1
The G0 Experiment:Parity Violation in e-N Scattering
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
Colleen EllisThe University of Maryland
The G0 Collaboration:
Hall C Meeting 18 January 2008
2
G0 Graduate Students Carissa Capuano : W&M, USA Maud Versteegen: LPSC, France. Alexandre Coppens: Manitoba, CanadaMathew Muether: Illinois, USA Colleen Ellis : Maryland, USA John Schaub : NMSU, USA.Juliette Mammei Virginia Tech. USA.
3
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
4
GE ,M,p QqGE ,M
q 2
3GE ,M
u,p
qu,d ,s
1
3GE ,M
d ,p 1
3GE ,M
s,p
GE ,MZ ,p 2T3
q 4Qq sin2 W GE ,Mq
qu,d ,s
GE ,Ms 1 4sin2 W GE ,M
,p GE ,M,n GE ,M
Z ,p
Assume: Isospin symmetry
GE ,Mu,p GE ,M
d ,p GE ,Mu
GE ,Md ,p GE ,M
u,n GE ,Md
GE ,Ms,p GE ,M
s,n GE ,Ms
Strange Form Factors
Electron scattering involves EM and Weak interactions
Known G0 measures
How does s quark contribute to electromagnetic properties of the nucleon?
5
APVproton
( 5 ,10 )
APVproton
( 110 )
APVdeuterium
( 110 )
GEs (Q2)
GMs (Q2)
GAe (Q2)
A
MM Z
M 2 {AE AM AA
0
}
AE GEGE
Z
AM GM GM
Z
AA (1 4sin2W)'GAeGM
0 (GE )2 (GM
)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
Model Independent Form FactorsPV asymmetries from EM and weak interference terms
Q2
4M 2
1 2(1 )tan2 2
1
' 1 2 1
6
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)
7
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
– Required for complete separation of
and
GEs GM
s 4 2
GFQ2
GEp 2
GMp 2
GEp 1 RV
(0) Aphys ANVS
GEs
GMs
(GeV2)
8
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
• Particle detection and identification :– 16 Focal Plan Detectors – 9 Cryostat Exit Detectors
elastic and inelastic electron separation
– Additional Cerenkov detectors electron and pion separation
Backward Angle Configuration
e- beam
target
CED + Cerenkov
FPD
e- beamline
9
G0 Backangle
Superconducting Magnet (SMS)
Detectors:Ferris Wheel
(FPDs)Detectors:
Mini-Ferris wheel(CEDs+Cerenkov)
Target Service Module
G0 Beam Monitoring
10
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
11
Blinding Factor
Rate Corrections for Electronics --Deadtime and Random Coincidences
Helicity Correlated Beam Corrections
Raw Yields and Blinded Asymmetries
by target and Q2
Raw Yields and Blinded Asymmetries
by target and Q2
Corrections from inelastic electronsBackground from target walls
Pion Asymmetry Contamination
EM Radiative Corrections(via Simulation)
EM Radiative Corrections(via Simulation)
Beam PolarizationCorrection
G0 Backangle Analysis Approach
Aphys
Unblind
Forward Angle
GES GM
S
GES
GMS
GAe
Q2 Determination
12
measure raw yield for each helicity state (+ or -) apply rate corrections (electronic deadtime and random coincidences):
correct for beam correlated effects :
form asymmetry :
correct for background contribution :
correct for beam polarization (P)
EQyxPPP
YY yxii
i i
ccc ,,,,,
Am Ycc
Ycc
Ycc Ycc
belbbelelm ffAfAfA 1
Ael
P ao a1GE
s a2GMs a3GA
e(T 1)
Yc Ym
1 fR
Forming Asymmetry
fR 7%
fR 9 13%
LH2
LD2
Afalse < 4 ppb
Am ~ 10 ppm
fb < 10 %
13
LH2, 687 MeV
LD2, 687 MeV
LH2, 362 MeV
LD2, 362MeV
Electron Yields (Hz/uA)
90 C 120 C
70 C 45 C
Quasi Elastic
Inelastic
Elastic
Inelastic
Elastic
Inelastic
14
LH2 362
LD2 362
IHWP IN OUTElastic Electron AsymmetriesP
RE
LIM
INA
RY
RA
W B
LIN
DE
D
15
LH2 687
PR
EL
IMIN
AR
Y R
AW
BL
IND
ED
LD2 687
Elastic Electron Asymmetries
16
G0 Backward Angle : Beam Specifications
Beam Parameter Achieved (IN-OUT)/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
Acor Ameas -1
2Yi
Y
Pi
Pi
• Beam parameters specifications were set to assure:
• Helicity correlated beam properties
false asymmetry
Correction : linear regression
APi
false 5%.Astatmeas
All Møller measurements during run)
P=85.78 +/- 0.07 (stat) +/-1.38 (sys) %
17
LD2 687 Field Scan (Octant 1)•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.
Random subtracted Electron Yield vs SMS Current (2 sample cells)
AElasticConstantBackground
ConstantBackground
Ae YYY
YYf
3500@
3500@
Cell by Cell dilutions extracted as:
18
Cerenkov Efficiencies
• Electron detection efficiency• Determined using three different techniques• Does not change asymmetry
Four Cerenkov
Detectors
CED/FPD
Coincidence
electron
pion
19
Measured Cerenkov Efficiencies
20
EM Radiative Effects
•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
•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.
21
LH2 687 RC Yield Simulations
With RC Effects
Without RC Effects
Without RC Effects
22
Expected G0 Results
23
G0: N → Measurement: Parity-violating asymmetry of electrons scattered
inelasticallyANΔ gives direct access to GA
NΔ
Directly measure the axial (intrinsic spin) response during N →Δ+ transition
First measurement in neutral current process
Elast
ic R
egio
n: G0
Inel
astic
Reg
ion:
Inel
astic
Reg
ion:
NN
BLINDED
Asymmetry (ppm) vs Octant (LH2 @ 687MeV)
Raw Asymmetry (averaged over inelastic region)
IN
OUT
Octant
Asy
mm
etry
(pp
m)
Ainel GFQ2
4 2(1)
(3) (3)
(1)
2(1 2sin2 W ) 1 (Standard Model)
(2) non- resonant contribution (small)
(3) 2(1 4 sin2 W )F (Q2,s) (N resonance)
Data: Inelastic electrons Scattered from both LH2 and LD2,
each at two energies (362MeV & 687MeV)
24
Transverse Polarization 2-Exchange
•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
2
Im
M
MMAn
25
G0 362 MeV LH2 Transverse Asymmetry
• BLINDED ---no corrections for helicity correlated beam parameters, deadtime, …
Octant
27
Parity Violating Photoproduction of - on the
Delta Resonance• PV asymmetry for pion photoproduction
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 non-zero asymmetry at Q2 = 0. (Siegert’s theorem)
A
28
Pion Yield Measurement
Rate corrections :
• fr ~15% (2/3 deadtime, 1/3 random coincidences)
• Longitudinal A is small
Analysis well underwayFPD
CE
D
LD2, 687 MeV
Pion Yields
Hz/uA
29
Summary• G0 Forward Angle and G0 Backward Angle
Measurement allows model independent determination of
• Analysis underway; good progress • Above specification beam and well-understood
detector performance• Other Backward Angle Physics Topics Analysis
well underway
GMS
GES
GAe
30