We review the main results obtained by the BRAHMS collaboration on the properties of hot and dense hadronic and partonic matter produced in ultrarelativistic heavy ion collisions at RHIC. A particular focus of this paper is to discuss to what extent the results collected so far by BRAHMS, and by the other three experiments at RHIC, can be taken as evidence for the formation of a state of deconfined partonic matter, the so called quark-gluon-plasma (QGP). We also discuss evidence for a possible precursor state to the QGP, i.e. the proposed Color Glass Condensate.

The centrality dependence of the charged-particle multiplicity density at midrapidity in Pb-Pb collisions at ffiffiffiffiffiffiffiffi s NN p ¼ 2:76 TeV is presented. The charged-particle density normalized per participating nucleon pair increases by about a factor of 2 from peripheral (70%-80%) to central (0%-5%) collisions. The centrality dependence is found to be similar to that observed at lower collision energies. The data are compared with models based on different mechanisms for particle production in nuclear collisions.

We report on a study of the transverse momentum dependence of nuclear modification factors R dAu for charged hadrons produced in deuteron + gold collisions at √ sNN = 200 GeV, as a function of collision centrality and of the pseudorapidity (η = 0, 1, 2.2, 3.2) of the produced hadrons. We find a significant and systematic decrease of R dAu with increasing rapidity. The mid-rapidity enhancement and the forward rapidity suppression are more pronounced in central collisions relative to peripheral collisions. These results are relevant to the study of the possible onset of gluon saturation at energies reached at BNL RHIC.

The first measurement of the charged-particle multiplicity density at midrapidity in Pb-Pb collisions at a center-of-mass energy per nucleon pair √ S NN = 2.76 TeV is presented. For an event sample corresponding to the most central 5% of the hadronic cross section, the pseudorapidity density of primary charged particles at midrapidity is 1584 ± 4(stat) ± 76(syst), which corresponds to 8.3 ± 0.4(syst) per participating nucleon pair. This represents an increase of about a factor 1.9 relative to pp collisions at similar collision energies, and about a factor 2.2 to central Au-Au collisions at √ S NN = 2.76 TeV. This measurement provides the first experimental constraint for models of nucleus-nucleus collisions at LHC energies.

The transverse momentum (p T ) spectrum and nuclear modification factor (R AA ) of reconstructed jets in 0-10% and 10-30% central Pb-Pb collisions at √ s NN = 2.76 TeV were measured. Jets were reconstructed using the anti-k T jet algorithm with a resolution parameter of R = 0.2 from charged and neutral particles, utilizing the ALICE tracking detectors and Electromagnetic Calorimeter (EMCal). The jet p T spectra are reported in the pseudorapidity interval of |η jet | < 0.5 for 40 < p T, jet < 120 GeV/c in 0-10% and for 30 < p T, jet < 100 GeV/c in 10-30% collisions. Reconstructed jets were required to contain a leading charged particle with p T > 5 GeV/c to suppress jets constructed from the combinatorial background in Pb-Pb collisions. The leading charged particle requirement applied to jet spectra both in pp and PbPb collisions had a negligible effect on the R AA . The nuclear modification factor R AA was found to be 0.28 ± 0.04 in 0-10% and 0.35 ± 0.04 in 10-30% collisions, independent of p T, jet within the uncertainties of the measurement. The observed suppression is in fair agreement with expectations from two model calculations with different approaches to jet quenching.

The p t-differential production cross sections of the prompt (B feed-down subtracted) charmed mesons D 0 , D + , and D * + in the rapidity range |y| < 0.5, and for transverse momentum 1 < p t < 12 GeV/c, were measured in proton-proton collisions at √ s = 2.76 TeV with the ALICE detector at the Large Hadron Collider. The analysis exploited the hadronic decays

A measurement of the transverse momentum spectra of jets in Pb-Pb collisions at √ s NN = 2.76 TeV is reported. Jets are reconstructed from charged particles using the anti-k T jet algorithm with jet resolution parameters R of 0.2 and 0.3 in pseudorapidity |η| < 0.5. The transverse momentum p T of charged particles is measured down to 0.15 GeV/c which gives access to the low p T fragments of the jet. Jets found in heavy-ion collisions are corrected event-by-event for average background density and on an inclusive basis (via unfolding) for residual background fluctuations and detector effects. A strong suppression of jet production in central events with respect to peripheral events is observed. The suppression is found to be similar to the suppression of charged hadrons, which suggests that substantial energy is radiated at angles larger than the jet resolution parameter R = 0.3 considered in the analysis. The fragmentation bias introduced by selecting jets with a high p T leading particle, which rejects jets with a soft fragmentation pattern, has a similar effect on the jet yield for central and peripheral events. The ratio of jet spectra with R = 0.2 and R = 0.3 is found to be similar in Pb-Pb and simulated PYTHIA pp events, indicating no strong broadening of the radial jet structure in the reconstructed jets with R < 0.3.

The p T -differential production cross section of prompt Λ + c charmed baryons was measured with the ALICE detector at the Large Hadron Collider (LHC) in pp collisions at √ s = 7 TeV and in p-Pb collisions at √ s NN = 5.02 TeV at midrapidity. The Λ + c and Λ -c were reconstructed in the hadronic decay modes Λ + c → pK − π + , Λ + c → pK 0 S and in the semileptonic channel Λ + c → e + ν e Λ (and charge conjugates). The measured values of the Λ + c /D 0 ratio, which is sensitive to the c-quark hadronisation mechanism, and in particular to the production of baryons, are presented and are larger than those measured previously in different colliding systems, centre-of-mass energies, rapidity and p T intervals, where the Λ + c production process may differ. The results are compared with the expectations obtained from perturbative Quantum Chromodynamics calculations and Monte Carlo event generators. Neither perturbative QCD calculations nor Monte Carlo models reproduce the data, indicating that the fragmentation of heavy-flavour baryons is not well understood. The first measurement at the LHC of the Λ + c nuclear modification factor, R pPb , is also presented. The R pPb is found to be consistent with unity and with that of D mesons within the uncertainties, and consistent with a theoretical calculation that includes cold nuclear matter effects and a calculation that includes charm quark interactions with a deconfined medium. Keywords: Heavy Ion Experiments, Quark gluon plasmaArXiv ePrint: 1712.09581Open Access, Copyright CERN, for the benefit of the ALICE Collaboration. Article funded by SCOAP 3 .https://doi.org/10.1007/JHEP04(2018)108 The ALICE collaboration 40 JHEP04(2018)108 IntroductionThe study of charm production at the Large Hadron Collider (LHC) is an important tool to test predictions obtained from perturbative Quantum Chromodynamics (pQCD) calculations for proton-proton (pp) collisions. These calculations are based on the factorisation approach that describes heavy-flavour production as a convolution of the parton distribution functions, the parton hard-scattering cross section and the fragmentation function. The cross section for heavy-flavour hadron production can be obtained from perturbative calculations at next-to-leading order with next-to-leading-log resummation, like the General-Mass Variable-Flavour-Number Scheme (GM-VFNS [1, 2]) and 4]) approaches. No predictions are, however, available for baryons in the latter approach due to lack of knowledge of the fragmentation function of charm quarks into baryons. Cross section calculations are available also with the k T factorisation framework [5]. These theoretical calculations generally describe within uncertainties the measurements at the LHC, with the central predictions for beauty production lying closer to data than the central predictions for charm production [6]. The measured -1 - JHEP04(2018)108transverse momentum differential cross section of charm mesons lies in the upper part of the FONLL uncertainty band and is systematically below th...