New observations and Multiquark Candidates at BESII Xu-Ai ZHUANG (for BES Collaboration) Institute of High Energy Physics (IHEP) zhxa@mail.ihep.ac.cn QCD and Hadronic interactions La Thuile , Mar. 20, 2006

Outline Multi-quark Candidates at BESII  and  observation A possible bound state: mass threshold enhancement in and new observation of X(1835). mass threshold enhancement in  mass threshold enhancement in J/    and  observation

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 They have been searched for experimentally for a very long time, but none is established.

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+p2) z = 2.3 cm

World J/ and (2S) Samples (106) Largest from BES J/ (2S) 2001 2002

A possible ppbar bound state

Observation of an anomalous enhancement near the threshold of mass spectrum at BES II J/ygpp acceptance weighted BW +3 +5 -10 -25 M=1859 MeV/c2 G < 30 MeV/c2 (90% CL) c2/dof=56/56 0.1 0.2 0.3 Phys. Rev. Lett. 91, 022001 (2003)    M(pp)-2mp (GeV) 3-body phase space acceptance

This narrow threshold enhancement is NOT observed in B decays The structure in B decays is obviously different from the BES observation: Belle The structure in B decays is much wider and is not really at threshold. It can be explained by fragmentation mechanism. BES II 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: From Crystal Ball result, we estimate: 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)? 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) Datta, P.J. O’Donnell, PLB 567, 273 (2003)] M.L. Yan et al., hep-ph/0405087 B. Loiseau et al., hep-ph/0411218 G.J. Ding and M.L. Yan, Phys. Rev. C 72, 015208 (2005) deuteron: baryonium: attractive nuclear force attractive force? + n + - loosely bound 3-q 3-q color singlets with Md = 2mp- e loosely bound 3-q 3-q color singlets with Mb = 2mp-d ? Observations of this structure in other decay modes are desirable.

New Observation of X(1835) in PRL 95, 262001 (2005)

Observation of X(1835) in X(1835) 5.1  X(1835) 6.0  phase space

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

Comparison of two decay modes Mass and width from m=1827.48.1MeV/c2 , =54.234.5MeV/c2 m=1836.37.9MeV/c2 , =70.323.1MeV/c2 The mass, width and branching fractions obtained from two different decay modes are consistent with each other.

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 = 1830.6  6.7 MeV  < 153 MeV @90%C.L. 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 Phys. Rev. Lett. 93, 112002 (2004)   

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

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

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

A strong enhancement is observed near the mass threshold of MK at BES II. Preliminary PWA with various combinations of possible N* and Λ* in the fits —— The structure Nx*has: Mass 1500~1650MeV Width 70~110MeV JP favors 1/2- The most important is: It has large BR(J/ψ  pNX*) BR(NX* KΛ) 2 X 10-4 , suggesting NX* 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. Phys.Rev. C71 (2005) 064001 Ecm – ( MΛ+MK ) (MeV)

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

Observation of  mass threshold enhancement Submitted to Phys. Rev. Lett., hep-ex/0602031

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

Clear  and  signals    -sideband recoiling against 

Dalitz Plot

A clear mass threshold enhancement is observed Acceptance

Side-bands do not have mass threshold enhancement

0++ is favored over 0-+ and 2++ Partial Wave Analysis is performed 0++ is favored over 0-+ and 2++

Further look in , K*K*,  …. are desirable ! Is it the same 0++ observed in KK mass of J/KK? It is a multiquark state, a glueball, or a hybrid? Further look in , K*K*,  …. are desirable ! hep-ph/0602172, hep-ph/0602190

σ and κ observations

The  pole in at BESII   Averaged pole position: MeV M() M(+-) 0   Averaged pole position: MeV Phys. Lett. B 598 (2004) 149

κ Phys. Lett. B 633 (2006) 681

Summary (I) A very narrow and strong mass threshold enhancement is observed in decays at BES II: 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.

Summary (II) mass threshold enhancement was observed in Evidence of NX(1610) was observed near KΛ mass threshold, suggesting a KΛ bound or resonant state. An  mass threshold enhancement f0(1810) was observed in J/    and  are observed at BESII

Thank You!

f0(1710) PWA analysis shows one scalar in 1.7 GeV region Phys. Rev. D 68 (2003) 052003

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

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(1830)) seems very hard to be explained by a conventional qqbar meson.

NO strong dynamical threshold enhancement in cross sections (at LEAR) 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 ! |M|2 |M|2 BES BES Both arbitrary normalization Both arbitrary normalization

Any inconsistency? NO! For example: with Mres = 1859 MeV, Γ = 30 MeV, J=0, BR(ppbar) ~ 10%, an estimation based on: At Ecm = 2mp + 6 MeV ( i.e., pLab = 150 MeV ), in elastic process, the resonant cross section is ~ 0.6 mb : much smaller than the continuum cross section ~ 94  20 mb .  Difficult to observe it in cross sections experimentally.

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

In ppbar collision, the background is much larger (II) 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 I=1 S-wave P-wave I=0 S-wave A.Sibirtsev, J. Haidenbauer, S. Krewald, Ulf-G. Meißner, A.W. Thomas, Phys.Rev.D71:054010, 2005

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: “JP, I” filter

Pure FSI disfavored (I) Theoretical calculation (Zou and Chiang, PRD69 034004 (2003)) shows: The enhancement caused by one-pion-exchange (OPE) FSI is too small to explain the BES structure. The enhancement caused by Coulomb interaction is even smaller than one-pion-exchange FSI. |M|2 |M|2 BES BES Both arbitrary normalization Both arbitrary normalization one-pion-exchange FSI 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 P-wave I=0 S-wave

Pure FSI disfavored (II) I=0 S-wave FSI CANNOT fit the BES data.

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

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

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 = 1830.6  6.7 MeV  = 0  93 MeV

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.

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

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

This enhancement is NOT observed in process at SAPHIR

It is unlikely to be N*(1535). Non-observation of NX in suggests an evidence of new baryon (personal view): It is unlikely to be N*(1535). If NX 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, NX would also have almost the largest BR to KΛ. Also, the EM transition rate of NXto proton is very low.

Discussion on KΛ mass threshold enhancement NX(1610) NX(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 NX to KΛ is very small, so such a big BR shows NX has very strong coupling to KΛ, indicating it has a big hidden ssbar component. (5-quark system)

   sideband w signal with multiple entries w signal after best BES II Preliminary   M(+-0)(GeV/c2) M(+-0)(GeV/c2) M(+-0)(GeV/c2) M(+-0)(GeV/c2)  sideband w signal with multiple entries w signal after best candidate selection (best w masses)

BES II Preliminary M() Eff. curve Side-band Phase Space

PWA Fit results Total f2(1910) f2(160) M() dominated by (1760) BES II Preliminary Total (1760) > 10  f0 6.5  f0(1710) f0(1790) f0(1810) (1760) f0(1710) Using observed mass and width for f0(1810) in J/y  g w f2(1910) f2(1910) 5.8  f2(160) f2(1640) 5.5  BG BG M() dominated by (1760) a 0++ is needed (6.5 ) Being submitted to PRD.