The Qweak experiment, which took data at Jefferson Lab in the period 2010 - 2012, will precisely determine the weak charge of the proton by measuring the parity-violating asymmetry in elastic e-p scattering at 1.1 GeV using a longitudinally polarized electron beam and a liquid hydrogen target at a low momentum transfer of Q2 = 0.025 (GeV/c)2. The weak charge of the proton is predicted by the Standard Model and any significant deviation would indicate physics beyond the Standard Model. The technical challenges and experimental apparatus for measuring the weak charge of the proton will be discussed, as well as the method of extracting the weak charge of the proton. The results from a small subset of the data, that has been published, will also be presented. Furthermore an update will be given of the current status of the data analysis.

The Q-weak experiment at Jefferson Laboratory measured the parity violating asymmetry (A P V ) in elastic electron-proton scattering at small momentum transfer squared (Q 2=0.025 (G e V/c)2), with the aim of extracting the proton’s weak charge (\({Q^p_W}\)) to an accuracy of 5 %. As one of the major uncertainty contribution sources to \({Q^p_W}\), Q 2 needs to be determined to ∼1 % so as to reach the proposed experimental precision. For this purpose, two sets of high resolution tracking chambers were employed in the experiment, to measure tracks before and after the magnetic spectrometer. Data collected by the tracking system were then reconstructed with dedicated software into individual electron trajectories for experimental kinematics determination. The Q-weak kinematics and the analysis scheme for tracking data are briefly described here. The sources that contribute to the uncertainty of Q 2 are discussed, and the current analysis status is reported.

A subset of results from the recently completed Jefferson Lab Qweak experiment are reported. This experiment, sensitive to physics beyond the Standard Model, exploits the small parity-violating asymmetry in elastic ep scattering to provide the first determination of the protons weak charge Qweak(p). The experiment employed a 180 uA longitudinally polarized 1.16 GeV electron beam on a 35 cm long liquid hydrogen target. Scattered electrons corresponding to Q2 of 0.025 GeV2 were detected in eight Cerenkov detectors arrayed symmetrically around the beam axis. The goals of the experiment were to provide a measure of Qweak(p) to 4.2 percent (combined statistical and systematic error), which implies a measure of sin2(thetaw) at the level of 0.3 percent, and to help constrain the vector weak quark charges C1u and C1d. The experimental method is described, with particular focus on the challenges associated with the worlds highest power LH2 target. The new constraints on C1u and C1d provided by the subset of the experiments data analyzed to date will also be shown, together with the extracted weak charge of the neutron.

In order to constrain weak coupling constants between nucleons, the Neutron Spin Rotation (NSR) collaboration has placed an experimental upper bound on the parity-violating spin rotation of transversely polarized neutrons transmitted through liquid helium. These measurements also place limits on the existence of possible long-range parity-odd forces [1]. Particular attention has been paid to reducing possible systematic errors below the statistical precision of the measurement. In addition, simulations of the beam transport and target interactions have been used to investigate systematic errors from small-angle scattering in the target and help plan the next generation experiment. The recent experiment performed on the NG6 neutron beam at the NIST Center for Neutron Research (NCNR) yielded a statisticallylimited rotation angle of dφ/dz = [+1.7 ± 9.1(stat.)± 1.4(sys.)] × 10−7 rad/m [2]. The NSR collaboration is currently upgrading the apparatus to accept the higher flux and increased phase-space of the new NGC beam at the NCNR.

A Mott polarimeter with a design optimized for 5.5 MeV/c has been in routine use at the CEBAF accelerator for well over a decade, providing polarization measurements approaching 1% accuracy. Measurements with different target materials (Au, Ag, Cu) over a range of target thicknesses (100 – 10,000 Å), and beam energies between 2 and 8 MeV allow us to determine the effective analyzing power with a high degree of certainty. Recent and planned improvements in our polarimeter configuration, detectors and data acquisition system, coupled with a low 31 MHz repetition rate beam allow us to distinguish and suppress electrons that do not originate from the target foil. This work coupled with a significant effort to produce a detailed GEANT4 model of the polarimeter is part of an effort to determine systematic uncertainties at the level of the theoretically calculated analyzing power. We describe our activities and a series of planned measurements that will allow us to demonstrate and possibly improve the precision and accuracy of polarization measurements at JLab, as required for future parity violation experiments.

The Q(weak) experiment has measured the parity-violating asymmetry in ep elastic scattering at Q(2)=0.025(GeV/c)(2), employing 145 μA of 89% longitudinally polarized electrons on a 34.4 cm long liquid hydrogen target at Jefferson Lab. The results of the experiment's commissioning run, constituting approximately 4% of the data collected in the experiment, are reported here. From these initial results, the measured asymmetry is A(ep)=-279±35 (stat) ± 31 (syst) ppb, which is the smallest and most precise asymmetry ever measured in ep scattering. The small Q(2) of this experiment has made possible the first determination of the weak charge of the proton Q(W)(p) by incorporating earlier parity-violating electron scattering (PVES) data at higher Q(2) to constrain hadronic corrections. The value of Q(W)(p) obtained in this way is Q(W)(p)(PVES)=0.064±0.012, which is in good agreement with the standard model prediction of Q(W)(p)(SM)=0.0710±0.0007. When this result is further combined with the Cs atomic parity violation (APV) measurement, significant constraints on the weak charges of the up and down quarks can also be extracted. That PVES+APV analysis reveals the neutron's weak charge to be Q(W)(n)(PVES+APV)=-0.975±0.010.

In May 2012, the \(Q^{p}_{\rm Weak}\) collaboration completed a two year measurement program to determine the weak charge of the proton \({Q^{p}_W} = ( 1 - 4\sin^2{\theta_{W}})\) at the Thomas Jefferson National Accelerator Facility (TJNAF). The experiment was designed to produce a 4.0 % measurement of the weak charge, via a 2.5 % measurement of the parity violating asymmetry in the number of elastically scattered 1.165 GeV electrons from protons, at forward angles. At the proposed precision, the experiment would produce a 0.3 % measurement of the weak mixing angle at a momentum transfer of Q 2 = 0.026 GeV2, making it the most precise stand alone measurement of the weak mixing angle at low momentum transfer. In combination with other parity measurements, \(Q^{p}_{\rm Weak}\) will also provide a high precision determination of the weak charges of the up and down quarks. At the proposed precision, a significant deviation from the Standard Model prediction could be a signal of new physics at mass scales up to ≃ 6 TeV, whereas agreement would place new and significant constraints on possible Standard Model extensions at mass scales up to ≃ 2 TeV. This paper provides an overview of the physics and the experiment, as well as a brief look at some preliminary diagnostic and analysis data.

A large set of cross sections for semi-inclusive electroproduction of charged pions ($\pi^\pm$) from both proton and deuteron targets was measured. The data are in the deep-inelastic scattering region with invariant mass squared $W^2$ > 4 GeV$^2$ and range in four-momentum transfer squared $2 < Q^2 < 4$ (GeV/c)$^2$, and cover a range in the Bjorken scaling variable 0.2 < x < 0.6. The fractional energy of the pions spans a range 0.3 < z < 1, with small transverse momenta with respect to the virtual-photon direction, $P_t^2 < 0.2$ (GeV/c)$^2$. The invariant mass that goes undetected, $M_x$ or W', is in the nucleon resonance region, W' < 2 GeV. The new data conclusively show the onset of quark-hadron duality in this process, and the relation of this phenomenon to the high-energy factorization ansatz of electron-quark scattering and subsequent quark --> pion production mechanisms. The x, z and $P_t^2$ dependences of several ratios (the ratios of favored-unfavored fragmentation functions, charged pion ratios, deuteron-hydrogen and aluminum-deuteron ratios for $\pi^+$ and $\pi^-$) have been studied. The ratios are found to be in good agreement with expectations based upon a high-energy quark-parton model description. We find the azimuthal dependences to be small, as compared to exclusive pion electroproduction, and consistent with theoretical expectations based on tree-level factorization in terms of transverse-momentum-dependent parton distribution and fragmentation functions. In the context of a simple model, the initial transverse momenta of $d$ quarks are found to be slightly smaller than for $u$ quarks, while the transverse momentum width of the favored fragmentation function is about the same as for the unfavored one, and both fragmentation widths are larger than the quark widths.

The weak interaction between nucleons leads to parity violation in various reaction observables. Neutron spin rotation, the rotation of the plane of polarization of a transversely polarized neutron beam passing through unpolarized matter, is an especially clear example of a breakdown in mirror symmetry. The Neutron Spin Rotation (NSR) Collaboration is engaged in an experimental program to observe parity-odd neutron spin rotation. We recently completed the first phase of an experiment to measure parity violating neutron spin rotation in He-4. Our result for the neutron spin rotation angle per unit length in He-4, d phi/dz = (+ 1.7 +/- 9.1(stat.) +/- 1.4(sys.)) x 10(-7) rad/m, is the most sensitive search for neutron weak optical activity yet performed and represents a significant advance in precision in comparison to past measurements in heavy nuclei. This experiment was performed at the NG-6 slow neutron beamline at the National Institute of Standards and Technology (NIST) Center for Neutron Research. The systematic uncertainty is small enough to proceed to the second phase of the He-4 measurement at the new NG-C slow neutron beamline under construction at NIST. The projected intensity of this beam is high enough to see parity odd neutron spin rotation in He-4 and to seriously consider a future experiment to measure neutron spin rotation in hydrogen.

We propose a new precision measurement of parity-violating electron scattering on the proton at very low Q^2 and forward angles to challenge predictions of the Standard Model and search for new physics. A unique opportunity exists to carry out the first precision measurement of the proton's weak charge, Q_W =1 - 4sin^2θ_W. A 2200 hour measurement of the parity violating asymmetry in elastic ep scattering at Q^2=0.03 (GeV/c)^2 employing 180 μA of 85 determine the proton's weak charge with approximately 4 and systematic errors. The Standard Model makes a firm prediction of Q_W, based on the running of the weak mixing angle from the Z0 pole down to low energies, corresponding to a 10 sigma effect in this experiment.

We present new measurements of electron scattering from high-momentum nucleons in nuclei. These data allow an improved determination of the strength of two-nucleon correlations for several nuclei, including light nuclei where clustering effects can, for the first time, be examined. The data also include the kinematic region where three-nucleon correlations are expected to dominate.

We have determined the transparency of the nuclear medium to kaons from $A(e,e^{'} K^{+})$ measurements on $^{12}$C, $^{63}$Cu, and $^{197}$Au targets. The measurements were performed at the Jefferson Laboratory and span a range in four-momentum-transfer squared Q$^2$=1.1 -- 3.0 GeV$^2$. The nuclear transparency was defined as the ratio of measured kaon electroproduction cross sections with respect to deuterium, ($\sigma^{A}/\sigma^{D}$). We further extracted the atomic number ($A$) dependence of the transparency as parametrized by $T= (A/2)^{\alpha-1}$ and, within a simple model assumption, the in-medium effective kaon-nucleon cross sections. The effective cross sections extracted from the electroproduction data are found to be smaller than the free cross sections determined from kaon-nucleon scattering experiments, and the parameter $\alpha$ was found to be significantly larger than those obtained from kaon-nucleus scattering. We have included similar comparisons between pion- and proton-nucleon effective cross sections as determined from electron scattering experiments, and pion-nucleus and proton-nucleus scattering data.

We report an upper bound on parity-violating neutron spin rotation in He-4. This experiment is the most sensitive search for neutron-weak optical activity yet performed and represents a significant advance in precision in comparison to past measurements in heavy nuclei. The experiment was performed at the NG-6 slow-neutron beamline at the National Institute of Standards and Technology (NIST) Center for Neutron Research. Our result for the neutron spin rotation angle per unit length in He-4 is d phi/dz = [+ 1.7 +/- 9.1(stat.) +/- 1.4(sys.)] x 10(-7) rad/m. The statistical uncertainty is smaller than current estimates of the range of possible values of d phi/dz in n+He-4.

Cross sections for the H-1(e,e'pi(+))n process on H-1, H-2, C-12, Al-27, Cu-63, and Au-197 targets were measured at the Thomas Jefferson National Accelerator Facility ( Jefferson Lab) to extract nuclear transparencies. Data were taken from Q(2) = 1.1- 4.7 GeV2 for a fixed center-of-mass energy of W = 2.14 GeV. The ratio of sigma(L) and sigma(T) was extracted from the measured cross sections for H-1, H-2, C-12, and Cu-63 targets at Q(2) = 2.15 and 4.0 GeV2, allowing for additional studies of the reaction mechanism. In this article, we present the experimental setup and the analysis of the data in detail, including systematic uncertainty studies. Differential cross sections and nuclear transparencies as a function of the pion momentum at different values of Q(2) are presented. Our results are consistent with the predicted early onset of color transparency in mesons. Global features of the data are discussed and the data are compared with model calculations for the H-1(e,e'pi(+)) n reaction from nuclear targets.

N. Fomin, 2 J. Arrington, D. B. Day, D. Gaskell, A. Daniel, J. Seely, R. Asaturyan, ∗ F. Benmokhtar, W. Boeglin, B. Boillat, P. Bosted, A. Bruell, M. H. S. Bukhari, M. E. Christy, E. Chudakov, B. Clasie, S. H. Connell, M. M. Dalton, D. Dutta, 14 R. Ent, L. El Fassi, H. Fenker, B. W. Filippone, K. Garrow, C. Hill, R. J. Holt, T. Horn, 4 M. K. Jones, J. Jourdan, N. Kalantarians, C. E. Keppel, 11 D. Kiselev, M. Kotulla, R. Lindgren, A. F. Lung, S. Malace, P. Markowitz, P. McKee, D. G. Meekins, T. Miyoshi, H. Mkrtchyan, T. Navasardyan, G. Niculescu, Y. Okayasu, A. K. Opper, C. Perdrisat, D. H. Potterveld, V. Punjabi, X. Qian, P. E. Reimer, J. Roche, 4 V.M. Rodriguez, O. Rondon, E. Schulte, E. Segbefia, K. Slifer, G. R. Smith, P. Solvignon, V. Tadevosyan, S. Tajima, L. Tang, 11 G. Testa, R. Trojer, V. Tvaskis, W. F. Vulcan, C. Wasko, F. R. Wesselmann, S. A. Wood, J. Wright, and X. Zheng 3 University of Virginia, Charlottesville, VA, USA University of Tennessee, Knoxville, TN, USA Physics Division, Argonne National Laboratory, Argonne, IL, USA Thomas Jefferson National Laboratory, Newport News, VA, USA University of Houston, Houston, TX, USA Massachusetts Institute of Technology, Cambridge, MA, USA Yerevan Physics Institute, Armenia University of Maryland, College Park, MD, USA Florida International University, Miami, FL, USA Basel University, Basel, Switzerland Hampton University, Hampton, VA, USA University of Johannesburg, South Africa Mississippi State University, Jackson, MS, USA Duke University, Durham, NC, USA Kellogg Radiation Laboratory, California Institute of Technology, Pasadena, CA, USA TRIUMF, Vancouver, British Columbia, Canada Tohoku University, Sendai, Japan James Madison University, Harrisonburg, VA, USA Ohio University, Athens, OH, USA College of William and Mary, Williamsburg, VA, USA Norfolk State University, Norfolk, VA, USA (Dated: January 1, 2013)

Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. 197 Au targets were measured at the Thomas Jefferson National Accelerator Facility (Jefferson Lab) to extract nuclear transparencies. Data were taken from Q 2 = 1.1–4.7 GeV 2 for a fixed center-of-mass energy of W = 2.14 GeV. The ratio of σ L and σ T was extracted from the measured cross sections for 1 H, 2 H, 12 C, and 63 Cu targets at Q 2 = 2.15 and 4.0 GeV 2 , allowing for additional studies of the reaction mechanism. In this article, we present the experimental setup and the analysis of the data in detail, including systematic uncertainty studies. Differential cross sections and nuclear transparencies as a function of the pion momentum at different values of Q 2 are presented. Our results are consistent with the predicted early onset of color transparency in mesons. Global features of the data are discussed and the data are compared with model calculations for the 1 H(e,e π +)n reaction from nuclear targets.