Measurements of the EMC effect in the tritium and helium-3 mirror nuclei are reported. The data were obtained by the MARATHON Jefferson Lab experiment, which performed deep inelastic electron scattering from deuterium and the three-body nuclei, using a cryogenic gas target system and the high resolution spectrometers of the Hall A Facility of the Lab. The data cover the Bjorken x range from 0.20 to 0.83, corresponding to a squared four-momentum transfer Q^{2} range from 2.7 to 11.9  (GeV/c)^{2}, and to an invariant mass W of the final hadronic state greater than 1.84  GeV/c^{2}. The tritium EMC effect measurement is the first of its kind. The MARATHON experimental results are compared to results from previous measurements by DESY-HERMES and JLab-Hall C experiments, as well as with few-body theoretical predictions.

We propose a new measurement of the ratio of positron-proton to electron-proton elastic scattering at DESY. The purpose is to determine the contributions beyond single-photon exchange, which are essential for the Quantum Electrodynamic (QED) description of the most fundamental process in hadronic physics. By utilizing a 20 cm long liquid hydrogen target in conjunction with the extracted beam from the DESY synchrotron, we can achieve an average luminosity of $2.12\times10^{35}$ cm$^{-2}\cdot$s$^{-1}\cdot$sr$^{-1}$ ($\approx200$ times the luminosity achieved by OLYMPUS). The proposed TPEX experiment entails a commissioning run at 2 GeV, followed by measurements at 3 GeV, thereby providing new data up to $Q^2=4.6$ (GeV/$c$)$^2$ (twice the range of current measurements). We present and discuss the proposed experimental setup, run plan, and expectations.

We propose a new measurement of the ratio of positron-proton to electron-proton, elastic scattering at DESY to determine the contributions beyond single-photon exchange, which are essential to the QED description of the most fundamental process in hadronic physics. A 20~cm long liquid hydrogen target together with the extracted beam from the DESY synchrotron would yield an average luminosity of $2.12\times10^{35}$~cm$^{-2}\cdot$s$^{-1}\cdot$sr$^{-1}$ ($\sim200$ times the luminosity achieved by OLYMPUS). A commissioning run at 2 GeV followed by measurements at 3 GeV would provide new data up to $Q^2=4.6$~(GeV/$c$)$^2$ (twice the range of current measurements). Lead tungstate calorimeters would be used to detect the scattered leptons at polar angles of $30^\circ$, $50^\circ$, $70^\circ$, $90^\circ$, and $110^\circ$. The measurements could be scheduled to not interfere with the operation of PETRA. We present rate estimates and simulations for the planned measurements including background considerations. Initial measurements at the DESY test beam facility using prototype lead tungstate calorimeters in 2019, 2021, and 2022 were made to check the Monte Carlo simulations and the performance of the calorimeters. These tests also investigated different readout schemes (triggered and streaming). Various upgrades are possible to shorten the running time and to make higher beam energies and thus greater $Q^2$ ranges accessible.

The ratio of the nucleon F_{2} structure functions, F_{2}^{n}/F_{2}^{p}, is determined by the MARATHON experiment from measurements of deep inelastic scattering of electrons from ^{3}H and ^{3}He nuclei. The experiment was performed in the Hall A Facility of Jefferson Lab using two high-resolution spectrometers for electron detection, and a cryogenic target system which included a low-activity tritium cell. The data analysis used a novel technique exploiting the mirror symmetry of the two nuclei, which essentially eliminates many theoretical uncertainties in the extraction of the ratio. The results, which cover the Bjorken scaling variable range 0.19<x<0.83, represent a significant improvement compared to previous SLAC and Jefferson Lab measurements for the ratio. They are compared to recent theoretical calculations and empirical determinations of the F_{2}^{n}/F_{2}^{p} ratio.

Positron beams, both polarized and unpolarized, are identified as important ingredients for the experimental programs at the next generation of lepton accelerators. In the context of the hadronic physics program at Jefferson Lab (JLab), positron beams are complementary, even essential, tools for a precise understanding of the electromagnetic structure of nucleons and nuclei, in both the elastic and deep-inelastic regimes. For instance, elastic scattering of polarized and unpolarized electrons and positrons from the nucleon enables a model independent determination of its electromagnetic form factors. Also, the deeply-virtual scattering of polarized and unpolarized electrons and positrons allows unambiguous separation of the different contributions to the cross section of the lepto-production of photons and of lepton-pairs, enabling an accurate determination of the nucleons and nuclei generalized parton distributions, and providing an access to the gravitational form factors of the energy-momentum tensor. Furthermore, positron beams offer the possibility of alternative tests of the Standard Model of particle physics through the search of a dark photon, the precise measurement of electroweak couplings, and the investigation of charged lepton flavor violation. This document discusses the perspectives of an experimental program with high duty-cycle positron beams at JLab.

The ratio of the nucleon $F_2$ structure functions, $F_2^n/F_2^p$, is determined by the MARATHON experiment from measurements of deep inelastic scattering of electrons from $^3$H and $^3$He nuclei. The experiment was performed in the Hall A Facility of Jefferson Lab and used two high resolution spectrometers for electron detection, and a cryogenic target system which included a low-activity tritium cell. The data analysis used a novel technique exploiting the mirror symmetry of the two nuclei, which essentially eliminates many theoretical uncertainties in the extraction of the ratio. The results, which cover the Bjorken scaling variable range $0.19<x<0.83$, represent a significant improvement compared to previous SLAC and Jefferson Lab measurements for the ratio. They are compared to recent theoretical calculations and empirical determinations of the $F_2^n/F_2^p$ ratio.

The ratio of the nucleon $F_2$ structure functions, $F_2^n/F_2^p$, is determined by the MARATHON experiment from measurements of deep inelastic scattering of electrons from $^3$H and $^3$He nuclei. The experiment was performed in the Hall A Facility of Jefferson Lab and used two high resolution spectrometers for electron detection, and a cryogenic target system which included a low-activity tritium cell. The data analysis used a novel technique exploiting the mirror symmetry of the two nuclei, which essentially eliminates many theoretical uncertainties in the extraction of the ratio. The results, which cover the Bjorken scaling variable range $0.19<x<0.83$, represent a significant improvement compared to previous SLAC and Jefferson Lab measurements for the ratio. They are compared to recent theoretical calculations and empirical determinations of the $F_2^n/F_2^p$ ratio.

We report the first measurement of the (e,e^{'}p) three-body breakup reaction cross sections in helium-3 (^{3}He) and tritium (^{3}H) at large momentum transfer [⟨Q^{2}⟩≈1.9  (GeV/c)^{2}] and x_{B}>1 kinematics, where the cross section should be sensitive to quasielastic (QE) scattering from single nucleons. The data cover missing momenta 40≤p_{miss}≤500  MeV/c that, in the QE limit with no rescattering, equals the initial momentum of the probed nucleon. The measured cross sections are compared with state-of-the-art ab initio calculations. Overall good agreement, within ±20%, is observed between data and calculations for the full p_{miss} range for ^{3}H and for 100≤p_{miss}≤350  MeV/c for ^{3}He. Including the effects of rescattering of the outgoing nucleon improves agreement with the data at p_{miss}>250  MeV/c and suggests contributions from charge-exchange (SCX) rescattering. The isoscalar sum of ^{3}He plus ^{3}H, which is largely insensitive to SCX, is described by calculations to within the accuracy of the data over the entire p_{miss} range. This validates current models of the ground state of the three-nucleon system up to very high initial nucleon momenta of 500  MeV/c.

Positron beams, both polarized and unpolarized, are identified as essential ingredients for the experimental program at the next generation of lepton accelerators. In the context of the Hadronic Physics program at the Jefferson Laboratory (JLab), positron beams are complementary, even essential, tools for a precise understanding of the electromagnetic structure of the nucleon, in both the elastic and the deep-inelastic regimes. For instance, elastic scattering of (un)polarized electrons and positrons off the nucleon allows for a model independent determination of the electromagnetic form factors of the nucleon. Also, the deeply virtual Compton scattering of (un)polarized electrons and positrons allows us to separate unambiguously the different contributions to the cross section of the lepto-production of photons, enabling an accurate determination of the nucleon Generalized Parton Distributions (GPDs), and providing an access to its Gravitational Form Factors. Furthermore, positron beams offer the possibility of alternative tests of the Standard Model through the search of a dark photon, the precise measurement of electroweak couplings, or the investigation of lepton flavor violation. This white paper discusses the perspectives of an experimental program with positron beams at JLab.

Positron beams, both polarized and unpolarized, are identified as essential ingredients for the experimental program at the next generation of lepton accelerators. In the context of the Hadronic Physics program at the Jefferson Laboratory (JLab), positron beams are complementary, even essential, tools for a precise understanding of the electromagnetic structure of the nucleon, in both the elastic and the deep-inelastic regimes. For instance, elastic scattering of (un)polarized electrons and positrons off the nucleon allows for a model independent determination of the electromagnetic form factors of the nucleon. Also, the deeply virtual Compton scattering of (un)polarized electrons and positrons allows us to separate unambiguously the different contributions to the cross section of the lepto-production of photons, enabling an accurate determination of the nucleon Generalized Parton Distributions (GPDs), and providing an access to its Gravitational Form Factors. Furthermore, positron beams offer the possibility of alternative tests of the Standard Model through the search of a dark photon or the precise measurement of electroweak couplings. This letter proposes to develop an experimental positron program at JLab to perform unique high impact measurements with respect to the two-photon exchange problem, the determination of the proton and the neutron GPDs, and the search for the $A^{\prime}$ dark photon.

We report the first measurement of the ( e , e (cid:2) p ) reaction cross-section ratios for Helium-3 ( 3 He), Tritium ( 3 H), and Deuterium ( d ). The measurement covered a missing momentum range of 40 ≤ p miss ≤ 550 MeV / c , at large momentum transfer ( (cid:5) Q 2 (cid:6) ≈ 1 . 9 (GeV/c) 2 ) and x B > 1, which minimized contributions from non quasi-elastic (QE) reaction mechanisms. The data is compared with plane-wave impulse approximation (PWIA) calculations using realistic spectral functions and momentum distributions. The measured and PWIA-calculated cross-section ratios for 3 He/ d and 3 H/ d extend to just above the typical nucleon Fermi-momentum ( k F ≈ 250 MeV/c) and differ from each other by ∼ 20%, while for 3 He/ 3 H they agree within the measurement accuracy of about 3%. At momenta above k F , the measured 3 He/ 3 H ratios differ from the calculation by 20% − 50%. Final state interaction (FSI) calculations using the generalized Eikonal Approximation indicate that FSI should change the 3 He/ 3 H cross-section ratio for this measurement by less than 5%. If these calculations are correct, then the differences at large missing momenta between the 3 He/ 3 H experimental and calculated ratios could be due to the underlying NN interaction, and thus could provide new constraints on the previously loosely-constrained short-distance parts of the NN interaction.

We report the first measurement of the (e, e'p) reaction cross-section ratios for Helium-3 (He-3), Tritium (H-3), and Deuterium (d). The measurement covered a missing momentum range of 40 <= p(miss) <= 550 MeV/c, at large momentum transfer (< Q(2)> approximate to 1.9 (GeV/c)(2)) and x(B) > 1, which minimized contributions from non quasi-elastic (QE) reaction mechanisms. The data is compared with plane-wave impulse approximation (PWIA) calculations using realistic spectral functions and momentum distributions. The measured and PWIA-calculated cross-section ratios for He-3/d and H-3/d extend to just above the typical nucleon Fermi-momentum (k(F) approximate to 250 MeV/c) and differ from each other by similar to 20%, while for He-3/H-3 they agree within the measurement accuracy of about 3%. At momenta above k(F), the measured He-3/H-3 ratios differ from the calculation by 20% - 50%. Final state interaction (FSI) calculations using the generalized Eikonal Approximation indicate that FSI should change the He-3/H-3 cross-section ratio for this measurement by less than 5%. If these calculations are correct, then the differences at large missing momenta between the He-3/H-3 experimental and calculated ratios could be due to the underlying NN interaction, and thus could provide new constraints on the previously loosely-constrained short-distance parts of the NN interaction. (C) 2019 The Author(s). Published by Elsevier B.V.

The J-PARC E36 experiment is searching for lepton universality violation with a stopped kaon beam by measuring the ratio of the K+ decay widths Γ(Ke2)∕Γ(Kμ2)=Γ(K+→e+νe)∕Γ(K+→μ+νμ). Since the radiative K+→e+νeγ decays are backgrounds to be removed in this measurement, the radiated γ rays were detected in a CsI(Tl) calorimeter. The energy calibration for the 768 CsI(Tl) modules was performed using mono-chromatic μ+s from the Kμ2 decays. The delayed e+ signals from the muon decays were required in order to improve the S/N ratio of the Kμ2 peak by suppressing background events. In addition, a new energy calibration method of the CsI(Tl) calorimeter using stopped cosmic muons has been established.

81 We propose a measurement of double polarized 2H(− →e , e′− →n ) 82 at a four-momentum transfer Q = 4.5 (GeV/c). The ra83 tio of electric to magnetic elastic form factors GE/GM will 84 be extracted from the ratio of transverse and longitudi85 nal components of the spin polarization Px/Pz, which is 86 transferred to the recoiling neutron from an incident, lon87 gitudinally polarized electron. 88 The experiment will be performed in Hall-A of Jeffer89 son Laboratory, utilizing common components of the Su90 per BigBite apparatus. It will include apparatus to imple91 ment neutron polarimetry, using both np → pn (charge92 exchange) and np→ np scattering to analyze the neutron 93 polarization . The electron arm will be the BigBite spec94 trometer. The hadron arm will be the neutron polarimeter 95 consisting of a Cu block (the analyzer), GEM charged par96 ticle trackers, the CDet coordinate detector, the hadron 97 calorimeter HCAL and a set of scintillation counters. The 98 bulk of this apparatus is currently under commissioning for 99 other approved SBS experiments. The polarimeter will be 100 sensitive both to high-momentum forward-angle protons, 101 to enable it to measure charge-exchange np→ pn scatter102 ing, and to large-angle, low-momentum protons, to enable 103 it to measure np→ np scattering. A recent measurement 104 at JINR Dubna has shown that np → pn on a relatively 105 heavy nucleus has a sizable analyzing power, similar to 106 np → pn scattering on the proton. The proposed po107 larimeter will yield valuable information on the figure of 108 merit for both np→ pn and np→ np scattering channels. 109 The present experiment, which we propose to run con110 currently with E12-09-019, will yield GE/GM at the high111 est Q kinematic point yet recorded. The technical infor112 mation on the polarimetry will be used to optimize future 113 measurements of GE/GM in Hall A and/or Hall C to reach 114 Q values as high as 9.3 (GeV/c) using recoil polarimetry 115 techniques. 116

The difference in proton radii measured with $\mu p$ atoms and with $ep$ atoms and scattering remains an unexplained puzzle. The PSI MUSE proposal is to measure $\mu p$ and $e p$ scattering in the same experiment at the same time. The experiment will determine cross sections, two-photon effects, form factors, and radii independently for the two reactions, and will allow $\mu p$ and $ep$ results to be compared with reduced systematic uncertainties. These data should provide the best test of lepton universality in a scattering experiment to date, about an order of magnitude improvement over previous tests. Measuring scattering with both particle polarities will allow a test of two-photon exchange at the sub-percent level, about a factor of four improvement on uncertainties and over an order of magnitude more data points than previous low momentum transfer determinations, and similar to the current generation of higher momentum transfer electron experiments. The experiment has the potential to demonstrate whether the $\mu p$ and $ep$ interactions are consistent or different, and whether any difference results from novel physics or two-photon exchange. The uncertainties are such that if the discrepancy is real it should be confirmed with $\approx$5$\sigma$ significance, similar to that already established between the regular and muonic hydrogen Lamb shift.

We describe the current status of the DarkLight experiment at Jefferson Laboratory. DarkLight is motivated by the possibility that a dark photon in the mass range 10 to 100 MeV/c$^2$ could couple the dark sector to the Standard Model. DarkLight will precisely measure electron proton scattering using the 100 MeV electron beam of intensity 5 mA at the Jefferson Laboratory energy recovering linac incident on a windowless gas target of molecular hydrogen. The complete final state including scattered electron, recoil proton, and e+e- pair will be detected. A phase-I experiment has been funded and is expected to take data in the next eighteen months. The complete phase-II experiment is under final design and could run within two years after phase-I is completed. The DarkLight experiment drives development of new technology for beam, target, and detector and provides a new means to carry out electron scattering experiments at low momentum transfers.

P. Hansson Adrian, S. Andreas, T. Averett, O. Baker, B. Batell, M. Battaglieri, J. Beacham, T. Beranek, J. D. Bjorken, F. Bossi, J. R. Boyce, G. D. Cates , A. Celentano, A. S. Chou, R. Cowan, F. Curciarello, H. Davoudiasl, P. deNiverville, R. De Vita, A. Denig, R. Dharmapalan, B. Dongwi, B. Döbrich, B. Echenard, D. Espriu, S. Fegan, P. Fisher, G. B. Franklin, A. Gasparian, Y. Gershtein, M. Graham, P. W. Graham, A. Haas, A. Hatzikoutelis, M. Holtrop, I. Irastorza, E. Izaguirre, J. Jaeckel, Y. Kahn, N. Kalantarians, M. Kohl, G. Krnjaic, V. Kubarovsky, H-S. Lee, A. Lindner, A. Lobanov, W. J. Marciano, D. J. E. Marsh, T. Maruyama, D. McKeen, H. Merkel, K. Moffeit, P. Monaghan, G. Mueller, T. K. Nelson, G.R. Neil, M. Oriunno, Z. Pavlovic, S. K. Phillips, M. J. Pivovaroff, R. Poltis, M. Pospelov, S. Rajendran, J. Redondo, A. Ringwald, A. Ritz, J. Ruz, K. Saenboonruang, P. Schuster, M. Shinn, T. R. Slatyer, J. H. Steffen, S. Stepanyan, D. B. Tanner, J. Thaler, M. E. Tobar, N. Toro, A. Upadye, R. Van de Water, B. Vlahovic, J. K. Vogel, D. Walker, A. Weltman, B. Wojtsekhowski, S. Zhang, K. Zioutas

Dark sectors, consisting of new, light, weakly-coupled particles that do not interact with the known strong, weak, or electromagnetic forces, are a particularly compelling possibility for new physics. Nature may contain numerous dark sectors, each with their own beautiful structure, distinct particles, and forces. This review summarizes the physics motivation for dark sectors and the exciting opportunities for experimental exploration. It is the summary of the Intensity Frontier subgroup "New, Light, Weakly-coupled Particles" of the Community Summer Study 2013 (Snowmass). We discuss axions, which solve the strong CP problem and are an excellent dark matter candidate, and their generalization to axion-like particles. We also review dark photons and other dark-sector particles, including sub-GeV dark matter, which are theoretically natural, provide for dark matter candidates or new dark matter interactions, and could resolve outstanding puzzles in particle and astro-particle physics. In many cases, the exploration of dark sectors can proceed with existing facilities and comparatively modest experiments. A rich, diverse, and low-cost experimental program has been identified that has the potential for one or more game-changing discoveries. These physics opportunities should be vigorously pursued in the US and elsewhere.