E12-09-019 Precision Measurement of the Neutron Magnetic Form Factor up to Q 2 =13.5 (GeV/c) 2 by...

1

E12-09-019

Precision Measurement of the Neutron Magnetic Form Factor up to

Q2=13.5 (GeV/c)2 by the Ratio Method

B. Quinn, J. Annand, R. Gilman, B. Wojtsekhowski Hall A collab

Approved by PAC 34 (except Q2=16 and 18 (GeV/c)2 points)

PAC 35 allocated 25 of requested 31 days.

B.Quinn Carnegie Mellon Univ.Hall A collab. mtg. Dec. 15/11

B. Quinn Carnegie Mellon Univ. 2

Introduction

2 2MottE M

dG G

d

Elastic scattering from spin ½ target (one photon exchange approx.)

/2)(sin21

1

2

NME

211 2(1 )tan ( / 2)

22 4/ NMQwhere:

2 2( ) and ( ) are Sachs Electric and Magnetic form factors.Nucleon: E MG Q G Q

• Among the simplest, most fundamental of hadronic observables• Helps provide definitive test of nucleon structure theory(With proton form factor, gives access to isovector form factor, which is ‘easy’ case for lQCD.)• Sets sum rule for GPD measurements/predictions


TechniqueRatio Method:Measure quasi-elastic scattering from deuteron tagged by coincident nucleon: d(e,e’p) and d(e,e’n)

1

2 2Mott

( , ')

nEn

MG

d

d

G

e ep

Many systematic effects (experimental and theory) cancel in ratio.

nucl.

corr.

( , ' ) ( , ')

( , ' ) ( , ')

''

d d

d d

d d

d d

d e e e e

d e e e e

n

p p

nR

Expect very small correction for Electric because small form factor and large kinematic weighting of Magnetic

neutron

Electric

2

Mott/

( , ')

d

d

Mn

e ep

GR


HCal replaces BIG-Hand for nucleon detection

BIG Bite instrumented with GEMS for higher luminosity


SystematicsRatio Method is insensitive to:•Target thickness•Target density•Beam current•Beam structure•Live time•(electron) trigger efficiency•Electron track reconstruction•Electron acceptance

Important to understand:•Neutron efficiency/proton efficiency HCal almost ideal•Neutron acceptance/proton acceptance

( , ' )

( , ' )

''

d

d

d

d

nd e e

d e pe

R


Apparatus BigBite Spectrometer

Instrumented with: -GEM planes from SBS-Electromagnetic Cal.-Gas Cerenkov-timing planesAllows high luminosity

38 22.8 10 / A /s/cm

(44 A on d) L


Apparatus (2)Hadron Calorimeter (HCal) to be built for Super BigBite Spectrometer (SBS) provides ideal replacement for BigHAND stack• Iron/scintillator sampling calorimeter• 11X22 identical modules• High energy-deposition

- High threshold- High Luminosity

• Excellent spatial resolution

- Tight cut on nucleondirection (wrt q-vector)

• High efficiency for n and p

- Nearly equal (cancelin ratio)

Iron

Scintillator


Apparatus (3)

Results from Geant4 Simulation(matches published results forCOMPASS calorimeter).Assume identical modules, 3X3 clustersize for hits With fast scint./WLS

(5X5 cluster)


Apparatus (4)


2 23.5 (GeV/c)Q 2 26.0 (GeV/c)Q

HRS used for neutron calibration via ( , )p n

11

SimulationQuasi-elastic• On shell spectator(Struck nucleon off shell)• Boost to struck nucleon rest frame• Isotropic distribution

-Weighted by cross section (using Bodek parameterization)• Boost back to lab• Fold in resolution, make (weighted) increment of spectra

Inelastic• GENEV physics Monte Carlo (Genoa/CLAS)• On-shell initial nucleons (with Fermi momentum)• Boost to struck nucleon rest frame• Generate GENEV event (with boosted beam energy)• Boost back to lab• Fold in resolution, increment (un-weighted) spectra

Inelastic normalized empirically to quasi-elastic


Hours requested 739Approved 600

Fixed overhead 214

Fraction of running time 74%


Projected results


•Introduction

•Technique

•Systematics

•Apparatus

•Simulation/Projected results

E12-09-019 Precision Measurement of the Neutron Magnetic Form Factor up to Q 2 =13.5 (GeV/c) 2 by...

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