Overview of Nucleon Form Factor Experiments with 12 GeV at Jefferson Lab

Transcription

1 Overview of Nucleon Form Factor Experiments with 12 GeV at Jefferson Lab E. Cisbani INFN Rome Sanità Group and Italian National Institute of Health Outlook Overview of Form Factors Experimental Status JLab and the future FF experiments Details on p and n G E /G M at large Q 2 Expected results 1/Oct/2013 E. Cisbani / Nucleon JLab12 Note: Space-like, electromagnetic Form Factors only in this talk 1

2 Form Factors: discovery and formalism R.W. McAllister, R. Hofstadter Phys. Rev. 102 (1956) 851 First measurement of the proton electromagnetic radius : RMS E/M radius of =(0.74 ± 0.24) cm Nucleon electromagnetic current operator has two unknown functions (Dirac and Pauli FFs) that describe the internal structure of the nucleon (one photon exchange): In terms of Sachs FFs: Elastic Cross section (Rosenbluth): Measurements of proton Form Factors at higher Q 2 driven the discovery of the DIS and the parton model of the nucleon in the 60 1/Oct/2013 E. Cisbani / Nucleon JLab12 2

3 FF «traditional» extraction method Rosenbluth separation (the traditional method for the first ~30 years); in Born approximation (Za<<1) Measure cross sections for different e (different scattering angles) and make linear fits Can determine both absolute values of G E and G M t kinematically suppress G E at high Q 2 and G M at low Q 2 1/Oct/2013 E. Cisbani / Nucleon JLab12 3

4 Q 2 0 G p n E 1 0 Form Factor Dipole Scaling «standard» dipole approx. M m p m n Exp. Nucleon Spatial distribution Large uncertainties at large Q 2 for G E p, at small Q 2 for G M p 1/Oct/2013 E. Cisbani / Nucleon JLab12 4

5 FF at small Q 2 : the nucleon E,M radius Nucleon size (radius) can be derived from Form Factors at low Q 2 (similar for the magnetic radius) 2010 (Pohl et al.): Large disagreement between the proton charge radius from ep scattering/spectroscopy and mp Lamb shift FS 2 1/10 p n Downie, Gasparian <r 2 > E [fm] /Oct/2013 E. Cisbani / Nucleon JLab12 5

6 The power of spin Akhiezer et al. Sov. Phys. JETP 6, 588 (1958), Sov. PHys. Dokl. 14 (1968),... Form factor accuracy can be improved by measuring interference term G E G M by means of beam helicity asymmetry with polarized target or recoil polarimetry About 30 years to get the needed technologies: Polarized e beam (high current 100 ma and polarization>70%) and beam polarimetry (~3%) Highly polarized targets Efficient recoil polarimeters (resonably high analyzing power) 1/Oct/2013 E. Cisbani / Nucleon JLab12 6

7 many systematics (theory and exp.) cancel in ratio FF «modern» extraction methods Recoil polarization: P t, P l = trans. and long. polarization of the recoil proton Beam-Target polarization asymmetry: Ratio method: d(e,e n)/d(e,e p), nuclear corrections needed for d (but tend to cancel) 1/Oct/2013 E. Cisbani / Nucleon JLab12 7

8 Proton G E /G M an «unexpected» discrepancy d d t e 2 2 G Ep G Mp Rosenbluth Separation: assume single photon approximation Prior to JLab/2000, expectations were that proton G E /G M fairly constant with Q 2 G m G Ep Mp Pt m P l ( Ebeam e e E 2M p ) tan 2 Polarization transfer from the incident electron to the scattered proton At JLab, new class of experiments show proton G E /G M decreasing linearly with Q 2 1/Oct/2013 E. Cisbani / Nucleon JLab12 8

9 Two Photon Exchange favorite candidate Rosenbluth at 1g approx. Rosenbluth separation does not provide simple relation on form factors; Its interpretation not fully understood Systematics can be large Considerable literature P.G. Blunder et al. Phys. Rev. C72 (2005) 1/Oct/2013 E. Cisbani / Nucleon JLab12 9

10 Two Photon Exchange - experiments Three e asymmetry experiments to search for 2g effect Novosibirsk (E= GeV) JLab (E=0.5 4 GeV) data analysis DESY (E=2 GeV) data analysis started Run I (2009) / E=1.6 GeV Run II ( ) / E=1 GeV A. Gramolin Trento/2013 1/Oct/2013 E. Cisbani / Nucleon JLab12 10

11 Proton G E /G M - Theoretical models Many theoretical models VMD (Iachello, Lomon, Bijker), generally good description of all FF Relativistic CQM (Miller, Gross,...) spin dependent quark density Lattice QCD, start to give prediction Dyson-Schwinger, dressed quarks, diquark correlation,... pqcd-based: scaling behavior at large Q 2 (next slide) From C.F. Perdrisat / Trento 2013 GPD-based: direct connection to quark OAM, FF s constraint GPD s Most of them agree with current data but diverges at higher, unexplored, Q 2 1/Oct/2013 E. Cisbani / Nucleon JLab12 11

12 Proton F 2 /F 1 scaling pqcd prediction Quark Helicity conservation Counting rules: 1/Q 2 for gluon line 1/Q 2 for helicity flip Photon absorbed by one «collinear» quark which interacts with the other two quarks by 2xgluon exchange > F 1 ~ 1/Q 4, F 2 ~ 1/Q 6 NS 5 2/10 Fanelli - RCS Modified pqcd: Belitsky et al. & other models include quark orbital angular momentum and gluon pololarization effects to explain F 2 /F 1 at large Q 2 1/Oct/2013 E. Cisbani / Nucleon JLab12 12

13 Neutron G E /G M All double polarization Rosenbluth separation affected by large nuclear structure correlations. Importance to go to high Q 2 Different models reproduce data, but at large Q 2 models predictions diverge pqcd log scaling too large for neutron Relativistic CQM works better but tends to overestimate 1/Oct/2013 E. Cisbani / Nucleon JLab12 13

14 FF Flavor decomposition Enough FF data to separate the different flavor u and d contributions (assume negligible s-quark) up to Q 2 ~ 3.5 GeV NS 7 3/10 Wojtsekhowski Different Q 2 scaling of the two flavors F1 and F2 constant! Diquark evidence? 1/Oct/2013 E. Cisbani / Nucleon JLab12 14

15 Plenary 1/10 Guidal - GPD Form Factors and GPDs GPDs provide a framework that links FF and DIS parton distributions FF constraint 2 of the 4 GPDs through sum rules: Strong hints for the estimation of the quark OAM (Ji decomposition of nucleon spin): 1/Oct/2013 E. Cisbani / Nucleon JLab12 15

16 Summary - why and where are FF interesting Fundamental properties of the nucleon Many fundamental models able to calculate them Large Q 2 : Distinguish important models for GE/GM Scaling behaviour (flavor form factors components) Constrain GPD at high x (valence quark dominate) Smaller effective mass of the quarks Toward pqcd dominated regime Very Low Q 2 : Pion cluod effects Precise nucleon size estimation 1/Oct/2013 E. Cisbani / Nucleon JLab12 16

17 6 GeV CEBAF (< 2013) Max Current: 200 ma Max Energy: GeV Long. Polarization: 75-85% Jefferson Lab - CEBAF after 2013 add Hall D (and beam line) Upgrade magnets and power supplies CHL-2 Plenary 4/10 Mckeown - JLab IFAE 2012 / Ferrara 12 GeV CEBAF (>2013) Max Current: 90 ma Max Energy Hall A,B,C: 10.9 GeV Max Energy Hall D: 12 GeV Long. Polarization: 75-85% E. Cisbani / Experimental Physics 17

18 JLab Experimental Halls from 2014 Hall A Hall B/CLAS12 Hall C Hall D/GLUEX + 1 large angular and momentum, high lumi spectrometer with hadron ID + Solid detector + Möller detector New beam line New ~2p toroid detector with extended hadron ID + lumi cm -2 s -1 + forward tagger for quasi-real photons + targets with large thickness + long/trans polarized H/D target + super high momentum spectrometer + dedicated equipment Excellent hermetic coverage, Solenoid field High multiplicity reconstruction + lumi cm -2 s linearly polarized <12 GeV real photons/s hallaweb.jlab.org E. Cisbani / Experimental Physics 18 IFAE 2012 / Ferrara

19 Nucleon Form Factor JLab/12GeV Hall Experiment Title Q 2 A E Precision Measurement of the Proton Elastic Cross Section at High Q 2 up to 18 A E Large Acceptance Proton Form Factor Ratio Measurements at 13 and 15 (GeV/c) 2 using Recoil Polarization Method A E Precision Measurement of the Neutron Magnetic Form Factor up to Q 2 = 18.0 (GeV/c) 2 by the Ratio Method up to 12 up to 14 A E Measurement of the Neutron Electromagnetic Form Factor Ratio GnE /GnM at High Q 2 up to 10 B E Measurement of the Neutron Magnetic Form Factor at High Q 2 Using the Ratio Method on Deuterium C E The Neutron Electric Form Factor at Q 2 up to 7 (GeV/c) 2 from the Reaction 2 H(e,e n) 1 H via Recoil Polarimetry up to 14 up to 7 B E High precision measurement of the proton charge radius FS 2 1/10 Gasparian 1/Oct/2013 E. Cisbani / Nucleon JLab12 19

20 Hall A experiment layouts GMn GEp5 H 2 From P. Rossi Trento/2013 1/Oct/2013 E. Cisbani / Nucleon JLab12 20 GEn

21 Large luminosity New SuperBigbite Spectrometer in Hall A Moderate acceptance Forward angles Reconfigurable detectors High photon up to 250 MHz/cm 2 and electron 160 khz/cm 2 background... and more Support event rate 10x higher than with standard small acceptance spectrometer GEM chambers to handle the high rate of the background MPGD August Kobe E. Cisbani et al. GEM JLab Hall A

22 Proton G E /G M at large Q 2 by polarization transfer GEp5 Coordinate Detector Beam: Current= 75 ma, Polarization= 85% long. Energy= 6, 8 and 11 GeV Target: H 2 Liquid Length= 40 cm L = Detectors: P-arm: SBS + Polarimeter E-arm: BigCal + Coordinate GOAL: Extend the measurement of the proton form factor ratio G E /G M to the maximum Q 2 that is possible with 11 GeV beam with constraints: Absolute error < 0.1 Beam time = 60 days 1/Oct/2013 E. Cisbani / Nucleon JLab12 22

23 GEp5: Polarization Transfer Polarized Electron Beam Unpolarized proton target Scattered proton Scattered Electron P t P l Transverse component of scattered proton spin Longitudinal component of scattered proton spin Intrinsic small systematic errors: Measuring ratio P t /P l No cross section measurement needed Fixed energy and angle Require: Measurement of the recoil proton polarization 1/Oct/2013 E. Cisbani / Nucleon JLab12 23

24 GEp5: Proton Polarimeter (PP) Use azimuthal asymmetry of the proton scattering off matter induced by spin-orbit coupling Number of scattered protons: where refers to electron beam helicity A (a.u.) P y pp P x pp Track in Track in Track out Track out Polarimeter only measures components of proton spin that are transverse to the proton s momentum direction N=number of scattered proton, P e beam polarization Require: Dipole magnet to precess P l at target to P y pp 1/Oct/2013 E. Cisbani / Nucleon JLab12 Maximize P e 24

25 GEp5: From polarization ratio to Form Factor ratio Back propagate P pp x, y ratio to the vertex to get P t /P l (under geometric approximation): Proton track deflection Proton spin precession in dispersive + non dispersive plane g p Q 2 is big good accuracy of Df is needed Note: the analyzing power cancel out in ratio, but is important in overall statistics (as well as beam polarization) 1/Oct/2013 Consolidated experience with GEp(1), GEp(2) and GEp(3) E. Cisbani / Nucleon JLab12 25

26 GEp5: Experiment s Figure of Merit For polarization transfer experiment with recoil polarization measurement: e pp = Proton polarimeter efficiency P e = Beam polarization ; Using DQ 2 /Q 2 =10% as baseline (due to fast fall of statistics with Q 2 ) Maximize Luminosity (L) and polarimeter efficiency (e pp ) Match electron and hadron acceptances 1/Oct/2013 E. Cisbani / Nucleon JLab12 26

27 Elastic process selection / p 0 Background Suppression Dominant background expected from p 0 photo-production (as in previous GEP experiments) (eh,p 0 gp) Maximal exploitation of two-body kinematic correlations Proton arm: - momentum resolution: 1 % - angular resolution: 1 mrad - vertex reconstruction: 5 mm Red: p 0 photoproduction Black: Elastics Blue: Sum For E miss <0.35 GeV, remaining background: 10% (Background is going ~ quadratically respect to angular resolution) Energy difference between electron calorimeter and the one expected from hadron arm (in elastic kinematics) 1/Oct/2013 E. Cisbani / Nucleon JLab12 27

28 Neutron G E /G M by double polarization Polarized Beam SBS Magnet used to deflect proton to improve charge identification (+ GEM veto) GEM detector in BigBite to accomodate large luminosity Challenging polarized 3 He target 1/Oct/2013 E. Cisbani / Nucleon JLab12 28

29 Polarized 3 He target development G. Cates/Uva target 1/Oct/2013 E. Cisbani / Nucleon JLab12 29

30 Neutron GE/GM via Recoil polarization in HallC 1/Oct/2013 E. Cisbani / Nucleon JLab12 30

31 JLab FF experiments - expected results and accuracy E : Polarization transfer E /E : Polarization Plenary 3/10 Roberts E /E : Cross section ratio Extended measurements of p/n Form Factors at high (and very low) Q 2 Test different models (including different contributions from the quark OAM) Investigate the transition region (perturbative / non perturbative) Constraint the H and E GPDs... and more 1/Oct/2013 E. Cisbani / Nucleon JLab12 31