A revised many-body potential energy function for the description of the H3O+(H2O)n clusters

A revised potential energy function that has been fitted to the latest set of Kebarle and coworkers [1982, J. Am. chem. Soc, 104, 1462] entropy and enthalpy measurements at T = 300 K is presented. The model assumes a rigid hydronium unit and accounts for all orders of many-body interactions explicitly. The difference with the older function that had been based on earlier measurements by Kebarle and co-workers [1972, J. Am. chem. Soc, 94, 7627; 1967, J. Am. chem. Soc, 89, 6393] is that more compact clusters are generated. We have studied the structural properties of water clusters in the size range 5–80 at T = 250 K within the framework of the (μPT) Grand Canonical ensemble. Clusters with sizes less than about 10 water molecules consist of a four-coordinated first shell, where the fourth water molecule is hydrogen bonded to the oxygen atom of the hydronium ion. The hydration number goes through a minimum value ～1.6, for a cluster size around 50, and it starts increasing again with further cluster growth, to ～2.5 for a cluster size of 250 water molecules, which is the largest cluster examined. On the other hand the water molecule coordination number shows a monotonic increase with cluster size. In small clusters, less than 10, water molecules prefer to be arranged in a chain-like fashion; at sizes around 50, tri-coordinated clathrate-like structures dominate whereas with further size increase the coordination number eventually levels off to the experimental bulk value, at 4.6.     

Nanoflare heating model for collisionless solar corona

The problem of coronal heating remains one of the greatest unresolved problems in space science. Magnetic reconnection plays a significant role in heating the solar corona. When two oppositely directed magnetic fields come closer to form a current sheet, the current density of the plasma increases due to which magnetic reconnection and conversion of magnetic energy into thermal energy takes place. The present paper deals with a model for reconnection occurring in the solar corona under steady state in collisionless regime. The model predicts that reconnection time in the solar corona varies inversely with the cube of magnetic field and varies directly with the Lindquist number. Our analysis shows that reconnections are occurring within a time interval of 600 s in the solar corona, producing nanoflares in the energy range 10 ²¹–10 ²³ erg /s which matches with Yohkoh X-ray observations.     

Low-<Emphasis Type="Italic">p</Emphasis><Subscript>T</Subscript> proton-proton physics at low luminosity at LHC

This review of low-p _T proton-proton physics at low luminosity at the large hadron collider (LHC) should cover all LHC experiments, but in practice, is mainly related to ALICE, for reasons which will be explained. However, the relevance to other LHC experiments is clear, as low-p_T. phenomena represent an important component of the background to their high-p_T. phenomena which needs to be calibrated. The ALICE collaboration will study proton-proton collisions as part of their heavy-ion programme, where most signals are relative to the proton-proton system. In addition, the ALICE detector’s unique acceptance at low p_T as well as its unique particle identification capability will make it possible to carry out a program of genuine proton-proton physics complementary to those of other LHC experiments.     

Z0 Boson measurement with the ALICE central barrel in pp collisions at 14TeV

The possibility to detect the Z⁰ in the ALICE central barrel is studied via the electronic decay channel Z⁰ → e ⁺ e ⁻. The signal and the background are simulated with the leading order event generator PYTHIA 6. The total cross-sections are taken from NLO calculations. Based on test beam data, the electron identification performance of the Transition Radiation Detector is extrapolated to high momenta. The expected yields for minimumbias pp collisions at 14TeV are presented. An isolation cut on the single electron, together with a minimum transverse momentum cut, allows to obtain a clear signal. The expected background is of the order of 1% with the main contribution coming from misidentified pions from jets.     

Reaction plane determination by means of the ALICE zero degree calorimeters

A possible way to estimate the reaction plane of the ion-ion collision is to measure the sideward deflection of the spectator neutrons. In the ALICE experiment this kind of measurement can be performed by means of the two neutron zero degree calorimeters (ZN), which are located at opposite sides with respect to the beam intersection point (IP). In fact the ZN calorimeters, thanks to their segmentation in four towers, are position sensitive devices. Concerning their localizing capability, a spatial resolution of ∼3 mm has been measured for a 100 GeV/c hadron beam. This performance will be used to reconstruct, event by event, the centroid coordinate of the spectator neutron spot on the ZN front-face, which is sensitive to the directed flow (“bounce off”) of spectator neutrons. The measurement of the centroid will therefore allow to reconstruct the 1st-order event plane azimuth. A simulation is performed in order to estimate the dependence of the event plane resolution on the magnitude of the directed flow v₁ of the spectator neutrons and on the neutron multiplicity (event centrality). In particular, it will be shown that the event plane resolution is not dominated by the smearing on the centroid measurement, but by the smearing due to the transverse lead beam divergence at the IP. Finally a possible tool to select events with small lead beam divergence at IP is discussed, using the information coming from both the ZN calorimeters.     

Centrality dependence of the charged-particle multiplicity density at midrapidity in Pb-Pb collisions at sqrt[s(NN)] = 2.76 TeV

The centrality dependence of the charged-particle multiplicity density at midrapidity in Pb-Pb collisions at sqrt[s_{NN}]=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.     

The ALICE experiment at the LHC

After a general introduction on the Quark Gluon Plasma and a short overview of the experimental results obtained so far with heavy-ion collisions at the SPS and at the RHIC, the physics goals of the ALICE experiment at the LHC are presented. The text was submitted by the author in English.     

Jet measurements by ALICE at LHC

Jets are collimated sprays of particles originating from fragmentation of high energy partons produced in a hard collision. They are an important diagnostic tool in studies of the Quark Gluon Plasma (QGP). The modification of the jet fragmentation pattern and its structure is a signature for the influence of hot and dense matter on the parton fragmentation process. Jet measurements in proton-proton collisions provide a baseline for similar measurements in heavy-ion collisions, while studies in proton-nucleus system allow to estimate cold nuclear matter effects. Here we present jet studies in different colliding systems (p–p, p–Pb, Pb–Pb) performed by the ALICE collaboration at LHC energies. Results on jet spectra, cross sections, nuclear modification factors, jet structure and other kinematic observables will be presented.