The 1 H e; e 0 n cross section was measured at four-momentum transfers of Q 2 1:60 and 2:45 GeV 2 at an invariant mass of the photon nucleon system of W 2:22 GeV. The charged pion form factor (F ) was extracted from the data by comparing the separated longitudinal pion electroproduction cross section to a Regge model prediction in which F is a free parameter. The results indicate that the pion form factor deviates from the charge-radius constrained monopole form at these values of Q 2 by one sigma, but is still far from its perturbative quantum chromodynamics prediction. DOI: 10.1103/PhysRevLett.97.192001 PACS numbers: 14.40.Aq, 11.55.Jy, 13.40.Gp, 25.30.Rw A fundamental challenge in nuclear physics is the description of hadrons in terms of the constituents of the underlying theory of strong interactions, quarks, and gluons. Properties such as the total charge and magnetic moments are well described in a constituent quark framework, which effectively takes into account quark-gluon interactions. However, charge and current distributions, which are more sensitive to the underlying dynamic processes, are not well described.Hadronic form factors provide important information about hadronic structure. The coupling of a virtual photon to structureless particles is completely determined by their charge and magnetic moments. However, for composite particles one must account for the internal structure, which is accomplished by momentum transfer dependent functions. Examples of these functions are the electromagnetic form factors, which describe the distribution of charge and current.One of the simplest hadronic systems available for study is the pion, whose valence structure is a bound state of a quark and an antiquark. The electromagnetic structure of a spinless particle such as the pion is parametrized by a single form factor. Asymptotically, the pion charge form factor, F , is given in perturbative quantum chromodynamics (pQCD) [1]:

The charged pion form factor, F π (Q 2 ), is an important quantity that can be used to advance our knowledge of hadronic structure. However, the extraction of F π from data requires a model of the 1 H(e, e π + )n reaction and thus is inherently model dependent. Therefore, a detailed description of the extraction of the charged pion form factor from electroproduction data obtained recently at Jefferson Lab is presented, with particular focus given to the dominant uncertainties in this procedure. Results for F π are presented for Q 2 = 0.60-2.45 GeV 2 . Above Q 2 = 1.5 GeV 2 , the F π values are systematically below the monopole parametrization that describes the low Q 2 data used to determine the pion charge radius. The pion form factor can be calculated in a wide variety of theoretical approaches, and the experimental results are compared to a number of calculations. This comparison is helpful in understanding the role of soft versus hard contributions to hadronic structure in the intermediate Q

Cross sections for the reaction 1 H(e, e π + )n were measured in Hall C at Thomas Jefferson National Accelerator Facility (JLab) using the high-intensity Continuous Electron Beam Accelerator Facility (CEBAF) to determine the charged pion form factor. Data were taken for central four-momentum transfers ranging from Q 2 = 0.60 to 2.45 GeV 2 at an invariant mass of the virtual photon-nucleon system of W = 1.95 and 2.22 GeV. The measured cross sections were separated into the four structure functions σ L , σ T , σ LT , and σ TT . The various parts of the experimental setup and the analysis steps are described in detail, including the calibrations and systematic studies, which were needed to obtain high-precision results. The different types of systematic uncertainties are also discussed. The results for the separated cross sections as a function of the Mandelstam variable t at the different values of Q 2 are presented. Some global features of the data are discussed, and the data are compared with the results of some model calculations for the reaction 1 H(e, e π + )n.

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