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 (e, e 0 p) reaction was studied on targets of C, Fe, and Au at momentum transfers squared Q 2 of 0.6, 1.3, 1.8, and 3.3 GeV 2 in a region of kinematics dominated by quasifree electron-proton scattering. Missing energy and missing momentum distributions are reasonably well described by plane wave impulse approximation calculations with Q 2 and A dependent corrections that measure the attenuation of the final state protons. [S0031-9007 (98) The (e, e 0 p) reaction with nearly free electron-proton kinematics (quasifree) has proven to be a valuable tool to study the propagation of nucleons in the nuclear medium [1][2][3]. The relatively weak interaction of the electron with the nucleus allows the electrons to penetrate the nuclear interior and knock out protons. These studies complement nucleon-induced measurements of proton propagation in nuclei which give more emphasis to the nuclear surface. This paper reports the first results of a systematic study of the quasifree knockout of protons of 300-1800 MeV kinetic energy from carbon, iron, and gold targets. This energy range includes the minimum of the nucleon-nucleon (N-N) total cross section, the rapid rise in this cross section with energy above the pion production threshold, and extends to the long plateau in the energy dependence of the N-N total cross section. These features of the N-N interaction would be expected to be reflected in the energy dependence of attenuation of protons as they pass 5072 0031-9007͞98͞80(23)͞5072(5)$15.00

A newly obtained sample of inclusive electron-nucleon scattering data has been analyzed for precision tests of quark-hadron duality. The data are in the nucleon resonance region, and span the range 0.3 , Q 2 , 5.0 ͑GeV͞c͒ 2 . Duality is observed both in limited and extended regions around the prominent resonance enhancements. Higher twist contributions to the F 2 structure function are found to be small on average, even in the low Q 2 regime of ഠ0.5 ͑GeV͞c͒ 2 . Using duality, an average scaling curve is obtained. In all cases, duality appears to be a nontrivial property of the nucleon structure function.PACS numbers: 13.60. Hb, 12.38.Qk The interpretation of the resonance region in inclusive electron-proton scattering and its possible connection with deep inelastic scattering has been a subject of interest for nearly three decades since quark-hadron duality ideas, which successfully described hadron-hadron scattering [1], were first extended to electroproduction. Bloom and Gilman [2] showed that it was possible to equate the nucleon resonance region structure function nW 2 ͑n, Q 2 ͒ (at some typically low Q 2 value) to the structure function F 2 in the deep inelastic regime of electron-quark scattering (at some higher value of Q 2 ). These structure functions are obtained from inclusive electron-nucleon scattering where the substructure of the nucleon is probed with virtual photons of mass-squared 2Q 2 and energy n. The resonance structure function was demonstrated to be equivalent in average to the deep inelastic one, with these averages obtained over the same range in a scaling variable v 0 1 1 W 2 ͞Q 2 , where W is the invariant mass. Bloom and Gilman's quark-hadron duality qualitatively explained the data in the range 1 # Q 2 # 10 ͑GeV͞c͒ 2 . This relationship between resonance electroproduction and the scaling behavior observed in deep inelastic scattering suggests a common origin for both phenomena. Inclusive deep inelastic scattering on nucleons is a firmly established tool for the investigation of the quark-parton model. At large enough values of W and Q 2 , quantum chromodynamics (QCD) provides a rigorous description of the physics that generates the Q 2 behavior of the nucleon structure function F 2 nW 2 . The well-known logarithmic scaling violations in the F 2 structure function of the nucleon, predicted by asymptotic freedom, played a crucial role in establishing QCD as the accepted theory of strong interactions [3,4]. However, as Q 2 decreases, the description of the nucleon's structure cannot be expressed in terms of single parton densities with simple logarithmic behavior in Q 2 . Inverse power violations in Q 2 , physically representing initial and final state interactions between the struck quark and the remnants of the target (termed higher twist effects), must be taken into account as well.An analysis of the resonance region in terms of QCD was first presented in [5,6], where Bloom and Gilman's approach was reinterpreted, and the integrals of the average scaling curves were equated to the ...