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

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

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

BaBar detector DIRC Quartz Cherenkov radiator Covers 80% of solid angle Particle ID above 600 MeV/c 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 EMC: 6580 CsI(Tl) crystals Covers 91% of solid angle E resolution ~2 % at high E. V.Druzhinin

Event selection At least 2 tracks with Data: 232 fb-1 collected during 1999-2004 Process Nexpect pp 4k +- 1000k +- 2000k K+K- 100k 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

Event selection K p p K V.Druzhinin

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. data +- K+K- +- e+e- 4025 5.9 ± 2.5 2.5 ± 1.1 < 11 0.8 ± 0.8 V.Druzhinin

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

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. Mpp <2.5 2.5-3.0 3.0-4.5 >4.5 Ndata 3166 322 57 4 Npp 171 ± 29 33 ± 11 25 ± 8 5 ± 3 V.Druzhinin

ISR background e+e-qq with other than pp0 final state 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

Background subtraction +- K+K- pp0 pp0 uds pp data N1 5.9 ± 2.5 2.5 ± 1.0 229 ± 32 13 ± 3 26 ± 4 3737± 75 4025 N2 4.2 ± 1.8 1.3 ± 0.5 29 ± 5 20 ± 3 37 ± 5 179 ± 12 290  0.71 ± 0.05 0.52 ± 0.04 0.13 ± 0.01 1.53 ± 0.25 1.44 ± 0.30 0.048 ± 0.003 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

Mass spectrum V.Druzhinin

Angular distribution Mpp, GeV/c2 N Nbkg |GE/GM| 1.877-1.950 533 2±7 1.950-2.025 584 37±12 2.025-2.100 602 50±15 2.100-2.200 705 42±14 2.200-2.400 592 61±16 2.400-3.000 464 45±12 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

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

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

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

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

Efficiency corrections Effect 1.9 GeV/c2 3.0 GeV/c2 4.5 GeV/c2 2 cuts -0.7 ± 1.0 -1.1 ± 1.0 -1.7 ± 1.0 Track loss 0.8 ± 3.0 1.1 ± 3.0 1.0 ± 3.0 PID 2.5 ± 3.3 3.2 ± 2.4 3.5 ± 2.7 Photon inefficiency -1.3 ± 0.1 Photon conversion 0.4 ± 0.2 Trigger -0.6 ± 0.3 — Total 1.1 ± 4.6 2.3 ± 4.0 1.9 ± 4.2 V.Druzhinin

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

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

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

J/ and (2S) decays N ,% eeBpp, eV J/ 438 ± 22 17.2±0.7 12.0±0.6±0.5 (2S) 22.2 ± 5.5 16.5±0.7 0.70±0.17±0.03 (2S) V.Druzhinin

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