Study of Charmonium States formed in pp annihilations: results from Fermilab E835

1. Study of Charmonium States formed inpp annihilations: results from Fermilab E835 Diego Bettoni INFN – Sezione di Ferrara for the Fermilab E835 Collaboration International Conference on Low Energy Antiproton Physics Yokohama, March3-7, 2003

2. Outline • Introduction • Experimental Method • Physics results • Charmonium decays to J/+X • pp  1,2 J/  • pp  0 J/  • Charmonium decays to  • c(11S0) • 2  • 0  • Search for the c(21S0) • Search for the hc(1P1) • Radiative transitions of the J(3PJ) charmonium states • Proton e.m. form factors in the time-like region • Summary and outlook

3. Introduction Ever since its discovery in 1974 the charmonium system has been considered a powerful tool for the understanding of the strong interaction, and this is why it has often been called the positronium of QCD. Non relativistic potential models + relativistic corrections + perturbative QCD make it possible to calculate masses, widths and branching ratios to be compared with experiment. s  0.3 2  0.2

4. In e+e- annihilations only states with the quantum numbers of the photon JPC = 1-- can be formed directly via the process e+e- * cc. States with JPC  1-- are usually studied from radiative decays, e.g. e+e-    +  In this case the measurement accuracy ( for masses and widths ) is limited by the detector. In pp annihilations all quantum numbers are directly accessible. The resonance parameters are determined from the beam parameters and do not depend on the detector energy and momentum resolution. Whypp ?

5. The charmonium spectrum

6. Experimental Method The accumulated beam of antiprotons (up to 51011p) is decelerated from the injection momentum of 8.9 GeV/c to the formation energy of the resonance to be studied. The beam momentum is then varied in small steps ( 150 KeV/c) and a scan across the resonance is made. The formation of charmonium is detected through e+ e – X and  final states.The resonance parameters (MR, R, BinBout) are extracted from the excitation curve using a maximum likelihood fit.

7. The E835 Detector

8. E835 e+e– EVENT SELECTION Hardware 1st level trigger: candidate events are selected requiring two electrons (defined by scintillators and Cerenkov counter concidences) and two high invariant mass CCAL clusters (> 2 GeV). Online software filter selects “golden” candidates by requiring an inclusive invariant mass > 2.4 GeV Event selection is based on electron id, event topology, timing and kinematic fit. • Electron ID is done using 8 variables from hodoscope counters dE/dx, Cerenkov photoelectrons, CCAL cluster shape variables. Background can be reduced to a few pb while keeping efficiency around 85% • CCAL cluster timing (TDC) is used to reject pile-up clusters due to high intensity • Event topology and kinematic fit are used to classify final states

9. 1 2 Ecm(MeV) Ecm(MeV)

10. pp 0 J/ +  e+e- + 

11. E835  event selection • Two high-energy deposits in the Central Calorimenter. • Kinematical fit to  hypothesis has > 5 % probability. • |cos*|<(s) to improve signal to background ratio. • Backgrounds • pp  00  4; two of the photons missing (510-3 ). • pp  0  3; one of the photons missing (0.03 ). • Photons are lost either below the CCAL energy threshold of 20 MeV or outside • its acceptance (10o <  < 70o). • Background from the above sources can be studied by first measuring the 00 • and 0 cross sections, then using a Monte Carlo to predict their contribution to • the background.

12. c(11S0)    PRELIMINARY

13. pp 2

14. pp 2 The results from previous experiments have been re-calculated using the BR values from 2002 PDG The systematic difference between e+e- and pp experiments has become smaller with the new PDG value for B(2J/)

15. pp 0 PRELIMINARY

16. pp 0 PRELIMINARY using

17. pp 0 PRELIMINARY The results from previous experiments have been re-calculated using the BR values from 2002 PDG

18. The c(21S0) • The mass difference  between the c and the  can be related to the mass difference  between the c and the J/ : • First candidate observed by the Crystal Ball in the radiative decay of the . • Recent discovery by BELLE in hadronic B meson decays. Mass incompatible with Crystal Ball. •  not seen in  collisions at LEP

19. c(21S0) search 2 Both E760 and E835 searched for the c in the energy region: using the process: but no evidence of a signal was found Crystal Ball

20. c(21S0) E760/E835 limits Upper limits on the product of the branching ratios into the initial and final states can be set by fitting the data to spin 0 resonance + a power law background for different values of the total width. From the combined E760/E835 data we get: In the restricted energy region 3589-3599 MeV:

21. c(21S0) search inother channels

22. Expected properties of the hc(1P1) • Quantum numbers JPC=1+-. • The mass is predicted to be within a few MeV of the center of gravity of the c(3P0,1,2) states • The width is expected to be small (hc)  1 MeV. • The dominant decay mode is expected to be c+, which should account for  50 % of the total width. • It can also decay to J/: J/ + 0 violates isospin J/ + +- suppressed by phase space and angular momentum barrier

23. The hc(1P1)E760 candidate A signal in the hc region was seen by E760 in the process: Due to the limited statistics E760 was only able to determine the mass of this structure and to put an upper limit on the width:

24. The hc(1P1)E835 search E835 has performed a search for the hc, in the attempt to confirm the E760 results and possibly add new decay channels. So far E835 has been unable to confirm or deny the E760 result, despite the presence of a clear J/ signal in the hc region.

25. Radiative transitions of the J(3PJ) charmonium states The measurement of the angular distributions in the radiative decays of the c states provides insight into the dynamics of the formation process, the multipole structure of the radiative decay and the properties of the cc bound state. Dominated by the dipole term E1. M2 and E3 terms arise in the relativistic treatment of the interaction between the electromagnetic field and the quarkonium system. They contribute to the radiative width at the few percent level. The angular distributions of the 2 and 2 are described by 4 independent parameters:

26. Angular Distributions of the c states • The coupling between the set of  states and pp is described by four independent helicity amplitudes: • 0 is formed only through the helicity 0 channel • 1 is formed only through the helicity 1 channel • 2can couple to both • The fractional electric octupole amplitude, a3E3/E1, can contribute only to the 2 decays, and is predicted to vanish in the single quark radiation model if the J/ is pure S wave. • For the fractional M2 amplitude a relativistic calculation yields: where c is the anomalous magnetic moment of the c-quark.

27. c1(13P1) AND c2(13P2) ANGULAR DISTRIBUTIONS

28. c1(13P1) AND c2(13P2) ANGULAR DISTRIBUTIONS 2144 c1 events

29. c1(13P1) AND c2(13P2) ANGULAR DISTRIBUTIONS 6028 c2 events

30. c1(13P1) AND c2(13P2) ANGULAR DISTRIBUTIONS Interesting physics. Good test for models Predicted to be 0 or negligibly small

31. c1(13P1) AND c2(13P2) ANGULAR DISTRIBUTIONS McClary and Byers (1983) predict that ratio is independent of c-quark mass and anomalous magnetic moment

32. Angular Distributions of the c states The angular distributions in the radiative decay of the 1 and 2 charmonium states have been measured for the first time by the same experiment in E835. While the value of a2(2) agrees well with the predictions of a simple theoretical model, the value of a2(1) is lower than expected (for c=0) and the ratio between the two, which is independent of c, is 2 away from the prediction. This could indicate the presence of competing mechanisms, lowering the value of the M2 amplitude at the 1. Further, high-statistics measurements of these angular distributions are clearly needed to settle this question.

33. Proton e.m. form factorsin the time-like region The electromagnetic form factors of the proton in the time-like region can be extracted from the cross section for the process: pp  e+e- First order QED predicts: Data at high Q2 are crucial to test the QCD predictions for the asymptotic behavior of the form factors and the spacelike-timelike equality at corresponding values of Q2.

34. Proton magnetic form factor The dashed line is the PQCD fit: s(GeV2)

35. Summary E760 and E835 have been very successful in producing a wealth of new measurements: • High precision measurements of 1 and 2 masses and widths • High precision measurements of 2  and 0  • Best measurements of 0 mass and width • Best measurements of 1 and 2 angular distributions • First observation of the c in pp annihilation, measurement of its mass, total width and partial width to  • Observation of a signal in the hc region • New limits on the c • Measurement of the proton form factors at the highest timelike q2. Coming up from E835: • Measurement of the  decays • Measurement of the   e+e- angular distribution • A new measurement of the 1 width • Study of the 00,0 and  final states

36. Outlook Still there remains a lot to be done: • Improve measurement of c mass (error still bigger than 2 MeV), width and branching ratios. Detect hadronic decay channels. • Identify unambiguously the c , measure its parameters accurately, detect hadronic decay modes. • Confirm/Find the hc(1P1) • Find the states above the DD threshold • Improve measurement of  states angular distributions • Measure the form factor of the proton at even higher q2. • ....... We look forward to carry out all these measurements at PANDA the proposed new experiment at the future GSI HESR.