arXiv subjects

Charles Gale

Publications and source records attributed to Charles Gale.

At least 109 records · Page 6Linked to original sources

arXiv2017

Hydrodynamic Predictions for Pb+Pb Collisions at 5.02 A TeV

Predictions and comparisons of hadronic flow observables for Pb+Pb collisions at 2.76 A TeV and 5.02 A TeV are presented using a hydrodynamics + hadronic cascade hybrid approach. Initial conditions are generated via a new formulation of the IP-Glasma model and then evolved using relativistic viscous hydrodynamics and finally fed into transport cascade in the hadronic phase. The results of this work show excellent agreement with the recent charged hadron anisotropic flow measurements from the ALICE collaboration of Pb+Pb collisions at 5.02 A TeV. Event-by-event distributions of charged hadron v n , flow event-plane correlations, and flow factorization breaking ratios are compared with existing measurements at 2.76 A TeV, and are predicted at 5.02 A TeV. Further predictions of identified hadron observables (for both light and multi-strange hadrons), such as p T -spectra and anisotropic flow coefficients, are presented.

hep-ph↗

arXiv2017

Multistage Monte-Carlo simulation of jet modification in a static medium

The modification of hard jets in an extended static medium held at a fixed temperature is studied using three different Monte-Carlo event generators (LBT, MATTER, MARTINI). Each event generator contains a different set of assumptions regarding the energy and virtuality of the partons within a jet versus the energy scale of the medium, and hence, applies to a different epoch in the space-time history of the jet evolution. For the first time, modeling is developed where a jet may sequentially transition from one generator to the next, on a parton-by-parton level, providing a detailed simulation of the space-time evolution of medium modified jets over a much broader dynamic range than has been attempted previously in a single calculation. Comparisons are carried out for different observables sensitive to jet quenching, including the parton fragmentation function and the azimuthal distribution of jet energy around the jet axis. The effect of varying the boundary between different generators is studied and a theoretically motivated criterion for the location of this boundary is proposed. The importance of such an approach with coupled generators to the modeling of jet quenching is discussed.

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arXiv2017

A Detailed Study and Synthesis of Flow Observables in the IP-Glasma+MUSIC+UrQMD Framework

In this work we use the IP-Glasma+MUSIC+UrQMD framework to systematically study a wide range of hadronic flow observables at 2.76 TeV. In addition to the single particle spectra and anisotropic flow coefficients $v_n$ previously studied in \cite{1609.02958}, we consider event-plane correlations, non-linear response coefficients $χ_{npq}$, and event shape engineering. Taken together, these observables provide a wealth of insight into the collective behavior of the QGP and initial state fluctuations. They shed light on flow fluctuations, flow at fixed system size but different initial geometries, as well as the non-linear hydrodynamic response to the initial state spatial eccentricities. By synthesizing this information we can gain further insight into the transport properties of the QGP as well as the fluctuation spectrum of the initial state.

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arXiv2017

Pre-equilibrium Longitudinal Flow in the IP-Glasma Framework for Pb+Pb Collisions at the LHC

In this work, we debut a new implementation of IP-Glasma and quantify the pre-equilibrium longitudinal flow in the IP-Glasma framework. The saturation physics based IP-Glasma model naturally provides a non-zero initial longitudinal flow through its pre-equilibrium Yang-Mills evolution. A hybrid IP-Glasma+MUSIC+UrQMD frame- work is employed to test this new implementation against experimental data and to make further predictions about hadronic flow observables in Pb+Pb collisions at 5.02 TeV. Finally, the non-zero pre-equilibrium longitudinal flow of the IP-Glasma model is quantified, and its origin is briefly discussed.

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arXiv2017

A hybrid approach to relativistic heavy-ion collisions at the RHIC BES energies

Using a hybrid (viscous hydrodynamics + hadronic cascade) framework, we model the bulk dynamical evolution of relativistic heavy-ion collisions at Relativistic Heavy Ion Collider (RHIC) Beam Energy Scan (BES) collision energies, including the effects from non-zero net baryon current and its dissipative diffusion. This framework is in full (3+1)D, which allows us to study the non-trivial longitudinal structure and dynamics of the collision systems, for example baryon stopping and transport, as well as longitudinal fluctuations. For the first time, the quantitative effect of net-baryon diffusion on hadronic observables is addressed. Finally, we propose a dynamical initialization scheme to study the importance of the pre-equilibrium stage at the RHIC BES energies.

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arXiv2017

Renormalization Group Flow, Stability, and Bulk Viscosity in a Large N Thermal QCD Model

The ultraviolet completion of a large N QCD model requires introducing new degrees of freedom at certain scale so that the UV behavior may become asymptotically conformal with no Landau poles and no UV divergences of Wilson loops. These UV degrees of freedom are represented by certain anti-branes arranged on the blown-up sphere of a warped resolved conifold in a way that they are separated from the other set of branes that control the IR behavior of the theory. This separation of the branes and the anti-branes creates instability in the theory. Further complications arise from the curvature of the ambient space. We show that, despite these analytical hurdles, stability may still be achieved by switching on appropriate world-volume fluxes on the branes. The UV degrees of freedom, on the other hand, modify the RG flow in the model. We discuss this in details by evaluating the flow from IR confining to UV conformal. Finally we lay down a calculational scheme to study bulk viscosity which, in turn, would signal the inherent non-conformality in this model.

hep-th↗

arXiv2017

Dilepton radiation and bulk viscosity in heavy-ion collisions

Starting from IP-Glasma initial conditions, we investigate the effects of bulk pressure on thermal dilepton production at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) energies. Though results of the thermal dilepton $v_2$ under the influence of both bulk and shear viscosity is presented for top RHIC energy, more emphasis is put on LHC energy where such a calculation is computed for the first time. The effects of the bulk pressure on thermal dilepton $v_2$ at the LHC are explored through bulk-induced modifications on the dilepton yield.

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arXiv2016

Rapidity-dependent jet energy loss in small systems with finite-size effects and running coupling

Longitudinal dynamics of particle production and rapidity-dependent jet energy loss are investigated in small and asymmetric colliding systems. We utilize an improved version of \textsc{martini} in which two improvements are implemented to calculate the effect of the strongly coupled QGP droplet on jet energy loss. We show that those realistic prescriptions improve the results of nuclear modification factor calculations. We also observe visible energy loss of jets in a thermal background of high-multiplicity p-Pb collisions, and a clear correlation between the energy loss and elliptic flow coefficients for energetic particles. We conclude that systematic measurements of jet quenching in central collisions of small systems can support the formation of the QGP droplet.

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arXiv2016

Electromagnetic radiation and collectivity in small quark-gluon droplets

We study the multiplicity and rapidity dependence of thermal and prompt photon production in p+Pb collisions at 5.02 TeV, using a (3+1)D viscous hydrodynamic framework. Direct photon anisotropic flow coefficients $v^γ_{2,3}$ and nuclear modification factor $R^γ_\mathrm{pPb}(p_T)$ are presented in both the p-going (backward) and the Pb-going (forward) directions. The interplay between initial state cold nuclear effect and final state thermal enhancement at different rapidity regions is discussed. The proposed rapidity dependent thermal photon enhancement and direct photon anisotropic flow observables can elucidate non-trivial longitudinal dynamics of hot quark-gluon plasma droplets created in small collision systems.

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arXiv2016

Bulk viscous corrections to photon production in the quark-gluon plasma

Photons radiated in heavy-ion collisions are a penetrating probe, and as such can play an important role in the determination of the quark-gluon plasma (QGP) transport coefficients. In this work we calculate the bulk viscous correction to photon production in two-to-two scattering reactions. Phase-space integrals describing the bulk viscous correction are evaluated explicitly in order to avoid the forward scattering approximation which is shown to be poor for photons at lower energies. We furthermore present hydrodynamical simulations of AA collisions focusing on the effect of this calculation on photonic observables. Bulk corrections are shown to reduce the elliptic flow of photons at higher $p_T$.

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arXiv2016

Collectivity and electromagnetic radiation in small systems

Collective behaviour has been observed in hadronic measurements of high multiplicity proton+lead collisions at the Large Hadron Collider (LHC), as well as in (proton, deuteron, helium-3)+gold collisions at the Relativistic Heavy-Ion Collider (RHIC). To better understand the evolution dynamics and the properties of the matter created in these small systems, a systematic study of the soft hadronic observables together with electromagnetic radiation from these collisions is performed, using a hydrodynamic framework. Quantitative agreement is found between theoretical calculations and existing experimental hadronic observables. The validity of the fluid dynamical description is estimated by calculating Knudsen and inverse Reynolds numbers. Sizeable thermal yields are predicted for low $p_T$ photons. Further predictions of higher order charged hadron anisotropic flow coefficients and of thermal photon enhancement are proposed.

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arXiv2016

The production of photons in relativistic heavy-ion collisions

In this work it is shown that the use of a hydrodynamical model of heavy ion collisions which incorporates recent developments, together with updated photon emission rates greatly improves agreement with both ALICE and PHENIX measurements of direct photons, supporting the idea that thermal photons are the dominant source of direct photon momentum anisotropy. The event-by-event hydrodynamical model uses IP-Glasma initial states and includes, for the first time, both shear and bulk viscosities, along with second order couplings between the two viscosities. The effect of both shear and bulk viscosities on the photon rates is studied, and those transport coefficients are shown to have measurable consequences on the photon momentum anisotropy.

hep-ph↗

arXiv2016

Electromagnetic radiation as a probe of the initial state and of viscous dynamics in relativistic nuclear collisions

The penetrating nature of electromagnetic signals makes them suitable probes to explore the properties of the strongly-interacting medium created in relativistic nuclear collisions. We examine the effects of the initial conditions and shear relaxation time on the spectra and flow coefficients of electromagnetic probes, using an event-by-event 3+1D viscous hydrodynamic simulation (MUSIC).

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arXiv2016

Direct photon production and jet energy-loss in small systems

Two types of penetrating probes, direct photon and QCD jets, are investigated in the background of a small and rapidly expanding droplet of quark-gluon plasma. The additional thermal electromagnetic radiation results in a $\sim$50\% enhancement of the direct photons. In high multiplicity p+Pb collisions, jets can lose a sizeable fraction of their initial energy, leading to a charged hadron $R_\mathrm{pA}$ of $\sim$0.8 at a transverse momentum around 10\,GeV. Those two proposed measurements can help understand the apparent collective behaviour observed in small collision systems.

hep-ph↗

arXiv2015

Dilepton emission in high-energy heavy-ion collisions with dissipative hydrodynamics

In this contribution we study the effects of three transport coefficients of dissipative hydrodynamics on thermal dilepton anisotropic flow observables. The first two transport coefficients investigated influence the overall size and growth rate of shear viscous pressure, while the last transport coefficient governs the magnitude of net baryon number diffusion in relativistic dissipative fluid dynamics. All calculations are done using state-of-the-art 3+1D hydrodynamical simulations. We show that thermal dileptons are sensitive probes of the transport coefficients of dissipative hydrodynamics.

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arXiv2015

Probing the dissipative properties of a strongly interacting medium with dileptons

We investigate the effects of the presence of a non-vanishing net baryon number density and its diffusion on dilepton production, within a hydrodynamical description of the medium created at $\sqrt{s_{NN}}=7.7$ GeV collision energy. This energy value is explored within the Beam Energy Scan (BES) program at Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory. Particular attention is devoted to a new dissipative degree of freedom: the net baryon number diffusion ($V^μ$), and to the net baryon number conductivity ($κ$) | a transport coefficients governing the overall magnitude of $V^μ$. The effects of $κ$ on dilepton production are assessed, with an outlook on how future experimental dilepton data can be used to learn more about $κ$.

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arXiv2015

Infrared Dynamics of a Large N QCD Model, the Massless String Sector and Mesonic Spectra

A consistency check for any UV complete model for large N QCD should be, among other things, the existence of a well-defined vector and scalar mesonic spectra. In this paper, we use our UV complete model in type IIB string theory to study the IR dynamics and use this to predict the mesonic spectra in the dual type IIA side. The advantage of this approach is two-fold: not only will this justify the consistency of the supergravity approach, but it will also give us a way to compare the IR spectra and the model with the ones proposed earlier by Sakai and Sugimoto. Interestingly, the spectra coming from the massless stringy sector are independent of the UV physics, although the massive string sector may pose certain subtleties regarding the UV contributions as well as the mappings to actual QCD. Additionally, we find that a component of the string landscape enters the picture: there are points in the landscape where the spectra can be considerably improved over the existing results in the literature. These points in the landscape in-turn also determine certain background supergravity components and fix various pathologies that eventually lead to a consistent low energy description of the theory.

hep-th↗

arXiv2015

The domain of validity of fluid dynamics and the onset of cavitation in ultrarelativistic heavy ion collisions

If the bulk viscosity of QCD matter is large, the effective pressure of the hot and dense matter created in ultrarelativistic heavy ion collisions can become negative, leading to instabilities in the evolution of the plasma. In the context of heavy ion collisions, this effect is sometimes referred to as cavitation. In this contribution we discuss the onset of cavitation in event-by-event hydrodynamic simulations of ultrarelativistic heavy ion collisions at LHC energies. We estimate how large the bulk viscosity of the QGP has to be in the QCD (pseudo) phase transition region in order for the effective pressure of the system to actually become negative.

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