Estudo hidrodinâmico de correlações de partículas e fluxo coletivo em colisões de íons pesados relativísticos
Arquivos
Data
2019-09-12
Autores
Wen, Dan [UNESP]
Título da Revista
ISSN da Revista
Título de Volume
Editor
Universidade Estadual Paulista (Unesp)
Resumo
O sucesso da descrição hidrodinâmica das colisões de íons pesados relativísticos desempenha um papel vital para entender as propriedades da matéria QCD. A essência da evolução hidrodinâmica, em geral, foi atribuída à resposta dinâmica às condições iniciais flutuantes. Em particular, as características observadas nas correlações de duas partículas, referidas como ``cume'' e ``ombro'', mostraram ser reproduzidas com sucesso por simulações hidrodinâmicas com condições iniciais flutuantes evento a evento, mas não por condições iniciais médias. Posteriormente, leva ao entendimento atual, através de extensos estudos de análise hidrodinâmica/transporte baseada em eventos por eventos, que as correlações de duas partículas para o momento transversal inferior podem ser interpretadas principalmente em termos de harmônicos de fluxo $ v_n $. Notavelmente, o fluxo triangular, $ v_3 $, é atribuído principalmente à aparência da estrutura do ``ombro'' no lado externo da partícula acionadora. Além disso, entende-se que esses coeficientes harmônicos estão intimamente associados aos correspondentes $ \varepsilon_n $, as anisotropias da distribuição inicial de energia. No entanto, a linearidade entre $ v_n $ e $ \varepsilon_n $ se torna menos evidente para harmônicos maiores que $ n = 2 $. Isso sugere que as próprias flutuações de evento a evento carregam informações importantes, além da linearidade observada. Se alguém se restringe apenas à análise das relações/correlações médias de eventos entre $ v_n $ e $ \varepsilon_n $, então alguns sinais hidrodinâmicos genuínos das flutuações locais em cada evento individual podem ser desbotados ou ocultos por trás de algumas correlações muito complicadas entre os harmônicos. Portanto, esperamos explorar de um ângulo alternativo que possa explicar de maneira simples a origem física do padrão de fluxo anisotrópico. Nesta tese, estudamos as correlações de duas partículas em relação a um modelo de tubo periférico, entre os outros. De nossa perspectiva, as principais características das correlações de duas partículas observadas são atribuídas às flutuações de multiplicidade e à distribuição de uma partícula perturbada localmente. A última é associada à resposta hidrodinâmica às flutuações geométricas nas condições iniciais. Investigamos as propriedades das condições iniciais e do fluxo coletivo em relação ao modelo proposto. É mostrado que os dados experimentais podem ser reproduzidos por simulações hidrodinâmicas usando condições iniciais adequadamente construídas. Além disso, em vez de calibração numérica, extraímos os parâmetros do modelo de acordo com suas respectivas interpretações físicas e mostramos que os valores numéricos obtidos estão de fato qualitativamente de acordo com os dados observados. Além disso, como a hidrodinâmica é conhecida por suas características altamente não lineares, vários estudos foram realizados para explorar esse aspecto. Em particular, muitos esforços foram dedicados à relação entre excentricidades iniciais do estado e anisotropias do estado final. No contexto da descrição hidrodinâmica evento a evento, analisamos as implicações para dois modelos caracterizados por condições iniciais distintas. A densidade de energia inicial do primeiro modelo adota uma distribuição do tipo Gaussiana, enquanto as do segundo modelo são características de tubos periféricos de alta energia. Calibramos as condições iniciais de ambos os modelos para que suas excentricidades iniciais sejam praticamente idênticas. As distribuições escalonadas de probabilidade do fluxo coletivo e as correlações entre os coeficientes harmônicos e excentricidades do fluxo são investigadas. Além disso, os cálculos são realizados para correlações de partículas em relação aos cumulantes simétricos, coeficientes de resposta não lineares. Embora as correlações resultantes de duas partículas possuam formas aparentemente semelhantes, os cálculos numéricos indicam uma diferença substancial entre os dois modelos. Para ser específico, a diferença reside em observáveis mais detalhados, como o coeficiente de correlação de Pearson entre harmônicos de ordem superior. Discutimos vários aspectos essenciais relativos à linearidade e não linearidade entre excentricidades iniciais e anisotropias no estado final. Implicações adicionais são abordadas.
The success of the hydrodynamic description of relativistic heavy-ion collisions plays a vital part in our ongoing endeavor to understand the properties of QCD matter. The essence of hydrodynamical evolution, generally, has been attributed to the dynamic response to fluctuating initial conditions. For instance, the observed features in two-particle correlations referred to as ``ridge'' and ``shoulders'', were shown to be successfully reproduced by hydrodynamical simulations with event-by-event fluctuating initial conditions but not by averaged initial conditions. Subsequently, it leads to the current understanding through extensive studies of event-by-event based hydrodynamic/transport analysis. In other words, the two-particle correlations for the lower transverse momenta can be mostly interpreted in terms of flow harmonics $v_n$. In particular, the triangular flow, $v_3$, is mostly attributed to for the appearance of the ``shoulder'' structure on the away side of the trigger particle. Moreover, it is understood that the flow harmonics are closely associated with the corresponding $\varepsilon_n$, the anisotropies of the initial energy distribution. However, numerical calculations have demonstrated that the linearity between $v_n$ and $\varepsilon_n$ become less evident for harmonics higher than $n=2$. This suggests that the event-by-event fluctuations themselves carry essential information besides the observed linearity. To be specific, if one restricts himself only to the analysis of the event-averaged relations/correlations among $v_n$ and $\varepsilon_n$, then some genuine hydrodynamic signals from the local fluctuations in each individual event might be washed out, or hidden behind some very complicated correlations among the harmonics. Therefore, we hope to explore from an alternative angle, which may explain in a simple way the physical origin of the anisotropic flow pattern. In this thesis, we study the collective flow, particle correlations, and nonlinear response in terms of, among others, a peripheral tube model. From our perspective, the main characteristics of the observed two-particle correlations are attributed to the multiplicity fluctuations and the locally disturbed one-particle distribution. The latter is associated with the hydrodynamic response to the geometric fluctuations in the initial conditions. Also, we investigate the properties of the initial conditions and collective flow concerning the proposed model. It is shown that the experimental data can be reproduced by hydrodynamical simulations using appropriately constructed initial conditions. Besides, instead of numerical calibration, we extract the model parameters according to their respective physical interpretations and show that the obtained numerical values are indeed qualitatively in agreement with the observed data. Moreover, as hydrodynamics is known for its high nonlinearity, various studies have been carried out to explore this aspect. In particular, many efforts have been devoted to the relationship between initial state eccentricities and final state anisotropies. In the context of event-by-event hydrodynamic description, we analyze two different models characterized by distinct initial conditions. The initial energy density of the first model adopts a Gaussian-type distribution, while those of the second model are features by high energy peripheral tubes. We calibrate the initial conditions of both models so that their initial eccentricities are mostly identical. The scaled probability distributions of collective flow and the correlations between flow harmonic and eccentricity coefficients are investigated. Besides, the calculations are carried out for particle correlations regarding the symmetric cumulant, nonlinear response coefficients. Although the resultant two-particle correlations possess seemingly similar shapes, numerical calculations indicate a substantial difference between the two models. To be specific, the difference resides in more detailed observables such as the Pearson correlation coefficient between higher-order harmonics. We discuss several essential aspects concerning the linearity and nonlinearity between initial eccentricities and final state anisotropies. Further implications are addressed.
The success of the hydrodynamic description of relativistic heavy-ion collisions plays a vital part in our ongoing endeavor to understand the properties of QCD matter. The essence of hydrodynamical evolution, generally, has been attributed to the dynamic response to fluctuating initial conditions. For instance, the observed features in two-particle correlations referred to as ``ridge'' and ``shoulders'', were shown to be successfully reproduced by hydrodynamical simulations with event-by-event fluctuating initial conditions but not by averaged initial conditions. Subsequently, it leads to the current understanding through extensive studies of event-by-event based hydrodynamic/transport analysis. In other words, the two-particle correlations for the lower transverse momenta can be mostly interpreted in terms of flow harmonics $v_n$. In particular, the triangular flow, $v_3$, is mostly attributed to for the appearance of the ``shoulder'' structure on the away side of the trigger particle. Moreover, it is understood that the flow harmonics are closely associated with the corresponding $\varepsilon_n$, the anisotropies of the initial energy distribution. However, numerical calculations have demonstrated that the linearity between $v_n$ and $\varepsilon_n$ become less evident for harmonics higher than $n=2$. This suggests that the event-by-event fluctuations themselves carry essential information besides the observed linearity. To be specific, if one restricts himself only to the analysis of the event-averaged relations/correlations among $v_n$ and $\varepsilon_n$, then some genuine hydrodynamic signals from the local fluctuations in each individual event might be washed out, or hidden behind some very complicated correlations among the harmonics. Therefore, we hope to explore from an alternative angle, which may explain in a simple way the physical origin of the anisotropic flow pattern. In this thesis, we study the collective flow, particle correlations, and nonlinear response in terms of, among others, a peripheral tube model. From our perspective, the main characteristics of the observed two-particle correlations are attributed to the multiplicity fluctuations and the locally disturbed one-particle distribution. The latter is associated with the hydrodynamic response to the geometric fluctuations in the initial conditions. Also, we investigate the properties of the initial conditions and collective flow concerning the proposed model. It is shown that the experimental data can be reproduced by hydrodynamical simulations using appropriately constructed initial conditions. Besides, instead of numerical calibration, we extract the model parameters according to their respective physical interpretations and show that the obtained numerical values are indeed qualitatively in agreement with the observed data. Moreover, as hydrodynamics is known for its high nonlinearity, various studies have been carried out to explore this aspect. In particular, many efforts have been devoted to the relationship between initial state eccentricities and final state anisotropies. In the context of event-by-event hydrodynamic description, we analyze two different models characterized by distinct initial conditions. The initial energy density of the first model adopts a Gaussian-type distribution, while those of the second model are features by high energy peripheral tubes. We calibrate the initial conditions of both models so that their initial eccentricities are mostly identical. The scaled probability distributions of collective flow and the correlations between flow harmonic and eccentricity coefficients are investigated. Besides, the calculations are carried out for particle correlations regarding the symmetric cumulant, nonlinear response coefficients. Although the resultant two-particle correlations possess seemingly similar shapes, numerical calculations indicate a substantial difference between the two models. To be specific, the difference resides in more detailed observables such as the Pearson correlation coefficient between higher-order harmonics. We discuss several essential aspects concerning the linearity and nonlinearity between initial eccentricities and final state anisotropies. Further implications are addressed.
Descrição
Palavras-chave
modelo hidrodinâmico, correlação de partículas, harmônicas de fluxo, hydrodynamical model, particle correlation, flow harmonics, Hidrodinâmica, Partículas (Física nuclear), Flutuação (Física)