Deep exclusive electroproduction ofπ+from data measured with the HERMES detector at DESY

Abstract

Deeply virtual electroproduction of pions in an exclusive reaction p(e,e π + )n is studied using a two-component model that includes soft hadronic and hard partonic reaction mechanisms. The results are compared with the experimental data measured at HERMES in the deep inelastic region for values of Q 2 > 1 GeV 2 and W 2 > 10 GeV 2 . At forward angles the π + cross section is longitudinal and is dominated by exchange of Regge poles, with π being the dominant trajectory. The off-forward region with −t > 1 GeV 2 is transverse and shows the dominance of partonic subprocesses. An implication of the present results for the future JLab facilities is briefly discussed.Exclusive electroproduction of pions in the reactionat high values of photon virtuality Q 2 and invariant mass W of produced hadronic final state provides an interesting tool to study a space-time pattern of partonic interactions in deep inelastic scattering (DIS). It may further reveal the partonic substructure of participating hadrons, correlating the longitudinal momentum fraction carried by quarks to transverse coordinates. This latter property of the hard exclusive reaction N (e,e π )N follows from the QCD factorization theorem that was proven for hard Q 2 2 QCD electroproduction of mesons by longitudinal photons γ * L [1]. Predictions for the production by transverse virtual photons γ * T are absent because no factorization theorem has been proven for such photons. However, their contribution to the cross section is expected to be suppressed by at least a power of 1/Q 2 . Yet, above the resonance region W 2 > 4 GeV 2 the exclusive reaction (γ * ,π ± ) with charged pions provides important information concerning the electromagnetic form factor of the pion at momentum transfer Q 2 being much bigger than in the direct scattering of pions from atomic electrons [2]. Experimentally, the differential cross sections in the exclusive reaction p(γ * ,π + )n has been measured above the resonance region at CEA [3], Cornell [4], DESY [5], and recently at JLab [6]. At JLab a separation of cross sections into different transverse and longitudinal components has been carried out. The HERMES data at DESY [7] largely extend the kinematic region to much higher values of W toward the true DIS region Q 2 1 GeV 2 and much higher values of −t. Theoretically, there is a long-standing issue concerning the reaction mechanisms contributing to the single pion N(e,e π )N production at high energies and photon virtualities. Just above the resonance region around the onset of the deep inelastic regime the models describing the exclusive pion production p(e,e π + )n in terms of hadronic degrees of freedom fail to reproduce the large transverse cross section σ T observed in this reaction. For instance, the hadron-exchange * murat.kaskulov@theo.physik.uni-giessen.de models, which are generally considered to be a guideline for the experimental analysis and extraction of the pion form factor, underestimate grossly σ T at the highest values of Q 2 measured at JLab [6], whereas the longit...

Deep exclusive π+ electroproduction off the proton at CLAS

“…Figures 13 and 14 show the results of this model compared to our data where very good agreement is found. This calculation was also found to give a good description of the L/T-separated Hall C and unseparated HERMES data [45,46].…”

“…In fig. 17, we compare our data to the Laget [42] and the Kaskulov et al [45,46] models. The Laget calculation gives a reasonable description of the data although it seems to have a slightly steeper Q 2 dependence than our data (particularly in the x B =0.37 bin).…”

“…The second model with which we compare our data is the "hybrid" two-component hadron-parton model proposed in refs. [45,46]. Like in the Laget model, it is based on the exchange of the π + and ρ + Regge trajectories in the t-channel.…”

Charged-pion electroproduction above the resonance region

“…The framework developed in Refs. [6,7] deals with the N(e, e π ± )N reaction at the cross section level. In Ref.…”

Hadrons in Nuclear Matter

“…It turned out that the available, widely used model for the calculation of elementary electroproduction of pions on nucleons was seriously deficient: while the longitudinal part of the cross section was described very well, the transverse part was significantly underestimated (by about 1 order of magnitude!). We have, therefore, developed a new model that describes both reaction components over a very wide kinematical range extremely well [17,21,22]. In particular, the new model is able to describe the observed onset of transverse strength with increasing momentum transfer Q 2 , which is in contradition to simple pQCD based expectations for exclusive reaction channels.…”