Search

We report the first measurement of the parity-violating elastic electron scattering asymmetry on 27Al. The 27Al elastic asymmetry is A_PV = 2.16 +- 0.11 (stat) +- 0.16 (syst) ppm, and was measured at =0.02357 +- 0.0001 GeV^2, = 7.61 +- 0.02 degrees, and = 1.157 GeV with the Qweak 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. The 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_wk - R_ch = -0.04 +- 0.15 fm. The weak form factor at this Q^2 is F_wk = 0.39 +- 0.04., Comment: Revised version after referee's comments and suggestions. Some improvements and clarification to the text, no changes to the figures, tables, results or conclusions. 7 pages, 3 figures, 3 tables

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. 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_lab = 7.9 degrees and a mean energy of 1.149 GeV. The asymmetry result is B_n = -5.194 +- 0.067 (stat) +- 0.082 (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_lab -> 0) where they should be most reliable., Comment: 6 pages, 3 figures; Slightly revised version, after referee's comments; accepted in PRL

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 inclusive $\pi^-$ 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., Comment: 18 pages, 9 figures, version accepted in Physical Review C

The fields of particle and nuclear physics have undertaken extensive programs to search for evidence of physics beyond that explained by current theories. The observation of the Higgs boson at the Large Hadron Collider completed the set of particles predicted by the Standard Model (SM), currently the best description of fundamental particles and forces. However, the theory's limitations include a failure to predict fundamental parameters and the inability to account for dark matter/energy, gravity, and the matter-antimater asymmetry in the universe, among other phenomena. Given the lack of additional particles found so far through direct searches in the post-Higgs era, indirect searches utilizing precise measurements of well predicted SM observables allow highly targeted alternative tests for physics beyond the SM. Indirect searches have the potential to reach mass/energy scales beyond those directly accessible by today's high-energy accelerators. The value of the weak charge of the proton Q_W^p is an example of such an indirect search, as it sets the strength of the proton's interaction with particles via the well-predicted neutral electroweak force. Parity violation (invariance under spatial inversion (x,y,z) -> (-x,-y,-z)) is violated only in the weak interaction, thus providing a unique tool to isolate the weak interaction in order to measure the proton's weak charge. Here we report Q_W^p=0.0719+-0.0045, as extracted from our measured parity-violating (PV) polarized electron-proton scattering asymmetry, A_ep=-226.5+-9.3 ppb. Our value of Q_W^p is in excellent agreement with the SM, and sets multi-TeV-scale constraints on any semi-leptonic PV physics not described within the SM., Comment: Direct link to Nature Version "https://rdcu.be/954U"

The cross section of atomic electron Compton scattering $\gamma + e \rightarrow \gamma^\prime + e^\prime $ was measured in the 4.40--5.475 GeV photon beam energy region by the {\em PrimEx} collaboration at Jefferson Lab with an accuracy of 2\% and less. The results are consistent with theoretical predictions that include next-to-leading order radiative corrections. The measurements provide the first high precision test of this elementary QED process at beam energies greater than 0.1 GeV., Comment: 6 pages, 4 figures

Neutron spin rotation is expected from quark-quark weak interactions in the Standard Model, which induce weak interactions among nucleons that violate parity. We present the results from an experiment searching for the effect of parity violation via the spin rotation of polarized neutrons in a liquid $^{4}$He medium. The value for the neutron spin rotation angle per unit length in $^{4}$He, $d\phi/dz =(+2.1 \pm 8.3 (stat.) \pm 2.9 (sys.))\times10^{-7}$ rad/m, is consistent with zero. The result agrees with the best current theoretical estimates of the size of nucleon-nucleon weak amplitudes from other experiments and with the expectations from recent theoretical approaches to weak nucleon-nucleon interactions. In this paper we review the theoretical status of parity violation in the $\vec{n}+^{4}$He system and discuss details of the data analysis leading to the quoted result. Analysis tools are presented that quantify systematic uncertainties in this measurement and that are expected to be essential for future measurements., Comment: 15 pages, 7 figures

We have performed a novel comparison between electron-beam polarimeters based on M{\o}ller and Compton scattering. A sequence of electron-beam polarization measurements were performed at low beam currents ($<$ 5 $\mu$A) during the $Q_{\rm weak}$ experiment in Hall C at Jefferson Lab. These low current measurements were bracketed by the regular high current (180 $\mu$A) operation of the Compton polarimeter. All measurements were found to be consistent within experimental uncertainties of 1% or less, demonstrating that electron polarization does not depend significantly on the beam current. This result lends confidence to the common practice of applying M{\o}ller measurements made at low beam currents to physics experiments performed at higher beam currents. The agreement between two polarimetry techniques based on independent physical processes sets an important benchmark for future precision asymmetry measurements that require sub-1% precision in polarimetry.

We report on the highest precision yet achieved in the measurement of the polarization of a low energy, $\mathcal{O}$(1 GeV), electron beam, accomplished using a new polarimeter based on electron-photon scattering, in Hall~C at Jefferson Lab. A number of technical innovations were necessary, including a novel method for precise control of the laser polarization in a cavity and a novel diamond micro-strip detector which was able to capture most of the spectrum of scattered electrons. The data analysis technique exploited track finding, the high granularity of the detector and its large acceptance. The polarization of the $180~\mu$A, $1.16$~GeV electron beam was measured with a statistical precision of $<$~1\% per hour and a systematic uncertainty of 0.59\%. This exceeds the level of precision required by the \qweak experiment, a measurement of the vector weak charge of the proton. Proposed future low-energy experiments require polarization uncertainty $<$~0.4\%, and this result represents an important demonstration of that possibility. This measurement is also the first use of diamond detectors for particle tracking in an experiment., Comment: 9 pages, 7 figures, published in PRX

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 ${\vec{e}}$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 microA 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 degrees and 11.6 degrees were bent in the toroidal magnetic field of a resistive copper-coil magnet. The electrons inside this acceptance were focused onto eight fused silica Cerenkov 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., Comment: 48 pages, 36 figures. Accepted by Nuclear Instruments and Methods A

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., Comment: 8 pages, 4 figures, INPC2013

The Qweak experiment has measured the parity-violating asymmetry in polarized e-p elastic scattering at Q^2 = 0.025(GeV/c)^2, employing 145 microamps of 89% longitudinally polarized electrons on a 34.4cm long liquid hydrogen target at Jefferson Lab. The results of the experiment's commissioning run are reported here, constituting approximately 4% of the data collected in the experiment. From these initial results the measured asymmetry is Aep = -279 +- 35 (statistics) +- 31 (systematics) ppb, which is the smallest and most precise asymmetry ever measured in polarized e-p scattering. The small Q^2 of this experiment has made possible the first determination of the weak charge of the proton, QpW, by incorporating earlier parity-violating electron scattering (PVES) data at higher Q^2 to constrain hadronic corrections. The value of QpW obtained in this way is QpW(PVES) = 0.064 +- 0.012, in good agreement with the Standard Model prediction of QpW(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 QnW(PVES+APV) = -0.975 +- 0.010., Comment: 7 pages, 3 figures

The parity-violating asymmetry arising from inelastic electron-nucleon scattering at backward angle (~95 degrees) near the Delta(1232) resonance has been measured using a hydrogen target. From this asymmetry, we extracted the axial transition form factor G^A_{N\Delta}, a function of the axial Adler form factors C^A_i. Though G^A_{N\Delta} has been previously studied using charged current reactions, this is the first measurement of the weak neutral current excitation of the Delta using a proton target. For Q^2 = 0.34 (GeV/c)^2 and W = 1.18 GeV, the asymmetry was measured to be -33.4 \pm (5.3)_{stat} \pm (5.1)_{sys} ppm. The value of G^A_{N\Delta} determined from the hydrogen asymmetry was -0.05 \pm (0.35)_{stat} \pm (0.34)_{sys} \pm (0.06)_{theory}. These findings agree within errors with theoretical predictions for both the total asymmetry and the form factor. In addition to the hydrogen measurement, the asymmetry was measured at the same kinematics using a deuterium target. The asymmetry for deuterium was determined to be -43.6 \pm (14.6)_{stat} \pm (6.2)_{sys} ppm., Comment: 8 pages, 3 figures, submitted to Physics Letters B

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, $Q_W =1 - 4\sin^2\theta_W$. A 2200 hour measurement of the parity violating asymmetry in elastic ep scattering at Q^2=0.03 (GeV/c)^2 employing 180 $\mu$A of 85% polarized beam on a 35 cm liquid Hydrogen target will determine the proton's weak charge with approximately 4% combined statistical 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., Comment: Proposal to Jefferson Lab Program Advisory Committee, December 2007. 93 pages, 44 figures. Uploaded here so a permanent copy is available, and for other papers to use arXiv version as reference First four authors are the spokespersons

The parity-violating (PV) asymmetry of inclusive $\pi^-$ 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 $\Delta^0$ resonance. In the context of heavy-baryon chiral perturbation theory (HB$\chi$PT), this asymmetry is related to a low energy constant $d_\Delta^-$ that characterizes the parity-violating $\gamma$N$\Delta$ coupling. Zhu et al. calculated $d_\Delta^-$ in a model benchmarked by the large asymmetries seen in hyperon weak radiative decays, and predicted potentially large asymmetries for this process, ranging from $A_\gamma^-=-5.2$ to $+5.2$ ppm. The measurement performed in this work leads to $A_\gamma^-=-0.36\pm 1.06\pm 0.37\pm 0.03$ 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 $N-\Delta$ axial transition form-factors using PV electron scattering., Comment: 5 pages, 3 figures

We present studies of single-spin asymmetries for neutral pion electroproduction in semi-inclusive deep-inelastic scattering of 5.776 GeV polarized electrons from an unpolarized hydrogen target, using the CEBAF Large Acceptance Spectrometer (CLAS) at the Thomas Jefferson National Accelerator Facility. A substantial $\sin \phi_h$ amplitude has been measured in the distribution of the cross section asymmetry as a function of the azimuthal angle $\phi_h$ of the produced neutral pion. The dependence of this amplitude on Bjorken $x$ and on the pion transverse momentum is extracted with significantly higher precision than previous data and is compared to model calculations., Comment: to be submitted PLB

We have measured the beam-normal single-spin asymmetries in elastic scattering of transversely polarized electrons from the proton, and performed the first measurement in quasi-elastic scattering on the deuteron, at backward angles (lab scattering angle of 108 degrees) for Q2 = 0.22 GeV^2/c^2 and 0.63 GeV^2/c^2 at beam energies of 362 MeV and 687 MeV, respectively. The asymmetry arises due to the imaginary part of the interference of the two-photon exchange amplitude with that of single photon exchange. Results for the proton are consistent with a model calculation which includes inelastic intermediate hadronic (piN) states. An estimate of the beam-normal single-spin asymmetry for the scattering from the neutron is made using a quasi-static deuterium approximation, and is also in agreement with theory.

In the G0 experiment, performed at Jefferson Lab, the parity-violating elastic scattering of electrons from protons and quasi-elastic scattering from deuterons is measured in order to determine the neutral weak currents of the nucleon. Asymmetries as small as 1 part per million in the scattering of a polarized electron beam are determined using a dedicated apparatus. It consists of specialized beam-monitoring and control systems, a cryogenic hydrogen (or deuterium) target, and a superconducting, toroidal magnetic spectrometer equipped with plastic scintillation and aerogel Cerenkov detectors, as well as fast readout electronics for the measurement of individual events. The overall design and performance of this experimental system is discussed., Comment: Submitted to Nuclear Instruments and Methods

In an experiment with a 65 MeV/nucleon polarized deuteron beam on a liquid-deuterium target at KVI, several multi-body final states in deuteron-deuteron scattering were identified. For these measurements, a unique and advanced detection system, called BINA, was utilized. We demonstrate the feasibility of measuring vector and tensor polarization observables of the deuteron break-up reaction leading to a three-body final-state. The polarization observables were determined with high precision in a nearly background-free experiment. The analysis procedure and some results are presented., Comment: 9 pages, 9 figures, accepted for publication in Phys. Rev. C

We successfully identified a few final states in deuteron-deuteron scattering at 65 MeV/nucleon at KVI using a unique and advanced detection system called BINA. This facility enabled us to perform cross sections and polarization measurements with an excellent statistical and systematical precision. The analysis procedure and part of the results of the three-body break-up channel in deuteron-deuteron scattering at 65 MeV/nucleon are presented in this paper., Comment: proceedings of the 19th International IUPAP Conference on Few-Body Problems in Physics, Bonn University, 31.08 - 05.09.2009, Bonn, GERMANY

A liquid helium target system was designed and built to perform a precision measurement of the parity-violating neutron spin rotation in helium due to the nucleon-nucleon weak interaction. The measurement employed a beam of low energy neutrons that passed through a crossed neutron polarizer--analyzer pair with the liquid helium target system located between them. Changes between the target states generated differences in the beam transmission through the polarizer--analyzer pair. The amount of parity-violating spin rotation was determined from the measured beam transmission asymmetries. The expected parity-violating spin rotation of order $10^{-6}$ rad placed severe constraints on the target design. In particular, isolation of the parity-odd component of the spin rotation from a much larger background rotation caused by magnetic fields required that a nonmagnetic cryostat and target system be supported inside the magnetic shielding, while allowing nonmagnetic motion of liquid helium between separated target chambers. This paper provides a detailed description of the design, function, and performance of the liquid helium target system., Comment: V2: 29 pages, 14 figues, submitted to Nucl. Instrum. Meth. B. Revised to address reviewer comments

Observables in elastic proton-deuteron scattering are sensitive probes of the nucleon-nucleon interaction and three-nucleon force effects. The present experimental data base for this reaction is large, but contains a large discrepancy between data sets for the differential cross section taken at 135 MeV/nucleon by two experimental research groups. This paper reviews the background of this problem and presents new data taken at KVI. Differential cross sections and analyzing powers for the $^{2}{\rm H}(\vec p,d){p}$ and ${\rm H}(\vec d,d){p}$ reactions at 135 MeV/nucleon and 65 MeV/nucleon, respectively, have been measured. The data differ significantly from previous measurements and consistently follow the energy dependence as expected from an interpolation of published data taken over a large range at intermediate energies., Comment: 5 pages, 4 figures, submitted to PRC

High precision cross-section data of the deuteron-proton breakup reaction at 130 MeV deuteron energy are compared with the theoretical predictions obtained with a coupled-channel extension of the CD Bonn potential with virtual Delta-isobar excitation, without and with inclusion of the long-range Coulomb force. The Coulomb effect is studied on the basis of the cross-section data set, extended in this work to about 1500 data points by including breakup geometries characterized by small polar angles of the two protons. The experimental data clearly prefer predictions obtained with the Coulomb interaction included. The strongest effects are observed in regions in which the relative energy of the two protons is the smallest., Comment: 9 pages, 3 figures, submitted to Physics Letters B

High precision cross-section data of the deuteron-proton breakup reaction at 130 MeV are presented for 72 kinematically complete configurations. The data cover a large region of the available phase space, divided into a systematic grid of kinematical variables. They are compared with theoretical predictions, in which the full dynamics of the three-nucleon (3N) system is obtained in three different ways: realistic nucleon-nucleon (NN) potentials are combined with model 3N forces (3NF's) or with an effective 3NF resulting from explicit treatment of the Delta-isobar excitation. Alternatively, the chiral perturbation theory approach is used at the next-to-next-to-leading order with all relevant NN and 3N contributions taken into account. The generated dynamics is then applied to calculate cross-section values by rigorous solution of the 3N Faddeev equations. The comparison of the calculated cross sections with the experimental data shows a clear prefernce for the predictions in which the 3NF's are included. The majority of the experimental data points is well reproduced by the theoretical predictions. The remaining discrepancies are investigated by inspecting cross sections integrated over certain kinematical variables. The procedure of global comparisons leads to establishing regularities in disagreements between the experimental data and the theoretically predicted values of the cross sections. They indicate deficiencies still present in the assumed models of the 3N system dynamics., Comment: 27 pages, 18 figures, submitted to Physical Review C