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G0 Backward Angle Request: Q 2 = 0.23, 0.48 GeV 2. Main points G0 goal is to measure G E s , G M s and G A e over range of momentum transfers with best possible precision Forward angle measurements complete/published PRL, Sept. 2, 2005; nucl-ex/0506021 - PowerPoint PPT Presentation
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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