Initial State Helicity Correlation in Wide Angle Compton Scattering E Donal Day and Bogdan Wojtsekhowski, co-spokespersons

Transcription

1 Initial State Helicity Correlation in Wide Angle Compton Scattering E5- Donal Day and Bogdan Wojtsekhowski, co-spokespersons Hall C Workshop January 7, 6 Initial State Helicity Correlation in Wide Angle Compton Scattering p./3

2 Outline Introduction Theoretical Motivation Why do we want to do this experiment? Experimental Technique How we will do the experiment? Beam Request What did we request from the PAC? Physics goals of this experiment complement those of E99-4 and benefits from its experience with the RCS experimental technique. Initial State Helicity Correlation in Wide Angle Compton Scattering p./3

3 Real Compton Scattering: Introduction Key element in Program of Hard Exclusive Reactions RCS Elastic Form Factors DVCS DVMP Common issues: interplay between hard and soft processes Onset of asymptotic regime Role of hadron helicity flip Uniqueness Vary both s and t Weighting of quarks, e q independent integral of GPD s, x Initial State Helicity Correlation in Wide Angle Compton Scattering p.3/3

4 Real Compton Scattering: Introduction Compton Scattering off nucleons provides information on the substructure of nucleon in terms of quark and gluon d.o.f. extremely complicated Compton scattering in various kinematical regions low energy dominated by nucleon as a whole deeply virtual CS; low t, large Q handbag diagram involving skewed parton distributions wide angle CS; low Q, large t and s ensures dominance of short distance behaviour What is the reaction mechanism? Initial State Helicity Correlation in Wide Angle Compton Scattering p.4/3

5 What is the reaction mechanism? 3 active quarks hard gluons hard gluon exchange 3-body "form factor" active quark hard gluons handbag -body "form factor" Which, if either, dominates at few GeV? We will be able to distinguish among the competing mechanisms. Initial State Helicity Correlation in Wide Angle Compton Scattering p.5/3

6 Asymptotic (pqcd) Mechanism Brodsky/Lepage Kronfeld, Nizic Vanderhaeghen, Guichon hard gluon exchange Brooks, Dixon,... momentum shared by hard gluon exchange 3 active quarks valence configuration dominates soft physics in distribution amplitudes, Φ(x,x,x 3 ), Φ(y,y,y 3 ) constituent scaling: dσ dt = f(θ CM)/s 6 Must dominate at "sufficiently" high energy(?) Has predictions for polarization observables, K LL = A LL Initial State Helicity Correlation in Wide Angle Compton Scattering p.6/3

7 Constituent Scaling γp γp Approximate scaling dσ dt = f(θ CM)/s 6 Cornell data approximately support scaling but... 9 JLab Cornell Radyushkin(98) n(θ cm ) 8 7 pqcd 6 n=6 two gluon exchange Brooks&Dixon Phys.Rev.D6:4, θ cm [deg] Asymptotically we expect pqcd to be dominant, but when? Initial State Helicity Correlation in Wide Angle Compton Scattering p.7/3

8 Handbag Mechanism for (s, t, u) M Radyushkin Diehl, Feldman, Jakob, Kroll handbag One active parton Momentum shared by soft overlap Feynman mechanism struck quark nearly real (x ) (co-linear with proton) Form factor like expression dσ dt = dσ dt KN f(t) Straightforward predictions for polarization observables Initial State Helicity Correlation in Wide Angle Compton Scattering p.8/3

9 Handbag Approach and GPD s Radyushkin Diehl, Feldman, Jakob, Kroll Compton form factors: R V (t) = a e a dx x Ha (x,, t) R A (t) = a e a dx x sign(x) Ĥa (x,, t) handbag R T (t) = a e a dx x Ea (x,, t) Factorize into hard scattering on single quark and moments of GPD s at skewness ξ = hard scattering: Klein-Nishina from nearly on-shell parton Soft physics: Compton form factors R V (t), R A (t) and R T (t) relating emission and reabsorption of struck quark in the proton Elastic form factors: F (t) = e a dx H a (x,, t) a G A (t) = a F (t) = a dx sign(x) Ĥ a (x,, t) e a dx E a (x,, t) Initial State Helicity Correlation in Wide Angle Compton Scattering p.9/3

10 Handbag Predictions for WACS Cross section from E99-4 ] dσ/dt [nb/gev dσ dt = dσ KN dt - R V [f V R V (t) + ( f V )RA (t) ] Kroll et al. -t [GeV ] Cornell JLab s = 6.9 GeV s = 8.9 GeV s =. GeV s= 6.9 GeV s= 8.9 GeV s=. GeV Polarization Observables A LL dσ dt = [ dσ(++) dt dσ(+ ) dt A LL = K LL K KN R A (t) LL R V (t) Related to u at moderate to u high x. K_LL Polarization Transfer K_LL preliminary E99 4 KN KS COZ CZ hand bag JML ].6 AS t [GeV ] proton angle in cm system Initial State Helicity Correlation in Wide Angle Compton Scattering p./3

11 Handbag in CQM..8.6 K LL from E99 4 K LL (Miller) handbag Miller in IA approximation of handbag. Massive quark Model wave function same as for E/M form factors Orbital angular momentum and nonconservation of proton helicity Good agreement with cross section data But A LL K LL, backward angles A LL K LL K LL and A LL A LL (Miller) Proposed A LL measurements K LL and A LL (Kroll) Θ cm [deg] Initial State Helicity Correlation in Wide Angle Compton Scattering p./3

12 Physics Goals Measure A LL (never been measured) at two scattering angles: θ CMS γ = 7 corresponding to t =.4 (GeV/c) θ CMS γ = 4 corresponding to t = 6.4 (GeV/c) Provide an experimental test of the RCS reaction mechanism: does the photon interact with a constituent or a current quark? Provide an additional test for hadron helicity conservation and pqcd Initial State Helicity Correlation in Wide Angle Compton Scattering p./3

13 Experimental Layout Kinematic Range E γ = 4.3 GeV, s = 9 GeV θ cms = 7,4 mixed e γ beam Require HMS trigger only e p/rcs discrimination needed control of backgrounds good angular resolution Polarized target Radiator Beam Target Proton High Momentum Spectrometer Beam Dump photon intensity Photon acceptance electron intensity Photon Electron BigCal Calorimeter energy Initial State Helicity Correlation in Wide Angle Compton Scattering p.3/3

14 Calorimeter 75 lead glass blocks, TF- type Arranged as 56 rows in 3 columns Approximately. meters by. meters Built by GEP-III, to be used by SANE and SemiSANE (BETA) and E3-3 Initial State Helicity Correlation in Wide Angle Compton Scattering p.4/3

15 Discrimination of e p/rcs Deflection of electrons by magnetic field. Initial State Helicity Correlation in Wide Angle Compton Scattering p.5/3

16 Polarized Target % P LN Microwave Input To Pumps Liquid Helium NMR Signal Out Refrigerator To Pumps Liquid Helium Signal LN Frequency frozen(doped) NH 3 4 He evaporation refrigerator 5T polarizing field remotely movable insert dynamic nuclear polarization e Beam Magnet Target (inside coil) K NMR Coil B 5T 6 n o i t a z i r a l o A te s u Mi n Initial State Helicity Correlation in Wide Angle Compton Scattering p.6/3

17 Kinematics kin. t θ lab γ θ cm γ θ lab p E lab γ p p L P# (GeV/c) degree degree degree GeV GeV/c m P P kin. θ lab γ t θ cm γ dω γ dω p D N RCS A LL P# degree (GeV/c) degree total P P kin. θ e θ p V V HMS p(proton) θ rms P# degree degree degree GeV/c mrad Initial State Helicity Correlation in Wide Angle Compton Scattering p.7/3

18 Beam Request Kin. beam, time P# Procedure na hours P BigCal calibration 8 P RCS data taking 9 76 P RCS data taking 9 4 Packing Fraction Measurements 9 6 Moller Measurements 8 Beam Time 458 BigCal angle change 8 Target Anneals 5 Stick Changes 36 Overhead Time 96 Requested Time 56 Initial State Helicity Correlation in Wide Angle Compton Scattering p.8/3

19 Error Budget Asymmetry measurement relaxes demands on some systematic error sources (solid angles etc) which cancel but requires attention to others. The largest sources are: Target polarization % Beam polarization % π subtraction(shape) 3% epγ subtraction % Total 4.% Initial State Helicity Correlation in Wide Angle Compton Scattering p.9/3

20 Conclusions Experiment straightforward - based on experimental data and extensive experience. Test onset of handbag approach in terms of GPD s. Positive indications for handbag allows extraction of non-perturbative structure of hadrons in form of GPD s. Explore role of finite quark masses in polarization observables. Shed light on nature of quark helicity flip processes. As byproduct A π LL will also be measured. Scheduling with SANE and Semi-SANE captures setup savings. Initial State Helicity Correlation in Wide Angle Compton Scattering p./3

21 PAC 8 Recommendation Merely due to lack of available beam time, the PAC recommends that only the kinematic point in the backward hemisphere be measured. Approved with A rating for 4 days. Initial State Helicity Correlation in Wide Angle Compton Scattering p./3

22 Simulation Presence of radiator creates unique conditions Beam blows up Large number of secondary particles (electrons, photons) - implications for rates in calorimeter; where to place shielding. Include target magnetic field Physics backgrounds Elastic electron scattering Quasielastic electron scattering π γ from proton and target materials Include target magnetic field Initial State Helicity Correlation in Wide Angle Compton Scattering p./3

23 Status GEANT4 - Justin Wright, UVA graduate student Electromagentic part moving along well Second part hindered by lack of the physics in GEANT4 Also by our unfamiliarity with the standard practice for incorporating new physics. Initial State Helicity Correlation in Wide Angle Compton Scattering p.3/3

24 GEANT4 Simulation Geometry Upstream beam pipe Downstream beam pipe (Helium bag or flaring Aluminum tube) Upstream copper radiator (%) Target can (simplified), including the target cell and magnet Big Cal Simple plane detectors to represent the solid angle openings of the Calorimeter and the HMS Fields The target magnet s field (read in from a table) Electromagnetic processes as currently implemented by the Geant4 collaboration Electron Ionization Electron Bremsstrahlung Photo Electric Effect Compton Scattering (from electron) Pair Production Annihilation Data collection and analysis Each primary electron represents a single event All daughter particles are tracked fully All physical objects can be treated as perfect detectors, recording all interactions Separate code converts this data into root trees or paw ntuples Initial State Helicity Correlation in Wide Angle Compton Scattering p.4/3

25 Pure Photon Beam Initial State Helicity Correlation in Wide Angle Compton Scattering p.5/3

26 Details HMS resolution <.% HMS acceptance HMS p p ±9% Angle Resolution HMS vertex resolution Photon fluence BigCal block sizes ±7(h) ± 7(v) mr.9 mr (h).9 mr(v) ± mm dk k ( ) 4 4 cm Lucite Cerenkov hodoscope thick horiz. vert. # cm cm cm x BigCal σ E BigCal 5%/ E.. m y p.e. % r.l., x rms = 3.6 cm Möller <.5% Target thickness Multiple scattering.5 g/cm of NH3,.3 of He.7 (P),.8 (P) mr kin. t u θ lab γ θ cm γ θ lab p E lab γ p p L P# (GeV/c) (GeV/c) degree degree degree GeV GeV/c m P P Initial State Helicity Correlation in Wide Angle Compton Scattering p.6/3

27 Coils restrict access ! 7! 6! 5! 4! 3!! C M S a n g l e t tv s C M S a n g l e > 8 > 7 > 6 > 5 > 4 > 3 > > C M S a n g l e u u v s C M S a n g l e Magnet coils restrict access to range of angles: here the field direction is along the beam line. Initial State Helicity Correlation in Wide Angle Compton Scattering p.7/3

28 π photons Initial State Helicity Correlation in Wide Angle Compton Scattering p.8/3

29 Dilution from other materials - Hall A Al data LH target z target [cm] LH target [cm] + y x hhe Entries 96 Mean x 743 Mean y 38.8 RMS x RMS y Energy deposited in the calorimeter [MeV] Empty target Empty target [cm] + y x hale Entries 53 Mean x 9 Mean y 5.3 RMS x RMS y z target [cm] Energy deposited in the calorimeter [MeV] Run Number Target Charge Thickness Integrated N pion Coulomb g/cm e N Luminosity and Electrons 377 Al (total) 39 LH N quasi /N free (P N + P He )/P free =. (7 +.4)/3.6 Initial State Helicity Correlation in Wide Angle Compton Scattering p.9/3

30 Dilution from other materials - Simulation results F = Nπ p N π freep T p T π CL (P N + P He )/P free = (7 +.4)/3. Initial State Helicity Correlation in Wide Angle Compton Scattering p.3/3

31 Rate test Counting rate vs. threshold /8/ Counting rate (Hz) 3 Beam energy : Beam current: 3.3 GeV µα Target: 5 cm LH Calorimeter angle : O 35 Calorimeter to target :.6 m Solid angle :.36 msr threshold (GeV) E elastic threshold (mv). Initial State Helicity Correlation in Wide Angle Compton Scattering p.3/3

32 Miller approach compared to Huang et al. Miller constituent quark model soft physics embodied in wave function (power law) m q 35 MeV non-zero quark-helicity flip K LL A LL Huang et al. current quarks proton helicity flip non-zero Φ = Φ 6 ; double-flip amplitudes Φ,Φ 6 are non-zero with α s corrections, without both are zero. K LL = A LL Miller s quark helicity flip implies Φ Φ 6 even with α s corrections, and large compared to non-helicity flip Initial State Helicity Correlation in Wide Angle Compton Scattering p.3/3