Two Photon Exchange (TPE) and the Proton Form Factor Problem

Transcription

1 Two Photon Exchange (TPE) and the Proton Form Factor Problem Larry Weinstein Old Dominion University With Dasuni Adikaram, Dipak Rimal, Robert Bennett, Puneet Khetarpal, Mauri Ungaro, Brian Raue, Will Brooks, John Arrington, 1. Problem: Factor of 3 discrepancy in G E p 2. Solution: Compare H(e±,e±) cross sections to measure TPE 1. Important problem high protile competition 3. Aggressive Measurement: make high energy e+ in Hall B 4. Challenging Analysis 5. Preliminary Results (much better than published data) 6. Summary

2 The Proton Charge Formfactor Why do we care? To compare to models of the proton: The ratio of the electric and magnetic formfactors, G E (Q 2 )/G M (Q 2 ), gives information about the relative distributions of the quark longitudinal momenta and transverse positions Nonrelativistically, G E2 (Q 2 ) is the Fourier transform of the charge distribution. To build models of the nucleus and answer questions like: is the proton moditied in the nucleus? where does color transparency start?

3 σ R = εg 2 E Q 2 τ = Q2 4m p G E p and Rosenbluth SeparaAon ( ) + τg M 2 ( Q 2 ) = [1+ 2(1+ τ )tan 2 (θ e / 2)] 1 Measure H(e,e ) reduced cross section as a function of ε (angle) for Tixed values of Q 2. G E p and PolarizaAon Transfer H( e, e p) G E G M σ red = P T P L large θ Measure transverse (P T ) and longitudinal (P L ) polarization of outgoing proton. ( E + E ') tan θ 2m p 2 TPE Jefferson Lab Oct ε small θ

4 The Proton Charge Form Factor G E p Rosenbluth Separations Super- Rosenbluth Separations OOPS!! Polarization Measurements This discrepancy needs to be resolved TPE Jefferson Lab Oct

5 RadiaAve CorrecAons to Single Photon Exchange Standard radiative corrections Hard two- photon exchange (TPE) Smaller by a factor of α TPE Jefferson Lab Oct

6 The TPE Formalism General 1- and 2- photon exchange amplitude 2: 1: General 1- and 2- photon exchange cross section 1: 2: Thus we have Another ε dependent term ModiTied G E and G M Guichon and Vanderhaegen, PRL 91 (03) TPE Jefferson Lab Oct

7 Possible Effect of Two Photon Exchange on Rosenbluth SeparaAon 6% of σ R Small (few %) TPE effects can dramatically change G E 2 TPE Jefferson Lab Oct

8 TPE CalculaAons are Hard Doubly- virtual Compton scattering Need to integrate over (VVCS)! All intermediate virtual baryonic states All photon energy sharing wide range of masses of those states Photocouplings of most states are not known Typical approximations 1. Consider only the nucleon and the delta 2. Use GPDs for baryon structure We need to measure it! TPE Jefferson Lab Oct

9 Compare H(e ±,e ± ) to Measure TPE A Born ~ e ± changes sign σ (e ± ) A Born + A 2γ + 2 A 2 ~ (e ± ) 2 always positive σ (e ± ) A Born 2 ± 2A Born Re(A 2γ ) R = σ (e+ p) σ (e p) 1+ 4A BornRe(A 2γ ) A Born 2 R measures the real part of the two photon amplitude The real part contributes directly to the G E p problem. TPE Jefferson Lab Oct

10 Phenomenological TPE ExtracAons (to make Rosenbluth and PolarizaAon Transfer G E p agree) Parametrize the TPE amplitude and then Tit the e+/e- ratio to the Rosenbluth and polarization transfer data Different e+/e- ratios can explain the G E p discrepancy ε Qattan, Alsaad and Arrington, ArXiv arxiv: TPE Jefferson Lab Oct

11 Recap: The Problem: Rosenbluth/Polarization Transfer discrepancy in G E (Q 2 ) The Probable Cause: Two- Photon Exchange contributions of a few percent How to test it: Compare electron and positron elastic scattering from the proton to better than a few percent - existing data is at low Q 2 and too imprecise Now where did I leave those positrons TPE Jefferson Lab Oct

12 The CompeAAon: VEPP- 3 (Novosibirsk) " E = 0.6, 1 and 1.6 GeV " Alternating e+ and e- beams " Internal target " Separate large and small angle detectors " Non- magnetic spectrometer: identical e+/e- detector acceptance " Preliminary results arxiv: TPE Jefferson Lab Oct

13 VEPP- 3 Preliminary Results Theory: Blunden et al, Phys. Rev. C 72, (2005) Radiative corrections have been applied. Systematic uncertainties (0.3%) not shown. 13

14 OLYMPUS: BLAST Detector at DESY " E = 2 GeV " Alternating e+ and e- beams " Internal target atom/cm 3 BEAM TARGET DRIFT CHAMBERS " Forward angle (12 o ) luminosity monitor COILS " Continuous coverage 20 o 80 o " Taking data NOW! e CERENKOV COUNTERS BEAM p,d NEUTRON COUNTERS SCINTILLATORS TPE Jefferson Lab Oct

15 CLAS: Making Positrons in Hall B converter lead wall calorimeter radiator 2.5 cm thick steel wall beam monitor na 5.7 GeV e beam hits 0.9% radiator, makes photons Electrons dumped in tagger dump 2. Photon beam hits 9% converter, makes e+/e pairs e+/e beams split by 3- dipole chicane Photon beam blocked Low energy leptons blocked Lepton beams recombined 3. Simultaneous e+/e beams hit 30- cm 6- cm diam hydrogen target in CLAS TPE Jefferson Lab Oct

16 Experiment Features/Bugs Simultaneous e+/e beams (~100 pa each) Continuous beam energy distribution Wide Q 2 and angle ( ) coverage e+ and e- beam position and energy measurements Simultaneous e + p and e p cross section measurements Minimize systematic uncertainty Reverse Torus magnetic Tield to cancel acceptance effects Reverse chicane magnetic Tield to cancel beam asymmetries Six independent measurements in the 6 CLAS sectors Overdetermined ep kinematics allows background rejection TPE Jefferson Lab Oct

17 Experiment Comparison CLAS VEPP-3 OLYMPUS Beam energy 0.8 to 4 GeV 0.6, 1, 1.6 GeV 2 GeV e+/e- swapping frequency simultaneous 0.5 hour 8 hour e+/e- luminosity Calorimeter Elastic low Q 2 Elastic low Q 2 Moller/Bhabha ε (angular) coverage Discrete 25 o 140 o 0.4, o 80 o Scattered e energy magnet calorimeter magnet Proton PID kinematics ΔE/E, TOF TOF e+/e- detector acceptance Not-identical Identical Not-identical Luminosity cm -2 s (projected) TPE Jefferson Lab Oct

18 7 years of simulations to design the shielding 100 tons of shielding 20 tons placed by hand Collimator Collimator Convertor Chicane Collimator Dipoles CLAS Fiber BPM Calorimeter Beam line Tagger Target Shielding Tagger Dump TPE Jefferson Lab Oct

19 The JLab CLAS Almost 4π coverage Six independent sectors

20 Sectors 1 and 4 CLAS Cross SecAon TOF DC1 DC2 DC3 Beamline CER CAL

21 Centering the Lepton Beams Beam ProTile Fiber Monitor (FIU) Block one lepton beam Scan chicane dipoles 1&3 Watch the beam move Repeat for the other beam Beam position (mm) Repeated each chicane Tlip 1 st and 3 rd Dipole Current (A) Chicane currents were reproducible 21 TPE Jefferson Lab Oct 2012

22 Trigger EC (minimum ionizing) & TOF (θ<45 o ) Any opposite sector TOF EC needed in trigger to reduce rates TPE Jefferson Lab Oct

23 Q 2 (GeV 2 ) Q 2 (GeV 2 ) e- p e- p ε Q 2 (GeV 2 ) Q 2 (GeV 2 ) e+p e+p 0 0 ε ε Θ=180 o Θ=0 o Θ=180 o Θ=0 o ε ElasAc KinemaAc Coverage Positive torus B Tield Trigger holes (no particles with θ<45 o ) Negative torus B Tield 23

24 Analysis issues: Unknown beam energy for a given detected event Non- standard particle ID (no CC or EC for lepton ID) Need charge- symmetric event analyzer Analysis solutions: IdenAfying ElasAc Events Rewrote event analyzer to eliminate electron bias Select two charged particles (++ or +- ) in opposite sectors Elastic kinematic cuts 6 measured quantities (θ l,φ l,p l, θ p,φ p,p p ) 3 free quantities (e.g., θ l,φ l, θ p ) 3 orthogonal cuts (actually we make 4 cuts) TPE Jefferson Lab Oct

25 ElasAc Event IdenAficaAon 1. Reconstructed beam energy: " E 1 = m p cot θ e 2 cotθ 1 % $ p ' # & E 2 = p e cosθ e + p p cosθ p 2. Scattered lepton energy: ΔE beam = E 1 E 2 ΔE ' e = E ' e meas E 1 beam m p E 1 beam ( 1 cosθ ) e + m p 3. Proton momentum: Δp p = p p meas p e sinθ e sinθ p 4. Coplanarity: Δφ = φ p φ e TPE Jefferson Lab Oct

26 (backward angle) ΔΦ: Cut on other 3 ε = 0.3 ε = 0.6 ε = 0.75 ε = 0.85 ε = (forward angle) Δϕ Very little background for ε > 0.7 TPE Jefferson Lab Oct

27 (backward angle) + ΔP p : Cut on other 3 (forward angle) ε = 0.3 ε = 0.6 ε = 0.75 ε = 0.85 ε = ΔP p (GeV/c) TPE Jefferson Lab Oct

28 ElasAc Event IdenAficaAon ΔE beam and ΔE e are correlated so make cuts on ΔE+ = ΔE beam + Δ E ΔE = ΔE beam Δ E ΔE ' e = E ' e meas E 1 beam beam m p E 1 ( 1 cosθ e ) + m p ΔE beam = E beam (θ e,θ p ) E beam (p e z + p p z ) 0.2 ΔE e beam (GeV) ΔE e (GeV) TPE Jefferson Lab Oct

29 ElasAc Event IdenAficaAon ΔE+ (GeV) ΔE (GeV) TPE Jefferson Lab Oct

30 (backward angle) + ΔE+ = ΔE beam + ΔE Cut on other 3 (forward angle) ε = 0.3 ε = 0.6 ε = 0.75 ε = 0.85 ε = ΔE+ (GeV) Fit gaussians to determine peak widths for each bin Make 3- sigma cuts on each variable TPE Jefferson Lab Oct

31 (backward angle) + ΔE- = ΔE beam - ΔE Cut on other 3 (forward angle) ε = 0.3 ε = 0.6 ε = 0.75 ε = 0.85 ε = ΔE- (GeV) TPE Jefferson Lab Oct

32 Background SubtracAon Worst Case + ΔE cut on other 3 Fit region Sampled background ++ ΔE (GeV) 1. Measure background in ΔE- tails 2. Fit height to ΔΦ tails 3. Subtract from peak ΔΦ (gaussian Tit also shown) 32

33 Data Analysis: Acceptance Fiducial cuts to select regions of high detector efficiency and complete overlap between e + & e - Elastic ratio for given torus polarity: R 1 ± = N + N Proton acceptance cancels Flip torus polarity, form double ratio for given chicane setting: R 2 ± = R 1 + R 1 Lepton acceptance cancels Flip chicane polarity, form quadruple ratio: R = R 2 + R 2 = σ (e+ p) Beam asymmetries cancel σ (e p) Acceptance matching (swimming) e+ p e- Also doing acceptance corrections using GSIM (simulation) TPE Jefferson Lab Oct

34 Data: Low Q 2 and High ε Acceptance CorrecAons 0.4 < Q 2 < 0.6 GeV < ε < 0.95 (forward angle) Q 2 (GeV 2 ) ε Quadruple ratio Acceptance corrections (GSIM) Swimming corrections No corrections ε Acceptance corrections are small, even in small bins. TPE Jefferson Lab Oct

35 Data: Low Q 2 and High ε Beam Asymmetries 0.4 < Q 2 < 0.6 GeV < ε < 0.95 (forward angle) Negative chicane double ratio Q 2 (GeV 2 ) ε Quadruple ratio Ratio Positive chicane double ratio ε Does the beam asymmetry cancel in the super ratio? 35

36 Beam energy measurements USM/ODU Calorimeter 30 module shashlik (Pb/scint) calorimeter Positioned downstream of CLAS on forward carriage Measure beam energy during low luminosity runs Measure beam energy for e+ and e- separately every time we Tlipped the chicane Calibrated with cosmic rays (~450 MeV e- equivalent) centered at channel 1000 Incident Lepton Energy (Channel #) TPE Jefferson Lab Oct

37 Beam asymmetries cancel in the super raao Normalized to incident beam charge Left side of chicane: Ratio of e+ to e- Right side of chicane: Ratio of e+ to e- Combined e+/e- ratio χ 2 /ndf = 44/39 p0 = 0.998± Incident lepton energy (GeV approx) TPE Jefferson Lab Oct 2012 The chicane has a left/ right asymmetry, not an e+/e- asymmetry 37

38 Stability: cycles 2, 3, 4 We took four full magnet cycles of data torus /+ chicane /+ Q 2 Negative chicane double ratio Cycle 2 Cycle 3 ε Quadruple ratio Positive chicane double ratio Cycle 4 Ratios consistent for both magnet cycles ε 38

39 Preliminary Results e+/e- ratio e+/e- ratio Q GeV 2 (varies) ε 0.5 (varies) 1 Q 2 (GeV 2 ) ε 2 Q 2 Q 2 ε Binning 75% of data No acceptance corrections No radiative corrections ε 39

40 Comparison to world data at Q 2 > 1 VEPP- 3 (Novosibirsk) Preliminary No radiative corrections CLAS: 50% of data No acceptance corrections No radiative corrections VEPP Back angle Forward angle TPE Jefferson Lab Oct

41 Tasks to do: 1. Include remaining data 2. Lots more data points at large ε and varied Q 2 3. Acceptance corrections for all bins 1. Swimming 2. GSIM- based 4. Charge dependent radiative corrections 5. Determine systematic uncertainties (anticipate 1-2%) 6. Get CLAS analysis approval 7. Publish 8. Enjoy the plaudits of an admiring populace! TPE Jefferson Lab Oct

42 Summary 1. Very serious discrepancy in G ep, resolvable by H(e±,e±) 2. Major competition from OLYMPUS@DESY and Novosibirsk 3. First ever experiment using simultaneous beams of high energy e+ and e- 4. Measured σ(e+p)/σ(e- p) over a wide range of Q 2 and ε 1. Our measurements can be applied directly to correct the Rosenbluth G e p measurements 5. Innovative analysis 1. CLAS charge dependent acceptances cancel when torus magnet polarity Tlipped 2. Beam charge asymmetries cancel when chicane polarity Tlipped 6. Initial results appear consistent with e+/e- ratios needed to explain the G E p discrepancy 7. Final results expected in 3-6 months TPE Jefferson Lab Oct