Electromagnetic Form Factors John Arrington Argonne National Lab

Electromagnetic Form Factors John Arrington Argonne National Lab Long Range Plan QCD Town Meeting Piscataway, NJ, 12 Jan 2007

Nucleon Form Factors n Fundamental properties of the nucleon – Connected to charge, magnetization distribution – Crucial testing ground for models of the nucleon internal structure – Necessary input for experiments probing nuclear structure, or trying to understand modification of nucleon structure in nuclear medium n Recent revolution in experiments: last 5 -10 yrs – Dramatically improved precision, Q 2 coverage – New program of parity-violating measurements – Revelation of importance of two-photon exchange n Driving renewed activity on theory side – Models trying to explain all four electromagnetic form factors – Trying to explain data at both low and high Q 2 – Progress in QCD based calculations 2

Status Ten Years Ago (end of 1997) Proton Neutron Range allowed by e -d elastic 3

Unpolarized Elastic e-N Scattering n Nearly all of these measurements used Rosenbluth separation s. R = ds/d. W [e(1+t)/s. Mott] = t. GM 2 + e. GE 2 t = Q 2/4 M 2 Reduced sensitivity to… • GM if Q 2 << 1 • GE if Q 2 >> 1 • GE if GE 2<<GM 2 (e. g. neutron) GE 2 Form factor extraction is very sensitive to angle-dependent corrections in these cases Lack of a free neutron target – correct for nuclear effects (FSI, MEC) and proton contributions t. GM 2 q=180 o q=0 o 4

New techniques: Polarization and A(e, e’N) n Mid ’ 90 s brought measurements using improved techniques – Polarized beams with polarized target or recoil polarimeter – Large, efficient neutron detectors for 2 H(e, e’n) – Improved models for nuclear corrections L/T: t. GM 2 + e. GE 2 Pol: GE/GM BLAST at MIT-Bates Polarized 3 He target Focal plane polarimeter – Jefferson Lab 5

Example: GE /GM from Recoil Polarization Similar expressions for cross section asymmetry from polarized target 6

Progress in the last decade (since 1997) Magenta: underway or approved 7

Results from BLAST (unpublished) 2 H(e, e’n): 1 H(e, e’p): GEn GEp / GMp RY A IN M ELI R P 2 H(e, e’): GMn 8

Insight from New Measurements n New information on proton structure – GE, GM differ for the proton: different charge, magnetization distributions – Connection to GPDs: spin-space-momentum correlations Model-dependent extraction of charge, magnetization distribution of proton: A. Belitsky, X. Ji, F. Yuan, PRD 69: 074014 (2004) G. Miller, PRC 68: 022201 (2003) J. Kelly, Phys. Rev. C 66, 065203 (2002) x=0. 1 x=0. 4 x=0. 7 1 fm 9

Insight from New Measurements n Can test models with data on both proton and neutron form factors – Previously, precise data and large Q 2 range only for GMp, lower precision and limited Q 2 range for GEp, GMn, little data for GEn n Data for all FFs at low Q 2 – GEp, GMn, GEn known to greater precision – discrepancies resolved n Soon, FFs known to 4 -5 Ge. V 2 – GEp changed dramatically, GMp also modified – Complete data set in “quark core” and “pion cloud” region 10

Small Sample of Recent Calculations 11

Pion Form Factor: Fπ n The pion form factor is of fundamental importance to our understanding of hadronic structure n The pion is the lightest QCD system and one of the simplest – “The positronium atom of QCD” – Excellent test case for non-perturbative models of hadronic structure n Test case for study of transition between non-perturbative and perturbative regions of QCD Fπ is experimentally challenging to determine • Above Q 2>0. 3 Ge. V 2, one must employ the 1 H(e, e’π+)n reaction • At small –t < 0. 2 Ge. V 2, the t-channel diagram dominates σL; In the t-pole approximation 12

Projected JLab 12 Ge. V Data A program that can only be performed at Jefferson Lab • Experiments performed in 1997 and 2003 established the validity of the experimental technique and extended measurements to Q 2=2. 45 Ge. V 2 Higher Q 2 data will challenge QCD-based models in the most rigorous manner and provide a real advance in our understanding of light quark systems 12 Ge. V JLab upgrade and proposed forward-angle SHMS spectrometer are essential to the measurement 13

Parity Violating Elastic e-p Scattering n Nucleon charge, mag. distributions determined by quark distributions Experiment SAMPLE HAPPEX G 0 PVA 4 Q 2 0. 1* 0. 04* 0. 5 0. 1 -1 0. 4* 0. 7* 0. 1 0. 2* APV [ppm] 6 ppm 7 2 15 2 6 1 -10 1 5 - Notes 1997 deuterium 4 He * = backward angle Magneta for planned or ongoing measurements 14

Present Status n Recent and near-future measurements: 1997 -2007 – Most of the world’s high-Q 2 data, most of the world’s high-precision data – Demonstrated problems with previous GEp AND GMp data – New program of parity violating elastic scattering n For isovector (proton–neutron) form factors or flavor decomposition, need precise data covering similar Q 2 range, careful understanding of systematics, including correlations between measurements n TPE contributions – Large effect on GEp (up to 100+%), smaller effect on GMp – Corrections can propagate from proton to neutron (as extracted from 2 H) – While direct TPE corrections to parity violation are small, the effect of TPE corrections to the EM FFs changes the expected asymmetry 15

Two-Photon Exchange n Proton form factor measurements – Comparison of precise Rosenbluth and Polarization measurements of GEp/GMp show clear discrepancy at high Q 2 n Two-photon exchange corrections believed to explain the discrepancy P. A. M. Guichon and M. Vanderhaeghen, PRL 91, 142303 (2003) n Compatible with e+/e- ? – Yes: previous data limited to low Q 2 or small scattering angle n Still lack direct evidence of effect on cross section – Beam normal spin asymmetry the only observable in elastic e-p where TPE observed M. K. Jones, et al. , PRL 84, 1398 (2000) O. Gayou, et al. , PRL 88, 092301 (2003) I. A. Qattan, et al. , PRL 94, 142301 (2005) 16

Two-Photon Exchange Measurements n Comparisons of e+-p and e--p scattering [VEPP-III, JLab-Hall B] Evidence (3 s level) for TPE in existing data J. Arrington, PRC 69, 032201(R) (2004) World’s data Novosibirsk JLab – Hall B n e dependence of polarization transfer and unpolarized se-p [JLab-Hall C] – More quantitative measure of the discrepancy – Test against models of TPE at both low and high Q 2 n TPE effects in Born-forbidden observables [JLab-Hall A, Hall C, Mainz] – Target single spin asymmetry, Ay in e-n scattering – Induced polarization, py, in e-p scattering – Vector analyzing power, AN, in e-p scattering 17

Two-Photon Exchange Calculations n Significant progress in theoretical understanding – Hadronic calculations appear sufficient up to 2 -3 Ge. V 2 – GPD-based calculations used at higher Q 2 n Experimental program will quantify TPE for several e-p observables – Precise test of calculations – Tests against different observables n Want calculations well tested for elastic e-p, reliable enough to be used for other reactions Before TPE After TPE (Blunden, et al) 18

TPE Beyond the Elastic Cross Section n Two-photon exchange (TPE) corrections – Direct impact on extraction of form factors – Important direct and indirect consequences on other experiments • Neutron form factor measurements • Strangeness from parity violation • High-precision quasi-elastic experiments • - N scattering measurements P. Blunden, et al, PRC 72, 034612 (2005) • Proton charge radius, hyperfine splitting A. Afanasev, et al. , PRD 72, 013008 (2005) A. Afanasev and C. Carlson, PRL 94, 212301 (2005) J. Arrington and I. Sick, nucl-th/0612079 D. Dutta, et al. , PRC 68, 064603 (2003) J. Arrington, PRC 69, 022201(R) (2004) H. Budd, A. Bodek, and J. Arrington, hep-ex/0308005 P. Blunden and I. Sick, PRC 72, 057601 (2005) S. Brodsky, et al. , PRL 94, 022001 (2005) 19

Summary: Next few years n Data being analyzed – BLAST – JLab: GEn at high Q 2 n Upcoming experiments – GEp/GMp at high Q 2 (zero crossing? ) – TPE corrections • Cross section, polarization, Born-forbidden observables – Parity measurements (HAPPEX, G 0, A 4) n New experiments being planned – Extend GMn to higher Q 2 – Improve GEp/GMp precision at low Q 2 n Global analysis of form factor, TPE measurements – Extract corrected proton, neutron, and strangeness form factors – Precise, complete data set for nucleon form factors to moderate Q 2 – Constraints for GPDs, proton and neutron, extending to high Q 2 20

Extensions with JLab 12 Ge. V Upgrade ~8 Ge. V 2 n BLUE = CDR or PAC 30 approved, GREEN = new ideas under development 21

Electromagnetic Form Factors n Part of the mission of Hadronic physics – 2002 Long Range Plan, Hadronic physics milestone (2010) • Electromagentic form factors up to 3. 5 Ge. V 2 • Parity measurements up to 1 Ge. V 2 – These measurements completed or currently in progress – Driving rapid progress in theory – Pion form factor measurements to challenge QCD-based calculations n Delivered, and still delivering, new insight and surprises – Decrease of GE/GM at high Q 2 • Reexamination and modification of p. QCD predictions • Emphasized effects of relativity, quark angular momentum – Two-photon exchange • Complicated task of making precise extractions • Will be thoroughly tested in next few years – High Q 2 extensions probe quark structure, provide input to GPDs, sensitive to relativity and quark angular momentum – High precision data at lower Q 2, probing “pion cloud” contributions 22

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