Search

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.

Measurements of parity-violating neutron spin rotation can provide insight into the poorly understood nucleon–nucleon weak interaction. Because the expected rotation angle per unit length is small (10 −7 rad/m), several properties of the polarized cold neutron beam phase space and the neutron optical elements of the polarimeter must be measured to quantify possible systematic effects. This paper presents (1) an analysis of a class of possible systematic uncertainties in neutron spin rotation measurements associated with the neutron polarimetry, and (2) measurements of the relevant neutron beam properties (intensity distribution, energy spectrum, and the product of the neutron beam polarization and the analyzing power as a function of the beam phase space properties) on the NG-6 cold neutron beam-line at the National Institute of Standards and Technology Center for Neutron Research. We conclude that the phase space nonuniformities of the polarimeter in this beam are small enough that a parity-violating neutron spin rotation measurement in n- 4 He with systematic uncertainties at the 10 −7 rad/m level is possible.

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.

Recoil polarimetry was used to extract the ratio of the proton electromagnetic form factors, μ p G E p / G M p = 0.878 ± 0.064 (stat) ± 0.012 (sys) , at Q 2 = 1.13 ( GeV / c ) 2 from the reaction H 1 ( e → , e p → ) . This was an ancillary measurement in which the proton polarization was determined as part of a larger program utilizing a stand-alone polarimeter designed to measure μ n G E n / G M n . This measurement complements previous recoil polarimetry measurements of μ p G E p / G M p made at the Thomas Jefferson National Accelerator Facility.

The ratio of the electric to the magnetic form factor of the neutron, \(g \equiv G_{En}/G_{Mn}\), was measured via recoil polarimetry (R.G. Arnold, C.E. Carlson, F. Gross, Phys. Rev. C 23, 363 (1981)) from the quasielastic 2H\((\mathop{e}\limits^{\scriptstyle\to},e' \mathop{n}\limits^{\scriptstyle\to} )\)1H reaction at three values of Q2 (viz, 0.45, 1.15, and 1.47 (GeV/c)2) in Hall C of the Thomas Jefferson National Accelerator Facility. The data reveal that GEn continues to follow the Galster parameterization up to Q2 = 1.15 (GeV/c)2 and rises above the Galster parameterization at Q2 = 1.47 (GeV/c)2.

The Jefferson Lab Q weak experiment determined the weak charge of the proton by measuring the parity-violating elastic scattering asymmetry of longitudinally polarized electrons from an unpolarized liquid hydrogen target at small momentum transfer. A custom apparatus was designed for this experiment to meet the technical challenges presented by the smallest and most precise e → p asymmetry ever measured. Technical milestones were achieved at Jefferson Lab in target power, beam current, beam helicity reversal rate, polarimetry, detected rates, and control of helicity-correlated beam properties. The experiment employed 180 μA of 89% longitudinally polarized electrons whose helicity was reversed 960 times per second. The electrons were accelerated to 1.16 GeV and directed to a beamline with extensive instrumentation to measure helicity-correlated beam properties that can induce false asymmetries. Moller and Compton polarimetry were used to measure the electron beam polarization to better than 1%. The electron beam was incident on a 34.4 cm liquid hydrogen target. After passing through a triple collimator system, scattered electrons between 5.8° and 11.6° were bent in the toroidal magnetic field of a resistive copper-coil magnet. The electrons inside this acceptance were focused onto eight fused silica Cherenkov detectors arrayed symmetrically around the beam axis. A total scattered electron rate of about 7 GHz was incident on the detector array. The detectors were read out in integrating mode by custom-built low-noise pre-amplifiers and 18-bit sampling ADC modules. The momentum transfer Q 2 =0.025 GeV 2 was determined using dedicated low-current ( ~ 100 pA ) measurements with a set of drift chambers before (and a set of drift chambers and trigger scintillation counters after) the toroidal magnet.

The Q weak 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 Q 2 = 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.

We report an upper bound on parity-violating neutron spin rotation in $^{4}\mathrm{He}$. 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 $^{4}\mathrm{He}$ is $d\ensuremath{\phi}/\mathit{dz}=[+1.7\ifmmode\pm\else\textpm\fi{}9.1(\text{stat.})\ifmmode\pm\else\textpm\fi{}1.4(\text{sys.})]\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$ rad/m. The statistical uncertainty is smaller than current estimates of the range of possible values of $d\ensuremath{\phi}/\mathit{dz}$ in $n{+}^{4}$He.

The importance of high quality spin-observable data in intermediate energy nuclear physics requires that techniques be developed for obtaining accurate and reliable measurements of incident beam polarizations in a nondestructive manner. For this purpose, we have constructed, installed, and calibrated two transmission-type polarimeters at IUCF, in which p+d elastic scattering is used to determine the two transverse polarization components of a high energy proton beam. In this report, we discuss some of our experimental design considerations, and provide technical details on the actual implementation schemes chosen. Representative spectra are shown, and typical performance capabilities are described. We have also completed a calibration of the effective analyzing power of the polarimeter (Apol) as a function of the incident (beam) proton energy, and find Apol ≈ 0.51 over the entire proton energy range studied in this work ( ∼ 150–200 MeV) when θdlab = 42.6°.

The analyzing power ${\mathit{A}}_{\mathit{y}}$ for proton elastic scattering on $^{12}\mathrm{C}$ has been precisely and absolutely determined at three scattering angles (${\mathrm{\ensuremath{\theta}}}_{\mathrm{lab}}$=16.3\ifmmode^\circ\else\textdegree\fi{}, 17.3\ifmmode^\circ\else\textdegree\fi{}, and 18.3\ifmmode^\circ\else\textdegree\fi{}) at an incident proton energy of 188.9 MeV. The technique employed requires statistically high quality polarization transfer measurements, combined with the constraints imposed on polarization observables for reactions with the spin structure 1/2+0\ensuremath{\rightarrow}1/2+0. These results represent some of the most accurately known spin observables at intermediate energies and serve as calibration points for secondary standards.

Measurements of the unpolarized triple differential cross section and the {ital A}{sub {ital y}{ital y}} tensor analyzing power for the {sup 1}H({ital d},{ital pp}){ital n} reaction were made using a 94.5 MeV polarized deuteron beam at the Indiana University Cyclotron Facility. Scattering angles ({theta} and {phi}) and energy information were recorded for the two emerging protons using large-area wire chambers backed by stopping plastic scintillator detectors. Events were selected that were close to the symmetric constant relative energy geometry in order to enhance the sensitivity of the observables to off-shell and three-body effects. The measurements covered values of {alpha}, the center-of-mass angle between the incoming proton and the outgoing neutron, from 72{degree} to 180{degree}. Comparisons are made to Faddeev calculations that use either separable potentials or an exact treatment of the {ital S}-wave nucleon-nucleon interaction in conjunction with a perturbative treatment of higher partial waves. While none of these calculations, which use only two-nucleon interactions, is completely satisfactory, there remains too much variation among different theoretical treatments to demonstrate the need for including additional dynamical features in the three-body model.

The NuMI neutrino beam line and the MINOS experiment represent a major investment of US High Energy Physics in the area of neutrino physics. The forthcoming results could decisively establish neutrino oscillations as the underlying physics mechanism for the atmospheric $\numu$ deficit and provide a precise measurement of the corresponding oscillation parameters, $\dmsq23$ and $\sinsq2t23$.neutrino sector may well be within our reach. The full potential of the NuMI neutrino beam can be exploited by complementing the MINOS detector, under construction, with a new detector(s) placed at some off-axis position and collecting data in parallel with MINOS. The first phase of the proposed program includes a new detector, optimized for $\nue$ detection, with a fiducial mass of the order of 20 kton and exposed to neutrino and antineutrino beams. In a five year run its sensitivity to the $\numutonue$ oscillations will be at least a factor of ten beyond the current limit. The future direction of the program will depend on the results of this first phase, but it is very likely that it will be a combination of a significant increase of the neutrino beam intensity via an upgraded proton source and an increase of the detector mass by a factor of five or so. Depending on the circumstances, the goals of Phase II may be a further increase of the sensitivity of a search for $\numutonue$ oscillations, or, perhaps, a measurement of the CP violating phase $\delta$ in the lepton sector.

Distorted wave calculations of large l n -transfer (d, p) reactions near 90 MeV indicate that far-side dominance reduces the number of effective amplitudes at large angles to two or three, making many spin observables redundant. Comparisons of A y with A yy data for 116 Sn(d, p) 117 Sn, and A yy with p y data for 66 Zn(d, p) 67 Zn, do not confirm this expectation.