RRecent results from Belle
T. V. Uglov a ∗ a Institute for Theoretical and Experimental PhysicsB. Cheremushkinskaya 25, Moscow, Russia
Abstract
The review on experimental results on charmonium and charmonium-like spectroscopyfrom B-factories is presented. Main theoretical interpretations, such as conventional char-monium, molecular state, hybrids, tetraquarks and others are discussed.
Operation of two B-factories, Belle [1] and BaBar [2], is likely the most significant success in theexperimental particle physics of the last decade. Their brilliant work on CP-violation studiesin B-mesons leads to Nobel Prize winning by Kobayashi and Maskawa. In addition to theCPV studies B-factories also made a huge contribution to the hadron spectroscopy, τ -, charmand two-photon physics. One of the most prominent discovery made by the B-factories is theobservation of new charmonium-like states, so called X , Y and Z mesons. In this paper anexperimental review of these states discovery and studies at B-factories is presented, principaltheoretical theoretical interpretations are discussed. The Belle detector is a large-solid-angle magnetic spectrometer that consists of a silicon vertexdetector (SVD), a 50-layer central drift chamber (CDC), an array of aerogel threshold Cherenkovcounters (ACC), a barrel-like arrangement of time-of-flight scintillation counters (TOF), andan electromagnetic calorimeter (ECL) comprised of CsI(Tl) crystals located inside a supercon-ducting solenoid coil that provides a 1 . K L mesons and to identify muons (KLM). Two inner detectorconfigurations were used. A beampipe at radius 2 . ∼ f b − , while a 1 . ∼ f b − . The charmed particles are known already more than for 35 years. The first state,
J/ψ wasobserved by Ting and Richter [3] in 1974, ten years after the c -quark theoretical prediction.There was a great progress in charmonium spectroscopy during next 6 years. Another 9 ( c ¯ c )states shown in Fig 1 were discovered: ψ (2 S ) η c , χ c , χ c , χ c , ψ (3770), ψ (4040), ψ (4160)and ψ (4415). Between 1980 and 2002 not a single new charmonium state was observed. Thebeginning of the new charmonium boom coincides with start of the B-factories operation.There are several processes of charmonium production at the B-factories: ∗ e-mail : [email protected] a r X i v : . [ h e p - e x ] N ov . production in B meson decays;2. production in e + e − annihilation through ISR photon;3. double charmonium production in e + e − annihilation;4. production in two photon collisions.Some of these mechanisms allow to fix quantum numbers of the final states. All charmoniumstates produced in e + e − annihilation have photon quantum numbers J P C = 1 −− ; ( c ¯ c ) pairsfrom the double charmonium production have opposite C -parities. Two photon production fix C = +1 and forbids J = 1.Most of more than ten new charmonium-like states discovered at B-factories do not fitthe conventional charmonium spectroscopy scheme. Observation of charmonium-like stateswith forbidden quantum numbers, charged or extremely narrow width is the direct evidence ofthe non-conventional spectroscopy. Thus the studies of the new states could lead to the newrevolution in the hadron physics. ( ) The first and may be the most mysterious charmonium-like particle, X (3872), was found byBelle collaboration in 2003 [4] in B + → J/ψπ + π − K + exclusive decay. Except for well-known ψ (2 S ) resonance, a new peak of more than 10 σ significance at mass (3872 . ± . ± .
5) MeV /c was found in the J/ψπ + π − mass spectrum. Mass of the new state and presence of ( c ¯ c ) pairin the decay products indicated charmonium nature of X (3872). However it’s compatible withzero width (Γ tot < . CL ) as well as mass equal within errors to the sum of D ∗ and D mesons rise doubts on this interpretation. Until now it is unknown whether the massof X (3872) is above or below D D ∗ threshold: ∆ m = − . ± .
40 MeV /c . An existence andproperties of X (3872) was confirmed by CD, D0 and BaBar collaborations [5].So narrow width for the state ∼
138 MeV above the DD threshold and absence of X (3872) → DD decay (Γ( X → DD ) / Γ( X → J/ψπ + π − ) < CL [6]) means thatthe latter is forbidden either by parity conservation (if X (3872) has unnatural spin-parity J P = 0 − , + , − , . . . ) or high orbital momentum, e.g. J P = 3 − , + , . . . Presence of gluonor pair of the light quarks except for ( c ¯ c ) pair in X (3872) could also leads to suppression.Observed enhancement at higher mass region in m ( π + π − ) spectrum could be interpreted as X → J/ψρ decay [7]. Since charmonium decay to J/ψρ violates isospin symmetry observationof this decay would be a strong argument against charmonium nature of X (3872) state. Firstevidence of X (3872) radiative decays was reported by Belle in 2005 [8] and confirmed by BaBarcollaboration in 2009 [9]. Decay X (3872) → J/ψγ was observed with signal significance above3 . σ , thus fixing C ( X ) = +1. Positive charge parity of X supports hypothesis of J/ψρ decay.Direct quantum numbers measurements are possible with angular analysis. Performed bythe Belle collaboration [7] it excludes J P C = 0 ++ and 0 + − possibilities. More accurate analysisdone by CDF [10] excludes all J P C for J ≤
3, except for 1 ++ and 2 − + . Recent BaBar studies [11]of X (3872) → J/ψω decay insignificantly favours negative parity assignment. ( ) Another puzzling state, Y (3940), was found by Belle collaboration [12] in B + → J/ψωK + decays. Near-threshold event excess in J/ψω spectrum was interpreted as positive parityresonance of mass (3943 ± ±
13) MeV /c and width (87 ± ±
26) MeV. Three yearslater this state was confirmed by BaBar [13], however measured resonance mass and width,(3914 . ± ± .
9) MeV /c and width (34 +12 − ±
6) MeV, respectively, differs from previously2igure 1: Charmonium spectroscopy.reported by Belle. Signal was found both for the charged and neutral B meson decays withsurprising ratio for this two channels N ( B ± ) /N ( B ) = 0 . +0 . − . , about 3 σ below predictionsbased on isospin symmetry. Observation of two-photon Y production [14] with mass and widthsimilar to the BaBar values fix J Y to 0 or 2. Quantum numbers of Y could be determined viaangular analysis. These results will clear up the nature of the resonance. Double charmonium production is another source of charmonium-like states. Studying recoilmass spectrum against
J/ψ , Belle collaboration found [15], among known charmonium states, η c , χ c and η c (2 S ), another narrow with respect to the resolution Γ tot <
52 MeV @ 90% CL peak of at mass (3943 ± ±
6) MeV /c . The discovery was soon confirmed by exclusive processreconstruction in e + e − → J/ψDD ∗ channel [16]. Studies of similar final state J/ψD ∗ D ∗ leadsto discovery of another narrow state of width of (139 +111 − ±
21) MeV with mass (4156 +25 − ) MeV /c decaying to D ∗ D ∗ . Although mass and width of new X (4160) resonance agrees with those of3nown ψ (4160) charmonium state, charge parity, fixed by their production mechanisms, isopposite. /ψϕ resonances: Y ( ) & Y ( ) Last year CDF reported evidence for the new narrow state [17] with mass of 4143 . ± . ± . /c and width 11 . +0 . − . ± . . σ significance was observedin B → J/ψϕK decays of B mesons produced in p ¯ p at √ s ∼ .
96 TeV. Belle collaborationdoes not confirm this peak [18], however low efficiency near the threshold disallows to reportthe contradiction. Search for this state in γγ collisions also fails [19]. At the same time a newnarrow structure J/ψϕ mass spectrum of 3 . σ significance was observed at 4 .
35 GeV /c . Beinginterpreted as resonance, this structure has mass and width of (4350 . +4 . − . ± . /c ) and(13 . +17 . − . ± . Y (4140) and Y (4350) states areless than 4 σ , thus their confirmation is required. ( ) , Y ( ) , Y ( ) and Y ( ) First state of this family was discovered in 2005. Inspired by X (3940) observation, BaBarcollaboration undertake a search [23] for the resonance in J/ψπ − π + spectrum in the e + e − → J/ψπ − π + γ ISR process. A clear peak with mass and width of (4264 ±
12 MeV /c ) and (83 ±
22) MeV, respectively, was found. Similar analysis done by Belle confirms Y (4260), besidesthem another structure at mass of (4008 ± +114 − ) MeV /c and width of (226 ± ±
87) MeVwas found. Fit of the mass distribution with two interfering Brait-Wigner shapes results inunavoidable splitting: there are two solution of the same significance, resonance masses andwidths, but with different amplitudes.In 2007 repeating the analysis for ψ (2 S ) π + π − γ ISR final state [20] BaBar found an evidencefor a new resonance decaying to ψ (2 S ) π + π − . Belle confirms [21] this peak and found another oneat higher mass. The masses and widths of new Y (4350) and Y (4660) states were determinedto be M = 4355 +9 − ± /c , Γ = 103 +17 − ±
11 MeV and M = 4661 +9 − ± /c , Γ =47 +17 − MeV, respectively. The latter states was also found in Λ c Λ c mass spectrum in e + e − → Λ c Λ c γ ISR process [22].
Probably the most speculating charmonium-like states are those of Z-family. The first evidencefor charged charmonium-like state was found by Belle in 2008 [24]. Studying ψ (2 S ) π + massspectrum for B → ψ (2 S ) π + K decay after discarding B → ψ (2 S ) K ∗ events a narrow structure of6 . σ significance near 4 .
43 GeV /c was found. Fit to this distribution returns mass and widthof the new Z + (4430) resonance of (4433 ± ±
2) MeV /c and (45 +18 − − ) MeV, respectively.Next year same data were analysed using more sophisticated technique [25]. Fit to the Dalitzdistribution M ( ψ (2 S ) π + ) vs. M ( Kπ + ) results in similar Z ’s mass and slightly higher widthof (107 +86 − − ) MeV. However, no significant signal of Z (4430) was found by the BaBar collab-oration [26]: only the upper limit have been set B ( B → Z + K ) · B ( Z + → ψ (2 S ) π + ) < . · − ,which is lower than measured by Belle (4 . ± . ± . · − . Search for Z ’s in B → χ c π + K process turns out even more puzzling. Dalitz plot fit to broad structure in χ c π + mass re-sults in two charged charmonium-like resonances with masses of (4051 ± +20 − MeV /c ) and(4248 +44 − − ) MeV /c , and widths of (82 +21 − − ) MeV /c and (177 +54 − − ) MeV, respectively.A hypothesis of two resonance is ∼ . σ more preferable in respect with one Z .4 There are several general interpretations of the new charmonium-like states are now underdiscussion. Their current theoretical and experimental status is summarized in Table 1.State M , MeV /c Γ, MeV J P S
Possible interpretation X (3872) 3871 . ± . < . − + Molecule, χ (cid:48) c , η c (2 S ), tetraquark X (3915) † ± ± / ++ Y (3940) Z (3930) † ± ±
10 2 ++ χ c (2 P ) X (3940) 3942 ± ±
17 0 +? η c (3 S ) Y (3940) 3943 ±
17 87 ±
34 ? ?+ Conventional ( c ¯ c ), hybrid Y (4008) 4008 +82 − +97 − −− non-res J/ψπ + π − X (4160) 4156 ±
29 139 +113 − ?+ η c (4 S ) Y (4260) 4264 ±
12 83 ±
22 1 −− D Y (4350) 4361 ±
13 74 ±
18 1 −− Molecule,tetraquark, hadrocharmonium, hybrid X (4630) † +9 − +41 − −− Y (4660) Y (4660) 4664 ±
12 48 ±
15 1 −− Molecule,tetraquark, hadrocharmonium, hybrid Z (4050) 4051 +24 − +51 − ? Molecule,tetraquark, hadrocharmonium, hybrid Z (4250) 4248 +185 − +320 − ? Molecule,tetraquark, hadrocharmonium, hybrid Z (4430) 4433 ± +35 − ? Molecule,tetraquark, hadrocharmonium, hybridTable 1: Charmonium-like states and their interpretation. Only states shown with † sign arewell interpreted. • Conventional charmonium is the most well known and the only experimentally provedmodel tuned to describe charmonium below the DD threshold. It constrains quantumnumbers of the state: J = L + S ; P = ( − L +1 ; C = ( − L + S .In conventional charmonium model X (3872) state could be interpreted as χ (cid:48) c . Howeverthe measured decay rate B ( X → J/ψγ ) / B ( X → J/ψππ ) < . ∼ X (3872) treatment as η c (2 S ) expects large Γ( X → gg ) andtiny Γ( X → J/ψππ ) which contradicts to the experiment.Interpretation of Y (3940) as conventional charmonium also faced the problems. As-suming that probability of B → Y (3940) K decay is less than 10 − , which is usual for B → ( c ¯ c ) K ; ( c ¯ c ) = η c , J/ψ, χ c , χ c , ψ (2 S ) decays, one could calculate Γ( Y (3940) → J/ψω ) ∼ DD and D ∗ D decays is also counts against charmonium interpretation.A lack of D ( ∗ ) D ( ∗ ) decays also prevents to the Y (4260) interpretation as conventional3 D ( c ¯ c ) state.For X (3940) and X (4160) , candidates for 3 S and 4 S states, respectively, the dis-crepancies between the predicted and measured masses reach ∼
100 MeV /c and ∼
250 MeV /c , respectively.For two states observed in ISR, Y (4360) and Y (4660) there are no vacant 1 −− states inthe conventional charmonium spectroscopy scheme and they are too broad for it. • Molecular state is two mesons loosely bounded by pion or gluon exchange.Molecule is the most popular interpretation of X (3940). Belief that fine coincidencebetween X ’s mass and D ∗ D threshold is not accidental, observed decays to D ∗ D and a tinyradiative decay to J/ψ rate strongly supports molecular hypothesis.
Contra argumentsare too large X → ψ (2 S ) γ decay width and too small binding energy: D mesons are too5ar from each other to be produced in p ¯ p collisions as an entity. However a possibility tomix with χ (cid:48) c state solves all these problems.No evidence for Y (4140) production in two-photon collisions is found for now. Small γγ width disfavours its molecular interpretation.Molecule composed of DD or D D ∗ is a good hypothesis for X (3940) and X (4160) states.Charged Z ’s could be treated as D ( ∗ )( s ) D ( ∗ )( s ) molecule. • Tetraquark is tightly bound four quark state.Tetraquark interpretation predicts a list of new states with small mass spiting, especiallyfor ( c ¯ u )(¯ cu ) , ( c ¯ d )(¯ cd ) and ( c ¯ u )(¯ cd ) combinations. Intensive searches for X (3872) find noevidence neither for charges nor for neutral partners.Some resonances from Y family, Y (4360) and Y (4660) could be treated as (¯ cq )( c ¯ q ) tetraquark, Y (4140) and Y (4350) as ( s ¯ sc ¯ c ) diquark-antidiquark state. For Z ’s there is ( c ¯ q )(¯ cq ) in-terpretation. • Hybrid — meson with excited gluon degree of freedom. Interpretations of Y (4360) and Y (4660) states as hybrids is supported by lattice calculations, however in such a model,contrary to experiment, D ( ∗ ) D ∗∗ decays should dominate. • Hadrocharmonium — charmonium coated by excited hadronic matter. Y (4360) and Y (4660) could be treated as ( c ¯ c )and excited light meson hadrocharmonium. Excitedcharged light meson coating ψ (2 S ) or χ c could give Z ’s. • There are discussion on dynamical nature of new states.
Threshold effects caused byvirtual states near the threshold could be responsible for these structures.
11 Conclusions: a challenge
Results of B-factories, Belle and BaBar, Tevatron experiments (CDF, D0) as well as manyothers (BES etc.) start a new exciting era in the hadron spectroscopy. Seven years passed afterdiscovery of X (3872), first charmonium-like state. More than ten another puzzling structureswere observed since that time and only a tiny fraction of them are interpreted for now. Newhadronic ’zoo’ is a great challenge for theoreticians to explain the nature of these states and forexperimentalist to measure their properties with a highest possible precision. New data from theupcoming Super-B-factories [27] would illuminate the mystery of this charmonium-like familyand hopefully solve XYZ puzzle.
12 Acknowledgements
This work is done with partial support of the Presidental grant MK-4646.2009.2.
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