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Deeply Virtual Compton Scattering on the neutron with CLAS12 at 11 GeV

k’. Deeply Virtual Compton Scattering on the neutron with CLAS12 at 11 GeV. q’. k. Silvia Niccolai. n. n’. GPDs. CLAS12 Workshop, Paris, March 8th 2011. The CLAS collaboration. Deeply Virtual Compton Scattering on the neutron with CLAS12 at 11 GeV. Saclay.

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Deeply Virtual Compton Scattering on the neutron with CLAS12 at 11 GeV

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  1. k’ Deeply Virtual Compton Scatteringon the neutron with CLAS12 at 11 GeV q’ k Silvia Niccolai n n’ GPDs CLAS12 Workshop, Paris, March 8th 2011

  2. The CLAS collaboration Deeply Virtual Compton Scatteringon the neutron with CLAS12 at 11 GeV Saclay • Presentedat PAC37 (January 2011) and accepted • Goal: BSA for nDVCS • 90 days of beam time requested Co-spokespersons: A. El Alaoui (Argonne), M. Mirazita (INFN Frascati), S. Niccolai (IPN Orsay), V. Kubarovsky (Jefferson Lab)

  3. Sensitivity to GPDs of DVCS spin observables g ~ Polarized beam, unpolarizedtarget: Im{Hp, Hp, Ep} f e’ ~ DsLU~ sinfIm{F1H+ x(F1+F2)H-kF2E}df e leptonic plane N’ ~ ~ Unpolarized beam, longitudinaltarget: Polarized beam, longitudinaltarget: hadronic plane Re{Hp, Hp} Im{Hp, Hp} ~ ~ DsUL ~ sinfIm{F1H+x(F1+F2)(H+ xB/2E) –xkF2E+…}df DsLL~ (A+Bcosf)Re{F1H+x(F1+F2)(H+ xB/2E)…}df Unpolarized beam, transversetarget: Im{Hp, Ep} DsUT~ sinfIm{k(F2H – F1E) + …..}df x= xB/(2-xB) k=-t/4M2 ProtonNeutron ~ Im{Hn, Hn, En} ~ ~ Im{Hn, En, En} ~ Re{Hn, En, En} Im{Hn}

  4. DVCS measurements at JLab GPDs Reaction Obs. Expt.Ee Status ep→epγBSA CLAS 4.2 GeV Published PRL BSA CLAS 4.8- 5.75 GeVPublished PRC (σ, Ds) Hall A 5.75 GeV Published PRL BSA CLAS 5.75 GeV Published PRL ep→epγTSA (L) CLAS 5.65 GeV Published PRL (longitudinal) TSA (L) CLAS 5.9 GeV Analysis ongoing DSA(L) CLAS 5.9 GeV Analysis ongoing ep→epgTSA (T) CLAS 6 GeV Data taking this year en→engDsHall A 5.75 GeVPublished PRL ep(n)→ep(n)g s Hall A 4.82/6 GeV Data just taken • For JLab@12 GeV, approvedDVCS experiments: • CLAS12: BSA and TSA (longitudinal target) on the proton • Hall A for Ds(polarizedbeam) on the proton No experiments have been so far proposed for DVCS on the neutron at 11 GeV

  5. Flavor separation of GPDs GPDsdepend on quark flavor: proton and neutron GPDs are linearcombinations of quark GPDs (H,E)p(ξ, ξ, t) = 4/9 (H,E)u(ξ, ξ, t) + 1/9 (H,E)d(ξ, ξ, t) (H,E)n(ξ, ξ, t) = 1/9 (H,E)u(ξ, ξ, t) + 4/9 (H,E)d(ξ, ξ, t) Combinedanalysis of DVCS observables for proton and neutron targetsisnecessary to perform a flavorseparationof GPDs (H,E)u(ξ, ξ, t) = 9/15[4(H,E)p(ξ, ξ, t) – (H,E)n(ξ, ξ, t)] (H,E)d(ξ, ξ, t) = 9/15[4(H,E)n(ξ, ξ, t) – (H,E)p(ξ, ξ, t)] Measurements of DVCS on neutron target are crucial for the completion of a comprehensive GPD program for JLab@12 GeV

  6. BSA for DVCS at 11 GeV: sensitivity to E DVCS on the proton Ju=.3, Jd=.1 DsLU/s Ju=.1, Jd=.1 Ju=.5, Jd=.1 Ju=.3, Jd=.3 Ju=.3, Jd=-.1 f= 60° xB = 0.2 Q2 = 2 GeV2 t = -0.2 GeV2 VGG Model (calculations by M. Guidal) Ee = 11 GeV

  7. BSA for DVCS at 11 GeV: sensitivity to E DVCS on the neutron Ju=.3, Jd=.1 DsLU/s Ju=.1, Jd=.1 Ju=.5, Jd=.1 Ju=.3, Jd=.3 Ju=.3, Jd=-.1 f= 60° xB = 0.17 Q2 = 2 GeV2 t = -0.4 GeV2 VGG Model (calculations by M. Guidal) Ee = 11 GeV • The beam-spin asymmetry for nDVCS is: • very sensitive to E • depends strongly on the kinematics→ wide coverage needed • maximum at low xB→11 GeV beam energy is necessary

  8. BSA for DVCS at 11 GeV: sensitivity to E DVCS on the neutron Ju=.3, Jd=.1 DsLU/s Ju=.1, Jd=.1 • We propose to initiate an experimental program of • DVCS on the neutron by measuring the beam-spin asymmetry • CLAS12 willprovide the large acceptance and highluminosity to cover a wide phase space • The 11 GeV CEBAF electronbeamallow to cover a large Q2, xB, t range Ju=.5, Jd=.1 Ju=.3, Jd=.3 Ju=.3, Jd=-.1 f= 60° xB = 0.17 Q2 = 2 GeV2 t = -0.4 GeV2 VGG Model (calculations by M. Guidal) Ee = 11 GeV • The beam-spin asymmetry for nDVCS is: • very sensitive to E • depends strongly on the kinematics→ wide coverage needed • maximum at low xB→11 GeV beam energy is necessary

  9. Neutron DVCS setup For the detection of the scatteredelectron and of the DVCS photon: CLAS12 + ForwardCalorimeter CTOF Central Tracker CTOF DC EC CND Central Detector For the detection of the recoil neutron: Central Neutron Detector (CND) • Acceptance for • charged particles: • Central (CD), 40o<q<135o • Forward (FD), 5o<q<40o Forward Calorimeter LTCC • Acceptance for photons: • FC 2.5o<q< 5o • EC, 5o<q<40o (HTCC removed for clarity)

  10. CND: requirements <pn>~ 0.4 GeV/c Detected in forward CLAS12 Not detected ed→e’ng(p) Detected in EC, FC Detected in CND Central Detector pμe + pμn + pμp = pμe′ + pμn′ + pμp′ + pμg In the hypothesis of absence of FSI: pμp = pμp’ → kinematics are complete detecting e’, n (p,q,f), g FSI effects will be estimated measuring eng, epg, on deuteron in this same experiment and compare with free-proton data More than 80% of the neutrons have q>40° → Neutron detector in the CD Resolution on MM(eng)studiedwithnDVCSeventgenerator+ electron and photon resolutionsobtainedfromCLAS12 FastMC + design specs for ForwardCalorimeter → dominated by photon resolutions • → The CND must ensure: • good neutron identificationfor 0.2<pn≤1 GeV/c → s(TOF) ~ 150 psfor n/gb-separation • momentumresolutionup to 10% • no stringentrequirements for angularresolutions

  11. CND: constraints and chosen design • CTOF can also be used for neutron detection • Central Tracker (SVT+MM): vetofor charged particles • limited space available (~10 cm thickness) • limited neutron detection efficiency • no space for light guides upstream • strong magnetic field (~5 T) → problems for light readout • Three kinds of B-field-resistant photodetectorstested: SIPMs, APDs, MCP-PMs Final design: scintillator barrel 3 radial layers, 48 bars per layer coupled two-by-two by “u-turn” lightguides The light comes out onlyat the upstreamside of the CND, goesthroughbent light guides (1.5m) arriving to ordinaryPMTs, placed in the low-fieldregion

  12. CND: performances Efficiency for differentthresholds on depositedenergy Efficiency ~ 8-10% for a threshold of 2 MeV, TOF<8 ns and pn = 0.2 - 1 GeV/c Efficiency Hit position DT x x x x x N 2 1 0 -1 -2 New: cheaperPMTstested (R9779) Momentum (GeV/c) • GEANT4 simulations done for: • efficiency • PID (neutron/photon separation) • momentum and angular resolutions • definition of reconstruction algorithms • background studies • Cosmic-rays measurements on a prototype • Measured values of s(TOF) and light loss • due to u-turn implemented in the simulation

  13. CND: performances Equal n/g yields assumed n/gmisidentification for pn <1 GeV/c Error bars on the β - axis represent 3 σ b b p (GeV/c) s (q) Dp/p ~ 4-10% Dq ~ 2-4° p (GeV/c)

  14. Backgrounds in the CND • Electromagneticbackground rates and spectra • in the CND have been studied with GEANT4: • After reconstruction cuts background rate ~ 30 KHz • Assuming a 1-KHz rate of eg events in the CLAS12 • rate of accidental coincidences ~ 0.05 Hz Energydeposition in CND of background photons • Physicalbackground from photons coming from • asymmetric meson decays studied with DIS simulation and CLAS12 acceptance: • requiring an electron and a photon (Eg>1 GeV) in the FD • applying “DVCS-like” cut MM(eg)<1.1 GeV • assuming 30% of acceptance + efficiency for electron and photon in the CLAS12, and 10% photon efficiency in the CND • → 0.6 Hz of photon rate on the CND • Expected integrated nDVCS-BH neutrons rate ~ 4 Hz

  15. ed→enp0(p) background For each (Q2, xB, t, f)bin, the background comingfromenp0(p)events, whereonly one of the twop0decay photons isdetected, willbesubtracted in the analysis as follows: Background contamination estimatedusingnDVCS-BH and ed→enp0(p) generators + FASTMC (realistic CLAS12, FT and CND resolutions and acceptances): ~15% (19%) enp0generator: Regge-based model (Laget) reproducing Hall A and CLAS proton data at 6 GeV • Issue raised by a PAC reader: • background fromΔVCS on the proton • ed→e(n)Δ+γ→ e(n)np+g • cross section comparable withnDVCS • central tracker to vetop+ • simulation studiesongoing • possibility to cross check thischannel • usingBoNuS to detect soft p+ FT

  16. nDVCS with CLAS12 + CND: count-rate estimate N = ∆t ∆Q2 ∆x ∆f L Time Racc Eeff Beam-spin asymmetry for nDVCS VGG predictions • L = 1035cm-2s-1per nucleon • Time = 80 days • Racc= bin-by-bin acceptance for eg(10%-40%) • Eeff = neutron detection efficiency (10%) Count rates computed with nDVCS+BH event generator + CLAS12 acceptance from FastMC + CND efficiency from GEANT4 simulation Ju=.3, Jd=.1 Ju=.1, Jd=.1 Ju=.3, Jd=.3 Ju=.3, Jd=-.1 <t> ≈ - 0.35 GeV2 <Q2> ≈ 2.75GeV2 <x> ≈ 0.225 • 4 bins in Q2 1.5, 2.75, 4.25, 7.5 GeV2 • 4 bins in −t 0.1, 0.35, 0.65, 1 GeV2 • 4 bins in xB 0.1, 0.225, 0.375, 0.575 • 12 bins in φ, each 30owide 588 accessible bins Dt = 0.3 GeV2, DQ2=1.5 GeV2, DxB= 0.15, Df = 30°

  17. f → Projected number of counts/bin and coverage DN/N=0.05%-10% The final gridwillbeoptimized depending on the actual value of the BSA

  18. Summary of setup and beam-time request Plan for CND: Spring 2011: finalize R&D, with tests on 3-layer prototype and final mechanical design 2ndsemester 2011: detailed engineering drawing 2012- first half of 2013: construction 2ndsemester 2013: assembly → ready to beinstalled in the CD by spring 2014 Talk by Daria Sokhan on status of the CND (Wednesdayat 5PM) • Experimental setup: • CLAS12 + ForwardCalorimeter • Liquiddeuteriumtarget • Central Neutron Detector Testing and commissioning 7 days Production data taking at L = 1035 cm−2s−1/nucleon 80 days Moellerpolarimeterruns 3 days The detector willbefinanced by the proposingeuropean institutions, with a stronger contribution fromIN2P3 Beamenergy: 11 GeV Beampolarization: 85% Total requested 90 days

  19. Conclusions • nDVCS is a key reaction for the JLab GPD experimental program: measuring its beam-spin asymmetrycan give access to E and therefore to the quark total angular momentum • (via Ji’ssum rule), and it is a first step towards flavor separation of GPDs • A large kinematical coverage is necessary to sample the phase-space, as the BSA is expected to vary stronglyand be maximum atlow xB→11 GeV beam + CLAS12are necessary • The detection of the recoil neutron ensures exclusivity, reduces background andkeeps • systematic uncertainties under control • The nDVCS recoil neutrons are mostly going at large angles(qn>40°), therefore a neutron detectormust be added to the CLAS12 Central Detector using the available space • Using scintillator as detector material, “u-turn” downstream and long light guides with PMTs upstream, detection of nDVCSneutrons with ~10% of efficiencyand n/g separation for pn≤ 1 GeV/cwill be achieved in the CND • With 90 days of beam time at L=1035cm−2s−1/nucleon, using CLAS12+CND+FC, we’ll extract BSA on a wide phase space and with sufficient accuracy to allow GPD analysis • Simulation studies underway to address PAC concerns on background from ΔVCS on the proton For an update on the status of the CND, don’t miss Daria’s talk tomorrow

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