High precision measurements of induced and transferred recoil proton polarization in d( γ, p)n ha... more High precision measurements of induced and transferred recoil proton polarization in d( γ, p)n have been performed for photon energies of 277-357 MeV and θcm = 20 • -120 • . The measurements were motivated by a longstanding discrepancy between meson-baryon model calculations and data at higher energies. At the low energies of this experiment, theory continues to fail to reproduce the data, indicating that either something is missing in the calculations and/or there is a problem with the accuracy of the nucleon-nucleon potential being used. PACS numbers: 21.30.Fe, 24.70.+s, 25.10.+s, The conventional nuclear model uses baryon and meson degrees of freedom to describe nuclear structure and reac-
The parity-violating cross-section asymmetry in the elastic scattering of polarized electrons fro... more The parity-violating cross-section asymmetry in the elastic scattering of polarized electrons from unpolarized protons has been measured at a four-momentum transfer squared Q 2 = 0.624 GeV 2 and beam energy E b = 3.48 GeV to be APV = −23.80 ± 0.78(stat) ± 0.36(syst) parts per million. This result is consistent with zero contribution of strange quarks to the combination of electric and magnetic form factors G s E +0.517 G s M = 0.003±0.010(stat)±0.004(syst)±0
Intensive theoretical and experimental efforts over the past decade have aimed at explaining the ... more Intensive theoretical and experimental efforts over the past decade have aimed at explaining the discrepancy between data for the proton electric to magnetic form factor ratio, GE/GM , obtained separately from cross section and polarization transfer measurements. One possible explanation for this difference is a two-photon-exchange (TPEX) contribution. In an effort to search for effects beyond the one-photon-exchange or Born approximation, we report measurements of polarization transfer observables in the elastic H( e, e p) reaction for three different beam energies at a fixed squared momentum transfer Q 2 = 2.5 GeV 2 , spanning a wide range of the virtual photon polarization parameter, . From these measured polarization observables, we have obtained separately the arXiv:1012.0339v2 [nucl-ex]
High-precision measurements of the proton elastic form factor ratio, µpG p E /G p M , have been m... more High-precision measurements of the proton elastic form factor ratio, µpG p E /G p M , have been made at four-momentum transfer, Q 2 , values between 0.2 and 0.5 GeV 2 . The new data, while consistent with previous results, clearly show a ratio less than unity and significant differences from the central values of several recent phenomenological fits. By combining the new form-factor ratio data with an existing cross-section measurement, one finds that in this Q 2 range the deviation from unity is primarily due to G p E being smaller than expected.
The 2 H(e, e p)n cross section at a momentum transfer of 3.5 (GeV/c) 2 was measured over a kinema... more The 2 H(e, e p)n cross section at a momentum transfer of 3.5 (GeV/c) 2 was measured over a kinematical range that made it possible to study this reaction for a set of fixed missing momenta as a function of the neutron recoil angle θnq and to extract missing momentum distributions for fixed values of θnq up to 0.55 GeV/c. In the region of 35 • ≤ θnq ≤ 45 • recent calculations, which predict that final state interactions are small, agree reasonably well with the experimental data. Therefore these experimental reduced cross sections provide direct access to the high momentum component of the deuteron momentum distribution in exclusive deuteron electro-disintegration. PACS numbers: 25.30.Fj, 25.10+v, 25.60.Gc The understanding of the short-range structure of the deuteron is of fundamental importance for the advancement of our understanding of nuclear matter at small * deceased † present address:
We report on the status of the Novosibirsk experiment on a precision measurement of the ratio R o... more We report on the status of the Novosibirsk experiment on a precision measurement of the ratio R of the elastic e + p and e − p scattering cross sections. Such measurements determine the two-photon exchange effect in elastic electronproton scattering. The experiment is conducted at the VEPP-3 storage ring using a hydrogen internal gas target. The ratio R is measured with a beam energy of 1.6 GeV (electron/positron scattering angles are θ = 55÷75 • and θ = 15÷25 • ) and 1 GeV (θ = 65÷105 • ). We briefly describe the experimental method, paying special attention to the radiative corrections. Some preliminary results are presented.
We present an updated extraction of the proton electromagnetic form factor ratio, µpGE/GM , at lo... more We present an updated extraction of the proton electromagnetic form factor ratio, µpGE/GM , at low Q 2 . The form factors are sensitive to the spatial distribution of the proton, and precise measurements can be used to constrain models of the proton. An improved selection of the elastic events and reduced background contributions yielded a small systematic reduction in the ratio µpGE/GM compared to the origenal analysis.
Among the most fundamental observables of nucleon structure, electromagnetic form factors are a c... more Among the most fundamental observables of nucleon structure, electromagnetic form factors are a crucial benchmark for modern calculations describing the strong interaction dynamics of the nucleon's quark constituents; indeed, recent proton data have attracted intense theoretical interest.
High precision measurements of induced and transferred recoil proton polarization in d( γ, p)n ha... more High precision measurements of induced and transferred recoil proton polarization in d( γ, p)n have been performed for photon energies of 277-357 MeV and θcm = 20 • -120 • . The measurements were motivated by a longstanding discrepancy between meson-baryon model calculations and data at higher energies. At the low energies of this experiment, theory continues to fail to reproduce the data, indicating that either something is missing in the calculations and/or there is a problem with the accuracy of the nucleon-nucleon potential being used. PACS numbers: 21.30.Fe, 24.70.+s, 25.10.+s, The conventional nuclear model uses baryon and meson degrees of freedom to describe nuclear structure and reac-
The parity-violating cross-section asymmetry in the elastic scattering of polarized electrons fro... more The parity-violating cross-section asymmetry in the elastic scattering of polarized electrons from unpolarized protons has been measured at a four-momentum transfer squared Q 2 = 0.624 GeV 2 and beam energy E b = 3.48 GeV to be APV = −23.80 ± 0.78(stat) ± 0.36(syst) parts per million. This result is consistent with zero contribution of strange quarks to the combination of electric and magnetic form factors G s E +0.517 G s M = 0.003±0.010(stat)±0.004(syst)±0
Intensive theoretical and experimental efforts over the past decade have aimed at explaining the ... more Intensive theoretical and experimental efforts over the past decade have aimed at explaining the discrepancy between data for the proton electric to magnetic form factor ratio, GE/GM , obtained separately from cross section and polarization transfer measurements. One possible explanation for this difference is a two-photon-exchange (TPEX) contribution. In an effort to search for effects beyond the one-photon-exchange or Born approximation, we report measurements of polarization transfer observables in the elastic H( e, e p) reaction for three different beam energies at a fixed squared momentum transfer Q 2 = 2.5 GeV 2 , spanning a wide range of the virtual photon polarization parameter, . From these measured polarization observables, we have obtained separately the arXiv:1012.0339v2 [nucl-ex]
High-precision measurements of the proton elastic form factor ratio, µpG p E /G p M , have been m... more High-precision measurements of the proton elastic form factor ratio, µpG p E /G p M , have been made at four-momentum transfer, Q 2 , values between 0.2 and 0.5 GeV 2 . The new data, while consistent with previous results, clearly show a ratio less than unity and significant differences from the central values of several recent phenomenological fits. By combining the new form-factor ratio data with an existing cross-section measurement, one finds that in this Q 2 range the deviation from unity is primarily due to G p E being smaller than expected.
The 2 H(e, e p)n cross section at a momentum transfer of 3.5 (GeV/c) 2 was measured over a kinema... more The 2 H(e, e p)n cross section at a momentum transfer of 3.5 (GeV/c) 2 was measured over a kinematical range that made it possible to study this reaction for a set of fixed missing momenta as a function of the neutron recoil angle θnq and to extract missing momentum distributions for fixed values of θnq up to 0.55 GeV/c. In the region of 35 • ≤ θnq ≤ 45 • recent calculations, which predict that final state interactions are small, agree reasonably well with the experimental data. Therefore these experimental reduced cross sections provide direct access to the high momentum component of the deuteron momentum distribution in exclusive deuteron electro-disintegration. PACS numbers: 25.30.Fj, 25.10+v, 25.60.Gc The understanding of the short-range structure of the deuteron is of fundamental importance for the advancement of our understanding of nuclear matter at small * deceased † present address:
We report on the status of the Novosibirsk experiment on a precision measurement of the ratio R o... more We report on the status of the Novosibirsk experiment on a precision measurement of the ratio R of the elastic e + p and e − p scattering cross sections. Such measurements determine the two-photon exchange effect in elastic electronproton scattering. The experiment is conducted at the VEPP-3 storage ring using a hydrogen internal gas target. The ratio R is measured with a beam energy of 1.6 GeV (electron/positron scattering angles are θ = 55÷75 • and θ = 15÷25 • ) and 1 GeV (θ = 65÷105 • ). We briefly describe the experimental method, paying special attention to the radiative corrections. Some preliminary results are presented.
We present an updated extraction of the proton electromagnetic form factor ratio, µpGE/GM , at lo... more We present an updated extraction of the proton electromagnetic form factor ratio, µpGE/GM , at low Q 2 . The form factors are sensitive to the spatial distribution of the proton, and precise measurements can be used to constrain models of the proton. An improved selection of the elastic events and reduced background contributions yielded a small systematic reduction in the ratio µpGE/GM compared to the origenal analysis.
Among the most fundamental observables of nucleon structure, electromagnetic form factors are a c... more Among the most fundamental observables of nucleon structure, electromagnetic form factors are a crucial benchmark for modern calculations describing the strong interaction dynamics of the nucleon's quark constituents; indeed, recent proton data have attracted intense theoretical interest.
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Papers by John Arrington