Abstract: Angular correlations between heavy quarks provide a unique probe of the quark-gluon plasma created in ultrarelativistic heavy-ion collisions. Results are presented of a measurement of the azimuthal angle correlations between muons originating from semileptonic decays of heavy quarks produced in 5.02 TeV Pb + Pb and pp collisions at the LHC. The muons are measured with transverse momenta and pseudorapidities satisfying p(T)(mu) > 4 GeV and vertical bar eta(mu)vertical bar < 2.4, respectively. The distributions of azimuthal angle separation Delta Phi for muon pairs having pseudorapidity separation vertical bar Delta eta vertical bar > 0.8, are measured in different Pb + Pb centrality intervals and compared to the same distribution measured in pp collisions at the same center-of-mass energy. Results are presented separately for muon pairs with opposite-sign charges, same-sign charges, and all pairs. A clear peak is observed in all Delta Phi distributions at Delta Phi similar to Pi, consistent with the parent heavy-quark pairs being produced via hard-scattering processes. The widths of that peak, characterized using Cauchy-Lorentz fits to the Delta Phi distributions, are found to not vary significantly as a function of Pb + Pb collision centrality and are similar for pp and Pb + Pb collisions. This observation will provide important constraints on theoretical descriptions of heavy-quark interactions with the quarkgluon plasma.

Abstract: We investigated decays of 51,52,53Kat the ISOLDE Decay Station at CERN in order to understand the mechanism of the β-delayed neutron-emission (βn) process. The experiment quantified neutron and gamma-ray emission paths for each precursor. We used this information to test the hypothesis, first formulated by Bohr in 1939, that neutrons in the βn process originate from the structureless “compound nucleus.” The data are consistent with this postulate for most of the observed decay paths. The agreement, however, is surprising because the compound-nucleus stage should not be achieved in the studied β decay due to insufficient excitation energy and level densities in the neutron emitter. In the 53 K βn decay, we found a preferential population of the first excited state in 52 Ca that contradicted Bohr's hypothesis. The latter was interpreted as evidence for direct neutron emission sensitive to the structure of the neutron-unbound state. We propose that the observed nonstatistical neutron emission proceeds through the coupling with nearby doorway states that have large neutron-emission probabilities. The appearance of “compound- nucleus” decay is caused by the aggregated small contributions of multiple doorway states at higher excitation energy.

Abstract: We propose multiloop vacuum amplitudes in the loop-tree duality (LTD) as the optimal building blocks for efficiently assembling theoretical predictions at high-energy colliders. This hypothesis is strongly supported by the manifestly causal properties of the LTD representation of a vacuum amplitude. The vacuum amplitude in LTD, acting as a kernel, encodes all the final states contributing to a given scattering or decay process through residues in the on-shell energies of the internal propagators. Gauge invariance and the wave function renormalization of the external legs are naturally incorporated. This methodological approach, dubbed LTD causal unitary, leads to a novel differential representation of cross sections and decay rates that is locally free of ultraviolet and infrared singularities at all orders in perturbation theory. Threshold singularities also match between different phase-space residues. Most notably, it allows us to conjecture for the first time the local functional form of initial-state collinear splitting functions. The fulfillment of all these properties provides a theoretical description of differential observables at colliders that is well defined in the four physical dimensions of the space-time.

Abstract: A measurement of time-dependent CP violation in the decays of B-0 and (B) over bar (0) mesons to the final states J/psi(-> mu(+) mu(-))K-S(0), psi(2S)(-> mu(+) mu(-))K-S(0) and J/psi(-> e(+)e(-))K-S(0) with K-S(0) -> pi(+)pi(-) is presented. The data correspond to an integrated luminosity of 6 fb(-1) collected at a center-of-mass energy of root s = 13 TeV with the LHCb detector. The CP-violation parameters are measured to be S-psi KS0 = 0.717 +/- 0.013(stat) +/- 0.008(syst) and C-psi KS0 = 0.008 +/- 0.012(stat) +/- 0.003(syst). This measurement of S-psi KS0 represents the most precise single measurement of the CKM angle beta to date and is more precise than the current world average. In addition, measurements of the CP-violation parameters of the individual channels are reported and a combination with the LHCb Run 1 measurements is performed.

Abstract: The production rate of Lambda(0)(b) baryons relative to B-0 mesons in pp collisions at a center-of-mass energy root s = 13 TeV is measured by the LHCb experiment. The ratio of Lambda(0)(b) to B-0 production cross sections shows a significant dependence on both the transverse momentum and the measured charged-particle multiplicity. At low multiplicity, the ratio measured at LHCb is consistent with the value measured in ethorne- collisions, and increases by a factor of similar to 2 with increasing multiplicity. At relatively low transverse momentum, the ratio of Lambda(0)(b) to B-0 cross sections is higher than what is measured in e(+)e(-) collisions, but converges with the ethorne- ratio as the momentum increases. These results imply that the evolution of heavy b quarks into finalstate hadrons is influenced by the density of the hadronic environment produced in the collision. Comparisons with several models and implications for the mechanisms enforcing quark confinement are discussed.