A Direct Measurement of Hard Two-Photon Exchange with Electrons and Positrons at CLAS12

International audienceOne of the most surprising discoveries made at Jefferson Lab has been the discrepancy in the determinations of the proton's form factor ratio $\mu_p G_E^p/G_M^p$ between unpolarized cross section measurements and the polarization transfer technique. Over two decades later, the discrepancy not only persists but has been confirmed at higher momentum transfers now accessible in the 12-GeV era. The leading hypothesis for the cause of this discrepancy, a non-negligible contribution from hard two-photon exchange, has neither been conclusively proven or disproven. This state of uncertainty not only clouds our knowledge of one-dimensional nucleon structure but also poses a major concern for our field's efforts to map out the three-dimensional nuclear structure. A better understanding of multi-photon exchange over a wide phase space is needed. We propose making comprehensive measurements of two-photon exchange over a wide range in momentum transfer and scattering angle using the CLAS12 detector. Specifically, we will measure the ratio of positron-proton to electron-proton elastic scattering cross sections, using the proposed positron beam upgrade for CEBAF. The experiment will use 2.2, 4.4, and 6.6 GeV lepton beams incident on the standard CLAS12 unpolarized hydrogen target. Data will be collected by the CLAS12 detector in its standard configuration, except for a modified trigger to allow the recording of events with beam leptons scattered into the CLAS12 central detector. The sign of the beam charge, as well as the polarity of the CLAS12 solenoid and toroid, will be reversed several times in order to suppress systematics associated with local detector efficiency and time-dependent detector performance. The proposed high-precision determination of two-photon effects will be..

Measurement of the helicity asymmetry E\mathbb{E} for the γ⃗p⃗→pπ0\vec{\gamma}\vec{p} \to p \pi^0 reaction in the resonance region

International audienceThe double-spin-polarization observable $\mathbb{E}$ for $\vec{\gamma}\vec{p}\to p\pi^0$ has been measured with the CEBAF Large Acceptance Spectrometer (CLAS) at photon beam energies $E_\gamma$ from 0.367 to $2.173~\mathrm{GeV}$ (corresponding to center-of-mass energies from 1.240 to $2.200~\mathrm{GeV}$) for pion center-of-mass angles, $\cos\theta_{\pi^0}^{c.m.}$, between -0.86 and 0.82. These new CLAS measurements cover a broader energy range and have smaller uncertainties compared to previous CBELSA data and provide an important independent check on systematics. These measurements are compared to predictions as well as new global fits from The George Washington University, Mainz, and Bonn-Gatchina groups. Their inclusion in multipole analyses will refine our understanding of the single-pion production contribution to the Gerasimov-Drell-Hearn sum rule and improve the determination of resonance properties

Measurement of the helicity asymmetry E\mathbb{E} for the γ⃗p⃗→pπ0\vec{\gamma}\vec{p} \to p \pi^0 reaction in the resonance region

International audienceThe double-spin-polarization observable $\mathbb{E}$ for $\vec{\gamma}\vec{p}\to p\pi^0$ has been measured with the CEBAF Large Acceptance Spectrometer (CLAS) at photon beam energies $E_\gamma$ from 0.367 to $2.173~\mathrm{GeV}$ (corresponding to center-of-mass energies from 1.240 to $2.200~\mathrm{GeV}$) for pion center-of-mass angles, $\cos\theta_{\pi^0}^{c.m.}$, between -0.86 and 0.82. These new CLAS measurements cover a broader energy range and have smaller uncertainties compared to previous CBELSA data and provide an important independent check on systematics. These measurements are compared to predictions as well as new global fits from The George Washington University, Mainz, and Bonn-Gatchina groups. Their inclusion in multipole analyses will refine our understanding of the single-pion production contribution to the Gerasimov-Drell-Hearn sum rule and improve the determination of resonance properties