Wed·May 22, 2019Wednesday·May 22, 2019

1. 01*

   Precision Measurement of the Weak Charge of the Proton

   D. Androic🇭🇷 · D. S. Armstrong🇺🇸 · A. Asaturyan🇦🇲 · T. Averett🇺🇸 · J. Balewski🇺🇸 · K. Bartlett🇺🇸 · J. Beaufait🇺🇸 · R. S. Beminiwattha🇺🇸 · J. Benesch🇺🇸 · F. Benmokhtar🇺🇸 · J. Birchall🇨🇦 · R. D. Carlini🇺🇸 and 87 other authors

   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.

   nucl-exNature(2018)·231 citations
2. 02*

   A study of ${}^{35}$Cl excited states via ${}^{32}$S($\alpha ,p$)

   Kiana Setoodehnia · John H. Kelley🇺🇸 · Caleb Marshall🇺🇸 · Federico Portillo Chaves · Richard Longland🇺🇸

   Presolar grains originating in oxygen-neon novae may be identified by their sulfur isotopic ratios compared with theoretical estimates. These ratios depend on reliable ${}^{33}$S($p,\gamma$)${}^{34}$Cl and ${}^{34}$S($p,\gamma$)${}^{35}$Cl reaction rates. The latter rate has recently been computed based on experimental input, and many new excited states in ${}^{35}$Cl were discovered above the proton threshold. The experimental ${}^{34}$S($p,\gamma$)${}^{35}$Cl rate was found to be 2 - 5 times smaller than the theoretical one, and the simulated ${}^{34}$S/${}^{32}$S isotopic ratio for nova presolar grains was thus predicted to be smaller than that of type II supernova grains by up to a factor of 3.7. The present study was performed to confirm the existence of these new resonances, and to improve the remaining uncertainties in the ${}^{34}$S($p,\gamma$)${}^{35}$Cl reaction rate. Energies and spin-parities of the ${}^{35}$Cl excited levels were investigated with an Enge split-pole spectrograph using the ${}^{32}$S($\alpha ,p$)${}^{35}$Cl reaction. Differential cross sections of the outgoing protons were measured at $E_{\alpha }$ = 21 MeV. The existence of the newly discovered states are largely confirmed, although a few states were not observed in this study. The spins and parities of several ${}^{35}$Cl states were assigned tentatively for the first time. The present ${}^{34}$S($p,\gamma$)${}^{35}$Cl experimental thermonuclear reaction rate is consistent within 1$\sigma$ with the previous evaluation. However, our rate uncertainty is larger due to a more realistic treatment of the experimental uncertainties. The uncertainty in the present rate is up to a factor of 3.5 at nova temperatures. We recommend future work to focus on the unknown properties of four excited states of ${}^{35}$Cl at 6643 keV, 6761 keV, 6780 keV, and 6800 keV.

   nucl-exastro-ph.SRPRC(2019)·7 citations

* Reconstructed cohort: no mailing for this day survives in the archive. Papers are grouped by their submission times and arXiv's announcement cut-off, assuming announcement without delay; positions follow identifier order. Validated at ~91% exact-day agreement against the archived era.