Positron-proton to electron-proton elastic cross section ratios from CLAS: Systematic uncertainties and Implications of the results

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1 Positron-proton to electron-proton elastic cross section ratios from CLAS: Systematic uncertainties and Implications of the results Dasuni Adikaram Old Dominion University Dipak Rimal, Larry Weinstein, Brian Raue APS April Meeting, Savannah, GA April 8, 2014

2 CLAS TPE Experiment Proton form factors, G E (Q 2 ) and G M (Q 2 ), describes its charge and magnetization distributions. Two methods to measure G E /G M : Rosenbluth separation method and polarization transfer technique. Super Rosenbluth separation Rosenbluth separation Polarization transfer Results Disagree. The possible explanation is the TPE corrections to the Rosenbluth separation measurements. The e + p/e - p is the only way to measure the TPE correction model-independently. CLAS TPE experiment measured e + p/e - p ratios using a mixed simultaneous electron-positron beam produced at Jefferson Lab Hall B. 2

3 Sources of systematic uncertainties e+/e beam luminosity chicane cycle variance CLAS detector imperfections sector variance Background fitting Elastic event selection and background subtraction Fiducial cuts Target vertex cuts 3

4 Systematics - e+/e luminosity difference The reconstructed electron and positron incident energy distributions are slightly different due to asymmetric beam transportation through beamline magnets (chicane). Reconstructed lepton incident energy distributions Positive chicane polarity e + p events e - p events Negative chicane polarity e + p events e - p events E incident (GeV) E incident (GeV) 4

5 Systematics - e+/e luminosity difference e+/e- - chicane left Energy distribution measured by TPE calorimeter. Chicane is not perfectly symmetric but e+-left is the same as e -left. e+/e- - chicane right Periodically flipping the beamline magnet polarity leads to symmetric luminosities. Uncertainty due to luminosity is measured by the comparison of magnet cycles. Energy (arb. Scale) 5

6 Systematic Uncertainty Due to Lepton Beam Variation Periodically reversed beamline and torus magnet polarities results four magnet cycles. Measure the e+/e- ratio for each chicane polarity and each magnet cycle. The measured variance of ratios (σ 2 total ) includes both statistical and systematic uncertainties. Systematic uncertainty: σ 2 syst = σ2 total σ2 stat Repeat this for the six CLAS sectors to determine the systematic uncertainties for dead detectors. 6

7 Proton form factor measurements at Q 2 = 1-2 GeV 2 Rosenbluth separation Andivahis (SLAC 1994) Q 2 = 1.75 GeV 2 µ p G E /G M = ± Polarization transfer Punjabi (Jlab 2000/2005) Q 2 = 1.77 GeV 2 µ p G E /G M = ±

8 Implications of the CLAS TPE measurements on the existing Rosenbluth measurements Lepton-proton elastic scattering cross section including TPE correction (e p) = Born (1 + 2 ) (e + p)= Born (1 2 ) R = (e+ p) (e p) =1 2 2 Estimate R from the linear fit Calculate δ 2γ Correct the electron-proton cross section measurements of Andivahis et al. Extract G E, G M and calculate µ p G E /G M 8

9 Implications of the CLAS TPE measurements on the existing Rosenbluth measurements Rosenbluth (Andivahis et al.) + TPE Slope = Intercept = Rosenbluth (Andivahis et al.) Slope = Intercept = Polarization transfer (Punjabi et al.) µ p G E /G M = ± Rosenbluth (Andivahis et al.) µ p G E /G M = ± Rosenbluth (Andivahis et al.) + TPE correction µ p G E /G M = ± exp TPE

10 Summary Proton form factor measurements obtained from Rosenbluth & polarization transfer methods disagree. The likeliest explanation is the two photon exchange corrections to the Rosenbluth measurements. The e + p/e - p ratio is the only way to measure the TPE correction to the electron-proton elastic cross section. CLAS TPE experiment measured e + p/e - p over wide range of Q2 and ε. The systematic uncertainties ~1%. Analysis note under review by the CLAS collaboration. Results agree with Blunden calculations which explain the form factor discrepancy at Q 2 = 2-3 GeV 2. TPE corrected Rosenbluth G E /G M agrees with the polarization G E /G M at Q 2 = 1.77 GeV 2. 10

11 Backup Slides 11

12 Ratio Variance for Magnet Cycles at Q 2 = 1.5 GeV 2 12

13 Variance of Ratios for Different Sectors Five independent measurements at five CLAS sectors. Q 2 = 1.5 GeV 2 Systematic uncertainty due to dead detector and other CLAS issues takes into account. Same procedure as magnet cycle variance. 13

14 Systematic Uncertainty Due to Elastic Event Selection Vary the widths of the elastic kinematic cuts: 3σ(nominal) and 3.5σ. Q 2 = 1.5 GeV 2 Varying the kinematic cuts changes the amount of background by a factor of 2. The effects due to the background subtraction is also taken into account. 14

15 Background Subtraction 3σ cuts 4σ cuts 15

16 Systematic Uncertainty Due to Target Vertex Cuts Q 2 = 1.5 GeV 2 Tightened target vertex cuts: change in ratio included as the systematic uncertainty. 16

17 Systematic Uncertainty Due to Fiducial Cuts Both in-bending and out-bending fiducial cuts were applied to all leptons to select regions of detector with 100% acceptance for both e+ and e-. ε = 0.88 Tightened fiducial cuts: change in ratio included as the systematic uncertainty. 17

18 Summary: Q 2 = 1.5GeV 2 18

19 Summary: ε =