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Register a new userElectron-Proton Scattering at High Q^2
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Hans-Christian Schultz-Coulon
Publication date
2002
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English
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Hans-Christian Schultz-Coulon
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The $e^{+}p$ and $e^{-}p$ scattering data recorded at HERA during the recent years offer the possibility to study electroweak effects in $ep$ interactions apparent at high momentum transfers, $Q^{2}$, and to reveal information on the proton parton densities at large values of the Bjorken scaling variable x. From the neutral current cross section measurements, H1 and ZEUS extract the generalized structure function $x{cal F}_{3}$, which can be related to the valence quark content of the proton. Individual quark densities are extracted by a global fit to the H1 neutral and charged current $e^{+}p$ and $e^{-}p$ data. The new results show the sensitivity of high $Q^{2}$ $ep$ data to the structure of the proton and indicate what to expect from a 1 fb$^{-1}$ data sample to be taken by H1 and ZEUS until 2006 at the upgraded HERA collider. Future perspectives concerning the investigation of electroweak effects and their utilization to extract the parton content of the proton are shortly discussed.
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Systematic differences in the the protons charge radius, as determined by ordinary atoms and muonic atoms, have caused a resurgence of interest in elastic lepton scattering measurements. The protons charge radius, defined as the slope of the charge form factor at Q$^2$=0, does not depend on the probe. Any difference in the apparent size of the proton, when determined from ordinary versus muonic hydrogen, could point to new physics or need for the higher order corrections. While recent measurements seem to now be in agreement, there is to date no high precision elastic scattering data with both electrons and positrons. A high precision proton radius measurement could be performed in Hall B at Jefferson Lab with a positron beam and the calorimeter based setup of the PRad experiment. This measurement could also be extended to deuterons where a similar discrepancy has been observed between the muonic and electronic determination of deuteron charge radius. A new, high precision measurement with positrons, when viewed alongside electron scattering measurements and the forthcoming MUSE muon scattering measurement, could help provide new insights into the origins of the proton radius puzzle, and also provide new experimental constraints on radiative correction calculations.
We report the measurement of the parity-violating asymmetry for the inelastic scattering of electrons from the proton, at $Q^2 = 0.082$ GeV$^2$ and $ W = 2.23$ GeV, above the resonance region. The result $A_{rm Inel} = - 13.5 pm 2.0 ({rm stat}) pm 3.9 ({rm syst})$~ppm agrees with theoretical calculations, and helps to validate the modeling of the $gamma Z$ interference structure functions $F_1^{gamma Z}$ and $F_2^{gamma Z}$ used in those calculations, which are also used for determination of the two-boson exchange box diagram ($Box_{gamma Z}$) contribution to parity-violating elastic scattering measurements. A positive parity-violating asymmetry for inclusive $pi^-$ production was observed, as well as positive beam-normal single-spin asymmetry for scattered electrons and a negative beam-normal single-spin asymmetry for inclusive $pi^-$ production.
High Q^2 NC and CC cross-sections as measured at HERA can give information on two distinct areas of current interest. Firstly, supposing that all the electroweak parameters are well known, these cross-sections may be used to give information on parton distributions at high x and high Q^2. Secondly, supposing that parton distributions are well known, after evolution in Q^2 from the kinematic regime where they are already measured, these cross-sections can be used to give information on electroweak parameters in a process where the exchanged boson is `spacelike rather than `timelike. WG1 addressed itself to clarifying the limits of our present and possible future knowledge on both these areas.
The GEp-III and GEp-2$gamma$ experiments, carried out in Jefferson Labs Hall C from 2007-2008, consisted of measurements of polarization transfer in elastic electron-proton scattering at momentum transfers of $Q^2 = 2.5, 5.2, 6.8,$ and $8.54$ GeV$^2$. These measurements were carried out to improve knowledge of the proton electromagnetic form factor ratio $R = mu_p G_E^p/G_M^p$ at large values of $Q^2$ and to search for effects beyond the Born approximation in polarization transfer observables at $Q^2 = 2.5$ GeV$^2$. The final results of both experiments were reported in a recent archival publication. A full reanalysis of the data from both experiments was carried out in order to reduce the systematic and, for the GEp-2$gamma$ experiment, statistical uncertainties. This technical note provides additional details of the final analysis omitted from the main publication, including the final evaluation of the systematic uncertainties.
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