BESII Results on Hadron Spectroscopy Shan JIN (for BES Collaboration) Institute of High Energy Physics (IHEP) QCD and Hadronic Physics Beijing, June 16, 2005

Multi-quark State, Glueball and Hybrid  Hadrons consist of 2 or 3 quarks ： Naive Quark Model ：  New forms of hadrons: Multi-quark states ： Number of quarks > ＝ 4 Hybrids ： qqg ， qqqg … Glueballs ： gg ， ggg … Meson （ q q ） Baryon （ q q q ）

Multi-quark states, glueballs and hybrids have been searched for experimentally for a very long time, but none is established. However, during the past two years, a lot of surprising experimental evidences showed the existence of hadrons that cannot (easily) be explained in the conventional quark model. Most of them are multi-quark candidates. Searching for multi-quark states becomes the hottest topic in the hadron spectroscopy.

J/  decays are an ideal factory to search for and study light exotic hadrons:  The production cross section of J/  is high.  The production BR of hadrons in J/  decays are one order higher than  ’ decays (“12% rule”).  The phase space to 1-3 GeV hadrons in J/  decays are larger than  decays.  Exotic hadrons are naively expected to have larger or similar production BR to conventional hadrons in J/  decays.  Clean background environment compared with hadron collision experiments, e.g., “J P, I” filter.

BESII VC:  xy = 100  m TOF:  T = 180 ps  counter:  r  = 3 cm MDC:  xy = 220  m BSC:  E/  E= 22 %  z = 5.5 cm  dE/dx = 8.5 %   = 7.9 mr B field: 0.4 T  p/p=1.7%  (1+p 2 )  z = 2.3 cm

World J/  and  (2S) Samples (10 6 ) J/   (2S)

Outline  Multi-quark Candidates at BESII A possible bound state: mass threshold enhancement in and new observation of X(1835). mass threshold enhancement in mass threshold enhancement in – evidence of a K  bound/resonant state. Search for pentaquark Θ + (1540)  Scalar Mesons at BESII σ, κ, f 0 (980), f 0 (1370), f 0 (1500), f 0 (1710), f 0 (1790)  Evidence of new N*(2065) at BESII

A possible ppbar bound state: ppbar mass threshold enhancement

Observation of an anomalous enhancement near the threshold of mass spectrum at BES II M=1859 MeV/c 2  < 30 MeV/c 2 (90% CL) J/    pp M(pp)-2m p (GeV) body phase space acceptance  2 /dof=56/56 acceptance weighted BW  10  25 BES II Phys. Rev. Lett. 91, (2003)

 With threshold kinematic contributions removed, there are very smooth threshold enhancements in elastic “matrix element” and very small enhancement in annihilation “matrix element”:  much weaker than what BES observed ! NO strong dynamical threshold enhancement in cross sections (at LEAR) |M| 2 BES Both arbitrary normalization

Any inconsistency? NO!  For example: with M res = 1859 MeV, Γ = 30 MeV, J=0, BR(ppbar) ~ 10%, an estimation based on: At E cm = 2m p + 6 MeV ( i.e., p Lab = 150 MeV ), in elastic process, the resonant cross section is ~ 0.6 mb : much smaller than the continuum cross section ~ 94  20 mb.  D ifficult to observe it in cross sections experimentally.

 J/  decays do not suffer large t-channel “background” as ppbar collision. >> In ppbar collision, the background is much lager (I)

A.Sibirtsev, J. Haidenbauer, S. Krewald, Ulf-G. Meißner, A.W. Thomas, hep- ph/ P-wave I=0 S-wave I=1 S-wave In ppbar elastic scattering, I=1 S-wave dominant, while in J/  radiative decays I=0 S-wave dominant. ppbar elastic cross section near threshold In ppbar collision, the background is much lager (II)

So, the mechanism in ppbar collision is quite different from J/  decays and the background is much smaller in J/  decays It would be very difficult to observe an I=0 S-wave ppbar bound state in ppbar collisions if it exists. J/  decays (in e+e- collider) have much cleaner environment: “J P, I” filter

This narrow threshold enhancement is NOT observed in B decays  The structure in B decays is obviously different from the BES observation: Belle BES II The structure in B decays is much wider and is not really at threshold. It can be explained by fragmentation mechanism. Threshold enhancement in J/  decays is obviously much more narrow and just at threshold, and it cannot be explained by fragmentation mechanism.

X(1860) has large BR to ppbar  We (BES) measured:  For a 0 -+ meson:  So we would have: (BR would be even larger if the width is wider) Considering that decaying into ppbar is only from the tail of X(1860) and the phase space is very small, such a BR indicates X(1860) has large coupling to ppbar !

The large BR to ppbar suggest it could be an unconventional meson  For a conventional qqbar meson, the BRs decaying into mesons are usually at least one order higher than decaying into baryons. There are many examples in PDG. E.g.  So the large BR to ppbar (with limited phase space from the tail of X(1860)) seems very hard to be explained by a conventional qqbar meson.

pp bound state (baryonium)? + n+  deuteron: loosely bound 3-q 3-q color singlets with M d = 2m p -  baryonium: loosely bound 3-q 3-q color singlets with M b = 2m p -  ? attractive nuclear force attractive force? There is lots & lots of literature about this possibility E. Fermi, C.N. Yang, Phys. Rev. 76, 1739 (1949) … I.S. Sharpiro, Phys. Rept. 35, 129 (1978) C.B. Dover, M. Goldhaber, PRD 15, 1997 (1977) … A.Datta, P.J. O’Donnell, PLB 567, 273 (2003)] M.L. Yan et al., hep-ph/ B. Loiseau et al., hep-ph/ …

ppbar bound state in NNbar potential  Paris NNbar potential: ( Paris 93, B. Loiseau et al., hep-ph/ , ) For 11 S 0, there is a bound state: E = i 26.3 MeV quite close to the BES observation.  However, Julich NNbar model: ( A. Sibirtsev et al., hep-ph/ ) For 11 S 0 : E = i 413 MeV seems quite far away from BES observation. They both predict an 11 S 0 ppbar bound state, although they are quantitatively different. Observations of this structure in other decay modes are desirable.

A possible ppbar bound state: New Observation of X(1835) in

Analysis of X(1835) 5.1  BESII Preliminary BESII Preliminary

Analysis of X(1835) 6.0  BESII Preliminary

Observation of X(1835) in The  +  -  mass spectrum for  decaying into  +  -  and   Statistical Significance 7.7  BESII Preliminary

Mass spectrum fitting 7.7  The  +  -  mass spectrum for  decaying into  +  -  and   BESII Preliminary

X(1835) could be the same structure as X(1860) indicated by ppbar mass threshold enhancement  X(1835) mass is consistent with the mass of the s-wave resonance X(1860) indicated by the ppbar mass threshold enhancement. Its width is 1.9  higher than the upper limit of the width obtained from ppbar mass threshold enhancement.  On the other hand, if the FSI effect is included in the fit of the ppbar mass spectrum, the width of the resonance near ppbar mass threshold will become larger.

Fit to J/   p pbar including FSI Include FSI curve from A.Sirbirtsev et al.(hep-ph/ ) in the fit (I=0) M =  6.7 MeV  = 0  93 MeV In good agreement with X(1835)

A Possible ppbar Bound State  X(1835) could be the same structure as ppbar mass threshold enhancement.  It could be a ppbar bound state since it dominantly decays to ppbar when its mass is above ppbar mass threshold.  Its spin-parity should be 0 -+ : this would be an important test.

Signature of a ppbar Bound State   ’  mode is expected to be the most favorable decay mode for a ppbar bound state below ppbar mass threshold ( G.J. Ding and M.L. Yan, hep-ph/ ) : There are gluon contents in proton and anti-proton.  ’ has strong coupling to gluons.

Observation of mass threshold enhancement in

Observation of an anomalous enhancement near the threshold of mass spectrum at BES II BES II 3-body phase space For a S-wave BW fit: M = 2075  12  5 MeV Γ = 90  35  9 MeV Phys. Rev. Lett. 93, (2004)

Similar enhancement also observed in 4  away from phase space.

Possible Interpretations  FSI ？ Theoretical calculations are needed.  Conventional K* or a multiquark resonance ？ Search for its Kπ 、 Kππ decay modes would help to understand its nature. We are now studying J/   KKπ 、 KKππ

K  mass threshold enhancement – evidence of a K  bound/resonant state

PS, eff. corrected Observation of a strong enhancement near the threshold of mass spectrum at BES II (Arbitrary normalization) BES II NX*NX*

 A strong enhancement is observed near the mass threshold of M K  at BES II.  Preliminary PWA with various combinations of possible N* and Λ* in the fits —— The structure N x *has: Mass 1500~1650MeV Width 70~110MeV J P favors 1/2 - The most important is: It has large BR(J/ψ  pN X *) BR(N X *  KΛ)  2 X 10 -4, suggesting N X * has strong coupling to KΛ.

This enhancement is NOT observed in process at SAPHIR

Discussion on KΛ mass threshold enhancement N X (1610)  N X (1610) has strong coupling to KΛ: From (S&D-wave decay) and is a P-wave decay, we can estimate From BESII, The phase space of N X to KΛ is very small, so such a big BR shows N X has very strong coupling to KΛ, indicating it has a big hidden ssbar component. (5-quark system)

Non-observation of N X in suggests an evidence of new baryon:  It is unlikely to be N*(1535). If N X were N*(1535), it should be observed in process, since: From PDG, for the N* in the mass range 1535~1750 MeV, N*(1535) has the largest, and from previous estimation, N X would also have almost the largest BR to KΛ.  Also, the EM transition rate of N X to proton is very low.

A ΛK resonance predicted by chiral SU(3) quark model  Based on a coupled- channel study of ΛK and ΣK states in the chiral SU(3) quark model, the phase shift shows the existence of a ΛK resonance between ΛK and ΣK mass threshold. ( F. Huang, Z.Y. Zhang et al. hep-ph/ ) E cm – ( M Λ +M K ) (MeV)

 The KΛ mass threshold enhancement N X (1610) could be a KΛ bound/resonant state.

Is the STRONG threshold enhancement universal/naïve in J/  decays ? —— NO !  Actually in many other cases we do NOT see STRONG threshold enhancements !  For example: In J/  decays at BES II

Searching for pentaquark Θ + (1540) at BESII

Searching for Θ + (1540) at BESII Upper 90% C.L. BR (  (2S)     (K S p)(K - n) + (K S p)(K + n)) < 0.84X10 -5 BR ( J/      (K S p)(K - n) + (K S p)(K + n)) < 1.1 X10 -5  (2S)J/  Phys. Rev. D 70, (2004)

Light Scalar Mesons at BESII: σ, κ, f 0 (980), f 0 (1370), f 0 (1500), f 0 (1710), f 0 (1790)

Why are light scalar mesons interesting?  There have been hot debates on the existence of σ and κ.  σ, κ and f 0 (980) are also possible mutiquark states. They are all near threshold.  Lattice QCD predicts the 0 ++ scalar glueball mass ~ 1.6 GeV. f 0 (1500) and f 0 (1710) are good candidates.

σ at BES  BES II observed σ in J/    +  -.  Pole position from PWA: BES II Phys. Lett. B 598, 149 (2004)

 BES II observed  in J/  K*K  K  K .  Preliminary PWA result Pole position: κ at BES BES II Preliminary

 Important parameters from PWA fit:  Large coupling with KK indicates big component in f 0 (980) f 0 (980) at BES f 0 (980) Phys. Lett. B 607, 243 (2005)

 There has been some debate whether f 0 (1370) exist or not.  f 0 (1370) clearly seen in J/   , but not seen in J/   . PWA 0 ++ components f 0 (1370) NO f 0 (1370) f 0 (1370) at BES

 Clear f 0 (1710) peak in J/    KK.  No f 0 (1710) observed in J/    ! f 0 (1710) at BES f 0 (1710) NO f 0 (1710) Phys. Lett. B 603, 138 (2004)

 A clear peak around 1790 MeV is observed in J/   .  No evident peak in J/    KK. If f 0 (1790) were the same as f 0 (1710), we would have: Inconsistent with what we observed in J/   ,  KK New f 0 (1790) at BES? f 0 (1790) ?  f 0 (1790) is a new scalar. Phys. Lett. B 607, 243 (2005)

Scalars in J/   ,  KK  Two scalars in J/    : One is around 1470 MeV, may be f 0 (1500)? The other is around 1765 MeV, is it f 0 (1790) or a mixture of f 0 (1710) and f 0 (1790)?  One scalar f 0 (1710) in J/    KK. BES II Preliminary Phys. Rev. D 68, (2003)

f 0 (1500) at BES  One scalar with a mass = 1466  6  16 MeV is needed in J/   , is it f 0 (1500)? Why the mass is different from PDG?  No peak directly seen in ,  KK, ,  KK.

OZI rule and flavor tagging in J/  hadronic decays  In J/  hadronic decays, an  or Φ signal determines the or component, respectively.  OZI rule

Unusual properties of f 0 (1370), f 0 (1710) and f 0 (1790)  f 0 (1710): It dominantly decays to KK (not to  )  It is mainly produced together with  (not  )  What is it ?  f 0 (1370) and f 0 (1790) They dominantly decays to  (not to KK)  It is mainly produced together with  (not  )  What are they ?  Scalar Puzzle  Q.Zhao’s talk

Evidence of new N*(2065) at BES II N*(1440)? N*(1520) N*(1535) N*(1650) N*(1675) N*(1680) ? BES II Preliminary hep-ex/

Summary (I)  BES II has observed several strong mass threshold enhancements in J/  decays.  Why strong mass threshold structures are important? Multiquark states may be only observable near mass thresholds with limited decay phase space.  Otherwise, it might be too wide to be observed as a resonance since it can easily fall apart into two or more mesons. I can see f 0 (980) broad resonance or phase space? any broad resonance under other peaks? I can see broad  under other peaks

Summary (II)  A unique very narrow and strong mass threshold enhancement is observed in decays at BES II: It is not observed in cross sections. It is not observed in B decays. Its large BR to suggests it be a bound state.  X(1835) is observed in It could be same structure as the ppbar mass threshold enhancement, i.e., it could be a ppbar bound state.

 mass threshold enhancement was observed in  Evidence of N X (1610) was observed near KΛ mass threshold, suggesting a KΛ bound/resonant state.  Scalar mesons have been studied carefully (via PWA) in J/ψ hadronic and radiative decay at BESII. We need to understand the scalar puzzle. J/ψ decay is an ideal place to study exotic structures. Summary (III)

For other interesting results from BES II: There are three BES talks in the parallel sessions:  S.S. FANG: Recent results on J/  decays  Z. WANG: Recent results on  (2S) decays  J.C. CHEN: Recent results on  (3770) and D decays

谢 谢！ Thank You!

MARK-III & DM2 Results Threshold enhancement Claimed in Phys. Rep. 174(1989) Too small statistics to draw any conclusion on the threshold enhancement, e.g., cannot exclude known particles such as  (1760) MARK-III DM2

 With threshold kinematic contributions removed, there are very smooth threshold enhancements in elastic “matrix element” and very small enhancement in annihilation “matrix element”:  much weaker than what BES observed ! NO strong dynamical threshold enhancement in cross sections (at LEAR) |M| 2 BES Both arbitrary normalization

Final State Interaction ? —— Not favored 1.Theoretical calculation (Zou and Chiang, PRD (2003)) shows: The enhancement caused by one-pion-exchange (OPE) FSI is too small to explain the BES structure. 2.The enhancement caused by Coulomb interaction is even smaller than one-pion-exchange FSI ! BES one-pion-exchange FSI |M| 2 Both arbitrary normalization BES Both arbitrary normalization Coulomb interaction

 One-pion-exchange FSI may be too simple, however, FSI does not contradict with ppbar bound state interpretation.  Keep in mind: We need to explain its large BR to ppbar.

Final State Interaction ? —— Not favored Theoretical calculation might be unreliable, however, according to Watson’s theorem, we can use elastic scattering experiments to check the FSI effect, i.e., if the BES structure were from FSI, it should be the same as in elastic scattering : But it is NOT !  FSI cannot explain the BES structure. elastic scattering |M| 2 BES Both arbitrary normalization

 No matter what are the interpretations for the recent new surprising observations, these discoveries certainly open a new window for understanding the strong interaction and hadron spectroscopy.  We need to have a global picture if there are new forms of hadrons beyond the naïve quark model —— any pentaquark, tetraquark, molecular state or other multiquark states cannot stay alone ! A lot of opportunities for BES III and other experiments working on hadron spectroscopy !!! Prospects

BCK subtracted PS corrected

BCK subtracted PS corrected

BCK subtracted PS corrected

Photon angular distributions in  For X(1835) signal MC (assuming 0 -+ ) MC– MCMADE MC – After Selection

Photon angular distributions in  For Data – it is not inconsistent with statistics is too low. X(1835) region X(1835) sideband Sideband Subtracted & Eff. corrected

 f 0 (980) is observed in    f 0 (980)    at KLOE (Background: ISR, FSR and  ).  The results support that f 0 (980) has large coupling with KK. f 0 (980) at KLOE KLOE Preliminary f 0 (980) Background subtracted

 ZEUS observed a narrow peak around 1730 MeV in K S K S mode:  Its width is much narrower than BES and others. Spin-parity needed.  How many mesons in this mass range? f 0 (1710) (?) at ZEUS f 0 (1710) ???