1
Strange Quark Contribution to the
Electromagnetic Properties of the
Nucleon,
G0 Results
Fatiha Benmokhtar
Carnegie Mellon University
Hall C Summer workshop, Aug 6-7th 2009
F. Benmokhtar
2
• Physics Motivation
• G0 Backward angle Setup
• Some Analysis
• From Physics Asymmetry to Strange FFs
• Results
• Summary
Outline
F. Benmokhtar
3
Nucleon Constituents
- The sea contains all flavors, but
• the u and d sea are hard to distinguish from the valence
• the heavier quarks (c,b,t) are too heavy to contribute much
• Strange quark is the natural candidate to study the sea.
With how much do virtual strange pairs contribute
to the structure of the nucleon ?
.....+ gssdduuuudp
« sea= virtual pairs »valence
F. Benmokhtar
4
Strange Quark Contribution to the Nucleon Properties
NssNσ s Hyp ->
p-N -> Up to 30 % with big theoretical
uncertainties.
DI -Nucleon scattering (NuTeV)
%~dx))x(s)x(s(x 421
0
• Mass:
• Longitudinal Momentum:
F. Benmokhtar
5
09.015.0 s
N|ss|N 5
. -p elastic scattering (E734 BNL)
• Polarized Semi-Inclusive DIS (HERMES)
• PQCD predicts -0.1
** Accepted JLab HallB 12GeV proposal ( E09-007 )
(H. Avagian, F. Benmokhtar, K. Hafidi, A. El-Alaoui, M. Marazita )
F. Benmokhtar
0.02 0.006 0.029 0.007s
• Spin:
6
Strangeness Contribution to the Nucleon Electromagnetic
Properties
??NssN s
M
s
EGG ,
Goal: Determine the contributions of the strange quark sea ( )
to the charge and current distributions in the nucleon :
“strange form factors” GsE and Gs
M
ss
How do we measure them?F. Benmokhtar
7
Strange Form Factors
Z
sin2qW = 0.2312 ± 0.00015
Charge symmetry ->
measured
?
(see G. A. Miller PRC 57 (98) 1492.)
F. Benmokhtar
s
MEW
d
MEW
u
MEW
Z
ME GGGG
,
2
,
2
,
2
, sin3
41sin
3
41sin
3
81
q
q
q
s
ME
d
ME
u
ME
p
ME GGGG ,,,
,
,3
1
3
1
3
2
nspsnupdndpu GGGGGG ,,,,,, ;;
pZnp
W
sMEMEMEME GGGG ,,,2
,,,, sin41 q
8
Parity Violation Asymmetry
2
Z
2
EM
NC
PV
LR
LRPV
M
Q~
M
M~
σσ
σσA
• Scatter polarized electrons off unpolarized target,
• Asymmetries (R,L) of the order of ppm
Electric Magnetic Axial
5
Z
10M
M,
• Proton target
• Deuteron target
(static case)
• Helium target
)(2sin
2
22
n
E
p
E
s
EW
FPV
GG
GQGA q
p
-Complete calculation by R. Schiavilla et al.,
is available ( priv. communication)
- Enhanced sensitivity to Axial form factor.
p
AME
2
FPV
σ
AAA
24π
QGA
s
EG
np
nnpp
PV
AAA
ss
ss
e
AM
e
AWA
s
MM
Z
MM
s
EE
Z
EE
GGG)sin(A
GGGA
GGG)(A
q
t
q
241
Left
Handed
spi
nRight
HandedDirection of motion
Left
Handed
spinRight
HandedDirection of motion
F. Benmokhtar
9
PV. Experiments
H, DHH, 4HeHH,DTarget
F/BF/BFFBAngle
Ges, GM
s , GA(p+n)GE
s, GMsGE
s , GMsGE
s + 0.4 GM
sGM
s, GA
(p+n) Separation
0.12 - 1.0
(0.23, 0.62)
0.1, 0.23
(0.23,0.6)0.10.480.04, 0.1Q2 (GeV/c)2
G0
(JLab)
2003-2007
PVA4
(MAMI)
2002-2008
HAPPEX II
(JLab)
2004-2005
HAPPEX I
(Jlab)
1998-2002
SAMPLE
(MIT-Bates)
1998-2002
Beam
LH2Target
SuperconductingCoils Particle
Detectors
• HAPPEX-III in preparation: 2009 at 0.6 GeV2
e
AM
e
AWA
s
MM
Z
MM
s
EE
Z
EE
GGG)sin(A
GGGA
GGG)(A
q
t
q
241
Forward
Backward
F. Benmokhtar
)1()1(
tan)1(21
4
2
12
2
2
2
tt
t
t
q
M
Q
10
Results from the Forward Angle
Compared to ANVS (“No VectorStrange”)
EM form factors : Kelly PRC 70 (2004) 068202
D.S. Armstrong, et al., PRL 95, 092001 (2005)
G0 Backward
NVSphys
V
p
E
p
M
p
E
F
s
M
s
E AARG
GG
QGGG
)0(
22
2 1
24
tp
Examining full data set,
probability that
GEs+GM
s ≠ 0 is 89%F. Benmokhtar
11
G0 Backward Angle Experiment
(06-07)
F. Benmokhtar
12
12
Experimental Setup
• Electron Beam: 362 and 687 MeV -> Q2: 0.23 and 0.62 GeV2
• Electron off 20 cm LH2 or LD2 target, electron detection : Θ =108°, W ~ 0.5 sr.
•Turn-around of the magnet, change polarity
• Add Cryostat Exit Detectors (9 CEDs per Octant)-> separate elastic
and inelastic electrons in the CED*FPD space.
• Aerogel Cerenkov detector per octant for p/e separation. (pp < 500 MeV/c)
Cerenkov
CED
FPD
electrons
pions
FPDC
ED
CE
D
Elastic
Inelastic
13
G0 beammonitoring
Spokesman
Target service module
Detectors(Mini-Ferris wheel)
CED+Cherenkov
Sup. Cond. Magnet(SMS)
Detectors(Ferris Wheel)
FPD
14
Beam Specifications
Acor Ameas -1
2Yi
Y
PiPi
• Helicity correlated beam properties
-> false asymmetry.
Correction : linear regression:
• 2ns beam structure
• 86 % longitudinal polarization
• Helicity changed every 1/30 sec (MPS).
• Form Asym. from a pseudo-random
quartet structure in helicity (+--+ or -++-).
• Slow helicity reversal: 1/2 wave plate IN and OUT
~0.1ppm
F. Benmokhtar 10-6
< 0.3 x 10-6
Beam halo
34 eV2.5 +/- 0.5Energy difference
4 nrad0.0 +/- 0.1y angle difference
4 nrad-0.8 +/- 0.2x angle difference
40 nm-17 +/- 2y position difference
40 nm-19 +/- 3x position difference
2 ppm0.09 +/- 0.08Charge asymmetry
“Specs”Achieved (IN-OUT)/2Beam Parameter
1515
Recorded Data(Hz/μA/Oct)
45 C
LH2, 687 MeV
LD2, 687 MeV
LH2, 362 MeV
LD2, 362 MeV
90 C 120 C
70 C 45 C
1616
Raw Blinded Asymmetries
per Kinematics
octant # (azimuthal distribution)
F. Benmokhtar
LH2 362 MeV
LD2 687 MeVLH2 687 MeV
LD2 362 MeVLH2 362 MeV
1717
Blinding Factors
X0.75-1.25
H, D Raw Asymmetries, Ameas
CorrectionsScaler counting correction
Rate corrections from electronics
Helicity-correlated corrections
Background asymmetry
Beam polarization
Nucleon EM form factors
Q2 Determination
Analysis Strategy
H, D Physics Asymmetries, Aphys
Unblinding
F. Benmokhtar
~10-50 ppm
P= 0.8578 ± 2.1(1.4)%
< 0.1 ppm
< 1% of Aphys
~3.5% on asymmetry
± 0.003 GeV2
, ,s s e
E M AG G G
1 to 10%
0.5 to 1.5% on asymmetry
Electromagnetic radiative corrections
ElectroWeakradiative corrections Forward measurements
181818
CFD
CFD
CFD
CFD
MT
MT
Coincidence
pion
electron
CED
FPD
• DT from Constant Fraction Discriminators,
• DT from Mean Timers,
• DT from Trigger module , MultiHits.
• DT and randoms in the cerenkov electronics.
• Random coincidences subtraction: contamination from pion signal in the scintillators
in coincidence with random signals from Cerenkovs.
Rate Corrections
• Rate correction study for counting experiment is complicated!
- Tools:- Simulate the full electronics chain. (inputs from real measurements)
- Study rate changes with current scans. - Introduce large charge Asymmetry and see what is the effect on the
detector asymmetry (Well’s plot).- Delay the cerenkov signal to measure random coincidences. F. Benmokhtar
1919F. Benmokhtar 19
A
Corrected
Measured
Electron Measured-Corrected Yield Electron Measured-Corrected Yield
LD
2-6
87 M
eV
(
Hz/
A)
A
Measured
Corrected
No Dead Time
No Dead Time
• Current Scans:
• Random coincidences:
– LH2 randoms small
– LD2 randoms significant
• higher pion rate
– Direct (out-of-time) measurement
Examples
• Multihit corrections smaller (yield)Arise because of trigger:(any CED) and (any FPD)
2020F. Benmokhtar 20
Dataset
Correction to Yield (%)
AsymmetryCorrection
(ppm)
systematic error (ppm)
H 362 6 0.3 0.06
H 687 7 1.4 0.17
D 362 13 0.7 0.2
D 687 9 6 1.8
•Linear regression will then take care of any residual corrections.
Total rate correction is up to ~ 10 % to the asymmetry
Total Rate Correction:
LH2, 687
MeV
LH2, 362 MeV
Deadtimes (%) (yield)
contamMHTrigglesyst AAAAA sin
21F. Benmokhtar
Backgrounds: Magnetic Field ScansD-687CED 7, FPD 13
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
• Primary background from Al
target windows:
- about 12% of yield for all
target/energy combinations
• p- contamination in D 687 MeV
-5% contribution (measured),
nearly 0 asymmetry (measured)
< 1%
22F. Benmokhtar
Physics Asymmetry (Preliminary Results)
Data Set Asymmetry Stat Sys pt Global
H 362 -11.01 0.84 0.26 0.37
D 362 -16.50 0.79 0.39 0.19
H 687 -44.76 2.36 0.80 0.72
D 687 -54.03 3.22 1.91 0.62
H: systematic uncertainties dominated by beam polarizationD: rate corrections also contribute to uncertainty.
all entries in ppm
23
From Asymmetry to Strange Form Factors
R. Schiavilla (priv. communication)
See: J. Liu PhD. thesis, UMD 2006.
• Proton
q
TTLL
A
TTW
V
TT
V
LLF
RvRv
qRvqRvqRvQGA
),(')sin41(),(),(
22
22
• Deuteron
s
A
Te
A
s
M
s
E GaGaGaGaaA 4
)1(
3210
Isoscalar
strange ffto be extracted23F. Benmokhtar
e
A
p
MW
)(
V
s
M
s
E
p
E
n
V
n
M
p
M
n
E
p
E
p
V
p
M
p
EWp
M
p
E
F
GGθε
RηGGεGRGGτGGε
RGτGεθGτGεπα
QGA
2
0
222
22
2
sin41
11
1sin411
24
• Proton&Deuteron
Choice of Nucleon Form Factors
• These results: Kelly’s form factors from (PRC 70 (2004)) simple parameterization +
Galster for GnE
- used in R. Schiavilla deuterium calculation.
24F. Benmokhtar
• Including all published low Q2 data:- Arrington , Melnitchouk & Tjon
for proton (Phys. Rev. C 76035205(2007))
- Arrington & Sick for the neutron
(Phys. Rev. C 76, 035201 (2007) )
-> Changes are from 0.5 to 1% to the
Asymmetry in our Backward and Forward
angle kinematics.
- With the addition of the preliminary Hall A unpublished proton FFs, the
asymmetry changes within only fraction of a % from AMT.
25
Electroweak Radiative Corrections and Two Boson Exchanges
F. Benmokhtar
e
A
p
MW
)(
V
s
M
s
E
p
E
n
V
n
M
p
M
n
E
p
E
p
V
p
M
p
EWp
M
p
E
F
GGθε
RηGGεGRGGτGGε
RGτGεθGτGεπα
QGA
2
0
222
22
2
sin41
11
1sin411
24
W
Wp
V
du
n
V
R
R
q
qr
r
2
2
11
sin41
sin411
,221
-PDG values of (r,) contain vertex corrections, corrections to the propagators,
and Z exchange corrections at Q2=0. They do not contain corrections or recent evolutions on the Z box diagrams. - Be Careful of double counting!
Marciano et al., Phys. Rev. D 29, 75 - 88 (1984)
2626
Addition of two Boson Corrections
11
1
Z Z
meas Born BornA A A
26F. Benmokhtar
Two groups use hadronic calculation with finite size of the nucleon:- Blunden, Melnitchouk & Tjon (Phys. Rev. C 79, 055201 (2009))
- Nagata, Zhou, Kao & Yang (Phys.Rev.C79.062501,2009)
-To avoid double counting, one has to remove the ‘hadronic’ part of the Z box diagram from r,
r, ) = (0.988210, 1.003520), and feed them back the Asymmetry then apply the new corrections as follows:
For G0 Kinematics:
H forw H back D backLow Q2 -0.64% 1.46% 0.41%High Q2 -0.16% 1.20% 0.62%
27
Form Factor Results• Using interpolation of G0 forward measurements
Global uncertainties
G0 forward/backward
SAMPLE
Zhu, et al. PRD 62 (2000)
G0 forward/backward
PVA4: PRL 102 (2009)
Q2 = 0.1 GeV2 combined
F. Benmokhtar
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 (Q2 = 0)
28
Global uncertainties
u
EGd
EGs
EGp
EG
n
EG
28
u
MGd
MGs
MGp
MGn
MG
Contributions to the Magnetic FFContributions to the Charge FF
Flavor Decomposition
Global uncertainties
F. Benmokhtar
2 1 2 1
;3 3 3 3
p u d s n d u s
E E E E E E E EG G G G G G G G 2 1 2 1
;3 3 3 3
p u d s n d u s
M M M M M M M MG G G G G G G G
2929
Additional Program to the
G0 Experiment
F. Benmokhtar
• PV Asymmetry in the N- transition:– Inelastic electron asymmetry from 687MeV H is being used to extract
the axial transition form factor GAN
– Represents the first measurement of GAN in the neutral current sector
• Pion Photoproduction:– Pion data from 362MeV D is being used to determine the scale of the
low energy constant characterizing the PV N coupling– Represents the first time this constant has been studied experimentally
• Transverse Measurements:– Transverse beam polarization data is being used to study the 2
exchange– Represents one of the first measurements with neutron
Results to come soon!
30
Summary
• Comparison of electromagnetic and weak neutral elastic form factors allows determination of strange quark contribution to the proton:
• Small and positive at higher Q2
• consistent with zero
• First interesting measurements of up to high Q2
• Publication is underway!
• Other results from G0 program will come soon! (publish apart)
s
EG
e
AG
s
MG
F. Benmokhtar
3131
The G0 Backward Angle
Collaboration
G0 Spokesperson: Doug Beck (UIUC)
California Institute of Technology, Carnegie-Mellon University, College of William and Mary, Grinnell College, IPN Orsay, JLab,
LPSC Grenoble, Louisiana Tech. Univ., New Mexico State University, Ohio University, TRIUMF, University of Illinois,
University of Kentucky, University of Manitoba, University of Maryland, University of Winnipeg, Virginia Tech, Yerevan Physics
Institute, University of Zagreb
Analysis Coordinator: Fatiha Benmokhtar Carnegie Mellon (Maryland)
Students:
Carissa Capuano (W&M), Alexandre Coppens (Manitoba), Colleen Ellis(Maryland) , Juliette Mammei (VaTech), Mathew Muether (UIUC), John
Schaub (NMSU), Maud Versteegen (LPSC),
Stephanie Bailey (Ph.D. W&M, Jan ’07)
32
Backup Slides
F. Benmokhtar
33
0sinˆ ss
ss
nen
m AnpAA
)()F~
F~
Im()F~
F~
Im(GA 2
5453Mn t
)(
1)1(22
22
1
2
EM
e
GGQ
m
t
t
iF~
-contains intermediate
hadronic state information
magnitude of transverse asymmetry
depends on direction of transverse beam polarization
Transverse Polarization Data
(G0 backward)
F. Benmokhtar
34
π
)(
π
)(
π
)(F
inel Δ+Δ+Δπα
QG=A 321
2
24
(1) = 2(1sin2qW) = 1 (Standard Model)
(2) = non-resonant contrib. (small)
(3) = 2(14sin2qW) F(Q2,s) (N- resonance)
)(QGs),F(Q A
NΔ
22
At tree-level:
expected precision
•F contains kinematic information & all weak transition form factors
→Extract GAN from F
Measurement of theParity Violating Asymmetry
in the N →Δ Transition
BLINDED
Asymmetry (ppm) vs Octant (LH2 @ 687MeV)
Raw Blinded Asymmetry (averaged over
inelastic region)
Q2 Range of G0 Measurement
22 ) ( QvsQGA
N
F. Benmokhtar
3535
Scaler Counting issue • An occasional bit drop in a North American scaler
was traced down to trigger electronics. At high
rates: LD2 target at 362 MeV. This was fixed during
the run (Jan07)
• Problem blind to helicity.
Exaggerated
re-production
with a Pulser
-Pulser data
-Simulation
5s cut removes ~1% of our data for
362MeV LD2,which is the worst case!
Uncut 7s 6s 5s 4s 3s
Cut on yield
• Test by cutting data; compare with
French octants.
• Confirmed by unchanged
asymmetry after fix
Effect on Asymmetry F. Benmokhtar
• asymmetry is <0.01 Aphys
36
Helicity-correlated Beam Properties: Linear Regression
• Cross check with sensitivities determined from natural beam motion and from deliberate ‘coil-pulsing’ runs
• Sensitivities smaller at backward angles
– ~ 1/5 of slopes at forward angles
– Kinematic correlation (momentum/angle) combined with spectrometer acceptance
1
% /Y
mmY x
1
% /Y
mmY y
Simulation
37
Anapole form factor FA(Q2)
2. FA(Q2) is flat
(Riska, NPA 678 (2000) 79)
1. FA(Q2) is like GA(Q2)
3. FA(Q2) ~ 1 + Q2
(Maekawa, Viega, van Kolck, PLB 488 (2000) 167) – (shown here are the most extreme set of model parameters)
Three scenarios
383838
Scaler Counting issue • An occasional bit drop in a North American scaler
was traced down to trigger electronics. At high
rates: LD2 target at 362 MeV. This was fixed during
the run (Jan07)
• Problem blind to helicity.
Exaggerated
re-production
with a Pulser
-Pulser data
-Simulation
5s cut removes ~1% of our data for
362MeV LD2,which is the worst case!
Uncut 7s 6s 5s 4s 3s
Cut on yield
• Test by cutting data; compare with
French octants.
• Confirmed by unchanged
asymmetry after fix
Effect on Asymmetry F. Benmokhtar
39
s Predictions Jan 2000
Model number
40
G0 Predictions 2003
Recommended
