Charmonium decays to baryon-antibaryon pairs

Charmonium decays to baryon-antibaryon pairs Zepu Mao For the BES Collaboration IHEP Beijing MENU Sep. 2, 2004

OUTLINE • Introduction • cJ  ΛΛ • cJ  PP • J/ PP • Summary

BES (BEijing Spectrometer) Construction: 1983-1989, Data Taking: 1989-1994 BESI , 1996-2004 BESII Research OBJ:J/ , , , D, Ds, R-Scan VC:xy = 100mBTOF:T = 180 ps MDC:xy = 200 mBSC:E/E= 22 % dE/dx=8.4 %z= 2.3 cm p/p = 1.6(1+p2) (GeV) counter: z = 5.5 cm

World J/ and (2S) Samples (×106) Largest from BES 58 M J/ 14 M (2S) 2002 2001

COM for cJB B • B(cj pp) input fromBES, PRL81, 3091 (1998); • E760, NPB373, 35 (1992); R704, PLBB172, 455 (1986) • Nucleon wave function at small Q2: J.Bolz, P. Kroll, Z. Phys. A356, 327 (1996) J.Bolz, P. Kroll, Eur. Phys. J. C2, 454 (1998) For each s quark, introduces S. M. Wong, Eur. Phys. J. C14, 643 (2000) BES measures them

Event Selection for(2s)  cJ  ΛΛ p - p + First Evidence ofcJ  ΛΛ NCHRG=4, MFIT=2, Q=0 |cos|<0.75 Low Momentum track:   Higher Momentum track: p/p photon by 4C fit and take the 2 smallest one Charged Particle ID of  and pp by TOF and dE/dX, and Prob>1%. 4C kinematic fit used for (2s)   p - p +, and Prob4c>1%

(2s)  cJ  ΛΛ  p - p + Monte Carlo Data  Clear Λ Λ Signal

cJ Signal Data/Monte Carlo χc2 χc1 χc0

Fit the mass spectrum MC efficiencies χc1 χc2 χc0 ΛΛ-bar Mass (GeV)

Systematic Errors Extensive Comparison Between Data and Monte Carlo Simulation! ~20%!

Results This is Is

Measurement of (2s)  cJ( selectionfinial state  p p events) One photon at least and two good tracks photon: the angle of photon between the track>150 , to ensure radiated from (2s) Tracks: two tracks opposite and |cos|<0.75 ESCp<0.7GeV, BSC and MU information used to refuse e+e- and  background events. Particle ID: Wp/ Wp >0.7 (refuse  and K) Wi=Pi/iPi, Pi =jPji (xj), i= , k, p and p by the likelihood method, X is the variables of the time in TOF and the dE/dX in MDC. 2 of 4C kinematic fit >1%

Final states: pp c0 c1 c2

System error and numbers used for B

RB Comparison with COM RBBES= B(cJ → ΛΛ)/ B(cj →pp) Many systematic errors cancel! * The RB on c1 and c2 decays only agree marginally with model predictions. * No prediction for c0 . Phys.Rev.D69:092001,2004

Precise measurement of J/ PP The angular distribution of two baryon final states can be written as: dN/dcosB  1 +  cos2B Where B is the angle between the baryon direction and the positron beam direction.

 predicted by Diff. models from first-order QCDTheo1: 1.0 Theo2: 0.46 Theo3: 0.69, 0.70 measured in Previous experiments Theo1: S.L.Brodsky, G.P.Lepage, Phys. ReV. D24, 2848(1981) Theo2: M.Claudson, S.L.Glashow,M.B.Wise, Phys. ReV. D25, 1345(1982) Theo3: C. Carimalo, Int. J. Mod. Phys. A2, 249(1987)

Good Charged track No.=2 only • two-track Opening angle>1750 • |cos |<0.8 • P=(Pm-1.232)<110. MeV/c • |T+-T-|<4ns •  and k refused by TOF Event Selection P Mom. P Mom. P

BES Results(Phys. Lett. B591（2004）42－48) =0.676±0.036±0.042 Sys. Error Br=(2.26 ±0.01 ±0.14) ×10-3 Sys. Error • The Br is within one  of PDG Ave. value. • Angular Dist. is in good agreement with “Theo3”

Summary • BES has first measurements for cJ → ΛΛ • The ratio of the Br on c1 and c2 decaystoΛΛand PPonly agree marginally with model predictions. • The Br of J/  → pp is within one  of PDG Ave. value. • The angular distribution of J/  → pp is in good agreement with theoretical prediction(Theo3.)

谢谢! Thanks!

CSM and COM(Color Singlet Mechanism and Color Octet Mec.) CDF, Phys. Rev. Lett. 69, 3704 (1992) CDF, Phys. Rev. Lett. 71, 2537 (1993) COM “Rigorous QCD analysis of inclusive annihilation and production of heavy quarkonium”, G.T. Bodwin, E. Braaten, G.P. Lepage Phys. Rev. D51, 1125 (1995) CSM P.L. Cho, A.K. Leibovich, Phys. Rev. D53, 150 (1996) Phys. Rev. D53, 6203 (1996)

Charmonium decays to baryon-antibaryon pairs