Future Measurements of the Nucleon Elastic Electromagnetic Form Factors atJefferson Lab

G.P. GilfoyleUniversity of Richmond, Richmond, VA USA

SM - WW H L.

I Nucleon elastic electromagnetic form factors (EEFFs) describe the distribution ofcharge and magnetization in the nucleon.

I They can reveal the internal landscape of the nucleon and nuclei.I They rigorously test QCD in the non-perturbative regime with different models,

constituent quarks, lattice QCD...I Map the transition from the hadronic picture to QCD.

S N B

EEFFs cross section is described with Dirac (F1) and Pauli (F2) form factors or Sachsform factors (GE and GM).

dσdΩ

= σMott

[(F2

1 + κ2τF22)

+ 2τ (F1 + κF2)2 tan2

(θe

2

)]=

σMott

ε(1 + τ)

(εG2

E + τG2M

)where

σMott =α2E′ cos2(θe

2 )

4E3 sin4(θe2 )

GE = F1 − τF2 GM = F1 + F2 ε =

[1 + 2(1 + τ) tan2 θe

2

]−1

and κ is the anomalous magnetic moment, E (E′) is the incoming (outgoing) electronenergy, θ is the scattered electron angle and τ = Q2/4M2.

W AW N?I Gp

M reasonably well known over large Q2 range.I The ratio Gp

E/GpM from recoil polarization measurements

diverged from previous Rosenbluth separations.I Two-photon exchange (TPE).I Effect of radiative corrections.

I The neutron form factor ratio GnE/Gn

M opens up flavordecomposition.

I Neutron magnetic FF GnM still follows dipole.

Eur. Phys. J., 73, 2397 (2013)

PRL 104, 242301 (2010)

PRL 105, 262302 (2010)

PRL, 102, 192001 (2009)

I Theory approaches like Vector Meson Domi-nance and dispersion analyses fit EEFFs, but usemany parameters.

I Constituent Quark Models highlight relativity,but don’t capture all of QCD.

I EEFFs are an early test of lattice QCD becauseisovector form does not have disconnected dia-grams.

P.E.Shanahan et al.

arXiv:1401.5862v2 [hep-lat]CSSM and QCDSF/UKQCDCollaborations

W AW G?

I Flavor DecompositionI With all four EEFFs we can unravel the u and dquark structure.

I Assume charge symmetry, no s quarks and use(Miller et al. Phys. Rep. 194, 1 (1990))

Fu1(2) = 2Fp

1(2)+Fn

1(2) Fd1(2) = 2Fn

1(2)+Fp1(2)

I The u and d quarks have different, unexplainedQ2 dependence - evidence of di-quarks?

I Dyson-Schwinger EquationsI Infinite set of coupled integral equa-tions.

I Inherently relativistic, and nonper-turbative.

I Connected to confinement, dynam-ical chiral symmetry breaking.

I Proton form factor ratio sensitive to the shape ofthe dressed-quark mass function.

PRL 111,101803 (2013)

PRL 106, 252003 (2011).

H AW G T? - U A

Halls A

CEBAF

Racetrack

Hall D

B C

Continuous Electron Beam Ac-celerator Facility (CEBAF)

I Superconducting ElectronAccelerator (338 cavities)

I 100% duty cycle.I Emax = 11 GeV (Halls A, B, and

C) and 12 GeV (Hall D)I∆E/E ≈ 2× 10−4

I Isummed ≈ 90 µA, Pe > 80%.

H AW G T? - N D

Hall A - High Resolution Spec-trometer (HRS) pair, SuperBig-Bite (SBS), neutron detector, andothers. Hall B - CLAS12 high lumi-

nosity, large acceptance spec-trometer with forward andcentral detectors.

Hall C - Super High MomentumSpectrometer paired with exist-ing High Momentum Spectrom-eter.

Hall D - New GlueX detector will search for exotic states,but not for EEFFs.

TM A R

JLab Program Advisory Committee (PAC) has ap-proved six experiments to measure all four EEFFs(some twice) with 224 days of running.

Anticipated Hall Apoints

Experiment E12-07-108 in Hall A (Arrington,Christy, Gilad, Silkovsky, Wojtsekhowski)will make a precise measurement of the epelastic cross section and extract Gp

M.

Experiment E12-07-109 (GEp) in Hall A (Cis-bani et al.) will use the polarization transfermethod on H(~e, e′~p) to measure the form fac-tor ratio Gp

EGp

M= −Pt

Pl

E+E′2M tan

(θe2

).

Two experiments E12-07-104 (Gilfoyle,Hafidi, and Brooks) in Hall B and E12-09-019 (Annand, Gilman, Quinn, Wo-jtsekhowski) in Hall A will use thequasielastic e − n/e − p ratio to extracta precision measurement of Gn

M.

Experiment E12-09-016 in Hall A (Cates,Wojtsekhowski, Riordan) will use thedouble Polarization Asymmetry AV

en from3 ~He(~e, e′n)pp to extract Gn

E/GnM.

Experiment E12-11-009 in Hall C (Arring-ton, Kohl, Sawadsky, Semenov) will use po-larization transfer off 2H(~e, e′~n)p to measureGn

E/GnM complementary to the other Gn

E/GnM

experiment in Hall A.

S C

I The proton cross section measurement (GpM) will run in spring, 2016 and the form

factor ratio (GpE/Gp

M) in 2018.I Remaining elastic form factor measurements will be done in 2018 or later.I Large recent gains in understanding of the EEFFs.I Major changes in our understanding of nucleon structure.I Jefferson Lab will mount a broad campaign on the EEFFs and will significantly

expand our physics reach.I Discovery potential in mapping out nucleon structure and understanding QCD.

University of Richmond Jerry Gilfoyle