Parity-violating Asymmetry Research Articles

Characterization of Cherenkov detectors for the MOLLER experiment

The MOLLER (Measurement Of Lepton Lepton Electroweak Reaction) experiment is aiming to measure the parity-violating asymmetry (Apv) in electron–electron (Møller) scattering with unprecedented precision. The flux of Møller scattered electrons from the liquid hydrogen target is measured using Cherenkov detectors and the longitudinal polarization of the incoming electron beam is rapidly flipped to extract the right-left fractional flux difference and thence Apv. The Cherenkov detector prototypes were tested at the MAMI accelerator facility in Mainz, Germany with an electron beam of energy ∼ 855 MeV.A brief overview of the experimental goals, design and performance of the prototype detectors with the electron beam are presented in this article.

Fundamental tests of P and CP symmetries using octet baryons at the J/ψ threshold

We systematically investigate tests of the parity P and the combined parity and charge-conjugate CP symmetries from differential angular distributions of J/ψ decaying into the lowest-lying baryon pairs at BESIII and the next-generation super tau-charm facilities (STCFs). Large corrections from Z and W exchange induced parity-violating effects are found for J/ψ decays with large logarithms resummed up to O(αs). The parity-violating asymmetries on the production and the decay sides of J/ψ are both estimated to be of O(10−4), thus barely observable with the 10 billion J/ψ events currently collected at BESIII. Nevertheless, these asymmetries utilizing the current BESIII data already permit a measurement of the weak mixing angle with an absolute uncertainty δsw2≈0.09, corresponding to the first determination of sw2 at the J/ψ threshold. In the future, STCFs are estimated to improve this bound by a factor of ∼20 to δsw2≈0.005 within one year based on luminosity rescaling. We also obtain the 95% confidence level upper bounds on the electric dipole moments of the octet baryons, which are of O(10−18) e cm for BESIII and O(10−19) e cm for STCFs. These bounds are improved by 2 to 3 orders of magnitude in comparison with the only existing one on Λ from Fermilab. The method discussed in this work also paves a way for a first and direct measurement of the Ξ and Σ electric dipole moments. Published by the American Physical Society 2024

High Voltage Monolithic Active Pixel Sensors for the MOLLER experiment

The MOLLER experiment [1] aims to measure the parity-violating asymmetry (APV) in electron–electron scattering at 11 GeV, and thus determine the weak charge of the electron to a high precision (2.4%). Two of the key sub-systems in the experimental apparatus are the main detector array, consisting of 224 fused silica detector modules and an electron beam Compton polarimeter. The main detector array can be operated in either pulsed mode for particle tracking, or integration mode for the asymmetry measurement at high rate detection. A subset of these detector modules will be equipped with an array of High Voltage Monolithic Active Pixel Sensors (HVMAPS), to be used in particle tracking and allowing for real-time monitoring of the event profile in the main Møller region. HVMAPS are also used for the Compton polarimeter, consisting of four-plane planes of HVMAPS chips, housed in a vacuum chamber upstream of the interaction point. An overview of the implementation of the HVMAPS for the main detectors and Compton polarimeter is provided.

Simultaneous extraction of the weak radius and the weak mixing angle from parity-violating electron scattering on C12

We study the impact of nuclear structure uncertainties on a measurement of the weak charge of C12 at the future Mainz Energy Recovering Superconducting Accelerator (MESA) facility in Mainz. Information from a large variety of nuclear models, accurately calibrated to the ground-state properties of selected nuclei, suggest that a 0.3% precision measurement of the parity-violating asymmetry at forward angles will not be compromised by nuclear structure effects, thereby allowing a world-leading determination of the weak charge of C12. Furthermore, we show that a combination of measurements of the parity-violating asymmetry at forward and backward angles for the same electron-beam energy can be used to extract information on the nuclear weak charge distribution. We conclude that a 0.34% precision on the weak radius of C12 may be achieved by performing a 3% precision measurement of the parity-violating asymmetry at backward angles. Published by the American Physical Society 2024

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.

Determination of the Neutron Skin of ^{208}Pb from Ultrarelativistic Nuclear Collisions.

Emergent bulk properties of matter governed by the strong nuclear force give rise to physical phenomena across vastly different scales, ranging from the shape of atomic nuclei to the masses and radii of neutron stars. They can be accessed on Earth by measuring the spatial extent of the outer skin made of neutrons that characterizes the surface of heavy nuclei. The isotope ^{208}Pb, owing to its simple structure and neutron excess, has been in this context the target of many dedicated efforts. Here, we determine the neutron skin from measurements of particle distributions and their collective flow in ^{208}Pb+^{208}Pb collisions at ultrarelativistic energy performed at the Large Hadron Collider, which are mediated by interactions of gluons and thus sensitive to the overall size of the colliding ^{208}Pb ions. By means of state-of-the-art global analysis tools within the hydrodynamic model of heavy-ion collisions, we infer a neutron skin Δr_{np}=0.217±0.058 fm, consistent with nuclear theory predictions, and competitive in accuracy with a recent determination from parity-violating asymmetries in polarized electron scattering. We establish thus a new experimental method to systematically measure neutron distributions in the ground state of atomic nuclei.

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.

Semi-inclusive deeply inelastic electron nucleus scattering in the eN collinear frame

Our understanding of the parton-level structure in the nucleon comes predominantly from the electron deeply inelastic scattering measurements with very high precision. It deserves further study. We therefore calculate the neutral current jet production semi-inclusive deeply inelastic scattering process in this paper. Neutral current implies that interactions can be mediated by the photon, Z0 boson and their interference. The initial electron is assumed to be polarized and then scattered off by a target particle with spin 1. After obtaining the differential cross section, we calculate sets of quantities, including structure functions, azimuthal asymmetries, parity-violating asymmetries and charge asymmetries. They can be measured in the experiments. We find that these quantities are expressed in terms of the parton distribution functions and the electroweak coefficients. These coefficients are different from that in the γ⁎N collinear frame. As a result, our calculation provides a set of new quantities for determining the electroweak couplings as well as the parton distribution functions in the deeply inelastic scattering. It is helpful to test the universality of the parton distribution functions or to understand the hadronic weak interactions and strong interactions simultaneously.

Combined Theoretical Analysis of the Parity-Violating Asymmetry for ^{48}Ca and ^{208}Pb.

The recent experimental determination of the parity violating asymmetry A_{PV} in ^{48}Ca and ^{208}Pb at Jefferson Lab is important for our understanding on how neutrons and protons arrange themselves inside the atomic nucleus. To better understand the impact of these measurements, we present a rigorous theoretical investigation of A_{PV} in ^{48}Ca and ^{208}Pb and assess the associated uncertainties. We complement our study by inspecting the static electric dipole polarizability in these nuclei. The analysis is carried out within nuclear energy density functional theory with quantified input. We conclude that the simultaneous accurate description of A_{PV} in ^{48}Ca and ^{208}Pb cannot be achieved by our models that accommodate a pool of global nuclear properties, such as masses and charge radii, throughout the nuclear chart, and describe-within one standard deviation-the experimental dipole polarizabilities α_{D} in these nuclei.

Measurement of the parity-odd angular distribution of γ rays from polarized neutron capture on Cl35

We report a measurement of two energy-weighted γ cascade angular distributions from polarized slow neutron capture on the Cl35 nucleus, one parity-odd correlation proportional to sn⃗·kγ⃗ and one parity-even correlation proportional to sn⃗·kn⃗×kγ⃗. A parity-violating asymmetry can appear in this reaction due to the weak nucleon-nucleon interaction, which mixes opposite parity S- and P-wave levels in the excited compound Cl36 nucleus formed upon slow neutron capture. If parity-violating (PV) and parity-conserving (PC) terms both exist, the measured differential cross section can be related to them via dσdΩ∝1+Aγ,PVcosθ+Aγ,PCsinθ. The PV and PC asymmetries for energy-weighted γ cascade angular distributions for polarized slow neutron capture on Cl35 averaged over the neutron energies from 2.27–9.53 meV were measured to be Aγ,PV=(−23.9±0.7)×10−6 and Aγ,PC=(0.1±0.7)×10−6. These results are consistent with previous experimental results. Systematic errors were quantified and shown to be small compared to the statistical error. These asymmetries in the angular distributions of the γ rays emitted from the capture of polarized neutrons in Cl35 were used to verify the operation and data analysis procedures for the NPDGamma experiment, which measured the parity-odd asymmetry in the angular distribution of γs from polarized slow neutron capture on protons.5 MoreReceived 1 October 2021Accepted 17 May 2022DOI:https://doi.org/10.1103/PhysRevC.106.015504©2022 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasNuclear spin & parityNuclear tests of fundamental interactionsPolarization phenomenaRadiative captureProperties20 ≤ A ≤ 38TechniquesParticle sources & targetsNuclear Physics

Precision Determination of the Neutral Weak Form Factor of ^{48}Ca.

We report a precise measurement of the parity-violating (PV) asymmetry A_{PV} in the elastic scattering of longitudinally polarized electrons from ^{48}Ca. We measure A_{PV}=2668±106(stat)±40(syst) parts per billion, leading to an extraction of the neutral weak form factor F_{W}(q=0.8733 fm^{-1})=0.1304±0.0052(stat)±0.0020(syst) and the charge minus the weak form factor F_{ch}-F_{W}=0.0277±0.0055. The resulting neutron skin thickness R_{n}-R_{p}=0.121±0.026(exp)±0.024(model) fm is relatively thin yet consistent with many model calculations. The combined CREX and PREX results will have implications for future energy density functional calculations and on the density dependence of the symmetry energy of nuclear matter.

Searching for new physics from SMEFT and leptoquarks at the P2 experiment

The P2 experiment aims at high-precision measurements of the parity-violating asymmetry in elastic electron-proton and electron-$^{12}$C scatterings with longitudinally polarized electrons. We discuss here the sensitivity of P2 to leptoquarks, which within the P2 energy range can be described in the language of Standard Model Effective Field Theory (SMEFT). We give the expected P2 limits on the SMEFT operators and on the leptoquark parameters, which will test energy scales up to 15 TeV. In many cases those limits exceed current constraints from LHC and atomic parity violation (APV) experiments. We also demonstrate that degeneracies of different SMEFT operators can partially be resolved by use of APV experiments and different targets (protons and $^{12}$C) at P2. Moreover, we show that P2 could confirm or resolve potential tensions between the theoretical and experimental determinations of the weak charge of $^{133}$Cs.

Information Content of the Parity-Violating Asymmetry in ^{208}Pb.

The parity-violating asymmetry A_{PV} in ^{208}Pb, recently measured by the PREX-2 Collaboration, is studied using modern relativistic (covariant) and nonrelativistic energy density functionals. We first assess the theoretical uncertainty on A_{PV} which is intrinsic to the adopted approach. To this end, we use quantified functionals that are able to accommodate our previous knowledge on nuclear observables such as binding energies, charge radii, and the dipole polarizability α_{D} of ^{208}Pb. We then add the quantified value of A_{PV} together with α_{D} to our calibration dataset to optimize new functionals. Based on these results, we predict a neutron skin thickness in ^{208}Pb r_{skin}=0.19±0.02 fm and the symmetry-energy slope L=54±8 MeV. These values are consistent with other estimates based on astrophysical data and are significantly lower than those recently reported using a particular set of relativistic energy density functionals. We also make a prediction for the A_{PV} value in ^{48}Ca that will be soon available from the CREX measurement.

Revisiting quark and gluon polarization in the proton at the EIC

We present a comprehensive impact study of future Electron-Ion Collider (EIC) data for parity-conserving and parity-violating polarization asymmetries on quark and gluon helicity distributions in the proton. The study, which is based on the JAM Monte Carlo global QCD analysis framework, explores the role of the extrapolation uncertainty and SU(3) flavor symmetry constraints in the simulated double-spin asymmetry, $A_{LL}$, at small parton momentum fractions $x$ and its effect on the extracted parton polarizations. We find that different assumptions about $A_{LL}$ extrapolations and SU(3) symmetry can have significant consequences for the integrated quark and gluon polarizations, for polarized proton, deuteron and $^3$He beams. For the parity-violating asymmetry, $A_{UL}$, we study the potential impact on the polarized strange quark distribution with different extrapolations of $A_{UL}$, finding the constraining power to be ultimately limited by the EIC machine luminosity.

First analysis of world polarized DIS data with small- x helicity evolution

We present a Monte Carlo based analysis of the combined world data on polarized lepton-nucleon deep-inelastic scattering at small Bjorken $x$ within the polarized quark dipole formalism. We show for the first time that double-spin asymmetries at $x<0.1$ can be successfully described using only small-$x$ evolution derived from first-principles QCD, allowing predictions to be made for the $g_1$ structure function at much smaller $x$. Anticipating future data from the Electron-Ion Collider, we assess the impact of electromagnetic and parity-violating polarization asymmetries on $g_1$ and demonstrate an extraction of the individual flavor helicity PDFs at small $x$.

Searching for Z\u2032 bosons at the P2 experiment

The P2 experiment aims at high-precision measurements of the parity-violating asymmetry in elastic electron-proton and electron-12C scatterings with longitudinally polarized electrons. We discuss here the sensitivity of P2 to new physics mediated by an additional neutral gauge boson Z′ of a new U(1)′ gauge symmetry. If the charge assignment of the U(1)′ is chiral, i.e., left- and right-handed fermions have different charges under U(1)′, additional parity-violation is induced directly. On the other hand, if the U(1)′ has a non-chiral charge assignment, additional parity-violation can be induced via mass or kinetic Z-Z′ mixing. By comparing the P2 sensitivity to existing constraints, we show that in both cases P2 has discovery potential over a wide range of Z′ mass. In particular, for chiral models, the P2 experiment can probe gauge couplings at the order of 10−5 when the Z′ boson is light, and heavy Z′ bosons up to 79 (90) TeV in the proton (12C) mode. For non-chiral models with mass mixing, the P2 experiment is sensitive to mass mixing angles smaller than roughly 10−4, depending on model details and gauge coupling magnitude.

Accurate Determination of the Neutron Skin Thickness of ^{208}Pb through Parity-Violation in Electron Scattering.

We report a precision measurement of the parity-violating asymmetry A_{PV} in the elastic scattering of longitudinally polarized electrons from ^{208}Pb. We measure A_{PV}=550±16(stat)±8(syst) parts per billion, leading to an extraction of the neutral weak form factor F_{W}(Q^{2}=0.00616 GeV^{2})=0.368±0.013. Combined with our previous measurement, the extracted neutron skin thickness is R_{n}-R_{p}=0.283±0.071 fm. The result also yields the first significant direct measurement of the interior weak density of ^{208}Pb: ρ_{W}^{0}=-0.0796±0.0036(exp)±0.0013(theo) fm^{-3} leading to the interior baryon density ρ_{b}^{0}=0.1480±0.0036(exp)±0.0013(theo) fm^{-3}. The measurement accurately constrains the density dependence of the symmetry energy of nuclear matter near saturation density, with implications for the size and composition of neutron stars.

Exploiting electron parity violation: from Standard Model tests to dark matter detection predictions

There has been recent interest in low energy, high luminosity polarized electron beams for studies of parity-violating (PV) electron scattering, such as the MESA accelerator at Mainz or an upgraded FEL facility at Jefferson Lab. Accurate measurements of the PV asymmetry in elastic electron scattering from nuclei can be used to determine Standard Model couplings, such as the weak-mixing angle or higher-order radiative corrections, as well as to extract specific information on the nuclear and nucleon structure. To this end, low uncertainties are required from modelling some confounding nuclear and nuclear structure effects, including isospin mixing, nucleon strangeness content or Coulomb distortion of electron wave functions. We estimate the sizes and theoretical uncertainties of such effects for a carbon 12 target. An experimental precision in the PV asymmetry of a few tenths of a percent may be reachable under certain kinematic conditions, that are also discussed for the same nuclear target.This high precision PV asymmetry in elastic electron scattering can also be used to relate in a very simple manner the elastic electron-nucleus scattering cross section with the elastic neutrino-nucleus cross section. This novel relationship allows us to exploit experimentally well-determined quantities to predict unknown or recently measured observables, such as coherent neutrino-nucleus cross sections. This idea can be extended to link electron scattering to an even more uncertain magnitude: the direct detection rate of hypothetical weak-interacting dark matter particles through axial and/or vector elastic interactions with nuclei.

A measurement of parity-violating asymmetry in polarized cold neutron capture on 3He

The n3He experiment measured the parity-violating directional asymmetry in the proton emission direction relative to the initial neutron polarization in the capture of polarized cold neutrons in the reaction n + 3He → T + p. Data taking was completed at the end of 2015 at the SNS, and two independent analyses of the proton parity asymmetry have since been completed and are in agreement. I will present the methods used to calculate the asymmetry, and the final results of the experiment.

Measurement of Longitudinal Single-Spin Asymmetry for W± Production in Polarized Proton+Proton Collisions at STAR

The contribution from the sea quark polarization to the nucleon spin is an important piece for the complete understanding of the nucleon spin structure. The production of W ± bosons in longitudinally polarized p+p collisions at the RHIC collider at Brookhaven National Laboratory provides a unique probe of the sea quark polarization, through the parity-violating single-spin asymmetry, AL . At the STAR experiment, the W bosons that decay through the W → ev channel at mid-rapidity (|η <1.3) can be effectively determined with the Electromagnetic Calorimeters and Time Projection Chamber. The STAR measurements of AL for W boson from datasets taken in 2011 and 2012 at =510 GeV have been included in the global analysis of polarized parton distribution functions, and provided significant constraints on the helicity distribution functions of and quarks. The final AL results from 2013 STAR data sample are reported, which is about three times larger than the total integrated luminosity of previous years. The combined results of AL for 2011-2013 data are also given. A flavor asymmetry of light sea quark helicity distribution, , is confirmed from a re-weighting of global analysis NNPDFpol1.1 after including the new AL results. In addition, results on the double-spin asymmetries ALL for W ±, and AL for Z/γ* production are also reported.