Abstract: This paper presents measurements of charged-hadron spectra obtained in pp, p+Pb, and Pb+Pb collisions at root s or root s(NN) = 5.02TeV, and in Xe+Xe collisions at root s(NN) = 5.44TeV. The data recorded by the ATLAS detector at the LHC have total integrated luminosities of 25 pb(-1), 28 nb(-1), 0.50 nb(-1), and 3 μb(-1), respectively. The nuclear modification factors RpPb and R-AA are obtained by comparing the spectra in heavy-ion and pp collisions in a wide range of charged-particle transverse momenta and pseudorapidity. The nuclear modification factor RpPb shows a moderate enhancement above unity with a maximum at p(T) approximate to 3 GeV; the enhancement is stronger in the Pb-going direction. The nuclear modification factors in both Pb+Pb and Xe+Xe collisions feature a significant, centrality-dependent suppression. They show a similar distinct p(T)-dependence with a local maximum at p(T) approximate to 2 GeV and a local minimum at p(T) approximate to 7 GeV. This dependence is more distinguishable in more central collisions. No significant eta |-dependence is found. A comprehensive comparison with several theoretical predictions is also provided. They typically describe RAA better in central collisions and in the pT range from about 10 to 100 GeV.

Abstract: The production ratio of psi(2S) to J/psi charmonium states is presented as a function of multiplicity in proton-lead collisions at a centre-of-mass energy of root s(NN) = 8.16TeV, for both prompt and nonprompt sources. The total luminosity recorded by the LHCb experiment corresponds to 13.6 nb(-1) for pPb collisions and 20.8 nb(-1) for Pbp collisions, where the first particle corresponds to the particle traveling towards the detector. Measurements are performed in the dimuon final state at forward (backward) centre-of-mass rapidity, with respect to the proton direction, 1.5 < y* < 4.0 (-5.0 < y* < -2.5) for pPb (Pbp) collisions. A multiplicity dependence of the prompt production ratio is observed in pPb collisions, whereas no dependence is found in nonprompt production, nor in either prompt or nonprompt production in Pbp collisions. These results suggest that in the Pb-going direction additional suppression mechanisms beyond comover effects may be present, possibly related to the formation of quark-gluon plasma. This highlights a transition from small to large collision systems and provides important insight into the suppression of charmonia in proton-nucleus collisions.

Abstract: The invariant-mass distribution for the coherent photoproduction of dipions in ultraperipheral PbPb collisions is measured using data, corresponding to an integrated luminosity of 224.6 +/- 9.6 μb(-1), collected by the LHCb experiment in 2018 at a nucleon-nucleon centre-of-mass energy root s(NN) = 5.02TeV. In the mass range from 400 to 1200MeV, the results are consistent with previous experiments, with the spectrum dominated by the rho(0) meson, which interferes with a nonresonant component, together with a smaller omega meson contribution. In an extended mass range up to 2300MeV, models previously used do not fit the data and a consistent description requires the introduction of two resonances at masses of 1350 +/- 20MeV and 1790 +/- 20MeV with widths of about 300MeV. The cross-section for each meson is measured differentially in twelve bins of rapidity from 2.05 to 4.90. The rho(0) cross-section increases with rapidity from about 400 to 600 mb and is measured with a typical precision of 8%, while the cross-section times branching fraction for the omega, rho' and rho '', with the statistical precision of the data, do not have a pronounced rapidity dependence and are between 0.5 and 1.5mb, with uncertainties up to 30%. A large nuclear suppression is observed for the rho(0) meson compared to expectations based on photoproduction on the proton that use the impulse approximation. Significant suppression is also observed compared to that predicted by elastic scattering described in the Glauber approach, or with the addition of inelastic scattering in a Gribov-Glauber model.

Abstract: The prompt production of +c baryons is studied in proton- lead collisions collected with the LHCb detector at the LHC. The data sample corresponds to an integrated luminosity of 1 : 58 nb recorded at a nucleon- nucleon centre- of- mass energy of p sNN = 5 : 02TeV. Measurements of the di ff erential cross- section and the forwardbackward production ratio are reported for +c baryons with transverse momenta in the range 2 < pT < 10 GeV =c and rapidities in the ranges 1 : 5 < y < 4 : 0 and 4 : 5 < y < 2 : 5 in the nucleon- nucleon centre- of- mass system. The ratio of cross- sections of +c baryons and D0 mesons is also reported. The results are compared with next- to- leading order calculations that use nuclear parton distribution functions.

Abstract: This article presents the first measurement of the differential Z-boson production cross-section in the forward region using proton-lead collisions with the LHCb detector. The dataset was collected at a nucleon-nucleon centre-of-mass energy of root(NN)-N-s = 8.16TeV in 2016, corresponding to an integrated luminosity of 30.8 nb(-1). The forward-backward ratio and the nuclear modification factors are measured together with the differential crosssection as functions of the Z boson rapidity in the centre-of-mass frame, the transverse momentum of the Z boson and a geometric variable phi*. The results are in good agreement with the predictions from nuclear parton distribution functions, providing strong constraining power at small Bjorken-x.