arXiv subjects

Subhasis Samanta

Publications and source records attributed to Subhasis Samanta.

At least 19 recordsLinked to original sources

arXiv2026

Isospin-symmetry violation -- kaons and beyond (ISO-BREAK 25: summary and outlook)

This report summarizes the presentations and discussions during the ISO-BREAK 25 Workshop ``Isospin symmetry violation: kaons and beyond'', which was held at Jan Kochanowski University in Kielce on October 23-25, 2025. We address the current status of the isospin-symmetry breaking discovered by NA61/SHINE in nucleus-nucleus collisions at the CERN SPS, its confirmation by other experiments and studies in \ee and deep inelastic scattering. In addition, we discuss the theoretical status as well as we outline experimental and theoretical priorities towards understanding this currently unexplained phenomenon.

nucl-ex↗

arXiv2024

Spin-orbit entangled moments and magnetic exchange interactions in cobalt-based honeycomb magnets BaCo$_2$($X$O$_4$)$_2$ ($X$ = P, As, Sb)

Co-based honeycomb magnets have been actively studied recently for the potential realization of emergent quantum magnetism therein such as the Kitaev spin liquid. Here we employ density functional and dynamical mean-field theory methods to examine a family of the Kitaev magnet candidates BaCo$_2$($X$O$_4$)$_2$ ($X$ = P, As, Sb), where the compound with $X$ = Sb being not synthesized yet. Our study confirms the formation of Mott insulating phase and the $J_{\rm eff}$ = 1/2 spin moments at Co$^{2+}$ sites despite the presence of a sizable amount of trigonal crystal field in all three compounds. The pnictogen substitution from phosphorus to antimony significantly changes the in-plane lattice parameters and direct overlap integral between the neighboring Co ions, leading to the suppression of the Heisenberg interaction. More interestingly, the marginal antiferromagnetic nearest-neighbor Kitaev term changes sign into a ferromagnetic one and becomes sizable at the $X$ = Sb limit. Our study suggests that the pnictogen substitution can be a viable route to continuously tune magnetic exchange interactions and to promote magnetic frustration for the realization of potential spin liquid phases in BaCo$_2$($X$O$_4$)$_2$.

cond-mat.str-el↗

arXiv2023

Evidence of isospin-symmetry violation in high-energy collisions of atomic nuclei: Theoretical and Phenomenological considerations

Recently, the NA61/SHINE collaboration at the CERN SPS reported evidence of isospin-symmetry violation in high-energy nuclear collisions [Nature Commun. 16, 2849 (2025)]. The effect was observed in the relative yields of charged and neutral kaons and cannot be explained by known sources of isospin symmetry breaking. In this work, we extend the theoretical and phenomenological aspects of that study. We discuss the historical background and introduce the concepts of isospin transformations and symmetry. Importantly, we relate isospin symmetry to the QCD flavor symmetry, and we present both conceptual and analytical proofs demonstrating the equality of the mean multiplicities of charged and neutral kaons for an initial ensemble of colliding systems that is invariant under charge-symmetry transformation.

nucl-th↗

arXiv2023

Emergence of flat bands and ferromagnetic fluctuations via orbital-selective electron correlations in Mn-based kagome metal

Kagome lattice has been actively studied for the possible realization of frustration-induced two-dimensional flat bands and a number of correlation-induced phases. Currently, the search for kagome systems with a nearly dispersionless flat band close to the Fermi level is ongoing. Here, by combining theoretical and experimental tools, we present Sc$_3$Mn$_3$Al$_7$Si$_5$ as a novel realization of correlation-induced almost-flat bands in the kagome lattice in the vicinity of the Fermi level. Our magnetic susceptibility, $^{27}$Al nuclear magnetic resonance, transport, and optical conductivity measurements provide signatures of a correlated metallic phase with tantalizing ferromagnetic instability. Our dynamical mean-field calculations suggest that such ferromagnetic instability observed originates from the formation of nearly flat dispersions close to the Fermi level, where electron correlations induce strong orbital-selective renormalization and manifestation of the kagome-frustrated bands. In addition, a significant negative magnetoresistance signal is observed, which can be attributed to the suppression of flat-band-induced ferromagnetic fluctuation, which further supports the formation of flat bands in this compound. These findings broaden a new prospect to harness correlated topological phases via multiorbital correlations in 3$d$-based kagome systems.

cond-mat.str-el↗

arXiv2022

Active orbital degree of freedom and potential spin-orbit-entangled moments in Kitaev magnet candidate BaCo$_2$(AsO$_4$)$_2$

Candidate materials for Kitaev spin liquid phase have been intensively studied recently because of their potential applications in fault-tolerant quantum computing. Although most of the studies on Kitaev spin liquid have been done in 4$d$ and 5$d$ based transition metal compounds, recently there has been a growing research interest in Co-based quasi-two-dimensional honeycomb magnets, such as BaCo$_2$(AsO$_4$)$_2$ because of formation of spin-orbit-entangled $J_{\rm eff}$ = 1/2 pseudospin moments at Co$^{2+}$ sites and potential realizations of Kitaev-like magnetism therein. Here, we obtain high-accuracy crystal and electronic structure of BaCo$_2$(AsO$_4$)$_2$ by employing a combined density functional and dynamical mean-field theory calculations, which correctly capture the Mott-insulating nature of the target system. We show that Co$^{2+}$ ions form a high spin configuration, $S=3/2$, with an active $L_{\rm eff}=1$ orbital degree of freedom, in the absence of spin-orbit coupling. The size of trigonal distortion within CoO$_6$ octahedra is found to be not strong enough to completely quench the orbital degree of freedom, so that the presence of spin-orbit coupling can give rise to the formation of spin-orbit-entangled moments and the Kitaev exchange interaction. Our finding supports recent studies on potential Kitaev magnetism in this compound and other Co-based layered honeycomb systems.

cond-mat.str-el↗

arXiv2022

Study of bound states in a thermal gas using the S-matrix formalism

We have studied the formation of bound states in a thermal gas in the context of quantum field theory (QFT). We have considered a scalar QFT with $\varphi^4$ interaction, where $\varphi$ is a scalar particle with mass $m$. We have observed the formation of a bound state of $\varphi$-$\varphi$ type when the coupling constant is negative and its modulus is larger than a certain critical value. We have calculated the contribution of the bound state to the pressure of the thermal gas of the system by using the S-matrix formalism. Our analysis is based on a unitarized one-loop resumed approach in which the theory is finite and well defined for each value of the coupling constant. We have observed that the total pressure as a function of the coupling constant is continuous also at the critical coupling: the jump in pressure due to the sudden appearance of the bound state is exactly cancelled by an analogous jump (but with opposite sign) of the interaction contribution to the pressure.

hep-ph↗

arXiv2021

Multiplicity dependence freeze-out scenarios in pp collisions at $\sqrt{s}$ = 7 TeV

The data on transverse momentum integrated hadron yields in different multiplicity classes of p+p collisions at $\sqrt{s}=7$ TeV have been analyzed to extract the chemical freeze-out parameters using a thermal model. The chemical freeze-out parameters have been extracted for three different freeze-out schemes: i. unified freeze-out for all hadrons in complete thermal equilibrium (1CFO), ii. unified freeze-out for all hadrons with an additional parameter $\gamma_S$ which accounts for possible out-of-equilibrium production of strange hadrons (1CFO$+\gamma_S$), and iii. separate freeze-out for hadrons with and without strangeness content (2CFO). It has been observed that 1CFO$+\gamma_S$ scheme gives the best description of the hadronic yields at midrapidity when multiplicity ($\langle dN_{ch}/d\eta \rangle$) of the collision is less than 10. This indicates that the strangeness is out of equilibrium in most of the multiplicity classes of p+p collisions. All the three parameters of this CFO scheme, temperature ($T$), radius of the fireball ($R$) and strangeness suppression factor ($\gamma_S$) increase with the increase of $\langle dN_{ch}/d\eta \rangle$. Further, we have compared applicability of different CFO schemes considering two more colliding system p+Pb at $\sNN$ = 5.02 and Pb+Pb at $\sNN$ = 2.76 TeV along with p+p collisions at $\sqrt{s}=7$ TeV. We observe a freeze-out volume (or multiplicity) dependence of CFO schemes regardless of colliding ions. The 1CFO+$\gamma_S$, 1CFO and 2CFO schemes provide best description of the data when the dimension less quantity $VT^3$ approximately satisfies the conditions $VT^3 <50$, $50 < VT^3 < 100$ and $VT^3 > 100$ respectively or the corresponding multiplicity satisfies the conditions $\langle dN_{ch}/d\eta \rangle<30$, $30 < dN_{ch}/d\eta < 60$ and $\langle dN_{ch}/d\eta \rangle>100$ respectively.

hep-ph↗

arXiv2021

Role of bound states and resonances in scalar QFT at nonzero temperature

We study the thermal properties of quantum field theories (QFT) with three-leg interaction vertices $g\varphi^{3}$ and $gS\varphi^{2}$ ($\varphi$ and $S$ being scalar fields), which constitute the relativistic counterpart of the Yukawa potential. We follow a non-perturbative unitarized one-loop resummed technique for which the theory is unitary and well-defined for a large range of values of the coupling constant $g$. Using the partial wave decomposition of two-body scattering we calculate the phase shifts, whose derivatives are used to infer the pressure of the system at nonzero temperature by using the so-called phase shift formalism. A $\varphi \varphi$ bound state is formed when the coupling $g$ is larger than a certain critical value. As the main outcomes of this work, we estimate the influence of particle interaction on the pressure (both without and with the bound state), and we demonstrate that the latter is always continuous as a function of the coupling constant $g$ (no sudden jumps occurs when the bound state forms), and we show that the contribution of the bound state to the pressure does not count as \textit{one} state in the thermal gas, since a cancellation with the residual $\varphi \varphi$ interaction occurs. The amount of this cancellation depends on the details of the model and its parameters and a variety of possible scenarios is presented. %Moreover, even when no bound state occurs, we estimate the role of the interaction (including a resonance in the $gS\varphi^{2}$ theory), which is in general non-negligible. We also show how the overall effect of the interaction, including eventual resonances and bound states, can be formally described by a unique expression that makes use of the phase shift continued below the threshold.

hep-ph↗

arXiv2021

Impact of momentum resolution on factorial moments due to power-law correlations between particles

The effect of momentum resolution on factorial moments due to the power-law correlation function is studied. The study is motivated by the search for the critical point of the strongly interacting matter in heavy-ion collisions using the intermittency method. We observe that factorial moments are significantly affected by the finite momentum resolution. The effect is superficially significant compared to intuitive expectations. The results depend on the power of the correlation function and the number of uncorrelated particles.

nucl-th↗

arXiv2021

Scaling of factorial moments in cumulative variables

A search for power-law fluctuations within the framework of the intermittency method is ongoing to locate the critical point of the strongly interacting matter. In particular, experimental data on proton and pion production in heavy-ion collisions are analyzed in transverse-momentum, $p_T$, space. In this regard, we have studied the dependence of the second scaled factorial moment $F_2$ of particle multiplicity distribution on the number of subdivisions of transverse momentum-interval used in the analysis. The study is performed using a simple model with a power-law two-particle correlation function in $p_T$. We observe that $F_2$ values depend on the size and position of the $p_T$ interval. However, when we convert the non-uniform transverse-momentum distribution to uniform one using cumulative transformation, $F_2$ calculated in subdivisions of the cumulative $p_T$ becomes independent of the cumulative-$p_T$ interval. The scaling behaviour of $F_2$ for the cumulative variable is observed. Moreover, $F_2$ follows a power law with the number of subdivisions of the cumulative-$p_T$ interval with the intermittency index close to the correlation function's exponent.

nucl-th↗

arXiv2021

Anomalous Hall signatures of nonsymmorphic nodal lines in doped chromium chalcospinel CuCr$_2$Se$_4$

An emerging phase of matter among the class of topological materials is nodal line semimetal, possessing symmetry-protected one-dimensional gapless lines at the (or close to) the Fermi level in $k$-space. When the $k$-dispersion of the nodal line is weak, van Hove singularities generated by the almost flat nodal lines may be prone to instabilities introduced by additional perturbations such as spin-orbit coupling or magnetism. Here, we study Cr-based ferromagnetic chalcospinel compound CuCr$_2$Se$_4$ (CCS) via first-principles electronic structure methods and reveal the true origin of its dissipationless anomalous Hall conductivity, which was not well understood previously. We find that CCS hosts nodal lines protected by nonsymmorphic symmetries, located in the vicinity of Fermi level, and that such nodal lines are the origin of the previously observed distinct behavior of the anomalous Hall signature in the presence of electron doping. The splitting of nodal line via spin-orbit coupling produces a large Berry curvature, which leads to a significant response in anomalous Hall conductivity. Upon electron doping via chemical substitution or gating, or rotation of magnetization via external magnetic field, steep change of anomalous Hall behavior occurs, which makes CCS a promising compound for low energy spintronics applications.

cond-mat.mtrl-sci↗

arXiv2020

QFT treatment of a bound state in a thermal gas

We investigate how to include bound states in a thermal gas in the context of quantum field theory (QFT). To this end, we use for definiteness a scalar QFT with a $\varphi^{4}$ interaction, where the field $\varphi$ represents a particle with mass $m$. A bound state of the $\varphi$-$\varphi$ type is created when the coupling constant is negative and its modulus is larger than a certain critical value. We investigate the contribution of this bound state to the pressure of the thermal gas of the system by using the $S$-matrix formalism involving the derivative of the phase-shift scattering. Our analysis, which is based on an unitarized one-loop resumed approach which renders the theory finite and well-defined for each value of the coupling constant, leads to following main results: (i) We generalize the phase-shift formula in order to take into account within a unique formal approach the two-particle interaction as well as the bound state (if existent). (ii) \textit{On the one hand}, the number density of the bound state in the system at a certain temperature $T$ is obtained by the standard thermal integral; this is the case for any binding energy, even if it is much smaller than the temperature of the thermal gas. (iii) \textit{On the other hand}, the contribution of the bound state to the total pressure is partly -- but not completely -- canceled by the two-particle interaction contribution to the pressure. (iv) The pressure as function of the coupling constant is \textit{continuous} also at the critical coupling for the bound state formation: the jump in pressure due to the sudden appearance of the bound state is exactly canceled by an analogous jump (but with opposite sign) of the interaction contribution to the pressure.

hep-ph↗

arXiv2020

Dynamics of QCD Matter -- current status

In this article, there are 18 sections discussing various current topics in the field of relativistic heavy-ion collisions and related phenomena, which will serve as a snapshot of the current state of the art. Section 1 reviews experimental results of some recent light-flavored particle production data from ALICE collaboration. Other sections are mostly theoretical in nature. Very strong but transient magnetic field created in relativistic heavy-ion collisions could have important observational consequences. This has generated a lot of theoretical activity in the last decade. Sections 2, 7, 9, 10 and 11 deal with the effects of the magnetic field on the properties of the QCD matter. There are several unanswered questions about the QCD phase diagram. Sections 3, 11 and 18 discuss various aspects of the QCD phase diagram and phase transitions. Recent years have witnessed interesting developments in foundational aspects of hydrodynamics and their application to heavy-ion collisions. Sections 12, 15, 16 and 17 of this article probe some aspects of this exciting field. Transport coefficients together with their temperature- and density-dependence, are essential inputs in hydrodynamical calculations. Sections 5, 8 and 14 deal with calculation/estimation of various transport coefficients (shear and bulk viscosity, thermal conductivity, relaxation times, etc.) of quark matter and hadronic matter. Sections 4, 6 and 13 deals with interesting new developments in the field. Section 4 discusses color dipole gluon distribution function at small transverse momentum in the form of a series of Bells polynomials. Section 6 discusses the properties of Higgs boson in the quark gluon plasma using Higgs-quark interaction. Section 13 discusses modification of coalescence model to incorporate viscous corrections and application of this model.

hep-ph↗

arXiv2020

Feasibility studies of conserved charge fluctuations in Au-Au collisions with CBM

We present the CBM physics performance study for measurements of the higher order cumulants of the net-proton multiplicity distributions. These observables are proxy for net-baryon fluctuations and are commonly used to study the phase structure of QCD phase diagram. The simulation is done for $Au+Au$ collision at beam kinetic energy $E_{lab} = 10$ AGeV. The cumulants of net-proton distributions have been calculated at midrapidity ($|\Delta y| =1$) for the transverse momentum range $0.2 < p_T < 2.0$ GeV/c. The centrality dependence of cumulants of net-proton upto order four is presented. The efficiency and detector effects are corrected using unfolding techniques. This work shows that the higher order cumulants of net-proton can be measured using the CBM detector.

nucl-ex↗

arXiv2020

Anisotropic transport properties of Hadron Resonance Gas in magnetic field

An intense transient magnetic field is produced in high energy heavy-ion collisions mostly due to the spectator protons inside the two colliding nucleus. The magnetic field introduces anisotropy in the medium and hence the isotropic scalar transport coefficients become anisotropic and split into multiple components. Here we calculate the anisotropic transport coefficients shear, bulk viscosity, electrical conductivity, and the thermal diffusion coefficients for a multicomponent Hadron- Resonance-Gas (HRG) model for a non-zero magnetic field by using the Boltzmann transport equation in a relaxation time approximation (RTA). The anisotropic transport coefficient component along the magnetic field remains unaffected by the magnetic field, while perpendicular dissipation is governed by the interplay of the collisional relaxation time and the magnetic time scale, which is inverse of the cyclotron frequency. We calculate the anisotropic transport coefficients as a function of temperature and magnetic field using the HRG model. The neutral hadrons are unaffected by the Lorentz force and do not contribute to the anisotropic transports, we estimate within the HRG model the relative contribution of isotropic and anisotropic transports as a function of magnetic field and temperature. We also give an estimation of these anisotropic transport coefficients for the hadronic gas at finite baryon chemical potential.

nucl-th↗

arXiv2020

Quantum expression for the electrical conductivity of massless quark matter and of the hadron resonance gas in the presence of a magnetic field

We have studied the classical and quantum expressions of electrical conductivity and their numerical estimation in the presence of a magnetic field for hadron resonance gas (HRG) and massless quark matter. Classical results of transport coefficients of HRG matter in the presence of a magnetic field were studied previously by Dash et al. [Phys. Rev. D 102, 016016 (2020)] using the standard relaxation time approximation in the Boltzmann equation. In the same reference, the transition from isotropic transport coefficients to anisotropic coefficients in the presence of a magnetic field was also estimated for massless and HRG matter. This led to an upper limit or Stefan-Boltzmann (SB) type limit to the nonperturbative domain transition of transport coefficients. In a similar context, the present work has concentrated on the classical to quantum transition of HRG transport from the domain of high temperature and low magnetic field to that of low temperature and high magnetic field. We have also compared the quantum modification of HRG results with that of massless quark matter, where we observed an opposite trend. A similar kind of quantum effect is also noticed between mesons and baryons due to their different particle distribution functions. Despite the fact that HRG contains both mesons and baryons, Landau quantization of its net magnetothermodynamic phase space reveals meson- or boson-dominated quantum modification. That is why the quantum modification of HRG results reveals the opposite trend from that of massless quark matter, which faces fermionic quantum modification.

nucl-th↗

arXiv2019

Viscosity calculations from Hadron Resonance Gas model: Finite size effect

We have attempted to review on microscopic calculation of transport coefficients like shear and bulk viscosities in the framework of hadron resonance gas model, where a special attention is explored on the effect of finite system size. The standard expressions of transport coefficients, obtained from relaxation time approximation of kinetic theory or diagrammatic Kubo-type formalism, carry mainly two temperature dependent components -- thermodynamical phase space and relaxation time of medium constituent. Owing to quantum effect of finite system size, thermodynamical phase space can be reduced as its momentum distribution will be started from some finite lower momentum cut-off instead of zero momentum. On the other hand, relaxation time of hadrons can also face finite size effect by considering only those relaxation scales, which are lower than the system size. Owing to these phenomenological issues, we have proposed a system size dependent upper bound of transport coefficients for ideal HRG model, whose qualitative technique may also be applicable in other models. This finite size prescription may guide to shorten the broad numerical band, within which earlier estimated values of transport coefficients for hadronic matter are located. It is also suspected that the hadronic matter may not be far from the (nearly) perfect fluid nature like the quark gluon plasma.

nucl-th↗

arXiv2019

Estimation of attractive and repulsive interactions from the fluctuation observables at RHIC using van der Waals hadron resonance gas Model

Experimental data on the moments of net-proton distribution in central Au-Au collisions for various center of mass energies ($\sqrt{\mathrm {s_{NN}}}$) measured by the STAR collaboration at the Relativistic Heavy-Ion Collider (RHIC) are compared to the corresponding results from a van der Waals type interacting hadron resonance gas (VDWHRG) model. The parameters representing the attractive and repulsive interactions in the VDWHRG model have been extracted by fitting the $\sigma^{2}$/$M$, $\it{S}\sigma$ and $\kappa\sigma^{2}$, where $M$ is the mean, $\sigma$ is the standard deviation, $\it{S}$ is the skewness and $\kappa$ the kurtosis of the net-proton distribution. Considering all the three moment products we observe that the strength of the repulsive interactions increases with decrease in $\sqrt{\mathrm {s_{NN}}}$ = 200 to 19.6 GeV while the strength of the attractive interaction is of the similar magnitude. For $\sqrt{\mathrm {s_{NN}}}$ = 11.5 and 7.7 GeV there is drop in the strength of both attractive and repulsive interactions relative to $\sqrt{\mathrm {s_{NN}}}$ = 19.6 GeV. On the other hand, if we consider only the higher order moment products, $\it{S}\sigma$ and $\kappa\sigma^{2}$, which are more sensitive to critical point physics, a segregation with respect to the strength of the attractive parameter is observed. The data for $\sqrt{\mathrm {s_{NN}}}$ = 19.6 and 27 GeV supports a larger attractive strength compared to other energies. For this latter case, the repulsive interaction values are of similar order for most of the beam energies studied, except for 7.7 GeV where the parameter value is not well constrained due to large uncertainties.

hep-ph↗