Backward meson production at CLAS

Transcription

1 Backward meson production at CLAS Alex Kubarovsky (RPI/Uconn) Kyungseon Joo (Uconn) Valery Kubarovsky (Jlab) Paul Stoler (RPI) Exclusive Meson Production and Short-Range Hadron Structure Jefferson Lab, January 22-24, 2015

2 Outline: Ø Physics mocvacon Ø CLAS spectrometer Ø Data analysis Ø DifferenCal cross seccons Ø Conclusion 2

3 The transicon distribucon amplitudes Generalized Parton DistribuCons (GPDs) offer a new way to access the quark and gluon nucleon structure. The nucleon to meson transicon distribucon amplitudes (TDAs) are extension of the GPD concept to three quark operators and the relevance of their nucleon to meson matrix elements which factorize in backward meson electroproduccon. J. P. Lansberg, B. Pire, L. Szymanowski Phys.Rev. D75 (2007) , Erratum- ibid. D77 (2008)

4 The leading- twist TDAs for the p π transicon is defined as: J. P. Lansberg, B. Pire, K. Semenov- Tian- Shansky, L. Szymanowski arxiv: [hep- ph] Phys.Rev. D85(2012)

5 Processes involving the TDAs γ*p pπ at high - t p π baryonic TDAs appear in the descripcon of backward electroproduccon of a π on a proton target. J. P. Lansberg, B. Pire, K. Semenov- Tian- Shansky, L. Szymanowski Phys.Rev. D84 (2011)

6 CLAS e1-6a experiment Ø Data taken years Ø Electron beam energy GeV Ø Beam current - 7nA Ø 5cm long liquid hydrogen target Ø Total accumulated charge - 21 mc Ø Torus current 3375 A VisualizaCon of the typical ep e pπ event on CLAS Event Display 6

7 Binning and kinemacc region Rectangular (Q 2,x B ) bins are used Ø Q 2 : 3 bins from 1.5 to 4.5 GeV 2 Ø x B : 3 bins from 0.15 to 0.60 Ø - t: 5 bins from up to 6.0 GeV 2 Ø W > 2.0 GeV (non- resonant region) 7

8 Events seleccon Ø Events with 1 electron, 1 proton and 2 photons Ø Electron hardware trigger, CC, EC, DC Ø Proton TOF, DC Ø Two photons EC, E γ > 0.5 GeV Ø W > 2 GeV Ø Exclusivity cuts: ü Angle between X(ep) and π cut ü M x 2 (ep) missing mass cut (3σ) ü Missing energy cut (3σ) ü M inv (γγ) invariant mass cut (3σ) ep e pπ γγ M(γγ)= GeV/c σ = GeV/c N events ~ η 8

9 Acceptance calculacon Ø RadiaCve generator aao_rad, ~ 10 8 events Ø GSIM Geant- 3 simulacon Ø RECSIS events reconstruccon Ø Analysis code 9

10 Data yield The following correccons were applied: Ø Electron momentum and angular correccons Ø Proton angular correccon Ø Proton energy loss correccon Ø Fiducial cuts 10

11 dσ/dt (γ*p pπ ) differencal cross seccon dσ = dt Γ( Q 2, x B ΔN 2 ) ΔQ Δx B Δt L ε R where Ø Γ (Q2,xB) virtual photon flux Ø L integrated luminosity Ø ε acceptance Ø R bin volume correccon dσ = dt A e Bt 11

12 t slope parameter B vs Q 2 and x B d dt σ = A Bt e t-slope parameter B is around 1GeV 2 B 0 when x B 1 B is almost independent of Q 2 that is the indication of scaling onset. 12

13 Comparison with theoreccal predisccons dσ dt, nb GeV CZ,BLWNNLO Q 2 =2 GeV 2 x B = t, GeV 2 dσ dt, nb GeV CZ,BLWNNLO Q 2 =3 GeV 2 x B = t, GeV 2 dσ dt, nb GeV CZ,BLWNNLO Q 2 =4 GeV 2 x B = t, GeV 2 Black CLAS data Red TDA model with Chernyak-Zhitinsky nucleon DAs as input Orange TDA model with Braun-Lenz-Wittmann nucleon DAs as input Comparison with theoreccal calculacons (K. Semenov- Tyan- Shansky) shows that magnitude seems to be correct. However data does not follow predicted dependence at high values of t. V.L. Chernyak and I.R. ZhiCnsky (IYF) Nucl.Phys. B246(1984) V.M. Braun, A. Lenz and M. Wi{mann (Regensburg U.) Phys.Rev. D73(2006)

14 Conclusion Ø CLAS has potencal to probe nucleon to meson transicon distribucon amplitudes (TDAs) in backward meson electroproduccon. Ø Independence of the t-slope parameter as a function of Q 2 is the indication of scaling onset. Ø TheoreCcal models show reasonable order of magnitude cross seccon Ø Cross seccon is small. More data needed in the region of high t. We can add e1f and e1- dvcs data sets to this analysis. Thank you 14