Observation of near-threshold enhancement at BES HongXun Yang Representing BES Collaboration IHEP September , 2004

Outline Introduction threshold enhancement in Summary

Introduction BESII Detector Data Sample

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)

Observation of threshold enhancement in pp bound state (baryonium)? Phys. Rev. Lett., 91 (2003)

Near pp threshold enhancement in enhancement cc

Fit Result 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 fitted peak location acceptance weighted BW  10  25

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

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.

Why can it be seen in J/  decays, but not in cross sections? Reason is simple: J/  decays do not suffer large t-channel “background”.  It is an s-channel effect ! >>

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

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

Threshold Enhancements in J/  decays and B decays They may come from different mechanism: There is “fragmentation” mechanism in B decays but NOT in J/  decays. Belle BES II “Threshold” enhancement 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.

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 Observations of this structure in other decay modes are desirable. 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. Yang et al., hep-ph/

Observation of threshold enhancement in Phys. Rev. Lett. 93, (2004)

High Purity of Signal after Selection It can be shown by the clean Λ signal MC background study: only 1~2% Dominantly from Data  Data/MC

Strong enhancement near the threshold of Phase Space

Observed in both of and

S-wave BW fit results M = (2075  12  5) MeV Γ = (90  35  9) MeV BR = (5.9  1.4  2.0)   2 /d.o.f = 31.1/26 ~ 7σ statistical significance

Similar enhancement also observed in Fix the parameters, 4  away from phase space.

MC (phase space) also show non-uniform and asymmetric distribution of. The enhancement is consistent with S wave. The distribution is consistent with S-Wave Err: Data His: MC

Observation of threshold enhancement in

Events/ 10 MeV NxNx NxNx NxNx PS, eff. corrected ( Arbitrary normalization) Near-threshold enhancement in M K 

We perform PWA studies on the KΛ mass threshold structure: The most important we want to study is its production BR

PWA is performed to  possible N* and  *states listed in PDG are fitted N(1720), N(1900),  (1520),  (1690), …  many different combinations are tried  different form factors are used  different J P of Nx is tried  also tried N(1535) to fit Nx

Mass and Width scan M  1520 – 1620 MeV   110 MeV J P = 1/2 - Total fit (S=-952) N event : Fraction N event N X 22% 1210 Mass scan(GeV/c 2 ) An example: Width scan(GeV/c 2 ) Ln L N(1720), N(1900),  (1520),  (1690) …. included in the PWA fit

Dalitz plot (data)Dalitz plot (PWA) Events/ 10 MeV Crosses: data Hist.: PWA fit projection

J P check with various combinations  J P ½- ½+ 3/2- 3/2+ non A B C D E F G

Fit results CasesMass(GeV)Width(MeV)Fraction(%)N event Log Likelyhood a1.52 ~ b ~ c d1.6 ~ e f ~ g ~ ~110>14.7>800

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 -  consistent with N*(1535) 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. indicating it could be a KΛ molecular state (5 - quark system).

A unique very narrow threshold enhancement is observed in decays at BES II: –It is not observed in elastic cross section  it cannot be explained by FSI. –It is obviously different from the structure observed in B decays and it cannot be explained by fragmentation. We need to understand the nature of the strong anomalous threshold enhancements in J/  decays: multiquark states or other dynamical mechanism ? (keeping in mind that there are no strong threshold enhancements in many cases) Summary

THANK YOU!

BES-I Result Threshold enhancement But NOT claimed in Phys. Rev. Lett. 76(1996) 3502 Too small statistics to draw any conclusion on the threshold enhancement

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 threshold enhancement in collision (at LEAR) |M| 2 BES Both arbitrary normalization

S-wave BW fit results M = (2075  12  5) MeV Γ = (90  35  9) MeV BR = (5.9  1.4  2.0)  M = (2044  17) MeV Γ = (20  45) MeV  2 /d.o.f = 32.5/26 P-wave BW fit results The systematic errors are carefully studied in S-wave case.  2 /d.o.f = 31.1/26 About 7σ statistical significance high L hypotheses fail

This is due to acceptance It can be shown in distribution, where is the decay angle of p in Why Dalitz plot not uniform for events MCDATA

Interference of excited baryons? PWA fits with pure N* and Λ* can hardly reproduce the enhancement. (with reasonable constrains production rate for excited baryons) PWA fit with X(2075) can easily reproduce the enhancement with high significance. (independent of constrains) It is unlikely that the enhancement is purely from  * and N* interference.

Systematic uncertainties

PWA of the near-threshold enhancement(N X ) in m K  PWA with: a: N X,N(1720),N(1900),  (1520),  (1570),  (1690),  (1810), X(2075) b: N X,N(1720),N(1900) ，  (1520),  (1690),  (1810), X(2075) c: N X,N(1720),N(1900),  (1520),  (1570),  (1690),  (1890),X(2075) d: N X,N(1720),N(1900),  (1520),  (1690),  (1890),X(2075) e: N X,N(1720),N(1900),  (1520),  (1570),  (1690),  (1810),  (1890),X(2075) f: N X,N(1720),N(1900),N(2050),  (1520),  (1570),  (1690),  (1810),  (1890),X(2075) g: N X,N(1720),N(1900),N(2050),  (1520),  (1570),  (1690),  (1810),  (1890) J(p) 1/2(-) 3/2(+) 3/2(+) 3/2(+) 3/2(-) 1/2(-) 3/2(-) 1/2(+) 3/2(+) 1(-) m (GeV)1.535,1.720, 1.900, 2.050, , 1.570, 1.690, 1.810, 1.890,  (GeV) 0.150, , 0.200, , 0.070, 0.060, 0.150, 0.100, h:N(1535),N(1650),N(1720),N(1900),  (1520),  (1570),  (1690),  (1810),  (1890),X(2075) i: N(1535),N(1650),N(1720),N(1900),  (1520),  (1690),  (1810),  (1890),X(2075) J(p) 1/2(-) 1/2(-), m N(1650) =1.650,  N(1650) =0.150 j: 18Res All possible N* and  *states listed in PDG and N(1900)(3/2-),N(2050)(1/2+,3/2+),  (1570), X(2075) N(1535 ） =N X

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

Conclusion Mass(GeV)Width(MeV)Fraction(%)N event Likelyhood a1.52~ (39.5)1210(2146)-940(-924) b1.5680~ (48.5)2412(2635)-845(-839) c (26.0)799(1413)-952(-932) d1.6~ (23.2)929(1260)-880(-873) e (31.3)1119(1701)-957(-946) f1.6270~9019.9(19.3)1081(1049)-970(-961) g (16.6)845(902)-954(-951) h(free) h(fixed) i(free) i(fixed) j Total1.5~1.6570~110>14.7>800