New Observations and Multiquark Candidates at BESII Shan JIN (for BES Collaboration) Institute of High Energy Physics (IHEP) Charm 2006 Beijing, June 6, 2006

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 ） How quarks/gluons form a hadron is far from being well understood.

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 one of the hottest topics 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.

Outline  A possible bound state: mass threshold enhancement in and new observation of X(1835).  mass threshold enhancement in   mass threshold enhancement in J/     New observation of a broad resonance in J/   K + K -  0

World J/  Samples (10 6 ) J/ 

A possible ppbar bound state

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)

Features of the enhancement near the threshold of mass spectrum at BES II J/    pp M(pp)-2m p (GeV) BES II  Peak position: ~ 0 MeV above threshold  “Width”: ~ 60M eV  Strong (“Height”): (S+B)/B ~ 2 The above features may help us easily to judge whether it is observed in other processes.

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.

 This result cannot be explained by pure FSI effect, since FSI is a universal effect. FSI interpretation of the narrow and strong ppbar threshold enhancement is disfavored.  This indicates that X(1860) has a production property similar to  ’ meson. c.f. : This narrow threshold enhancement is NOT observed in at CLEO No enhancement near threshold

Pure FSI disfavored I=0 S-wave FSI CANNOT fit the BES data. FSI * PS * eff + bck FSI curve from A.Sirbirtsev et al. ( Phys.Rev.D71:054010, 2005 ) in the fit (I=0)

So, pure FSI is strongly disfavored. However, we do not exclude the contribution from FSI.

Re-fit to J/    p pbar including FSI Include FSI curve from A.Sirbirtsev et al. ( Phys.Rev.D71:054010, 2005 ) in the fit (I=0) M =  6.7 MeV  = 0  93 MeV FSI * BW * PS * eff + bck

Crystal Ball results on inclusive photon spectrum of J/psi decays

X(1860) has large BR to ppbar  We (BES) measured:  From Crystal Ball result, we etimate:  So we would have: (This would be the largest BR to ppbar among all known mesons) 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 !

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/ … Observations of this structure in other decay modes are desirable.

What do we expect from J/psi  gamma ppbar results? The baryonium interpretation of the ppbar mass threshold enhancement predicts a new particle around 1.85 GeV which should be observed in other decay mode with full BW resonant structure.

New Observation of X(1835) in PRL 95, (2005)

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

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

Re-fit to J/    p pbar including FSI Include FSI curve from A.Sirbirtsev et al. ( Phys.Rev.D71:054010, 2005 ) in the fit (I=0) M =  6.7 MeV  = < 153 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.

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)

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

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.

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. Phys. Rev. C71: , 2005 ) E cm – ( M Λ +M K ) (MeV)

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

Observation of  mass threshold enhancement

 We studied DOZI process: J/    +  +    +  -  0 K + K -

Daliz Plot

A clear mass threshold enhancement is observed Acceptance

 The radiative decay of J/  has been observed in the 58M J/  data.  A significant structure of  has been found near the mass threshold.  PWA shows the structure favors 0 ++, with a mass, width 105  20  28 MeV, and the corresponding branch ration is (2.61  0.27  0.65)x  It could be a multiquark/hybrid/glueball state.  Its relation with f 0 (1710),f 0 (1790)?

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

New observation of a broad resonance in J/   K + K -  0

J/   K + K -  0 very clean  0 signal

Background J/   K + K -  0 PID and kinematic fit can significantly reduce the dominant background from J/    +  -  0.

 Four decay modes are included :  Amplitudes are defined by Covariant tensor formalism B.S. Zhou and D.V. Bugg, Eur. Phys. J. A16, 537(2003)  BW with energy-dependent width J.H. Kuhn, A. Satamaria, Z. Phys. C48, 445 (1990). Partial Wave Analysis of J/   K + K -  0 events

 Parity conservations in J/   K + K -  0 requires that spin-parity of K + K - should be 1 --,3 --,…  PWA fit with and phase space (PS) gives ( preliminary ):  ( can be ruled out by much worse likelihood )  X pole position   big destructive interference among and PS Partial Wave Analysis of J/   K + K -  0 events

Angular distributions for events with from PWA fit  Figures on the right:  (a),(c),(e) are polar angles in lab. reference frame  (b),(d),(f) are polar angles in CM frames of respectively

Broad X cannot be fit with known mesons or their interference  It is unlikely to be  (1450), because: The parameters of the X is incompatible with  (1450).  (1450) has very small fraction to KK. From PDG:  It cannot be fit with the interference of  (770),  (1900) and  (2150): The log-likelihood value worsens by 85 (  2 =170).

How to understand broad X(1580)?  Search of a similar structure in J/   K S K  will help to determine its isospin.  X(1580) could have different nature from conventional mesons: There are already many 1- - mesons nearby. Width is much broader than other mesons. Broad width is expected for a multiquark state.

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 very narrow and strong mass threshold enhancement is uniquely observed in decays at BES II: It is NOT observed in B or Y(1S) 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 or resonant state.  An  mass threshold enhancement was observed in J/   .  A very broad resonance X(1580) is observed in J/   K + K -  0. J/ψ decay is an ideal place to study exotic structures. Summary (III)

谢 谢！ Thank You!

 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.

Pure FSI disfavored (I) 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

FSI Factors Most reliable full FSI factors are from A.Sirbirtsev et al. ( Phys.Rev.D71:054010, 2005 ) ， which fit ppbar elastic cross section near threshold quite well. ppbar elastic cross section near threshold I=1 S-wave I=0 S-wave P-wave

 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, Phys.Rev.D71:054010, 2005 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

 pp near threshold enhancement is very likely due to some broad sub-threshold 0 -+ resonance(s) plus FSI. From B.S. Zou, Exotics 05: From A. Sirbirtsev :  FSI factors should be included in BW fit.

Discussion on I=1 S-wave FSI

Pure FSI disfavored (III) — I = 1 Pure I=1 S-wave FSI is disfavored by more than 3 . Pure FSI FSI + BW M = 1773  21 MeV  = 0  191 MeV

I=0 dominant in J/  radiative decays  Most I = 0 states have been observed in J/  radiative decays with big production rate ( especially for 0 -+ mesons ) such as ,  ’,  (1440),  (1760), f 2 (1270), f 2 (1525), f 0 (1500), f 0 (1710).  The only observed I=1 meson in J/  radiative decays is  0 with low production rate 4*10 – 5, e.g., no evidence for  (1800) in J/    3  process. It is unlikely to be from  (1800). I=1 S-wave FSI seems unlikely.

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.

BES II Preliminary No  (1800)

 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

The large BR to ppbar suggest it could be an unconventional meson  For a conventional qqbar meson, the BRs decaying into baryons are usually at least one order lower than decaying into mesons. 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.

Analysis of X(1835) 5.1 

Analysis of X(1835) 6.0 

Comparison of two decay modes  Mass and width from m=  8.1MeV/c 2,  =54.2  34.5MeV/c 2  Mass and width from m=  7.9MeV/c 2,  =70.3  23.1MeV/c 2      The mass, width and branching fractions obtained from two different decay modes are consistent with each other.

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

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.

Clear  and  signals    recoiling against 

Side-bands do not have mass threshold enhancement Side-bands