arXiv subjects

Matthew Luzum

Publications and source records attributed to Matthew Luzum.

At least 73 records · Page 4Linked to original sources

arXiv2014

Probing the non-equilibrium dynamics of hot and dense QCD with dileptons

It is argued that, in heavy ion collisions, thermal dileptons are good probes of the transport properties of the medium created in such events, and also of its early-time dynamics, usually inaccessible to hadronic observables. In this work we show that electromagnetic azimuthal momentum anisotropies do not only display a sensitivity to the shear relaxation time and to the initial shear-stress tensor profile, but also to the temperature dependence of the shear viscosity coefficient.

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arXiv2014

Probing the early-time dynamics of relativistic heavy-ion collisions with electromagnetic radiation

Using 3+1D viscous relativistic fluid dynamics, we show that electromagnetic probes are sensitive to the initial conditions and to the out-of-equilibrium features of relativistic heavy-ion collisions. Within the same approach, we find that hadronic observables show a much lesser sensitivity to these aspects. We conclude that electromagnetic observables allow access to dynamical regions that are beyond the reach of soft hadronic probes.

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arXiv2014

Initial State Fluctuations and Final State Correlations in Relativistic Heavy-Ion Collisions

We review the phenomenology and theory of bulk observables in ultra-relativistic heavy-ion collisions, focussing on recent developments involving event-by-event fluctuations in the initial stages of a heavy ion collision, and how they manifest in observed correlations. We first define the relevant observables and show how each measurement is related to underlying theoretical quantities. Then we review the prevailing picture of the various stages of a collision, including the state-of-the-art modeling of the initial stages of a collision and subsequent hydrodynamic evolution, as well as hadronic scattering and freeze-out in the later stages. We then discuss the recent results that have shaped our current understanding and identify the challenges that remain. Finally, we point out open issues and the potential for progress in the field.

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arXiv2014

Transverse momentum structure of pair correlations as a signature of collective behavior in small collision systems

We perform 3+1D viscous hydrodynamic calculations of proton-lead and lead-lead collisions at top LHC energy. We show that existing data from high-multiplicity p-Pb events can be well described in hydrodynamics, suggesting that collective flow is plausible as a correct description of these collisions. However, a more stringent test of the presence of hydrodynamic behavior can be made by studying the detailed momentum dependence of two-particle correlations. We define a relevant observable, $r_n$, and make predictions for its value and centrality dependence if hydrodynamics is a valid description. This will provide a non-trivial confirmation of the nature of the correlations seen in small collision systems, and potentially to determine where the hydrodynamic description, if valid anywhere, stops being valid. Lastly, we probe what can be learned from this observable, finding that it is insensitive to viscosity, but sensitive to aspects of the initial state of the system that other observables are insensitive to, such as the transverse length scale of the fluctuations in the initial stages of the collision.

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arXiv2014

Constraining models of initial state with $v_2$ and $v_3$ data from LHC and RHIC

We present a combined analysis of elliptic and triangular flow data from LHC and RHIC using viscous relativistic hydrodynamics. Elliptic flow $v_2$ in hydrodynamics is proportional to the participant eccentricity $\varepsilon_2$ and triangular flow is proportional to the participant triangularity $\varepsilon_3$, which means $v_n=C_n\varepsilon_n$, where $C_n$ is the linear response coefficient in harmonic n. Experimental data for $v_2$ and $v_3$ combined with hydrodynamic calculations of $C_n$ thus provide us with the rms values of initial anisotropies $\varepsilon_2$ and $\varepsilon_3$. By varying free parameters in the hydro calculation (in particular the shear viscosity), we obtain an allowed band in the (rms $\varepsilon_2$, rms $\varepsilon_3$) plane. Comparison with Monte-Carlo models of the initial state allows us to exclude several of these models. We illustrate that the effect of changing the granularity of the initial state is similar to changing the medium properties, making these effects difficult to disentangle.

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arXiv2014

Constraining models of initial conditions with elliptic and triangular flow data

We carry out a combined analysis of elliptic and triangular flow data using viscous relativistic hydrodynamics. We show that these data allow to put tight constraints on models of the early dynamics of a nucleus-nucleus collision. Specifically, the rms values of the initial ellipticity $\varepsilon_2$ and the initial triangularity $\varepsilon_3$ are constrained to lie within a narrow band for each centrality. We use these constraints as a filter for existing Monte-Carlo models of initial state, and provide a simple test that can be performed on any candidate model to determine its compatibility with data.

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arXiv2013

Solutions of Conformal Israel-Stewart Relativistic Viscous Fluid Dynamics

We use symmetry arguments developed by Gubser to construct the first radially-expanding explicit solutions of the Israel-Stewart formulation of hydrodynamics. Along with a general semi-analytical solution, an exact analytical solution is given which is valid in the cold plasma limit where viscous effects from shear viscosity and the relaxation time coefficient are important. The radially expanding solutions presented in this paper can be used as nontrivial checks of numerical algorithms employed in hydrodynamic simulations of the quark-gluon plasma formed in ultra-relativistic heavy ion collisions. We show this explicitly by comparing such analytic and semi-analytic solutions with the corresponding numerical solutions obtained using the MUSIC viscous hydrodynamics simulation code.

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arXiv2013

Eliminating experimental bias in anisotropic-flow measurements of high-energy nuclear collisions

We argue that the traditional event-plane method, which is still widely used to analyze anisotropic flow in ultrarelativistic heavy-ion collisions, should be abandoned because flow fluctuations introduce an uncontrolled bias in the measurement. Instead, one should use an alternative, such as the scalar-product method or cumulant method, which always measures an unambiguous property of the underlying anisotropic flow and therefore eliminates this bias, and does so without any disadvantages. It is known that this correction is important for precision comparisons of traditional v_n measurements requiring better than a few percent accuracy. However, we show that it is absolutely essential for correlations between different harmonics, such as those that have been recently measured by the ATLAS Collaboration, which can differ from the nominally-measured quantity by a factor two or more. We also describe how, using the corrected analysis method, the information from different subevents can be combined in order to optimize the precision of analyses.

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arXiv2013

Directed flow at midrapidity in sqrt(s_NN)=2.76 TeV Pb+Pb collisions

We analyze published data from the ALICE Collaboration in order to obtain the first extraction of the recently-proposed rapidity-even directed flow observable v1. An accounting of the correlation due to the conservation of transverse momentum restores the factorization seen by ALICE in all other Fourier harmonics and thus indicates that the remaining correlation gives a reliable measurement of directed flow. We then carry out the first viscous hydrodynamic calculation of directed flow, and show that it is less sensitive to viscosity than higher harmonics. This allows for a direct extraction of the dipole asymmetry of the initial state, providing a strict constraint on the non-equilibrium dynamics of the early-time system. A prediction is then made for v1 in Au-Au collisions at RHIC.

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arXiv2013

Breaking of factorization of two-particle correlations in hydrodynamics

The system formed in ultrarelativistic heavy-ion collisions behaves as a nearly-perfect fluid. This collective behavior is probed experimentally by two-particle azimuthal correlations, which are typically averaged over the properties of one particle in each pair. In this Letter, we argue that much additional information is contained in the detailed structure of the correlation. In particular, the correlation matrix exhibits an approximate factorization in transverse momentum, which is taken as a strong evidence for the hydrodynamic picture, while deviations from the factorized form are taken as a signal of intrinsic, "nonflow" correlations. We show that hydrodynamics in fact predicts factorization breaking as a natural consequence of initial state fluctuations and averaging over events. We derive the general inequality relations that hold if flow dominates, and which are saturated if the matrix factorizes. For transverse momenta up to 5 GeV, these inequalities are satisfied in data, but not saturated. We find at least as large factorization breaking in event-by-event ideal hydrodynamic calculations as in data, and argue that this phenomenon opens a new window on the study of initial fluctuations.

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arXiv2013

Anisotropic flow in event-by-event ideal hydrodynamic simulations of sqrt(s_{NN})=200 GeV Au+Au collisions

We calculate flow observables with the NeXSPheRIO ideal hydrodynamic model and make the first comparison to the complete set of mid-rapidity flow measurements made by the PHENIX collaboration in top energy Au+Au collisions. A simultaneous calculation of v_2, v_3, v_4, and the first event-by-event calculation of quadrangular flow defined with respect to the v_2 event plane (v_4{Psi_2}) gives good agreement with measured values, including the dependence on both transverse momentum and centrality. This provides confirmation that the collision system is indeed well described as a quark-gluon plasma with an extremely small viscosity, and that correlations are dominantly generated from collective effects. In addition we present a prediction for v_5.

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arXiv2012

Initial state fluctuations and final state correlations: Status and open questions

The recent appreciation of the importance of event-by-event fluctuations in relativistic heavy-ion collisions has lead to a large amount of diverse theoretical and experimental activity. In particular, there is significant interest in understanding the fluctuations in the initial stage of a collision, how exactly these fluctuations are propagated through the system evolution, and how they are manifested in correlations between measured particles. In order to address these questions a workshop was organized on "Initial State Fluctuations and Final State Correlations", held at ECT* in Trento, Italy during the week of 2--6 July, 2012. The goal was to collect recent work in order to provide a coherent picture of the current status of our understanding, to identify important questions that remain open, and to set a course for future research. Here we report the outcome of the presentations and discussions, focusing on the most important conclusions.

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arXiv2012

Characterizing the hydrodynamic response to the initial conditions

In hydrodynamics, the momentum distribution of particles at the end of the evolution is completely determined by initial conditions. We study quantitatively to what extent anisotropic flow is determined by predictors such as the initial eccentricity in a set of realistic simulations, and we also show the importance of nonlinear terms in order to correctly predict the quadrangular flow. This knowledge will be important for making a more direct link between experimental observables and hydrodynamic initial conditions, the latter being poorly constrained at present.

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arXiv2012

Extracting the shear viscosity of the quark-gluon plasma from flow in ultra-central heavy-ion collisions

We propose a method for extracting the shear viscosity over entropy density ratio (eta/s) of the quark-gluon plasma from experimental data. We argue that uncertainty due to poor knowledge of the earliest stages of a heavy-ion collision is smallest for ultra-central events. The most precise value of eta/s can thus be obtained from a global fit to p_T-integrated Fourier harmonics of azimuthal correlations. We further outline a method for quantifying the overall uncertainty in the extracted value. Only after a comprehensive and systematic accounting of all sources of uncertainty can a reliable measurement be claimed. In these proceedings we report preliminary results; full and final results will be presented in a separate publication.

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arXiv2012

New flow observables

Event-by-event fluctuations of the initial transverse density profile result in a collective flow pattern which also fluctuates event by event. We propose a number of new correlation observables to characterize these fluctuations and discuss how they should be analyzed experimentally. We argue that most of these quantities can be measured at RHIC and LHC.

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arXiv2012

Determining initial-state fluctuations from flow measurements in heavy-ion collisions

We present a number of independent flow observables that can be measured using multiparticle azimuthal correlations in heavy-ion collisions. Some of these observables are already well known, such as v2{2} and v2{4}, but most are new--in particular, joint correlations between v1, v2 and v3. Taken together, these measurements will allow for a more precise determination of the medium properties than is currently possible. In particular, by taking ratios of these observables, we construct quantities which are less sensitive to the hydrodynamic response of the medium, and thus more directly characterize the initial-state fluctuations of the event shape, which may constrain models for early-time, non-equilibrium QCD dynamics. We present predictions for these ratios using two Monte-Carlo models, and compare to available data.

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arXiv2012

Mapping the hydrodynamic response to the initial geometry in heavy-ion collisions

We investigate how the initial geometry of a heavy-ion collision is transformed into final flow observables by solving event-by-event ideal hydrodynamics with realistic fluctuating initial conditions. We study quantitatively to what extent anisotropic flow (v_n) is determined by the initial eccentricity epsilon_n for a set of realistic simulations, and we discuss which definition of epsilon_n gives the best estimator of v_n. We find that the common practice of using an r^2 weight in the definition of varepsilon_n in general results in a poorer predictor of v_n than when using r^n weight, for n > 2. We similarly study the importance of additional properties of the initial state. For example, we show that in order to correctly predict v_4 and v_5 for non-central collisions, one must take into account nonlinear terms proportional to (epsilon_2)^2 and (epsilon_2)*(epsilon_3), respectively. We find that it makes no difference whether one calculates the eccentricities over a range of rapidity, or in a single slice at z=0, nor is it important whether one uses an energy or entropy density weight. This knowledge will be important for making a more direct link between experimental observables and hydrodynamic initial conditions, the latter being poorly constrained at present.

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arXiv2011

Understanding anisotropy generated by fluctuations in heavy-ion collisions

Event-by-event fluctuations are central to the current understanding of ultrarelativistic heavy-ion collisions. In particular, fluctuations in the geometry of the early-time collision system are responsible for new phenomena such as triangular flow, which have solved important puzzles in existing data. We propose a simple model where initial fluctuations stem from independent flux tubes randomly distributed in the transverse plane. We calculate analytically the moments of the initial anisotropies (dipole asymmetry, eccentricity, triangularity), which are the sources of anisotropic flow, and their mutual correlations. Our analytic results are in good agreement with calculations from commonly-used Monte-Carlo codes, providing a simple understanding of the fluctuations contained in these models. Any deviation from these results in future experimental data would thus indicate the presence of non-trivial correlations between the initial flux tubes and/or extra sources of fluctuations that are not present in current models.

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