J/Psi production as a function of charged particle multiplicity in pp collisions at sqrt{s}=7 TeV with the ALICE experiment

We report on the first measurement of J/Psi production as a function of charged particle pseudo-rapidity density dN_ch/d_eta in pp collisions at sqrt{s} = 7 TeV with the ALICE experiment at the LHC. J/Psi mesons are detected down to p_T = 0 via their decays into e+e- pairs at mid-rapidity (|y| < 0.9) and into mu+mu- pairs at forward rapidity (2.5<y<4). dN_ch/d_eta is measured within |eta| < 1. We compare results in the two different J/Psi rapidity ranges. Preliminary PYTHIA simulations are also presented.Comment: contribution to the MPI@LHC 2011 proceedings, 8 pages, 4 figure

Recent results from the ALICE experiment on open heavy flavours in hadronic collisions at the LHC

ALICE is the LHC experiment devoted to the study of the Quark-Gluon Plasma (QGP). To probe this high energy density state of strongly interacting matter expected to be produced in heavy-ion collisions at high energies, measurements performed in various systems (pp, p-Pb and Pb-Pb) are compared to each other. Heavy quarks are produced in initial hard partonic scatterings on a short time scale and participate in the subsequent evolution of the medium. This makes them sensitive probes of the QGP. With ALICE, open heavy flavours are studied using D mesons (D$^0$, D$^+$, D$^{*+}$) reconstructed via their hadronic decay channels in the mid-rapidity region ($|y|<0.5$) and with heavy-flavour decay leptons reconstructed in the electronic (muonic) channel in the central rapidity region $|y|<0.9$ (forward rapidity region $2.5<y<4$). We present ALICE measurements of the nuclear modification factor $R_{\rm{AA}}$ and the elliptic flow $v_2$ for D mesons and heavy-flavour decay leptons in Pb-Pb collisions at $\sqrt{s_{\rm{NN}}}=2.76$ TeV. The corresponding measurements in p-Pb collisions at $\sqrt{s_{\rm{NN}}}=5.02$ TeV are also discussed. First results on the azimuthal correlations between heavy-flavour particles and hadrons are presented as well as the charged-particle multiplicity dependence of heavy-flavour production in pp collisions at $\sqrt{s}=7$ TeV.Comment: Proceedings of 52nd International Winter Meeting on Nuclear Physics conference (Bormio 2014), 27-31 January 201

New Developments of EPOS 2

Since 2006, EPOS hadronic interaction model is being used for very high energy cosmic ray analysis. Designed for minimum bias particle physics and used for having a precise description of SPS and RHIC heavy ion collisions, EPOS brought more detailed description of hadronic interactions in air shower development. Thanks to this model it was possible to understand why there were less muons in air shower simulations than observed in real data. With the start of the LHC era, a better description of hard processes and collective effects is needed to deeply understand the incoming data. We will describe the basic physics in EPOS and the new developments and constraints which are taken into account in EPOS 2.Comment: Contributed presentation to the XVI International Symposium on Very High Energy Cosmic Ray Interactions (ISVHECRI 2010), Batavia, IL, USA (28 June 2 July 2010). 4 pages, 6 figure

Collective flow in (anti)proton-proton collision at Tevatron and LHC

Collective flow as a consequence of hydrodynamical evolution in heavy ion collisions is intensively studied by theorists and experimentalists to understand the behavior of hot quark matter. Due to their large mass, heavy ions suffer collective effects even at low (SPS) or intermediate energies (RHIC). In case of light systems such as (anti)proton-proton interactions, collective effects was not expected. Within a global model such as EPOS, where light and heavy systems are treated using the same physics, it appears that Tevatron data are better described if a flow is introduced. Then the extrapolation to LHC can easily be done and we can compare to first data from ATLAS experiment.Comment: 4 pages, 6 figures, Proceeding of the 45th Rencontres de Moriond QC

Jets, Bulk Matter, and their Interaction in Heavy Ion Collisions at Several TeV

We discuss a theoretical scheme that accounts for bulk matter, jets, and the interaction between the two. The aim is a complete description of particle production at all transverse momentum ($p_{t}$) scales. In this picture, the hard initial scatterings result in mainly longitudinal flux tubes, with transversely moving pieces carrying the $p_{t}$ of the partons from hard scatterings. These flux tubes constitute eventually both bulk matter (which thermalizes and flows) and jets. We introduce a criterion based on parton energy loss to decide whether a given string segment contributes to the bulk or leaves the matter to end up as a jet of hadrons. Essentially low $p_{t}$ segments from inside the volume will constitute the bulk, high $p_{t}$ segments (or segments very close to the surface) contribute to the jets. The latter ones appear after the usual flux tube breaking via q-qbar production (Schwinger mechanism). Interesting is the transition region: Intermediate $p_{t}$ segments produced inside the matter close to the surface but having enough energy to escape, are supposed to pick up q-qbar pairs from the thermal matter rather than creating them via the Schwinger mechanism. This represents a communication between jets and the flowing bulk matter (fluid-jet interaction). Also very important is the interaction between jet hadrons and the soft hadrons from the fluid freeze-out. We employ the new picture to investigate Pb-Pb collisions at 2.76 TeV. We discuss the centrality and $p_{t}$ dependence of particle production and long range dihadron correlations at small and large $p_{t}$

Producing Hard Processes Regarding the Complete Event: The EPOS Event Generator

Jet cross sections can be in principle compared to simple pQCD calculations, based on the hypothesis of factorization. But often it is useful or even necessary to not only compute the production rate of the very high pt jets, but in addition the "rest of the event". The proposed talk is based on recent work, where we try to construct an event generator fully compatible with pQCD which allows to compute complete events, consisting of high pt jets plus all the other low pt particles produced at the same time. Whereas in "generators of inclusive spectra" like Pythia one may easily trigger on high pt phenomena, this is not so obvious for "generators of physical events", where in principle one has to generate a very large number of events in order to finally obtain rare events (like those with a very high pt jet). We recently developped an independnat block method which allow us ta have a direct access to dedicated variables 1. We will present latest results concerning this approach.Comment: Moriond 2010 Porceedin

On the Role of Initial Conditions and Final State Interactions in Ultrarelativistic Heavy Ion Collisions

We investigate the rapidity dependence of the elliptical flow in heavy ion collisions at 200 GeV (cms), by employing a three-dimensional hydrodynamic evolution, based on different initial conditions, and different freeze-out scenarios. It will be shown that the form of pseudo-rapidity ($\eta$) dependence of the elliptical flow is almost identical to space-time-rapidity ($\eta_{s}$) dependence of the initial energy distribution, independent of the freeze-out prescriptions

J/ΨJ/\Psi yield vs. multiplicity in proton-proton collisions at the LHC

We address the question of understanding the production of $J/\psi$ particles regarding the global underlying event in proton-proton collisions. To do so, we propose to look at a new observable: the $J/\psi$ production as a function of the charged particles multiplicity of the event. We demonstrate the interest for an experimental measurement by varying the model of multiple interactions in the PYTHIA generator.Comment: 4 pages, 2 figures, QUARKONIUM 2010: Three Days Of Quarkonium Production in pp and pA Collisions, 29-31 Jul 2010, Palaiseau, Franc