Nuclear Experiment

Higher-order cumulants of the net-baryon multiplicity distributions are predicted to be sensitive to the properties of the nuclear matter created in high-energy nuclear collisions. In this talk, we present the collision centrality and acceptance (rapidity and transverse momentum) dependence of the ratio of the 6 th - to the 2 nd -order cumulant ratio ( C 6 / C 2 ) of net-proton in Au+Au collisions at s NN − − − √ =54.4 and 200 GeV measured by the STAR detector at RHIC. The new results are compared to hadron transport model and lattice QCD calculations.

We report on a comparison between the theoretically predicted and experimentally measured spectra of the first-forbidden non-unique β -decay transition 137 Xe(7/ 2 − )→ 137 Cs(7/ 2 + ) . The experimental data were acquired by the EXO-200 experiment during a deployment of an AmBe neutron source. The ultra-low background environment of EXO-200, together with dedicated source deployment and analysis procedures, allowed for collection of a pure sample of the decays, with an estimated signal-to-background ratio of more than 99-to-1 in the energy range from 1075 to 4175 keV. In addition to providing a rare and accurate measurement of the first-forbidden non-unique β -decay shape, this work constitutes a novel test of the calculated electron spectral shapes in the context of the reactor antineutrino anomaly and spectral bump.

Experimental searches for Chiral Magnetic Effect (CME) in heavy-ion collisions have been going on for a decade, and so far there is no conclusive evidence for its existence. Recently, the Signed Balance Function (SBF), based on the idea of examining the momentum ordering of charged pairs along the in- and out-of-plane directions, has been proposed as a probe of CME. In this approach, a pair of observables is invoked: one is r rest , the out-of-plane to in-plane ratio of ΔB measured in pair's rest frame, where ΔB is the difference between signed balance functions; The other is a double ratio, R B = r rest / r lab , where r lab is a measurement similar to r rest but measured in the laboratory frame. These two observables give opposite responses to the CME-driven charge separation compared to the background correlations arising from resonance flow and global spin alignment. Both r rest and R B being larger than unity can be regarded as a case in favor of the existence of CME. It is found experimentally that r rest , r lab and R B are larger than unity in Au+Au collisions at 200 GeV, and larger than realistic model calculations with no CME implemented. These findings are difficult to be explained by a background-only scenario.

We propose to measure the high-energy behavior of the integrand of the Gerasimov-Drell-Hearn (GDH) sum rule on the proton and the neutron up to 12 GeV. The convergence of the GDH integral will be investigated for the first time and to high precision. The validity of the GDH sum rule on the neutron will be accurately tested for the first time, while for the proton the uncertainty will be improved by 25% relative. The data will allow precision testing of Regge phenomenology in the polarized domain. The a 1 and f 1 Regge trajectory intercepts will be obtained to an order of magnitude higher precision than the current best estimates. The data will also contribute to the determination of the real and imaginary parts of the spin-dependent Compton amplitude, the polarizability correction to hyperfine splitting in hydrogen, and to studying the transition between polarized DIS and diffractive regimes. The experiment will require a circularly polarized photon beam (produced from a longitudinally polarized electron beam) with a flux approximately 1/3 of the GlueX-II experiment E12-13-003. The experiment will run in two configurations which require two different CEBAF beam energies. A new longitudinal polarized proton and deuteron target will be needed in Hall D. The experiment will require 21 PAC days at the nominal CEBAF energy and another 12 PAC days at an energy 1/3 to 1/2 of the nominal.

In this Letter, we report the first measurement of the inelastic cross section for antideuteron-nucleus interactions at low particle momenta, covering a range of 0.3≤p<4 GeV/ c . The measurement is carried out using p-Pb collisions at a center-of-mass energy per nucleon-nucleon pair of s NN − − − √ = 5.02 TeV, recorded with the ALICE detector at the CERN LHC and utilizing the detector material as an absorber for antideuterons and antiprotons. The extracted raw primary antiparticle-to-particle ratios are compared to the results from detailed ALICE simulations based on the GEANT4 toolkit for the propagation of antiparticles through the detector material. The analysis of the raw primary (anti)proton spectra serves as a benchmark for this study, since their hadronic interaction cross sections are well constrained experimentally. The first measurement of the inelastic cross section for antideuteron-nucleus interactions averaged over the ALICE detector material with atomic mass numbers ⟨A⟩ = 17.4 and 31.8 is obtained. The measured inelastic cross section points to a possible excess with respect to the Glauber model parameterization used in GEANT4 in the lowest momentum interval of 0.3≤p<0.47 GeV/ c up to a factor 2.1. This result is relevant for the understanding of antimatter propagation and the contributions to antinuclei production from cosmic ray interactions within the interstellar medium. In addition, the momentum range covered by this measurement is of particular importance to evaluate signal predictions for indirect dark-matter searches.

The aCORN experiment measures the neutron decay electron-antineutrino correlation ( a -coefficient) using a novel method based on an asymmetry in proton time-of-flight for events where the beta electron and recoil proton are detected in delayed coincidence. We report the data analysis and result from the second run at the NIST Center for Neutron Research, using the high-flux cold neutron beam on the new NG-C neutron guide end position: a=??.10758±0.00136(stat)±0.00148(sys) . This is consistent within uncertainties with the result from the first aCORN run on the NG-6 cold neutron beam. Combining the two aCORN runs we obtain a=??.10782±0.00124(stat)±0.00133(sys) , which has an overall relative standard uncertainty of 1.7 \%. The corresponding result for the ratio of weak coupling constants λ= G A / G V is λ=??.2796±0.0062 .

Measuring the spin structure of nucleons (protons and neutrons) extensively tests our understanding of how nucleons arise from quarks and gluons, the fundamental building blocks of nuclear matter. The nucleon spin structure is typically probed in scattering experiments using polarized beams and polarized nucleon targets, and the results are compared with predictions from Quantum Chromodynamics directly or with effective theories that describe the strong nuclear force. Here we report on new proton spin structure measurements with significantly better precision and improved coverage than previous data at low momentum transfer squared between 0.012 and 1.0 GeV 2 . This kinematic range provides unique tests of effective field theory predictions. Our results show that a complete description of the nucleon spin remains elusive. They call for further theoretical works that include the more fundamental lattice gauge method. Finally, our data agree with the Gerasimov-Drell-Hearn sum rule, a fundamental prediction of quantum field theory.

We report on the W and Z/ γ ∗ differential and total cross sections as well as the W + / W − and ( W + + W − ) / (Z/ γ ∗ ) cross-section ratios measured by the STAR experiment at RHIC in p+p collisions at s √ =500 GeV and 510 GeV. The cross sections and their ratios are sensitive to quark and antiquark parton distribution functions. In particular, at leading order, the W cross-section ratio is sensitive to the d ¯ / u ¯ ratio. These measurements were taken at high Q 2 ∼ M 2 W , M 2 Z and can serve as input into global analyses to provide constraints on the sea quark distributions. The results presented here combine three STAR data sets from 2011, 2012, and 2013, accumulating an integrated luminosity of 350 pb −1 . We also assess the expected impact that our W + / W − cross-section ratios will have on various quark distributions, and find sensitivity to the u ¯ − d ¯ and d ¯ / u ¯ distributions.

Cross-sections for nat Pb(p,xn) 201−207 Bi , 204 Pb(p,1−4n) 201−204 Bi and 206 Pb(p,3n) 204 Bi reactions have been determined in the astrophysical energy range 20−30MeV . The analysis were performed by γ -spectroscopy associated with half-lives measurements. The results were compared with previously experimental data, when available, and with theoretical calculations performed using TALYS code. We report a possible new γ transition from 204m Pb and other theoretical discrepancies, probably due overestimation of the Coulomb barrier and neutron binding energy. Also reported are new determinations of the energies and intensities of the γ 's emitted in the decays of 205,206 Bi . We discuss possible implications for the r/rp-processes in neutrino-driven wind in supernovae and neutron stars mergers.

The production of Ξ(1321 ) − and Ξ ¯ ¯ ¯ ¯ (1321 ) + hyperons in inelastic p+p interactions is studied in a fixed target experiment at a beam momentum of 158 GeV/textit{c}. Double differential distributions in rapidity y and transverse momentum p T are obtained from a sample of 33M inelastic events. They allow to extrapolate the spectra to full phase space and to determine the mean multiplicity of both Ξ − and Ξ ¯ ¯ ¯ ¯ + . The rapidity and transverse momentum spectra are compared to transport model predictions. The Ξ − mean multiplicity in inelastic p+p interactions at 158~\GeVc is used to quantify the strangeness enhancement in A+A collisions at the same centre-of-mass energy per nucleon pair.