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Study of e + e -  pp process using initial state radiation with BaBar

Detailed study on mass spectrum of pp system in e+e- -> ppγ reaction related to cross section of e+e- -> pp reaction. Ratio of form factors GE/GM obtained from proton angular distribution analysis. Advantage of ISR method over conventional measurements discussed.

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Study of e + e -  pp process using initial state radiation with BaBar

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  1. Study of e+e- pp process using initial state radiation with BaBar V. Druzhinin (BINP,Novosibirsk) for BaBar Collaboration

  2. ISR method Mass spectrum of pp system in the e+e- pp reaction is related to cross section of e+e- pp reaction at E=m. e+e- pp cross section depends on two form factors, electric GE and magnetic GM. The ratio of form factors |GE/GM| can be obtained from the analysis of the proton angular distribution. The terms corresponding GM and GE have angular dependence close to 1+cos2 and sin2, respectively. V.Druzhinin

  3. Existing data The existing data come from DCI, ADONE and BEPC e+e- experiments and from pp experiments at LEAR and Fermilab. All data were obtained under |GE|=|GM| assumption. The |GE/GM| ratio was measured at LEAR and found to be compatible to unity. We are going to measure the e+e-pp from threshold up to 4.5 GeV. The advantage of ISR method over conventional e+e- and pp measurements is low dependence of the detection efficiency on mass and proton angle. V.Druzhinin

  4. BaBar detector DCH 40 layers, axial and stereo wires. Covers 92% of solid angle dpt/pt ~ 0.5 -1.5 % Particle ID up to 600 MeV/c • DIRC • Quartz Cherenkov radiator • Covers 80% of solid angle • Particle ID above 600 MeV/c EMC: • 6580 CsI(Tl) crystals • Covers 91% of solid angle • E resolution ~2 % at high E. V.Druzhinin

  5. Event selection Data: 232 fb-1 collected during 1999-2004 • At least 2 tracks with • Rxy < 2.5 cm • z < 6cm • p > 0.1 GeV/c • 0.45 <  < 2.4 • A photon with ECM > 3 GeV and 0.375 <  < 2.4 • Two tracks must pass proton selector • The kinematic fit is applied imposing energy and momentum conservation under different mass hypotheses V.Druzhinin

  6. Event selection K p p K V.Druzhinin

  7. K+K-, +-, +-, e+e- background K+K- background contribution is estimated from the number of events with two identified protons but with 2K <20. The scale factor is obtained from the event sample with one identified photon and MC simulation. The similar method is used for +- background. The e+e- and +- background is estimated using the difference between their mass distributions and pp mass distribution for mass greater than 4.5 GeV. V.Druzhinin

  8. K+K-, +-, +-, e+e- background data K   V.Druzhinin

  9. e+e- pp 0 background The most considerable source of background is the process e+e-pp0 with lost soft photon or with merged photons from 0 decay. These events have 2 distribution peaked at low 2 and can not be separated from the signal events. The events of the e+e-pp0 process were selected using 2 of the kinematic fit to e+e-pp hypothesis and requirement on two-photon invariant mass. The mass spectrum of pp0 events passed our standard selection is determined as KMC(dN/dM)data. The scale factor found from simulation is about 3. V.Druzhinin

  10. ISR background The main source of ISR background is e+e-pp0 process. Using the kinematic fit to pp0 hypothesis we select 847±31 events of this process. From the ratio of the detection efficiencies for pp and pp0 selections, 0.015±0.002, we estimate the number of e+e-pp0 events in pp event sample is 13±3. The similar procedure is used for e+e-pp20 process. The 560±30 selected events of this process translate to 0.5 ±0.3 events passed pp selection. N(1440) e+e-qq with other than pp0 final state is studied with JETSET simulation. Simulation yields 26±4 events. Two final states, pp20 and pp, dominate with 17 and 5 events, respectively. V.Druzhinin

  11. Background subtraction N1 and N2 are numbers of events with 2 < 30 and 30 < 2 <60, respectively. =N1/N2 Since the result of JETSET for rare processes requires confirmation we use the method of background subtraction based on the difference of 2 distributions for signal and background events. pp Nbkg=50±20 is in an agreement with the estimation from simulation, 39±5. pp0 pp0 V.Druzhinin

  12. Mass spectrum V.Druzhinin

  13. Angular distribution The distribution over the angle between the proton momentum in the pp rest frame and the momentum of pp system in the e+e- c.m. frame is fitted by the sum of histograms, obtained from two simulation event samples, one with GE=0 and the other with GM=0. These distributions are close to 1+cos2p and sin2p. V.Druzhinin

  14. Angular distribution The simulated angular distribution are corrected for data-MC difference in PID , tracking and photon efficiencies. About 10% variation of detection efficiency is explained by complex momentum dependence of PID efficiency which has minimum at momentum 1.5 GeV/c. The sources of |GE/GM| systematic error: • background subtraction (0.16) • MC statistics (0.08) • |GE/GM|mass dependence (0.01) • efficiency correction (0.02) V.Druzhinin

  15. Angular distribution,cross-checks To cross-check our method of |GE/GM| measurement we compare the data and simulated distributions over cos K for e+e-   K+K-  process. We also study angular distribution for J/ pp decay which is described by 1+cos2p with =0.672±0.034. Our result is V.Druzhinin

  16. |GE/GM|ratio Hep:-ph/0507085 BaBar |GE/GM| measurements vs previous ones and dispersion relation prediction (yellow) based on JLab space-like GE/GM and analyticity DM2+FENICE • BaBar • LEAR • hep-ph/0507085 E835 V.Druzhinin

  17. Detection efficiency To first approximation the detection efficiency is taken from simulation. The model dependence due to uncertainty in |GE/GM| ratio is about 1% for Mpp<3 GeV/c2, and 10% for higher masses where we use |GE|=|GM| assumption. This MC efficiency must be corrected to account for data-MC difference in detector response: V.Druzhinin

  18. Efficiency corrections V.Druzhinin

  19. Cross section With chosen mass bin the resolution correction insignificantly changes the shape of mass spectrum but leads to about 20% increase in the errors and their correlation. The radiative correction factor is calculated with use of generator level simulation. We evaluate the ratio of the mass spectrum generated with higher-order radiative correction included (structure function method) to pure Born mass spectrum. The theoretical uncertainty is about 1%. V.Druzhinin

  20. Cross section In reasonable agreement with e+e- previous measurements Negative steps at M ~ 2.2 and 3 GeV (!?) V.Druzhinin

  21. Effective proton form factor • Steep behaviour at threshold confirmed • Similar behaviourat threshold is seen in other processes with different quantum numbers V.Druzhinin

  22. J/ and (2S) decays J/ (2S) V.Druzhinin

  23. Summary • The e+e- pp cross section and proton form factor have been measured from threshold up to 4.5 GeV. • The form factor have complex mass dependence. We confirm near-threshold enhancement observed in LEAR experiment. There are also two mass regions, near 2.25 and 3 GeV, with the rapid decrease of the form factor. • The |GE/GM| ratio has been measured from threshold to 3GeV. We have observe noticeable deviation of this ratio from unity, in disagreement with previous LEAR measurement. V.Druzhinin

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