible hadronic decay channels this dibaryon statecould be established as a genuine s -channel resonance with I ( J P ) = 0(3 + ) at 2370 - 2380 MeV. Itsdynamic decay properties point to an asymptotic ∆∆ configuration, bound by 80 - 90 MeV. Its widthof only 70 MeV – being more than three times smaller than expected for a conventional ∆∆ systemexcited by t -channel meson exchange – points to an exotic origin like hidden-color effects in the compacthexaquark system.Though the observation of such a state came for many as a surprise, it was predicted properlyalready as early as 1964 by Dyson and Xuong and more recently by Z. Y. Zhang et al. , who also canreproduce all measured decay properties. Most recently this state has been also seen in lattice QCDcalculations.Though there is meanwhile added evidence for a number of dibaryonic states near the ∆ N threshold— all of them with large widths — d ∗ (2380) remains so far the only established resonance with asurprisingly small width pointing to a compact hexaquark structure of this state.Key informations about the (unflavored) dibaryon states discussed in this review are summarized inTable 2. For a number of them their existence is not (yet) certain. The column ”evidence” gives a starrating for the presently collected experimental evidence of the envisaged state. It is based on the authors’personal judgment and may serve just as a kind of guideline. The experimentally best established oneis certainly the isoscalar resonance d ∗ (2380) followed by the isovector ∆ N near-threshold state with J P = 2 + .The column ”structure” denotes the asymptotic configuration of the particular state in the courseof its decay into the hadronic channels — or also its hindrance in case of hidden color. The column”experimental information” summarizes recent references to corresponding experimental work. Thecolumn ”theoretical calculation” gives references to theoretical calculations for the particular dibaryonstate, be it predictions or ”postdictions”. From the measurements of the double-pionic fusion to He [33] and He [34] we know that d ∗ (2380)obviously survives in nuclear surroundings. There are several other remarkable enhancements inducedby np pairs inside nuclei. One is seen in di-electron pairs [145, 146] in heavy ion collisions. Thismay be partly due to d ∗ (2380) production inside nuclei [147]. Another is that the np short-rangecorrelation is found to be about 20 times higher than the pp in ( p, p (cid:48) ) and ( e, e (cid:48) ) scattering off nuclei[152, 153, 154]. The question arises, whether an intermediate formation of isoscalar dibaryon states like d ∗ (2380) in the course of the interaction between the nucleons in the nucleus may be an explanationfor this phenomenon. This would be in line with the N N -interaction ansatz by Kukulin et.al. , wherethe short- and intermediate range part of the
N N -interaction is assumed to be due to virtual s -channeldibaryon formation [155, 156]. In fact, inclusion of d ∗ (2380) leads to a quantitative description of the D − G phase shifts in both its real and imaginary parts [157]. Similar good results are obtained formost of the partial waves with low orbital momentum, when the N N -coupled dibaryon states given inTable 2 are included [116, 156, 157]. 19able 2: Unflavored Dibaryon Resonances (including candidates discussed in this review)below or near ∆(1232) N , N ∗ (1440) N and ∆(1232)∆(1232) thresholds. The resonance statesare characterized by their isospin I , spin-parity J P , involved N N partial wave ( S +1 L J withspin S , orbital angular momentum L and total angular momentum J ), mass m and widthΓ. The column ”evidence” shows a star rating based on the collected experimental evidencefor the envisaged state (or candidate). The column ”structure” denotes the asymptoticconfiguration of the particular state in the course of its decay into the hadronic channels— or also its hindrance in case of hidden color. The column ”experimental information”summarizes recent references to corresponding experimental work. The column ”theoreticalcalculation” gives references to theoretical work for the particular dibaryon state.I J P ( S +1 L J ) NN m (MeV) Γ (MeV) evidence asympt. experimental theoreticalstructure information calculation0 1 + 3 S − D N ∗ N [116, 117] [116]0 3 + 3 D − G (cid:42)(cid:41) [11, 35, 36, 37] [82, 52, 59, 73] hexaquark [40, 51, 53, 54] [79, 84, 86, 88] hidden color [68, 70, 65, 66] [97, 98, 94, 95][57, 58, 62, 55][56, 107, 100, 101][102, 157]0 ? ? 2469(2) 120(3) * ? [69]0 ? ? 2632(3) 132(5) * ? [69]1 0 + 1 S N ∗ N [116] [116]1 0 − P N [111]1 2 − P − F N [16, 21, 111] [156]1 2 + 1 D N [5, 16, 18, 19] [12, 56, 110, 150][156]1 3 − F N [16, 21] [156]1 4 − F − H N [16, 21]2 1 + 3 P + π N [148, 149] [12, 56, 150]3 0 + 1 S + ππ d ∗ (2380) appears to exist in nuclear matter, it can influence the nuclear equation of state,especially in compact stellar objects like neutron stars. A study finds d ∗ (2380) to appear at densitiesthree times the saturation density and to constitute around 20% of the matter in the center of neutronstars [158] depending on the assumed interaction of d ∗ (2380) with its surroundings [159].Also, since dibaryons are bosons, one may think about a Bose-Einstein condensate formed by d ∗ (2380) hexaquarks. In a first study of such a scenario it has been pointed out that stable d ∗ (2380)condensates could have formed in the early universe constituting even a candidate for dark matter [160]. We acknowledge valuable discussions with M. Bashkanov, Stanley J. Brodsky, Y. B. Dong, A. Gal, T.Goldman, Ch. Hanhart, Fei Huang, V. Kukulin, E. Oset, M. Platonova, P. N. Shen, I. I. Strakovsky,Fan Wang, C. Wilkin, R. Workman and Z. Y. Zhang. This work has been supported by DFG (CL214/3-3). One of us (H. Cl.) appreciates the support by the Munich Institute for Astro- and ParticlePhysics (MIAPP) which is funded by the Deutsche Forschungsgemeinschaft (DFG, German ResearchFoundation) under Germany’s Excellence Strategy – EXC-2094 - 390783311.
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