arXiv subjects

Sukanya Mitra

Publications and source records attributed to Sukanya Mitra.

At least 19 recordsLinked to original sources

arXiv2026

Hydrodynamics without a relaxation gap: memory effects, nonlocality, and superdiffusion

By studying a simple model of relativistic particles propagating through a background medium with an energy-dependent relaxation time that is unbounded from above, we investigate how long-term memory obstructs the emergence of local hydrodynamics in systems with a gapless non-hydrodynamic sector. In the RTA matching frame, we show that the full gradient expansion is generically divergent in most flows, even for Fourier modes, and that any resummation necessarily retains an infinite set of slow non-hydrodynamic degrees of freedom. The divergence of the gradient expansion therefore reflects a more fundamental breakdown of hydrodynamic locality caused by persistent non-hydrodynamic memory. We finally show that sufficiently singular relaxation spectra can invalidate ordinary diffusion itself. In these regimes, the diffusivity diverges, and the late-time dynamics become superdiffusive, governed by intrinsically nonlocal constitutive relations.

nucl-th↗

arXiv2026

Poles from the conserved kinetic equation: The emerging gradient structure and causality riddle of relativistic hydrodynamics

In this work, the poles and the resulting dispersion spectra from the relativistic kinetic equation have been analyzed with the help of a proposed collision kernel that conserves both the energy-momentum tensor and particle current by construction. The dispersion relations, which originally come out in the form of logarithmic divergences, in the long wavelength limit exhibit the systematic gradient structure of the relativistic hydrodynamics. The key result is that, in the derivative expansion series, the spatial gradients appear in perfect unison with the temporal gradients in the non-local relaxation operator like forms. It is then shown that this dispersion structure, including non-local temporal derivatives, is essential for the preservation of causality of the theory truncated at any desired order.

nucl-th↗

arXiv2026

Relativistic Dispersion Spectra across Lorentz boosted frames: Spurious modes and the enigma of causality

The analysis of excitation spectra in gradient-expanded relativistic fluid theories frequently leads to pathologies under Lorentz boosts. However, extracting the dispersion modes in a Lorentz boosted inertial frame can be nontrivial. Motivated by this problem, we develop a general framework for deriving the linearized dispersion spectra in Lorentz boosted frames using only information from the local rest-frame dispersion structure, particularly its mode-expansion coefficients. We observe that, under a Lorentz transformation from the local rest frame, additional boosted solutions may appear that conflict with the causality of the theory; we refer to these as "spurious modes." The key developments presented here are: (i) a convenient method for obtaining dispersion spectra across inertial frames, bypassing the traditional procedure of solving the boosted polynomial, and (ii) the establishment of a direct connection between mode conservation and the causality of a theory, supported by a detailed proof.

hep-th↗

arXiv2026

Necessary conditions for causality from linearized stability at ultra-high boosts

In this work, we provide a novel method to constrain the causal parameter space of a relativistic hydrodynamic system exclusively from its linear stability analysis at non-zero momenta. Our approach exploits the Lorentz-invariant stability property of causal theories. In boosted frames, the dispersion relation exhibits a feature that we call ``$γ$-suppression,'' whereby the higher-order terms in the wavenumber expansion are increasingly suppressed beyond leading order at large boosts. As a consequence, at near-luminal values of Lorentz boost, stability criteria at the spatially homogeneous limit are sufficient to identify the region of the parameter space that satisfies the necessary conditions of causality, even at non-zero momenta. After presenting the general hydrodynamic framework, we test the method in conformal Müller-Israel-Stewart theory and show that it provides an efficient way of deriving the necessary conditions of causality while remaining within the low-energy regime of hydrodynamic validity.

hep-th↗

arXiv2025

On the Approach Towards Equilibrium Through Momentum-Dependent Relaxation:Insights from Evolution of the Moments in Kinetic Theory

We investigate the impact of momentum-dependent relaxation time approximation in the Boltzmann equation within the Bjorken flow framework by analyzing the moments of the single-particle distribution function. The moment equations, which form an infinite hierarchy, provide important insights about the system dynamics and the approach towards equilibrium for systems far from equilibrium. The momentum-dependent collision kernel couples moments through both the energy exponents and the angular dependence via various-order Legendre polynomials, resulting in an intricate system of infinitely coupled equations. A naive truncation of the coupled equations results in diverging moments at late times. We outline strategies for solving the coupled system, including a novel approach for managing the divergences and non-integer moments. We show a significant influence of momentum dependent relaxation time on the time evolution of the moments, particularly for higher-order moments and system with smaller shear viscosity over entropy density, emphasizing the importance of incorporating such dependence for a more accurate description of the system dynamics with low shear viscosity such as the quark-gluon-plasma produced in high-energy heavy-ion collisions.

nucl-th↗

arXiv2025

Causality and Stability in Relativistic Hydrodynamics

The causality and stability of a relativistic hydrodynamic theory is shown to require a consensus between, either (i) newer degrees of freedom apart from the fundamental fluid fields, or (ii) a general hydrodynamic frame other than the Landau or Eckart compromising the field's first principle definition, unless the non-equilibrium derivative correction goes to infinity. Any finitely truncated derivative correction (no matter how high it is) is shown to lead to an acausal theory, unless the corrections are infinitely summed up to include all orders. From an underlying microscopic theory, an exact form of relativistic hydrodynamics has been derived which establishes that the resummation of all order temporal derivatives is essential for causality, which finally 'integrated in' as newer degrees of freedom.

nucl-th↗

arXiv2025

Transport coefficients of hot and dense matter in relativistic heavy-ion collisions

The transport coefficients are known as the measure of system interactions, as well as the dynamical input of the hydrodynamic evolution equations of an expanding system created in the relativistic heavy ion collisions. In the current analysis, they have been evaluated for strongly interacting quark-gluon plasma system as well as hot hadronic media. The medium effects for an interacting QCD system have been introduced through a quasiparticle model. The same for a hadronic system has been captured via the in-medium self-energy correction using thermal field theory. The resulting behavior of the transport coefficients shows significant modification with respect to the estimations made in vacuum or using ideal equation of state.

nucl-th↗

arXiv2025

Field redefinition and its impact in relativistic hydrodynamics

In this paper, we explore the impact of field redefinition on the spectrum of linearized perturbations in relativistic hydrodynamics. We observe that the spectrum of hydrodynamics modes is never affected by the local field redefinition, however, the spectrum of the non-hydrodynamic modes is affected. Through an appropriate all-order redefinition, non-hydrodynamic modes can be eliminated, leading to a new frame where the spectrum contains only hydrodynamic modes. We also observe that the resulting stress-energy tensor may have an infinite series in momentum space, with a convergence radius linked to the eliminated non-hydrodynamic mode. In certain special cases, higher-order terms in the stress-energy tensor under field redefinition may cancel, indicating that non-hydrodynamic modes are mere artefacts of the fluid variable choice and hold no physical significance, even if they appear to violate physical constraints. Using a special toy example, we find a criterion to distinguish between physical and unphysical non-hydrodynamic modes.

nucl-th↗

arXiv2024

Why do newer degrees of freedom appear in higher-order truncated hydrodynamic theory?

An exact derivation of relativistic hydrodynamics from an underlying microscopic theory has been shown to be an all-order theory. From the relativistic transport equation of kinetic theory, the full expressions of hydrodynamic viscous fluxes have been derived which turn out to include all orders of out-of-equilibrium derivative corrections. It has been shown, that for maintaining causality, it is imperative that the temporal derivatives must include all orders, which can be resummed in non-local, relaxation operator-like forms and finally `integrated in' introducing newer degrees of freedom. The theory can of course be truncated at any higher spatial orders, but the power over the the infinite temporal sum increases correspondingly such that the causality is respected. As a result, the theory truncated at any higher order of spatial gradient, requires newer degrees of freedom for each increasing order.

nucl-th↗

arXiv2024

Relativistic fluid dynamics in a 'hydro' frame

In this letter, we investigate how field redefinition influences the spectrum of linearized perturbations in relativistic fluid dynamics. We show that the hydrodynamic modes do not get affected under local field redefinition, whereas the non-hydrodynamic modes do. These non-hydrodynamic modes can be removed through a suitable all-order field redefinition. This process leads to a new frame containing only hydrodynamic modes, which we refer to as the hydro frame. Additionally, we demonstrate that the resulting stress-energy tensor may constitute an infinite series in momentum space, with the radius of convergence associated with the removed non-hydrodynamic mode, highlighting its role in the hydrodynamic expansion's validity.

nucl-th↗

arXiv2024

Frame transformation and stable-causal hydrodynamic theory

In this work, a connection has been indicated between the different existing formulations of relativistic hydrodynamic theories, which, in order to be causal and stable, (i) either requires `non-fluid' variables apart from velocity and temperature to be promoted to new degrees of freedom, or, (ii) needs to be in a generalized hydrodynamic frame other than those given by Landau or Eckart. The BDNK stress tensor (originally in a general frame) has been rewritten in the Landau frame using linearized all-order gradient-corrected redefinitions of the temperature and velocity fields. The redefinitions indicate that, while the BDNK formalism has a finite number of derivatives in the general frame, when written in the Landau frame, it either has an infinite number of derivatives, or one has to introduce MIS-like `non-fluid' variables by summing the infinite number of derivatives in the field redefinitions. There can be non-unique ways of performing these infinite-order summations. Finally, the dispersion relations and the corresponding spectra of these different systems of MIS type equations have been analyzed to check that the systems of equations presented here are indeed equivalent to the BDNK formalism, at least in the hydrodynamic regime.

nucl-th↗

arXiv2024

Causality and stability in relativistic hydrodynamic theory -- a choice to be endured

In this work, it has been indicated that the key features requisite for preserving causality and stability of the popularly existing relativistic hydrodynamic theories, can be translated into each other. It has been shown here, that a generic `fluid frame transformation' including all orders of gradient corrections can recast a stable-causal hydrodynamic theory that (i) only includes fundamental fluid variables (velocity and temperature) but requires to be in a general hydrodynamic frame other than the Landau or Eckart, to a theory that is (ii) pathology free in Landau frame but needs newer degrees of freedom. Since frame choice provides the first principle field definitions and degrees of freedom indicate the number of conserved quantities, the causality and stability of a theory seem to require a consensus between the two unless the derivative correction goes to infinity. The key finding of this work is to indicate a connection between these different formalisms, that lead to a causal and stable hydrodynamic theory.

nucl-th↗

arXiv2024

Causality Criteria from Stability Analysis at Ultra-High Boost

In this work, we have exclusively employed the linear stability analysis at ultra-high boost on two well-known stable-causal theories - second-order MIS and first-order BDNK, to identify the region of parameter space over which they are frame-invariantly stable and obey causal signal propagation. It has been shown that at near-luminal boost, stability criteria alone can provide the causality constraints on transport coefficients, which are identical to the asymptotic causality conditions, without actually going to the asymptotic limit of the theories. Thus, we present an alternative approach to derive the causality constraints, which is more appropriate for low-energy effective theories like relativistic hydrodynamics.

hep-th↗

arXiv2024

Acausality in truncated 'MIS'-type theory

The causal-stable Muller-Israel-Stewart (MIS) theory is known to have a finite number of out of equilibrium derivative order corrections but requires treating the viscosity tensor as a separate degree of freedom with its own equations of motion, apart from the fundamental fluid degrees of freedom like velocity and temperature. In this work, I will show that it is possible to rewrite the MIS theory only in terms of velocity and temperature, but the resultant constitutive relation for dissipation must include all orders of gradient corrections. In this work I will argue that an all-order resummation of gradient contributions is equivalent to introducing new `non-fluid' degrees of freedom in the MIS theory. It will also be shown, using the relativistic quantum causality condition, that any finitely truncated order of derivative correction, however high it is, leads to a theory that is acausal, unless the corrections are infinitely summed up to all orders.

gr-qc↗

arXiv2023

An expedition to the islands of stability in the first-order causal hydrodynamics

The recently proposed connection between the Lorentz invariance of stability and the speed of signal propagation has been tested for a first-order relativistic dissipative hydrodynamic theory. The fact that the stability situation in different reference frames agrees with each other only as long as the signal propagation respects causality, has been explicitly established for the theory, which is microscopically derived from the covariant kinetic equation in general hydrodynamic frames with arbitrary momentum-dependent interactions.

nucl-th↗

arXiv2022

Dynamics of Hot QCD Matter -- Current Status and Developments

The discovery and characterization of hot and dense QCD matter, known as Quark Gluon Plasma (QGP), remains the most international collaborative effort and synergy between theorists and experimentalists in modern nuclear physics to date. The experimentalists around the world not only collect an unprecedented amount of data in heavy-ion collisions, at Relativistic Heavy Ion Collider (RHIC), at Brookhaven National Laboratory (BNL) in New York, USA, and the Large Hadron Collider (LHC), at CERN in Geneva, Switzerland but also analyze these data to unravel the mystery of this new phase of matter that filled a few microseconds old universe, just after the Big Bang. In the meantime, advancements in theoretical works and computing capability extend our wisdom about the hot-dense QCD matter and its dynamics through mathematical equations. The exchange of ideas between experimentalists and theoreticians is crucial for the progress of our knowledge. The motivation of this first conference named "HOT QCD Matter 2022" is to bring the community together to have a discourse on this topic. In this article, there are 36 sections discussing various topics in the field of relativistic heavy-ion collisions and related phenomena that cover a snapshot of the current experimental observations and theoretical progress. This article begins with the theoretical overview of relativistic spin-hydrodynamics in the presence of the external magnetic field, followed by the Lattice QCD results on heavy quarks in QGP, and finally, it ends with an overview of experiment results.

nucl-th↗

arXiv2022

Is first-order relativistic hydrodynamics in general frame stable and causal for arbitrary interaction?

We derive a first-order, stable and causal, relativistic hydrodynamic theory from the microscopic kinetic equation using the gradient expansion technique in a general frame. The general frame is introduced from the arbitrary matching conditions for hydrodynamic fields. The interaction is introduced in the relativistic Boltzmann equation through the momentum-dependent relaxation time approximation (MDRTA) with the proposed collision operator that preserves the conservation laws. We demonstrate here for the first time that not only the general frame choice, but also the momentum dependence of microscopic interaction rate, captured through MDRTA, is imperative for producing the essential field corrections that give rise to a causal and stable first-order relativistic theory.

nucl-th↗

arXiv2022

Causality and stability analysis of first-order field redefinition in relativistic hydrodynamics from kinetic theory

In this work, the causality and stability of a first-order relativistic dissipative hydrodynamic theory, that redefines the hydrodynamic fields from a first principle microscopic estimation, have been analyzed. A generic approach of gradient expansion for solving the relativistic transport equation has been adopted using the Chapman-Enskog iterative method. Next, the momentum dependent relaxation time approximation (MDRTA) has been employed to quantify the collision term for analytical estimation of the field correction coefficients from kinetic theory. At linear regime, in local rest frame the dispersion relations are observed to produce a causal propagating mode. However, the acausality and instability reappear when a boosted background is considered for linear analysis. These facts point out relevant aspects regarding the methodology of extracting the causal and stable first order hydrodynamics from kinetic theory and indicate the appropriate approach to construct a valid first order theory with proper justification.

nucl-th↗