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1. Ultrahigh-precision Compton polarimetry at 2 GeV

   https://doi.org/10.1103/PhysRevC.109.024323  

   Zec, A; Premathilake, S; Cornejo, J C; Dalton, M M; Gal, C; Gaskell, D; Gericke, M; Halilovic, I; Liu, H; Mammei, J; et al (February 2024, Physical Review C)

   We report a high precision measurement of electron beam polarization using Compton polarimetry. The measurement was made in experimental Hall A at Jefferson Lab during the CREX experiment in 2020. A total uncertainty of 𝑑⁢𝑃/𝑃=0.36% was achieved detecting the back-scattered photons from the Compton scattering process. This is the highest accuracy in a measurement of electron beam polarization using Compton scattering ever reported, surpassing the groundbreaking measurement from the SLD Compton polarimeter. Such uncertainty reaches the level required for the future flagship measurements to be made by the MOLLER and SoLID experiments. 

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2. Parametric study of the polarization dependence of nonlinear Breit–Wheeler pair creation process using two laser pulses

   https://doi.org/10.1063/5.0165788  

   Qian, Qian; Seipt, Daniel; Vranic, Marija; Grismayer, Thomas E; Blackburn, Thomas G; Ridgers, Christopher P; Thomas, Alexander_G R (October 2023, Physics of Plasmas)

   With the rapid development of high-power petawatt class lasers worldwide, exploring physics in the strong field QED regime will become one of the frontiers for laser–plasma interactions research. Particle-in-cell codes, including quantum emission processes, are powerful tools for predicting and analyzing future experiments where the physics of relativistic plasma is strongly affected by strong field QED processes. The spin/polarization dependence of these quantum processes has been of recent interest. In this article, we perform a parametric study of the interaction of two laser pulses with an ultrarelativistic electron beam. The first pulse is optimized to generate high-energy photons by nonlinear Compton scattering and efficiently decelerate electron beam through the quantum radiation reaction. The second pulse is optimized to generate electron–positron pairs by the nonlinear Breit–Wheeler decay of photons with the maximum polarization dependence. This may be experimentally realized as a verification of the strong field QED framework, including the spin/polarization rates. 

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3. Constraint of the MINERνA medium energy neutrino flux using neutrino-electron elastic scattering

   Valencia, E.; Jena, D.; Nuruzzaman, Nofirstname; Akbar, F.; Aliaga, L.; Andrade, D. A.; Ascencio, M. V.; Bashyal, A.; Bellanton, L.; Bercellie, A.; et al (November 2019, Physical review)

   Elastic neutrino scattering on electrons is a precisely known purely leptonic process that provides a standard candle for measuring neutrino flux in conventional neutrino beams. Using a total sample of 810 neutrino-electron scatters after background subtraction, the measurement reduces the normalization uncertainty on the νμ NuMI beam flux between 2 and 20 GeV from 7.6 to 3.9%. This is the most precise measurement of neutrino-electron scattering to date, will reduce uncertainties on MINERνA’s absolute cross section measurements, and demonstrates a technique that can be used in future neutrino beams such as long baseline neutrino facility. 

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4. Proton form factor ratio \mu_p G_E^p/G_M^p from double spin asymmetry

   Liyanage, A.; Armstrong, W.; Kang, H.; Maxwell, J.; Mulholland, J.; Ndukum, L.; Ahmidouch, A.; Albayrak, I.; Asaturyan, A.; Ates, O.; et al (March 2020, Physical review)

   The ratio of the electric to magnetic form factors of the proton, μpGEp/GMp, has been measured for elastic electron-proton scattering with polarized beam and target up to four-momentum transfer squared Q2=5.66(GeV/c)2 using double spin asymmetry for target spin orientation aligned nearly perpendicular to the beam momentum direction. This measurement of μpGEp/GMp agrees with the Q2 dependence of previous recoil polarization data and reconfirms the discrepancy at high Q2 between the Rosenbluth and the polarization-transfer method with a different measurement technique and systematic uncertainties uncorrelated to those of the recoil-polarization measurements. The form factor ratio at Q2=2.06(GeV/c)2 has been measured as μpGEp/GMp=0.720±0.176stat±0.039sys, which is in agreement with an earlier measurement using the polarized target technique at similar kinematics. The form factor ratio at Q2=5.66(GeV/c)2 has been determined as μpGEp/GMp=0.244±0.353stat±0.013sys, which represents the highest Q2 measurement reached using double spin asymmetries with polarized target to date. 

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5. A Method to Measure Positron Beam Polarization Using Optically Polarized Atoms

   https://doi.org/10.3390/atoms11040065  

   Machacek, Joshua R.; Hodgman, Sean; Buckman, Stephen; Gay, T. J. (April 2023, Atoms)

   We outline an experimental technique for measuring the degree of polarization of a positron beam using an optically pumped, spin-polarized Rb target. The technique is based on the production and measurement of the ortho- and para-positronium fractions through positron collisions with the Rb atoms as a function of their polarization. Using realistic estimates for the cross sections and experimental parameters involved, we estimate that a polarization measurement with an uncertainty of 3% of the measured value can be achieved in an hour. 

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