Parity Violating Electron Scattering Research Articles

Global fit of electron and neutrino elastic scattering data to determine the strange quark contribution to the vector and axial form factors of the nucleon

We present a global fit of neutral-current elastic (NCE) neutrino-scattering data and parity-violating electron-scattering (PVES) data with the goal of determining the strange quark contribution to the vector and axial form factors of the proton. Previous fits of this form included data from a variety of PVES experiments (PVA4, HAPPEx, G0, SAMPLE) and the NCE neutrino and anti-neutrino data from BNL E734. These fits did not constrain the strangeness contribution to the axial form factor GAs(Q2) at low Q2 very well because there was no NCE data for Q2<0.45 GeV2. Our new fit includes for the first time MiniBooNE NCE data from both neutrino and antineutrino scattering; this experiment used a hydrocarbon target and so a model of the neutrino interaction with the carbon nucleus was required. Three different nuclear models have been employed: a relativistic Fermi gas model, the superscaling approximation model, and a spectral function model. We find a tremendous improvement in the constraint of GAs(Q2) at low Q2 compared to previous work, although more data is needed from NCE measurements that focus on exclusive single-proton final states, for example from MicroBooNE. Published by the American Physical Society 2024

Electron scattering and neutrino physics

A thorough understanding of neutrino–nucleus scattering physics is crucial for the successful execution of the entire US neutrino physics program. Neutrino–nucleus interaction constitutes one of the biggest systematic uncertainties in neutrino experiments—both at intermediate energies affecting long-baseline deep underground neutrino experiment, as well as at low energies affecting coherent scattering neutrino program—and could well be the difference between achieving or missing discovery level precision. To this end, electron–nucleus scattering experiments provide vital information to test, assess and validate different nuclear models and event generators intended to test, assess and validate different nuclear models and event generators intended to be used in neutrino experiments. Similarly, for the low-energy neutrino program revolving around the coherent elastic neutrino–nucleus scattering (CEvNS) physics at stopped pion sources, such as at ORNL, the main source of uncertainty in the evaluation of the CEvNS cross section is driven by the underlying nuclear structure, embedded in the weak form factor, of the target nucleus. To this end, parity-violating electron scattering (PVES) experiments, utilizing polarized electron beams, provide vital model-independent information in determining weak form factors. This information is vital in achieving a percent level precision needed to disentangle new physics signals from the standard model expected CEvNS rate. In this white paper, we highlight connections between electron- and neutrino–nucleus scattering physics at energies ranging from 10 s of MeV to a few GeV, review the status of ongoing and planned electron scattering experiments, identify gaps, and lay out a path forward that benefits the neutrino community. We also highlight the systemic challenges with respect to the divide between the nuclear and high-energy physics communities and funding that presents additional hurdles in mobilizing these connections to the benefit of neutrino programs.

Constraints on the dark photon from parity violation and the W mass

We present an analysis of the experimental data for parity-violating electron scattering (PVES) and atomic parity-violation, including the effects of a dark photon. We derive the favored region of dark photon parameter space, which provides a good description of the experimental data from the Qweak Collaboration and the Jefferson Lab PVDIS Collaboration and simultaneously relieves the tension between the neutron skin thickness determined in the PREX-II experiment and nuclear-model predictions. In addition, we extract the parameter region required to explain the latest W-boson mass anomaly. Our results indicate that a heavy dark photon with mass above the Z boson mass is favored, while other sources of new physics beyond the Standard Model in addition to the dark photon would also be expected.

Sensitivity of Parity-Violating Electron Scattering to a Dark Photon.

We explore the sensitivity of the parity-violating electron scattering (PVES) asymmetry in both elastic and deep-inelastic scattering to the properties of a dark photon. Given advances in experimental capabilities in recent years, there are interesting regions of parameter space where PVES offers the chance to discover new physics in the near future. There are also cases where the existence of a dark photon could significantly alter our understanding of the structure of atomic nuclei and neutron stars as well as parton distribution functions.

Disentangling Standard Model EFT operators with future low-energy parity-violating electron scattering experiments

We study the potential of future Parity-Violating Electron Scattering (PVES) data to probe the parameter space of the Standard Model Effective Field Theory (SMEFT). We contrast the constraints derived from Drell-Yan data taken at the Large Hadron Collider (LHC) with projections of the planned PVES experiments SoLID and P2. We show that the PVES data can complement the bounds set by the LHC data in the dimension-6 operator space since it probes different combinations of operators than Drell-Yan. The lower characteristic energy of P2 and SoLID also helps disentangle effects of dimension-6 and dimension-8 operators that are difficult to resolve with LHC Drell-Yan data alone.

Theoretical study on neutron distribution of 208Pb by parity-violating electron scattering**Supported by the National Natural Science Foundation of China (11505292, 11175085, 11235001, 11447226), by the Shandong Provincial Natural Science Foundation, China (BS2014SF007), by the Fundamental Research Funds for the Central Universities (15CX02072A, 15CX02070A, 15CX05026A, 13CX10022A, 14CX02157A).

abstractThe precise determination of neutron distribution has important implications for both nuclear structure and nuclear astrophysics. The purpose of this paper is to study the characteristics of neutron distribution of 208Pb by parity-violating electron scattering (PVS). Parity-violating asymmetries of 208Pb with different types of neutron skins are systematically calculated and compared with the experimental data of PREx. The results indicate that the PVS experiments are very sensitive to the nuclear neutron distributions. From further PVS measurements, detailed information on nuclear neutron distributions can be extracted.

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.

Nuclear Symmetry Energy: constraints from Giant Quadrupole Resonances and Parity Violating Electron Scattering

Experimental and theoretical efforts are being devoted to the study of observables that can shed light on the properties of the nuclear symmetry energy. We present our new results on the excitation energy [1] and polarizability of the Isovector Giant Quadrupole Resonance (IVGQR), which has been the object of new experimental investigation[2]. We also present our theoretical analysis on the parity violating asymmetry at the kinematics of the Lead Radius Experiment (PREx [3]) and highlight its relation with the density dependence of the symmetry energy [4].

Electroweak charge density distributions with parity-violating electron scattering

Parity-violating electron scattering (PVS) is an accurate and model-independent way to investigate the weak-charge density distributions of nuclei. In this paper, we study parity-violating electron scattering with the Helm model where the effects of spin-orbit currents on nuclear weak skins are taken into account. The conditions of two PVS measurements to constrain the surface thickness ${\ensuremath{\sigma}}_{W}$ of Helm weak-charge densities are investigated. According to the plane wave Born approximation, ${A}_{\mathrm{pv}}$ is expressed in terms of parameters of the corresponding Helm charge and weak-charge densities. After fitting the results of ${A}_{\mathrm{pv}}$ calculated from the phase-shift analysis method where the Coulomb distortion effects are incorporated, an empirical formula in terms of Helm model parameters for calculating ${A}_{\mathrm{pv}}$ is obtained. If two PVS measurements with different scattering angles are carried out, the modeled weak-charge density distributions with two parameters could be extracted from this empirical formula.

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.

Theoretical study on neutron skin thickness of Ca isotopes by parity-violating electron scattering

The parity-violating electron scattering (PVS) experiment of Ca-48 has been proposed in Jefferson Lab recently. In this paper, the relation between neutron radius of Ca-48 and corresponding parity-violating asymmetry A(pv) is investigated. With this relation, we can extract the neutron radius of Ca-48 from measurement at only one single momentum transfer. The statistical error for measuring A(pv) of Ca-48 is also calculated at the kinematics of CREx. Besides, with the study of linear dependence between Delta R-np and L, it enable us to constrain the slope L of the symmetry energy. Parity-violating electron scattering off Ca isotopes is also studied because some experimental errors can be canceled by combining PVS measurements of isotopes. To analyze the differences of neutron skin thickness of Ca isotopes, we fit the mean-field densities with 2pF distributions and study the bulk and surface contributions. By comparing the half density radius and surface parameter along Ca isotopes, we note there may exist neutron halo nuclei in the extreme neutron rich range. Crown Copyright (C) 2013 Published by Elsevier B.V. All rights reserved.

Parity-Violating Electron Scattering and the Electric and Magnetic Strange Form Factors of the Nucleon

Measurements of the neutral weak-vector form factors of the nucleon provide unique access to the strange quark content of the nucleon. These form factors can be studied by using parity-violating electron scattering. A comprehensive program of experiments has been performed at three accelerator laboratories to determine the role of strange quarks in the electromagnetic form factors of the nucleon. This article reviews the remarkable technical progress associated with this program, describes the various methods used in the different experiments, and summarizes the physics results along with recent theoretical calculations.

Neutron Skin ofPb208, Nuclear Symmetry Energy, and the Parity Radius Experiment

A precise determination of the neutron skin Δr(np) of a heavy nucleus sets a basic constraint on the nuclear symmetry energy (Δr(np) is the difference of the neutron and proton rms radii of the nucleus). The parity radius experiment (PREX) may achieve it by electroweak parity-violating electron scattering (PVES) on (208)Pb. We investigate PVES in nuclear mean field approach to allow the accurate extraction of Δr(np) of (208)Pb from the parity-violating asymmetry A(PV) probed in the experiment. We demonstrate a high linear correlation between A(PV) and Δr(np) in successful mean field forces as the best means to constrain the neutron skin of (208)Pb from PREX, without assumptions on the neutron density shape. Continuation of the experiment with higher precision in A(PV) is motivated since the present method can support it to constrain the density slope of the nuclear symmetry energy to new accuracy.

Parity Violating Electron Scattering in the Relativistic Eikonal Approximation

The parity violating electron scattering is investigated in the relativistic Eikonal approximation. The parity violating asymmetry parameters for many isotopes are calculated. In calculations the proton and neutron densities are obtained from the relativistic mean-field theory. We take Ni isotopes as examples to analyse the behaviour of the parity violating asymmetry parameters. The results show that the parity violating asymmetry parameter is sensitive to the difference between the proton and neutron densities. The amplitude of the parity violating asymmetry parameter increases with the distance between the minima of proton and neutron form factors. Our results are useful for future parity violating electron scattering experiments. By comparing our results with experimental data one can test the validity of the relativistic mean-field theory in calculating the neutron densities of nuclei.

Effects of spatial neutron distributions onlow-energy parity-violating observables

We investigate the optimal kinematic conditions for extractinginformation about the spatial distribution of neutrons in heavy spin-0nuclei from a hypothetical parity-violating electron scattering (PVES)experiment. We further investigate the extent to which presentuncertainties in the shape of the neutron distribution affect theextraction of the rms neutron radius (the first moment of the distributionRn). We conclude that present theoretical uncertainties in theshape of the neutron distribution will not preclude a PVES measurement ofRn at the 1% level in lead. We also calculate the additionalconstraints provided by measurements at two different values of themomentum transfer. Finally, we examine the connection to atomic paritynon-conservation(PNC) observables and find that a single optimally designed PVESexperiment could eliminate nuclear structure uncertainties in futureprecision tests of the standard model at the sub-1% level.

Effects of Isospin-Symmetry Violation on Tests of the Standard Model Using Parity-Violating Electron Scattering

The effects of isospin-symmetry breaking on the observables for parity-violating electron scattering are investigated within the framework of the nuclear shell model for 12C, 16O, and 28Si. Contributions due to mixing with low-lying states as well as admixtures of 1p-1h configurations (via the radial wave functions) are accounted for. It is found that isospin-mixing can be important, and the consequences regarding precision tests of the standard model are addressed.

Strange-quark vector currents and parity-violating electron scattering from the nucleon and from nuclei.

Measurements of the processes p(π,π), p(ν,ν)/p(ν¯,ν¯), and deep-inelastic p→(μ→,μ’) can be interpreted in a manner which requires a significant strange-quark contribution to proton matrix elements. In this paper some implications of strange-quark contributions to proton vector currents and their manifestation in parity-violating electron-scattering experiments are examined. It is found that strange-quark currents of plausible magnitude significantly affect the parity-violating elastic electron scattering from the nucleon in certain kinematic regimes. It is also shown that, while the effects in on-going parity-violating experiments on 9Be and 12C are small, significant strange-quark contributions might be expected in experiments with nuclear targets at higher momentum transfer.