Deeply Virtual Compton Scattering on the neutron with CLAS12 at 11 GeV e’ g e n n’ GPDs Silvia Niccolai (IPN Orsay), for the CLAS Collaboration INPC 2019, Glasgow (UK) - August 1 st , 2019
Deeply Virtual Compton Scattering and quark GPDs • Q 2 = - (k- k’) 2 e’ • x B = Q 2 /2M n n=E e -E e ’ g t • x+ξ , x- ξ long. mom. fract. e • t = D 2 = (p- p’) 2 g * (Q 2 ) • x x B /(2-x B ) factorization x+ξ x- ξ At leading order QCD, twist 2, chiral-even ~ ~ H, H, E, E ( x,ξ,t ) (quark helicity is conserved), quark sector → 4 GPDs for each quark flavor N(p’) N(p) Quark angular momentum (Ji’s sum rule) xp b 1 1 1 D D z xdx ( H ( x , , t 0 ) E ( x , , t 0 )) J L x 2 2 1 X. Ji, Phy.Rev.Lett.78,610(1997) Nucleon tomography D 2 d D D i b 2 q ( x , b ) e H ( x , 0 , ) 2 ( 2 ) 0 D 2 d ~ D D D i b 2 q ( x , b ) e H ( x , 0 , ) 2 ( 2 ) 0 M. Burkardt, PRD 62, 71503 (2000)
Accessing GPDs through DVCS DVCS allows access to 4 complex GPDs-related quantities: Compton Form Factors CFF( ,t) 1 GPDs ( x , , t ) T DVCS ~ P dx i GPDs ( , , t ) x 1 1 1 1 2 H e q q Re P H ( x , , t ) H ( x , , t ) dx q q x x 0 2 s DVCS BH 2 ~ T T q q H e Im H ( , , t ) H ( , , t ) q q D s s s I DVCS BH Proton Neutron ~ Im { H p , H p , E p } Polarized beam, unpolarized target: ~ ~ Ds LU ~ sin f Im{F 1 H + (F 1 +F 2 ) H -kF 2 E+… } Im { H n , H n , E n } g ~ Unpolarized beam, longitudinal target: Im { H p , H p } e’ f ~ ~ Ds UL ~ sin f Im{F 1 H + (F 1 +F 2 )( H + x B /2 E ) – kF 2 E } Im { H n , E n } e ~ N’ Re { H p , H p } Polarized beam, longitudinal target: ~ Ds LL ~ (A+Bcos f Re{F 1 H + (F 1 +F 2 )( H + x B /2 E )+… } Re { H n , E n } Im { H p , E p } Unpolarized beam, transverse target: Ds UT ~ cos f sin f s f Im{k(F 2 H – F 1 E ) +… } Im { H n }
Summary of proton-DVCS spin observables and tomography Beam charge asymmetry Beam spin asymmetry Transverse R. Dupré, M. Guidal, target spin M.Vanderhaeghen, asymmetry PRD95, 011501 (2017) Beam-spin and Proton DVCS at JLab@12 GeV tr. target spin asymmetry Longitudinal target spin asymmetry Beam-spin and long. target spin asymmetry
Interest of DVCS on the neutron A combined analysis of DVCS observables for proton and neutron targets is necessary for flavor separation of GPDs 9 ( H , E ) ( , , t ) 4 H , E ( , , t ) H , E ( , , t ) 15 u p n 9 ( H , E ) ( , , t ) 4 H , E ( , , t ) H , E ( , , t ) 15 d n p Moreover, the beam-spin asymmetry for nDVCS is the most sensitive observable to the GPD E → Ji’s sum rule for Quarks Angular Momentum Polarized beam, unpolarized target: f ~ ~ Im{ H n , H n , E n } D s f H H E ~ sin Im F F F kF d LU 1 1 2 2 Neutron Unpolarized beam, transversely polarized target: Proton f Im{ H p , E p } D s f H E ~ cos Im k ( F F ) ... d UT 2 1 The BSA for nDVCS: • is complementary to the TSA for pDVCS on transverse target, aiming at E • depends strongly on the kinematics → wide coverage needed • is smaller than for pDVCS → more beam time needed to achieve reasonable statistics
DVCS on the neutron in Hall A at 6 GeV → ed →eγ (np) F. Cano, B. Pire, Eur. Phys. J. A19 (2004) 423 ~ Ds LU ~ sin f Im {F 1 H + (F 1 +F 2 ) H -kF 2 E } M. Mazouz et al., PRL 99 (2007) 242501 S. Ahmad et al., PR D75 (2007) 094003 VGG, PR D60 (1999) 094017 Q 2 =1.9 GeV 2 and x B =0.36 NEW! Hall-A experiment E08-025 (2010) • Beam-energy « Rosenbluth » separation of • E03-106 : First-time measurement of Ds LU for nDVCS CS using an LD2 target and two nDVCS, model-dependent extraction of J u , J d different beam energies • First observation of non-zero nDVCS CS 1 1 xdx ( H ( x , , t 0 ) E ( x , , t 0 )) J • Results recently submitted for publication 2 1
E12-11-003: nDVCS on the neutron with CLAS12 at 11 GeV ~ Ds LU ~ sin f Im {F 1 H + (F 1 +F 2 ) H -kF 2 E }d f JLab PAC: high-impact experiment The most sensitive observable to the GPD E → ed → e(p)n g Projections for 90 PAC days -t 0 1.2 Fully exclusive final state: 0.2 CLAS12 +Forward Tagger +Central Neutron Detector BSA Liquid deuterium target Beam polarization =85% L = 10 35 cm −2 s −1 /nucleon -0.2 Model predictions (VGG) for different values of quarks’ angular momentum J u =.3, J d =.1 J u =.1, J d =.1 J u =.3, J d =.3 J u =.3, J d =-.1 Hall-A@6 GeV kinematics
CLAS12 Run Group B Electroproduction on deuterium with CLAS12 DIS nDVCS Elastic SIDIS Scattering + J/psi photoproduction + Short Range Correlations 2019 schedule: first part of RG B in Febuary 6th - March 25th 2019, second part in November 1st – December 19th → ~44.5 PAC days (~1/2 of approved run time) Statistics for the spring run: • 237 « good » production runs + various ancillary runs First round of preliminary • « Production » beam current: 50 nA calibrations done • ~9.7 B triggers at 10.6 GeV , 11.7 B triggers at 10.2 GeV Reconstruction ongoing • Average beam polarization ~86% (22 Moeller runs) • ~25% of the approved beam time
CLAS12 Run group B: experimental setup Forward Central RICH Detector Detector Forward Tagger CLAS12 baseline BAND Central Neutron Detector
CND: characteristics and performances with RGB data Purpose: detect the recoiling neutron in nDVCS CND design: scintillator barrel - 3 radial Requirements/performances : layers, 48 bars per layer coupled two-by-two • good neutron/photon separation for 0.2<p n <1 GeV/c downstream by a “u - turn” lightguide, 144 → ~150 ps time resolution long light guides with PMTs upstream • momentum resolution d p/p < 10% • neutron detection efficiency ~10% S.N. et al ., NIM A 904, 81 (2018) CND hits matched to charged tracks CND hits not matched to charged tracks Photons Neutrons (<p>~0.5 GeV) Background
First glance at nDVCS from RGB spring data • Very preliminary calibrations and reconstruction • 11 full runs, at both beam energies (10.6 and 10.2 GeV) • ~5% of the spring run statistics (1.25% of the approved beam time) Final state: en g reconstructed using basic CLAS12 PID • → ed→eγ n(p) • no refined PID, no fiducial cuts, no corrections • Photons are reconstructed in FT and FEC • Minimum energy 1 GeV • The highest-energy photon of the event is chosen • Neutrons are reconstructed in CND and FEC • The neutron having momentum closest to the average expected momentum for nDVCS (~0.6 GeV) is taken • nDVCS simulation on deuteron (GPD based generator) • Same event selection as for the data • Helps determine optimal detection topology and exclusivity cuts
Kinematics ( q vs p): electron, neutrons, photons Base cuts: • Q 2 >1 GeV 2 DATA MC • q (e)>5 ° • p(e)>1 GeV • vz(e) cut • p n >0.3 GeV • No other charged particles detected A lot of EC neutrons, at high p nDVCS neutrons mainly in CND A lot of low-energy nDVCS photons mainly in photons, mainly in FEC FT, and with high energy
Kinematics ( q vs p): electron, neutrons, photons Base cuts: • Q 2 >1 GeV 2 DATA MC • q (e)>5 ° • p(e)>1 GeV • vz(e) cut • p n >0.3 GeV • No other charged particles detected • Neutrons in CND E g >2 GeV cut nDVCS photons mainly in to remove FT, and with high energy background
Exclusivity variables Base cuts + E g >2 GeV E(x) ( ed→e’n g X) (GeV) M(X) 2 ( ed→e’n g X) (GeV 2 ) en g events with CND neutron en g events with CND neutron, FT photon p(x) ( ed→e’n g X) (GeV) M(X) 2 ( en→e’n’ g X) (GeV 2 ) Df ( ° ) Cone angle qg X ( ° )
en g yield vs f , after exclusivity cuts Very preliminary Q 2 (GeV 2 ) Very preliminary Data exclusivity cuts: nDVCS MC 0<M(X) 2 <2 GeV 2 0<E(X)<2 GeV 0<p(X)<1 GeV -2 ° < Df <2 ° -1< D t<1 GeV 2 -t (GeV 2 ) x B To-do list: • Refine calibrations • Reconstruct all data • Data, all photons Data Refine exclusivity cuts Data, photons in FT nDVCS MC • Study other topologies (FD…) • Beam-helicity asymmetry 0 background • subtraction • .... f ( ° ) f ( ° )
Future experiment: nDVCS, target-spin asymmetry First time measurement of longitidunal target-spin asymmetry and double (beam-target) spin asymmetry ~ ~ Ds UL ~ sin f Im {F 1 H + (F 1 +F 2 )( H + x B /2 E ) – kF 2 E+… } ~ ~ Ds LL ~ ( A+Bcos f Re {F 1 H + (F 1 +F 2 )( H + x B /2 E ) – kF 2 E+… } → 3 observables ( including BSA), constraints on real and imaginary CFFs of various neutron GPDs eND 3 → e(p)n g CLAS12 + Longitudinally polarized target + CND L = 3/20∙10 35 cm -2 s -1 Run time = 40 days Will run in 2021 P t = 0.4; P b = 0.85
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