Dissertationen zum Thema „ALICE at the LHC“

K*(892)± resonance with the ALICE detector at LHC Author: Kunal Garg PhD Cycle XXXI, University of Catania It has been established that ultra-relativistic heavy-ion collisions produce a hot and dense QCD system which behaves like a perfect fluid. The study of the Quark Gluon Plasma created in these collisions is important to understand the cosmic evolution of our Universe. The study of strange hadronic resonances in pp collisions contributes to the study of strangeness production in small systems. Usually, measurements in pp collisions constitute a reference for the study in larger colliding systems and provide constraints for tuning QCD-inspired event generators and then to test specific aspects of QCD in the non-perturbative sector. However recent observations at the LHC have shown striking similarities between Pb-Pb collisions and high-multiplicity p-Pb and pp collisions. In the elementary collisions a large variation of the characteristics of the event and of the strange particle production rate has been observed as a function of the charged particle multiplicity density. In particular it has been observed as particle production depends only from the event multiplicity and it is independent of the system size and collision energy. This thesis reports about first measurement of K^{*}(892)^{\pm} in pp collisions at \sqrt{s} = 13 TeV in inelastic pp collisions and in different charged particle multiplicity classes. In particular the transverse momentum (p_{T}) spectrum, the integrated yield, the mean p_{T} and the ratio to stable hadrons as pions and kaons have been measured. Moreover the K^{*}(892)^{0} p_{T} spectrum in inelastic pp collisions at the same energy has been also measured. Similar results have been obtained for charged and neutral K^{*}. The K*(892)± p_{T} spectrum has been compared to the predictions of some event generators as PYTHIA6, PYTHIA8 and EPOS-LHC. Furthermore, the comparison of the p_{T} spectrum with the one obtained at different energies has shown a hardening of the spectra with increasing energy of the collisions. Increase of the K*(892)± yield and mean p_{T} when growing the event multiplicity, confirms the independence of the particle yields from the collision system or energy. From the distribution of the K^{*}/K ratio as a function of the charged particle multiplicity, a hint of suppression of the K* production has been observed in high multiplicity pp collisions. This in an analogy to the K^{*}/K results in heavy-ion collisions, is consistent with the presence of re-scattering effects in an hadronic phase in high multiplicity pp collisions.

Quantum chromodynamics, the quantum field theory that describes the strong interaction, demonstrates a property known as asymptotic freedom which weakens the strong coupling constant αs at high energies or short distances. The measurement of particles containing heavy quarks, i.e. charm and beauty, in high-energy particle collisions is a stringent test of the theory of quantum chromodynamics in the regime where αs is small. In addition, asymptotic freedom leads to a phase transition of nuclear matter at high temperatures or energy densities to a phase known as the Quark-Gluon Plasma, where quarks and gluons are deconfined, and this state of matter can be studied in relativistic heavy-ion collisions. Particles containing heavy quarks, i.e. charm and beauty, have been proposed as probes of the properties of the Quark Gluon Plasma, where the measure- ment of mesons and baryons can offer insight into the transport properties of the medium and mechanisms related to the formation of hadrons during the transition back to ‘confined’ quark states. Proton-proton and proton- lead collisions offer a crucial benchmark for these measurements, and can also reveal important insights into particle production and interaction mechanisms. The goal of this thesis is to investigate the production of the charmed baryon Λc+ in high-energy particle collisions with the ALICE detector at the Large Hadron Collider. The measurements presented will test pre- dictions utilising perturbative (small αs) and non-perturbative (large αs) methods, will test possible cold-nuclear-matter modifications of the Λc+ yield in proton-lead collisions, and will set the stage for future measure- ments in lead-lead collisions. The measurements are carried out by recon- structing the hadronic decay channel Λc+ → pK−π+, making selections on its decay topology, extracting the signal via an invariant mass analysis, and finally correcting for its selection and reconstruction efficiency. A multivariate technique (Boosted Decision Trees) has been developed and is utilised in order to improve the signal extraction by optimally com- bining discriminating variables related to the Λc+ decay topology. This technique has also been investigated as a possible approach to measuring the Λc+ baryon in lead-lead collisions in the future, after the upgrade of the ALICE Inner Tracking System will make this measurement possible. The transverse momentum dependence of the Λc+ baryon production cross section has been measured in proton-proton collisions at a centre-of-mass energy of 7 TeV and proton-lead collisions at a centre-of-mass energy per nucleon-nucleon collision of 5.02 TeV, in the transverse momentum range 2 < pT < 12 GeV/c, and is shown to be under-predicted by theoretical calculations. The baryon-to-meson ratio Λc+ /D0 has been measured to be consistent in proton-proton collisions and proton-lead collisions and under-predicted by theoretical calculations. The nuclear modification fac- tor RpPb is measured to be consistent with unity and in agreement with the D meson RpPb, indicating no significant modification of the Λc+ yield in proton-lead collisions with respect to proton-proton collisions within the experimental uncertainties. Finally, Boosted Decision Trees have been shown to significantly improve the statistical precision with which the measurement of the Λc+ baryon can be made in lead-lead collisions with the ALICE detector in the future.

The ALICE experiment at the CERN Large Hadron Collider (LHC) will explore a primordial state of matter that existed in the early Universe. Resonance production at the LHC is of great interest in the study of the phase state of hadronic matter known as Quark Gluon Plasma (QGP). Results are presented on the prospects for φ (1020) meson production in pp collisions. A careful analysis of background subtraction methods with particular attention of φ meson production during the first physics run is also presented. A discussion about the discrepancies between different versions of the PYTHIA event generator in charged-particle multiplicity and its implications in φ production is given. An overview of the physics of strongly interacting matter at high energy densities, and the ALICE experimental apparatus is given. A description of the software development of the ALICE trigger system that allows the determination of transmission quality parameters of cables, and the results obtained from bit-error rate measurements are also given. A feasibility study of the electromagnetic process pp → ppe+e- is presented. Results of the potential backgrounds and signals after full reconstruction along with the trigger rate for such measurement of the luminosity is given.

This thesis discusses the production of the multi-strange, charged Ξ and Ω baryons in proton-lead (p-Pb) collisions at a centre-of-mass energy of 5.02 TeV. The transverse momentum, P\(_T\), distributions are analysed as a function of event multiplicity. A hydrodynamical model based on statistical physics reproduces the shapes of the multi-strange p\(_T\)spectra, in conjunction with the spectra of lighter hadrons, in high multiplicity data. The good agreement is an indication of collective behaviour by all particles inside a system in thermal equilibrium, consistent with the picture of the build {up of a radially outward expansion due to an initially dense medium. These results are reminiscent of the observations made in lead-lead (Pb-Pb) collisions, which are explained by the formation of a Quark-Gluon Plasma. In addition, the p\(_T\)-integrated yields of the hyperons are reported on, revealing a steady increase as a function of multiplicity. An enhancement with respect to non-strange hadrons is observed, and the Ξ/π and Ω/π ratios in high multiplicity p-Pb data approach those measured in central Pb-Pb collisions. The Ξ/π ratio is comparable with the calculations from a thermal model for strangeness saturation, whereas the Ω/π ratio deviates from that value by 2σ.

No presente trabalho é explorada a performance do FoCal, um calorímetro eletromagnético frontal, com alta resolução espacial, proposto como um upgrade para o experimento ALICE, no LHC. O principal objetivo deste detector é fazer a medida de fótons diretos na região de pseudo-rapidez 2.5 < < 4.5. Essa região frontal é dominada por fótons provenientes do decaimento de hadrons, principalmente do píon neutro , e portanto uma eficiente medida de fótons diretos está diretamente associada à eficiência em se identificar fótons provenientes do decaimento do píon neutro. Para distinguir fótons diretos de fótons de decaimento do píon neutro, são explorados três diferentes métodos de análise, a saber, massa invariante, análise da forma do chuveiro e isolamento, sendo cada um deles adequado a uma diferente faixa de energia do píon neutro. O método da massa invariante permitiu identificar fótons de píon neutro com uma eficiência de 95% num ambiente de partículas únicas, para píon neutro com energia entre 0 e 300 GeV. Num ambiente pp, esse método mostra uma eficiência de 85%. A análise da forma do chuveiro foi capaz de identificar, com um dado corte no parâmetro de forma Width1, fótons diretos com uma eficiência acima de 90% e de rejeitar fótons de píon neutro com uma eficiência que varia de 65% a 95%, no intervalo de energia de 300 GeV a 500 GeV, num ambiente de partículas únicas. Essas eficiências podem ser melhoradas se for aplicado um corte dependente da energia e a rejeição de píon neutro pode também ser melhorada se os eventos mais assimétricos puderem ser melhor reconstruídos pelo detector no futuro. No método de isolamento, aplicado ao ambiente pp e PbPb, é explorado o melhor valor de raio de isolamento Riso a ser usado em cada ambiente. Por este método podem ser atingidas altas eficiências em detectar fótons diretos, mas o método se mostra limitado pelo grande fundo de fótons de decaimento, e a pureza dos cortes testados não atinge valores maiores que cerca de 10%. E mostrado que o uso de um trigger em 40 GeV pode melhorar a pureza dos cortes de 0.01 % para cerca de 10%. Foi feita a otimização de alguns parâmetros de análise, permitindo melhor desempenho desses métodos de análise explorados
In the present work is explored the performance of the FoCal, a forward electromagnetic calorimeter, with high granularity, proposed as an upgrade for the ALICE experiment, in LHC. The main goal of the detector is to be able to identify direct photons in the pseudo-rapidity region 2.5 < < 4.5. This forward region is dominated by decay photons, mainly photons from neutron pion decay, so an efficient mesurement of direct photons is directly associated to the efficiency in identify photons from neutron pion decay. To separate direct photons from neutron piondecay photons, it is explored three different analysis methods, the invariant mass, shower shape and isolation, each one useful in a different energy range of the neutron pion. The invariant mass method allowed to identify photons from neutron pion with an efficiency around 95% in a single particle environment, for neutron pion with energy between 0 and 300 GeV. In a pp environment, this method showed an efficiency of 85%. The shower shape method was able to identify, by a given cut in the shower shape parameter Width1, direct photons with an efficiency of 90% and to reject 65% to 95% of the photons from neutron pion in the energy range of 300 GeV to 500 GeV, in a single particle environment. This efficiencies can be improved if there is aplied a cut that depends on the energy and the neutron pion rejectioncan also be improved if the more assimetric decays can be better reconstructed by the detector in the future. The isolation method is aplied to a pp and PbPb environments, and it is explored the best value of isolation radius Riso to be used in each environment. By this method, it can be reached high efficiencies in detecting direct photons, but the method is limited by the high background of decay photons, and the purity of the cuts is not better than about 10%. It is showed that a trigger in 40 GeV can improve the purity from 0.01% to about 10%. It was made an optimization of some of the used analysis parameters, leading to an improvement of the explored analysis methods.

The aim of relativistic heavy-ion collisions is to investigate the properties of the Quark-Gluon Plasma (QGP) phase, that is achieved at high-enough temperatures and/or densities. In this context, light on heavy-ion collisions (e. g. p-Pb) are used to assess Cold Nuclear Matter effects (CNM), while elementary hadronic collisions (e. g. proton-proton) provide tests for QCD (Quantum Chromodynamics) based calculations and baseline for studies with heavy- ions. Heavy quarks, i. e. charm and beauty, are very convenient in the characterization of the QGP. They are produced via initial hard parton-parton scatterings at the early stages of the collision and, therefore, they are a self-generated probe for the system created in the reaction. In this work the angular correlation between electrons from heavy-flavour hadron decays and charged particles was studied in pp (2.76 and 7 TeV) and p-Pb (5.02 TeV) collisions at the CERN Large Hadron Collider, using the ALICE detector. The correlation strengths were evaluated as a function of multiplicity in p-Pb collisions. In pp collisions the relative beauty (and charm) contribution to the total heavy-flavour decay electron yield was estimated using the measured correlation distribution and Monte Carlo templates.
O próposito de colisões entre íons pesados relativísticos é investigar as propriedades do plasma de quarks e gluons (QGP, do inglês Quark-Gluon Plasma). A transição de fase, de um estado hadrônico para o QGP, ocorre em regimes onde a temperatura e/ou densidade atingem um valor suficientemente alto. Neste contexto, colisões entre íons pesados e leves (por exemplo, p-Pb) permitem acessar efeitos devido à matéria nuclear fria (CNM, do inglês Cold Nuclear Matter) e colisões elementares (por exemplo, próton-próton) são usadas como referência para estudos com íons pesados, além de proporcionarem testes para cálculos de QCD perturbativa. Quarks pesados, isto é charm e bottom, são ferramentes muito convenientes no estudo e caracterização do QGP. Essas partículas são produzidas através de espalhamento duro nos instantes iniciais da colisão e, portanto, elas atuam como uma sonda externa para o sistema criado na reação. Esse trabalho consiste no estudo da correlação angular entre elétrons oriundos de quarks pesados e partículas carregadas, em colisões pp (2.76 e 7 TeV) e p-Pb (5.02 TeV), no acelerador LHC (do inglês Large Hadron Collider) do CERN, usando o detector ALICE (do inglês A Large Ion Collider Experiment). A distribuição angular mencionada foi medida em função da multiplicidade do evento, no caso de colisões p-Pb. Em colisões pp, a contribuição relativa de quarks charm e bottom para o total de elétrons provenientes de quarks pesados foi estimada usando a função de correlação obtida.

Questa tesi si propone di studiare l’invecchiamento dei sensori che compongono il Beam Condition Monitor System (BCM), il sistema di protezione di ALICE da eventuali problematiche legate al fascio di particelle, come i beam failures o beam losses. ALICE (A Large Ion Collider Experiment) è uno dei grandi esperimenti presenti ad LHC (Large Hadron Collider) presso il CERN di Ginevra, dedicato allo studio delle proprietà dello stato della materia chiamato Quark Gluon Plasma (QGP). Per riuscire in questo studio, ALICE si serve di una grande quantità di dati raccolti durante collisioni tra ioni pesanti di piombo (Pb) e protoni. ALICE è composto da numerosi e sofisticati rivelatori, alcuni dei quali posti in prossimità del fascio di particelle. Ogni rivelatore che compone l’esperimento è dedicato alla misura di un particolare segnale fisico o di una caratteristica della collisione analizzata. Il danneggiamento di anche solo uno di tali rivelatori potrebbe quindi limitare le performance di tutto l’esperimento, o addirittura arrestare la presa dati. I detector sono progettati per essere estremamente sensibili alle particelle che vengono generate nelle collisioni studiate: risultano così vulnerabili alle imperfezioni che si possono presentare nel fascio o nella sua iniezione nell’anello di LHC dopo essere stato accelerato dai precedenti acceleratori. Il sistema di protezione dal fascio presso ALICE è dato da una combinazione di "hardware interlocks" che compongono il Beam Interlock System (BIS) di LHC e del Beam Condition Monitor System. Le analisi effettuate in questa tesi, che hanno come oggetto di studio la risposta degli 8 sensori al diamante pCVD che compongono la Stazione 0 del BCM in funzione della luminosità dell’esperimento, hanno permesso di evidenziare come la risposta dei sensori sia variata negli anni, indicando quindi un loro invecchiamento.

Le jet est l'une des sondes permettant de tester les prédictions de la théorie de l'interaction forte, la QCD, et d'extraire les propriétés physiques d'un état particulier de la matière nucléaire : le Plasma de Quarks et de Gluons (PQG). Ce travail de thèse a pour but de montrer les capacités d'ALICE à mesurer les jets issus des collisions produites au Large Hadron Collider (LHC).La détection des particules constituant les jets, leur association à l'aide d'algorithmes de reconstruction ainsi que la construction d'observables telles que le spectre en pT des jets ou le Plateau Bossu est un travail complexe. Nous détaillons ces différentes étapes à partir de simulations permettant d'appréhender le taux de jets réels à attendre pour notre analyse etd'évaluer l'impact de la mesure expérimentale sur les observables finales. Nous présentons finalement les spectres en pT et Plateaux Bossus issus des premières collisions p+p du LHC auxquels ont été appliquées des corrections moyennes.

O Grande Colisor de Hádrons (Large Hadron Collider - LHC) é um acelerador de prótons e íons pesados localizado no CERN (Conseil Européen pour la Recherche Nucléaire). Em um de seus experimentos, ALICE (A Large Ion Collider Experiment ), está sendo projetado um detector dedicado a explorar os aspectos únicos de colisões núcleo-núcleo. A principal finalidade do ALICE é estudar a formação de um novo estado da matéria, o plasma de quarks e glúon. Para isto devem-se ter medidas precisas de hádrons, elétrons, múons e fótons produzidos em colisões chumbo-chumbo. Assim está sendo proposto um calorímetro frontal (Foward Calorimeter - FoCal) como um upgrade para o ALICE. A função deste calorímetro é o estudo das funções de distribuição de pártons (Partons distribuction Functions - PDF) no regime de pequenos valores do x de Bjorken. Nesta região é esperado que estas PDFs tenham um comportamento não linear devido ao processo de saturação de glúons. Para o estudo desta região é necessária a medida de fótons diretos produzidos na colisão. Estes, por sua vez, ficam mascarados pelo fundo de fótons provenientes do decaimento de píon, o que leva a uma necessidade de suas identificações. Com isto surge a oportunidade para a utilização do método de clusterização que é uma ferramenta de mineração de dados. Este trabalho contribuiu para o desenvolvimento inicial de um algoritmo de clusterização para o calorímetro FoCal.
The Large Hadron Collider (LHC) is a CERN\'s accelerator that collides protons and heavy ions. One of its experiments, ALICE, is building a new detector to explore new aspects of heavy ions collisions. The Alice\'s main goal is to study the formation of quark-gluon plasma (QGP). To do that it\'s necessary to get accurate data on hadrons, electrons, muons and gammas created in lead-lead collision. So, to accomplish that a new calorimeter is in study to scan the foward region of experiment, the Foward Calorimeter (FoCal). It\'s proposed to study Parton Distribution Functions (PDFs) in a regime of very small Bjorken-x, where it is expected that the PDFs evolve non-linearly due to the high gluon densities, a phenomena referred to as gluon saturation.But to do that it\'s required to measure the direct gammas created on collision. These fotons are blended on by fotons descendant of pion. So there\'s a need to separate it from the direct gammas. One way to solve this problem is to use clustering methods (a type of mining data algorithm). This work helped on early stages of development that clustering algorithm.

Un des objectifs de la physique des ions lourds ultra-relativistes est la mise en évidence et la caractérisation d'une nouvelle phase de la matière nucléaire : le plasma de quarks et de gluons (Q. G. P. ). Le détecteur ALICE permettra d'accéder à la plupart des signatures du Q. G. P. , permettant ainsi une étude systématique des propriétés du milieu formé lors d'une collision d'ions lourds aux énergies du L. H. C. En particulier, un spectromètre dimuons a été développé pour permettre la mesure de la production des résonances de haute masse (J/psi, upsilon) via leur désintégration dimuonique. Le système de déclenchement du spectomètre dimuons ("trigger") est basé sur des chambres à plaques résistives (R. P. C. ). Il permet de réaliser une sélection des événements contenant une ou plusieurs traces de muons provenant du vertex d'interaction. En outre, une coupure sur l'implusion transverse de ces traces est appliquée grâce à des mémoires statiques à accès aléatoire ou "Look Up tables" (LUT). Une méthode d'optimisation des LUT a été mise au point et est décrite dans ce document. Celle-ci s'appuie sur des simulations complètes du dispositif expérimental d'ALICE réalisées par le biais d'AliRoot. D'autre part, un prototype miniature du système de déclenchement a été construit et testé dans la zone d'irradiation aux gammas (GIF) du CERN, avec les muons du S. P. S. L'efficacité de reconstruction de traces de muons simples, la robustesse du système au bruit de fond non-corrélé ainsi que les aspects de mise en temps ont été particulièrement approfondis. Enfin, des simulations de la production de dimuons, dans le spectromètre d'ALICE, pour des collisions Pb _ Pb et en fonction de la centralité, ont été menées. Les spectres en masse ainsi que les rapports signal/ bruit et significance sont donnés et commentés pour les diverses tranches en centralité considérées

2008/2009
The ALICE experiment at CERN will study the medium formed in very high energy lead-lead collisions at the LHC. According to the Quantum Chromo Dynamics theory of the strong interaction, a phase transition to a state where quarks and gluons are not confined into hadrons (Quark-Gluon Plasma) can occur in these collisions. Heavy quarks (charm and beauty) are produced in hard scattering processes in the first stages of the collisions. While travelling through the medium they can lose energy by means of gluon radiation. This affects the momentum spectra of the hadrons produced in the subsequent hadronization. To study the energy loss mechanism and its dependence on the parton nature (quark/gluon) and mass (light/heavy quark), hadron momentum spectra observed in heavy-ion collisions are compared to the same spectra observed in proton-proton collisions, where the formation of a thermalized medium is not expected. In this thesis, the measurement of charm production in proton-proton collisions via the exclusive reconstruction of the D0 meson (cū) decay in the D0->Kπ channel is presented. An invariant mass analysis of selected pairs of particles with opposite charges is performed, exploiting the high spatial resolution of the Inner Tracking System silicon detector, that assures the tracking precision needed to resolve the D0 decay vertices from the interaction point of the colliding protons (primary vertex). The estimate of the fraction of secondary D0 mesons coming from the decay of B mesons, if based on theoretical calculations, introduces a relevant systematic error. A method to extract this contribution directly from data, via the analysis of the impact parameter distribution, i.e. the distance between the reconstructed meson trajectory and the primary vertex, is presented. The presence of the ITS assures the possibility to highlight the different shape of the impact parameter distribution typical of secondary D0 mesons, influenced by the relative long lifetime of B mesons (cτ~460-490 μm). Due to construction and mounting finite precision, the modules composing the ITS can be displaced from their designed positions by large amounts if compared with the expected spatial resolutions, of the order of tens of micron. This deteriorates the detector performances, in particular the tracking spatial and momentum resolutions. The alignment of the ITS, that is, the a posteriori determination of more than 13000 parameters defining the real position and orientation in space of the modules, is extensively discussed. The results obtained with 2008 cosmic-ray data are presented. The promising alignment status allowed to test the analysis for the D0 reconstruction on proton-proton collision data simulated with realistic detector properties.
L'esperimento ALICE al CERN studierà il mezzo formato in collisioni piombo-piombo ad altissime energie a LHC. Secondo la Cromodinamica Quantistica, teoria che descrive l'interazione forte, in queste collisioni può avvenire una transizione di fase ad uno stato in cui quark e gluoni non siano confinati dentro gli adroni (Quark-Gluon Plasma). I quark pesanti (charm e beauty) vengono prodotti negli istanti iniziali delle collisioni, in processi con alto momento trasferito. Attraversando il mezzo possono perdere energia tramite radiazione di gluone. Per studiare il meccanismo con cui la perdita di energia avviene, in particolare la sua dipendenza dalla natura del partone (quark/gluone) e dalla sua massa (quark leggeri/pesanti), gli spettri in momento degli adroni osservati in collisioni tra ioni pesanti vengono confrontati con gli stessi spettri osservati in collisioni protone-protone, dove non ci si aspetta la formazione di un mezzo termalizzato. In questa tesi è presentata la misura della produzione di charm in collisioni protone-protone tramite la ricostruzione esclusiva del decadimento del mesone D0 (cū) nel canale D0->Kπ. A questo scopo viene effettuata un'analisi in massa invariante di coppie di tracce con carica opposta opportunamente selezionate, sfruttando l'elevata precisione spaziale dell'Inner Tracking System, rivelatore al silicio che assicura la precisione di tracciamento necessaria per risolvere i vertici di decadimento dei mesoni D0 dal punto di interazione dei protoni dei fasci (vertice primario). La stima della frazione di D0 secondari, provenienti dal decadimento di un mesone B, introduce un errore sistematico consistente, se basata su calcoli teorici. Viene presentato un metodo per valutare questo contributo direttamente dai dati, tramite l'analisi della distribuzione del parametro d'impatto, cioè la distanza tra la traiettoria ricostruita del mesone e il vertice primario. La presenza dell'ITS assicura la possibilità di evidenziare la diversa forma della distribuzione in parametro d'impatto tipica dei D0 secondari, influenzata dalla vita media relativamente lunga dei mesoni B (cτ~460-490 μm). A causa della precisione finita nelle operazioni di costruzione e montaggio, la posizione e l'orientazione dei moduli che compongono l'ITS possono differire da quelle di progetto. Se paragonate alle risoluzioni spaziali attese, dell'ordine di decine di micron, le differenze possono essere notevoli. Questo degrada le prestazioni del rivelatore, soprattutto le risoluzioni spaziale e in momento del tracciamento. L'allineamento dell'ITS, ovvero la determinazione a-posteriori dei più di 13000 parametri necessari per definire le reali posizioni e orientazioni dei moduli viene discussa in dettaglio nella tesi e vengono presentati i risultati ottenuti utilizzando i dati da raggi cosmici raccolti nel 2008. Il promettente stato dell'allineamento ha permesso di testare l'analisi per la ricostruzione del mesone D0 su dati da simulazioni di collisioni protone-protone in cui le proprietà del rivelatore fossero descritte realisticamente.
XXII Ciclo
1981

2011/2012
The subject of the present PhD thesis is the study of the production of light hypernuclei in ultra-relativistic Pb-Pb collisions with ALICE (A Large Ion Collider Experiment), one of the four major experiments at the LHC (Large Hadron Collider). The main physics goal of the ALICE experiment is the investigation of the properties of the strongly interacting matter at high energy density ($>$ 10 GeV/fm$^3$) and high temperature ($\approx$ 0.2 GeV) conditions. According to the lattice Quantum Chromo Dynamics (QCD) calculations, under these conditions (i.e. high temperature and large energy density) hadronic matter undergoes a phase transition to a ``plasma'' of deconfined quarks and gluons (Quark Gluon Plasma, QGP). In the first chapter of the thesis a general introduction to the heavy-ion physics will be given. Then the main quantities related to QGP formation (i.e. \textit{probes}) will be described. Finally the most important results obtained at SPS, RHIC and LHC experiments will be shown and discussed. In the second chapter a short description of the LHC and its experimental conditions will be reported and an overview of the ALICE experiment will be given. A description of the different detectors and their performances during data taking will be described; in addition a description of the computing framework will be given. The third chapter will be devoted to an introduction of the (anti)(hyper)nuclei production in heavy-ion collisions. The two main approaches which are believed to govern nuclei production (i.e. coalescence and thermal models) will be described, and an overview on the results at different energies will be shown. A comparison of the theoretical results will be also shown, with particular regards to the energies at the LHC. The fourth chapter is devoted to the description of the analysis method used to get (anti)hypertriton production yield in \PbPb~collisions at $\sqrt{s_{\rm{NN}}}$ = 2.76~TeV with the ALICE experiment via its mesonic decay \hyp~$\rightarrow$ \he + \pim (\antihyp $\rightarrow$ \antihe + \pip). In the beginning of the chapter the analysis technique used for particle identification and for the determination of secondary vertices will be described. The analysis will be divided into two distinct parts: the first one based on the data sample collected by the ALICE experiment during the first LHC heavy-ion run held at the end of 2010, while the second one based on data collected at the end of 2011. A detailed description of the study on efficiency evaluation and signal extraction will be shown for both analysis, together with a study of the systematic uncertainties. The results on the production yield of (anti)hypertriton will also be shown. The estimation of the hypertriton lifetime will be provided in the final section of the chapter.\\ In the fifth chapter the method used to obtain the \pt~spectrum of \he~will be presented. The raw spectra, the efficiency evaluation, systematic errors and feed-down from \hyp~will be presented. The final spectrum will be used to evaluate the production yield of \he(\antihe) in the whole \pt~region, from 0 to $\infty$. \\ Finally, in the last chapter, the present experimental results will be compared with published relevant results and with the most recent theoretical findings. Moreover, the measurement of the ``Strangeness Population Factor'' [S$_{3}$= \hyp/\he/($\Lambda$/p)] at the LHC energies will be provided. This quantity is a valuable tool to probe the nature of dense matter created in high-energy heavy-ion collisions and to validate theoretical models.
-------------------------------------------------------------------------------------------------- Questa tesi è dedicata allo studio della produzione di ipernuclei leggeri in collisioni ultra-relativistiche di ioni piombo (Pb) con l'esperimento ALICE (A Large Ion Collider Experiment), uno dei quattro grandi esperimenti del Large Hadron Collider (LHC) del CERN. Il principale obiettivo scientifico dell'esperimenento ALICE è lo studio delle proprietà della materia in condizioni estreme di energia (> 10 GeV/fm^3) e di temperatura (~ 0.2 GeV) mediante lo studio di collisioni di ioni piombo. Calcoli di Cromo Dinamica Quantistica (QCD) su reticolo prevedono, infatti, che in condizioni di alta temparatura e grande energia la materia adronica subisca un transizione di fase verso un ``plasma'' di quark e gluoni deconfinati (Quark Gluon Plasma, QGP). Nel primo capitolo della tesi verranno descritte in maniera generale la fisica degli ioni pesanti e le grandezze caratterische usate per provare la formazione del QGP (probes). Verranno quindi mostrati e discussi i risultati sperimentali che possono provare l'esistenza di uno stato deconfinato della materia nucleare ottenuti agli esperimenti a SPS, RHIC e LHC. Nel secondo capitolo saranno brevemente presentati il Large Hadron Collider (LHC) e le condizioni sperimentali di lavoro durante i primi tre anni di presa dati; in seguito verrà data un'ampia panoramica dell'esperimento ALICE. Saranno descritti i differenti sotto-rivelatori che formano l'esperimento e verranno inoltre mostrate le loro performance durante l'acquisione dati; inoltre verrà fornita una descrizione del framework di calcolo utilizzato nell'analisi dei dati. Il terzo capitolo sarà dedicato alla descrizione dei maccanismi di produzione di (anti)(iper)nuclei in collisioni di ioni pesanti: verranno descritti i due meccanismi di produzione che si ritiene governino la loro produzione (coalescenza e modello termico) e verrà mostrata una panoramica sui risultati ottenuti a diverse energie. Inotre saranno presentati diversi calcoli teorici, ponendo particolare attenzione ai risultati aspettati all'energia di LHC. Il quarto capitolo contiene la descrizione del metodo di analisi utilizzato per valutare lo yield di pruduzione dell'(anti)ipertritone attraverso il suo canale di decadimento mesonico \hyp~$\rightarrow$ \he + \pim (\antihyp $\rightarrow$ \antihe + \pip) in collisioni \PbPb~con energia nel centro di massa $\sqrt{s_{\rm{NN}}}$ = 2.76~TeV. Inizialmente verrà descritta la tecnica di analisi utilizzata per l'identificazione di particelle e dei vertici secondari, quindi sarà fornita la descrizione dettagliata della tecnica di analisi. L'analisi dei dati è stata siddivisa in due distinte parti: la prima è dedicata alla descrizione della procedura utilizzata per l'analisi dei dati raccolti da ALICE durante la prima acquisizione di collisioni Pb--Pb alla fine del 2010; nella seconda parte, invece, verrà descritta la procedura di analisi dei dati raccolti durante la seconda presa dati nel Dicembre 2011. Verranno quindi descritte in modo dettagliato l'estrazione del segnale, lo studio del fondo combinatoriale e gli errori sistematici. Infine, nella parte finale del capitolo, varrà fornita una stima della vita media dell'ipertritone.\\ Nel quinto capitolo sarà presentato il metodo usato per ottenere lo spettro in pT di (anti-3He)3He. Verranno descritti: la procedura di estrazione del segnale, la stima dell'efficienza in funzione del momento trasverso, la valutazione degli errori sistematici e la procedure usata per sottrarre il feed-down dovuto al decadimento dell'ipertitone. Lo spettro verrà quindi utilizzato per valutare lo yield di produzione di (anti-3He) 3He. Infine, nel sesto e ultimo capitolo, i risultati sperimentali ottenuti verranno confrontati con i risultati teorici discussi nel Capitolo 3.
XXV Ciclo
1985

Con il lavoro svolto in questa tesi si propone un primo modello di analisi delle prestazioni del rivelatore TOF di ALICE usando dati di raggi cosmici. L’elaborato verte brevemente sugli aspetti salienti della fisica degli ioni pesanti, dell’esperimento ALICE e dell’apparato sperimentale (LHC, TOF), per poi concentrarsi sull’analisi del campione di dati di raggi cosmici e sui risultati inerenti alle prestazioni del rivelatore. L'analisi ha permesso di evidenziare potenziali anomalie nella ricostruzione delle tracce di muoni cosmici da parte degli algoritmi dei rivelatori, e il suo completamento ha richiesto lo sviluppo di un metodo di ricostruzione manuale delle tracce e di un algoritmo di minimizzazione che, in definitiva, hanno permesso di determinare dei valori plausibili di risoluzione temporale globale del TOF e individuale dei suoi supermoduli. I metodi proposti sono arbitrariamente estensibili ad unità più piccole del rivelatore, in vista di ulteriori analisi o campioni di dati maggiormente certificati. Complessivamente, i risultati ottenuti mostrano lievi differenze rispetto ai valori misurati a partire da dati provenienti da collisioni Pb-Pb ed eventi ultraperiferici, presumibilmente a causa del fatto che il rivelatore e gli algoritmi di ricostruzione sono rispettivamente calibrati ed ottimizzati per prese dati su collisioni, la cui topologia differisce in maniera sostanziale da quella di eventi di raggi cosmici. Ciononostante, gli esiti evidenziano con successo le ottime caratteristiche temporali del TOF. Il quesito sull'imperfezione nella ricostruzione di tracce cosmiche da parte degli algoritmi di ALICE rimane ad ogni modo aperto e, in attesa di ulteriori riscontri o eventuali smentite, si propone il presente modello di analisi.

Le plasma de quarks et de gluons est un état de matière obtenu lors de températures et de densités d’énergie extrêmes où les quarks et les gluons sont libres. Cette matière chaude et dense peut être créée dans les collisions d’ions lourds ultra-relativistes. Les mesures des spectres des particules identifiées produites dans des jets représentent un outil majeur permettant d'étudier les propriétés du plasma créé dans les collisions et d'ainsi comprendre les relations entre divers mécanismes contribuant à la production de particules dans ce milieu. Cette thèse présente une analyse des spectres en impulsion transverse des baryons Λ et mésons K0S produits dans des jets chargés lors de collisions Pb–Pb centrales à l'énergie sqrt(sNN) = 2.76 TeV mesurées avec ALICE au LHC. Les résultats sont utilisés pour discuter l'origine de l'augmentation du rapport entre baryons et mésons observée pour la production inclusive des particules dans les collisions d’ions lourds ultra-relativistes
Quark–gluon plasma is a state of matter existing under extreme energy densities and temperatures where quarks and gluons are deconfined. This hot and dense strongly interacting matter can be created in ultra-relativistic heavy-ion collisions. Measurements of spectra of identified particles produced in jets represent a major tool for studying properties of the plasma created in the collisions and for understanding the interplay of various mechanisms which contribute to the particle production in this medium. In this thesis, we present the measurement of the transverse-momentum spectra of Λ baryons and K0S mesons produced in charged jets in central Pb–Pb collisions at the centre-of-mass energy sqrt(sNN) = 2.76 TeV, measured with ALICE at the LHC. The results of the analysis are used to discuss the origin of the enhancement of the baryon-to-meson ratio observed for the inclusive production of light-flavour particles in ultra-relativistic heavy-ion collisions

We present an analysis of the properties of jets in proton-proton and lead-lead collisions at the ALICE experiment at the CERN Large Hadron Collider. The jet width and transverse momentum imbalance were studied in minimum bias proton collisions. The width was found to be independent of hadron transverse momentum and to have a larger value than measurements from earlier experiments at lower beam energies, consistent with theoretical expectations. The imbalance was found to increase with collision energy and also with the transverse momentum of the hadrons in the jet. The dependence of jet yield on the produced multiplicity was studied in proton and ion collisions, with a view to identifying suppression through interaction with a quark-gluon plasma. This suppression was identified in lead collisions as expected, but no suppression was observed in high multiplicity proton collisions. The jet width and tranverse momentum imbalance were also studied as a function of multiplicity. In proton collisions, the jet width was measured to be independent of multiplicity and hadron transverse momentum and consistent with minimum bias measurements. The width was observed to decrease in lead collisions of increasing centrality; this was interpreted as a suppression of the high momentum hadrons in the jet.

Alice (a large ion collider experiment) etudiera, a partir de 2005, les collisions d'ions lourds a des energies ultrarelativistes aupres de l'accelerateur lhc (large hadron collider) du cern. Dans le cadre de cette experience, un important programme de r&d sur les chambres de trajectoire qui equiperont le spectrometre dimuons a ete realise a l'institut de physique nucleaire d'orsay. Il s'est concretise par la construction de trois prototypes de chambres proportionnelles multifils a cathode segmentee, dont un grandeur reelle de 1m 2. Dans cette these, nous presentons les tests experimentaux ainsi que les simulations qui nous ont permis de connaitre et d'optimiser les performances de nos detecteurs. Les prototypes ont ete testes a plusieurs reprises aupres des accelerateurs du ps et du sps au cern. Ils ont eu pour role de valider les choix effectues en terme de mecanique de construction, de parametres geometriques, de melange gazeux et d'electronique de lecture. L'analyse des nombreuses donnees collectees a montre que les performances de ce type de detecteur repondaient au cahier des charges etabli. Les resultats concernant les caracteristiques du detecteur, resolution spatiale, efficacite, gain et homogeneite sont donnes. Des resolutions spatiales inferieures a trente microns ont notamment ete obtenues. En parallele a ces essais sous faisceau, plusieurs simulations ont ete developpees afin d'avoir une meilleure comprehension de la reponse des detecteurs. Elles ont permis, entre autres, de definir la segmentation du plan cathodique, d'etudier l'algorithme de reconstruction de la position et d'etablir le cahier des charges de la future electronique.

Cette etude s'inscrit dans le cadre de la recherche du plasma de quarks et de gluons (qgp) dans les collisions d'ions lourds ultrarelativistes au lhc avec le detecteur alice. Le spectrometre dimuons de cette experience doit mesurer la production des resonances de la famille du j/psi et du upsilon via leur desintegration muonique. Le systeme de declenchement associe ou trigger dimuons selectionne les evenements interessants en effectuant des coupures sur l'impulsion transverse des traces. Le signal de trigger dimuons consiste en deux traces validees de signe oppose. La decision de trigger est prise par une electronique dediee qui utilise les informations de chambres de detection de type rpc (resistive plate chamber). Nous avons effectue l'etude de ce dispositif en procedant dans un premier temps a des tests experimentaux des rpc. Nous montrons les performances du detecteur obtenues lors des tests, notamment pour une rpc de basse resistivite fonctionnant en mode streamer. Les exigences de alice sont respectees au niveau de la tenue au flux, la taille de clusters et la resolution en temps. Nous avons optimise le dispositif du trigger en effectuant des simulations comprenant une description detaillee des plans de lecture et de la logique de trigger (algorithme). En particulier, un algorithme de declustering est propose et valide. De meme, nous avons introduit une methode, appelee reduction ds, qui permet de limiter les effets du bruit de fond combinatoire sur les taux de trigger. Les efficacites et les taux de trigger sont obtenus pour les collisions pb-pb, ca-ca et p-p au lhc. Des coupures plus elaborees, sur la masse invariante par exemple, utilisant les informations du trigger sont testees par simulation mais n'ameliorent pas significativement l'efficacite du systeme.

Sonder le plasma de quarks et de gluons et explorer ses propriétés thermiques comme son évolution dynamique sont les principaux objets des collisions d'ions lourds ultra-relativistes. La sonde électromagnétique dure est un outil expérimental bien établi. ALICE est l'une des quatre expériences principales du LHC, qui est dédiée aux collisions d'ions lourds d'énergie √SNN = 5,5 TeV. Les prises de données initiales avec des collisions pp à 900GeV(2009) et 7TeV(2010) sont des points de référence nécessaires. Dans la présente thèse, trois parties sont principalement concernées : 1) les taux de production brut de mésons neutres π0 (η) en 2γ et ω (782) -> π0γ-> 3 γ, ont été estimés, avec une configuration réaliste de détecteurs. Ces taux nous ont donné une référence importante avant la prise de données d'ALICE. Elle prouve également la possibilité de mesurer la η et ω (782) en dépit de l'acceptance limitée du calorimètre PHOS. 2) La chaîne de traitement, des données brutes à l'analyse physique des données, est étudiée pour une meilleure compréhension des détecteurs. Selon la conception spécifique de chaque calorimètre (PHOS et EMCal), ainsi que les stratégies d'extraction des mésons neutres par analyse de masse invariante, la forme de la gerbe et les coupures d'isolement sont étudiées. 3) D'après la statistique actuelle obtenue pour les collisions pp à 7TeV, ~ 390M événements min-bias sont analysés. En utilisant l'analyse de la masse invariante, le spectre des π0 est extrait à l'aide de PHOS, jusqu'à de hautes impulsions (pT~25GeV/c). Les incertitudes systématiques sont pour l'essentiel dues à l'étalonnage du niveau d'énergie absolue, la non-linéarité de la réponse de détecteur, les canaux défectueux et le pertes de π0 par conversion dans la matière. La section efficace de production des π0 est comparée aux calculs NLO. Les pics de masse du η et du ω(782) sont également observés. Les mesures initiales des mésons neutres sont cruciales pour l'étalonnage des calorimètres électromagnétiques. Elle est également liée à la quasi-totalité de la physique photon, comme l'excès aux photons directs, la mesure de flot et la physique des jets, etc. Dans les années qui suivent, ALICE va poursuivre la prise de données avec des collisions pp et PbPb. Une plus grande statistique sera accumulée, ouvrant ainsi la porte à une physique très riche
Probing of the quark gluon plasma and exploring its thermal properties and dynamic evolution are the main objects in ultra-relativistic heavy-ion collisions. The hard electromagnetic probe is a well built detection. A Large Ion Collider Experiment (ALICE) is one of four main experiments at the LHC, which is dedicated on the heavy ion collisions at a top energy √SNN=5. 5 TeV. The initial runs with pp collisions at 900 GeV (2009) and 7 TeV (2010) are necessary reference points. In the thesis, three parts are mainly involved : 1) the raw yield of neutral mesons by π0(η)->2γ and ω(782)->π0γ->3 γ with the realistic geometrical configuration were estimated. It gave us an important reference before the ALICE data-taking. It also proves the possibility to measure the η and ω(782) despite of its limited acceptance for PHOS. 2) The data processing chain from raw data to the physics analysis oriented are studied for a better understanding to the detectors. Depending on the different design of the PHOS and EMCAL, the strategies of the neutral mesons extraction by invariant mass analysis, shower shape analysis and isolation cut method are studied. 3) From the current statistics with pp collisions at 7 TeV, ~ 390 million min-bias events are analyzed. By using the invariant mass analysis, π0 spectrum is extracted up to pT range to 25~GeV/c with PHOS. The systematic uncertainties are estimated due to the absolute energy scale calibration, non-linearity response, bad channels and π0 conversion loss. The final π0 cross-section is compared with the NLO calculation. The η and ω(782) peak are also observed. The initial measurements of neutral mesons are crucial for the calibration of electromagnetic calorimeters. It is also related to almost all the photon physics, such as direct photon excess, flow measurement and jet physics etc. In the coming years, ALICE will continue with the data taking with pp and PbPb collisions. And more statistics and rich physics will come forth

In this thesis the analysis to reconstruct the transverse momentum p_{t} spectra for pions, kaons and protons identified with the TOF detector of the ALICE experiment in pp Minimum Bias collisions at $\sqrt{s}=7$ TeV was reported. After a detailed description of all the parameters which influence the TOF PID performance (time resolution, calibration, alignment, matching efficiency, time-zero of the event) the method used to identify the particles, the unfolding procedure, was discussed. With this method, thanks also to the excellent TOF performance, the pion and kaon spectra can be reconstructed in the 0.5