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 •2contribution to elastic scattering • (Single spin asymmetry in pion photoproduction) D. Beck Mar. 2012
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The G0 Experiment Parity-violating electron scattering from the nucleon –Longitudinal beam polarization Strange quark contribution to elastic form factors.
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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
• spin carried by strange quarks– as determined in sum rule
s ~ -0.1 - 0
– as determined in semi-inclusive s(x)
• vector matrix elements
4.01.0~2 NdduuNNssN
06.007.042.02 dus
xsxs HERMES: Phys. Rev. D 89 (2014)
097101
0.02 0.006 0.029 0.007s
S. Alekhin et al. arXiv:1404.6469
Parity-Violating Electron Scattering
• Interference term violates parity: use
where
pZG ,
e,e
• contributes to electron scattering
- interference term: large x small
2ZMM
M ZM
22
2
5
24
10~
ME
AMEF
LR
LRPV
GG
AAAQG
A
e
AMWA
ZMMM
ZEEE
GGA
GGAGGA
2sin41
,
e p
Z
e p
2
2
2
12
11
,4
,2/tan121
pM
Q
:,, ,,, npZp GGG
Quark Currents in the Nucleon• Measure
– e.g.
– note
then
NqqeNG iii
i ~
psME
pdME
puME
pME GGGG ,
,,,
,,
,, 3
1
3
2
nsps
nupd
ndpu
GG
GG
GG
,,
,,
,,
charge symmetry
(see B. Kubis & R. Lewis, PRC 74 (06) 015204 and G. A. Miller PRC 57 (98) 1492.)
pZ
MEnME
pMEW
sME
pZME
nME
pMEW
dME
pZME
pMEW
uME
GGGG
GGGG
GGG
,,
,,
,,
2,
,,
,,
,,
2,
,,
,,
2,
sin41
sin42
sin43
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)
– 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: APV(inel)
• Like electroproduction, different flavor structure
• Pion production– Measure inclusive - from D target, dominated by
photoproduction– Asymmetry at Q2 =0 not zero (current conservation)– drelated to the anomalous S = 1 hyperon decays
S.-L. Zhu, C. Maekawa, B. Holstein & M. Ramsey-Musolf PRL 87 (2001) 201802
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”