Transcript Weak probe of the nucleon in electron scattering

Weak probe of the nucleon
in electron scattering
•Weak Current far from
the Z pole
•History
•Elastic Scattering and strange
content of the nucleon
•Perspectives in charged
current and DIS kinematics
•Conclusion
David Lhuillier , CEA Saclay
Principle
e’
e’
e
e-
N
e- X
e
g(Q2)
=
N
+
X
Q2 << MZ2 at typical “hadronic scale”
Z0
N
X
Nice perturbative probe
But weak interaction violates P
 Build a pseudo-scalar observable, prop. to MZ:
ALR~GFQ2/4pa
ALR~10-4 at Q2=1 (GeV/c)2
David Lhuillier , CEA Saclay
2
A Bit of History: E122 @ SLAC
DIS off deuteron target:
e’
e
g
2H
•Isoscalar target: fu(x)=fd(x)
•Neglect sea distributions: x > 0.2
David Lhuillier , CEA Saclay
x=Q2/2Mn
y=n/E
,Z0
X
x dependence factorizes out
A bit of History: E122 @ SLAC
First constraint establishing
the Weinberg-Salam Model:
Hybrid
C.Y.Prescot et al,
Phys. Lett. 84B, 524 (1979)
sin2qW=
0.224+/-0.020
Extensions:
•Low x: probe sea contribution with sin2qW as input
•Larger x: precise test of sin2qW
(Jlab LOI, complementary to E158 and Qweak)
David Lhuillier , CEA Saclay
Factor 1000 !
Parity Violation in e- Scattering
Test SM
Elastic
e--N
Mutual enrichment between test of SM and hadronic structure
Impressive performances of polarized beam
David Lhuillier , CEA Saclay
Experimental Techniques
Key elements:
Injector
Source
•High Pe and I
•Rapid helicity flip
•Feedbacks
Beam line
Hall
l/2
“Table-Top” Experiments
Half-wave plate reversals
IN
OUT
Charge asymmetry
at 1 GeV
Final: AI=-8.4+/-7.8 ppb (10-9) !
David Lhuillier , CEA Saclay
Elastic Scattering
e’
e’
e
e-
e- N
N
2
e
g(Q2)
=
N
+
N
Z0
N
N
Electromagnetic FF
Weak FF
ALR=
GFQ2
2pa 2
David Lhuillier , CEA Saclay
f(F1g,F2g,F1Z,F2Z,GA)
few ppm
at Q2<1 (GeV/c)2
Strange content of the Nucleon
Sum over 3 quarks flavours (u,d,s)
weighted by elec. and weak charges
Charge symmetry
Extraction of the strange quarks contribution
3x3 equations
Same relations
stand for GE/GM…
Motivation triggered by DIS results on Ds but the measured matrix elements
are independent: vector <N|sgms|N> instead of pseudo-vector <N|sgmg5s|N>
David Lhuillier , CEA Saclay
Leading Non Zero Moments
Measurements at low Q2
Scale: <r2n>Sachs=0.12 fm2
Strange Magnetic Moment:
Scale: mN
David Lhuillier , CEA Saclay
<rs2>Sachs (fm2)
Strange Charge Radius:
Elastic e-p Asymmetry
ms= -0.3
<rs2>Sachs= 0.33 fm2
Forward angle:
GsE,GsM
Backward angle:
GsM,GA
David Lhuillier , CEA Saclay
Experimental Strategy
Complete Separation GE/GM/GA
over Q2=0.11 (GeV/c)2
Rosenbluth Method
Forward and backward measurements with the same detector setup
G0 @ Jlab
Q2=0.3-0.8 (Gev/c)2
PVA4 @ MAMI
Q2=0.1 and 0.25 (Gev/c)2
but not enough lever arm to extract the 3 contributions
David Lhuillier , CEA Saclay
Experimental Strategy
Complete Separation GE/GM/GA
over Q2=0.11 (GeV/c)2
Quasi-elastic e-Deuteron
Different targets
Elastic e-4He:
SAMPLE
MIT-Bates
HAPPEx2
JLab
Q2=0.1 GeV/c2
David Lhuillier , CEA Saclay
Isoscalaire Transition pure GE
Experimental Program
Q2 dependence
G0, PVA4, HAPPEx1
0.2 < Q2 < 0.8 (GeV/c)2
First moments
SAMPLE, HAPPEx2, PVA4
Q2 = 0.1 (GeV/c)2
•First Constraint on ms:
ms=0.37+/-0.20stat+/-0.26syst+/-0.15theo
•Expected combined accuracy:
dms ~ 0.16 mN
David Lhuillier , CEA Saclay
d<rs2>Sachs ~ 0.020 fm2 ~ <rn2>Sachs/6
Weak Charged Current
in e-p scattering
0
W- Z W+
The NEPTUNE experiment
n
e-
Letter Of Intent at JLab (A. Deur)
W-
p
n
V
GE,M GA GP
David Lhuillier , CEA Saclay
+
•High luminosity
•Direct measurement
-
•Only detected particle = n
NEPTUNE Experimental Setup
•Background reduction:
Backward kinematic
Sweepnig magnet
Recoil detector
Pulsed beam
e-
S/B~0.1
•APV=10%
Symmetric w.r.t. beam axis
(not represented)
David Lhuillier , CEA Saclay
Accurate
normalization
despite bad S/B.
NEPTUNE Projected Results
n
e-pnn
p electroproduction
GA
E=1-2 GeV
qe=120deg
Q2=0.8-2.7
Q2
(GeV/c)2
Accurate data in an unexplored kinematical domain
of a fundamental observable of the nucleon
David Lhuillier , CEA Saclay
Parity Violating Asymmetry in e-p DIS
e’
e+,e-
g ,Z0
p
Extract G3  separate valence and sea contributions …
David Lhuillier , CEA Saclay
X
Parity Violating Asymmetry in e-p DIS
Single spin asymmetry:
•G3 term is suppressed
•but ratio G2/F2 depends on d/u
•ALR ~ 10-5 Q2, allows accurate
measurements
Charge asymmetry:
[se+-se-] a xG3
David Lhuillier , CEA Saclay
•First data from HERA,
Stat. Limited
•Intense neutrino beams
Conclusion
Weak Neutral Current in e- scattering:
•New probe of the nucleon structure
•Important experimental program ongoing to extract
the contribution of strange quarks to the vector
matrix element <N|sgms|N>.
Development of experimental techniques
•Perspectives in charged current (GA)
and DIS neutral current (d/u).
•Precise test of the interaction itself
David Lhuillier , CEA Saclay
Back to the Future
•New results from ALR in Moller scattering from E158 at SLAC.
•Parity violating asymmetries allow precise test of SM at low energy
David Lhuillier , CEA Saclay