The G0 Experiment Parity-violating electron scattering from the nucleon –Longitudinal beam polarization Strange quark contribution to elastic form factors Effective axial-vector elastic form factor Axial-vector N-  transition Weak interaction contribution to pion photoproduction Single-spin asymmetries –Transverse beam polarization 2  contribution to elastic scattering (Single spin asymmetry in pion photoproduction) D. Beck Mar. 2012

Strange Quark Observables scalar matrix element –. momentum carried by strange quarks –. NuTEV hep-ex/ –. spin carried by strange quarks –as determined in sum rule   s ~ –as determined in semi-inclusive  s(x) vector matrix elements HERMES: Phys. Rev. D 89 (2014) S. Alekhin et al. arXiv:

Parity-Violating Electron Scattering Interference term violates parity: use where contributes to electron scattering - interference term: large x small ep Z ep 

Quark Currents in the Nucleon Measure –e.g. –note then charge symmetry (see B. Kubis & R. Lewis, PRC 74 (06) and G. A. Miller PRC 57 (98) 1492.) dropping the p superscripts on the left

California Institute of Technology, Carnegie Mellon University, College of William and Mary, Grinnell College, Hampton University, Hendrix College, Institut de Physique Nucléaire d'Orsay, J. Stefan Institute, Laboratoire de Physique Subatomique et de Cosmologie-Grenoble, Louisiana Tech University, New Mexico State University, Ohio University, Thomas Jefferson National Accelerator Facility, TRIUMF, University of Illinois, University of Kentucky, University of Manitoba, University of Maryland, University of Northern British Columbia, University of Winnipeg, University of Zagreb, Virginia Tech, Yerevan Physics Institute G0 Collaboration Graduate Students: C. Capuano (2011 W&M), A. Coppens (2010 Manitoba), C. Ellis (2010 Maryland), J. Mammei (2010 VaTech), M. Muether (2010 Illinois), J. Schaub (2010 New Mexico State), M. Versteegen (2009 Grenoble); S. Bailey (2007 W&M) Analysis Coordinator: F. Benmokhtar - Carnegie Mellon (Maryland) SUPPORT: DOE, NSF, IN2P3, NSERC – Thank-you!

G0 Collaboration

G0 Experiment Overview Measure, –different linear combination of u, d and s contributions than e.m. form factors →strange quark contributions to sea (few % of overall G’s) Measure forward and backward asymmetries –recoil protons for forward measurement –electrons for backward measurements elastic/inelastic for 1 H, quasi-elastic for 2 H E beam = 3.03 GeV, GeV I beam = 40  A, 80  A P beam = 75%, 80%  = 52 – 76 0,  = 0.9 sr, 0.5 sr l target = 20 cm L = 2.1, 4.2 x cm -2 s -1 A ~ -1 to -50 ppm, -12 to -70 ppm GMGM Z GEGE Z

G0 in Hall C beam monitoring girder superconducting magnet (SMS) scintillation detectors cryogenic supply cryogenic target ‘service module’ electron beamline

What Do We See? Different components separated by t.o.f. Background under elastic peak is main analysis issue Background correction involves ‘dilution factor’, f and background asymmetry, A back

Back Angle Layout Elastic, inelastic electron separation using FPD, CED Electron/pion separation using aerogel Cherenkov Cherenkov Electron Beam Cryostat Exit Detector Focal PlaneDetector Shielding Cryotarget Single Octant Schematic

Back Angle Apparatus Back angle detector package Target system installation FPD CED Cherenkov Superconducting Magnet

LH2, 678 MeV LH2, 687 MeV LH2, 362 MeV LD2, 687 MeV LD2, 362 MeV Electron Yields (quasi) elastic electrons inelastic electrons background regions

Combined Results (2010) Using interpolation of G0 forward measurements G0 forward/backward PVA4: PRL 102 (2009) Q 2 = 0.1 GeV 2 combined Global uncertainties G0 forward/backward SAMPLE Zhu, et al. PRD 62 (2000) also assuming Androić, et al. PRL 104 (2010)

HAPPEx III Result Z. Ahmed, et al. PRL 108 (2012) Q 2 = 0.62 GeV 2

Relative to Overall Form Factors Z. Ahmed, et al. PRL 108 (2012)

Combined Results with Theory Androić, et al. PRL 104 (2010)

PV: Inelastic Scattering/Pion Production N-  transition –Dominant magnetic dipole (M1) transition in photoproduction Coupling to vector current Corresponds roughly to quark “spin flip” (via magnetic moment) –Also study spin flip in axial current e.g. charged pion electroproduction ep → e  -  ++ A. Liesenfeld et al, Phys. Lett. B468 (1999) 20. “Pure” spin flip accompanied by flavor change –We measure neutral current coupling to nucleon axial transition current: A PV (inel) Like electroproduction, different flavor structure Pion production –Measure inclusive  - from D target, dominated by photoproduction –Asymmetry at Q 2 =0 not zero (current conservation) –d   related to the anomalous  S = 1 hyperon decays S.-L. Zhu, C. Maekawa, B. Holstein & M. Ramsey-Musolf PRL 87 (2001)

Inelastic Asymmetry C. Capuano (thesis, W&M, 2011) Proton Deuteron: A = -43.6±14.6±6.2 ppm

Pion Asymmetries Constrain small photoproduction asymmetry “d  ” (OUT – IN)/2 = ± 1.03 ppm [OUT + IN = 3.84 ± 2.15 ppm] A. Coppens (thesis, Manitoba, 2010), arXiv: , PRL 108 (2012)

Transverse Asymmetries Elastic scattering –Second order e.m. effects generate single-spin asymmetries –Easy to measure with PV apparatus Pion production –Inclusive pion yield also has azimuthal dependence –Involves longitudinal/transverse interference (“5 th structure fn.) –Suppressed by ~m e /E –Calculation by A. Afanasev, et al.; T. S. H. Lee, et al. Pasquini & Vanderhaeghen PRC 70 (2004) –Interference of 1  and 2  amplitudes; time reversal invariance requires A n to involve the imaginary part of M 2  –Real part of M 2  related to important contribution to longitudinal scattering: G E /G M ratio E i = 570 MeV

Preliminary Transverse Asymmetries

Comparison of G0 Results with Theory Proton J. Mammei (thesis, VT, 2010), Androić, et al. PRL 107 (2011) Neutron: B n (362 MeV) = +87±41 ppm (sign from G M )

Preliminary Pion Transverse Asymmetry A. Coppens (Manitoba)

Summary Comparison of electromagnetic and weak neutral elastic form factors allows determination of strange quark contribution –large distance scale dynamics of the sea Contributions of and less than few % over measured range –why s, sbar not separated? First measurements of neutral current N  transition around Q 2 = 0.3 GeV 2 First measurements of PV asymmetry in inclusive  - production at low Q 2 – large d  ruled out First measurements of transverse asymmetries in –back angle elastic scattering from H, D targets: Im(2  ) –Inclusive  - production: R LT’

Another Holt Measurement MDC ISO 400 gate valve has diameter of 16 in!