M. Guilbaud

We present a systematic procedure for analyzing cumulants to arbitrary order in the context of heavy-ion collisions. It generalizes and improves existing procedures in many respects. In particular, particles which are correlated are allowed to belong to different phase-space windows, which may overlap. It also allows for the analysis of cumulants at any order, using a simple algorithm rather than complicated expressions to be derived and coded by hand. In the case of azimuthal correlations, it automatically corrects to leading order for detector non-uniformity, and it is useful for numerous other applications as well. We discuss several of these applications: anisotropic flow, event-plane correlations, symmetric cumulants, net baryon and net charge fluctuations.

Different mass formulae derived from the liquid drop model and the pairing and shell energies of the Thomas–Fermi model have been studied and compared. They include or not the diffuseness correction to the Coulomb energy, the charge exchange correction term, the curvature energy, different forms of the Wigner term and powers of the relative neutron excess I=(N−Z)/A. Their coefficients have been determined by a least square fitting procedure to 2027 experimental atomic masses (G. Audi et al. (2003) [1]). The Coulomb diffuseness correction Z2/A term or the charge exchange correction Z4/3/A1/3 term plays the main role to improve the accuracy of the mass formula. The Wigner term and the curvature energy can also be used separately but their coefficients are very unstable. The different fits lead to a surface energy coefficient of around 17–18 MeV. A large equivalent rms radius (r0=1.22–1.24 fm) or a shorter central radius may be used. An rms deviation of 0.54 MeV can be reached between the experimental and theoretical masses. The remaining differences come probably mainly from the determination of the shell and pairing energies. Mass predictions of selected expressions have been compared to 161 new experimental masses and the correct agreement allows to provide extrapolations to masses of 656 selected exotic nuclei.

We present the measurement of the charged-particle pseudorapidity (η) density distribution, dNchdη, for a number of centrality bins in Pb–Pb collisions at sNN=2.76TeV over a wide pseudorapidity range. Using the innermost pixel layers of the ALICE tracking system and the ALICE forward detectors (VZERO and FMD), we cover the pseudorapidity range: −5<η<5.5. The analysis is performed using a dedicated technique utilizing the collisions with LHC ‘satellite’ bunches. These collisions have displaced vertices in the range −187.5<Zvtx<375cm. The resulting distributions allow for an estimate of the total number of produced charged particles, Nch, in Pb–Pb collisions at the LHC energy.

The first comparison of anisotropy harmonics (vn, n = 2–4) and event-by-event correlations of different orders between p–p (13 TeV), p–Pb (5.02 and 8.16 TeV) and Pb–Pb (5.02 TeV) as a function of multiplicity is presented. The vn coefficients are extracted via long-range (|Δη|>2) two-particle correlations reaching a very-high-multiplicity region. Event-by-event correlations among v2, v3 and v4 are measured using the four-particle symmetric cumulant (SC(n,m), n = 2, m = 3, 4). For high-multiplicity (more than 100 tracks) events, v2 is found to have a negative correlation with the v3, while the v2 and v4 are positively correlated. Normalized by the two-particle vn, the SC(2,3) are quantitatively similar for p–Pb and Pb–Pb data, while a strong system size dependence is observed for SC(2,4). These new data provide important insights to the origin of collectivity observed in small collision systems.

The future opportunities for high-density QCD studies with ion and proton beams at the LHC are presented. Four major scientific goals are identified: the characterisation of the macroscopic long wavelength Quark-Gluon Plasma (QGP) properties with unprecedented precision, the investigation of the microscopic parton dynamics underlying QGP properties, the development of a unified picture of particle production and QCD dynamics from small (pp) to large (nucleus--nucleus) systems, the exploration of parton densities in nuclei in a broad ($x$, $Q^2$) kinematic range and the search for the possible onset of parton saturation. In order to address these scientific goals, high-luminosity Pb-Pb and p-Pb programmes are considered as priorities for Runs 3 and 4, complemented by high-multiplicity studies in pp collisions and a short run with oxygen ions. High-luminosity runs with intermediate-mass nuclei, for example Ar or Kr, are considered as an appealing case for extending the heavy-ion programme at the LHC beyond Run 4. The potential of the High-Energy LHC to probe QCD matter with newly-available observables, at twice larger center-of-mass energies than the LHC, is investigated.

The CMS Level-1 calorimeter trigger is being upgraded in two stages to maintain performance as the LHC increases pile-up and instantaneous luminosity in its second run. In the first stage, improved algorithms including event-by-event pile-up corrections are used. New algorithms for heavy ion running have also been developed. In the second stage, higher granularity inputs and a time-multiplexed approach allow for improved position and energy resolution. Data processing in both stages of the upgrade is performed with new, Xilinx Virtex-7 based AMC cards.

Most studies of anisotropy flow phenomena have assumed a global flow phase angle (or event plane angle) that is boost invariant in pseudorapidity ($\eta$). It was realized in recent years that this assumption may not be valid in presence of initial-state fluctuations, especially along the longitudinal direction. The effect of eta-dependent event plane fluctuations would break the factorization relation of Fourier coefficients from two-particle azimuthal correlations into a product of single-particle anisotropy Fourier harmonics as a function of $\eta$. First study of factorization breakdown effect in $\eta$ is carried out using the CMS detector, which covers a wide $\eta$ range of 10 units. A novel method is employed to suppress nonflow correlations at small pseudorapidity gaps of two particles. Significant eta-dependent factorization breakdown is observed in both PbPb and high-multiplicity pPb collisions. The measurements are presented for various orders of flow harmonics as a function of centrality or event multiplicity classes in PbPb and pPb, and are also compared to three-dimensional hydrodynamic calculations with longitudinal fluctuations. The new results presented here provide new insights into the longitudinal dynamics of relativistic heavy ion collisions, and help improve the three-dimensional modeling of the evolution of the strongly-coupled quark gluon medium.

The Compact Muon Solenoid (CMS) experiment is currently installing upgrades to their Calorimeter Trigger for LHC Run 2 to ensure that the trigger thresholds can stay low, and physics data collection will not be compromised. The electronics will be upgraded in two stages. Stage-1 for 2015 will upgrade some electronics and links from copper to optical in the existing calorimeter trigger so that the algorithms can be improved and we do not lose valuable data before stage-2 can be fully installed by 2016. Stage-2 will fully replace the calorimeter trigger at CMS with a micro-TCA and optical link system. It requires that the updates to the calorimeter back-ends, the source of the trigger primitives, be completed. The new system's boards will utilize Xilinx Virtex-7 FPGAs and have hundreds of high-speed links operating at up to 10 Gbps to maximize data throughput. The integration, commissioning, and installation of stage-1 in 2015 will be described, as well as the integration and parallel installation of the stage-2 in 2015, for a fully upgraded CMS calorimeter trigger in operation by 2016.

We present a systematic procedure for analyzing cumulants to arbitrary order in the context of heavy-ion collisions. It generalizes and improves existing procedures in many respects. In particular, particles which are correlated are allowed to belong to different phase-space windows, which may overlap. It also allows for the analysis of cumulants at any order, using a simple algorithm rather than complicated expressions to be derived and coded by hand. In the case of azimuthal correlations, it automatically corrects for detector non-uniformity, and it is useful for numerous other applications as well. We discuss several of these applications: anisotropic flow, event-plane correlations, symmetric cumulants, net baryon and net charge fluctuations.

Different Liquid Drop Model mass formulae have been studied. They include a Coulomb diffuseness correction Z2/A term and pairing and shell energies of the Thomas-Fermi model. The influence of the selected charge radius, the curvature energy and different forms of the Wigner term has been investigated. Their coefficients have been determined by a least square fitting procedure to 2027 experimental atomic masses. The different fits lead to a surface energy coefficient of 17-18 MeV. A large equivalent rms radius (r0 = 1.22 − 1.24 fm) or a shorter central radius may be used. A rms deviation of 0.54 MeV can be reached between the experimental and theoretical masses. The remaining differences come from the determination of the shell and pairing energies. Mass predictions are given for exotic nuclei.

La matiere que nous connaissons est composee de hadrons dont les quarks et les gluons sont les composants elementaires. Ces derniers n'existent pas libres dans la matiere ordinaire et sont donc en permanence confines dans les hadrons. Cependant, d'apres les predictions theoriques, quelques microsecondes apres le Big Bang, la temperature etait suffisamment elevee pour que les quarks et les gluons ne soient pas contenus dans les hadrons. Il s'agit d'une phase deconfinee de la matiere hadronique appelee Plasma de Quarks et Gluons (QGP). Le Large Hadron Collider (LHC) au CERN (Geneve) est un accelerateur de particules permettant d'accelerer, entre autres, des ions et de produire des collisions a des energies dans le centre de masse par nucleons allant jusqu'a plusieurs TeraelectronVolts. Il est ainsi possible d'atteindre des temperatures permettant de recreer cette phase de QGP pour en etudier les proprietes. C'est dans ce cadre que se place l'experience ALICE (A Large Ion Collider Experiement) qui est dediee a l'etude des collisions d'ions lourds ultra-relativistes. Le temps de vie du QGP etant trop faible, il n'est pas possible de l'etudier directement. Il est alors necessaire d'utiliser des observables indirectes. Ce travail de these s'inscrit directement dans ce programme de physique par le biais de l'etude des collisions d'ions lourds a 2.76 TeV. Deux observables sont abordees : la densite de particules chargees par unite de pseudorapidite et les mesons vecteurs de basse masse (rho, omega et phi) dans le canal dimuons. La premiere observable permet d'acceder a des informations sur les conditions initiales et la dynamique sous-jacente des mecanismes de production de particules. La mesure est realisee sur la gamme en pseudo-rapidite la plus large jamais atteinte au LHC (10 unites) grâce au developpement d'une methode d'analyse originale dite methode des vertex deplaces . La technique employee et les resultats obtenus sont decrits dans le chapitre 3. L'etude des mesons vecteurs de basse masse permet d'acceder a la production d'etrangete via le meson phi et a la symetrie chirale a travers la modification de la fonction spectrale du rho. L'analyse a ete menee a l'aide du spectrometre a muons d'ALICE et les resultats obtenus sur le taux de production du meson phi par rapport au mesons rho et omega sont presentes dans le chapitre 4. Dans ce chapitre, une etude sur la sensibilite du detecteur aux effets lies a la restauration de la symetrie chirale est aussi menee.