The G0 Experiment:
Parity Violation in e-N Scattering
Colleen Ellis
The University of Maryland
The G0 Collaboration:
CalTech, Carnegie-Mellon,William & Mary, Hendrix,
IPN-Orsay, LPSC-Grenoble, JLab, LaTech, NMSU,
Ohio University,TRIUMF, U Conn, UIUC, U
Manitoba, U Maryland, U Mass, UNBC,
U Winnipeg, VPI, Yerevan, Zagreb
Hall C Meeting 18 January 2008
1
0
G
Graduate Students
Carissa Capuano :
W&M, USA
Maud Versteegen:
LPSC, France.
Alexandre Coppens:
Manitoba, Canada
Mathew Muether:
Illinois, USA
Colleen Ellis :
Maryland, USA
John Schaub : NMSU,
USA.
Juliette Mammei
Virginia Tech. USA.
2
Overview
• Physics Introduction
• G0 Forward Angle
• G0 Backward Angle--Elastic Electron Scattering
–
–
–
–
Experimental Set-up
Analysis Overview
Preliminary Data
Detector Performance
• Other Backward Angle Physics Topics
– Inelastic e-p measurement to measure parity violation in
N- transition
– Elastic e-p scattering with transverse beam polarization
to investigate 2 photon exchange
– PV pion photoproduction on the  resonance
3
Strange Form Factors
How does s quark contribute to electromagnetic properties of the nucleon?
G
 ,p
E,M


Qq G
q
E,M
q u,d ,s
Z ,p
GE,M

2 u, p 1 d , p 1 s, p
 GE,M  GE,M  GE,M
3
3
3
Electron scattering
involves EM and
Weak interactions
q
2
q
2T

4Q
sin

G
 3
q
W  E,M
q u,d,s
Assume: Isospin
symmetry
u,p
d ,p
u
GE,M
 GE,M
 GE,M
d ,p
u,n
d
GE,M
 GE,M
 GE,M
s,p
s,n
s
GE,M
 GE,M
 GE,M
s
 ,p
 ,n
Z ,p
GE,M
 1 4 sin 2 W GE,M
 GE,M
 GE,M

Known

G0 measures
4

Model Independent Form Factors
M M Z
  
A

  
M




2
{
AE  AM  AA
0
PV asymmetries
from EM and
weak interference
terms
}
AA  (1 4sin 2W )'GAe GM
Q2

4M2
1





  1 2(1  )tan 2  
2 

 0  (GE ) 2   (GM ) 2
'

AE  GE G
Z
E
AM  GM GMZ
1   1  
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
proton
APV
GEs (Q2 )
(  5,10)
 proton
 s 2
APV (  110)  GM (Q )
 deuterium
 e 2
(  110)
APV
GA5(Q )
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)
6
G0 Forward Angle Results
• Forward Angle
–
–
–
–
700 hrs of data taking
101 C.
18 Q2 measurements
Good agreement with
other experiments
(HAPPEx and PVA4)
• Backward Angle
– Two Q2 measurements
0.23 and 0.62 GeV2
(GeV2)
GEs  GMs 
p2
E
p
E
– Required for complete
separation of
G Es and G Ms
p 2
M
(0)  phys 
V
4 2 G  G
GF Q2 G 1 R

A
 ANVS 
7
G0 Backward Angle
• Hydrogen and deuterium targets
• Electron beam energy of :
– 362 MeV : Q2=0.23 (GeV/c)2
– 687 MeV : Q2=0.62 (GeV/c)2
• Detection of scattered electrons
~ 108º
Backward Angle Configuration
CED + Cerenkov
FPD
e- beam
e- beamline
• Particle detection and
identification :
– 16 Focal Plan Detectors
– 9 Cryostat Exit Detectors
elastic and inelastic electron
separation
– Additional Cerenkov detectors
electron and pion separation
target
8
G0 Backangle
Superconducting
Magnet
(SMS)
Target Service
Module
G0 Beam
Monitoring
Detectors:
Ferris Wheel
(FPDs)
Detectors:
Mini-Ferris wheel
(CEDs+Cerenkov)
9
Collected Data
• Longitudinal
–
–
–
–
LH2 362MeV 90 C
LD2 362MeV 70 C
LH2 687MeV 120 C
LD2 687MeV 45 C
•Transverse
–LH2 362MeV 3.6 C
–LD2 362MeV 2.1 C
–LH2 687MeV 1.0 C
•Special Runs Types
•pion matrix
•random matrix
•magnet scans
10
G0 Backangle Analysis Approach
Raw Yields
and Blinded Asymmetries
by target and Q2
Blinding
Factor
Q2 Determination
Forward Angle
G  G
S
E
S
M
Aphys
G
S
E
G
S
M
Rate Corrections for Electronics
--Deadtime and
Random Coincidences
Helicity Correlated Beam Corrections
G Ae
Corrections from inelastic electrons
Background from target walls
Pion Asymmetry Contamination
Unblind

Beam Polarization
Correction
EM Radiative Corrections
(via Simulation)
11
Forming Asymmetry
 measure raw yield for each helicity state (+ or -) apply rate corrections (electronic
deadtime and random coincidences):
Ym
Yc 
1 f R
f R  7%
f R  9 13%
LH2
LD2
correct for beam correlated effects :
Ycc 
i
Yc
Pi
Pi
 form asymmetry :
Pi  x, y, x , y , Q, E 


Ycc  Ycc
Am  
Ycc  Ycc
Afalse < 4 ppb
Am ~ 10 ppm
 correct for background contribution :
Am  fel Ael  fb Ab

1  fel  fb
fb < 10 %
correct for beam polarization (P)
Ael
 ao  a1GEs  a2GMs  a3GAe(T 1)
P
12
Electron Yields (Hz/uA)
Elastic
Elastic
Inelastic
90 C
120 C
LH2, 362 MeV
LH2, 687 MeV
Quasi Elastic
Inelastic
70 C
LD2, 362MeV
Inelastic
45 C
13
LD2, 687 MeV
IHWP IN OUT
LH2 362
LD2 362
14
PRELIMINARY RAW BLINDED
Elastic Electron Asymmetries
LH2 687
LD2 687
15
PRELIMINARY RAW BLINDED
Elastic Electron Asymmetries
G0 Backward Angle : Beam Specifications
Beam Parameter
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)
•
•
Achieved (IN-OUT)/2 “Specs”
< 0.3 x 10
-6
-6
10
Beam parameters specifications were set
meas
to assure:APfalse  5%.Astat
i
Helicity correlated beam properties
All Møller measurements during run)
P=85.78 +/- 0.07 (stat) +/-1.38 (sys) %
false asymmetry
Correction : linear regression
Acor  Ameas - 
i
1 Y
Pi
2Y Pi
16
LD2 687 Field Scan (Octant 1)
Random subtracted Electron Yield vs
SMS Current (2 sample cells)
•Ramped SMS from 1900A to
4900A
•Cell by cell fits made using a
Gaussian (blue) for low momentum
“background” and 2 Gaussians (with
shared width) (red) for the elastic
peak. A constant (lt. green) is also
added to the fit to remove any field
independent rate.
Cell by Cell dilutions extracted as:
f e @ 3500A 
YBackground  YConstant
YBackground  YConstant  YElastic
@ 3500 A
17
Cerenkov Efficiencies
• Electron detection efficiency
• Determined using three different
techniques
• Does not change asymmetry
Four Cerenkov
Detectors
CED/FPD
Coincidence
electron
pion
18
Measured Cerenkov Efficiencies
19
EM Radiative Effects
•Net effect is to reduce the energy
of the scattered electron so elastic
peak now has a low energy tail due to
events which have “radiated” out of
the peak.
•Follow process of Tsai [SLAC=PUB-848] 1971.
•Compute asymmetry [
] based on the kinematics
at the reaction vertex after the radiative emission.
•This is compared to Born asymmetry calculation
[
] with
20
LH2 687 RC Yield Simulations
Without RC Effects
Without
RC Effects
With RC
Effects
21
Expected G0 Results
22
G0: N → 
Measurement: Parity-violating asymmetry
of electrons scattered inelastically
ANΔ gives direct access to GANΔ
Directly measure the axial (intrinsic spin) response
during N →Δ+ transition
First measurement in neutral current process
GF Q 2
Ainel  
(1)  (3)  (3) 

4  2
(1)  2(1 2sin 2 W )  1 (Standard Model)
(2)  non - resonant contribution (small)
(3)  2(1 4 sin 2 W )F(Q 2 ,s)  (N   resonance)
IN
Asymmetry (ppm)
Asymmetry (ppm) vs Octant (LH2 @ 687MeV)
OUT
BLINDED
Data: Inelastic
electrons Scattered
from both LH2 and LD2,
each at two energies
(362MeV & 687MeV)
Octant
Raw Asymmetry (averaged over inelastic region)
23
Transverse Polarization 2-Exchange
An 
M  Im M 
M
2
•When a transversely polarized electron scatters
from a proton, the scattering rate has an
azimuthal dependence arising from two-photon
exchange contributions
•This beam normal single spin asymmetry is of the
same order of magnitude as the PV asymmetry; it
can introduce a background asymmetry if the
beam polarization has a transverse component
24
G0 362 MeV LH2 Transverse
Asymmetry
• BLINDED ---no corrections for helicity
correlated beam parameters, deadtime, …
Octant
25
Parity Violating Photoproduction
of - on the Delta Resonance
• PV asymmetry for pion photoproduction A  may
be as large as 5 ppm (based on hyperon model)
with several ppm statistical uncertainty
• Can access this from inclusive -asymmetries at
kinematics. (Zhu et al, Phys. Rev. Lett.)
• Electroweak radiative corrections generate a nonzero asymmetry at Q2 = 0. (Siegert’s theorem)
27
Pion Yield Measurement
Analysis well underway
LD2, 687 MeV
CED
Rate corrections :
• fr ~15% (2/3 deadtime,
1/3 random coincidences)
• Longitudinal A is small
Pion Yields
Hz/uA
FPD
28
Summary
• G0 Forward Angle and G0 Backward Angle
Measurement allows model independent
determination of
GMS
GES
GAe
• Analysis underway; good progress
• Above specification beam and well-understood
detector performance
• Other Backward Angle Physics Topics
Analysis well
underway
29
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