We measured the g1 spin structure function of the deuteron at low Q 2 , where QCD can be approxim... more We measured the g1 spin structure function of the deuteron at low Q 2 , where QCD can be approximated with chiral perturbation theory (χP T). The data cover the resonance region, up to an invariant mass of W ≈ 1.9 GeV. The generalized GDH sum, the moment Γ d 1 and the spin polarizability γ d 0 are precisely determined down to a minimum Q 2 of 0.02 GeV 2 for the first time, about 2.5 times lower than that of previous data. We compare them to several χP T calculations and models. These results are the first in a program of benchmark measurements of polarization observables in the χP T domain.
We report a measurement of a beam-target double-polarisation observable (E) for the γ n(p) → K + ... more We report a measurement of a beam-target double-polarisation observable (E) for the γ n(p) → K + − (p) reaction. The data were obtained impinging the circularly-polarised energy-tagged photon beam of Hall B at Jefferson Lab on a longitudinally-polarised frozen-spin hydrogen deuteride (HD) nuclear target. The E observable for an effective neutron target was determined for centre-of-mass energies 1.70 ≤ W ≤ 2.30 GeV, with reaction products detected over a wide angular acceptance by the CLAS spectrometer. These new double-polarisation data give unique constraints on the strange decays of excited neutron states. Inclusion of the new data within the Bonn-Gatchina theoretical model results in significant changes for the extracted photocouplings of a number of established nucleon resonances. Possible improvements in the PWA description of the experimental data with additional "missing" resonance states, including the N(2120) 3/2 − resonance, are also quantified.
The photoproduction of ω mesons off the proton has been studied in the reaction γp → p ω using th... more The photoproduction of ω mesons off the proton has been studied in the reaction γp → p ω using the CEBAF Large Acceptance Spectrometer (CLAS) and the frozen-spin target (FROST) in Hall B at the Thomas Jefferson National Accelerator Facility. For the first time, the target asymmetry, T , has been measured in photoproduction from the decay ω → π + π − π 0 , using a transversely-polarized target with energies ranging from just above the reaction threshold up to 2.8 GeV. Significant non-zero values are observed for these asymmetries, reaching about 30-40 % in the third-resonance region. New measurements for the photon-beam asymmetry, Σ, are also presented, which agree well with previous CLAS results and extend the world database up to 2.1 GeV. These data and additional ω photoproduction observables from CLAS were included in a partial-wave analysis within the Bonn-Gatchina framework. Significant contributions from s-channel resonance production were found in addition to t-channel exchange processes.
This document will continue to evolve as the IR expands. Additional guidelines will be drafted, a... more This document will continue to evolve as the IR expands. Additional guidelines will be drafted, as needed, over the coming months.
Background: The electromagnetic form factors of the proton measured by unpolarized and polarized ... more Background: The electromagnetic form factors of the proton measured by unpolarized and polarized electron scattering experiments show a significant disagreement that grows with the squared four momentum transfer (Q 2). Calculations have shown that the two measurements can be largely reconciled by accounting for the contributions of two-photon exchange (TPE). TPE effects are not typically included in the standard set of radiative corrections since theoretical calculations of the TPE effects are highly model dependent, and, until recently, no direct evidence of significant TPE effects has been observed. Purpose: We measured the ratio of positron-proton to electron-proton elastic-scattering cross sections in order to determine the TPE contribution to elastic electron-proton scattering and thereby resolve the proton electric form factor discrepancy. Methods: We produced a mixed simultaneous electron-positron beam in Jefferson Lab's Hall B by passing the 5.6 GeV primary electron beam through a radiator to produce a bremsstrahlung photon beam and then passing the photon beam through a convertor to produce electron/positron pairs. The mixed electron-positron (lepton) beam with useful energies from approximately 0.85 to 3.5 GeV then struck a 30-cm long liquid hydrogen (LH2) target located within the CEBAF Large Acceptance Spectrometer (CLAS). By detecting both the scattered leptons and the recoiling protons we identified and reconstructed elastic scattering events and determined the incident lepton energy. A detailed description of the experiment is presented. Results: We present previously unpublished results for the quantity R2γ, the TPE correction to the elasticscattering cross section, at Q 2 ≈ 0.85 and 1.45 GeV 2 over a large range of virtual photon polarization ε. Conclusions: Our results, along with recently published results from VEPP-3, demonstrate a non-zero contribution from TPE effects and are in excellent agreement with the calculations that include TPE effects and largely reconcile the form-factor discrepancy up to Q 2 ≈ 2 GeV 2. These data are consistent with an increase in R2γ with decreasing ε at Q 2 ≈ 0.85 and 1.45 GeV 2. There are indications of a slight increase in R2γ with Q 2 .
This document will continue to evolve as the IR expands. Additional guidelines will be drafted, a... more This document will continue to evolve as the IR expands. Additional guidelines will be drafted, as needed, over the coming months.
This document will continue to evolve as the IR expands. Additional guidelines will be drafted, a... more This document will continue to evolve as the IR expands. Additional guidelines will be drafted, as needed, over the coming months.
We report the first beam-target double-polarization asymmetries in the γ+n(p)→π^{-}+p(p) reaction... more We report the first beam-target double-polarization asymmetries in the γ+n(p)→π^{-}+p(p) reaction spanning the nucleon resonance region from invariant mass W=1500 to 2300 MeV. Circularly polarized photons and longitudinally polarized deuterons in solid hydrogen deuteride (HD) have been used with the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab. The exclusive final state has been extracted using three very different analyses that show excellent agreement, and these have been used to deduce the E polarization observable for an effective neutron target. These results have been incorporated into new partial wave analyses and have led to significant revisions for several γnN^{*} resonance photocouplings.
MeV-GeV dark matter (DM) is theoretically well motivated but remarkably unexplored. This proposal... more MeV-GeV dark matter (DM) is theoretically well motivated but remarkably unexplored. This proposal presents the MeV-GeV DM discovery potential for a ∼1 m 3 segmented CsI(Tl) scintillator detector placed downstream of the Hall A beam-dump at Jefferson Lab, receiving up to 10 22 electrons-on-target (EOT) in 285 days. This experiment (Beam-Dump eXperiment or BDX) would be sensitive to elastic DM-electron and to inelastic DM scattering at the level of 10 counts per year, reaching the limit of the neutrino irreducible background. The distinct signature of a DM interaction will be an electromagnetic shower of few hundreds of MeV, together with a reduced activity in the surrounding active veto counters. A detailed description of the DM particle χ production in the dump and subsequent interaction in the detector has been performed by means of Monte Carlo simulations. Different approaches have been used to evaluate the expected backgrounds: the cosmogenic background has been extrapolated from the results obtained with a prototype detector running at INFN-LNS (Italy), while the beam-related background has been evaluated by GEANT4 Monte Carlo simulations. The proposed experiment will be sensitive to large regions of DM parameter space, exceeding the discovery potential of existing and planned experiments in the MeV-GeV DM mass range by up to two orders of magnitude. * * This algorithm already exists in the FADC250 firmware, and has been extensively tested for short (O(100 ns)-wide) pulses.
We report measurements of target-and double-spin asymmetries for the exclusive channel e p → eπ +... more We report measurements of target-and double-spin asymmetries for the exclusive channel e p → eπ + (n) in the nucleon resonance region at Jefferson Lab using the CEBAF Large Acceptance Spectrometer (CLAS). These asymmetries were extracted from data obtained using a longitudinally polarized NH3 target and a longitudinally polarized electron beam with energies 1.1, 1.3, 2.0, 2.3 and 3.0 GeV. The new results are consistent with previous CLAS publications but are extended to a low Q 2 range from 0.0065 to 0.35 (GeV/c) 2. The Q 2 access was made possible by a custom-built Cherenkov detector that allowed the detection of electrons for scattering angles as low as 6 •. These results are compared with the unitary isobar models JANR and MAID, the partial-wave analysis prediction from SAID and the dynamic model DMT. In many kinematic regions our results, in particular results on the target asymmetry, help to constrain the polarization-dependent components of these models.
Beam-target double-spin asymmetries and target single-spin asymmetries in exclusive π + and quasi... more Beam-target double-spin asymmetries and target single-spin asymmetries in exclusive π + and quasiexclusive π − electroproduction were obtained from scattering of 1.6-to 5.7-GeV longitudinally polarized electrons from longitudinally polarized protons (for π +) and deuterons (for π −) using the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab. The kinematic range covered is 1.1 < W < 2.6 GeV and 0.05 < Q 2 < 5 GeV 2 , with good angular coverage in the forward hemisphere. The asymmetry results were divided into approximately 40 000 kinematic bins for π + from free protons and 15 000 bins for π − production from bound nucleons in the deuteron. The present results are found to be in reasonable agreement with fits to previous world data for W < 1.7 GeV and Q 2 < 0.5 GeV 2 , with discrepancies increasing at higher values of Q 2 , especially for W > 1.5 GeV. Very large target-spin asymmetries are observed for W > 1.6 GeV. When combined with cross-section measurements, the present results can provide powerful constraints on nucleon resonance amplitudes at moderate and large values of Q 2 , for resonances with masses as high as 2.3 GeV.
This document will continue to evolve as the IR expands. Additional guidelines will be drafted, a... more This document will continue to evolve as the IR expands. Additional guidelines will be drafted, as needed, over the coming months.
Single-beam, single-target, and double-spin asymmetries for hard exclusive photon production on t... more Single-beam, single-target, and double-spin asymmetries for hard exclusive photon production on the proton e p → e p γ are presented. The data were taken at Jefferson Lab using the CLAS detector and a longitudinally polarized 14 NH3 target. The three asymmetries were measured in 165 4-dimensional kinematic bins, covering the widest kinematic range ever explored simultaneously for beam and target-polarization observables in the valence quark region. The kinematic dependences of the obtained asymmetries are discussed and compared to the predictions of models of Generalized Parton Distributions. The measurement of three DVCS spin observables at the same kinematic points allows a quasi-model-independent extraction of the imaginary parts of the H andH Compton Form Factors, which give insight into the electric and axial charge distributions of valence quarks in the proton.
The electromagnetic decay Σ 0 (1385) → Λγ was studied using the CLAS detector at the Thomas Jeffe... more The electromagnetic decay Σ 0 (1385) → Λγ was studied using the CLAS detector at the Thomas Jefferson National Accelerator Facility. A real photon beam with a maximum energy of 3.8 GeV was incident on a proton target, producing an exclusive final state of K + Σ * 0. We report the decay widths ratio Σ 0 (1385) → Λγ/ Σ 0 (1385) → Λπ 0 = 1.42 ± 0.12(stat) +0.11 −0.07 (sys)%. This ratio is larger than most theoretical predictions by factors ranging from 1.5-3, but is consistent with the only other experimental measurement. From the reported ratio we calculate the partial width and electromagnetic transition magnetic moment for Σ 0 (1385) → Λγ.
Data analysis techniques, differential cross sections, and spin density matrix elements for the r... more Data analysis techniques, differential cross sections, and spin density matrix elements for the reaction γ p → φ p
We measured the g1 spin structure function of the deuteron at low Q 2 , where QCD can be approxim... more We measured the g1 spin structure function of the deuteron at low Q 2 , where QCD can be approximated with chiral perturbation theory (χP T). The data cover the resonance region, up to an invariant mass of W ≈ 1.9 GeV. The generalized GDH sum, the moment Γ d 1 and the spin polarizability γ d 0 are precisely determined down to a minimum Q 2 of 0.02 GeV 2 for the first time, about 2.5 times lower than that of previous data. We compare them to several χP T calculations and models. These results are the first in a program of benchmark measurements of polarization observables in the χP T domain.
We report a measurement of a beam-target double-polarisation observable (E) for the γ n(p) → K + ... more We report a measurement of a beam-target double-polarisation observable (E) for the γ n(p) → K + − (p) reaction. The data were obtained impinging the circularly-polarised energy-tagged photon beam of Hall B at Jefferson Lab on a longitudinally-polarised frozen-spin hydrogen deuteride (HD) nuclear target. The E observable for an effective neutron target was determined for centre-of-mass energies 1.70 ≤ W ≤ 2.30 GeV, with reaction products detected over a wide angular acceptance by the CLAS spectrometer. These new double-polarisation data give unique constraints on the strange decays of excited neutron states. Inclusion of the new data within the Bonn-Gatchina theoretical model results in significant changes for the extracted photocouplings of a number of established nucleon resonances. Possible improvements in the PWA description of the experimental data with additional "missing" resonance states, including the N(2120) 3/2 − resonance, are also quantified.
The photoproduction of ω mesons off the proton has been studied in the reaction γp → p ω using th... more The photoproduction of ω mesons off the proton has been studied in the reaction γp → p ω using the CEBAF Large Acceptance Spectrometer (CLAS) and the frozen-spin target (FROST) in Hall B at the Thomas Jefferson National Accelerator Facility. For the first time, the target asymmetry, T , has been measured in photoproduction from the decay ω → π + π − π 0 , using a transversely-polarized target with energies ranging from just above the reaction threshold up to 2.8 GeV. Significant non-zero values are observed for these asymmetries, reaching about 30-40 % in the third-resonance region. New measurements for the photon-beam asymmetry, Σ, are also presented, which agree well with previous CLAS results and extend the world database up to 2.1 GeV. These data and additional ω photoproduction observables from CLAS were included in a partial-wave analysis within the Bonn-Gatchina framework. Significant contributions from s-channel resonance production were found in addition to t-channel exchange processes.
This document will continue to evolve as the IR expands. Additional guidelines will be drafted, a... more This document will continue to evolve as the IR expands. Additional guidelines will be drafted, as needed, over the coming months.
Background: The electromagnetic form factors of the proton measured by unpolarized and polarized ... more Background: The electromagnetic form factors of the proton measured by unpolarized and polarized electron scattering experiments show a significant disagreement that grows with the squared four momentum transfer (Q 2). Calculations have shown that the two measurements can be largely reconciled by accounting for the contributions of two-photon exchange (TPE). TPE effects are not typically included in the standard set of radiative corrections since theoretical calculations of the TPE effects are highly model dependent, and, until recently, no direct evidence of significant TPE effects has been observed. Purpose: We measured the ratio of positron-proton to electron-proton elastic-scattering cross sections in order to determine the TPE contribution to elastic electron-proton scattering and thereby resolve the proton electric form factor discrepancy. Methods: We produced a mixed simultaneous electron-positron beam in Jefferson Lab's Hall B by passing the 5.6 GeV primary electron beam through a radiator to produce a bremsstrahlung photon beam and then passing the photon beam through a convertor to produce electron/positron pairs. The mixed electron-positron (lepton) beam with useful energies from approximately 0.85 to 3.5 GeV then struck a 30-cm long liquid hydrogen (LH2) target located within the CEBAF Large Acceptance Spectrometer (CLAS). By detecting both the scattered leptons and the recoiling protons we identified and reconstructed elastic scattering events and determined the incident lepton energy. A detailed description of the experiment is presented. Results: We present previously unpublished results for the quantity R2γ, the TPE correction to the elasticscattering cross section, at Q 2 ≈ 0.85 and 1.45 GeV 2 over a large range of virtual photon polarization ε. Conclusions: Our results, along with recently published results from VEPP-3, demonstrate a non-zero contribution from TPE effects and are in excellent agreement with the calculations that include TPE effects and largely reconcile the form-factor discrepancy up to Q 2 ≈ 2 GeV 2. These data are consistent with an increase in R2γ with decreasing ε at Q 2 ≈ 0.85 and 1.45 GeV 2. There are indications of a slight increase in R2γ with Q 2 .
This document will continue to evolve as the IR expands. Additional guidelines will be drafted, a... more This document will continue to evolve as the IR expands. Additional guidelines will be drafted, as needed, over the coming months.
This document will continue to evolve as the IR expands. Additional guidelines will be drafted, a... more This document will continue to evolve as the IR expands. Additional guidelines will be drafted, as needed, over the coming months.
We report the first beam-target double-polarization asymmetries in the γ+n(p)→π^{-}+p(p) reaction... more We report the first beam-target double-polarization asymmetries in the γ+n(p)→π^{-}+p(p) reaction spanning the nucleon resonance region from invariant mass W=1500 to 2300 MeV. Circularly polarized photons and longitudinally polarized deuterons in solid hydrogen deuteride (HD) have been used with the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab. The exclusive final state has been extracted using three very different analyses that show excellent agreement, and these have been used to deduce the E polarization observable for an effective neutron target. These results have been incorporated into new partial wave analyses and have led to significant revisions for several γnN^{*} resonance photocouplings.
MeV-GeV dark matter (DM) is theoretically well motivated but remarkably unexplored. This proposal... more MeV-GeV dark matter (DM) is theoretically well motivated but remarkably unexplored. This proposal presents the MeV-GeV DM discovery potential for a ∼1 m 3 segmented CsI(Tl) scintillator detector placed downstream of the Hall A beam-dump at Jefferson Lab, receiving up to 10 22 electrons-on-target (EOT) in 285 days. This experiment (Beam-Dump eXperiment or BDX) would be sensitive to elastic DM-electron and to inelastic DM scattering at the level of 10 counts per year, reaching the limit of the neutrino irreducible background. The distinct signature of a DM interaction will be an electromagnetic shower of few hundreds of MeV, together with a reduced activity in the surrounding active veto counters. A detailed description of the DM particle χ production in the dump and subsequent interaction in the detector has been performed by means of Monte Carlo simulations. Different approaches have been used to evaluate the expected backgrounds: the cosmogenic background has been extrapolated from the results obtained with a prototype detector running at INFN-LNS (Italy), while the beam-related background has been evaluated by GEANT4 Monte Carlo simulations. The proposed experiment will be sensitive to large regions of DM parameter space, exceeding the discovery potential of existing and planned experiments in the MeV-GeV DM mass range by up to two orders of magnitude. * * This algorithm already exists in the FADC250 firmware, and has been extensively tested for short (O(100 ns)-wide) pulses.
We report measurements of target-and double-spin asymmetries for the exclusive channel e p → eπ +... more We report measurements of target-and double-spin asymmetries for the exclusive channel e p → eπ + (n) in the nucleon resonance region at Jefferson Lab using the CEBAF Large Acceptance Spectrometer (CLAS). These asymmetries were extracted from data obtained using a longitudinally polarized NH3 target and a longitudinally polarized electron beam with energies 1.1, 1.3, 2.0, 2.3 and 3.0 GeV. The new results are consistent with previous CLAS publications but are extended to a low Q 2 range from 0.0065 to 0.35 (GeV/c) 2. The Q 2 access was made possible by a custom-built Cherenkov detector that allowed the detection of electrons for scattering angles as low as 6 •. These results are compared with the unitary isobar models JANR and MAID, the partial-wave analysis prediction from SAID and the dynamic model DMT. In many kinematic regions our results, in particular results on the target asymmetry, help to constrain the polarization-dependent components of these models.
Beam-target double-spin asymmetries and target single-spin asymmetries in exclusive π + and quasi... more Beam-target double-spin asymmetries and target single-spin asymmetries in exclusive π + and quasiexclusive π − electroproduction were obtained from scattering of 1.6-to 5.7-GeV longitudinally polarized electrons from longitudinally polarized protons (for π +) and deuterons (for π −) using the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab. The kinematic range covered is 1.1 < W < 2.6 GeV and 0.05 < Q 2 < 5 GeV 2 , with good angular coverage in the forward hemisphere. The asymmetry results were divided into approximately 40 000 kinematic bins for π + from free protons and 15 000 bins for π − production from bound nucleons in the deuteron. The present results are found to be in reasonable agreement with fits to previous world data for W < 1.7 GeV and Q 2 < 0.5 GeV 2 , with discrepancies increasing at higher values of Q 2 , especially for W > 1.5 GeV. Very large target-spin asymmetries are observed for W > 1.6 GeV. When combined with cross-section measurements, the present results can provide powerful constraints on nucleon resonance amplitudes at moderate and large values of Q 2 , for resonances with masses as high as 2.3 GeV.
This document will continue to evolve as the IR expands. Additional guidelines will be drafted, a... more This document will continue to evolve as the IR expands. Additional guidelines will be drafted, as needed, over the coming months.
Single-beam, single-target, and double-spin asymmetries for hard exclusive photon production on t... more Single-beam, single-target, and double-spin asymmetries for hard exclusive photon production on the proton e p → e p γ are presented. The data were taken at Jefferson Lab using the CLAS detector and a longitudinally polarized 14 NH3 target. The three asymmetries were measured in 165 4-dimensional kinematic bins, covering the widest kinematic range ever explored simultaneously for beam and target-polarization observables in the valence quark region. The kinematic dependences of the obtained asymmetries are discussed and compared to the predictions of models of Generalized Parton Distributions. The measurement of three DVCS spin observables at the same kinematic points allows a quasi-model-independent extraction of the imaginary parts of the H andH Compton Form Factors, which give insight into the electric and axial charge distributions of valence quarks in the proton.
The electromagnetic decay Σ 0 (1385) → Λγ was studied using the CLAS detector at the Thomas Jeffe... more The electromagnetic decay Σ 0 (1385) → Λγ was studied using the CLAS detector at the Thomas Jefferson National Accelerator Facility. A real photon beam with a maximum energy of 3.8 GeV was incident on a proton target, producing an exclusive final state of K + Σ * 0. We report the decay widths ratio Σ 0 (1385) → Λγ/ Σ 0 (1385) → Λπ 0 = 1.42 ± 0.12(stat) +0.11 −0.07 (sys)%. This ratio is larger than most theoretical predictions by factors ranging from 1.5-3, but is consistent with the only other experimental measurement. From the reported ratio we calculate the partial width and electromagnetic transition magnetic moment for Σ 0 (1385) → Λγ.
Data analysis techniques, differential cross sections, and spin density matrix elements for the r... more Data analysis techniques, differential cross sections, and spin density matrix elements for the reaction γ p → φ p
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