A Precision Measurement of Elastic e+p Beam Normal Single Spin Asymmetry and Other Transverse Spin Measurements from Qweak

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1 A Precision Measurement of Elastic e+p Beam Normal Single Spin Asymmetry and Other Transverse Spin Measurements from Qweak Buddhini P. Waidyawansa For the Qweak Collaboration JLab Users Group Meeting June 3rd, 2014

2 Outline Electron Scattering Beyond Born Approximation Single Spin Asymmetries in Electron Scattering Beam Normal Single Spin Asymmetry Qweak Transverse Spin Program Analysis Overview Preliminary Results from the Proton Very Preliminary Results from Other Measurements 2

3 Elastic Electron Scattering in the Born Approximation Radiative Corrections Born + e+p Elastic cross section Source : A.Afansev, WSPC Proceedings, GE, GM form factors τ, ε, kinematic factors With radiative corrections 3

4 Elastic Electron Scattering Beyond the Born Approximation + Born +... Two-photon exchange Elastic cross section 4

5 Elastic Electron Scattering Beyond the Born Approximation + Born +... Two-photon exchange Elastic cross section Contributions from higher order processes with at least twophoton exchange 5

6 Two-photon Exchange Contribution two-photon exchange corrections on the electron-nucleon scattering cross-sections are not complete at all kinematics First noticed in Rosenbluth and polarization transfer methods used to extract protons electric and magnetic form factor ratio Without two-photon correction With two-photon correction Better agreement Two techniques starts to deviate Phys. Rev. C. 76 (2008) Observables of two-photon exchange can help to benchmark models 6

7 Single Spin Asymmetry Observable of the imaginary part of two-photon exchange process Parity conserving, time reversal invariant A. De Rajula et. al. Nucl. Phys. Vol. B35, Spin of beam or target normal to scattering plane Target normal = An ~ αem ~ 10-2 Beam normal = Bn ~ αem ~

8 Single Spin Asymmetry T1γ Calculable using elastic form factors of the nucleon T2γ Modeled using doubly virtual Compton scattering e k1 Q1 Q2 p 8

9 Single Spin Asymmetry Hadronic tensor gets contributions from ground state+excited states of the nucleon Excited States Ground State X X = pπ, pππ,.. On-shell nucleon intermediate states Exactly calculable using elastic electromagnetic form factors Resonant+non-resonant intermediate states Not exactly calculable due to lack of information on resonance states Rely on experimental inputs e.g. GPDs, electroproduction amplitudes etc. 9

10 Single Spin Asymmetry Hadronic tensor gets contributions from ground state+excited states of the nucleon Excited States Ground State X X = pπ, pππ,.. On-shell nucleon intermediate states Exactly calculable using elastic electromagnetic form factors Resonant+non-resonant intermediate states Not exactly calculable due to lack of information on resonance states Rely on experimental inputs e.g. GPDs, electroproduction amplitudes etc. Causes a model dependence on the asymmetry calculations 10

11 Beam Normal Single Spin Asymmetry - Model Calculations Electron spin transverse Contribution from excited states dominates Pasquini et.al Phys. Rev. C 70, (2004) Ground state Excited states Total Data point from A4 11

12 Beam Normal Single Spin Asymmetry - Calculations vs Measurements A4 [Source : PRL, 94, , (2005)] G0 forward [Source :PRL 99, (2007] D&M P&V G0 A&M P&V MG Model Intermediate state Input D&M Diaconescui & Musolf Nucleon Field theory calculation P&V Pasquini & Vanderhaeghen Nucleon+pion MAID electro-production amplitudes M. Gorchtein Nucleon+ multiple pions Photo-production cross-sections A&M Afanasev & Merenkov Nucleon+ multiple pions Photo-production cross-sections [Phys. Rev.C. 70, (3004)] [Phys.Rev. C70, (2004)] [Phys.Rev. C73, ;055201(2006)] [Phys.Lett. B 599,48 (2004)] Models are not in good agreement with measurements at all kinematics 12

13 Beam Normal Single Spin Asymmetry -Measurements There are no dedicated experiments Measuring a 1 ppm asymmetry is technically challenging All existing measurements come from parity violating electron scattering experiments (PVES) Background measurements to constrain the false asymmetry MIT Bates Jlab Jlab MAMI SLAC Jlab Jlab 13

14 Beam Normal Single Spin Asymmetry -Measurements Use transversely polarized electrons scattering from unpolarized nucleons s e r c to e te De s k2 k1 p Measured asymmetry has a an azimuthal dependence 14

15 Beam Normal Single Spin Asymmetry - Physics Interests Magnitude of Bn Imaginary part of 2-photon exchange Information on nucleon structure 15

16 Beam Normal Single Spin Asymmetry - Physics Interests Magnitude of Bn Two-photon corrections required by protons form factor ratio measurements γz box corrections required by electro-weak observables Nucleon structure information Information on nucleon structure Interpretation of radiative corrections Imaginary part of 2-photon exchange Complementary to direct measurements of GPDs, electroproduction amplitude etc. False asymmetry in parity violating asymmetry measurements 16

17 Beam Normal Single Spin Asymmetry - Physics Interests Magnitude of Bn Two-photon corrections required by protons form factor ratio measurements γz box corrections required by electro-weak observables Nucleon structure information Information on nucleon structure Interpretation of radiative corrections Imaginary part of 2-photon exchange Complementary to direct measurements of GPDs, electroproduction amplitude etc. False asymmetry in parity violating asymmetry measurements 17

18 BNSSA as a False Asymmetry in PVES Residual transverse polarization in the beam Broken azimuthal symmetry of the detectors Ameasured (φdet ) = PL APV + PT Bn sin(φdet φs ) +.. False asymmetry from Bn Φ=0 Detector system on the azimuthal plane 18

19 BNSSA as a False Asymmetry in PVES Residual transverse polarization in the beam Broken azimuthal symmetry of the detectors Ameasured (φdet ) = PL APV + PT Bn sin(φdet φs ) +.. False asymmetry from Bn Parity Violating Electron Scattering (PVES) experiments may need to correct for BNSSA Dedicated measurements with a transversely polarized beam are needed. Theoretical calculations are model dependent. 19

20 The Qweak Experiment Precision test of the Standard Model Measures the parity violating elastic electron proton asymmetry to extract the weak charge of the proton For details see Rakitha Beminiwattha's talk (JSA thesis prize): First Determination of the Weak Charge of the Proton through Parity Violating Electron Scattering Several ancillary measurements were taken to determine or constrain background processes or corrections 20

21 Qweak Transverse Spin Program Lots of interesting physics! Interaction Target Elastic e+p at E = GeV Analysis Status Hydrogen Ready for publication Aluminum Ongoing Carbon Ongoing Ongoing Elastic e+e at E=0.877 GeV Hydrogen, Al, C Hydrogen Deep inelastic e+p at W=2.5GeV Hydrogen Ongoing Pion electro-production at E=3.3GeV Hydrogen Ongoing Inelastic e+p with a Δ in the final state E=0.877 GeV and GeV Ongoing 21

22 Qweak Transverse Spin Program Covered in this talk Interaction Target Elastic e+p at E = GeV Analysis Status Hydrogen Ready for publication Aluminum Ongoing Carbon Ongoing Ongoing Elastic e+e at E=0.877 GeV Hydrogen, Al, C Hydrogen Deep inelastic e+p at W=2.5GeV Hydrogen Ongoing Pion electro-production at E=3.3GeV Hydrogen Ongoing Inelastic e+p with a Δ in the final state E=0.877 GeV and GeV Ongoing 22

23 Qweak Apparatus Kinematics Beam energy = 1.165GeV Q2= (GeV/c)2 <Scattering angle> = 7.80 Beam Polarization ~ 89% 8 Quartz bar detectors Toroidal Spectrometer 35cm LH2 target Polarized Electron Beam Acceptance-defining Pb collimator 23

24 Qweak Apparatus (before shielding) 24

25 Azimuthally Symmetric Detectors Maximizes rate and decrease sensitivity to beam motion ~50% of full azimuthal coverage Quartz Cernenkov detectors Simulation of rates on a detector Measured 2m 25

26 Analysis Overview Form individual bar asymmetries e.g. asymmetries from Hydrogen Remove helicity correlated changes in the beam position, angle and energy using linear regression 26

27 Analysis Overview Use an insertable half wave plate at the injector to remove helicity correlated false asymmetries. 3 2 Regressed asymmetries from Hydrogen using vertical transverse polarization 4 1 IHWP IN (IN+OUT)/2 IHWP OUT (IN+OUT)/2 ~ 0 indicates good cancellation of helicity correlated false asymmetries. 27

28 Analysis Overview Fit regressed detector asymmetries to get the measured physics asymmetry Not corrected for backgrounds, polarization and other systematics Amplitude of the fit = measured physics asymmetry 28

29 Analysis Overview Fit regressed detector asymmetries to get the measured physics asymmetry To extract Bn, correct the measured asymmetry for backgrounds, beam polarization and other systematics Backgrounds (Bkg) Aluminum target windows Inelastics Bkg dilutions (f) Systematics ( R ) Radiative corrections Acceptance averaging Q2 variation 29

30 BNSSA from elastic e+p scattering (PRELIMINARY) ~ 50 hrs of good data obtained from Qweak transverse spin measurements gives, Bn = ± 0.07 (stat) ± 0.15 (sys) ppm Vertex kinematics: Error source Polarization 2.2 % Statistics 1.3 % <E> = 1.155± GeV Q2 acceptance 1.2 % < θ > = 7.9 ± 0.30 Non-linearity 1.0 % Regression 0.9 % Backgrounds 0.3 % 2 <Q >= ± (GeV/c) Preliminary A 3% measurement 2 Soon to be published! 30

31 BNSSA from elastic e+p scattering (PRELIMINARY) Compare to world data on Bn Experiment Beam (GeV) Q2 (GeV/c)2 BNSSA (ppm) Precision A4 (Mainz) ± 0.89stat ± 0.79sys ~ 14% A4 (Mainz) ± 2.31stat ± 0.87sys ~ 30% ± 0.07stat ± 0.15sys ~ 3% ± 1.47stat±0.24sys ~ 23% ± 0.99stat± 0.63sys ~ 29% ± 1.87stat± 0.98sys ~ 44% Qweak (Jlab) [preliminary] HAPPEX (JLab) G0 forward (JLab) Most precise measurement of Bn by far! 31

32 BNSSA from elastic e+p scattering (PRELIMINARY) Compare to model calculations PRELIMINARY 32

33 BNSSA from elastic e+p scattering (PRELIMINARY) Compare to model calculations PRELIMINARY Using single pion electro-production amplitudes (MAID). PRELIMINARY 33

34 BNSSA from elastic e+p scattering (PRELIMINARY) Compare to model calculations PRELIMINARY Using single pion electro-production amplitudes (MAID). PRELIMINARY multi-pion intermediate states Input:photoproduction cross sections But: Different cross-section fits Different Compton slopes 34

35 BNSSA from elastic e+p scattering (PRELIMINARY) Compare to model calculations PRELIMINARY Using single pion electro-production amplitudes (MAID). PRELIMINARY multi-pion intermediate states Input:photoproduction cross sections But: Different cross-section fits Different Compton slopes Emphasizes the significant role played by multiple pion resonance intermediate states in two-photon exchange 35

36 BNSSA from elastic e+p scattering (PRELIMINARY) ~50% over/under prediction of Bn from models first observed by G0 forward angle measurement Qweak's precise measurement indicates at forward angles, Bn favors models with multiple pion excitations of the nucleon G0 forward [PRL 99, (2007] P&V A&M PRELIMINARY MG PRELIMINARY 36

37 BNSSA from elastic e+al and e+c (Analysis Ongoing) Preliminary estimates of the uncertainties of the measurements looks promising New Carbon data point Aluminum point will help to understand theory between A=12 and A=208. AIP Conf. Proc. 1563, 212 (2013) Theory theory from M. Gorchtein and C. J. Horowitz, Phys. Rev. C77, (2008) Pb data is from PRL 109, (2012) 37

38 BNSSA from Inelastic Scattering e+p e+δ (Analysis Ongoing) Kinematics : E = 877 MeV, θlab = Relative statistical precisions of the measurements looks promising Some of these are first time measurements Good candidates to test model calculations Target LH2 cell Aluminum Carbon Relative Statistical Precision ~ 3% ~ 5% ~ 3% 38

39 BNSSA from Moller Scattering e+e e+e (Analysis Ongoing) Kinematics : E = 877 MeV, θlab = Leading order QED prediction from PRD 69, (2004) Only statistical error shown. Points displayed at A = 0 39

40 Summary Qweak have several interesting transverse asymmetry measurements Some are first time measurements. Good candidates to test model calculations The 3% measurement of BNSSA from elastic e+p scattering is ready for publication! This is the most precise measurement of BNSSA to-date. Clearly emphasizes the role played by multi-pion resonance intermediate states in the two-photon exchange process A good candidate to test two-photon model calculations at low Q 2 Rest of the data analysis ongoing and will be published in the future. 40

41 Thank you! Thank you! 41

42 Backups 42

43 Elastic scattering cross-section in born approximation, Radiative Corrections Born e+p + Source : A.Afansev, WSPC Proceedings, Contributions from higher order processes with at least two-photon exchange 43

44 Why Did Qweak Need to Measure Transverse Asymmetry Residual transverse polarization in the beam. The symmetry breaking of the detector system was not known. Relying on existing models could have increased Qweak error goal Precision goal of Qweak is 4% Precision of Bn correction for Qweak should be 1%. Qweak had to measure Bn to get a better estimate with high precision. 44

45 Two-photon Exchange Observables Electron-proton, positron-proton scattering ratio probes the real part of the two-photon exchange amplitude. 1 for no two-photon contribution > 1 for two-photon contribution 45