S. Covrig

First Determination of the Weak Charge of the Proton

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 experiments 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 neutrons weak charge to be Q(W)(n)(PVES+APV)=-0.975±0.010.

NPDGamma: A measurement of the parity-violating gamma asymmetry A γ in n-p capture

The NPDGamma Experiment measures the parity-violating correlation Aγ between neutron spin and photon momentum in the reaction + p → d + γ. Knowledge of Aγ and other parity-violating observables in few-body nuclear systems will provide constraints for a parameterized description of ΔS = 0 parity-violating phenomena free from complications of nuclear structure. The NPDGamma experiment uses a polarized cold pulsed neutron beam, a liquid parahydrogen target, and a cylindrical array of 48 CsI(Tl) scintillation detectors operated in current mode to search for the asymmetry. NPDGamma recently completed the first phase of the program to measure Aγ at the Los Alamos Neutron Science Center with the preliminary result Aγ = (−1.2 ± 2.1(stat.) ± 0.1(sys.)) × 10−7, reproducing the previous upper limit from a measurement at a reactor facility. We discuss the theoretical background and experimental method and report on preliminary analysis of the LANSCE data. The second phase of the program to measure Aγ is in progress at the Spallation Neutron Source at Oak Ridge National Laboratory.

Primary beam steering due to field leakage from superconducting SHMS magnets

Simulations of the magnetic fields from the Super High Momentum Spectrometer in Hall C at Thomas Jefferson National Accelerator Facility show significant field leakage into the region of the primary beam line between the target and the beam dump. Without mitigation, these remnant fields will steer the unscattered beam enough to limit beam operations at small scattering angles. Presented here are magnetic field simulations of the spectrometer magnets and a solution using optimal placement of a minimal amount of shielding iron around the beam line.

Early Results from the Q{sub weak} Experiment

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 scattering to provide the first determination of the proton’s weak charge . The experiment employed a 180 μ A longitudinally polarized 1.16 GeV electron beam on a 35 cm long liquid hydrogen target. Scattered electrons in the angular range 6° θ 2 = 0.025 GeV 2 were detected in eight Cerenkov detectors arrayed symmetrically around the beam axis. The goals of the experiment were to provide a measure of to 4.2% (combined statisstatistical and systematic error), which implies a measure of sin 2 ( θ w ) at the level of 0.3%, and to help constrain the vector weak quark charges C 1 u and C 1 d . The experimental method is described, with particular focus on the challenges associated with the world’s highest power LH 2 target. The new constraints on C 1 u and C 1 d provided by the subset of the experiment’s data analyzed to date will also be shown, together with the extracted weak charge of the neutron.

Early results from the

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 scattering to provide the first determination of the proton’s weak charge . The experiment employed a 180 μ A longitudinally polarized 1.16 GeV electron beam on a 35 cm long liquid hydrogen target. Scattered electrons in the angular range 6° θ 2 = 0.025 GeV 2 were detected in eight Cerenkov detectors arrayed symmetrically around the beam axis. The goals of the experiment were to provide a measure of to 4.2% (combined statisstatistical and systematic error), which implies a measure of sin 2 ( θ w ) at the level of 0.3%, and to help constrain the vector weak quark charges C 1 u and C 1 d . The experimental method is described, with particular focus on the challenges associated with the world’s highest power LH 2 target. The new constraints on C 1 u and C 1 d provided by the subset of the experiment’s data analyzed to date will also be shown, together with the extracted weak charge of the neutron.