G0 Backward Angle Request: Q2 = 0.23, 0.48 GeV2

Main points

• G0 goal is to measure GEs, GM

s and GAe over range of

momentum transfers with best possible precision

– Forward angle measurements complete/published• PRL, Sept. 2, 2005; nucl-ex/0506021

– Requires backward angle H2 and D2 measurements

• Q2 = 0.8 GeV2 run scheduled, now starting Mar. 2006

• Based on forward results choose Q2 = 0.23 GeV2 then Q2 = 0.48 GeV2

D. Beck, UIUCPAC28, Aug. 2005

G0: Forward Angle Results (1)• Measurement over wide range of Q2: 0.12 – 1.0 GeV2

• Measure elastic asymmetries (recoil protons)– asymmetry: 1 – 40 ppm

NVSphysV

pE

pM

pE

F

sM

sE AA

RG

GG

QGGG

)0(

22

2 1

24

GEs GM

s= = 0

• Physics from comparison with ANVS

– “no vector strange” asymmetry, ANVS, calculated with for all Q2

pE

pM

i G

GEQ

,2

pE

pM

i G

GEQ

,2

Where Were We?• From HAPPEX H preprint nucl-ex/0506011

G0: Forward Angle Results (2)

PRL in press (Sept. 2), nucl-ex/0506021, http://www.npl.uiuc.edu/exp/G0/Forward

G0: Forward Angle Results (3)• Summary of conclusions: non-trivial Q2 dependence

• If has simple dipole falloff, rises monotonically to Q2 z 2

– At Q2 = 0, , at low Q2,

• Decrease of around Q2 = 0.2 GeV2 suggests

GMs GM

s

+ GEs GM = 0s + GE

s GM Q2s

+ GEs GM

s

GE < 0s

• Remember s-quark charge is factored out:– contributions to charge and magnetization distributions are

sM

sE G

3

1,G

3

1

= +0.62 0.31GMs

• Q2 = 0.1 GeV2 good agreement among all measurements

World Data @ Q2 = 0.1 GeV2

http://www.npl.uiuc.edu/exp/G0/Forward

= -0.013 0.028 GEs

GMs= +0.62 0.31

0.62 2

Contours

1, 2 68.3, 95.5% CL

Theories1. Leinweber, et al.

PRL 94 (05) 2120012. Lyubovitskij, et al.

PRC 66 (02) 0552043. Lewis, et al.

PRD 67 (03) 0130034. Silva, et al.

PRD 65 (01) 014016

• PVA4 measurement at Q2 = 0.23 GeV2

– consistent probable value for

– supports negative

World Data @ Q2 = 0.23 GeV2

GMs

GEs

http://www.npl.uiuc.edu/exp/G0/Forward

Background Overview• Measure yield and asymmetry

of entire spectrum

• Correct asymmetry according to

backelmeas AfAfA 1

where Ael is the raw elastic asymmetry,

meas

back

Y

Yf

• Actual analysis: f = f(t)– det. 1-14

• fit Yback (poly’l of degree 4), Gaussian for elastic peak

• then fit Aback (poly’l of degree 2), constant Ael

• uncertainties

– statistical contribution: f/(1-f)2 in 2Astat (20% for f = 15%)

– systematic contribution: ~ 0.5 Astat

Proposed Backward Measurements• Measurements at Q2 = 0.23, 0.48 GeV2

– motivated by present data: G0 + Mainz, G0 + HAPPEX, respectively• convincing picture at Q2 = 0.1 GeV2

– same setup as scheduled Q2 = 0.8 GeV2 run• new cryostat exit scintillators (CEDs), Cherenkov detector• regular beam structure (499 MHz)• higher beam current (80 A)

– requires lower beam energies

Target Q2 Energy Rate Asym.

(GeV2) (GeV) (MHz) (ppm)1H 0.23 0.360 3.1 -132H 0.23 0.360 4.7 -181H 0.48 0.585 0.72 -322H 0.48 0.585 1.1 -431H 0.8 0.799 0.19 -542H 0.8 0.799 0.27 -72

scheduled

Cherenkov

Electron incident

CED

FPD

Backward Measurements• Additional detectors

complete – final testing

• Target modifications complete– extension of support

Backward Measurements• Additional detectors

complete – final testing

CherenkovCEDs

CED PMTs

Cherenkov PMTs

Cherenkov PMTs

CED PMTs

CEDs

Backward Angle Detector Rotation Test

Backgrounds• “Direct”

– inelastic electrons, electrons from 0 decay– continuing development of MC– use of wire chamber to make careful separation of yields

• measures angle near focal surface

• “Indirect”– “hall background” - shower from target– main addition – lead insert downstream of target– careful shielding of exit beamline and dump tunnel

Direct Backgrounds• Asymmetries measured for

combinations of CEDs and focal plane detectors (FPDs)

Q2 = 0.23 GeV2

Direct Backgrounds• Asymmetries measured for

combinations of CEDs and focal plane detectors (FPDs)

Q2 = 0.48 GeV2

– contamination from inelastic electrons few % for Q2 = 0.48 GeV2

Direct Backgrounds• Asymmetries measured for

combinations of CEDs and focal plane detectors (FPDs)

Q2 = 0.8 GeV2

– contamination from inelastic electrons few % for Q2 = 0.48 GeV2

– electrons from 0 decay likely to dominate, especially at higher Q2

– measure trajectory angles with wire chamber at low beam current

• understand components of background yields

Direct Background ComponentsQ2 = 0.8 GeV2

Indirect Background

• GEANT code based on that of P. Degtiarenko

• Added detailed G0 geometry

• Careful shielding of dump

• Add lead insert downstream of target

• With this configuration, Q2 = 0.23 GeV2 background ~ same as at 0.8 GeV2

Beam Polarization Measurement• Beam polarization measured with Møller polarimeter

– forward angle: <Pe> = 73.71.0%

– use <Pe> = 75 1.5% for backward angle estimates

• Low energy running requires moving Q1 in Møller spectrometer– previous move by 6 in. successful ( )

Parity Quality Beam• Require ~ x2 looser specs compared to forward angle• Plan to use feedback for position differences

– hope to improve damping in injector• very small damping in forward measurement

– better matching in 1/4 cryo and injector cryomodule• promising solution tried recently (Y. Chao)

GEn

Expected Results

• Assumes single measurement 50 d LH2

– total background uncertainty 2% (stat. unc. 2.8%)

PVA4

G0 Forward

G0 Backward

stat

stat + sys

stat + sys + model

Expected Results

• Assumes two measurements 30 d each: LH2, LD2

– total background uncertainty 3% (stat. unc. 3.3%)

HAPPEX

G0 Forward

G0 Backward

stat

stat + sys

stat + sys + model

Axial Form Factor

• is important component of asymmetry at backward angles

• no information yet about Q2 dependence

GAe

Beam Request• Running periods

• Breakdown of auxiliary measurement time– forward measurement required about 10%– expect same for backward measurement - periodically

measure:• beam polarization• beam energy• charge monitor calibration

– recall 10 d commissioning time for detector, target tuneup, background studies, etc.

Target Q2 Energy Request Astat/A

(GeV2) (GeV) (days) (%)1H 0.23 0.360 50 2.81H/ 2H 0.48 0.585 60 3.3/2.7

Summary• May have glimpse of physics picture from SAMPLE,

forward angle measurements–

– may be negative

• Most interesting physics around Q2 = 0.2 GeV2

– best to make backward angle measurements where there are other data

– Q2 = 0.23 GeV2: G0 forward, PVA4 I– Q2 = 0.48 GeV2: G0 forward, HAPPEX I

• Detectors, target, electronics ready for first run at 0.8 GeV2

GMs= +0.62 0.31

GEs