aa r X i v : . [ h e p - ph ] J a n Noname manuscript No. (will be inserted by the editor) N ∗ Production from e + e − Annihilations
Bing-Song Zou
Received: date / Accepted: date
Abstract
Up to now, the N ∗ production from e + e − annihilations has beenstudied only around charmonium region. Charmonium decays to N ∗ s are anal-ogous to (time-like) EM form factors in that the charm quark annihilationprovides a nearly pointlike (ggg) current. Complementary to other sources,such as πN , eN and γN reactions, this new source for N ∗ spectroscopy hasa few advantages, such as an isospin filter and a low spin filter. The exper-imental results on N ∗ from e + e − annihilations and their phenomenologicalimplications are reviewed. Possible new sources on N ∗ production from e + e − annihilations are discussed. Keywords N ∗ spectrum · electron-positron annihilation · hadron structure Historically the study of spectroscopy at various microscopic levels of matterproves to be a powerful tool to explore the relevant structures and interac-tions. About a hundred years ago, the study of atomic spectroscopy revealedthe quantum physical picture for atoms and played a important role for thedevelopment of quantum mechanics. Around the middle of last century, thestudy of nuclear spectroscopy led to the two Nobel prize-winning works: nu-clear shell model and collective motion model. With the quark model developed
This work is supported in part by DFG and NSFC through funds provided to the Sino-German CRC 110 “Symmetry and the Emergence of Structure in QCD” (NSFC Grant No.11621131001), as well as an NSFC fund under Grant No. 11647601.Bing-Song ZouCAS Key Laboratory of Theoretical Physics, Institute of Theoretical Physics, and Universityof Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100190, ChinaTel.: +86-10-62562584Fax: +86-10-62562587E-mail: [email protected] Bing-Song Zou in the early 1960s, it became clear that the hadrons are not elementary par-ticles, but composed of quarks and antiquarks. In the classical quark model,a baryon is composed of three quarks and a meson is composed of one quarkand one antiquark. The only stable hadron is the proton which was proposedto be composed of two u-quarks and one d-quark. Since then, the protons wereused as targets to be bombarded by pion, electron, photo beams to explorethe spectroscopy of excited nucleons, N ∗ resonances [1,2,3,4].With accumulation of half century on the N ∗ spectroscopy, two outstand-ing problems appeared for the classical simple 3q constituent quark model.The first outstanding problem is that the mass-order for the lowest excitedstates is reversed. In the simple 3q constituent quark model, the lowest spa-tial excited baryon is expected to be a (uud) N ∗ state with one quark inorbital angular momentum L = 1 state, and hence should have negative par-ity. Experimentally [4], the lowest negative parity N ∗ resonance is found tobe N ∗ (1535), which is heavier than N ∗ (1440) of positive parity. The secondoutstanding problem is that in many of its forms the classical 3q quark modelpredicts a substantial number of ‘missing N ∗ states’ around 2 GeV/ c , whichhave not so far been observed [5].The first problem suggests that we should go beyond the simple 3q quenchedquark model. It can be reconciled by taking these N ∗ s as meson-baryon dynam-ically generated states [6,7,8,9,10,11] or considering large 5-quark componentsin them [12,13,14].For the second problem, non-observation of these ‘missing N ∗ states’ doesnot necessarily mean that they do not exist. Their couplings to πN and γN may be too weak to be observed by presently available πN and γN experi-ments [5]. Other production processes should be explored. Joining the efforton studying the excited nucleons, N ∗ baryons, BES started a baryon reso-nance program [15] at Beijing Electron-Positron Collider (BEPC) just beforethe start of new century. The J/ψ and ψ ′ experiments at BES provide anexcellent place for studying excited nucleons and hyperons – N ∗ , Λ ∗ , Σ ∗ and Ξ ∗ resonances [16,17].In the following, the major experimental results on N ∗ from e + e − an-nihilations for last 20 years and some of their interesting phenomenologicalimplications are reviewed. N ∗ production from ¯ cc decays Since 2001, BES/BESII/BESIII Collaborations have published their resultson N ∗ production from J/ψ → ¯ ppη [18], p ¯ nπ − + c.c. [19], p ¯ pπ [20], pK − ¯ Λ + c.c. [21], ¯ nK S Λ [22], ¯ ppω [23], ¯ ppφ [24], ¯ ppπ η [25], and ψ (2 S ) → ¯ ppη [26], p ¯ nπ − + c.c. [27], p ¯ pπ [28], ¯ pK + Σ [29], and χ cJ → p ¯ nπ − [30], p ¯ nπ − π [30],¯ pK + Λ [29], and ψ (3770) → p ¯ pπ [31]. Some interesting insights on the N ∗ shave been gained through this novel source of information.In Fig.1, the invariant mass data corrected by MC simulated efficiency andphase space versus pπ − (or ¯ pπ + ) invariant mass for J/ψ → p ¯ nπ − + c.c. and ∗ Production from e + e − Annihilations 3 p p - or p – p + invariant mass (GeV/c ) | A | Fig. 1
Data corrected by MC simulated efficiency and phase space versus pπ − (or ¯ pπ + )invariant mass for J/ψ → p ¯ nπ − + c.c. [19] (left) and ψ ′ → p ¯ nπ − + c.c. [27] (right). M(p(p – ) p ) E v e n t s / ( M e V / c ) GeV/c ) (GeV/c p p M ) E v e n t s / ( M e V / c Fig. 2 pπ (or ¯ pπ ) invariant mass for J/ψ → p ¯ pπ [20] (left) and ψ ′ → p ¯ pπ [28] (right). ψ ′ → p ¯ nπ − + c.c. are shown together for a comparison. Similarly, in Fig.2, pπ (or ¯ pπ ) invariant mass for J/ψ → p ¯ pπ and ψ ′ → p ¯ pπ are presented.Compared with N π invariant mass spectrum from πp or γp reactions, anobvious phenomena is that there are more N ∗ peaks meanwhile without thestrong ∆ peak. This is because ψ annihilates into a baryon-antibaryon pairthrough three gluons and conserves isospin. The N π recoiling an anti-protonis limited to be isospin 1/2. So the charmonium annihilation provides a niceisospin filter. Due to the non-presence of the strong ∆ peak in other reactions,the N ( πN invariantmass spectrum. Besides several well known N ∗ resonances around 1520 MeVand 1670 MeV, three new N ∗ resonances above 2 GeV were found throughdelicate partial wave analyses. They are N ∗ (2040)3 / + , N ∗ (2300)1 / + and Bing-Song Zou
Fig. 3 pK (left) and KΛ (middle) invariant mass spectra for J/ψ → pK − ¯ Λ +c.c., comparedwith phase space distribution; right: Dalitz plot for J/ψ → pK − ¯ Λ +c.c. [21] N ∗ (2570)5 / − . An additional advantage of this reaction is that it not onlyselects isospin 1/2 states but also suppresses high spin states due to the shortrange interaction involved in the ¯ cc annihilation that generates the N π system.The suppression of higher spin states greatly simplifies partial wave analysis.Another interesting phenomena is that the N ∗ (1440) is produced muchstronger from ψ (2 S ) than from J/ψ . There are two common features for ψ (2 S ) and N ∗ (1440): 1) they are supposed to be radial excitation of J/ψ and nucleon, respectively, in the simple quenched quark model; 2) they werefound experimentally to have large coupling to σJ/ψ and σN , respectively. Inunquenched quark models, radial excitations like to pull out ¯ q q (0+) fromsea, hence favor transition between each other. This unquenched picture notonly gives a natural explanation of much enhanced N ∗ (1440) production from ψ (2 S ) than J/ψ , may also explain the long-standing ρπ puzzle [17] from ψ (2 S )and J/ψ decays, i.e. , ψ (2 S ) tends to decay into ρ (2 S ) π while J/ψ tends todecay into ρπ . CLEO Collaboration also studied ψ (2 S ) → ¯ ppπ channel andgot a similar strong N ∗ (1440) peak [32]. There is no obvious N ∗ (1440) peakfor e + e − → p ¯ pπ in the vicinity of the ψ (3770) [31].In Fig.3, the Dalitz plot and corresponding invariant mass spectra are pre-sented for J/ψ → pK − ¯ Λ and ¯ pK + Λ channels [21]. There are clear Λ ∗ peaks at1.52 GeV, 1.69 GeV and 1.8 GeV in pK invariant mass spectrum, and N ∗ peaksnear KΛ threshold, 1.9 GeV and 2.05 GeV for KΛ invariant mass spectrum.The N ∗ peak near KΛ threshold is most probably due to N ∗ (1535). Com-bined with information on N ∗ (1535) from J/ψ → ¯ ppη [18] as well as COSYdata on pp → pK + Λ , a large coupling to KΛ was found for the N ∗ (1535) [13].This supports it to be a KΣ - KΛ dynamically generated state with large hid-den strangeness component. Extending this picture from strangeness to charmand beauty, super-heavy N ∗ with hidden charm [33] or hidden beauty [34] werepredicted to exist around 4.3 GeV and 11 GeV, respectively. Two super-heavy N ∗ states with hidden charm were later discovered by LHCb experiment [35]from Λ b decays. Their meson partners Z c states were also discovered by BESIIICollaboration [36,37] and other experiments as reviewed in Refs.[38,39]. ∗ Production from e + e − Annihilations 5 ) )(GeV/c - pL M( ) E ve n t s / ( M e V / c ) ) (GeV/c L - M(K ) E v e n t s / ( M e V / c ) ) (GeV/c L - M(K ) E v e n t s / ( M e V / c ) ) (GeV/c L - M(K ) E v e n t s / ( M e V / c ) ) (GeV/c L - M(K ) E v e n t s / ( M e V / c Fig. 4 Λπ − invariant mass for ψ (2 S ) → Λ ¯ Σ ± π ∓ [41] (left) and K − Λ invariant mass for ψ (2 S ) → K − Λ ¯ Ξ + + c.c. [42] (right). Besides N ∗ resonances, some hyperon resonances were also studied by BESIIIfrom J/ψ → γΛ ¯ Λ [40], and ψ (2 S ) → ¯ pK + Σ [29], Λ ¯ Σ ± π ∓ + c.c. [41], ψ (2 S ) → K − Λ ¯ Ξ + + c.c. [42].Two typical invariant mass plots for hyperon resonances are shown in Fig.4.Clear resonance peaks are observed for Σ ∗ and Ξ ∗ resonances. There is aclear Σ ∗ peak around 1580 MeV which can be fitted well with the 1-star Σ (1580)3 / − resonance of PDG [4]. In 2012, by analyzing K − p → π Λ data,we also found some evidence for a Σ ∗ (3 / − ) resonance around 1542 MeV [44].A Σ ∗ (3 / − ) around 1560 MeV was expected by unquenched quark model [14].For e + e − annihilations at energies above Λ c ¯ Λ c threshold, the Λ c decaysprovide a new source on the N ∗ and hyperon spectroscopy. Recently, BelleCollaboration observed a very narrow Λ ∗ peak around 1670 MeV in the pK invariant mass spectrum in Λ + c → pK − π + [45]. This is consistent with aprevious observation of a very narrow Λ ∗ (1670)1 / − from analyzing K − p → Λη data [46]. If it is confirmed, it would be a natural candidate of [ ud ] ss ¯ s pentaquark state which can only decay to Λη through strongly suppressed D-wave decay. It is important to check its existence through Λη invariant massspectrum of Λ + c → Ληπ + . For e + e − annihilations at energies above Λ b ¯ Λ b threshold at super-B or super-Z factories, its Λ b decays would provide a muchcleaner source than LHCb experiment to look for super-heavy N ∗ and hyperonresonances with hidden-charm.With further accumulation on charmonium decays, there are many moreinteresting channels can be explored, such as ¯ ΩΞ ¯ K , ¯ ΞΞπ , ¯
ΛΛγ , ¯
ΣΛγ , ¯
ΣΣγ ,¯ ΞΞγ , etc., with Ω → ΛK − and Ξ → Λπ . While CEBAF at JLab has advan-tage for studying radiative decays of N ∗ and ∆ ∗ , BESIII may have advantageto study radiative decays of Λ ∗ , Σ ∗ and Ξ ∗ . To complete N ∗ , Λ ∗ , Σ ∗ , Ξ ∗ spec- Bing-Song Zou tra and establish the lowest Λ ∗ , Σ ∗ , Ξ ∗ and Ω ∗ with partial wave analysis, asuper τ -charm factory may be needed. References
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