Qweak Transverse Asymmetry Measurements

Transcription

1 Qweak Transverse Asymmetry Measurements Buddhini Waidyawansa For the Qweak Collaboration Hall C Collaboration Meeting

2 Outline Physics of transverse asymmetries Qweak transverse data set Analysis overview Analysis updates 2

3 Transverse Asymmetry Generated by transversely polarized electrons scattering from unpolarized nucleons. s e s e k2 k1 p Has an azimuthal dependence; 3

4 Transverse Asymmetry Generated by transversely polarized electrons scattering from unpolarized nucleons. s e s e k2 k1 p Has an azimuthal dependence; Beam Normal Single Spin Asymmetry 4

5 Beam Normal Single Spin Asymmetry An observable of the two photon exchange process. Parity even and time reversal odd. Arise from the interference between one and two photon exchange processes when the beam is transversely polarized e.g. e+p scattering 5

6 Beam Normal Single Spin Asymmetry An observable of the two photon exchange process. Parity even and time reversal odd. Arise from the interference between one and two photon exchange processes when the beam is transversely polarized e.g. e+p scattering Direct access to the imaginary part of two-photon exchange process 6

7 Beam Normal Single Spin Asymmetry T1 - calculable using form factors of the nucleon. T2 - use Compton scattering from the nucleon to model two photon exchange. e d k1 1 µν AbsT2γ = l W µν ( 2π ) 3 2 Ek1 q12 q Leptonic tensor lμν Hadronic Tensor Wμν : absorptive part of VVCS tensor 7

8 Beam Normal Single Spin Asymmetry The hadronic tensor gets contributions from the ground state and the excited states of the nucleon Ground state Excited states X Exactly calculable - on-shell electromagnetic form factors X = p+pi, p+2pi, Dominant contributor Not exactly calculable. - large number of excited states - Rely on experimental inputs. e.g. GPDs, electroproduction amplitudes etc. 8

9 BNSSA Calculations vs Measurements A4 [Source : PRL, 94, , (2005)] G0 forward [Source :PRL 99, (2007] D&M P&V G0 A&M P&V Model Intermediate state Input D&M Diaconescui & Musolf [Phys. Rev.C. 70, (3004)] Nucleon Field theory calculation. No pions. P&V Pasquini & Vanderhaeghen [Phys.Rev. C70, (2004)] Nucleon+pion MAID electroproduction amplitude M. Gorchtein [Phys.Rev. C73, ;055201(2006)] Nucleon+Pion+Pion Photoproduction cross-section A&M Afanasev & Merenkov [Phys.Lett. B 599,48 (2004)] Nucleon+Pion+Pion Photoproduction cross-section MG 9

10 Physics Interest Magnitude of Bn Imaginary part of 2-photon exchange Information on nucleon structure Nucleon structure information. - GPDs, resonance form factors. Complementary to VCS, electroproduction results. Interpretation of radiative corrections. - Ratio of the proton s electric and magnetic form factors - Higher order box-graph corrections to weak interaction observables. Potential false asymmetry in precision parity violating experiments. 10

11 Bn as a False Asymmetry in PV Measurements Residual transverse polarization in the beam Broken azimuthal symmetry of the detectors Ameasured (φdet ) = PL APV + PT Bn sin(φdet φs ) +.. Additional piece from BNSSA 180 PV experiments may need to correct for Bn Dedicated measurements with a transversely polarized beam are needed 11

12 Qweak Transverse Program Summary Lots of interesting physics! Interaction Elastic e+p at E = GeV Target Analysis Status Hydrogen Ready for publication Aluminum Ongoing Carbon Ongoing Inelastic e+p with a Δ in the final state E=0.877 GeV and GeV Hydrogen, Al, C Ongoing Elastic e+e at E=0.877 GeV Hydrogen Ongoing Hydrogen Ongoing Hydrogen Ongoing Deep inelastic e+p at W=2.5GeV Pion photoproduction at E=3.3GeV 12

13 Qweak Transverse Program Summary Covered in this talk Interaction Elastic e+p at E = GeV Target Analysis Status Hydrogen Ready for publication Aluminum Ongoing Carbon Ongoing Inelastic e+p with a Δ in the final state E=0.877 GeV and GeV Hydrogen, Al, C Ongoing Elastic e+e at E=0.877 GeV Hydrogen Ongoing Hydrogen Ongoing Hydrogen Ongoing Deep inelastic e+p at W=2.5GeV Pion photoproduction at E=3.3GeV 13

14 Measurement Setup Kinematics Beam energy = 1.165GeV Q2= (GeV/c)2 Scattering angle = 7.80 Beam Polarization ~ 89% Transversely polarized (vertical/horizontal) Electron beam 14

15 Analysis Overview Form individual bar asymmetries e.g. asymmetries from Hydrogen Remove helicity correlated changes in position, angle and energy using linear regression. 15

16 Analysis Overview Check for false asymmetry cancellation with the insertable half wave plate (IHWP) e.g. regressed asymmetries from Hydrogen using vertical transverse polarization Cerenkov Detector Array (IN+OUT)/2 shows a good cancellation of helicity correlated false asymmetries. 6 16

17 Analysis Overview Extract the measured physics asymmetry by fitting the regressed detector asymmetries e.g. transverse asymmetries from elastic e+p Not corrected for backgrounds, polarization and other systematics Fit 17

18 Analysis Overview Extract the measured physics asymmetry by fitting the regressed detector asymmetries Correct for beam polarization, backgrounds and other systematics to extract Bn. Backgrounds (Bkg) Aluminum target windows Inelastics Systematics ( R ) Radiative corrections Acceptance averaging Q2 variation 18

19 BNSSA from elastic e+p (PRELIMINARY) From ~ 50 hrs worth of data. BNSSA from elastic e+p scattering : Bn = ± 0.07 (stat) ± 0.15 (sys) ppm A 3% Measurement. Vertex kinematics : Q2 = ± (GeV/c)2 Energy = ± GeV Scattering angle = 7.9 ± 0.3 degrees Soon to be published! Error source Preliminary Polarization 2.2 % Statistics 1.3 % Q2 acceptance 1.2 % Non-linearity 1.0 % Regression 0.9 % Backgrounds 0.3 % 19

20 BNSSA from elastic e+p (PRELIMINARY) Compare to world data on forward angle Bn measurements. Experiment Beam (GeV) Q2 (GeV/c)2 BNSSA (ppm) Precision A4 (Mainz) ± 0.89stat ± 0.79sys ~ 14% A4 (Mainz) ± 2.31stat ± 0.87sys ~ 30% Qweak (JLab) ± 0.07stat ± 0.15sys ~ 3% HAPPEX (JLab) ± 1.47stat±0.24sys ~ 23% G0 forward (JLab) ± 0.99stat± 0.63sys ~ 29% ± 1.87stat± 0.98sys ~ 44% Most precise measurement of Bn by far. 20

21 BNSSA from elastic e+p (PRELIMINARY) Comparison to model calculations 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 21

22 BNSSA from elastic e+al and e+c Ongoing analysis. Preliminary estimates of the uncertainties of the measurements looks promising. a new Carbon data point Al27 data point will help to understand theory between A=12 and A=208. Original theory and data plot from PRL 109, (2012) 22

23 Summary Qweak have several interesting transverse asymmetry measurements. Some are first time measurements Good candidates to test model calculations A 3% measurement of BNSSA from elastic e+p scattering is ready for publication! This is the most precise measurement of BNSSA to-date. Emphasizes the role played by multi-pion resonance intermediate states in the two-photon exchange process Rest of the data analysis is on going and can be expected to be finalized within the next year. 23

24 Thank you! Thank you! 24

25 Backup Slides 25

26 Beam Normal Single Spin Asymmetry Model calculations varies for different kinematics Model Kinematics region Input Diaconescui & Musolf [Phys. Rev.C. 70, (3004)] Threshold Field theory calculation. Pasquini & Vanderhaeghen [Phys.Rev. C70, (2004)] Resonance electroproduction amplitude M. Gorchtein [Phys.Rev. C73, ;055201(2006)] High energy forward scattering Photoproduction crosssection Afanasev & Merenkov [Phys.Lett. B 599,48 (2004)] M. Gorchtein, P.A.M. Guichon, M. Vanderhaeghen [Nuc.Phys. A 741: (2004)] Photoproduction crosssection Hard scattering GPDs 26