Production of He-4 and (4) in Pb-Pb collisions at root(NN)-N-S=2.76 TeV at the LHC

Results on the production of He-4 and (4) nuclei in Pb-Pb collisions at root(NN)-N-S = 2.76 TeV in the rapidity range vertical bar y vertical bar <1, using the ALICE detector, are presented in this paper. The rapidity densities corresponding to 0-10% central events are found to be dN/dy4(He) = (0.8 +/- 0.4 (stat) +/- 0.3 (syst)) x 10(-6) and dN/dy4 = (1.1 +/- 0.4 (stat) +/- 0.2 (syst)) x 10(-6), respectively. This is in agreement with the statistical thermal model expectation assuming the same chemical freeze-out temperature (T-chem = 156 MeV) as for light hadrons. The measured ratio of (4)/He-4 is 1.4 +/- 0.8 (stat) +/- 0.5 (syst). (C) 2018 Published by Elsevier B.V.Peer reviewe

Quelques études autour de la fission nucléaire

Studies on nuclear fission and its properties are still a challenge both theoretically and experimentally. Among the most studied physical observables, the mass and charge distributions of nuclei produced in fission, the so-called fission fragments, play a key role. First, these distributions allow us to understand how the large number of nucleons (protons and neutrons) of the fissioning nucleus are shared between the two fragments that are formed during fission.The understanding of the process requires a very broad experimental knowledge of the properties of the fission fragments, and this for a large number of fissioning systems and in very wide energy range. It is within this framework that an experimental program for measuring the fission fragment distributions of several actinides has been conducted at facilities as diverse as the Lohengrin spectrometer at ILL (Grenoble) in the thermal domain and using the VAMOS spectrometer at GANIL (Caen) in inverse kinematics at higher energy. These measures have made it possible to explore regions of mass still poorly known, which has necessitated the development of innovative experimental techniques, in particular through the use of gamma spectrometry. The experience acquired in these experimental developments has also made it possible to develop new instruments such as the FALSTAFF spectrometer which will be used, among others, with the NSF installation of SPIRAL2 at GANIL or, in a more exotic application, the FIDIAS TPC.The interpretation of the many experimental data available on fission fragment distributions makes use to different theoretical approaches. A theoretical approach that has been the subject of many developments is the statistical model of scission point. A new and original version of this model, called SPY and which uses a very advanced description of the microscopic properties of the nuclei, made it possible to derive the distributions of fission fragments in unexplored regions. This approach allowed, among other things, to shed interesting light on the role of the microscopic structure of fragments in the asymmetric fission of relatively light elements such as Mercury, as well as to propose an original interpretation on the role of fission of rare earth elements in explosive nucleosynthesis.Les études sur la fission nucléaire et ses propriétés constituent encore aujourd’hui un défis aussi bien théorique qu’expérimental. Parmi les observables physiques les plus étudiées, les distributions de masse et charge des noyaux issus de la fission, les fragments de fission, jouent un rôle clé car elles permettent de comprendre comment le grand nombre de nucléons (protons et neutrons) du noyau fissionnant se partage entre les deux fragments qui sont formés lors de la fission.Cette compréhension requiert une connaissance expérimentale très large des propriétés des fragments de fission, et ceci pour un grand nombre de système fissionnants et dans un large domaine en énergie. C’est dans ce cadre qu’un programme expérimental de mesure des distributions des fragments de fission de nombreux actinides à été mené auprès d’installations aussi diverses que les spectromètres Lohengrin à l’ILL (Grenoble) dans le domaine thermique et VAMOS au GANIL (Caen) en cinématique inverse à plus haute énergie. Ces mesures ont permis d’explorer des régions de masse encore peu connues et de développer des techniques expérimentales innovantes, en particulier grâce à la spectrométrie gamma. L’expérience acquise dans ces développements expérimentaux a également permis de développer des nouveaux instruments tels le spectromètre FALSTAFF qui sera utilisé, entre autres, auprès de l’installation NSF de SPIRAL2 au GANIL ou encore, dans une application plus exotique, la TPC FIDIAS.L’interprétation des nombreuses données expérimentales sur les distributions des fragments de fission fait appel à des approches théoriques diverses. Un voie théorique qui a fait l’objet de nombreux développements est le modèle statistique de point de scission dont une version nouvelle et originale, appelée SPY et qui utilise une description très avancée des propriétés microscopiques des noyaux, a permis de dériver les distributions des fragments de fission dans des régions inexplorées . Cette approche a apporté un éclairage intéressant sur le rôle de la structure microscopique des fragments dans la fission asymétrique du Mercure, ainsi que de proposer une interprétation originale sur le rôle de la fission des terres rares dans la nucléosynthèse explosive

Quelques questions ouvertes et points critiques… qui n’ont pas été traités dans les articles précédents

Lectrice, lecteur, peut-être avez-vous en tête un point important qui mériterait de contribuer au débat sur le nucléaire civil (français ou international), mais qui ne figure pas dans les pages précédentes. C’est frustrant, mais inévitable ; les contraintes de taille d’un dossier comme celui-ci nous interdisent de traiter le sujet de manière exhaustive. Nous esquissons ici une liste de ces points non traités. Par ailleurs, des questions, selon nous, restent encore très ouvertes ; nous tentons d’en identifier les aspects les plus critiques

Multiplicity dependence of light (anti-)nuclei production in p–Pb collisions at sNN=5.02 TeV

The measurement of the deuteron and anti-deuteron production in the rapidity range −1 < y < 0 as a function of transverse momentum and event multiplicity in p–Pb collisions at √sNN = 5.02 TeV is presented. (Anti-)deuterons are identified via their specific energy loss dE/dx and via their time-of- flight. Their production in p–Pb collisions is compared to pp and Pb–Pb collisions and is discussed within the context of thermal and coalescence models. The ratio of integrated yields of deuterons to protons (d/p) shows a significant increase as a function of the charged-particle multiplicity of the event starting from values similar to those observed in pp collisions at low multiplicities and approaching those observed in Pb–Pb collisions at high multiplicities. The mean transverse particle momenta are extracted from the deuteron spectra and the values are similar to those obtained for p and particles. Thus, deuteron spectra do not follow mass ordering. This behaviour is in contrast to the trend observed for non-composite particles in p–Pb collisions. In addition, the production of the rare 3He and 3He nuclei has been studied. The spectrum corresponding to all non-single diffractive p-Pb collisions is obtained in the rapidity window −1 < y < 0 and the pT-integrated yield dN/dy is extracted. It is found that the yields of protons, deuterons, and 3He, normalised by the spin degeneracy factor, follow an exponential decrease with mass number

Pseudorapidity densities of charged particles with transverse momentum thresholds in pp collisions at √ s = 5.02 and 13 TeV

The pseudorapidity density of charged particles with minimum transverse momentum (pT) thresholds of 0.15, 0.5, 1, and 2 GeV/c is measured in pp collisions at the center of mass energies of √s=5.02 and 13 TeV with the ALICE detector. The study is carried out for inelastic collisions with at least one primary charged particle having a pseudorapidity (η) within 0.8pT larger than the corresponding threshold. In addition, measurements without pT-thresholds are performed for inelastic and nonsingle-diffractive events as well as for inelastic events with at least one charged particle having |η|2GeV/c), highlighting the importance of such measurements for tuning event generators. The new measurements agree within uncertainties with results from the ATLAS and CMS experiments obtained at √s=13TeV.

Forward rapidity J/ψ production as a function of charged-particle multiplicity in pp collisions at s \sqrt{s} = 5.02 and 13 TeV

International audienceThe production of J/ψ is measured as a function of charged-particle multiplicity at forward rapidity in proton-proton (pp) collisions at center-of-mass energies $\sqrt{s}$ = 5.02 and 13 TeV. The J/ψ mesons are reconstructed via their decay into dimuons in the rapidity interval (2.5 < y < 4.0), whereas the charged-particle multiplicity density (dN$_{ch}$/dη) is measured at midrapidity (|η| < 1). The production rate as a function of multiplicity is reported as the ratio of the yield in a given multiplicity interval to the multiplicity-integrated one. This observable shows a linear increase with charged-particle multiplicity normalized to the corresponding average value for inelastic events (dN$_{ch}$/dη/〈dN$_{ch}$/dη〉), at both the colliding energies. Measurements are compared with available ALICE results at midrapidity and theoretical model calculations. First measurement of the mean transverse momentum (〈p$_{T}$〉) of J/ψ in pp collisions exhibits an increasing trend as a function of dN$_{ch}$/dη/〈dN$_{ch}$/dη〉 showing a saturation towards high charged-particle multiplicities.[graphic not available: see fulltext

Enhanced deuteron coalescence probability in jets

The transverse-momentum (pT) spectra and coalescence parameters B2 of (anti)deuterons are measured in pp collisions at s√=13 TeV for the first time in and out of jets. In this measurement, the direction of the leading particle with the highest pT in the event (pleadT>5 GeV/c) is used as an approximation for the jet axis. The event is consequently divided into three azimuthal regions and the jet signal is obtained as the difference between the Toward region, that contains jet fragmentation products in addition to the underlying event (UE), and the Transverse region, which is dominated by the UE. The coalescence parameter in the jet is found to be approximately a factor of 10 larger than that in the underlying event. This experimental observation is consistent with the coalescence picture and can be attributed to the smaller average phase-space distance between nucleons inside the jet cone as compared to the underlying event. The results presented in this Letter are compared to predictions from a simple nucleon coalescence model, where the phase space distributions of nucleons are generated using PYTHIA 8 with the Monash 2013 tuning, and to predictions from a deuteron production model based on ordinary nuclear reactions with parametrized energy-dependent cross sections tuned on data. The latter model is implemented in PYTHIA 8.3. Both models reproduce the observed large difference between in-jet and out-of-jet coalescence parameters, although the almost flat trend of the BJet2 is not reproduced by the models, which instead give a decreasing trend

Charged-particle multiplicity fluctuations in Pb–Pb collisions at √ sNN = 2.76 TeV

Measurements of event-by-event fluctuations of charged-particle multiplicities in Pb–Pb collisionsat √sNN = 2.76 TeV using the ALICE detector at the CERN Large Hadron Collider (LHC) are presented in the pseudorapidity range |η| < 0.8 and transverse momentum 0.2 < pT < 2.0 GeV/c. The amplitude of the fluctuations is expressed in terms of the variance normalized by the mean of the multiplicity distribution. The η and pT dependences of the fluctuations and their evolution with respect to collision centrality are investigated. The multiplicity fluctuations tend to decrease from peripheral to central collisions. The results are compared to those obtained from HIJING and AMPT Monte Carlo event generators as well as to experimental data at lower collision energies. Additionally, the measured multiplicity fluctuations are discussed in the context of the isothermal compressibility of the high-density strongly-interacting system formed in central Pb–Pb collisions