Timelike Compton Scattering with CLAS12 at Jefferson Lab Pierre - PowerPoint PPT Presentation

Timelike Compton Scattering with CLAS12 at Jefferson Lab Pierre Chatagnon Institut de Physique Nucleaire d’Orsay For the CLAS Collaboration chatagnon@ipno.in2p3.fr Palaiseau, September 17th, 2019 Pierre Chatagnon (IPNO) TCS with CLAS12 at JLab September 17th, 2019 1 / 15

From D eeply V irtual C ompton S cattering to T imelike C ompton S cattering DVCS ( γ ∗ p → γ p ) TCS ( γ p → γ ∗ p ) C ompton F orm F actors (CFF) P � 1 � � q e 2 − 1 dxH q ( x , ξ, t ) � 1 1 � + i π [ H q ( ξ, ξ, t ) − H q ( − ξ, ξ, t )] H = � ξ − x − q ξ + x Imaginary part Measured in DVCS asymmetries Accessible in TCS photon polarization asymmetry Real part Accessible in DVCS cross section Accessible in TCS in cross section angular modulation Pierre Chatagnon (IPNO) TCS with CLAS12 at JLab September 17th, 2019 2 / 15

Physics Motivations The CFFs dispersion relation at leading-order and leading twist : � 1 � � 1 1 Re H ( ξ, t ) = P ξ − x − Im H ( ξ, t )+ D ( t ) dx ξ + x − 1 D-term expansion � 1 D ( t ) = 1 dz D ( z , t ) Nature (2018) Burkert, Elouadrhiri, Girod 2 1 − z − 1 D ( z , t ) = (1 − z 2 )[ d 1 ( t ) C 3 / 2 ( z ) + ... ] 1 d 1 ( t ) is directly related to the pressure distribution in the nucleon. Measurement of photon polarization asymmetry will provide a test of universality of GPDs. DVCS phase space TCS phase space Bo¨ er, Guidal, Vanderhaeghen (2015) Pierre Chatagnon (IPNO) TCS with CLAS12 at JLab September 17th, 2019 3 / 15

TCS and Bethe-Heitler γ p → e + e − p γ p → e + e − p γ p → e + e − p Bethe-Heitler (Form Factors) TCS (GPDs) TCS cross section d 4 σ dQ ′ 2 dtd Ω = σ TCS + σ BH + σ INT TCS cross section not large enough to allow meaningful measurement Use interference term to access GPDs Berger, Diehl and Pire (2002) Pierre Chatagnon (IPNO) TCS with CLAS12 at JLab September 17th, 2019 4 / 15

γ p → e + e − p kinematics Q ′ 2 = ( k + k ′ ) 2 t = ( p ′ − p ) 2 L = ( Q ′ 2 − t ) 2 − b 2 L 0 = Q ′ 4 sin 2 θ b = 2( k − k ′ )( p − p ′ ) 4 4 Q ′ 2 Q ′ 2 t 0 = − 4 ξ 2 M 2 τ s = ( p + q ) 2 τ = 2 p · q = ξ = s − M 2 (1 − ξ 2 ) 2 − τ Pierre Chatagnon (IPNO) TCS with CLAS12 at JLab September 17th, 2019 5 / 15

γ p → e + e − p Cross section and CFFs Interference cross section d 4 σ INT − α 3 cos ( φ )1 + cos 2 ( θ ) 1 m p 1 L 0 Re ˜ em Re ˜ Re ˜ M −− M −− M −− = τ √ 1 − τ L [ cos ( φ ) cos ( φ ) + ... ] dQ ′ 2 dtd Ω 4 π s 2 − t Q ′ sin ( θ ) M −− = 2 √ t 0 − t 1 − ξ t � � → ˜ H − ξ ( F 1 + F 2 ) ˜ ˜ ˜ F 1 H H H H H − 4 M 2 F 2 E E E M 1 + ξ BH cross section dQ ′ 2 dtd Ω ≈ − α 3 d 4 σ BH 1 + cos 2 ( θ ) � ∆ 2 � 1 2 ) 2 t em ( F 2 4 M 2 F 2 − t + ( F 1 + F 2 ) 2 T 1 − sin 2 ( θ ) 2 π s 2 − t sin 2 ( θ ) sin 2 ( θ ) τ 2 BH cross section diverges at θ ≈ 0 ◦ and 180 ◦ Weighted cross section ratio � 2 π � 3 π/ 4 cos ( φ ) dS d φ cos ( φ ) cos ( φ ) R ( √ s , Q ′ 2 , t ) = dS d θ L d σ dQ ′ 2 dtd φ 0 dQ ′ 2 dtd φ = � 2 π L 0 dQ ′ 2 dtd φ d θ dS d φ π/ 4 dQ ′ 2 dtd φ 0 Pierre Chatagnon (IPNO) TCS with CLAS12 at JLab September 17th, 2019 6 / 15

CLAS12 at Jlab Central Detector ◮ Time-of-Flight (CTOF) ◮ Tracking (SVT and MM) ◮ Neutron detector (CND) Forward Detector ◮ Drift Chambers (DC) ◮ Time-of-Flight (FTOF) ◮ Calorimeters (Pre-Shower Calorimeter/2 layer EC) ◮ Cherenkov Counters (HTCC and LTCC) ◮ RICH ◮ Forward tagger (FT) Data Set First CLAS12 experiment, data were taken in the Spring and Fall 2018 Beam energy 10.56 GeV / Liquid hydrogen target Two torus magnetic field configurations (Inbending/Outbending electrons) Total accumulated charge in the Faraday cup for data shown here : 18 mC ∼ 3% of the proposed total data (100 days at 75nA). Total taken data corresponds to 50% of total proposed data Pierre Chatagnon (IPNO) TCS with CLAS12 at JLab September 17th, 2019 7 / 15

Data analysis ep → e ′ γ p → ( e ′ ) e + e − p ′ Final state Scattered electron Incoming photon The real photon is Use the CLAS12 Cut on scattered radiated by the beam reconstruction electron missing mass electron software PID Cut on missing Cuts on scattered Events with exactly transverse momentum electron constrain the one e + ,one e − and of ep → e + e − pX virtuality of the one proton are system photon selected Q 2 ∝ cos (Θ scattered ) Pierre Chatagnon (IPNO) TCS with CLAS12 at JLab September 17th, 2019 8 / 15

e + e − pX final state selection Protons Matching β calculated from Time-Of-Flight and momentum from tracking Leptons Number of Cherenkov photons > 2 Minimum energy deposited in the Pre-Shower Calorimeter (PCAL) Cuts on total calorimeters sampling fractions ( E deposited / p )) Pierre Chatagnon (IPNO) TCS with CLAS12 at JLab September 17th, 2019 9 / 15

Exclusivity cuts Scattered electron: p µ scattered e − = p µ beam + p µ target − p µ proton − p µ e + − p µ e − Pierre Chatagnon (IPNO) TCS with CLAS12 at JLab September 17th, 2019 10 / 15

Lepton-pair spectrum 3% of total proposed data Low e + e − invariant mass spectrum is dominated by vector meson photoproduction → Mass cut between the ρ region [ ρ (1450 MeV ) and ρ (1700 MeV ) ] and J /ψ (3 GeV ) → The resonance-free mass region between 2 GeV and 3 GeV will be used for the analysis Pierre Chatagnon (IPNO) TCS with CLAS12 at JLab September 17th, 2019 11 / 15

Projected results Experimental cross section φ modulation ratio � � 2 π cos ( φ ) Y φ R ( √ s , Q ′ 2 , t ) = dS d φ cos ( φ ) R ′ = dQ ′ 2 dtd φ φ L 0 N φ 1 where Y φ = � L 0 → � 2 π � θ A φ dS Y φ d φ dQ ′ 2 dtd φ θ θ 0 φ Estimate of CLAS12 acceptance with BH simulation Acceptance in the θ/φ plane ( A φ θ = N REC N GEN ) → Yellow lines are CLAS12 acceptance limits → Cut regions correspond to events where one lepton goes in the beam pipe (BH peaks are out of CLAS12 acceptance) Pierre Chatagnon (IPNO) TCS with CLAS12 at JLab September 17th, 2019 12 / 15

Projected results Generator developed by R. Paremuzyan at Jefferson Lab. → Double distribution GPD parametrization H ( x , ξ, t ) = H DD ( x , ξ, t ) + κ 1 N f Θ( ξ − | x | ) D ( x ξ , t ) R ′ is sensitive to D-term strength within CLAS12 acceptance. Full data set ( 50% of total proposed data) will provide enough statistics to give insight on D-term strengh (green points and associated error bars). Pierre Chatagnon (IPNO) TCS with CLAS12 at JLab September 17th, 2019 13 / 15

Projected results R ′ is sensitive to D-term strength BUT also depends on acceptance limits → difficulties to compare measurement with theoretical models Possibility to restore θ dependence of the interference cross-section We want to access the φ moment of the cross section. We can measure : � a ( φ ) d θ d 4 σ TOT dS TOT L dS BH dS INT dQ ′ 2 dtd φ = L 0 = dQ ′ 2 dtd φ + dQ ′ 2 dtd Ω dQ ′ 2 dtd φ b ( φ ) dS BH dQ ′ 2 dtd φ is calculable from form factors. The θ/φ dependance of the interference term is fully known : � a ( φ ) dQ ′ 2 dtd φ = − α 3 1 1 dS INT m p M −− + ... ] em (1 + cos 2 ( θ )) d θ · Re ˜ τ √ 1 − τ [ cos ( φ ) 4 π s 2 − t Q ′ b ( φ ) This method will be implemented at a later stage of the analysis, as it requires good accumulated luminosity estimation. Pierre Chatagnon (IPNO) TCS with CLAS12 at JLab September 17th, 2019 14 / 15

Conclusion Timelike Compton Scattering allows to investigate the real part of CFFs which is difficult to constrain with DVCS. No published results on TCS yet. Main resonances in the e + e − spectrum visible in CLAS12 data. Projected statistic will allow insight on the strength of the D-term. Outlook The analysis procedure leading to R’ has been developed. More statistics is coming from the data processing of the 2018 run. Dependence on acceptance limits of R ′ will be corrected to allow comparison with models and future TCS measurements. Pierre Chatagnon (IPNO) TCS with CLAS12 at JLab September 17th, 2019 15 / 15