2,543 research outputs found
ALICE Vzero Detector
No full textALICE is the only experiment at CERN specifically designed to study the Quark-Gluon Plasma, the hot and dense matter which is created in ultra relativistic heavy-ion collisions. - VZERO-A (CINVESTAV-UNAM Mexico): 2.8 328 cm away from Interaction Point - VZERO-C (IPN Lyon): -3.6 88 cm away from Interaction Poin
Growth and ferroelectricity of GeTe on Si(111)
No full textL’effet Rashba géant mesuré récemment dans GeTe a montré le potentiel pour la spintronique des matériaux ferroélectriques avec un fort couplage spin-orbite. Dans cette thèse nous avons réalisé la croissance de films minces de GeTe sur Si(111)-Sb par épitaxie par jet moléculaire dans une large gamme d’épaisseur. À 270°C, nous observons une croissance couche-par-couche et une relation d’épitaxie principale GeTe(111) ∥Si(111) et GeTe[110] ∥Si[110]. En outre des nanodomaines ferroélastiques apparaissent dont la fraction volumique ainsi que la taille peuvent être contrôlées en ajustant finement l’épaisseur et la température de dépôt. Leur maille est identique à celle du domaine principal (rhomboédrique avec ar = 0.429 nm et θ = 58.3°). L’axe [111] de leur maille et leur polarisation électrique sont inclinés d’environ 72° par rapport à l’axe [111] du domaine principal. Nous mettons en évidence par microscopie à effet tunnel à la surface des domaines ferroélastiques trois structures surfaciques différentes, En analysant de façon détaillée l’interface GeTe/Si par microscopie électronique en transmission, nous démontrons que les dislocations de désaccord paramétrique localisées à l’interface formées pendant la croissance jouent un rôle clé dans la stabilité des nanodomaines ferroélectriques. Finalement, nous avons caractérisé la structure de bande électronique des domaines majoritaires de GeTe et mesuré une constante Rashba αr = 0.49 eV.nm qui diminue avec l’épaisseur des couches minces. De premières mesures par spectroscopie d’émission de photo-électrons résolues angulairement en dichroïsme circulaire ont permis de caractériser la courbure de Berry des films minces de GeTe.The recently measured giant Rashba effect in GeTe has shown the potential for spintronics of ferroelectric materials with large spin-orbit coupling. In this thesis we have grown GeTe thin films on Si(111)-Sb by molecular beam epitaxy in a wide range of thickness. At 270°C, we observe a layer-by-layer growth and a main epitaxial relationship GeTe(111) ∥Si(111) and GeTe[110] ∥Si[110]. In addition, ferroelastic nanodomains appear which volume fraction and size can be controlled by fine tuning the thickness and the deposition temperature. Their unit cell is identical to that of the main domain (rhombohedral with ar = 0.429 nm and θ = 58.3°). The [111] axis of their unit cell and their electrical polarization are tilted by about 72° with respect to the [111] axis of the main domain. We show by scanning tunneling microscopy that the surface of the ferroelastic nanodomains have three different surface structures. By a detailed analysis of the GeTe/Si interface by transmission electron microscopy, we demonstrate that misfit dislocations localized at the interface formed during growth play a key role in the stability of ferroelectric nanodomains. Finally, we have characterized the electronic band structure of the GeTe majority domains and measured a Rashba constant αr = 0.49 eV.nm that decreases with film thickness. First measurements to characterize the Berry curvature are performed by circular dichroism in angle-resolved photoemission spectroscopy
Study of light-neutral meson production in the dimuon chanel in pp collisions at sqrt(s)=13 TeV at forward rapidity at the CERN LHC with ALICE
No full textLa matière qui nous entoure est formé de hadrons, eux-même constitués de quarks et de gluons. Ces derniers sont des composants élémentaires qui n'existent pas sous forme libre. Cependant nous savons à l'heure actuelle que la matière confinée dans des hadrons peut, dans des conditions de haute température et/ou de haute densité baryonique, se retrouver sous une forme déconfinée de plasma de quarks et de gluons. Pour réaliser expérimentalement les conditions permettant de former ce plasma de quarks et de gluons, nous avons besoin d'une machine capable de faire entrer en collision des noyaux à des énergies très élevées: cela est notamment possible au CERN, où se situe le plus grand accélérateur de particules du monde, le Large Hadron Collider, qui a permis de faire entrer en collisions des noyaux de plomb à une énergie par paire de nucléons de 2.76 et 5.02TeV, et des protons à des énergies allant de 0.9 à 13TeV. Les collisions entre noyaux de plomb permettent, en particulier, d'atteindre les conditions de densité d'énergie nécessaires à la formation de la phase de plasma de quarks et de gluons. Ce travail de thèse contribue à ce programme de physique par l'étude de la production de mésons neutres légers en collisions proton-proton à 13TeV, référence nécessaire pour comprendre les observations en collisions plomb-plomb. L'étude des mésons neutres légers a été menée dans le canal dimuonique par l'analyse du spectre de masse invariante des dimuons de masse inférieure à 1.5 GeV/c², permettant notamment de mesurer les sections efficaces des mésons eta, rho/omega et phiThe ordinary matter surrounding us is made of hadrons which in turn are composed of quarks and gluons. These latter are elementary constituents which cannot be observed in a free state. However it is at present recognized that this matter confined within hadrons can undergo, under extreme conditions of high temperature and/or highnet baryonic density, a transition to a state of deconfined quarks and gluons whichcalled quark gluon plasma. The conditions required to form this quark gluon plasma can be experimentallyachieved using a machine capable of colliding nuclei at very high energies: this is particularly the case at CERN where is located the world’s largest and most powerful particle accelerator, the Large Hadron Collider, which collided Pb ions at a center-of-mass energy of 2.76 to 5.02 TeV per nucleon pair and protons of 0.9 to 13 TeV. Pb-Pb collisionssuch relativistic energies definitely allow for the suitable density conditions to form the quark gluon plasma phase. This thesis work contributes to this physics program by studying the production of neutral light mesons in collisions of proton-proton at 13 TeV, which provides the necessary reference to understand further observations done in Pb-Pb collisions. This study has been performed in the dimuon decay channel by analyzing the dimuon invariant mass spectrum in the region of masses lower than 1.5 GeV/c2 , giving accessthe measurement of the cross sections of eta, rho/omega and phi meson
Thermal instability of silicon-on-insulator thin films measured by low-energy electron microscopy
No full textInternational audienceUsing low-energy electron microscopy (LEEM), we investigate the ultrahigh vacuum annealing of silicon-on-insulator (SOI) samples capped by a chemically-prepared oxide layer. Consistent with previous reports: (1) for T > 750 • C, the capping-oxide decomposes by void nucleation and growth, then (2) for T > 850 • C, the Si thin-film dewets from the SiO2 substrate. Here, we show that the morphological evolution of the surface during the dewetting process is dependent on the preparation of the SOI surface. Two dewetting pathways are evident in recent literature, we find that one evolution is characteristic of clean Si(100)-2 × 1 surfaces, while the other is correlated with surface contamination. Silicon thin-films, capped by ultra-thin oxide layers, are the basic building blocks of microelectronics. Microelectronic device fabrication requires thermal annealing steps, which may induce drastic morphological changes in these building blocks. Here, we explore the annealing behavior of oxide-capped silicon-on-insulator (SOI) films. During annealing at T > 750 • C, the oxide capping layer decomposes (Fig. 1(a)) by void nucleation and growth. Then at T > 850 • C, the Si thin film spontaneously dewets (Fig. 1(b)), forming an assembly of three dimensional Si nanocrystals. Previous works [1, 2, 3, 4, 5] have explored the thermal decomposition of ultrathin Si-oxide, and the subsequent dewetting of the Si (SOI) layer [6, 7]. Here, we report two results which, to our knowledge, have not been reported previously. During thermal decomposition of the capping oxide layer, the radii of the initial isolated voids obeys a r ∝ t 1/3 law. This exponent suggests that void growth is governed by diffusion of the decomposition product (SiO molecules) on the oxide outside the voids. The decomposition of the oxide exposes a 2 × 1 reconstructed surface, characteristic of clean Si(100), ideal for studying the mechanisms of the SOI dewetting process. The dewetting proceeds by the opening of square, crystallographically oriented holes, followed by a finger instability that leads to the formation of self-organized
Anomalous J/psi suppression in Pb-Pb interactions at 158 GeV/c per nucleon
No full textThe Drell-Yan and J/psi cross-sections measured in Pb-Pb collisions are compared with the values extrapolated from the results obtained in proton and light ion induced reactions. While the Drell-Yan production exhibits the normal expected behaviour, the yield of J/psi in Pb-Pb interactions is abnormally low, as it lies 9 standard deviations below the expected value. Moreover, the departure from the expected behaviour increases significantly from peripheral to central collisions. (C) 1997 Elsevier Science B.V
phi production in Pb-Pb collisions at 158 GeV/c per nucleon incident momentum
No full textThe production of vector mesons phi, rho and omega has been measured in Pb-Pb collisions at 158 GeV/c per nucleon incident momentum at the CERN/SPS. The muon spectrometer of experiment NA50 detects phi, rho and omega mesons via their mu(+)mu(-) decay channel in the collision center of mass rapidity range 0 less than or equal to gamma(CM) less than or equal to 1. The results reported here show that the relative production of the phi compared to the (rho + omega) and the phi multiplicity per participant nucleon (N-part) increase with the centrality of the collision. On the other hand, the (p + w) multiplicity per participant does not exhibit any N-part dependence within our errors. The inverse slope parameter as deduced from an exponential fit to the phi transverse mass distribution is 228 +/- 10 MeV. Our results are compared with those obtained by experiment NA49 and with theoretical calculations. (C) 2003 Elsevier Science B.V. All rights reserved
First measurement of Xi(0)(c) production in pp collisions at root s=7 TeV
No full textThe production of the charm-strange baryon Xi(0)(c) is measured for the first time at the LHC via its semileptonic decay into e(+) Xi(-) nu(e) in ppcollisions at root s = 7 TeV with the ALICE detector. The transverse momentum ( p(T)) differential cross section multiplied by the branching ratio is presented in the interval 1 < p(T)< 8 GeV/c at mid-rapidity, vertical bar y vertical bar < 0.5. The transverse momentum dependence of the Xi(0)(c) baryon production relative to the D-0 meson production is compared to predictions of event generators with various tunes of the hadronisation mechanism, which are found to underestimate the measured cross-section ratio. (C) 2018 The Author(s). Published by Elsevier B.V
Production et polarisation de Υ en collisions pp à √s = 13 TeV au LHC du CREN
No full textLes états de quarkonium (e.g. J/ψ or Υ et leurs états excités), états liés de quarks charme et anticharme (c ̄c) ou bottom et antibottom (b ̄b) fournissent un moyen efficace de tester notre compréhension de la chromodynamique quantique (QCD), à la fois en ce qui concerne leurs mécanismes de production dans le vide ou dans des environnements hadroniques de type vide. et leur interaction avec le milieu déconfiné produit en collisions d'ions lourds ultra-relativistes. Parmi les outils offerts pour caractériser la production de quarkonium en collisions proton-proton, la corrélation entre production de quarkonium et multiplicité de particules chargées est particulièrement pertinente pour mettre en évidence le rôle joué par les mécanismes durs et doux de production de quarkonium. Dans ce contexte, la multiplicité de particules chargées permet d'établir les propriétés de l'événement sous-jacent (y compris le rôle des interactions multi-partoniques), agissant comme un paramètre d'échelle effectif, déterminant l'écart des effets observés à la référence du vide, indépendamment de la taille du système. L'étude de la polarisation de quarkonium, i.e. l'alignement d'un état donné de quarkonium par rapport à un axe choisi, est un autre outil intéressant pour éclairer notre compréhension de la production de quarkonium. ALICE est une expérience installée au LHC (Large Hadron Collider) du CERN, dédiée à l'étude de collisions hadroniques de proton-proton à Pb-Pb, où un milieu chaud et dense peut être créé, nommé le Plasma de Quarks-Gluons (QGP). Cette thèse décrit la mesure des Υ(1S), Υ(2S) et Y(3S) en fonction de la densité de multiplicité de particules chargées dNch/dη, effectuée par l'expérience ALICE du LHC en collisions proton-proton à √s = 13 TeV. Les mésons Y sont mesurés à rapidité avant (2.5 < y < 4) dans le canal de désintégration dimuon, tandis que la multiplicité de particules chargées est mesurée aux rapidités centrales (|η| < 1). Ces deux quantités sont normalisées à leurs valeurs moyennes obtenues dans les événements de biais minimum. L'augmentation observée des taux de production des Y(1S), Y(2S) et Y(3S) en fonction de la multiplicité de particules chargées normalisée est compatible avec une évolution linéaire, compte-tenu des barres d'erreur. Les rapports états excités sur fondamentaux normalisés sont compatibles avec l'unité, compte tenu des barres d'erreur. De même, le double rapport normalisé Y(1S) sur J/ψ , tous deux aux rapidités avant, est compatible avec l'unité pour des multiplicités de particules chargées normalisées au delà de 1. Les mesures sont confrontées à des prédictions de modèles théoriques incorporant des effets d'états initiaux ou finaux. La polarisation du Y(1S) a aussi été mesurée en collisions proton-proton à √s = 13 TeV. Du fait de la taille réduite du lot de données collectées, l'analyse n'a pas pu être menée pour les états Y(2S) et Y(3S). Les paramètres de polarisation λθ, λφ et λθφ ont été extraits pour le Y(1S) en fonction de l'impulsion transverse dans les référentiels de référence de l'hélicité et de Collins-Soper. Aucun écart significatif du scénario de polarisation nulle a été observé pour le Y(1S) en collisions pp, conformément aux prédictions de calculs Next-to-Leading-Order QCD. Ce résultat est en accord avec une mesure effectuée par la collaboration LHCb en collisions pp à √s = 8 TeV dans une région cinématique similaire (2.2<y<4.5).Quarkonium (e.g. J/ψ or Υ and their excited states), bound states of charm and anticharm (c ̄c) or bottom and antibottom (b ̄b) quarks, represent an effective tool to test our understanding of quantum chromodynamics (QCD), concerning both their production mechanisms in vacuum or vacuum-like hadronic environments, and their interaction with the deconfined medium produced in ultra-relativistic heavy-ion collisions. Among the available tools to characterize quarkonium production in proton-proton collisions, the correlation between quarkonium production and the charged-particle multiplicity is particularly helpful in order to investigate the interplay between soft and hard production mechanisms of quarkonium. In this context, the charged-particle multiplicity allows one to characterize the properties of the underlying event (including the role of multiparton interactions), acting as an effective scaling parameter determining the deviation of the observed effects from the vacuum reference, independently of the collision mode size. Another interesting tool to constrain our understanding of quarkonium production, is the study of quarkonium polarization, i.e. the spin alignment of a given quarkonium state with respect to a chosen axis. ALICE is an experiment at the CERN Large Hadron Collider (LHC), dedicated to the study of hadronic collisions from pp to Pb-Pb, where a hot and dense medium can be created, named quark-gluon plasma. With the Run 2 data taking of the LHC, the highest collision energies ever delivered in the laboratory have become accessible. In this thesis, the measurement of Υ(1S), Υ(2S), and Υ(3S) yields as a function of the charged-particle multiplicity density dNch/dη, using the ALICE experiment at the LHC, is reported in pp collisions at √s = 13 TeV. The Υ meson yields are measured at forward rapidity (2.5 < y < 4) in the dimuon decay channel, whereas the charged-particle multiplicity is defined at central rapidity (|η| < 1). Both quantities are normalized to their average value in minimum bias events. The increase of the self-normalized Υ(1S), Υ(2S), and Υ(3S) yields is found to be compatible with a linear scaling with the self-normalized dNch/dη, within the uncertainties. The measured Υ excited-to-ground state self-normalized yield ratios are found to be compatible with unity within uncertainties. Similarly, the measured double ratio of the self-normalized Υ(1S) to the self-normalized J/ψ yields, both measured at forward rapidity, is compatible with unity for self-normalized charged-particle multiplicity beyond one. The measurements are compared with theoretical predictions incorporating initial or final state effects. The Υ(1S) polarization has been also measured in pp collisions at √s = 13 TeV; the analysis could not be extended to the excited states Υ(2S) and Υ(3S) due to the limited size of the available data sample. The Υ(1S) polarization parameters λθ, λφ and λθφ have been measured as a function of pT both in the Helicity and Collins-Soper reference frames. No significant deviation from a zero-polarization scenario is observed for the Υ(1S) in pp collisions, as expected from the Next-to-Leading-Order QCD calculations. This result is consistent with the measurement performed by LHCb in pp collisions at √s = 8 TeV in a similar kinematic region (2.2 < y < 4.5) within large uncertainties
Intermediate mass muon pair continuum in Pb-Pb collisions at 158 GeV/c
No full textThe NA50 experiment measures, at CERN SPS, the dimuon production in Pb-Pb collisions at 158 GeV/c incident momentum. In this paper an analysis of the intermediate mass region (1.5 less than or equal to M(mu mu)12.5 GeV/c(2)) is presented. Known dimuon sources in this interval of the spectrum are the e.m. annihilation of pairs (i.e, Drell-Yan) and the semileptonic decay of charmed hadrons. The contribution of these processes is calculated by means of a fit to the measured invariant mass distribution (for Drell-Yan) and of a linear extrapolation of p-A experimental results obtained by NA38 (for charm). It will he shown that known sources do not account for all of the dimuon yield measured by NA50, i.e, an excess is observed. The size of the excess measured in Pb-Pb is larger than the one seen by NA38 in S-U collisions. The centrality dependence of this effect, both in S-U and Pb-Pb, is also discussed
Measurement of the production cross section of prompt Xi(0)(c) baryons at midrapidity in pp collisions at root s=5.02 TeV
No full textTwo-particle Azimuthal correlations are measured with the ALICE apparatus in pp collisions at s=13 TeV to explore strangeness- and multiplicity-related effects in the fragmentation of jets and the transition regime between bulk and hard production, probed with the condition that a strange meson (KS0) or baryon (Λ) with transverse momentum pT> 3 GeV/c is produced. Azimuthal correlations between kaons or Λ hyperons with other hadrons are presented at midrapidity for a broad range of the trigger (3<20 GeV/c) and associated particle pT (1 GeV/
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