Precision measurement of the weak charge of the proton

Abstract

Large experimental programmes in the fields of nuclear and particle physics search for evidence of physics beyond that explained by current theories. The observation of the Higgs boson completed the set of particles predicted by the standard model, which currently provides the best description of fundamental particles and forces. However, this theory's limitations include a failure to predict fundamental parameters, such as the mass of the Higgs boson, and the inability to account for dark matter and energy, gravity, and the matter-antimatter asymmetry in the Universe, among other phenomena. These limitations have inspired searches for physics beyond the standard model in the post-Higgs era through the direct production of additional particles at high-energy accelerators, which have so far been unsuccessful. Examples include searches for supersymmetric particles, which connect bosons (integer-spin particles) with fermions (half-integer-spin particles), and for leptoquarks, which mix the fundamental quarks with leptons. Alternatively, indirect searches using precise measurements of well predicted standard-model observables allow highly targeted alternative tests for physics beyond the standard model because they can reach mass and energy scales beyond those directly accessible by today's high-energy accelerators. Such an indirect search aims to determine the weak charge of the proton, which defines the strength of the proton's interaction with other particles via the well known neutral electroweak force. Because parity symmetry (invariance under the spatial inversion (x, y, z) → (-x, -y, -z)) is violated only in the weak interaction, it provides a tool with which to isolate the weak interaction and thus to measure the proton's weak charge . Here we report the value 0.0719 ± 0.0045, where the uncertainty is one standard deviation, derived from our measured parity-violating asymmetry in the scattering of polarized electrons on protons, which is -226.5 ± 9.3 parts per billion (the uncertainty is one standard deviation). Our value for the proton's weak charge is in excellent agreement with the standard model and sets multi-teraelectronvolt-scale constraints on any semi-leptonic parity-violating physics not described within the standard model. Our results show that precision parity-violating measurements enable searches for physics beyond the standard model that can compete with direct searches at high-energy accelerators and, together with astronomical observations, can provide fertile approaches to probing higher mass scales.

Neutrino, electroweak, and nuclear physics from COHERENT elastic neutrino-nucleus scattering with refined quenching factor

“…Differently from Ref. [15], we use a more recent determination of the vector transition polarizability, namely The SM prediction is shown as the solid curve, together with experimental determinations in black at the Z-pole [29] (Tevatron, LEP1, SLC, LHC), from APV on Caesium [26,28], which has a typical momentum transfer given by Q 2.4 MeV, Møller scattering [30] (E158), deep inelastic scattering of polarized electrons on deuterons [31] (e 2 H PVDIS) and from neutrino-nucleus scattering [32] (NuTeV) and the new result from the proton's weak charge at Q = 0.158 GeV [33] (Q weak ). In red it is shown the result derived in this paper, obtained correcting the APV data point by the improved direct Caesium neutron rms radius determination obtained in this work.…”

Electroweak probes of ground state densities

“…In an innovative paper written three decades ago, Donnelly, Dubach, and Sick proposed the use of parity violating electron scattering (PVES) as a clean and model-independent probe of neutron densities [14]. Since then [60], many of the experimental challenges have been met, leading to a mature and enormously successful PVES program at JLab [7,15,61]. Moreover, the interest in measuring the neutron distribution of heavy nuclei (specifically of 208 Pb) was rekindled because of the impact that such a measurement could have in constraining the equation of state of neutron rich matter and ultimately the structure of neutron stars [24].…”

Polarized electron-beam acceleration driven by vortex laser pulses

“…Spin is an intrinsic form of angular momentum carried by elementary particles [1]. Numerous studies in particle physics and material science have been carried out using spin-polarized electron beams [2][3][4][5][6]. Generally, generating polarized electrons requires conventional accelerators (Storage ring or Linac) that are typically very large in scale and budget [2,7,8].…”