Results from the G0 Measurement Colleen Ellis, University of Maryland
2
e e pp
Parity-violating electron scattering from the nucleon Hydrogen and deuterium targets Strange quark contributions to electromagnetic form factors
1988 Kaplan and Manohar—sea quark contributions to ground state nucleon properties,
i.e. spin, charge, and magnetic movements from neutral weak probes ( νN scattering)
1989 McKeown then Beck outlined a PVES program that could measure strange quark contributions to the proton’s charge and magnetism.
D.Kaplan and A.V. Manohar, Nucl. Phys. B310, 527 (1988)R.D. McKeown, Phys Lett. B219, 140 (1989) D.H. Beck, Phys. Rev. D39, 3248 (1989)
Q (EM) QW
u 2/3d -1/3s -1/3
charge symmetry
Measured in G0
Access via form factors: contribution to nucleon charge and magnetism
Parity Violating elastic e-N scattering
Q2 behavior well-understood
3
€
APVproton
(θ ≈ 5°,10°)
APVproton
(θ ≈ 110°)
APVdeuteron
(θ ≈ 110°)
⎧
⎨ ⎪
⎩ ⎪
→
GEs (Q2)
GMs (Q2)
GAe (Q2)
⎧
⎨ ⎪
⎩ ⎪
€
A∝ σ + − σ −
σ + + σ −∝M γM Z
M γ 2 ∝{AE + AM + AA
AD}
€
AE = εGEγGE
Z
AM = τGMγ GM
Z
AA = −(1− 4sin2θW)ε 'GAeGM
γ
AD = ε(GEγ )2 + τ (GM
γ )2
€ can be varied between zero and unity for a fixed Q2 by varying the beam energy and electron scattering angle.
Two kinematics, two targets gives 3 linear combinations of EM and weak form factors
Parity Violating elastic e-N Form Factors PV asymmetries from EM and weak interference terms
€
τ = Q2
4M 2
ε = 1+ 2(1+ τ )tan2 θ2 ⎛ ⎝ ⎜
⎞ ⎠ ⎟
⎡ ⎣ ⎢
⎤ ⎦ ⎥
−1
ε '= 1−ε 2( )τ 1+ τ( )
2
e e pp
C. Ellis/UMD
Axial form factor seen by PV electron scattering
)())(()()( 2222 QGRQFQGQG sA
eA
ZA
eA =
e
e'Z 0 PV
e+ ...
e
Z+++ PV
e+ ...
e
Z+
but Q2 behavior is not well constrained
(FA + Re) at Q2=0 computed
by Zhu etal, PRD 62 (2000) 033008
SAMPLE deuterium asymmetry data agree well
4slide thanks to E. Beise
Asymmetries to Form Factors
5
0s s e
phys E E M M A AA a a G a G a G=
Electromagnetic form factors: Kelly (PRC 70 (2004))does not include new low Q2 data from BLAST or JLabeventually use new fits (Arrington & Melnitchouk for p, Arrington & Sick for n)differences in fits become 0.5 – 1 % in the asymmetryalso used in Schiavilla calculation for D(see Diaconescu, Schiavilla & van Kolck, PRC 63 (2001) 044007)
Two-boson exchange corrections to Asymmetry: 0.5 -1.2%(see Tjon, Blunden & Melnitchouk, arXiv:0903.2759v1)
11
1Z Z
meas Born BornA A A
= =
G0 –Path to extracting vector form factors
C. Ellis/UMD
€
AF
AB
Ad
⎛
⎝
⎜ ⎜ ⎜
⎞
⎠
⎟ ⎟ ⎟=
a1F a2F a3F
a1B a2B a3B
a1d a2d a3d
⎛
⎝
⎜ ⎜ ⎜
⎞
⎠
⎟ ⎟ ⎟
GEs
GMs
GAe
⎛
⎝
⎜ ⎜ ⎜
⎞
⎠
⎟ ⎟ ⎟+
a0F
a0B
a0d
⎛
⎝
⎜ ⎜ ⎜
⎞
⎠
⎟ ⎟ ⎟
-53.299.5012.4912.12Ad
-38.2812.1062.8421.57AB
-23.682.4642.8779.39AFat Q2 = 0.62 GeV2
At Q2 = 0.62 and 0.23 (GeV/c)2, there are three measurements:AF : forward angle H (recoil protons) (D.S. Armstrong, et al., PRL 95, 092001 (2005))
AB : backward angle H (D. Androic, et al., PRL 104, 012001 (2010))
Ad : backward angle D
a1 (ppm)
a0 (ppm)a2 (ppm) a3 (ppm)
€
A∝ σ + − σ −
σ + + σ −∝M γM Z
M γ 2 ∝{ AE + AM + AA
ε(GEγ )2 + τ (GM
γ )2 }∝ a0 + a1GEs + a2GM
s + a3GAe(T=1)
G0 Forward Angle Experiment
• Forward angle measurement completed May 04
• LH2 target, detect recoil protons • Q2 = 0.12-1.0 (GeV/c)2,
E=3.03GeV• Spectrometer sorts protons by Q2
in focal plane detectors (16 rings in total)
• Detector 16: “super-elastic”, crucial for measuring the background
• Beam bunches separated by 32 ns• Time-of-flight separates protons
from pions• Results published in : • D.S. Armstrong, et al.,
PRL 95, 092001 (2005)
77
G0 Forward angle Results
8
D.S. Armstrong et al., PRL 95 (2005) 092001
NVSphysV
pE
pM
pE
F
sM
sE AA
RGGG
QGGG
= )0(
22
2 124
τEM form factors: J.J.Kelly, PRC 70, 068202 (2004)
HAPPEX-3
G0 Backward
9
G0 Backward Angle
C. Ellis/UMD
• Hydrogen and deuterium targets• Electron beam energy :
• 362 MeV : Q2=0.23 GeV2 • 687 MeV : Q2=0.62 GeV2
• Completed running in March 2007• Particle detection and
identification :• 16 Focal Plan Detectors • 9 Cryostat Exit Detectors
elastic and inelastic electron separation
• Čerenkov detectors electron and pion separation
e- beam
target
CED + Cherenkov
FPD
e- beamline
Beam Parameter Achieved (OUT-IN)/2
“Specs”
charge asymmetry 0.09 +/- 0.08 2 ppm
x position difference -19 +/- 3 40 nm
y position difference -17 +/- 2 40 nm
x angle difference -0.8 +/- 0.2 4 nrad
y angle difference 0.0 +/- 0.1 4 nrad
energy difference 2.5 +/- 0.5 34 eV
Beam halo (out 6 mm) < 0.3 x 10-6 10-6
Sep.-Dec. 2006
April 2006 March 2007
687 MeV Møller Results
Polarized Beam Properties Measurements of Møller asymmetry
at 687 MeV Mott scattering for 362 MeV
measurements Mott asymmetry measured at 5 MeV
in injector New work on effect of background
improves agreement with Møller
Pe(687 MeV) = 85.78 ± 0.07 ± 1.38%Pe(362 MeV) = ± 2.05%
10
362 MeV
Hydrogen raw electron dataHz/uA
d
11
octant # (azimuthal distribution)
blinded raw asymmetries (ppm)
687 MeV Hz/uA
362 MeV
687 MeV
Deuterium raw electron datablinded raw asymmetries (ppm) Hz/uA
d
d
Hz/uA
12
octant # (azimuthal distribution)
Blinding Factors
× 0.75-1.25H, D Raw Asymmetries,
Ameas
CorrectionsScaler counting correctionRate corrections from
electronicsHelicity-correlated corrections
Background correctionsBeam polarization
Q2 Determination
(from simulation)
Analysis Strategy
H, D Physics Asymmetries, Aphys
UnblindElectromagnetic
radiative corrections
(from simulation)
P= 0.8578 ± 0.0007 (stat) ± .014 (sys)
< 0.1 ppm
13
eA
sM
sE GGG ,,
4% on asymmetry
~10-50 ppm
± 0.003 GeV2
< 1% of Aphys
Rate CorrectionsCorrect the yields for random
coincidences and electronic dead time prior to asymmetry calculation
Randoms small except for D-687 (due to higher pion rate)
Direct (out-of-time) randoms measured
Validated with simulation of the complete electronics chain
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Data set
Correction to Yield (%)
AsymmetryCorrection (ppm)
systematic error (ppm)
H 362 8 -0.31 0.08
H 687 4 -1.28 0.18
D 362 7 -0.58 0.21
D 687 -10 -7.0 1.8
CED
FPD
Cerenkov TriggerCFD
CFD
CFD
CFD
MT
MT
Coincidence1 CED1 FPD1 Trigger
pion
electron
Trigger CEDxFPD
Backgrounds: Magnetic Field Scans
Use simulation shapes to help determine dilution factors
Main contributions are Aluminum windows (~10%), pions (for D-687 data only).
D-687CED 7, FPD 13
15
Data set Asymmetry Correction (ppm)
systematic error (ppm)
H 362 0.50 0.37
H 687 0.13 0.78
D 362 0.06 0.02
D 687 2.03 0.37
Experimental “Physics” Asymmetries
16
Data Set Asymmetry Stat Sys pt Global
H 362 -11.25 0.86 0.27 0.43
D 362 -16.93 0.81 0.41 0.21
H 687 -45.9 2.4 0.8 1.00
D 687 -55.5 33 2.0 0.7
Largest correction is due to the 85% beam polarization--dominates H systematic uncertainties--for D-687, contributes about equally with rate corrections to
the systematic uncertainty
all entries in ppm
Form Factor Results• Using interpolation of G0 forward measurements
G0 forward/backwardPVA4: PRL 102 (2009)Q2 = 0.1 GeV2 combined(G0,HAPPEX, SAMPLE & PVA4)
Global uncertainties
G0 forward/backwardSAMPLEZhu, et al. PRD 62 (2000)
17
Some calculations: Leinweber, et al. PRL 97 (2006) 022001 Leinweber, et al. PRL 94 (2005) 152001 Wang, et al arXiv:0807.0944 (Q2 = 0.23 GeV2) Doi, et al, arXiv:0903.3232
Summary
18
• Q2 behavior of strangeness contribution to proton’s charge and magnetism: continue to be small• first results for the Q2 behavior of the anapole contributions to the axial form factor• other results to come soon from G0:
transverse beam spin asymmetries (2- exchange) in H and D PV in the N-D transition: axial transition f.f.
PV asymmetry in inclusive production
€
GE ,M(0) = 1
3(GE ,M
u +GE ,Md +GE ,M
s )
The G0 Collaboration (backward angle run)
19
Caltech, Carnegie Mellon, William and Mary, Hendricks College, Orsay, Grenoble, LA Tech, NMSU, Ohio, JLab, TRIUMF, Illinois, Kentucky, Manitoba, Maryland, Winnipeg,
Zagreb, Virginia Tech, Yerevan Physics InstitutePhD students: C. Capuano, A. Coppens, C. Ellis, J. Mammei, M. Muether, J. Schaub, M. Veerstegen, S. BaileyAnalysis Coordinator: F. Benmokhtar
• Backups
G0: N → DMeasurement: Parity-violating
asymmetry of electrons scattered inelasticallyANΔ gives direct access to GA
NΔDirectly measure the axial (intrinsic spin)
response during N →Δ+ transitionWill find GA
NΔ over a range of Q2
0.05 GeV/c2 < Q2 < 0.5 GeV/c2.First measurement in neutral current process
Data: Inelastic electrons measured by G0Scattered from both LH2 and LD2, each at
two energies (362MeV & 687MeV)
Elastic R
egion: G
0
Inelastic R
egion: N
D
BLINDED
Asymmetry (ppm) vs Octant (LH2 @ 687MeV)
Raw Asymmetry (averaged over inelastic region)
IN
OUT
Octant
Asym
met
ry (p
pm)
2
Im
M
MMAn ∝
Transverse Asymmetry 2 photon exchange
Inclusion of the real part of the 2γ exchange in the cross sectionmay account for the difference between measurements of GE/GM from unpolarized cross section and polarization transfer measurements
It also tests the theoretical framework that calculates the contribution of γZ and W+W- box diagrams that are important corrections to precision electroweak measurements
Asymmetry Uncertainties--Hydrogen, 687 MeV
Value(ppm)
Stat(ppm)
Sys Pt(ppm)
Sys Gl(ppm)
Total(ppm)
Measured Asymmetry -38.14 2.43Background Asymmetry -38.27 0.40Dilution Correction 0.47 0.52Transverse Correction 0.008Rate Correction -38.39 0.17Beam Polarization -44.76 0.52 0.53EM Radiative Correction -46.14 0.16Physics Asymmetry -46.14 2.43 0.84 0.75 2.68
Asymmetry Uncertainties--Deuterium, 687 MeV
Value(ppm)
Stat(ppm)
Sys Pt(ppm)
Sys Gl(ppm)
Total(ppm)
Measured Asymmetry -44.02 3.34
Background Asymmetry -46.05 0.050
Dilution Correction 0.38
Transverse Correction 0.009 0.008
Rate Correction -46.35 1.82
Beam Polarization -54.03 0.62 0.64
EM Radiative Correction -55.87 0.19
Physics Asymmetry -55.87 3.34 1.98 0.64 3.92
Correction due to misidentified pions
red: pionsblue: electrons
3 Ways to calculate • runs with special beam structure providing timing reference allowing particle TOF• PMT Multiplicity (two versus three of the cerenkov PMT’s firing)• Pulse heights (waveform digitized)
cerenkov detector4 pmts
aerogel
# single photo electrons
coun
ts
threshold
contamination to electron yield (A ~ 0)D 362: 0.5%D 687: 4%
687 MeV
25
26
€
A(q,ω,θe ) = a0 + a1GEs + a2GM
s + a3GAe(T=1) + a4GA
s
sAA
AVTT
VLL
elem GRRa
DRvRvAa
= 0
0
40
22
2
QGA F
elem =
),(),( qRvqRvD TTLL =
€
a2 = AelemvTRT
s
D× 1GM
s (Q2)
€
a3 = Aelem
(1− 4 sin2θW )vT ' RLTA( )
10+ RLT
A( )11
[ ]D
× 1GA
e(T=1)(Q2)
€
a4 = Aelem
(1− 4sin2θW )vT ' RLTA( )
00+ RLT
A( )01
[ ]D
× 1RA
0 +GAs (Q2)
⎡ ⎣ ⎢
⎤ ⎦ ⎥
absorbs isoscalar axial term
broken down by isospin
€
a1 = Aelem
vLRLs + vTRT
cs[ ]D
× 1GE
s (Q2)
D asymmetries: Calculation provided by Rocco Schiavilla
C. Ellis/UMD
(see Diaconescu, Schiavilla & van Kolck, PRC 63 (2001) 044007)
TTLL
ATTW
VTT
VLLF
e RvRvqRvqRvqRvQGqA
=),(')sin41(),(),(
22),,(
22
Quasielastic PV (ee’) in Deuterium
E. Beise, U Maryland
d
nnppd
AAA
=
Parity conserving nuclear corrections to the asymmetry are generally small, 1-3% at backward angles. Calculation provided to us by R. Schiavilla includes final state interactions and 2-body effects.
Use Quasielastic scattering from deuterium as lever arm for GAe(Q2)
Diaconescu, Schiavilla + van Kolck, PRC 63 (2001) 044007Schiavilla, Carlson + Paris, PRC 67 (2003) 032501
See also Hadjimichael, Poulis + Donnelly, PRC45 (1992) 2666 Schramm + Horowitz, PRC 49 (1994) 2777 Kuster + Arenhovel, NPA 626 (1997) 911 Liu, Prezeau, + Ramsey-Musolf,
PRC 67 (2003) 035501
27
Deuterium model comparison to cross section data
E. Beise, U Maryland 28
data from:S. Dytman, et al., Phys. Rev. C 38, 800 (1988)B. Quinn, et al., Phys. Rev. C 37, 1609 (1988)
calculation from R. Schiavillasee alsoR.S., J. Carlson, and M. Paris, PRC70, 044007 (2004).
• AV18 NN potential• relativistic kinematics• J.J. Kelly fit to nucleonform factors
2-body effects in the D asymmetry
E. Beise, U Maryland 29
leading termof the asymmetry
axial formfactor coefficienthas ~15% correction from2-body effects
calculations from R. Schiavilla, see also R.S., J. Carlson, and M. Paris, PRC70, 044007 (2004).
Scaler Counting Issue with Electronics
Electronics sorts detector coincidences (CEDi and FPDj) into separate scaler channels FPGA-based system in North American electronics (4
octants) Because of error in FPGA programming, two short
(~3 ns) pulses could be sent to scaler in < 7 ns ~ 1% of events have such miscounts
Such pulse pairs can cause scaler to drop or add bits Detailed simulation of ASIC with propagation delays
between (flip flop) elements Effect on asymmetry is <0.01 Aphys
Test by cutting data; compare with French octants
Data
Simulation
30
College of William and Mary, Institut de Physique Nucléaire d'Orsay, Yerevan Physics Institute, Laboratoire de Physique Subatomique et de
Cosmologie Grenoble, University of Illinois, University of Maryland, Thomas Jefferson National Accelerator Facility, University of Manitoba,
Carnegie Mellon University, California Institute of Technology, University of Kentucky, TRIUMF, Louisiana Tech University, Virginia Tech, University of Northern British Columbia, New Mexico State University ,University of Winnipeg, Ohio University, Hampton University, Hendricks College,
University of Zagreb
The Gzero Collaboration
U.S. G0 participation jointly supported by DOE and NSF. Significant additional contributions to G0 from NSERC (Canada) and CNRS (FR)
C. Ellis/UMD 31
Summary of the G0 Backangle Run
Beise Run Coordinator report March 2007
Run Coulombs Hours Gbytes dataH 687 86 400 600H 362 119 550 825D 362 66 520 800D 687 I 40 520 780D 687 II 15 240 340total 330 2300 3450
Run start to run end ~ 8940 hours
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