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The instrumentation in Hall A at the Thomas Jefferson National Accelerator Facility was designed to study electro-and photo-induced reactions at very high luminosity and good momentum and angular resolution for at least one of the reaction products. The central components of Hall A are two identical high resolution spectrometers, which allow the vertical drift chambers in the focal plane to provide a momentum resolution of better than 2 x 10(-4). A variety of Cherenkov counters, scintillators and lead-glass calorimeters provide excellent particle identification. The facility has been operated successfully at a luminosity well in excess of 10(38) CM-2 s(-1). The research program is aimed at a variety of subjects, including nucleon structure functions, nucleon form factors and properties of the nuclear medium. (C) 2003 Elsevier B.V. All rights reserved

The ratio of the electric and magnetic form factors of the proton G(E(p))/G(M(p)), which is an image of its charge and magnetization distributions, was measured at the Thomas Jefferson National Accelerator Facility (JLab) using the recoil polarization technique. The ratio of the form factors is directly proportional to the ratio of the transverse to longitudinal components of the polarization of the recoil proton in the elastic e(-->)p---> e(-->)p reaction. The new data presented span the range 3.5< Q(2)< 5.6 GeV(2) and are well described by a linear Q(2) fit. Also, the ratio sqrt[Q(2)] F(2(p))/F(1(p)) reaches a constant value above Q(2) = 2 GeV(2).

We report the results of a new Rosenbluth measurement of the proton electromagnetic form factors at Q2 values of 2.64, 3.20, and 4.10 GeV2. Cross sections were determined by detecting the recoiling proton, in contrast to previous measurements which detected the scattered electron. Cross sections were determined to 3%, with relative uncertainties below 1%. The ratio mu(p)G(E)/G(M) was determined to 4%-8% and showed mu(p)G(E)/G(M) approximately 1. These results are consistent with, and much more precise than, previous Rosenbluth extractions. They are inconsistent with recent polarization transfer measurements of similar precision, implying a systematic difference between the techniques.

Precise proton and neutron form factor measurements at Jefferson Lab, using spin observables, have recently made a significant contribution to the unraveling of the internal structure of the nucleon. Accurate experimental measurements of the nucleon form factors are a test-bed for understanding how the nucleon's static properties and dynamical behavior emerge from QCD, the theory of the strong interactions between quarks. There has been enormous theoretical progress, since the publication of the Jefferson Lab proton form factor ratio data, aiming at reevaluating the picture of the nucleon. We will review the experimental and theoretical developments in this field and discuss the outlook for the future.PACS. PACS-key 13.40.Gp -PACS-key 13.85.Dz

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Recent measurements of the ratio of the elastic electromagnetic form factors of the proton, GEp/GMp, using the polarization transfer technique at Jefferson Lab show that this ratio decreases dramatically with increasing Q 2 , in contradiction to previous measurements using the Rosenbluth separation technique. Using this new high quality data as a constraint, we have reanalyzed most of the world ep elastic cross section data. In this paper, we present a new empirical fit to the reanalyzed data for the proton elastic magnetic form factor in the region 0 < Q 2 < 30 GeV 2 . As well, we present an empirical fit to the proton electromagnetic form factor ratio, GEp/GMp, which is valid in the region 0.1 < Q 2 < 6 GeV 2 .The elastic electromagnetic form factors are crucial to our understanding of the proton's internal structure. Indeed, the differential cross section for elastic ep → ep scattering is described completely in terms of the Dirac and Pauli form factors, F 1 and F 2 , respectively, based solely on fundamental symmetry arguments. Further, the Sachs form factors, G Ep and G Mp , which are simply derived from F 1 and F 2 , reflect the distributions of charge and magnetization current within the proton.Until recently, the form factors of the proton have been determined experimentally using the Rosenbluth separation method [3], in which one measures elastic ep cross sections at constant Q 2 , and varies both the beam energy and scattering angle to separate the electric and magnetic contributions. In terms of the Sachs form factors, the differential cross section for elastic ep scattering has traditionally been written as, θ e is the in-plane electron scattering angle. For elastic ep scattering, the so-called nonstructure cross section, σ ns is given bywhere α em is the electromagnetic coupling constant, and E ′ (E) is the energy of the scattered (incident) electron. From the measured differential cross section, one typically derives a "reduced cross section", defined according towhere ǫ = {1 + 2(1 + τ ) tan 2 (θ e /2)} −1 is a measure of the virtual photon polarization. Equation 3 is known as the Rosenbluth formula, and shows that fits to reduced cross section measurements made at constant Q 2 but varying ǫ values may be used to extract both form factors independently.With increasing Q 2 , the reduced cross sections are increasingly dominated by the magnetic term G Mp ; at Q 2 ≈ 3 GeV 2 , the electric term contributes only a few percent of the cross section. Furthermore, referring to the open data points in the left panel of Fig. 1, one can see that the various Rosenbluth separation data sets [4][5][6][7][8][9] for the ratio µ p G Ep /G Mp , where µ p = 2.79 is the magnetic moment of the proton, are not consistent with one another for Q 2 > 1 GeV 2 . It is clear that a tremendous effort has gone into the analysis of these difficult experiments, however, one is forced to speculate that some of the experiments have underestimated the systematic errors. For example, the Rosenbluth experiments apply radiative correcti...

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]: