Time-like Compton Scattering with transversely polarized target

Transcription

1 Time-like Compton Scattering with transversely polarized target Vardan Tadevosyan AANSL (YerPhI) Foundation Arthur Mkrtchyan CUA

2 Outline Physics case and motivation Experimental setup Simulation results Latest developments Summary and Outlooks 2

3 Physics case DVCS TCS DVCS and TCS, limiting cases of double DVCS (DDVCS) q + P p = q + P p. TCS inverse to DVCS. At LO of S and leading twist amplitudes are complex conjugate, CFFs are same. TCS hard scale provided by virtuality of the final state photon. Comparison of DVCS and TCS data provides a test for universality of GPDs. Combine DVCS and TCS data reduce uncertainties of the fits for CFFs (provided the GPD universality is established!). 3

4 Physics case BH TCS BH produces same final state as TCS. M.Boer et al, arxiv: TCS interferes with BH: d 4 σ γp p e + e dq 2 dtdω = TBH + T TCS π 4 s M N 2 2 At Jlab energies σ BH >> σ TCS ( times diff.). TCS signal attainable in interference with BH (~30%). 4

5 TCS kinematics, definitions σ TCS = F Q 2, t, CM, ϕ CM Asymmetries: A UX target spin asymmetry for un-polarized beam, P T in hadron plane; A UY target spin asymmetry for un-polarized beam,p T to hadron plane; A X beam-target spin asymmetry for longitudinally polarized beam, P in hadron plane; A Y beam-target spin asymmetry for longitudinally polarized beam, P to hadron plane; 5

6 Physics case ξ = 0.2, Q 2 = 7 GeV 2, θ 45, 135 φ = 90 φ = 90 t = 0.4 t = 0.4 BH not sensitive to TSA (Im A 0). TSA due to Im A 0 in TCS. TSA can reach 20%, measurable! TSA sensitive to Im H and Im E. 6

7 Physics case ξ = 0.2, Q 2 = 7 GeV 2, t = 0.4 GeV 2, θ 45, 135 Double Spin Asymmetry (DSA): o Sensitive to real part of amplitude. o BH contributes! o Sensitive to all GPDs, particularly to real parts. o Sensitive to models. o ϕ CM shapes complex, very dependent on CM and GPDs entering TCS. o Extraction of GPDs more challenging. 7

8 Physics case Hall C TCS is focused on 1-st time measurement of spin asymmetries with transversely polarized target (access to Im H and Im E ). Double spin asymmetry with transversely polarized target and circularly polarized photon beam (sensitive to Re A TCS ) will be measured, provided the polarized photon beam available. Complementary to CLAS12 E , SoLID E A (TCS cross section and beam asymmetry measurements with unpolarized target) and other DVCS experiments. Will also measure TCS cross section for cross check with E (sensitive to Re CFFs, constrains GPDs). 8

9 Experimental Setup Side view 11 GeV e- beam (quasi-real photons) * Transversely (horizontally) polarized UVA target. Vertical arrangement of detectors: Trackers F 1 and F 2 Hodoscopes H 1 and H 2 PbWO calorimeters NPS 1 and NPS 2 Beam pipe of wide critical angle * Will use (polarized) photon beam if available. 9

10 TCS UVA Polarized Target Beam 5T Used in E93-026, E01-006, SANE. Target material: ammonia ( 15 NH 3) ), doped with paramagnetic centers, immersed in LHe (high polarizability, large nucleon content, resistance to rad. damage). 5T (uniform to 10-4 ) magnetic field generated by pair of superconducting Helmhotz coils (axis known to 0.1 ). Dynamic Nuclear Polarization (DNP) by 140 GHz, 20 W (max) RF field. Polarization decays due to radiation damage (from above 80% to 60% in 8 hours for 100 na beam current, typically). Target polarization monitored via NMR Q-meter. Needs annealing (heating to K for min). UVA polarized target, cross section view. (Adopted from J. Zhang) Will be rotated by 90 around vertical axis. Angular acceptance 17 horizontally, 26.5 vertically. 10

11 Trackers and Hodoscopes Trackers will be used for reconstruction of trajectories and as a start-time for TOF. Construction analogous to Scintillating Fibre Tracker (SFT) in HERMES Recoil Detector. Can be constructed from 1mm Kuraray SCSF-78 fibers with rad. resistance 100 Gy/yr. X and Y planes of cm 2 area fibers per plane. Accuracy 0.9 mm. Multi-anode phototubes (64 channel Hamamatsu) for read-out of fibers. High magnetic field at Trackers, 1.5 kg. Light from both sides transported to PMTs by 2.5 m long Wave-Length-Shifters, to where field is below 100 G (like in SANE). Hodoscopes for reconstruction of recoil proton (P p, θ p, φ p ). Crucial for determining t. Proton identification with TOF and de/dx. Expected time resolution 200 ps. X and Y planes from 1 cm thick scintillator. Eff. area cm 2 (150 cm from target) to cover ±20 horizontally, vertically. HODO TOF p HODO de/dx p K K 11

12 Calorimeters Detect and identify leptons, measure energy and X and Y coordinates. Define Q 2, ξ and τ. A pair of similar to the NPS PbWO calorimeters. Positions and sizes optimized to cover max. angular acceptance, and for highest BH rates. 2 options (at 150 cm from target): 1) Full angular acceptance (±18 horizontally, 6-28 vertically): cm 2 active area; = 1,392 blocks in active area; = 1,550 blocks total for each calorimeter. 2) ~NPS size (±15.3 horizontally, from 6 to 25.1 vertically): cm 2 active area; 40x25 = 1000 blocks in active area; Total: 42x27 = 1134 blocks for each calorimeter. Energy and coordinate resolutions of PrimEx PbWO calorimeter (from A.Gasparyan) 12

13 TCS Analysis Options Significant magnetic field from target, mostly transverse and confined in R < 20 cm. Bends vertically e +, e - by ~2.5, and p by 20! Will reconstruct tracks at vertex (provided field is mapped to good accuracy). Reconstructed momenta can be used in conjunction with β TOF for PID. Trackers (1.5 kg) Hodos (40 G) Calo. PMTs (30 G) Deflections of accepted tracks in the target magnetic field (BdL~0.7 Tm) relative to directions at target (from simulations). 13

14 TCS Analysis Options Quasi-real photoproduction of e + e pairs (q 2 0) from 100 na 11 GeV e beam (LOI). Will check recoil proton s complanarity with reaction plane to minimize background (inelastic) reactions. Will identify quasi-real photon by missing momentum analysis of ep e + e p e reaction 2 0). (p miss,m miss Quasi-Real Photoproduction From e1-6 analysis in Paremuzyan, R., Timelike Compton Scattering, Pn,D, Thesis, Yerevan,

15 Simulations GenTCS code from R.Paremuzyan to simulate BH events: Bremsstrahlung + quasi-real photons from 11 GeV e beam Luminosity = cm 2 (3cm ammonia target, 0.6 packing fraction, 90 na beam, 30 days beam time) Cuts: E [3 11] GeV; Q 2 < 0.3 GeV 2 /c 2 ; t < 1.2 GeV 2 /c 2. A Root/C++ code to track e +, e, p in the Hall C TCS setup: Transversely polarized UVA target Scattering chamber with windows 2 EM calorimeters for e +, e detection X, Y scintillator arrays for proton detection (before calorimeters) 2 trackers by scattering chamber windows Phase space cuts: t < 1 GeV 2 /c 2 ; s > 4 GeV 2 /c 2. No particle interaction with matter. 15

16 Kinematic Coverage Setup with full acceptance calorimeters. BH maxima avoided! Blue --sampled at target events, red -- accepted triple coincidence events. 16

17 Kinematic Coverage Setup with full acceptance calorimeters. Accepted triple coincidence events. 17

18 Phase Space Binning Phase space division. Will study Q 2, ξ and t dependences. Region 1 Region 2 Region 3 Region 4 Count rates in 14ϕ CM bins. 18

19 Latest developments The Hall C TCS project was presented at Di-lepton Production workshop (Trento, Italy, October of 2016), by M.Boer (theory aspects) and V.Tadevosyan (experimental aspects). Positive response from theorists, mixed response from experimentalists. A Geant4 simulation code under development. Includes key elements of the TCS setup: UVA target cell and material, magnet coils, magnetic field, scattering chamber, trackers, hodoscopes, calorimeters, a beam pipe. Works in 2 modes: beam mode (e- or photon beam incident); tcs mode (input from a BH/TCS generator). Scattering chamber XY Trackers Calorimeters Magnet coils Target cell XY Hodoscopes Beam pipe 19

20 Preliminary result from G4 simulation 11 GeV e- beam 11 GeV photon beam Beam mode Top Calo Top Calo Bottom Calo Bottom Calo Significantly lower background hit load from photon beam than from electron beam. 20

21 Preliminary result from G4 simulation Energy depositions TCS mode 21

22 Hall C TCS Summary and Outlook Studies on the NPS calorimeter s constituents (crystals, PMTs) proceed in good pace. LOI on TCS in Hall C at JLab presented before PAC 43, welcomed. Physics case established Design construction of setup outlined Results from preliminary simulations shown. On the way of developing a full proposal Update of the Physics Case is under way (M.Boer). Tuning of Generator_TCS code (TCS event generator from M.Boer) to be done. Development of Geant4 based simulation code of the TCS setup is in progress. Simulations of the measured asymmetries with Generator_TCS and G4 TCS setup simulation codes are needed. A thorough examination of expected results from experiment, from point of view of GPD analyses is needed. Fine tuning of the design setup is needed. Interested in a High Intensity Photon Source! Interested in the UVA target field modifications (B.Wojtsekhowski)! 22

23 Back up 23

24 TCS kinematics and cuts Analysis cuts: To have GPD interpretation of TCS: Q 2 m N 2 t Q 2 1 From DVCS and DIS: Q 2 > 2 GeV 2 σ TCS = F Q 2, t, CM, ϕ CM (keeps di-lepton system out of resonances) t < 1GeV 2 (or t Q 2 < 30%) 24