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

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

3. Im{ H n, E n, E n }  = x B /(2-x B ) k=-t/4M 2   leptonic plane hadronic plane N’ e’ e Unpolarized beam, longitudinal target:  UL ~ sin  Im{F 1 H +  (F 1 +F 2 )( H + x B /2 E ) –  kF 2 E+… }d  ~ Im{ H p, H p } ~  LU ~ sin  Im{F 1 H +  (F 1 +F 2 ) H -kF 2 E }d  ~ Polarized beam, unpolarized target: Im{ H p, H p, E p } ~ Unpolarized beam, transverse target:  UT ~ sin  Im{k(F 2 H – F 1 E ) + ….. }d  Im{ H p, E p } Sensitivity to GPDs of DVCS spin observables Polarized beam, longitudinal target:  LL ~ (A+Bcos  Re{F 1 H +  (F 1 +F 2 )( H + x B /2 E )…}d  ~ Re{ H p, H p } ~ Im{ H n, H n, E n } ~ Proton Neutron ~ Re{ H n, E n, E n } ~ Im{ H n } ~

4. ep→epγ BSA CLAS4.2 GeV Published PRL BSA CLAS4.8- 5.75 GeV Published PRC (σ,  ) Hall A5.75 GeV Published PRL BSA CLAS5.75 GeV Published PRL ep→epγ TSA (L) CLAS5.65 GeV Published PRL (longitudinal) TSA (L) CLAS5.9 GeV Analysis ongoing DSA(L) CLAS 5.9 GeV Analysis ongoing ep→ep   TSA (T) CLAS 6 GeV Data taking this year en→en   Hall A 5.75 GeV Published PRL ep(n)→ep(n)   Hall A 4.82/6 GeV Data just taken GPDs Reaction Obs. Expt. E e Status DVCS measurements at JLab For JLab@12 GeV, approved DVCS experiments:  CLAS12: BSA and TSA (longitudinal target) on the proton  Hall A for  (polarized beam) on the proton No experiments have been so far proposed for DVCS on the neutron at 11 GeV

5. (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) Combined analysis of DVCS observables for proton and neutron targets is necessary to perform a flavor separation of 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)] Flavor separation of GPDs GPDs depend on quark flavor: proton and neutron GPDs are linear combinations of quark GPDs Measurements of DVCS on neutron target are crucial for the completion of a comprehensive GPD program for JLab@12 GeV

6.  = 60° x B = 0.2 Q 2 = 2 GeV 2 t = -0.2 GeV 2 VGG Model (calculations by M. Guidal) DVCS on the proton J u =.3, J d =.1 J u =.1, J d =.1 J u =.5, J d =.1 J u =.3, J d =.3 J u =.3, J d =-.1 E e = 11 GeV BSA for DVCS at 11 GeV: sensitivity to E  LU 

7.  = 60° x B = 0.17 Q 2 = 2 GeV 2 t = -0.4 GeV 2 VGG Model (calculations by M. Guidal) DVCS on the neutron J u =.3, J d =.1 J u =.1, J d =.1 J u =.5, J d =.1 J u =.3, J d =.3 J u =.3, J d =-.1 E e = 11 GeV BSA for DVCS at 11 GeV: sensitivity to E  LU  The beam-spin asymmetry for nDVCS is: very sensitive to E depends strongly on the kinematics → wide coverage needed maximum at low x B → 11 GeV beam energy is necessary

8.  = 60° x B = 0.17 Q 2 = 2 GeV 2 t = -0.4 GeV 2 VGG Model (calculations by M. Guidal) DVCS on the neutron J u =.3, J d =.1 J u =.1, J d =.1 J u =.5, J d =.1 J u =.3, J d =.3 J u =.3, J d =-.1 E e = 11 GeV BSA for DVCS at 11 GeV: sensitivity to E  LU  The beam-spin asymmetry for nDVCS is: very sensitive to E depends strongly on the kinematics → wide coverage needed maximum at low x B → 11 GeV beam energy is necessary We propose to initiate an experimental program of DVCS on the neutron by measuring the beam-spin asymmetry CLAS12 will provide the large acceptance and high luminosity to cover a wide phase space The 11 GeV CEBAF electron beam allow to cover a large Q 2, x B, t range

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

10. Central Detector CND: requirements More than 80% of the neutrons have  >40° → Neutron detector in the CD ~ 0.4 GeV/c ed→e’n  (p) Detected in forward CLAS12 Detected in EC, FC Not detected Detected in CND In the hypothesis of absence of FSI: p μ p = p μ p’ → kinematics are complete detecting e’, n (p, ,  ),  p μ e + p μ n + p μ p = p μ e′ + p μ n′ + p μ p′ + p μ  FSI effects will be estimated measuring en , ep , on deuteron in this same experiment and compare with free-proton data Resolution on MM(en  ) studied with nDVCS event generator + electron and photon resolutions obtained from CLAS12 FastMC + design specs for Forward Calorimeter → dominated by photon resolutions Resolution on MM(en  ) studied with nDVCS event generator + electron and photon resolutions obtained from CLAS12 FastMC + design specs for Forward Calorimeter → dominated by photon resolutions → The CND must ensure: good neutron identification for 0.2<p n ≤1 GeV/c →  (TOF) ~ 150 ps for n/   separation momentum resolution up to 10% no stringent requirements for angular resolutions

11.  CTOF can also be used for neutron detection  Central Tracker (SVT+MM): veto for 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 photodetectors tested: SIPMs, APDs, MCP-PMs CND: constraints and chosen design The light comes out only at the upstream side of the CND, goes through bent light guides (1.5m) arriving to ordinary PMTs, placed in the low-field region Final design: scintillator barrel 3 radial layers, 48 bars per layer coupled two-by-two by “u-turn” lightguides

12. 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  (TOF) and light loss due to u-turn implemented in the simulation CND: performances Efficiency Efficiency for different thresholds on deposited energy Momentum (GeV/c) Efficiency ~ 8-10% for a threshold of 2 MeV, TOF<8 ns and p n = 0.2 - 1 GeV/c New: cheaper PMTs tested (R9779) DTDT N xxxxx Hit position 2 1 0 -1 -2

13. n/  misidentification for p n <1 GeV/c Error bars on the β - axis represent 3 σ  p/p ~ 4-10%  ~ 2-4° Equal n/  yields assumed CND: performances  p (GeV/c)  

14. Backgrounds in the CND  Electromagnetic background rates and spectra in the CND have been studied with GEANT4: After reconstruction cuts background rate ~ 30 KHz Assuming a 1-KHz rate of e  events in the CLAS12 rate of accidental coincidences ~ 0.05 Hz  Physical background from photons coming from asymmetric meson decays studied with DIS simulation and CLAS12 acceptance: requiring an electron and a photon (E  >1 GeV) in the FD applying “DVCS-like” cut MM(e  )<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 Energy deposition in CND of background photons

15. FT ed→en  0 (p) background For each (Q 2, x B, t,  bin, the background coming from en  0 (p) events, where only one of the two  0 decay photons is detected, will be subtracted in the analysis as follows: Background contamination estimated using nDVCS-BH and ed→en  0 (p) generators + FASTMC (realistic CLAS12, FT and CND resolutions and acceptances): ~15% (19%) en  0 generator: Regge-based model (Laget) reproducing Hall A and CLAS proton data at 6 GeV Issue raised by a PAC reader: background from ΔVCS on the proton ed→e(n)Δ + γ→ e(n)n  +  cross section comparable with nDVCS central tracker to veto  + simulation studies ongoing possibility to cross check this channel using BoNuS to detect soft  +

16. nDVCS with CLAS12 + CND: count-rate estimate  t = 0.3 GeV 2,  Q 2 =1.5 GeV 2,  x B = 0.15,  = 30° L = 10 35 cm -2 s -1 per nucleon Time = 80 days R acc = bin-by-bin acceptance for e  (10%-40%) E eff = neutron detection efficiency (10%) N = ∆t ∆Q 2 ∆x ∆  L Time R acc E eff Count rates computed with nDVCS+BH event generator + CLAS12 acceptance from FastMC + CND efficiency from GEANT4 simulation ≈ - 0.35 GeV 2 ≈ 2.75GeV 2 ≈ 0.225 Beam-spin asymmetry for nDVCS VGG predictions 4 bins in Q 2 1.5, 2.75, 4.25, 7.5 GeV 2 4 bins in −t 0.1, 0.35, 0.65, 1 GeV 2 4 bins in x B 0.1, 0.225, 0.375, 0.575 12 bins in φ, each 30 o wide 588 accessible bins J u =.3, J d =.1 J u =.1, J d =.1 J u =.3, J d =.3 J u =.3, J d =-.1

17. Projected number of counts/bin and coverage  N/N=0.05%-10% The final grid will be optimized depending on the actual value of the BSA  →

18. Summary of setup and beam-time request Testing and commissioning 7 days Production data taking at L = 10 35 cm −2 s −1 /nucleon 80 days Moeller polarimeter runs 3 days Beam energy: 11 GeV Beam polarization: 85% Plan for CND: Spring 2011: finalize R&D, with tests on 3-layer prototype and final mechanical design 2 nd semester 2011: detailed engineering drawing 2012- first half of 2013: construction 2 nd semester 2013: assembly → ready to be installed in the CD by spring 2014 Talk by Daria Sokhan on status of the CND (Wednesday at 5PM) Total requested 90 days The detector will be financed by the proposing european institutions, with a stronger contribution from IN2P3 Experimental setup: CLAS12 + Forward Calorimeter Liquid deuterium target Central Neutron Detector

19. Using scintillator as detector material, “u-turn” downstream and long light guides with PMTs upstream, detection of nDVCS neutrons with ~10% of efficiency and n/  separation for p n ≤ 1 GeV/c will be achieved in the CND Conclusions nDVCS is a key reaction for the JLab GPD experimental program: measuring its beam-spin asymmetry can give access to E and therefore to the quark total angular momentum (via Ji’s sum 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 strongly and be maximum at low x B → 11 GeV beam + CLAS12 are necessary The detection of the recoil neutron ensures exclusivity, reduces background and keeps systematic uncertainties under control The nDVCS recoil neutrons are mostly going at large angles (  n >40°), therefore a neutron detector must be added to the CLAS12 Central Detector using the available space For an update on the status of the CND, don’t miss Daria’s talk tomorrow Simulation studies underway to address PAC concerns on background from ΔVCS on the proton With 90 days of beam time at L=10 35 cm −2 s −1 /nucleon, using CLAS12+CND+FC, we’ll extract BSA on a wide phase space and with sufficient accuracy to allow GPD analysis

20. This proposal is devoted to a measurement of exclusive observables that should provide access to Generalized Parton Distributions (GPDs). GPDs provide novel tools for the description of nucleon structure. Being hybrids of usual parton distributions, form factors and distribution amplitudes, GPDs accumulate more detailed information about hadrons than these “older" functions alone. On the other hand, the task of extracting from the data functions depending on a larger number of variables is more challenging. The cleanest process for GPD extraction is deeply virtual Compton scattering (DVCS), the study of which forms a rather large program of investigation at Jefferson Lab, and the proposed experiment complements other experiments that are already approved. In particular, the proposed study of DVCS on the neutron complements the proton target experiments, and is necessary for flavor separation of GPDs. As noted in the proposal, beam-spin asymmetry (BSA) on the neutron (which is the main object of study in the proposal) is mainly governed by the GPD E, which, unlike GPDs H and ~H, is not constrained by information from the usual parton distributions. However, no details concerning why neutron BSA is dominated by the GPD E are given in the proposal. The relevant formula is not very long, so the authors could present it and give some discussion of the relative size of the terms which appear there (it contains not only E, but also H and ~H ), and the role of various suppression factors accompanying each contribution. Theory comments

21. The results for the neutron BSA generated by VGG are shown in Figure 2 of the proposal, and it is written that “one sees that these BSAs can extend from 10 up to 50%, with spectacular changes of sign depending on the relative signs of Ju and Jd, and therefore they can be as large, in magnitude, as the proton-DVCS beam-spin asymmetries that have been recently measured." The curves are given for different values of Ju and Jd, the parameters characterizing the normalization of Eu and Ed. As noted in the proposal, some of the chosen values of Ju and Jd are unrealistic. And it is these values that produce the curves reaching 50%. The curves corresponding to more realistic values Ju = 0.3; Jd = 0.1 do not reach even 10%. Here it may be mentioned that the values obtained by Guidal et al. (Phys. Rev. D 72, 054013 (2005)) from a fit of nucleon form factors are Ju = 0.29; Jd = -0.04 3 (and they agree with lattice results). Thus, it is quite possible that the neutron BSA is very small, and it would be very appropriate to discuss in the “physics motivation" part of the proposal what kind of information about GPDs could be obtained in such a situation. Finally, we note that measurements of the helicity-flip GPD E are of intrinsic interest for nucleon structure, aside from the connection with quark angular momentum through the Ji sum rule inferred above. The helicity-flip GPD E represents a fundamental twist-2 structure which describes how the nucleon's anomalous magnetic momentum is distributed over the quark momentum fraction x, and what partonic configurations contribute to the Pauli form factor at different values of t.

22. Technical Comments: 1) The neutron detector is not part of the base equipment and will likely be supplied by the proposers of the experiment. Optimally, the new detector would have to be installed in the CD before any of the other CD components can be installed. This puts significant time constraints on the development and construction of the neutron detector, as it must be ready for installation in the spring 2014 in order to not impact the overall 12 GeV schedule. 2) The space requirements for the CND will need to be defined consistent with the requirements of the CLAS12 central detector.

23. One-layer prototype 2 scintillators BC408 (700 mm long) coupled to two PMTs R2083 (R9779) by means of 1500 mm long light guide, wrapping with Al foil, semicircular light guide at the “u-turn” A factor of 2 loss of light due to the u-turn Light guide PMT D PMT N Scintillator xxxxx Hit position PMT D PMT N TRG  D)  [105, 136] ps  N)  [194, 262] ps

24. One-layer prototype 2 scintillators BC408 (700 mm long) coupled to two PMTs R2083 (R9779) by means of 1500 mm long light guide, wrapping with Al foil, semicircular light guide at the “u-turn”

27. Results from Orsay’s test bench σ 2 test =1/2 (σ 2 test,trig + σ 2 test,ref − σ 2 ref,trig ) Test Ref Trig Test = 1 SiPM Hamamatsu (MPPC 3x3mm 2 ) rise time ~5 ns (> capacitance) more noise than 1x1 mm 2 Test = 1 APD Hamamatsu (10x10 mm 2 )  TOF ~ 1.4 ns high noise, high rise time Test = 1 MCP-PMT Photonis/DEP (two MCPs)  TOF ~ 130 ps tested in B field at Saclay: big loss of gain Thi Nguyen Trung Bernard Genolini S. Pisano J. Pouthas Test = PMT  TOF < 90 ps nphe ~1600 Single pe Test = 1 SiPM Hamamatsu (MPPC 1x1 mm 2 )  TOF ~ 1.8 ns rise time ~ 1 ns nphe ~1 Test = 1 MPPC 1x1mm 2 Extruded scint. + WLS fiber  TOF ~ 1.4 ns WLS -> Width ~ 15 ns

28. Technical challenge: TOF resolution & B=5T SiPM - PROS: Insensitive to magnetic field High gain (10 6 ) Good intrinsic timing resolution (30 ps/pixel) Good single photoelectron resolution SiPM - CONS: Very small active surface (1-3 mm 2 ) → small amount of light collected (  TOF ~1/√N phel ) Noise SiPM APD – PROS: insensitive to magnetic field bigger surface than SiPM → more light collected APD – CONS: low gain at room temperature timing resolution? MCP-PMT – PROS: resistant to magnetic field ~1T big surface timing resolution ~ordinary PMT MCP-PMT – CONS: behavior at 5T not yet studied high cost (2K euros/PMT) lifetime? MCP-PMT

29. “LOI12-09-001: The PAC considers performing Deeply Virtual Compton Scattering on not only protons but also neutrons essential. For this a central neutron detector is required. This collaboration is strongly encouraged to complete the required R&D and bring forward a Proposal.” Letter of Intent presented to PAC34 (January 2009)

30. pDVCS & nDVCS: comparison of cross sections

32. FT IC

34. Mechanics and integration Semicircular light guides Scintillators Light guide PMTs

35. Mechanics and integration

36. Meeting - Nov 09 2010

37. Mechanics and integration Keeping volume Mechanic structure through the CND PMTs position according to the magnetic field Meeting - Nov 09 2010

38. Work plan for the next months  Test of the one-layer prototype with PMT R9779 (1/3 of the price of R2083)  Test of the prototype with 6 PMTs, 6 scintillators  Optimize length of light guides, PMT shieldings according to up-to-date B-field map

39.  H(x,ξ,t)dx = F 1 (t) (  ξ) H(x,0,0) = q(x), H(x,0,0) = Δq(x) ~ x Form factors: transverse quark distribution in coordinate space Parton distributions: longitudinal quark distribution in momentum space Accessible in hard exclusive processes  E(x,ξ,t)dx = F 2 (t) (  ξ) Generalized Parton Distributions GPDs: H, E, H, E Fully correlated quark distributions in both coordinate and momentum space ~~

40. t (Q 2 ) e ** x+ξ x-ξ H, H, E, E (x,ξ,t) ~ ~  N N’ Deeply Virtual Compton Scattering and GPDs e’ « Handbag » factorization valid in the Bjorken regime  x B /(2-x B )   0,x  ),(Ex q  2 1 Hxdx q  J G =  2 1 J q  1 1  )0,,( Quark angular momentum (Ji’s sum rule) X. Ji, Phy.Rev.Lett.78,610(1997)  4 GPDs for each quark flavor  two conserving nucleon spin, two flipping nucleon spin: Vector: H (x,ξ,t) Tensor: E (x,ξ,t) Axial-Vector: H (x,ξ,t) Pseudoscalar: E (x,ξ,t)  Connected to elastic form factors and ordinary PDFs GPDs: H, E, H, E Fully correlated quark and spin distributions in both coordinate and momentum space ~~ ~ ~

41. Central Neutron Detector for nDVCS: requirements nDVCS event generator + electron and photon resolutions obtained from CLAS12 FastMC + design specs for Forward Tagger or IC nDVCS event generator + electron and photon resolutions obtained from CLAS12 FastMC + design specs for Forward Tagger or IC Photon resolution contributes to 94% (97%) of the width of MM → The CND must ensure: good neutron identification for 0.2<p n <1.2 GeV/c →  (TOF)~150 ps for n/  separation using  momentum resolution below 10% no stringent requirements for angular resolutions FT IC

42. Chosen design: the “U – turn” Two neighboring bars are connected via a semi-circular light guide One of the two bars acts as the « light guide » for the other one PMTs location The light comes out only from one side of the CND, goes through bent light guides (1.5m) arriving to PMTs, placed in the low- field region

43. J u =.3, J d =.1 J u =.8, J d =.1 J u =.5, J d =.1  = 60° x B = 0.2 Q 2 = 2 GeV 2 t = -0.2 GeV 2 BSA for DVCS at 11 GeV: sensitivity to E VGG Model (calculations by M. Guidal) DVCS on the proton J u =.3, J d =.8 J u =.3, J d =-.5 E e = 11 GeV

44. First measurement of nDVCS: Hall A E e = 5.75 GeV/c P e = 75 % L = 4 · 10 37 cm -2 · s -1 /nucleon Q 2 = 1.9 GeV 2 x B = 0.36 0.1 GeV 2 < -t < 0.5 GeV 2 HRS Electromagnetic Calorimeter (PbF 2 ) LH 2 / LD 2 target  e’e’ e Subtraction of quasi-elastic proton contribution deduced from H 2 data convoluted with Fermi motion of the nucleon Analysis done in the impulse approximation: Active nucleon identified via missing mass Twist-2 M. Mazouz et al., PRL 99 (2007) 242501

45. nDVCS in Hall A: results S. Ahmad et al., PR D75 (2007) 094003 VGG, PR D60 (1999) 094017 M. Mazouz et al., PRL 99 (2007) 242501 Q 2 = 1.9 GeV 2 - x B = 0.36 Im(C I n ) compatible with zero (→ too high x B ?) Strong correlation between Im[C I d ] and Im[C I n ] Big statistical and systematic uncertainties (mostly coming from H 2 and  0 subtraction) Model dependent extraction of J u and J d F. Cano, B. Pire, Eur. Phys. J. A19 (2004) 423

46. Central Neutron Detector for nDVCS: requirements Varying  (  x200→  (MM) +30% Varying  (p)/p x200→  (MM)x40 nDVCS event generator with « smearing » on neutron kinematic variables. Studied effect of neutron resolutions on MM(en  ) MM(en  ) has little sensitivity to angular resolutions stronger sensitivity to momentum resolution → The CND must ensure: good neutron identification for 0.2<p n <1.2 GeV/c →  (TOF) ~ 150 ps for n/   separation momentum resolution up to 10% no stringent requirements for angular resolutions Smearing on e  n e  e n GeV 2 Photon resolution: 94% of  (MM) nDVCS event generator + electron and photon resolutions obtained from CLAS12 FastMC + design specs for Forward Calorimeter

47. Accessing GPDs through DVCS      I( DVCS·BH )  ~ |T DVCS +T BH | 2 I( DVCS·BH ) A =    | BH | 2 + | DVCS | 2 +I   Only  and t are accessible experimentally

48. H, H ~ What we learn from the CLAS data M. Guidal, PLB 689, 156 (2010) Fit at fixed x B, t, Q 2 of DVCS observables to extract CFFs using twist-2 LO DVCS/BH amplitude 7 CFFs VGG (different bounds) Fit of CLAS proton-DVCS BSAs and longitudinal TSA Few observables available: model-dependent assumptions still necessary No flavor separation possible at this stage