ar X iv : h ep - e x / 05 05 02 1 v 1 1 1 M ay 2 00 5 Physics of the Muon Spectrometer of the ALICE

The main goal of the ALICE Muon spectrometer experiment is the measurement of heavy quark production in p+p, p+A and A+A collisions at LHC energies, via the muonic channel. Physics motivations and expected performances have been presented in this talk. 1. Physics motivations Heavy ion collisions (HIC) at relativistic energies are a privileged tool for creating very hot and dense matter in a laboratory. In particular, lattice chromo-dynamics (lQCD) predicts a crossover toward a new state of matter called Quark Gluon Plasma (QGP) at a temperature ∼ 170 MeV for vanishing chemical potential μB [1, 2, 3]. Heavy ion collisions allow to experimentally study the properties of this new state of matter. This experimental program started in the mid 80’s with fixed target heavy ion experiments at the AGS and SPS [4] and continued with the physics program developed at the RHIC collider (BNL) [5]. Heavy ion collisions at the future Large Hadron Collider (LHC) at CERN will open new experimental insights in the study of hadronic matter at high temperature. The ALICE experiment will be the only experiment at the LHC devoted to heavy ion physics, whereas the ATLAS and the CMS experiments plan to develop a heavy-ion program [6, 7] in parallel with their main physics goals. The LHC collider will provide lead (argon) high luminosity beams < L >= 5 · 10cms (< L >= 5 ·10cms) at √ s =5.5A TeV ( √ s = 6.3A TeV). In addition, the LHC will deliver proton beams < L >= 3·10cms at √ s =14 TeV and d+Pb beams < L >= 8·10cms at √ s ∼6.2A TeV, providing a solid baseline for the study of medium effects in HIC [8] . At such ultra-relativistic energies new phenomena emerge, improving the experimental conditions for studying the hadronic matter in HIC: • Initial conditions. The initial conditions will be under control by the gluon saturation scenario. At these energies the initial nucleus-nucleus interaction can be viewed as weak interactions of a huge number of small x gluons which are freed in the beginning of the collision leading to the formation of a big gluonic ball [10, 11]. Up to 8000 gluons in the 1 Talk presented in the ICPAQGP Conference, February 8-12, 2005, Salt Lake City, Kolkata, India. Web page of the conference : http://www.veccal.ernet.in/∼icpaqgp/ 2 [email protected] 3 It should be noted that d+Pb and Ar+Ar beams will require an upgrade of the LHC collider. Reported luminosities are very preliminary [9]. early stage of the collisions are predicted [12]. Such a system will rapidly evolve toward a equilibrium [13]. Most of these processes (as secondary interaction of minijets) leading to thermalization will be governed by hard processes (αs < 1). • Equilibrated matter. After equilibration of the initial gluonic ball, a hot and long-lived hadronic matter will be formed (hotter and longer-lived than at lower energy HIC). The increase of the beam energy will favor the creation of vanishing baryonic potential hadronic matter with a temperature around 0.5-1 GeV, well above the critical temperature predicted by lQCD. • Observables. The experience acquired during the last 20 years of Heavy-Ion Physics in the relativistic regime has shown the necessity to measure most of the probes of the reaction dynamics, from hard processes and QGP formation observables, until the freezeout of the expanding hadron gas. A coherent explanation of the full set of observables will be the only way to study the properties of the ephemeral QGP. Final states probes like particle multiplicities, hadron pT distributions, particle ratios, strangeness production, azimuthal asymmetries, etc ... will shed light on the condition of the phase transition and the dynamical evolution of the expanding hadron gas. Penetrating probes, as real and virtual photon production, charmonium production, and light vector meson properties will provide informations about the QGP formation and properties. Moreover, the study of the QGP at LHC energies will be enriched by exploiting new probes which can be efficiently studied in this energy regime: (i) Hard processes leading to very fast leading partons will provide information about the QGP through the interaction of jets with the surrounding partonic matter [14]. Huge parton energy losses (around 1-3 GeV/fm) in the QGP will modify the hadronization process of these very energetic jets, inducing a jet-quenching and a suppression of high pT hadron production. At the LHC, jet production cross-section in the very high pT range (pT >50 GeV) will increase by more of 4 orders of magnitude with respect to RHIC beam energies; (ii) The Debye screening of bottomonium states in QGP will be studied. Suppression of the Upsilon family production will be exploited due to its sensitivity to the density of color charges in the medium [15]; (iii) Open charm and open beauty production will be exploited as new probes of the strong interacting system of partons [17, 16]. Open charm (beauty) production cross-section will increase by a factor 10 (100) with respect to RHIC energies; (iv) The production of massive electroweak bosons (W and Z) will open the possibility to check the validity of the glauber scaling in HIC. These bosons do not interact with the surrounding medium and they are produced in hard parton collisions; (v) Finally, the huge particle multiplicity at the LHC will make possible to measure a large number of observables on an event-by-event basis, increasing the sensitivity to non-statistical fluctuations predicted to occur in a phase transition scenario. 2. Heavy Quark Production at the LHC Heavy quarks (charm and beauty quarks) in HIC are produced in the first stages of the collisions and then they coexist with the surrounding medium due to their long life-time. Production rates, transverse momentum (pT ) and rapidity (y) distributions, quarkonia production rates, etc ... will allow for probing the properties of the medium, as the QGP. 2.1. Initial hard production Charm quarks will be copiously produced in HIC at LHC energies: more than 100 cc̄ pairs per central Pb+Pb collision and around 5 bb̄ pairs per collision are expected [19]. Production of heavy quarks will be dominated by prompt parton-parton scattering. Open heavy flavor production will be dominated by gluon fusion. Next leading-order (NLO) diagrams noticeably contribute to the heavy quark production cross-section [18] although the LO one particle differential cross-section shapes are not appreciably modified by the NLO contribution. It has been observed that pQCD calculations underestimate the measured differential cross-sections in pp̄ collisions at Tevatron energies, although they are compatible within uncertainties [20]. All those experimental data concerns high pT production (larger than 5 GeV/c). At LHC energies, heavy quark production will explore a x and transfered momentum Q domain which has not been well studied. For instance in rapidities 2.5 < |y| < 4 at LHC charm production will be sensible to x ∼ 10 and Q ∼ 10 GeV. In the case of HIC, one should expect gluon shadowing which will modify the heavy quark production based on glauber scaling. It is expected a heavy quark yield suppression of about 20% (10%) for charm (beauty) production in p+Pb collisions at LHC energies [21] due to gluon shadowing. About 1% of the heavy quark pairs will end in the formation of a colorless bound state: quarkonium. The process of direct formation of quarkonia from the initially produced QQ̄ pair is a non-perturbative process. Models like Color Evaporation Model (CEM) or non-relativistic QCD [19] have been successfully tested in pp and pp̄ collisions. In HIC direct production of quarkonia will take place inside the impinging nuclei, leading to an initial nuclear absorption of quarkonia (normal suppression in HIC). The nuclear absorption increases with the enhancement of the relative rapidity between the quarkonia and the impinging nuclei. For this reason, nuclear absorption becomes higher at LHC energies, a factor 2 higher than at SPS energies. The nuclear absorption exhibits a relatively flat distribution as a function of the HIC centrality for central and semi-central collisions and becomes maximal for central Pb+Pb collisions at the LHC, where the survival probability of the charmonium is ∼ 25% (50% for bottomonium) [19]. 2.2. Pre-equilibrium production Pre-equilibrium stage at LHC energies will be dominated by secondary mini-jet interactions which could noticeably contribute to the total production cross-section of charm quark pairs [22]. One could expect up to 200 cc̄ in central Pb+Pb collisions taking into account the production of heavy quarks during the thermalization phase. 2.3. Heavy quarks in hot and dense medium Heavy quarks will be then embedded in a matter mainly formed by gluons and light quarks (u, d, s). It is a open question how heavy quarks will behave in such a medium. Will heavy quarks thermalize? Will heavy quarks develop collective motion? Some models assume that heavy quarks behave like Brownian particles (mQ >mq) [23] and their transverse momentum (pT ) and rapidity distributions, collective motion ... will probe the properties of the QGP. Although pQCD heavy quark interaction cross-sections with the medium seems too small for reaching full thermalization of heavy quarks, non-perturbative effects in QGP could noticeably increase the heavy-quark cross-section, leading to their full thermalization [24]. Other models do not address the problem of thermalization and directly assume statistical coalescence of heavy quarks [31, 26, 28, 29]. The study of the heavy quark bound states will allow for probing the medium via the Debye screening [3, 27], the gluon dissociation of quarkonia [28], kinetic recombination of heavy quarks [29] and/or their statistical hadronization [31, 26]. It is expected that charmonium production in HIC at LHC will be dominated by recombination of cc̄ pairs in the QGP or at the hadronization. Due to the huge production rate of charmed quarks at LHC, these models predict a spectacular enhancement of the charmonium yield in central Pb+Pb collisions, more than a factor 10. Based on these models, charmonium yield will exhibit centrality dependence proportional to < Ncc̄ > 2 [26, 28, 29], which will be a very clear signature of QGP. In the case of charmonium, a non-negligible production could occur after hadronization of the QGP due to DD̄ annihilation [30, 31]. For the bottomonium production at the LHC, the recombination mechanism will play a less important role due to the lower bottom pair production rate. Recombination mechanism would compete with the suppression mechanism due to Debye screening and/or gluon dissociation. Charm measurements at RHIC would help to predict what should occur with beauty at LHC since production rates per central HIC are similar. 2.4. Secondary production There will be a production of J/ψ due to charmonium decay modes of the B mesons. At LHC energies the relative contribution of J/ψ from B-meson decay should be about 25% [17]. Upsilon family production will be increased by radiative decay of χb quarkonia. About 60% of the total Upsilon family production will originate from these resonances. 3. Production of electroweak bosons Electroweak bosons (W, Z) will be abundantly produced at LHC and this will allow for several precision measurements, for instance high precision measurement of the W mass [33]. However, proton luminosities in ALICE experimental region will be much lower (< L >= 3 · 10cms) than in other LHC experiments, due to ALICE detector performances. At leading-order, W bosons are produced in quark antiquark collisions: ud̄ → W, cs̄ → W etc ... The LO production cross-section of W obtained from Pythia [34] amounts to ∼17 nb in p+p collisions at 14 TeV [35], leading to a production of half a millions W bosons decaying in the muonic channel in one year of data taken in the ALICE interaction point. At these energies, contributions to W production from higher order diagrams should be about 10% [36]. In proton+proton collisions, there will be more u than d valence quarks, leading to a enhancement of W with respect W which is more pronounced at high rapidities where the W-boson production is dominated by valence-quarks. At mid-rapidity the production of W and W is almost symmetric. In HIC, W bosons production will not be affected by the medium effects since W bosons will leave the hot and dense bulk of matter without any secondary interaction. In this respect, W bosons production will be a very powerful tool for checking the validity of the glauber scaling hypothesis at these energies. Since shadowing could be different in quarks and gluons, measurements of W production in d+Pb (or p+Pb) will be necessary. W-boson production could provide a powerful normalization of very high pT (pT ∼30-40 GeV/c) beauty and charm production where a suppression could be expected due to jet quenching of heavy quarks. The asymmetry between W and W in HIC will be given by the isospin of the nuclei and the elementary asymmetry in p+p, n+n, p+n and n+p collisions [35]. 4. Muon Spectrometer In the framework of the ALICE physics program [6], the goal of the Muon spectrometer of ALICE [37] is the study of open heavy flavor production and quarkonia production (J/ψ, ψ and Υ(1S), Υ(2S) and Υ(3S)) via the muonic channel. For HIC the dependence with the collision centrality and with the reaction plane will also be studied. The study of electroweak bosons is also foreseen and preliminary studies are under progress [35]. The main experimental requirement is to measure the quarkonia production in central Pb+Pb collisions at LHC energies, down to very low pT , since low pT quarkonia will be sensitive to medium effects like heavy-quark potential screening. Since muons are passively identified by the absorber technique, a Lorentz boost is needed to be able to measure quarkonia at low pT . On the other hand, the Muon spectrometer has to be as close as possible to the physics of the QGP which occurs in the mid-rapidity region. As a compromise, the muon spectrometer allows for measuring muons and quarkonia production in an intermediate rapidity range −4.0 < y < −2.5. (GeV/c) T p 0 5 10 15 20 25 30 A cc ep ta n ce 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 ψ J/