Deeply virtual  0 electroproduction measured with CLAS

Silvia Niccolai for the CLAS Collaboration BARYONS07, Seoul, Introduction and motivation The e1-dvcs experiment Channel selection Kinematical coverage Preliminary cross sections Beam-spin asymmetry Conclusions and outlook

Deeply virtual  0 electroproduction e’ e 00  leptonic plane hadronic plane p’ Q 2 = - (e-e’) 2 x B = Q 2 /2M  =E e -E e’ t = (p-p’) 2  (angle between leptonic and hadronic plane) =  (Q 2,x B ) d4d4 dQ  dx B d  dt d2d2 d  dt reduced cross section for  *p→p  0 d  TT dt +  d2d2 d  dt = 1 22 ( dTdT dt dLdL +  d  LT dt +√ 2  +1) cos  cos2  +h√ 2  -1) d  L’T dt sin  Deeply virtual exclusive reactions: access to the structure of the nucleon Varying Q 2 (size of the probe) and t (transverse size of the target object) → explore transition between hadronic and partonic mechanisms

Hard exclusive meson production and GPDs 4 Generalized Parton Distributions (GPDs) H H conserve nucleon helicity E E flip nucleon helicity Vector mesons (  Pseudoscalar mesons (  ~ ~ ρ0ρ0 2u+d ω 2u-d ρ+ρ+ u-d quark flavor decomposition accessible via meson production x+ξ x-ξ t π, ρ, ω… (Q 2 ) e e’ L*L* Factorization proven only for longitudinally polarized virtual photons and valid at high Q 2 and small t 00 2  u+  d  2  u-  d H, H, E, E (x,ξ,t) ~ ~  0 )~ | ∫ dx H(x, ,t) | 2 ~

Continuous Electron Beam Accelerator Facility Jefferson Laboratory Newport News, USA Hall B E max = 6 GeV CEBAF Large Acceptance Spectrometer I max ~ 200  A Duty Factor ~ 100%  E /E ~ Beam Pol ~ 80%

The CLAS detector Toroidal magnetic field (6 supercond. coils) Drift chambers (argon/CO 2 gas, cells) Time-of-flight scintillators Electromagnetic calorimeters Cherenkov Counters (e/  separation) Performances: large acceptance for charged particles 8° 0.3 GeV/c, p  >0.1GeV/c good momentum and angular resolution  p/p ≤0.5%- 1.5%,  ≤ 1 mrad

The e1-dvcs experiment at CLAS Experiment: March - May 2005 E e = 5.77 GeV Polarization: 76% - 82% Current: nA Integrated luminosity: 3.33· 10 7 nb -1 ~ 7 TBytes of data Main goal of the experiment: measurement of DVCS (see talk by H.S. Jo)

Additions to CLAS for e1-dvcs Standard CLAS acceptance for photons: Inner calorimeter: IC solenoid CLAS Superconducting solenoid magnet (shielding for Moeller electrons)

Channel selection e’ p’   ep→e’p’  0 (  0 →  ) in CLAS 00 electron ID:  EC, CC, DC and TOF proton ID:  DC and TOF photon ID:  IC or EC DC EC CC IC TOF All 4 particles are detected

00 00   p Exclusivity cuts  3  cuts (for each 4-dim bin) on:  invariant mass ep missing mass squared e  0 missing mass ep  0 missing mass squared MM 2 (ep) MM(e   ) MM 2 (ep   ) IM(  ) M(GeV/c 2 ) MM 2 (ep))  -t (GeV 2 /c 2 ) DATA MC M 2 (GeV 2 /c 4 ) M(GeV/c 2 ) M 2 (GeV 2 /c 4 )

00 00   p Exclusivity cuts MM 2 (ep) MM(e   ) MM 2 (ep   ) IM(  ) M 2 (GeV 2 /c 4 ) M(GeV/c 2 ) 00 00 p

00 00   p Background subtraction Background subtraction: side-band method on M(  ) for each Q 2, x B, t,  bin  (°) Black: events within ±3σ from the mean Red: events from side bands (each 3σ wide) GeV/c 2 Invariant mass  Q 2 = 2.25 GeV 2 /c 2 x B = t = 0.12 GeV 2 /c 2

Kinematical coverage and binning W > 2 GeV/c 2 7 Q 2 bins ( GeV 2 /c 2 ) 7 x B bins (0.1 – 0.55) 8 -t bins (0.09 – 2 GeV 2 /c 2 ) 20  bins

Reduced cross sections vs  Preliminary 0.4<-t<0.6 (GeV/c) 2 Arbitrary units Statistical errors only  *p→p  0 d 2  /dtd 

Reduced cross sections vs  Preliminary 0.6<-t<1.0 (GeV/c) 2 Arbitrary units Statistical errors only  *p→p  0 d 2  /dtd 

Reduced cross sections: fits Each Q 2 -x B -t bin is fitted with the function: A+B√2  (  +1) cos  + C  cos2  d  TT dt +  dd d  dt = 1 22 ( dTdT dt dLdL +  d  LT dt +√ 2  +1) cos  cos2  NO  T /  L separation (°)(°)

Preliminary Arbitrary units Statistical errors only  T +  L vs. –t (GeV 2 /c 2 )

Preliminary Arbitrary units Statistical errors only  LT vs. –t (GeV 2 /c 2 )

Preliminary Arbitrary units Statistical errors only  TT vs. –t (GeV 2 /c 2 )

Q 2 = 2.25 (GeV/c) 2, x B = 0.34  T +  L  LT  TT Arbitrary units Statistical errors only Interference terms different from zero: transverse and longitudinal contributions Preliminary

Fit function: A√t-t min e -bt  T +  L vs. –t (GeV 2 /c 2 ) Preliminary

t-slopes b ~ constant as a function of Q 2 b decreasing as a function of x b  T +  L ~A√t-t min e -bt b ~ constant as a function of Q 2 Preliminary

t-slopes b decreasing as a function of x b Valence (high x) quarks at the center (small b) Sea (small x) quarks at the perifery (high b) IF GPD formalism applies!! y x z Guidal, Polyakov, Radyushkin, Vanderhaeghen (2005) b (fm) x

Model predictions J. M. Laget, Regge-inspired model Dashed line:  b 1 exchange (a) and elastic rescattering (b) Full line: (a), (b), and , , N intermediate states p(  *,  0 )p  T +  L  LT  TT Q 2 =2.3 (GeV/c) 2 W=2.269 GeV/c 2 x b =0.36 d  /dt (  b GeV 2 /c 2 ) -t (GeV 2 /c 2 )

Model predictions Fairly good description of the data p(  *,  0 )p  T +  L  LT  TT Q 2 =2.3 (GeV/c) 2 W=2.269 GeV/c 2 x b =0.36 d  /dt (  b GeV 2 /c 2 ) -t (GeV 2 /c 2 )  T +  L  LT  TT J. M. Laget, Regge-inspired model

Rita De Masi’s & Bo Zhao’s analysis d  TT dt +  dd d  dt = 1 22 ( dTdT dt dLdL +  d  LT dt +√ 2  +1) cos  cos2  +h√ 2  -1) d  L’T dt sin   (°) Beam-spin asymmetry BSA fit:  sin 

Beam-spin asymmetry Rita De Masi’s & Bo Zhao’s analysis  vs. t  ≈ 0.08 d  L’T dt non zero

Conclusions and outlook ep →e’p’   measured for the first time at W>2 and over a wide kinematical range with CLAS Preliminary  *p→   p differential cross sections extracted  L /  T separation not possible (accessible via Rosenbluth method with 12 GeV upgrade of CEBAF) Transverse contribution not negligible (from cross sections and BSA) Laget model predictions give good qualitative description of the data Final cross sections available soon, work in progress…