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. BeckMar. 2012
Strange Quark Observables• scalar matrix element
– .
• momentum carried by strange quarks– .
NuTEV hep-ex/9906037
– .
• 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)
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 1H,
quasi-elastic for 2H
Electron Beam
LH2 Target
SuperconductingCoils
Particle Detectors
Ebeam = 3.03 GeV, 0.33 - 0.93 GeV
Ibeam = 40 A, 80 A
Pbeam = 75%, 80%
= 52 – 760, 104 - 1160
= 0.9 sr, 0.5 sr
ltarget = 20 cm
L = 2.1, 4.2 x 1038 cm-2 s-1
A ~ -1 to -50 ppm, -12 to -70 ppm
GMZGE
Z
G0 in Hall C
beammonitoring 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
backelmeas AfAfA 1
• Background correction involves ‘dilution factor’, f and background asymmetry, Aback
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
SuperconductingMagnet
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)
Q2 = 0.1 GeV2 combined
Global uncertainties
G0 forward/backward
SAMPLE
Zhu, et al. PRD 62 (2000)
• also assuming
0 2 0 2,
0 2,
3
2
0 0.070
T T dipoleA NS A A
TA NS
F DG Q R G Q
G Q
Androić, et al. PRL 104 (2010) 012001
HAPPEx III Result
Z. Ahmed, et al. PRL 108 (2012) 102001
Q2 = 0.62 GeV2
Relative to Overall Form Factors
Z. Ahmed, et al. PRL 108 (2012) 102001
Combined Results with Theory
Androić, et al. PRL 104 (2010) 012001
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: 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”
(OUT – IN)/2 = -0.56 ± 1.03 ppm[OUT + IN = 3.84 ± 2.15 ppm]
A. Coppens (thesis, Manitoba, 2010), arXiv:1112.1720, PRL 108 (2012) 122002
0.37 1.06 0.37 0.03
(8.1 23.7 8.3 0.7)
A
d g
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 (“5th structure fn.)
– Suppressed by ~me/E
– Calculation by A. Afanasev, et al.; T. S. H. Lee, et al.
Pasquini & Vanderhaeghen PRC 70 (2004) 045206
– Interference of 1 and 2 amplitudes; time reversal invariance requires An to involve the imaginary part of M2
– Real part of M2 related to important contribution to longitudinal scattering: GE/GM ratio
Ei = 570 MeV
Preliminary Transverse Asymmetries
Comparison of G0 Results with Theory
Proton
J. Mammei (thesis, VT, 2010), Androić, et al. PRL 107 (2011) 022501
Neutron: Bn(362 MeV) = +87±41 ppm (sign from GM)
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 Q2 = 0.3 GeV2
• First measurements of PV asymmetry in inclusive - production at low Q2 – large d ruled out
• First measurements of transverse asymmetries in– back angle elastic scattering from H, D targets: Im(2)
– Inclusive - production: RLT’
sEG s
MG
Another Holt Measurement
• MDC ISO 400 gate valve has diameter of 16 in!