B-hadron cross-section measurement in pp collisions at $\sqrt{s}=14$ TeV with the ALICE muon spectrometer

ALICE (A Large Ion Collider Experiment) is the LHC detector designed to measure nucleus-nucleus (AA) collisions where the formation of the Quark Gluon Plasma is expected. The experiment will also study proton-proton (pp) collisions at 14 TeV. Amongst the relevant observables to be investigated in pp collisions, the B-hadron cross-section is particularly interesting since it provides benchmarks for theoretical models and it is mandatory for understanding heavy flavour production in AA collisions. The performances of the ALICE muon spectrometer to measure B-hadron cross-section in pp collisions at 14 TeV via single muons are presented.     

Exploring the LHC medium with direct photons

Heavy-ion collisions will enter a new era with the start of the CERN Large Hadron Collider (LHC). A first short run with proton-proton collisions at the injection energy of 0.9 TeV will be followed by a longer one with pp collisions at 10 TeV. First Pb-Pb collisions at  TeV will take place in 2009. Three experiments (ALICE, ATLAS, and CMS) will study both pp and Pb-Pb collisions. A selection of results showing the capabilities of the three experiments for the study of the LHC medium with direct photons is presented.     

Can the RHIC <Emphasis Type="Italic">J</Emphasis>/<Emphasis Type="Italic">ψ</Emphasis> puzzle(s) be settled at LHC?

One observes strong suppression effects for hard probes, e.g. the production of J/ψ or high-p _T particles, in nucleus–nucleus (AA) collisions at RHIC. Surprisingly, the magnitude of the suppression is quite similar to that at SPS. In order to establish whether these features arise due to the presence of a thermalized system of quarks and gluons formed in the course of the collision, one should investigate the impact of suppression mechanisms which do not explicitly involve such a state. We calculate shadowing for gluons in the Glauber–Gribov theory and propose a model invoking a rapidity-dependent absorptive mechanism motivated by energy-momentum conservation effects. Furthermore, final-state suppression due to interaction with comoving matter (hadronic or pre-hadronic) has been shown to describe the data at SPS. We extend this model by including the backward reaction channel, i.e. recombination of open charm, which is estimated directly from pp data at RHIC. Strong suppression of charmonium both in pA and AA collisions at LHC is predicted. This is in stark contrast with the predictions of models assuming QGP formation and thermalization of heavy quarks.     

Fine structure in the azimuthal transverse momentum correlations at $\sqrt{s_{NN}}=200$ GeV using the event shape analysis

The experimental results on transverse momentum azimuthal hadron correlations at RHIC have opened a rich field for parton energy loss analysis in heavy-ion collisions. Recently, a considerable amount of work has been devoted to study the shapes of the “away-side” jet which exhibit an interesting and unexpected “double hump” structure not observed in the analogous treatment of the pp data. Driven by the possibility that the latter result might just mean that such a structure exists already in the case of pp collisions, but that its relative intensity could be small, here we use the Event Shape Analysis to show that it is possible to identify and select well defined event topologies in pp collisions, among which a double hump structure for the away-side jet emerges. Using two shape parameters, the sphericity in the transverse plane and the recoil to analyze a sample of PYTHIA generated pp collisions at  GeV, we show that this structure corresponds to two jets emitted in the backward hemisphere. Finally, we show that Q-PYTHIA qualitatively reproduces the decrease in the yield of dijet events and the increase of the double hump structure in the away side observed in heavy-ion collisions. The implications for the treatment of parton energy loss in heavy-ion collisions are discussed.     

Low- x QCD physics from RHIC and HERA to the LHC

We present a summary of the physics of gluon saturation and non-linear QCD evolution at small values of the parton momentum fraction x in the proton and nucleus in the context of recent experimental results at HERA and RHIC. The rich physics potential of low-x studies at the LHC, especially in the forward region, is discussed and some benchmark measurements in pp, pA and AA collisions are introduced.     

Heavy-ion physics with the ATLAS detector

The CERN LHC collider will operate with lead ions at √s of 5.5 TeV/nucleon. The ATLAS detector, designed to study high-pT physics in the pp mode of the LHC, has the potential to study ultrarelativistic heavy-ion collisions in a full range of observables characterized extremely dense matter and the formation of a quark-gluon plasma. The ATLAS physics program includes global event measurements (particle multiplicities, transverse momentum), suppression of heavy-quarkonia production, jet quenching, and a study of ultraperipheral collisions. (on behalf of the ATLAS Collaboration) The text was submitted by the author in English.     

Matter–Antimatter Asymmetry in Heavy-Ion Collisions

Implementing modified phase space and distribution function in grand-canonical ensemble and taking into account the experimental acceptance, the ratios of antiparticle-to-particle over the whole range of energies are well reproduced by hadron resonance gas (HRG) model. We introduce a systematic study for antiparticle-to-particle ratios measured in various pp and AA collisions and find that the ratios of bosons and baryons get very close to unity indicating that the matter–antimatter asymmetry nearly vanishes at LHC energy. We predict that the LHC heavy-ion program will produce the same particle ratios as the pp program implying the dynamics and evolution of the system would not depend on the initial conditions.     

Production of secondaries in high-energy <Emphasis Type="Italic">d</Emphasis>+Au collisions

In the framework of the quark–gluon string model we calculate the inclusive spectra of secondaries produced in d+Au collisions at intermediate (CERN SPS) and at much higher (RHIC) energies. The results of numerical calculations at intermediate energies are in reasonable agreement with the data. At RHIC energies numerically large inelastic screening corrections (percolation effects) should be accounted for in the calculations. We extract these effects from the existing experimental data of RHIC on minimum-bias and central d+Au collisions. The predictions for p+Au interactions at LHC energy are also given.     

<Emphasis Type="Italic">J</Emphasis>/<Emphasis Type="Italic">ψ</Emphasis> suppression and <Emphasis Type="Italic">p</Emphasis><Subscript><Emphasis Type="Italic">T</Emphasis></Subscript> spectra in RHIC and LHC energy collisions

In a hydrodynamic model, we have studied J/ψ production in Au+Au/Cu+Cu collisions at RHIC energy, GeV. At the initial time, J/ψ’s are randomly distributed in the fluid. As the fluid evolves in time, the free streaming J/ψ’s are dissolved if the local fluid temperature exceeds a threshold temperature T _J/ψ . Sequential melting of charmonium states (χ _c , ψ ^′ and J/ψ), with melting temperatures , T _J/ψ ≈2T _c and feed-down fraction F≈0.3, explains the PHENIX data on the centrality dependence of J/ψ suppression in Au+Au collisions. J/ψ p _T spectra and the nuclear modification factor in Au+Au collisions are also well explained in the model. The model however overpredicts the centrality dependence of J/ψ suppression in Cu+Cu collisions by 20–30%. The J/ψ p _T spectra are underpredicted by 20–30%. The model predicts that in central Pb+Pb collisions at LHC energy,  GeV, J/ψ’s are suppressed by a factor of ∼10. The model predicted a J/ψ p _T distribution in Pb+Pb collisions at LHC is similar to that in Au+Au collisions at RHIC.     

Deuterons from 300 GeV proton collisions calculated from the Lund model

We present an alternative approach to describe deuteron production in high energy particle collisions. The phenomenological Lund-model has been used to give the number and momentum distribution of secondary nucleons produced in 300 GeV proton-proton collisions. Deuterons are assumed to be produced in a final state interaction between the nucleons inNN d reactions inside a volume of 1 F³. The results are compared to experimental data.Data became available for antideuteron production in e⁺e^– annihilation. * A Lund Monte Carlo calculation for the production of secondary nucleons in e⁺e^– collisions exist. The results from a final state interaction were too small by a factor 10³. A calculation for a coalescent model which uses the overlap of the proton-neutron momentum distribution with a distribution from a Hulthen wave function is in good agreement with the data. The coalescent model works well in this case. It is because the relative momentum of the coalescent nucleons produced for 10 GeV cm energy in e⁺e^– collisions is very much smaller than the relative momentum of the nucleons produced in 300 GeV fixed target energy ofpp collisions.     

Description of pp collisions at LHC energies

The Monte Carlo version of quark-gluon string model is employed to study the multiplicity, rapidity and pr spectra of particles in pp collisions at energies from √s = 200 GeV to 14 TeV. A good quantitative agreement with the experimental data is found in a broad energy range. It means that the general features of ultrarelativistic pp interactions can be well understood in terms of soft- and hard-Pomeron exchanges. Predictions are made for the top LHC energy √s = 14TeV.     

Quarkonia measurements with STAR

We report results on quarkonium production from the STAR experiment at the Relativistic Heavy-Ion Collider (RHIC). J/ψ spectra in p+p and Cu+Cu collisions at  GeV with transverse momenta in the range of 0.5–14 GeV/c and 5–8 GeV/c, respectively, are presented. We find that for p _T >5 GeV/c yields in p+p collisions are consistent with those in minimum-bias Cu+Cu collisions scaled with the respective number of binary nucleon-nucleon collisions. In this range the nuclear modification factor, R _AA , is measured to be 0.9±0.2 (stat). For the first time at RHIC, high-p _T J/ψ-hadron correlations were studied in p+p collisions. Implications from our measurements on J/ψ production mechanisms, constraints on open bottom yields, and J/ψ dissociation mechanisms at high-p _T are discussed. In addition, we give a brief status of measurements of ϒ production in p+p and Au+Au collisions and present projections of future quarkonia measurements based on an upgrades to the STAR detector and increased luminosity achieved through stochastic cooling of RHIC.     

Transverse-momentum dependence of <Emphasis Type="Italic">J</Emphasis>/<Emphasis Type="Italic">ψ</Emphasis> shadowing effects

We present a new approach to estimate the effect of gluon shadowing in nucleus + nucleus collisions and its consequences on the J/ψ production yield. Using kinematical information available from the measured J/ψ production in proton + proton collisions at  GeV, we build a Glauber Monte Carlo code which takes into account shadowing in two alternative ways: multiple-scattering corrections or Q ² evolution of parton densities. We exploit the dependence of these different parameterizations to the J/ψ transverse momentum and we give the first predictions on the resulting p _T dependence of the nuclear modification factor in deuteron + gold collisions at the same energy.     

Scaling of single photon production in hadronic collisions

Scaling of single photon production in pp and collisions is studied. It is empirically observed that the available data scales for and for larger . The NLO pQCD predictions for pp collisions at an of 200 and 5500 GeV, relevant for RHIC and LHC energies are seen to closely follow this scaling behavior. Implications for single photon production in heavy ion collisions are discussed. Received: 2 February 2001 / Revised version: 28 May 2001 / Published online: 5 October 2001     

Charmonium dissociation and recombination at RHIC and LHC

Charmonium production at heavy-ion colliders is considered within the comovers-interaction model. The formalism is extended by including possible secondary J/ψ production through recombination and an estimate of recombination effects is made without adjusting the model parameters. The comovers-interaction model also includes a comprehensive treatment of initial-state nuclear effects, which are discussed in the context of such high energies. With these tools, the model properly describes the centrality and the rapidity dependence of experimental data at RHIC energy,  GeV, for both Au+Au and Cu+Cu collisions. Predictions for LHC,  TeV, are presented and the assumptions and extrapolations involved are discussed.     

Diffractive vector meson production at large t in coherent hadronic interactions at CERN LHC

The diffractive vector meson photoproduction with hadron dissociation in coherent hadron-hadron interactions at LHC energies is studied assuming that the color singlet t-channel exchange carries large momentum transfer. We estimate the rapidity distribution and total cross-section for the processes pp → pVX and PbPb → PbVX (V = ρ or ϒ considering the non-forward solution of the BFKL equation at high energy and large momentum transfer. Our results indicate that the experimental identification of these processes can be feasible at the LHC.     

Dilepton-tagged $Q\overline{Q}+\mathrm{jet}$ events at the LHC

We propose a new method for identifying and isolating events through semileptonic decays of the pair. Employing these decay dileptons to tag the jet in a specific kinematic region provides a clean signature of jets associated with heavy-quark production. The measurement, in both pp and heavy-ion collisions, is essential for addressing heavy-quark fragmentation in vacuum and in a dense medium. We present next-to-leading order calculations of production (leading order in production) in TeV pp collisions at the LHC and discuss the feasibility of the measurement in heavy-ion collisions at TeV.     

Physics with the ALICE experiment

ALICE experiment at LHC collects data in pp collisions at 1497-1 = 0.9, 2.76, and 7 TeV and in PbPb collisions at 2.76 TeV. Highlights of the detector performance and an overview of experimental results measured with ALICE in pp and AA collisions are presented in this paper. Physics with protonproton collisions is focused on hadron spectroscopy at low and moderate p _t . Measurements with lead-lead collisions are shown in comparison with those in pp collisions, and the properties of hot quark matter are discussed.     

Probe of the Wtb coupling in t \bar{t} pair production at linear colliders

The Wtb vertex can be probed on future colliders in the processes of single top production (LHC, pp mode, NLC, mode) and of top pair production (NLC, mode). We analyze observables sensitive to anomalous Wtb couplings in the top pair production process of collisions. In particular, forward–backward and spin–spin asymmetries of the top decay products and the asymmetry of the lepton energy spectrum are considered. Possible bounds on anomalous couplings obtained are competitive to those expected from the upgraded Tevatron and LHC. The validity of the infinitely small width approximation for the three-body top decay is also studied in detail. Received: 20 January 2000 / Revised version: 12 April 2000 / Published online: 18 May 2000     

Charged particle identification with PHOS and central tracking of ALICE

The ALICE Photon Spectrometer (PHOS) has been designed to measure the photon spectrum produced in pp and AA collisions at LHC over a broad transverse momentum range from 0.5–100 GeV/c. We present a study on the matching of charged tracks, detected by the ALICE central tracking detectors, with clusters of hit PHOS elements. Matching efficiency and contamination due to false matches have been deduced for charged pions, muons and electrons. The study indicates also that material from other detectors in front of PHOS disturbs the photon detection.