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 momentum and helicity density distributions of the strange quark sea in the nucleon are obtained in leading order from charged-kaon production in deep-inelastic scattering on the deuteron. The distributions are extracted from spin-averaged K� multiplicities, and from K� and inclusive double-spin asymmetries for scattering of polarized positrons by a polarized deuterium target. The shape of the momentum distribution is softer than that of the average of the and quarks. In the region of measurement 0.021.0�GeV2, the helicity distribution is zero within experimental uncertainties

Multiplicities in semi-inclusive deep-inelastic scattering are presented for each charge state of pi(+/-) and K-+/- mesons. The data were collected by the HERMES experiment at the HERA storage ring using 27.6 GeV electron and positron beams incident on a hydrogen or deuterium gas target. The results are presented as a function of the kinematic quantities x(B), Q(2), z, and P-h perpendicular to. They represent a unique data set for identified hadrons that will significantly enhance our understanding of the fragmentation of quarks into final-state hadrons in deep-inelastic scattering. DOI: 10.1103/PhysRevD.87.07402

Separated longitudinal and transverse structure functions for the reaction 1 H͑e, e 0 p 1 ͒n were measured in the momentum transfer region Q 2 0.6 1.6 ͑GeV͞c͒ 2 at a value of the invariant mass W 1.95 GeV. New values for the pion charge form factor were extracted from the longitudinal cross section by using a recently developed Regge model. The results indicate that the pion form factor in this region is larger than previously assumed and is consistent with a monopole parametrization fitted to very low Q 2 elastic data. DOI: 10.1103/PhysRevLett.86.1713 The pion occupies an important place in the study of the quark-gluon structure of hadrons. This is exemplified by the many calculations that treat the pion as one of their prime examples [1][2][3][4][5][6][7][8]. One of the reasons is that the valence structure of the pion, being ͗qq͘, is relatively simple. Hence it is expected that the value of the four-momentum transfer squared Q 2 , down to which a perturbative QCD (pQCD) approach to the pion structure can be applied, is lower than, e.g., for the nucleon. Furthermore, the asymptotic normalization of the pion wave function, in contrast to that of the nucleon, is known from the pion decay.The charge form factor of the pion, F p ͑Q 2 ͒, is an essential element of the structure of the pion. Its behavior at very low values of Q 2 , which is determined by the charge radius of the pion, has been determined up to Q 2 0.28 ͑GeV͞c͒ 2 from scattering high-energy pions from atomic electrons [9]. For the determination of the pion form factor at higher values of Q 2 one has to use high-energy electroproduction of pions on a nucleon, i.e., employ the 1 H͑e, e 0 p 1 ͒n reaction. For selected kinematical conditions this process can be described as quasielastic scattering of the electron from a virtual pion in the proton. In the t-pole approximation the longitudinal cross section s L is proportional to the square of the pion form factor. In this way the pion form factor has been studied for Q 2 values from 0.4 to 9.8 ͑GeV͞c͒ 2 at CEA͞Cornell [10] and for Q 2 0.7 ͑GeV͞c͒ 2 at DESY [11]. In the DESY experiment a longitudinal͞transverse (L͞T ) separation was performed by taking data at two values of the electron energy. In the experiments done at CEA͞Cornell this was done in a few cases only, and even 0031-9007͞01͞86(9)͞1713(4)$15.00

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

The data analysis for the reaction 1 H(e, e ′ π + )n, which was used to determine values for the charged pion form factor Fπ for values of Q 2 =0.6-1.6 GeV 2 , has been repeated with careful inspection of all steps and special attention to systematic uncertainties. Also the method used to extract Fπ from the measured longitudinal cross section was critically reconsidered. Final values for the separated longitudinal and transverse cross sections and the extracted values of Fπ are presented.

Polarized deep-inelastic scattering data on longitudinally polarized hydrogen and deuterium targets have been used to determine double-spin asymmetries of cross sections. Inclusive and semi-inclusive asymmetries for the production of positive and negative pions from hydrogen were obtained in a reanalysis of previously published data. Inclusive and semi-inclusive asymmetries for the production of negative and positive pions and kaons were measured on a polarized deuterium target. The separate helicity densities for the up and down quarks and the anti-up, anti-down, and strange sea quarks were computed from these asymmetries in a "leading order" QCD analysis. The polarization of the up-quark is positive and that of the down-quark is negative. All extracted sea quark polarizations are consistent with zero, and the light quark sea helicity densities are flavor symmetric within the experimental uncertainties. First and second moments of the extracted quark helicity densities in the measured range are consistent with fits of inclusive data

Single-spin asymmetries have been measured for semi-inclusive electroproduction of pi(+), pi(-), pi(0) and K+ mesons in deep-inelastic scattering off a longitudinally polarised deuterium target. The asymmetries appear in the distribution of the hadrons in the azimuthal angle phi around the virtual photon direction, relative to the lepton scattering plane. The corresponding analysing powers in the sinphi moment of the cross section are 0.012 +/- 0.002(stat.) +/- 0.002(syst.) for pi(+), 0.006 +/- 0.003(stat.) +/- 0.002(syst.) for pi(-), 0.021 +/- 0.005(stat.) +/- 0.003(syst.) for pi(0) and 0.013 +/- 0.006(stat.) +/- 0.003(syst.) for K The sin 20 moments are compatible with zero for all particles. (C) 2003 Elsevier Science B.V All rights reserved