Deep exclusive π+ electroproduction off the proton at CLAS
0
INFN, Sezione di Genova, 16146 Genova,
Italy
1
INFN, Sezione di Ferrara, 44100 Ferrara,
Italy
2
INFN, Laboratori Nazionali di Frascati, 00044 Frascati,
Italy
3
The George Washington University
,
Washington, D.C. 20052, USA
4
Idaho State University
, Pocatello,
Idaho 83209, USA
5
Florida State University
, Tallahassee,
Florida 32306, USA
6
Universit`a di Genova
, 16146 Genova,
Italy
7
Fairfield University
, Fairfield Connecticut 06824,
USA
8
Florida International University
, Miami,
Florida 33199, USA
9
Institut de Physique Nucleaire ORSAY
, Orsay,
France
10
INFN, Sezione di Roma Tor Vergata,
00133 Rome, Italy
11
Institute of Theoretical and Experimental Physics
,
Moscow
, 117259,
Russia
12
James Madison University
, Harrisonburg,
Virginia 22807, USA
13
Kyungpook National University
, Daegu 702-701,
Republic of Korea
14
LPSC,
Universite Joseph Fourier
, CNRS/IN2P3, INPG, Grenoble,
France
15
University of New Hampshire
,
Durham, New Hampshire 03824-3568, USA
16
Norfolk State University
, Norfolk,
Virginia 23504, USA
17
California State University
, Dominguez Hills, Carson,
California 90747, USA
18
Arizona State University
, Tempe,
Arizona 85287-1504, USA
19
Edinburgh University
, Edinburgh EH9 3JZ,
UK
20
Argonne National Laboratory
, Argonne,
Illinois 60439, USA
21
CEA,
Centre de Saclay, Irfu/Service de Physique Nucleaire
, 91191 Gif-sur-Yvette,
France
22
Catholic University of America
,
Washington, D.C. 20064, USA
23
Carnegie Mellon University
,
Pittsburgh, Pennsylvania 15213, USA
24
Canisius College
, Buffalo,
New York 14208, USA
25
University of Connecticut
, Storrs,
Connecticut 06269, USA
26
Christopher Newport University
, Newport News,
Virginia 23606, USA
27
Thomas Jefferson National Accelerator Facility
, Newport News,
Virginia 23606, USA
28
Universidad Tecnica Federico Santa Mara
, Casilla 110-V Valparaso,
Chile
29
Skobeltsyn Nuclear Physics Institute, Skobeltsyn Nuclear Physics Institute
,
119899 Moscow, Russia
30
University of South Carolina
,
Columbia, South Carolina 29208, USA
31
University of Virginia
,
Charlottesville, Virginia 22901, USA
32
Ohio University
,
Athens, Ohio 45701, USA
33
University of Glasgow
, Glasgow G12 8QQ,
UK
34
Old Dominion University
, Norfolk,
Virginia 23529, USA
35
Virginia Polytechnic Institute and State University
, Blacksburg,
Virginia 24061-0435, USA
36
Rensselaer Polytechnic Institute
, Troy,
New York 12180-3590, USA
37
Yerevan Physics Institute
, 375036 Yerevan,
Armenia
38
Universit`a di Roma Tor Vergata
,
00133 Rome, Italy
39
College of William and Mary
, Williamsburg,
Virginia 23187-8795, USA
40
University of Richmond
,
Richmond, Virginia 23173, USA
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Received: 11 June 2012 / Revised: 7 December 2012
Published online: 30 January 2013
c The Author(s) 2013. This article is published with open access at Springerlink.com
Communicated by Z.-E. Meziani
Abstract. The exclusive electroproduction of + above the resonance region was studied using the CEBAF
Large Acceptance Spectrometer (CLAS) at Jefferson Laboratory by scattering a 6 GeV continuous electron
beam off a hydrogen target. The large acceptance and good resolution of CLAS, together with the high
luminosity, allowed us to measure the cross section for the p n+ process in 140 (Q2, xB, t) bins:
0.16 < xB < 0.58, 1.6 GeV2 < Q2 < 4.5 GeV2 and 0.1 GeV2 < t < 5.3 GeV2. For most bins, the statistical
accuracy is on the order of a few percent. Differential cross sections are compared to four theoretical
models, based either on hadronic or on partonic degrees of freedom. The four models can describe the
gross features of the data reasonably well, but differ strongly in their ingredients. In particular, the model
based on Generalized Parton Distributions (GPDs) contain the interesting potential to experimentally
access transversity GPDs.
1 Introduction incoming photon Q2 = (pe pe)2, which effectively rep
resents the transverse size of the probe, or the
momenOne of the major challenges in contemporary nuclear tum transfer to the nucleon t = (pN pN )2, which
effecphysics is the study of the transition between hadronic tively represents the transverse size of the target. Here, pe
and partonic pictures of the strong interaction. At asymp- and pe are the initial and scattered electron four-momenta
totically short distances, the strong force is actually weak and pN and pN are the initial and final nucleon
fourand the appropriate degrees of freedom are the quarks momenta, respectively. Figure 1 sketches the transition
and gluons (partons) whose interaction can be quantified regions that have been experimentally explored until now
very precisely by perturbative Quantum Chromodynam- (lightly shaded areas) as a function of these two variables,
ics (pQCD). However, at larger distances on the order Q2 and |t|. In this figure, we keep, quite arbitrarily, only
of one Fermi, effective theories that take hadrons as ele- the experiments for which |t| > 3 GeV2 in
photoproducmentary particles whose interactions are described by the t (...truncated)