1. Proton Charge Form Factor Measurement E. Cisbani INFN Rome – Sanità Group and Italian National Institute of Health 113/Oct/2011E. Cisbani / Proton FF

2. Proton Form Factors at high Q 2 As seen from G. Cates talk, Form Factors measurement at high Q 2 range are of paramount importance Approved Hall A experiment: E12-07-109 or GEp(5) : Large Acceptance Proton Form Factor Ratio Measurement at High Q 2 Using Recoil Polarization Method 2 Our 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 13/Oct/2011E. Cisbani / Proton FF

3. Elastic Electron Scattering In 1  approx., form factors of elastic electron scattering are a function of Q 2 and the elastic cross section is: At large Q 2 : –  is large and G M 2 terms dominate in cross section – Extraction of G E /G M from Rosenbluth separation becomes unreliable due to 2  exchange Need to exploit interference between G E and G M by means of double polarization methods: ● Polarized beam and target ● Polarization transfer from polarized beam to scattered proton Relative FOM at Q 2 up to 15 GeV2 is about 1 order of magnitude larger for the polarization transfer method (due to smaller achievable luminosity of a polarized target) 313/Oct/2011E. Cisbani / Proton FF  Use polarization transfer technique

4. Generalized Configuration 13/Oct/2011E. Cisbani / Proton FF4

5. Polarization Transfer 5 Require: Measurement of the recoil proton polarization  Determine the form factor ratio and relative sign of G E to G M  Intrinsic small systematic errors: Measuring ratio P t /P l No cross section measurement needed Fixed energy and angle Polarized Electron Beam Unpolarized proton target Scattered Electron Scattered proton PlPl PtPt Transverse component of scattered proton spin Longitudinal component of scattered proton spin 13/Oct/2011E. Cisbani / Proton FF

6. Proton Polarimeter (PP) Use azimuthal asymmetry of the proton scattering off matter induced by spin-orbit coupling 6 Number of scattered protons: Require: Dipole magnet to precess P l at target to P y pp Polarimeter only measures components of proton spin that are transverse to the proton’s momentum direction Track in Track out Track in Track out P y pp P x pp N=number of scattered proton, P e beam polarization where  refers to electron beam helicity 13/Oct/2011E. Cisbani / Proton FF Maximize P e A (a.u.)

7. From measured 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): 7 Proton spin precession in dispersive + non dispersive plane  p  Q 2 is big  good accuracy of  is needed Consolidated experience with GEp(1), GEp(2) and GEp(3) Note: the analyzing power cancel out in ratio, but is important in overall statistics (as well as beam polarization) Proton track deflection 13/Oct/2011E. Cisbani / Proton FF

8. Polarimeter Analyzing power For Q 2 = 12 GeV 2, p  7.3 GeV/c → 1/p  0.14 c/GeV  A y  0.08 Rather well known; A Y analyzing power decreases at larger p (or Q 2 ) A y  1/p  1/Q 2 813/Oct/2011E. Cisbani / Proton FF

9. General Requirements Select elastic events with multiple kinematic correlation cuts – to suppress the large inelastic background High Q 2 – our goal Operation at high luminosity Provide large acceptance Provide efficient polarimeter 9 to maximize statistics and achieve adequate accuracy 13/Oct/2011E. Cisbani / Proton FF

10. Experiment’s Figure of Merit For polarization transfer experiment with recoil polarization measurement: 10  pp = Proton polarimeter efficiency P e = Beam polarization ; Using  Q 2 /Q 2 =10% as baseline (due to fast fall of statistics with Q 2 ) Maximize Luminosity (L) and polarimeter efficiency (  pp ) Match electron and hadron acceptances 13/Oct/2011E. Cisbani / Proton FF

11. 11 E (GeV) Q 2 (GeV 2 )  E (deg) P e (GeV)  p (deg) P p (GeV) Days  G E /G M 6.65.025.33.9429.03.4810.023 8.88.025.94.5422.85.12100.032 11.012.028.24.6017.47.27300.074 Kinematics and projected data Assumed: I beam = 75 uA Beam Polarization = 85% Target Length = 40 cm Proton Polar. Efficiency = 50% Acceptance:  e = 130 msr (largest Q 2 )  E e > 4.0 GeV  E p > 3.5 GeV Last point is the most demanding 13/Oct/2011E. Cisbani / Proton FF

12. Polarimeter Requirements / Efficiency 12 Double-polarimeter provides ~50% efficiency gain relative to single- polarimeter of equivalent thickness Require: Double polarimeter  pp (deg) Number of scattered protons 13/Oct/2011E. Cisbani / Proton FF

13. Polarimeter Requirements / Acceptance 13 Figure of Merit, FOM pp =  pp ·A y 2 FOM  pp (deg) Peaks at 4 deg Shape of FOM versus p*sin  pp is found to be universal. Requirement: Polarimeter must cover  pp up to 10 deg 13/Oct/2011E. Cisbani / Proton FF

14. Maximal exploitation of two-body kinematic correlations Elastic process selection /  0 Background Suppression 14 Dominant background expected from  0 photo-production (as in previous GEP experiments) (eH,  0  p) For E miss <0.35 GeV, remaining background:  10% (Background is going ~ quadratically respect to angular resolution) Red:  0 photoproduction Black: Elastics Blue: Sum Proton arm: - momentum resolution: 1 % - angular resolution: 1 mrad - vertex reconstruction: 5 mm Energy difference between electron calorimeter and the one expected from hadron arm (in elastic kinematics) 13/Oct/2011E. Cisbani / Proton FF

15. High Luminosity, impact on Trigger / DAQ Must efficienty select electron elastic scattering by angular correlation First level (L1) from electron arm – Energy information (with cuts to reduce inelastic) – Rate (from SLAC high energy data and RCS experiments): Hadron Arm: – Energy information (with cuts to reduce inelastic) – Rate: 1.5 MHz Second level (L2) from two-arm coincidence: – in 30 ns gate: 9 kHz – AND geometrical correlation: 2 kHz Ethr/Emax %50758590 Rate [kHz]14002036038 1513/Oct/2011E. Cisbani / Proton FF Refer to A. Camson

16. 16 Electron spectrometer requirements in Proton Charge Form Factor Measurement Electron-nucleon luminosity 10 39 Hz/cm 2 Calorimeter rate*200 kHz Angular acceptance150 msr Momentum range4-5 GeV Energy resolution10% Central angle (range)25-30 degrees Angular resolution1 mrad Time resolution2 ns Proton arm requirements in Proton Charge Form Factor Measurement Electron-nucleon luminosity 10 39 Hz/cm 2 Front tracker rate500 kHz/cm 2 Calorimeter rate**1.5 MHz Angular acceptance40 msr Momentum range3-8 GeV Momentum resolution1% Central angle (range)17-30 degrees Angular resolution1 mrad Vertex reconstruction5 mm Time resolution1 ns Proton spin rotation90 +/- 30 degrees Accuracy of spin rotation In non-dispersive plane 0.1 mrad Proton polarimeteranalyzer 50 cm x 2 Polarimeter acceptance10 degrees * for threshold 0.75 E electron elastic ** for threshold 0.5 E proton elastic 13/Oct/2011E. Cisbani / Proton FF Requirements for Instrumentation in G E p /G M p

17. Backup slides 1713/Oct/2011E. Cisbani / Proton FF

18. Elastic scattering of longitudinally polarized electrons on proton Physics Process Investigated Cross section up to 2  exchange approximation: 18 Rosenbluth at 1  approx. Not negligible at high Q2 Rosenbluth separation does not provide “simple” relation on form factors; Its interpretation not fully understood Systematics can be large Varying angle and beam energy, keeping Q2 constant – that is vary epsilon, tau is constant 13/Oct/2011E. Cisbani / Proton FF

19. Beam and Target Beam – Highest energy (11 GeV) to measure up to the largest Q 2 (3 energy values, one for each Q 2 point) – Intensity as large as possible (75 uA) to maximize luminosity – polarization as large as possible (85% expected from JLab beam) Target – Hydrogen, as thick as possible (keep liquid) to maximize luminosity (compatible with spectrometers acceptances and resolution) 1913/Oct/2011E. Cisbani / Proton FF

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23. Systematic from  0 background 23 f is ratio of elastic to total events Q 2 = 12 13/Oct/2011E. Cisbani / Proton FF

24. Proton Polarimeter (PP) Use azimuthal asymmetry of the proton scattering off matter induced by spin-orbit coupling 24 Number of scattered protons: Extract P pp x /P pp y from trigonometric analysis Require Dipole magnet to precess P l at target to P y pp Polarimeter only measures components of proton spin that are transverse to the proton’s momentum direction Track in Track out Track in Track out P y pp P x pp N=number of scattered proton, P e beam polarization where  refers to electron beam helicity 13/Oct/2011E. Cisbani / Proton FF Maximize P e

25. Polarimeter Requirements / Efficiency 25 Double-polarimeter provides ~50% efficiency gain relative to single- polarimeter of equivalent thickness Require: Double polarimeter Tracker has to identify multiple tracks in small angular range Q 2 = 2.5 GeV 2 p*sin  fpp AyAy (GeV)  pp (deg) Number of scattered protons Comparison of A y at Q 2 = 2.5 for: Single outgoing track 72% of all tracks Multiple outgoing tracks 28% of all tracks Similar fractions at higher Q 2 13/Oct/2011E. Cisbani / Proton FF

26. Polarimeter Requirements / Acceptance 26 Figure of Merit, FOM = N*A y 2 where N = number of scattered protons FOM  pp (deg) Peaks at 4 deg  pp (deg) Integral of FOM FOM saturates at 10 deg Shape of FOM versus p*sin  pp is found to be universal. Require: Maximum  pp = 16 and 7.5 deg for Q 2 = 5 and 12 13/Oct/2011E. Cisbani / Proton FF