a measurement of gen at high momemtum transfer in hall...

A Measurement of GEn at High

Momentum Transfer in Hall A

Robert J. FeuerbachThe College of William and Mary

Spokepersons: Gordon Cates, Nilanga Liyanage, Bogdan Wojtsekhowski

Students: Sergey Abrahamyan,Brandon Craver, Aidan Kelleher,

Ameya Kolarkar, Jonathan Miller, Tim Ngo, Seamus Riordan,

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Elastic EM Form FactorsFor an extended spin-1/2 particle, the general vertex term is:

Elastic cross-section:

Or in terms of the Sachs Form factors:

2

Elastic EM Form FactorsFor an extended spin-1/2 particle, the general vertex term is:

Elastic cross-section:

Or in terms of the Sachs Form factors:

Dominate at large Q2

2

Double Polarization Measurement

3

Elastic EM Form factors: the Neutron

• GMn behavior well matched by

the dipole form up to Q2~4 GeV2

• GEn more sensitive than other

FF to details of the pion-cloud at low Q2

• GEn is not precisely measured

above 1.5GeV2

• Permits disentanglement of F2

4

]2 [GeV2Q

0 2 4

0.00

0.05

0.10

Miller

Miller (q-only)2)/Q2

!/2(Q2 ln" 1

/F2F

Galster fit

Herberg

OstrickMadey

Seimetz

Warren

Becker

Bermuth

nEG

F q1 (t) =

∫ +1

−1dx eqH

q(x, ξ, t)

F q2 (t) =

∫ +1

−1dx eqE

q(x, ξ, t)

Fn1 (t) =

23Fu

1 (t)− 23F d

1 (t)

Fn2 (t) =

23Fu

2 (t)− 23F d

2 (t)

Exclusive QE scattering: 3He(e,e′n)

e′

n

e

Q2 = 1.3, 1.7, 2.5, 3.5 GeV2

5


e′

n

2 veto planes

7 Iron/scintillatorsandwich planes

e

Q2 = 1.3, 1.7, 2.5, 3.5 GeV2

5


e′

n

2 veto planes

7 Iron/scintillatorsandwich planes

e

Beam polarization

84%

Target polarization

~50%

Q2 = 1.3, 1.7, 2.5, 3.5 GeV2

5

Data analysis: BigHand and BigBite

Charged hits Neutral hits

• σBH ~ <400ps timing resolution achieved

• σP/p ~ 0.8% for BigBite

pions

protons

6

hEntries 901

Mean -4.144

RMS 0.7831

/ ndf 2! 6.652 / 5

Constant 9.1± 177.2

Mean 0.02± -4.22

Sigma 0.0140± 0.3825

Time (ns)-8 -7 -6 -5 -4 -3 -2 -1 0

Co

un

ts

0

20

40

60

80

100

120

140

160

180

hEntries 901

Mean -4.144

RMS 0.7831

/ ndf 2! 6.652 / 5

Constant 9.1± 177.2

Mean 0.02± -4.22

Sigma 0.0140± 0.3825

Elastic Time Spectrum Kin #3: Q2=3.5 GeV2

Kin #4: Q2=1.7 GeV2

Recent Progress: Kin3 and revised Kin4

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•Improved BB optics elements by using different coeff.

•broken into 20-X, 6-Y bins at B-midplane

•get σP/p ~ 0.8%, Z-vertex res. < 5mm across 90% of BB’s acceptance

•ToF to BigHand timing calibration for kin3 at σT~380ps, kin4 will have same level

• Target document circulating

•Understanding of p→n contribution to ‘neutron’ yield being developed (30% variation with L now at10% for H2)

• Building understanding of BigHand triggering eff.

•Calculation of FSI needed (theoretical support)

QE Event Selection•Use “W” and missing 3-momentum to select QE events;

(here W assumes scattering from stationary nucleon)

For protons from 3He(e,e′p):

8

Data analysis

Selection of quasi-elastic He(e,e’n) scattering events

Raw helicity asymmetry for different the target or 1/2 wave plate conditions

Contributions to GEn at 1.7 GeV2

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Effective UncertaintyQuantity Value Relative to Gn

E

Raw Asymmetry 0.0439 5.5 %Instr. Asymmetry -0.006 0.1 %Accid. Background 0.002 1.5 %Beam Polarization Pe 0.84 3 %Target Polarization PHe 0.49 4 %Neutron Polarization Pn 0.86·Pe 2 %Dilution factor from N2 0.95 3 %Dilution due to p→ n in processCorrection for A|| in processFSI/nuclear correction factor 0.85 to 1 in processGn

M -0.170 1 %

Expect final systematic uncertainty < 15%

Status of Analysis, June 2007

BigBite -10%

Polarized Target - 20%

1.2 GeV2

Neutron Arm -10%

BigBite -98%


1.7 GeV2

Neutron Arm -90%

BigBite -10%


2.5 GeV2

Neutron Arm -10%

BigBite -50%


3.4 GeV2

Neutron Arm -30%

Four kinematics - indicated the level of analysis for subsystems

Status: analysis 90% completeMonte-Carlo under way Calculation of DilutionCalculation of FSI corrections

Status: analysis in progress

]2 [GeV2Q

0 2 4

0.00

0.05

0.10

Miller

Miller (q-only)

2)/Q2!/2(Q2 ln" 1

/F2F

Galster fit

Herberg

Ostrick

Madey

Seimetz

Warren

Becker

Bermuth

E02-013 (proj)

nEG

Impact

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GEn≥GGalster @ 1.7 GeV2

Summary and Outlook

•We have collected data for the first high-precision measurement of GE

n up to Q2=3.5 GeV2.

•Analysis of 1.7 GeV2 set is nearing completion, and 3.5 GeV2 is underway.

•The same experiment could be done at 4.5 GeV2, and (with “super-BigBite) up to 7.5 GeV2.

•The precision measurement at high Q2 will determine F1 and F2, and the related GPD’s.

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Additional Details

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QE Event selection: Neutrons

Neutrons

AccidentalBackground

A significant fraction of “neutron” background not from accidental coincidences, but are protons.

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Pperp (GeV/c)0 0.1 0.2 0.3 0.4 0.50

5000

10000

15000He(e,e’n)3

QE Event selection: Neutrons

Accidental Background

Quasi-elastic defined as: 0.8<W<1.15 GeV|Ppar-q|<250 MeV/cPperp<150 MeV/c

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Observed Asymmetry at 1.7 GeV2

Beam HWP/Target Polarization Setting2 4 6 8 10

Obs

erve

d As

ymm

etry

-0.1

-0.05

0

0.05

0.1He3Observed Asymmetry for Quasi-elastic Neutrons from

Beam HWP/Target Polarization Setting2 4 6 8 10

Obs

erve

d As

ymm

etry

-0.1

-0.05

0

0.05

0.1 Observed asymmetry is 0.0439 ± 0.0024

(5.5% relative statistical uncertainty)

Requires:• proton→neutron conv. • finite acceptance corr.• dilution factors• polarization factors

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a measurement of gen at high momemtum transfer in hall...

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