Lepton universality test in the photoproduction of e - e + versus " - " + pairs on a proton target

Transcription

1 PhD in theoretical physics under supervision of Prof. Dr. Marc Vanderhaeghen at Johannes Gutenberg University Mainz. Thesis title Light-by-light scattering and the muon s anomalous magnetic moment Postdoc at the Institute of Nuclear Physics in Mainz Project: Lepton universality test in the photoproduction of e - e + versus " - " + pairs on a proton target JLab Director's Theory Seminar, December 2, 2015 Lepton universality test in the photoproduction of e - e + versus " - " + pairs on a proton target Vladiszlav Pauk Thomas Jefferson National Accelerator Facility Newport News, VA, USA

2 Summary of research activity -01 Electromagnetic properties of hadrons and their role in precision observables and searches for New Physics. large discrepancy between theory and experiment sign of New Physics! ~ 7σ discrepancy ~ 3σ discrepancy H large uncertainties from unconstrained hadronic corrections H

3 Summary of research activity -02 Two-photon production and meson transition form factors TFF M analyticity unitarity gauge symmetry Sum rules: Z 1 s s + Q 2 =0 (1270) 2 helicity-2 TFF of f 1-1 Phys. Rev. D 85, Phys.Lett. B725, predictions for the TFF for f 2 (1270) as a function of the photon virtuality Q (GeV ) Next: update of the analysis using new data Belle (2015) FIG. 41: The measured helicity-2 TFF of the f 2 (1270) as a function of Q 2. Short (long) vertical bars indicate

4 Summary of research activity -03 TFF (g-2) µ Eur.Phys.J. C a 0, f 0, f 1, a 2, f 2 FNAL expected accuracy: Total contribution of single meson exchanges beyond pseudo scalars: δa μ a μ (a0, f0, f1, a2, f2) 6 11 model independent approach needed for the systematic account of the hadronic uncertainties new dispersive framework Phys.Rev. D90, relate (g-2) µ to the absorptive part of the Pauli form factor in the time-like region Next: realistic estimate of the contributions of the single- and two-pion thresholds

5 The proton radius -04 R 2 E def = 6 dg E dq 2 Q 2 =0 elastic scattering proton radius extraction agree for the electron (uncertainty ~0.6%) E L = 2 3 (0) 2 R 2 E atomic spectroscopy muon vs electron m µ 200 m e a 0 = ~2 m e e 2 stronger effect from the proton size Bohr radius x better precision

6 The puzzle -05 "H data: R E = ± fm Antognini et al.(2013) Pohl et al.(20) 7 σ difference!? ep/eh data: R E = ± fm CODATA(2012)

7 Lepton pair production on a proton target -06 Measure the ratio of the e vs µ cross sections R µ/e d (µ µ+ + e e + ) d (e e + ) 1, Phys. Rev. Lett. 115, Bethe-Heitler process Two-fold differential cross section of the Bethe-Heitler production to leading order d BH dt dm 2 ll = 3 (s M 2 ) 2 4 t 2 (M 2 ll t) ll C E G 2 Ep + C M G 2 Mp at small t the ratio R µ/e gives direct access to the ratio of the proton electric form factor in the µp versus ep scattering the deviation from the unity will be a sign of violation of the lepton universality

8 Lepton pair production on a proton target -07 Bethe-Heitler is highly peaked in the lepton forward region Detect the proton integrate over the lepton phase space increase the count rates the same systematics no lepton acceptance corrections needed large cancellation of radiative corrections recoil proton Lab angle cos lab p = M 2 ll + 2(s + M 2 ) 2(s M 2 ) p (1 + ). p lab (deg) t (GeV 2 ) proton Lab momentum ~p 0 lab =2M p (1 + ) p 0 lab = (0 174)MeV/c 0.01 = t/(4m) M ll 2 (GeV 2 )

9 Experimental feasibility -08 γp "-"+ p : events produced o () p 0 Compton 90 Eγ = 0.5 GeV Bethe-Heitler Com pton 2 p$ p$ p#+#- 0 0 p$+$ Adlarson (2015) 80 0 Energy p (MeV) 1

10 Lepton universality test -09 The cross section ratio R µ/e d (µ µ+ + e e + ) d (e e + ) 1, 1% difference in measured proton charge FF in electron vs muon observables leads to a 0.2% absolute effect for R "/e σ(e - e + +μ - μ + )/σ(e - e + ) E γ =0.5 GeV -t=0.03 GeV 2 lepton universality violation e =1.01 μ G Ep/GEp lepton universality μ e G Ep=GEp M ll 2 (GeV 2 ) the contribution of the TVCS 5x smaller than the effect due to the 1% variation in the value of G " Ep

11 JLab & JPAC - JPAC Double-Regge exchange model for # - p # - η p at COMPASS and explanation of J PG =1 -+ exotics FESR Study the double-regge limit of the 2 3 process and connect to to the resonance region by the finite energy sum rules JLab Dilepton production by detecting electrons and muons Forward-backward asymmetries and study of the TVCS access the proton form factor from the Bethe-Heitler process by analyzing angular distributions of the lepton pairs direct measurement of the interference between TVCS and BH to extract the information on the TVCS (polarizabilities, high-energy behavior)