Abstract: The spent fuel of current nuclear reactors contains fissile plutonium isotopes that can be combined with uranium to make mixed oxide (MOX) fuel. In this way the Pu from spent fuel is used in a new reactor cycle, contributing to the long-term sustainability of nuclear energy. However, an extensive use of MOX fuels, in particular in fast reactors, requires more accurate capture and fission cross sections for some Pu isotopes. In the case of Pu-242 there are sizable discrepancies among the existing capture cross-section measurements included in the evaluations (all from the 1970s) resulting in an uncertainty as high as 35% in the fast energy region. Moreover, postirradiation experiments evaluated with JEFF-3.1 indicate an overestimation of 14% in the capture cross section in the fast neutron energy region. In this context, the Nuclear Energy Agency (NEA) requested an accuracy of 8% in this cross section in the energy region between 500 meV and 500 keV. This paper presents a new time-of-flight capture measurement on Pu-242 carried out at nTOF-EAR1 (CERN), focusing on the analysis and statistical properties of the resonance region, below 4 keV. The Pu-242(n, gamma) reaction on a sample containing 95(4) mg enriched to 99.959% was measured with an array of four C6D6 detectors and applying the total energy detection technique. The high neutron energy resolution of nTOF-EAR1 and the good statistics accumulated have allowed us to extend the resonance analysis up to 4 keV, obtaining new individual and average resonance parameters from a capture cross section featuring a systematic uncertainty of 5%, fulfilling the request of the NEA.

Abstract: A search is presented for the direct pair production of the stop, the supersymmetric partner of the top quark, that decays through an R-parity-violating coupling to a final state with two leptons and two jets, at least one of which is identified as a b-jet. The data set corresponds to an integrated luminosity of 36.1 fb(-1) of proton-proton collisions at a center-of-mass energy of root s = 13 TeV, collected in 2015 and 2016 by the ATLAS detector at the LHC. No significant excess is observed over the Standard Model background, and exclusion limits are set on stop pair production at a 95% confidence level. Lower limits on the stop mass are set between 600 GeV and 1.5 TeV for branching ratios above 10% for decays to an electron or muon and a b-quark.

Abstract: In this paper we perform an amplitude analysis of eta ' -> pi(+)pi(-)gamma and confront it with the latest BESIII data. Based on the final-state interaction theorem, we represent the amplitude in terms of an Omnes function multiplied by a form factor that corresponds to the contributions from left-hand cuts and right-hand cuts in the inelastic channels. We also take into account the isospin violation effect induced by rho-omega mixing. Our results show that the anomaly contribution is mandatory in order to explain the data. Its contribution to the decay width of Gamma(eta ' -> pi pi gamma) is larger than that induced by isospin violation. Finally we extract the pole positions of the rho and omega as well as their corresponding residues.

Abstract: A detailed study of multiparticle azimuthal correlations is presented using pp data at root s = 5.02 and 13 TeV, and p+Pb data at root s(NN) = 5.02 TeV, recorded with the ATLAS detector at the CERN Large Hadron Collider. The azimuthal correlations are probed using four-particle cumulants c(n){4} and flow coefficients v(n){4} = (-c(n){4})(1/4) for n = 2 and 3, with the goal of extracting long-range multiparticle azimuthal correlation signals and suppressing the short-range correlations. The values of c(n){4} are obtained as a function of the average number of charged particles per event, < N-ch >, using the recently proposed two-subevent and three-subevent cumulant methods, and compared with results obtained with the standard cumulant method. The standard method is found to be strongly biased by short-range correlations, which originate mostly from jetswith a positive contribution to c(n){4}. The threesubevent method, on the other hand, is found to be least sensitive to short-range correlations. The three-subevent method gives a negative c(2){4}, and therefore a well-defined v(2){4}, nearly independent of < N-ch >, which implies that the long-range multiparticle azimuthal correlations persist to events with low multiplicity. Furthermore, v(2){4} is found to be smaller than the v(2){2} measured using the two-particle correlation method, as expected for long-range collective behavior. Finally, the measured values of v(2){4} and v(2){2} are used to estimate the number of sources relevant for the initial eccentricity in the collision geometry. The results based on the subevent cumulant technique provide direct evidence, in small collision systems, for a long-range collectivity involving many particles distributed across a broad rapidity interval.

Abstract: In this work, we analyze the spatial and time variation of the fine structure constant (alpha) upon the theoretical framework developed by Bekenstein (Phys. Rev. D 66, 123514 (2002)). We have computed the field psi related to alpha at first order of the weak-field approximation and have also improved the estimation of the nuclear magnetic energy and, therefore, their contributions to the source term in the equation of motion of psi. We obtained that the results are similar to the ones published in L. Kraiselburd and H. Vucetich, Int. J. Mod. Phys. E 20, 101 (2011) which were computed using the zero order of the approximation, showing that one can neglect the first order contribution to the variation of the fine structure constant. Through the comparison between our theoretical results and the observational data of the Eotvos-type experiments or the time variation of alpha over the cosmological time scale, we set constraints on the free parameter of the Bekenstein model, namely the Bekenstein length.