Qweak Experiment Research Articles

Neutron skin of 27Al with Skyrme and Korea-IBS-Daegu-SKKU density functionals

Recent measurement of the parity-violating (PV) asymmetry in the elastic electron scattering on [Formula: see text]Al target evokes the interest in the distribution of the neutron in the nucleus. In this work, we calculate the neutron skin thickness ([Formula: see text]) of [Formula: see text]Al with nonrelativistic nuclear structure models. We focus on the role of the effective mass, symmetry energy and pairing force. Models are selected to have effective masses in the range (0.58–1.05) M where M is the nucleon mass in free space, and stiffness of the symmetry energy is varied by choosing the slope of the symmetry energy in the range 9.4–100.5[Formula: see text]MeV. Effect of pairing force is investigated by calculating nuclear properties with and without pairing. With constant force pairing, we obtain [Formula: see text]–0.013[Formula: see text]fm. The result is independent of the effective mass and symmetry energy. However, [Formula: see text] is negative when the pairing force is switched off, so the pairing force plays an essential role to make [Formula: see text] positive and constrained in a narrow range. We also calculate the PV asymmetry ([Formula: see text]) in the elastic electron-[Formula: see text]Al scattering in the Born approximation at the kinematics of the Qweak experiment. We obtain a very narrow-ranged result [Formula: see text]. The result is consistent with the experiment and insensitive to the effective mass and symmetry energy.

The Q[formula omitted] high performance LH[formula omitted] target

A high-power liquid hydrogen target was built for the Jefferson Lab Qweak experiment, which measured the tiny parity-violating asymmetry in e→p scattering at an incident energy of 1.16 GeV, and a Q2=0.025 GeV2. To achieve the luminosity of 1.7×1039 cm−2 s−1, a 34.5 cm-long target was used with a beam current of 180 μA. The ionization energy-loss deposited by the beam in the target was 2.1 kW. The target temperature was controlled to within ±0.02 K and the target noise (density fluctuations) near the experiment’s beam helicity-reversal rate of 960 Hz was only 53 ppm. The 58 liquid liter target achieved a differential pressure (head) across the pump of 7.6 kPa (11.4 m) and a mass flow of 1.2 ± 0.3 kg/s (corresponding to a volume flow of 17.4 ± 3.8 l/s) at the nominal 29 Hz rotation frequency of the recirculating centrifugal pump. We describe aspects of the design, operation, and performance of this target, the highest power LH2target ever used in an electron scattering experiment to date.

Determination of the Proton's Weak Charge and Its Constraints on the Standard Model

This article discusses some of the history of parity-violation experiments that culminated in the Qweak experiment, which provided the first determination of the proton's weak charge [Formula: see text]. The guiding principles necessary to the success of that experiment are outlined, followed by a brief description of the Qweak experiment. Several consistent methods used to determine [Formula: see text] from the asymmetry measured in the Qweak experiment are explained in detail. The weak mixing angle sin2θw determined from [Formula: see text] is compared with results from other experiments. A description of the procedure for using the [Formula: see text] result on the proton to set TeV-scale limits for new parity-violating semileptonic physics beyond the Standard Model (BSM) is presented. By also considering atomic parity-violation results on cesium, the article shows how this result can be generalized to set limits on BSM physics, which couples to any combination of valence quark flavors. Finally, the discovery space available to future weak-charge measurements is explored.

Testing the Standard Model at the Precision Frontier with the Qweak Experiment

The standard model of particle physics (SM) has been wildly successful describing the strong, electromagnetic, and weak forces between visible matter. Despite this success, there is a host of phenomena it fails to describe, including neutrino oscillations, gravity, dark matter, and baryon asymmetry. Something is missing—the theory is not complete. This conviction has compelled physicists to develop ever-more-stringent tests in the hope of finding cracks in the SM that could point to where the beyond-the-standard-model (BSM) physics might lie. Direct searches at high-energy colliders have yielded no evidence for some of the most intriguing possibilities, like supersymmetry, and have not been able to shed much light on where to look next.

The tracking analysis in the Q-weak experiment

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.

Hadronic weak charges and parity-violating forward Compton scattering

The authors consider contributions to parity-violating electron-proton scattering from two-photon exchange, where the electromagnetic interaction of the nucleon has a parity-violating component. Unlike the standard short-ranged weak interaction conveyed by a massive $Z$ boson, the considered mechanism induces novel long-range parity-violating forces. This is relevant for upcoming measurements at electron accelerator facilities at the University of Mainz and Jefferson Laboratory, including the Q-Weak experiment. The results will allow access to the weak charge of the nucleon and thereby, potentially, to physics beyond the Standard Model.

Precision Electron-Beam Polarimetry at 1 GeV Using Diamond Microstrip Detectors

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.

The Qweak Experiment: First Direct Measurement of the Weak Charge of the Proton

The recently completed Qweak experiment at Jefferson Laboratory made the first direct determination of the proton's weak charge, QpW, via a measurement of the parity-violating asymmetry in elastic electron-proton scattering at low four-momentum transfer. The Standard Model (SM) makes a precise prediction of QpW (SM) = 0.0710 ± 0.0007. A deviation from this prediction could be an indicator of new physics. A longitudinally polarized electron beam was scattered off a liquid hydrogen target and detected in eight azimuthally symmetric fused silica detectors. The small asymmetry, Aep = -279 ± 35 (stat) ±31 (syst) ppb, was measured by observing the difference in rates seen in the detectors when the helicity of the electron beam was rapidly reversed. The measured asymmetry is the most precise and smallest asymmetry ever measured in an e⃗p scattering experiment. Combining this asymmetry with previous parity- violating electron scattering (PVES) data, we obtained a value of QpW(PVES) = 0.064 ± 0.012, which agrees well with the SM value. The results of the experiment's commissioning run, which constitutes about 4% of the total data set, are reported here. Analysis of the remainder of the data set is ongoing and will significantly reduce the statistical and systematic uncertainties; several aspects of this analysis will be highlighted.

Quark–hadron duality constraints on γZ box corrections to parity-violating elastic scattering

We examine the interference γZ box corrections to parity-violating elastic electron–proton scattering in the light of the recent observation of quark–hadron duality in parity-violating deep-inelastic scattering from the deuteron, and the approximate isospin independence of duality in the electromagnetic nucleon structure functions down to Q2≈1 GeV2. Assuming that a similar behavior also holds for the γZ proton structure functions, we find that duality constrains the γZ box correction to the proton's weak charge to be ℜe□γZV=(5.4±0.4)×10−3 at the kinematics of the Qweak experiment. Within the same model we also provide estimates of the γZ corrections for future parity-violating experiments, such as MOLLER at Jefferson Lab and MESA at Mainz.

How strange is pion electroproduction?

We consider pion production in parity-violating electron scattering (PVES) in the presence of nucleon strangeness in the framework of partial wave analysis with unitarity. Using the experimental bounds on the strange form factors obtained in elastic PVES, we study the sensitivity of the parity-violating asymmetry to strange nucleon form factors. For forward kinematics and electron energies above 1 GeV, we observe that this sensitivity may reach about 20% in the threshold region. With parity-violating asymmetries being as large as tens p.p.m., this study suggests that threshold pion production in PVES can be used as a promising way to better constrain strangeness contributions. Using this model for the neutral current pion production, we update the estimate for the dispersive γZ-box correction to the weak charge of the proton. In the kinematics of the Qweak experiment, our new prediction reads Re□γZV(E=1.165 GeV)=(5.58±1.41)×10−3, an improvement over the previous uncertainty estimate of ±2.0×10−3. Our new prediction in the kinematics of the upcoming MESA/P2 experiment reads Re□γZV(E=0.155 GeV)=(1.1±0.2)×10−3.

The Qweak experimental apparatus

The Jefferson Lab Qweak 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 180μA of 89% longitudinally polarized electrons whose helicity was reversed 960 times per second. The electrons were accelerated to 1.16GeV 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 34.4cm 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 7GHz 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=0.025GeV2 was determined using dedicated low-current (~100pA) measurements with a set of drift chambers before (and a set of drift chambers and trigger scintillation counters after) the toroidal magnet.

Parity violating elastic electron scattering fromAl27and the Qweak measurement

I calculate the parity violating asymmetry ${A}_{pv}$ for elastic electron scattering from $^{27}\mathrm{Al}$ in order to compare with the Qweak experiment ``background'' $^{27}\mathrm{Al}$ measurement. I find that Coulomb distortions, and the quadrupole form factor, reduce ${A}_{pv}$ near the diffraction minimum. The Qweak data can be used to confirm the neutron radius of $^{27}\mathrm{Al}$, if nuclear structure uncertainties are indeed small, as I suggest, and one can estimate inelastic and impurity contributions. This could provide an important check of the measurement, analysis, and theory.

Data Quality Control and Maintenance for the Qweak Experiment

Data Quality Control and Maintenance for the Qweak Experiment

Qweak: First Direct Measurement of the Weak Charge of the Proton

The Q-weak experiment at Hall C of Thomas Jefferson National Accelerator Facility has made the first direct measurement of the weak charge of the proton, Qweak(p), through a precision measurement of the parity-violating asymmetry in elastic e-p scattering at low momentum transfer Q^2= 0.025 (GeV/c)^2 with incident electron beam energy of 1.155 GeV. The Q-weak experiment, along with earlier results of parity violating elastic scattering experiments, is expected to determine the most precise value of Qweak(p) which is suppressed in the Standard Model. If this result is further combined with the 133Cs atomic parity violation (APV) measurement, significant constraints on the weak charge of the up quark, down quark, and neutron can be extracted. This data will also be used to determine the weak-mixing angle, sin^2 theta_?W, with a relative uncertainty of < 0.5% that will provide a competitive measurement of the running of sin^2 theta_?W to low Q^2. An overview of the experiment and its results using the commissioning dataset, constituting approximately 4% of the data collected in the experiment, are reported here.

Precise polarization measurements via detection of compton scattered electrons

The Qweak experiment at Jefferson Lab aims to make a 4% measurement of the parity-violating asymmetry in elastic scattering at very low Q2 of a longitudinally polarized electron beam off a proton target. One of the dominant experimental systematic uncertainties in Qweak will result from determining the beam polarization. A new Compton polarimeter was installed in the fall of 2010 to provide a non-invasive and continuous monitoring of the electron beam polarization in Hall C at Jefferson Lab. The Compton-scattered electrons are detected in four planes of diamond micro-strip detectors. We have achieved the design goals of <1% statistical uncertainty per hour and expect to achieve <1% systematic uncertainty.

First result from Qweak

Initial results are presented from the recently-completed Qweak experiment at Jefferson Lab. The goal is a precise measurement of the proton's weak charge Q p ,t o yield a test of the standard model and to search for evidence of new physics. The weak charge is extracted from the parity-violating asymmetry in elasticp scattering at low momentum transfer, Q 2 = 0.025GeV 2 . A 180 A longitudinally-polarized 1.16 GeV electron beam was scattered from a 35 cm long liquid hydrogen at small angles, 6 ◦ < < 12 ◦ . Scattered electrons were analyzed in a toroidal magnetic field and detected using an array of eight Cerenkov detectors arranged symmetrically about the beam axis. The initial result, from 4% of the complete data set, is Q p = 0.064 ± 0.012, in excellent agreement with the standard model expectation. Full analysis of the data is expected to yield a value for the weak charge to about 5% precision.

Early Results from the QweakExperiment

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.

Qweak, N → Δ, and physics beyond the standard model

The data-taking phase of the Qweak experiment ended in May of 2012 at the Thomas Jefferson National Accelerator Facility. Qweak aims to measure the weak charge of the proton, Q{sub W}{sup p}, via parity-violating elastic electron-proton scattering. The expected value of Q{sub W}{sup p} is fortuitously suppressed, which leads to an increased sensitivity to physics beyond the Standard Model.

A Measurement of the Weak Charge of the Proton through Parity Violating Electron Scattering using the Qweak Apparatus

After a decade of preparations, the Qweak experiment at Jefferson Lab is making the first direct measurement of the weak charge of the proton, Qweak9. Because this quantity is suppressed in the Standard Model, a 4% result will significantly constrain new physics at the TeV scale while providing the most precise measurement of sin2θW at low energies. Operationally, we measure the small (about −0.220ppm) parity violating asymmetry in electron–proton scattering in integrating mode while flipping the longitudinal polarization of the electrons up to 1000 times per second. Potential sources of new, parity violating interactions between electrons and light quarks include a Z′, lepto-quarks, and parity violating SUSY interactions. The result presented here is based on the data taken during an initial two weeks period which included a 16.7% measurement of the parity violating electron–proton (e→p) scattering asymmetry, A=−0.279±0.035(stat.)±0.031(syst.)ppm at Q2=0.0250±0.0006(GeV)2. The weak charge of the proton is extracted by performing a global analysis on parity violating electron scattering (PVES) asymmetries on nuclear targets and it is QWp=0.064±0.012. Then effective vector couplings of the up/down quarks (C1u/C1d) and weak charge of the neutron are extracted by combining precise 133Cs atomic parity violating (APV) measurement and PVES measurements. The result is a proof of principle for the analysis of the full Qweak data to be completed in the near future.

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 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.