13 Search Results

In this paper, we report the first measurement of the parity-violating elastic electron scattering asymmetry on ²⁷Al. The ²⁷Al elastic asymmetry is $$A_{\text{PV}}$$ = 2.16 ± 0.11(stat) ± 0.16(syst) ppm, and was measured at $$\langle Q^2\rangle$$ = 0.02357 ± 0.00010 GeV$^2$, $$\angleθ_{\text{lab}}$$ = 7.61° ± 0.02°, and $$\langle E_{\text{lab}}$$ = 1.157 GeV with the Q_weak apparatus at Jefferson Lab. Predictions using a simple Born approximation as well as more sophisticated distorted-wave calculations are in good agreement with this result. From this asymmetry the ²⁷Al neutron radius $$R_n$$ = 2.89 ± 0.12 fm was determined using a many-models correlation technique. Themore » corresponding neutron skin thickness $$R_n – R_p$$ = –0.04 ± 0.12 fm is small, as expected for a light nucleus with a neutron excess of only 1. This result thus serves as a successful benchmark for electroweak determinations of neutron radii on heavier nuclei. A tree-level approach was used to extract the ²⁷Al weak radius $$R_w$$ = 3.00 ± 0.15 fm, and the weak skin thickness $$R_{\text{wk}} – R_{\text{ch}}$$ = –0.04 ± 0.15 fm. The weak form factor at this $Q^2$ is $$F_{\text{wk}}$$ = 0.39 ± 0.04.« less

We report measurements of the parity-conserving beam-normal single-spin elastic scattering asymmetries B_n on ¹²C and ²⁷Al, obtained with an electron beam polarized transverse to its momentum direction. These measurements add an additional kinematic point to a series of previous measurements of B_n on ¹²C and provide a first measurement on ²⁷Al. The experiment utilized the Q_weak apparatus at Jefferson Lab with a beam energy of 1.158 GeV. The average lab scattering angle for both targets was 7.7°, and the average Q² for both targets was 0.02437 GeV² (Q = 0.1561 GeV). The asymmetries are B_n = -10.68 ± 0.90 (stat)more » ± 0.57 (syst) ppm ¹²C and B_n = -12.16 ± 0.58 (stat) ± 0.62 (syst) ppm for ²⁷Al. The results are consistent with theoretical predictions, and are compared to existing data. When scaled by Z/A, the Q dependence of all the far-forward angle (θ < 10°) data from ¹H to ²⁷Al can be described by the same slope out to Q ≈ 0.35 GeV. Larger-angle data from other experiments in the same Q range are consistent with a slope about twice as steep.« less

A beam-normal single-spin asymmetry generated in the scattering of transversely polarized electrons from unpolarized nucleons is an observable related to the imaginary part of the two-photon exchange process. Here we report a 2% precision measurement of the beam-normal single-spin asymmetry in elastic electron-proton scattering with a mean scattering angle of ${\theta }_{\mathrm{lab}}=7.9°$ and a mean energy of 1.149 GeV. The asymmetry result is $B_n=–5.194\pm 0.067\left(\mathrm{stat}\right)\pm 0.082$more » (syst) ppm. This is the most precise measurement of this quantity available to date and therefore provides a stringent test of two-photon exchange models at far-forward scattering angles ( ${\theta }_{\mathrm{lab}}\to 0$) where they should be most reliable.« less

We report the measurement of the parity-violating asymmetry for the inelastic scattering of electrons from the proton, at $Q^2 = 0.082$ GeV$^2$ and $ W = 2.23$ GeV, above the resonance region. The result $$A_{\rm Inel} = - 13.5 \pm 2.0 ({\rm stat}) \pm 3.9 ({\rm syst})$$ ppm agrees with theoretical calculations, and helps to validate the modeling of the $$\gamma Z$$ interference structure functions $$F_1^{\gamma Z}$$ and $$F_2^{\gamma Z}$$ used in those calculations, which are also used for determination of the two-boson exchange box diagram ($$\Box_{\gamma Z}$$) contribution to parity-violating elastic scattering measurements. A positive parity-violating asymmetry for inclusivemore » $$\pi^-$$ production was observed, as well as positive beam-normal single-spin asymmetry for scattered electrons and a negative beam-normal single-spin asymmetry for inclusive $$\pi^-$$ production.« less

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 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 asmore » the method of extracting the weak charge of the proton. Finally, 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.« less

The Nab experiment will measure the correlation a between the momenta of the beta particle and antineutrino in neutron decay as well as the Fierz term b which distorts the beta spectrum.

Here, the Q-weak experiment at Jefferson Laboratory measured the parity violating asymmetry (A$$_{PV}$$ ) 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 themore » 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.« less

The Jefferson Lab $Q_{\mathrm{weak}}$ 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 created for this experiment to meet the technical challenges presented by the smallest and most precise $\overrightarrow{e}\mathrm{p}$ 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 beammore » 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. Møller 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 Q²=0.025 GeV² was observed using dedicated low-current $(~100\mathrm{pA})$ measurements with a set of drift chambers before (and a set of drift chambers and trigger scintillation counters after) the toroidal magnet« less

The parity-violating (PV) asymmetry of inclusive π^- production in electron scattering from a liquid deuterium target was measured at backward angles. The measurement was conducted as a part of the G0 experiment, at a beam energy of 360 MeV. The physics process dominating pion production for these kinematics is quasi-free photoproduction off the neutron via the Δ⁰ resonance. In the context of heavy-baryon chiral perturbation theory (HBχPT), this asymmetry is related to a low energy constant d_Δ^- that characterizes the parity-violating γNΔ coupling. Zhu et al. calculated d_Δ^- in a model benchmarked by the large asymmetries seen in hyperon weakmore » radiative decays, and predicted potentially large asymmetries for this process, ranging from A_γ^- = -5.2 to +5.2 ppm. The measurement performed in this work leads to A_γ^- = -0.36 ± 1.06 ± 0.37 ± 0.03 ppm (where sources of statistical, systematic and theoretical uncertainties are included), which would disfavor enchancements considered by Zhu et al. proportional to V_ud/V_us. The measurement is part of a program of inelastic scattering measurements that were conducted by the G0 experiment, seeking to determine the N-Δ axial transition form-factors using PV electron scattering.« less