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Gzero ResultsFatiha Benmokhtar
Carnegie Mellon University
Jlab Users Group Meeting, June 9th 2009
F. Benmokhtar
2
• Physics Motivation
• G0 Backward angle Setup
• Some Analysis
• From Physics Asymmetry to Strange FFs
• Additional Physics programs to G0
Outline
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
• Spin:
NssNσ s Hyp ->
p-N -> Up to 30 % with big theoretical
uncertainties.
DI nm-Nucleon scattering (NuTeV)
. For x <0.1 %~dx))x(s)x(s(x 421
0
• Mass:
• Longitudinal Momentum:
09.015.0 s
N|ss|N 5 m
. n-p elastic scattering (E734 BNL)
• Polarized Semi-Inclusive DIS (HERMES)
- Future Jlab HallB 12GeV proposal: E09-007
• PQCD predicts -0.1F. Benmokhtar
0.02 0.006 0.029 0.007s
“Momentum “fraction”
5
Strangeness Contribution to the Nucleon Electromagnetic
Properties
??NssN ms
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
6
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
7
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
e
AM
e
AWA
s
MM
Z
MM
s
EE
Z
EE
GGG)sin(A
GGGA
GGG)(A
q
q
241
Left
Handed
spi
nRight
HandedDirection of motion
Left
Handed
spinRight
HandedDirection of motion
F. Benmokhtar
8
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 2009 at 0.6 GeV2
e
AM
e
AWA
s
MM
Z
MM
s
EE
Z
EE
GGG)sin(A
GGGA
GGG)(A
q
q
241
Forward
Backward
F. Benmokhtar
)1()1(
tan)1(21
4
2
12
2
2
2
q
M
Q
9
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
p
Examining full data set,
probability that
GEs+GM
s ≠ 0 is 89%F. Benmokhtar
10
G0 Backward Angle Experiment
(06-07)
F. Benmokhtar
11
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 CMU (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)
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 < 380 MeV/c)
Cerenkov
CED
FPD
electrons
pions
FPD
CE
DC
ED
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
15
Electron Yield (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 MeVLD2 362 MeV
LH2 687 MeVLH2 362 MeV
17
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
Forward measurementsEM form factors
Q2 Determination
Analysis Strategy
H, D Physics Asymmetries, Aphys
Unblinding
F. Benmokhtar
~10-50 ppm
P= 0.8578 ± 0.0007 ± .014
< 0.1 ppm
< 1% of Aphys
Electromagnetic radiative corrections
4% on asymmetry
± 0.003 GeV2
, ,s s e
E M AG G G
1 to 10%
1818
Asymmetry stability along the loci
-As
ym
metr
y (
pp
m)
18F. Benmokhtar
. These asymmetries are beam and rate corrected
. Note the stability along the elastic or the inelastic loci
. Non-locus cells are suppressed.
-As
ym
metr
y (
pp
m)
-As
ym
metr
y (
pp
m)
-As
ym
metr
y (
pp
m)
19F. 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)
20F. 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
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 extracted21F. 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 form factors from (PRC 70 (2004))
– used in Schiavilla deuterium calculation
22F. 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 0.5% from AMT.
23
Electroweak Radiative corrections and two boson corrections
• Use prescription of Tjon, Blunden & Melnitchouk
for applying 2 boson corrections to the asymmetry
11
1
Z Z
meas Born BornA A A
H forward H back D back
Low Q2 -0.64% 1.46% 0.41%High Q2 -0.16% 1.20% 0.62%
23F. Benmokhtar
Corrections from: J. W. Melnitchouk and J. Tjon, (Private communication, thanks!)
Two groups: - Blunden, Melnitchouk & Tjon (Phys. Rev. C 79, 055201 (2009))
- Keitaro, Zhou, Kao & Yang (Phys.Rev.C79.062501,2009)
24
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)
2525
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!
26
Summary
• Comparison of electromagnetic and weak neutral elastic form factors allows determination of strange quark contribution to the proton:
• Small positive at higher Q2
• consistent with zero,
• First interesting measurements of up to high Q2
• Other results from G0 program will come soon!
s
EG
e
AG
s
MG
F. Benmokhtar
27
Backup Slides
F. Benmokhtar
28
0sinˆ
nen
m AnpAA
)()F~
F~
Im()F~
F~
Im(GA 2
5453Mn
)(
1)1(22
22
1
2
EM
e
GGQ
m
iF~
-contains intermediate
hadronic state information
magnitude of transverse asymmetry
depends on direction of transverse beam polarization
Transverse Polarization Data
(G0 backward)
F. Benmokhtar
29
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
3030
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
5 cut removes ~1% of our data for
362MeV LD2,which is the worst case!
Uncut 7 6 5 4 3
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
31
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
323232
Contributions to Overall Form Factors
/10p
EGn
EG
/10p
MG
/10n
MG
- Error bars dominated by statistics.
- Systematic experimental: backgrounds Small
F. Benmokhtar
3333
• Dead Time and Randoms in CEDxFPD coincidences and cerenkov electronics.
• All the electronics chain was simulated.
mA
LD
2-3
62 M
eV
(
Hz/m
A)
Corrected
Measured
Electron Measured-Corrected Yield Electron Measured-Corrected Yield
LD
2-3
62 M
eV
(
Hz/m
A)
mA
Measured
Corrected
No Dead Time
No Dead Time
Correction: ~ 1-10% to the
asymmetry in the elastic locus.
Rate Correction for Electronic Related Effects
F. Benmokhtar
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
