Academic literature on the topic 'Collisioni ioni pesanti'

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

Low mass vector meson (ρ, ω,Φ ) production provides key information on the hot and dense state of strongly interacting matter produced in high-energy heavy ion collisions (called Quark Gluon Plasma). Strangeness enhancement is one of the possible signatures of the Quark Gluon Plasma formation and can be accessed through the measurement of Φ meson production with respect to ρ and Φ mesons, while the measurement of the Φ nuclear modification factor provides a powerful tool to probe the production dynamics and hadronization process in relativistic heavy ion collisions. Vector mesons can be detected through their decays into muon pairs with the ALICE muon spectrometer. This thesis presents the results on the measurement of the Φ differential cross section, as a function of the transverse momentum, in pp collisions at √s = 2.76 TeV; the measurement of the Φ fyield and of the nuclear modification factor RpA at forward and backward rapidity, as a function of the transverse momentum, in p-Pb collisions at √s = 5.02 TeV; the measurement of the Φ/ (ρ+ω) ratio, as well as of the Φ nuclear modification factors RAA and RCP , as a function of the number of participating nucleons, in Pb-Pb collisions at √sNN = 2.76 TeV.

In this thesis we study the possibility that QCD breaks parity at high temperatures and densities, a scenario that may be tested in heavy ion collisions. We present two different approaches to this problem. On the one hand, analytical studies with effective models suggest that QCD may break parity in dense systems due to condensation of pseudoscalar mesons in the isotriplet channel. On the other hand, P- and CP- odd bubbles may appear in a finite volume due to local large topological fluctuations in a hot medium. The presence of a large magnetic field or angular momentum may lead to the so-called Chiral Magnetic Effect. We assume topological fluctuations as the source of parity violation in heavy ion collisions throughout. This effect may be treated in a quasi-equilibrium description by means of a non-trivial axial chemical potential. We consider the 'two flavour' Nambu—Jona-Lasinio model in the presence of a vector and an axial external chemical potentials and study the phase structure of the model at zero temperature. The Nambu—Jona-Lasinio model is often used as a toy replica of QCD and it is therefore interesting to explore the consequences of adding external vector and axial chemical potentials in this model, mostly motivated by claims that such external drivers could trigger a phase where parity could be broken in QCD. We are also motivated by some lattice analysis that attempt to understand the nature of the so-called Aoki phase using this simplified model. Analogies and differences with the expected behaviour in QCD are discussed and the limitations of the model are pointed out. We also consider the low energy realization of QCD in terms of mesons when an axial chemical potential is present; a situation that may be relevant in heavy ion collisions. We demonstrate that the presence of an axial charge has profound consequences on scalar/pseudoscalar meson physics. The most notorious effect is the appearance of an explicit source of parity breaking. The eigenstates of strong interactions do not have a definite parity and interactions that would otherwise be forbidden compete with the familiar ones. We focus on scalars and pseudoscalars that are described by a generalized linear sigma model. We comment briefly on the screening role of axial vectors in formation of effective axial charge and on the possible experimental relevance of our results, whose consequences may have been already seen at RHIC. Finally, we investigate how local parity breaking may affect vector physics. A modified dispersion relation is derived for the lightest vector mesons rho and omega. They are characterised by a mass splitting depending on their polarization. This effect predicts a natural overproduction of lepton pairs in the vicinity of the rho-omega resonance peak as well as a polarization asymmetry around this peak. The dilepton excess seems relevant to explain the anomalous dielectron yield quoted by PHENIX/STAR. We present a detailed analysis of the angular distribution associated to the lepton pairs created from these mesons searching for polarization dependencies. Two angular variables are found to carry the main information related to the parity-breaking effect. Possible signatures for experimental detection of local parity breaking are discussed.
En esta tesis estudiamos la posibilidad de que QCD rompa paridad a altas temperaturas y densidades, un escenario que se puede examinar en colisiones de iones pesados. Estudios analíticos con modelos efectivos sugieren que QCD puede romper paridad en sistemas densos. Además, pueden aparecer burbujas con P y CP impar en un volumen finito debido a grandes fluctuaciones topológicas locales en un medio caliente. Este último efecto puede ser tratado en una descripción cercana al equilibrio mediante un potencial químico axial. Consideramos el modelo de Nambu—Jona-Lasinio con ‘dos sabores’ en presencia de un potencial químico vectorial y uno axial y estudiamos la estructura de fases del modelo a temperatura cero. Estas magnitudes podrían abrir una fase donde la paridad podría romperse en QCD. También nos motivan algunos análisis en el retículo que intentan entender la naturaleza de la llamada fase de Aoki usando este modelo simplificado. Se discuten analogías y diferencias con el comportamiento esperado en QCD además de las limitaciones del modelo. Consideramos también la realización de QCD a bajas energías en términos de mesones en presencia de un potencial químico axial. Demostramos que la presencia de una carga axial tiene profundas consecuencias en la física de mesones escalares y pseudoescalares. Los autoestados de la interacción fuerte no tienen una paridad bien definida y en consecuencia, algunas interacciones antes prohibidas, ahora compiten con las más familiares. Nos centramos en escalares y pseudoescalares que se describen mediante un modelo sigma lineal generalizado. Finalmente, investigamos cómo la ruptura de paridad local puede afectar la física de mesones vectoriales. Se deriva una relación de dispersión modificada para los mesones más ligeros rho y omega. Este efecto predice una sobreproducción natural de parejas de leptones en las inmediaciones del pico de resonancia rho-omega así como una asimetría en la polarización alrededor de este pico. El exceso de dileptones parece ser relevante para explicar la producción anómala de dielectrones citada por PHENIX/STAR. Presentamos un análisis detallado de la distribución angular asociada a las parejas de leptones creadas de estos mesones en busca de dependencias en la polarización. Se encuentran dos variables angulares que transportan la información principal relacionada con el efecto de ruptura de paridad.

In this thesis we have studied how high energetic excitations propagate through a non- abelian strongly coupled plasma. This new state of matter is produced at heavy ion collisions in our accelerators and allows us to study a stage of the evolution of our Universe that occurred during the first microseconds after the Big Bang. In this extreme conditions of temperature and density the ordinary matter that we are made of behaves as a an almost perfect fluid, the most perfect known by mankind up to now in fact. The theory of strong interactions is tested at an energy scale that even though it is high enough to melt hadrons, it does not get to the point where the coupling constant is low enough to allow a perturbative description. In the plasma, the partonic field content, the quarks and gluons, cease to be the relevant degrees of freedom and a microscopic description in terms of quasi-particles is not possible. A very useful tool to put to test the actual behaviour of this strongly coupled fluid is the analysis of jet modifications as a result of their interactions with the plasma. In a first introductory part we have given the concepts needed to picture how heavy ion collisions develop as we are able to understand it today. At weak coupling, the main mechanism responsible for energy loss is induced gluon emission and interesting interference phenomena occur that lead to a dependence on path length of as the squared distance. These are known as coherence effects and their study becomes richer by considering multi-gluon emission, as it is done done in Part III. The strongly coupled picture uses holography to map a dressed excitation moving through a strongly coupled plasma into a string propagating in a higher dimensional space containing a black hole. Since the non-abelian theory in which the calculation is done is not QCD, but N = 4 SYM, we take these results as an insight to describe energetic parton propagation in a model of jet quenching in heavy ion collisions. Even though we assume that the exchanges with the medium are soft enough to include non-perturbative effects, as described by gauge/gravity duality, the energetic partons that are produced in the collision generally have a high virtuality which they relax by successive splittings. The latter occur at length scales that are not resolvable by the medium, and they should proceed as in vacuum. This observation motivates us to adopt a hybrid description for the interplay between the multi scale jet and the QGP, using each description at the scale it is supposed to be valid. This phenomenological description has proven to be very successful in describing dijet and photon-jet data at different centralities, and predictions have been made for a wide range of observables for the coming data from run 2 of LHC, including a new observable, the ratio of the fragmentation functions of the leading and subleading jet in a dijet pair, which is highly sensitive to the specific energy loss mechanism. In the next part of the work we extend our hybrid model by the inclusion of two effects, broadening and medium response, which should help us better describe intra-jet observables. The first effect, broadening, is due to the Brownian motion that probes experience in a thermal bath, and it will tend to broaden the distribution of particles within the jet. As it turns out, the observable quantifying such modifications, the jet shapes, are rather insensitive to the inclusion of this effect. However, by restricting the range of the tracks entering this analysis,we have been able to produce a new observable which shows a remarkable dependence on the precise strength of the broadening mechanism. The second effect involves overall energy-momentum conservation. The rapidly thermalized energy deposited by the energetic partons modifies the plasma, inducing temperature and velocity fluctuations in the surrounding fluid cells. This perturbation propagates long distances in the form of a wake and eventually decays into soft hadrons, whose orientations keep a correlation with the jet direction and therefore produce a net effect even after background subtraction. The observable consequences are best noticed in intra-jet measurements such as jet shapes and fragmentation functions, where it is clearly seen that the inclusion of such physics is in good agreement with the observed experimental trend, and it becomes simply unavoidable when comparisons against global measurements are performed. Finally, we compute the inclusive two gluon stimulated emission within the context of perturbative QCD. By studying the full answer in different kinematical limits we arrive to the conclusion that jet propagation is perceived from the point of view of the plasma as a set of effective emitters depending on the resolution power, which for a thin plasma it is of the order of the Debye screening mass. This physics is a missing piece of the Monte Carlo jet quenching model presented in this thesis and its inclusion is expected to have important consequences for the more differential observables, a task that will be undertaken in future work. These are very exciting times for the physics of strong nuclear interactions. We have seen how the very fundamental questions about the nature of the high temperature, strongly coupled phase of ordinary matter can be addressed by the study of jet quenching and its observable consequences. This thesis represents an effort in the confrontation of the seductive ideas of holography with experiments. Having the means to quantitatively confront new ideas, as we have done throughout the presented work, new observables, and new data is critical if we are eventually to understand the properties of the strongly coupled liquid quark-gluon plasma that Nature has served us.
En aquesta tesi estudiem com excitacions d’alta energia es propaguen a través d’un plasma no abelià fortament acoblat. Aquest nou estat de la matèria es produeix a les col·lisions de ions pesats als nostres acceleradors i permet l’estudi d’una etapa de l’evolució del nostre Univers que va tenir lloc durant els primers microsegons després del Big Bang. En aquestes condicions extremes de temperatura i densitat, la matèria ordinària de la qual estem fets es comporta com un fluid perfecte, de fet el més perfecte conegut per la humanitat a dia d’avui. La teoria de les interaccions fortes és posada a prova a una escala d’energia que tot i ser prou alta per fondre els hadrons, no arriba al punt en què la constant d’acoblament és prou feble com per permetre una descripció pertorbativa. Al plasma, el contingut en camps partonics, els quarks i els gluons, deixen de ser els graus de llibertat rellevants i una descripciò microscòpica en termes de quasi-partícules no és possible. Una eina molt útil per posar a prova l’autèntic comportament d’aquest fluid fortament acoblat és l’anàlisi de les modificacions dels jets com a resultat de la seva interacció amb el plasma. Tot i que assumim que els intercanvis amb el medi són prou suaus com per incloure efectes no pertorbatius, tal com descriu la dualitat gauge/gravetat, els partons energètics que es produeixen a la col·lisió generalment tenen virtualitats altes, les quals relaxen a través de particions successives. Aquestes últimes ocorren a escales de longitut que no són resolubles pel medi, i per tant haurien de procedir com en el buit. Aquesta observació ens motiva a adoptar una descripció híbrida per a la interacció entre el jet i el QGP, fent servir cada descripció a l’escala on es suposa que es vàlida. Aquesta descripció fenomenològica ha demostrat ser molt exitosa en la descripció de les dades de dijets i fotó-jet per diferents centralitats, i hem produït prediccions per a un ample ventall d'observables a l'energia del run 2 del LHC, incloent un nou observable molt sensible al mecanisme específic de pèrdua d'energia. També incorporem els efectes d'eixamplament i resposta del medi per descriure els observables intra-jet tot obtenint importants millores qualitatives en la descripció de les correlacions a llarg abast entre el jets i el plasma. Finalment, calculem l’emissió estimulada de dos gluons en el context de QCD pertorbativa. Estudiant la resposta completa en diferents límits arribem a la conclusió que des del punt de vista del plasma la propagació del jet és percebuda com una col·lecció d’emissors efectius depenenent del poder de resolució, el qual per un plasma prim és de l’ordre de l’invers de la massa d’apantallament de Debye. La seva inclusió en el model híbrid s’espera que tingui conseqüències importants pels observables més diferencials, una tasca que s’emprendrà en un treball futur.