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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

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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.

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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

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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?

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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

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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)

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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)

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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

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G0 Backangle

Superconducting Magnet (SMS)

Detectors:Ferris Wheel

(FPDs)Detectors:

Mini-Ferris wheel(CEDs+Cerenkov)

Target Service Module

G0 Beam Monitoring

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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

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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

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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 %

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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

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LH2 362

LD2 362

IHWP IN OUTElastic Electron AsymmetriesP

RE

LIM

INA

RY

RA

W B

LIN

DE

D

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LH2 687

PR

EL

IMIN

AR

Y R

AW

BL

IND

ED

LD2 687

Elastic Electron Asymmetries

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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) %

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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:

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Cerenkov Efficiencies

• Electron detection efficiency• Determined using three different techniques• Does not change asymmetry

Four Cerenkov

Detectors

CED/FPD

Coincidence

electron

pion

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Measured Cerenkov Efficiencies

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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.

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LH2 687 RC Yield Simulations

With RC Effects

Without RC Effects

Without RC Effects

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Expected G0 Results

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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)

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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

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G0 362 MeV LH2 Transverse Asymmetry

• BLINDED ---no corrections for helicity correlated beam parameters, deadtime, …

Octant

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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

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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

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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

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