arXiv subjects

Sayantan Sharma

Publications and source records attributed to Sayantan Sharma.

At least 55 records · Page 3Linked to original sources

arXiv2016

The topological structures in strongly coupled QGP with chiral fermions on the lattice

The nature of chiral phase transition for two flavor QCD is an interesting but unresolved problem. One of the most intriguing issues is whether or not the anomalous U(1) symmetry in the flavor sector is effectively restored along with the chiral symmetry. This may determine the universality class of the chiral phase transition. Since the physics near the chiral phase transition is essentially non-perturbative, we employ first principles lattice techniques to address this issue. We use overlap fermions, which have exact chiral symmetry on the lattice, to probe the anomalous U(1) symmetry violation of 2+1 flavor dynamical QCD configurations with domain wall fermions. The latter also optimally preserves chiral and flavor symmetries on the lattice, since it is known that the remnant chiral symmetry of the light quarks influences the scaling of the chiral condensate in the crossover transition region. We observe that the anomalous U(1) is not effectively restored in the chiral crossover region. We perform a systematic study of the finite size and cut-off effects since the signals of U(1) violation are sensitive to it. We also provide a glimpse of the microscopic topological structures of the QCD medium that are responsible for the strongly interacting nature of the quark gluon plasma phase. We study the effect of these microscopic constituents through our first calculations for the topological susceptibility of QCD at finite temperature, which could be a crucial input for the equation of state for anomalous hydrodynamics.

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arXiv2016

Charm Degrees of Freedom in Quark Gluon Plasma

Lattice QCD studies on fluctuations and correlations of charm quantum number have established that deconfinement of charm degrees of freedom sets in around the chiral crossover temperature, $T_c$, i.e. charm degrees of freedom carrying fractional baryonic charge start to appear. By reexamining those same lattice QCD data we show that, in addition to the contributions from quark-like excitations, the partial pressure of charm degrees of freedom may still contain significant contributions from open-charm meson and baryon-like excitations associated with integral baryonic charges for temperatures up to $1.2~ T_c$. Charm quark-quasiparticles become the dominant degrees of freedom for temperatures $T>1.2~ T_c$.

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arXiv2015

The U(1)A anomaly in high temperature QCD with chiral fermions on the lattice

The magnitude of the $U_A(1)$ symmetry breaking is expected to affect the nature of $N_f=2$ QCD chiral phase transition. The explicit breaking of chiral symmetry due to realistic light quark mass is small, so it is important to use chiral fermions on the lattice to understand the effect of $U_A(1)$ near the chiral crossover temperature, $T_c$. We report our latest results for the eigenvalue spectrum of 2+1 flavour QCD with dynamical Mobius domain wall fermions at finite temperature probed using the overlap operator on $32^3\times 8$ lattice. We check how sensitive the low-lying eigenvalues are to the sea-light quark mass. We also present a comparison with the earlier independent results with domain wall fermions.

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arXiv2015

Divergences in the quark number susceptibility : The origin and a cure

Quark number susceptibility on the lattice, obtained by merely adding a $μN$ term with $μ$ as the chemical potential and $N$ as the conserved quark number, has a quadratic divergence in the cut-off $a$. We show that such a divergence already exist for free fermions with a cut-off regulator. While one can eliminate it in the free lattice theory by suitably modifying the action, as is popularly done, it can simply be subtracted off as well. Computations of higher order susceptibilities, needed for estimating the location of the QCD critical point, then need a lot fewer number of quark propagators at any order. We show that this method of divergence removal works in the interacting theory.

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arXiv2015

A simple idea for Lattice QCD at finite density

We pursue the idea of adding the naive $μN$ term, where $μ$ is the quark chemical potential and $N$ is the conserved quark number, to the lattice QCD action. While computations of higher order susceptibilities, required for estimating the location of the QCD critical point, need a lot fewer number of quark propagators at any order as a result, it has its problem. We discuss a solution, and examine if it works.

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arXiv2015

Microscopic Origin of \boldmath{$U_A(1)$} Symmetry Violation in the High Temperature Phase of QCD

We investigate the low-lying eigenmodes of the Dirac matrix with the aim to gain more insight into the temperature dependence of the anomalous $U_A(1)$ symmetry. We use the overlap operator to probe dynamical QCD configurations generated with (2+1)-flavors of highly improved staggered quarks. We find no evidence of a gap opening up in the infrared region of the eigenvalue spectrum even at $1.5\,T_c$, $T_c$ being the chiral crossover temperature. Instead, we observe an accumulation of near-zero eigenmodes. We argue that these near-zero eigenmodes are primarily responsible for the anomalous breaking of the axial symmetry still being effective. At $1.5\,T_c$, these near-zero eigenmodes remain localized and their distribution is consistent with the dilute instanton gas picture. At this temperature, the average size of the instantons is $0.223(8)\,\text{fm}$ and their density is $0.147(7)\,\text{fm}^{-4}$.

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arXiv2014

On curing the divergences in the quark number susceptibility

Adding chemical potential $μ$ linearly as $μN$ to the lattice QCD action, where $N$ is a conserved quark/baryon number, leads to a quadratic divergence as $a^{-2}$. We argue that it is inherited from the continuum theory and can be subtracted off on the lattice following a similar manner in the continuum. We test this idea for quenched quark number susceptibilities and demonstrate a finite continuum limit numerically.

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arXiv2014

The thermodynamics of heavy light hadrons at freezeout

In the discussion of hadronization at or close to the freeze-out curve statistical (hadron resonance gas) models play an important role. In particular, in the charmonium sector, regeneration models are considered which rely on the fact that charmonium states can form again already at temperatures well above the QCD transition or hadronization temperature. An important ingredient in these considerations is the regeneration or hadronization of open charm states. In this talk we report on a lattice QCD analysis of correlations of open strange and charm with other conserved quantum numbers like the net baryon number and electric charge. We analyze the temperature range in which an uncorrelated hadron resonance gas (HRG) provides an adequate description of such correlations. This limits the range of validity of HRG based thermodynamics in open flavor channels and provides an estimate for the melting temperature of heavy-light hadrons.

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arXiv2014

QCD thermodynamics on the lattice

A remarkable progress has been made in the understanding of the hot and dense QCD matter using lattice gauge theory. The issues which are very well understood as well those which require both conceptual as well as algorithmic advances are highlighted. The recent lattice results on QCD thermodynamics which are important in the context of the heavy ion experiments are reviewed. Instances of greater synergy between the lattice theory and the experiments in the recent years are discussed where lattice results could be directly used as benchmarks for experiments and results from the experiments would be a crucial input for lattice computations.

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arXiv2013

Investigation of the $U_A(1)$ in high temperature QCD on the lattice

In this project we study the effect of the $U_A(1)$ anomaly for (2+1)-flavour QCD at high temperature. We apply the overlap operator as a tool to probe the topological properties of gauge field configurations which have been generated within the Highly Improved Staggered Quark (HISQ) discretization scheme on lattices of size $32^3\times 8$ with $m_l/m_s=1/20$, commonly used for the study of QCD thermodynamics. Although we have at present, only results for one value of the quark masses and thus cannot monitor the change of the eigenvalue distributions with the light quark mass, the distribution of the low-lying eigenvalues of the overlap operator suggests that the $U_A(1)$ is not restored effectively even at 1.5 times the pseudo critical temperature. The corresponding low-lying eigenmodes show localization properties.

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arXiv2012

Exact Chiral Invariance at Finite Density on Lattice

A new lattice action is proposed for the overlap Dirac matrix with nonzero chemical potential. It is shown to preserve the full chiral invariance for all values of lattice spacing exactly. It is further demonstrated to arise in the domain wall formalism by coupling the chemical potential count only to the physically relevant wall modes.

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arXiv2011

A faster method of computation of lattice quark number susceptibilities

We compute the quark number susceptibilities in two flavor QCD for staggered fermions by adding the chemical potential as a Lagrange multiplier for the point-split number density term. Since lesser number of quark propagators are required at any order, this method leads to faster computations. We propose a subtraction procedure to remove the inherent undesired lattice terms and check that it works well by comparing our results with the existing ones where the elimination of these terms is analytically guaranteed. We also show that the ratios of susceptibilities are robust, opening a door for better estimates of location of the QCD critical point through the computation of the tenth and twelfth order baryon number susceptibilities without significant additional computational overload.

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arXiv2011

Exact chiral invariance at finite density on the lattice

We propose a lattice action for the overlap Dirac matrix with nonzero chemical potential which is shown to preserve the chiral invariance on the lattice exactly. We further demonstrate it to arise from the Domain wall by letting the chemical potential count only the physically relevant wall modes.

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arXiv2011

Introduction of the chemical potential in the overlap formalism

We investigate the possibility of coupling a chemical potential only to the physical chiral fermions on the lattice starting from the many body state description of overlap fermions. After developing the formalism for a chiral gauge theory, we focus our attention on the case of free fermions coupled to a vector like chemical potential and discuss the issue of zero temperature divergences.

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arXiv2010

Anomalies at finite density and chiral fermions

Using perturbation theory in the Euclidean (imaginary time) formalism as well as the non-perturbative Fujikawa method, we verify that the chiral anomaly equation remains unaffected in the presence of nonzero chemical potential, $μ$. We extend our considerations to fermions with exact chiral symmetry on the lattice and discuss the consequences for the recent Bloch-Wettig proposal for the Dirac operator at finite chemical potential. We propose a new simpler method of incorporating $μ$ and compare it with the Bloch-Wettig idea.

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arXiv2009

Anomaly at finite density and chiral fermions on lattice

Using both perturbation theory in the Euclidean formalism as well as the non-perturbative Fujikawa's method, we verify that the chiral anomaly equation remains unaffected in continuum QCD in the presence of nonzero chemical potential, μ. We extend our considerations to lattice fermions with exact chiral symmetry and discuss the consequences for the recent Bloch-Wettig proposal for the Dirac operator at finite chemical potential. We propose a new simpler method of incorporating μ.

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arXiv2008

Thermodynamics of free Domain Wall fermions

Studying various thermodynamic quantities for the free domain wall fermions for both finite and infinite fifth dimensional extent N_5, we find that the lattice corrections are minimum for $N_T\geq10$ for both energy density and susceptibility, for its irrelevant parameter M in the range 1.45-1.50. The correction terms are, however, quite large for small lattice sizes of $N_T\leq8$. We propose modifications of the domain wall operator, as well as the overlap operator, to reduce the finite cut-off effects to within 10% of the continuum results of the thermodynamic quantities for the currently used N_T=6-8 lattices. Incorporating chemical potential, we show that μ^2 divergences are absent for a large class of such domain wall fermion actions although the chiral symmetry is broken for $μ\neq0$.

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arXiv2008

Exact Chiral Fermions and Finite Density on Lattice

Any mu^2-divergence is shown analytically to be absent for a class of actions for Overlap and Domain Wall Fermions with nonzero chemical potential. All such actions are, however, shown to violate the chiral invariance. While the parameter M of these actions can be shown to be irrelevant in the continuum limit, as expected, it is shown numerically that the continuum limit can be reached with relatively coarser lattices for M in the range of 1.5-1.6.

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