arXiv subjects

Srimoyee Sen

Publications and source records attributed to Srimoyee Sen.

53 records · Page 3Linked to original sources

arXiv2018

Neutrino-nucleon scattering in the neutrino-sphere

We calculate the differential scattering rate for thermal neutrinos in a hot and dilute gas of interacting neutrons using linear response theory. The dynamical structure factors for density and spin fluctuations of the strongly interacting neutron matter, expected in the neutrino decoupling regions of supernovae and neutron star mergers, are calculated in the virial expansion for the first time. Correlations due to nucleon-nucleon interactions are taken into account using a pseudo-potential that reproduces measured nucleon-nucleon phase shifts, and we find that attractive s-wave interactions enhance the density response and suppress the spin response of neutron matter. The net effect of neutron correlations is to strongly suppress backscattering. Moreover, we find nearly exact scaling laws for the response functions, valid for the range $T = 5 - 10$ MeV and q < 30 MeV, allowing us to obtain analytic results for the dynamic structure factors at second-order in the fugacity of the neutron gas. We find that the modification of scattering rates depends on the energy and momentum exchanged, implying that dynamical structure factors are essential to describe neutrino decoupling in supernovae and neutron star mergers.

nucl-th↗

arXiv2017

Chiral Anomalous Dispersion

The linearized Einstein equation describing graviton propagation through a chiral medium appears to be helicity dependent. We analyze features of the corresponding spectrum in a collision-less regime above a flat background. In the long wave-length limit, circularly polarized metric perturbations travel with a helicity dependent group velocity that can turn negative giving rise to a new type of an anomalous dispersion. We further show that this chiral anomalous dispersion is a general feature of polarized modes propagating through chiral plasmas extending our result to the electromagnetic sector.

hep-th↗

arXiv2017

Order parameters and color-flavor center symmetry in QCD

Common lore suggests that $N$-color QCD with massive quarks has no useful order parameters which can be non-trivial at zero baryon density. However, such order parameters do exist when there are $n_f$ quark flavors with a common mass and $d\equiv\gcd(n_f,N) > 1$. These theories have a $\mathbb Z_d$ color-flavor center symmetry arising from intertwined color center transformations and cyclic flavor permutations. The symmetry realization depends on the temperature, baryon chemical potential and value of $n_f/N$, with implications for conformal window studies and dense quark matter.

hep-th↗

arXiv2017

Energy Conservation and the Chiral Magnetic Effect

We analyze the chiral magnetic effect in a homogeneous neutral plasma from the point of view of energy conservation, and construct an effective potential for the growth of maximally helical perturbations of the electromagnetic field. We show that a negative curvature at the origin of the potential, indicating instability of the plasma, is induced by a chiral asymmetry in electron Fermi energy, as opposed to number density, while the potential grows at large field value. It follows that the ground state for a plasma has zero magnetic helicity; a nonzero electron mass will allow an excited state of a plasma with nonzero helicity to relax to that ground state quickly. We conclude that a chiral plasma instability triggered by weak interactions is not a viable mechanism for explaining magnetic fields in stars except possibly when dynamics drives the system far from equilibrium.

hep-ph↗

arXiv2017

Chiral Shock Waves

We study the shock waves in relativistic chiral matter. We argue that the conventional Rankine-Hugoinot relations are modified due to the presence of chiral transport phenomena. We show that the entropy discontinuity in a weak shock wave is quadratic in the pressure discontinuity when the effect of chiral transport becomes sufficiently large. We also show that rarefaction shock waves, which do not exist in usual nonchiral fluids, can appear in chiral matter. The direction of shock wave propagation in a vorticity is found to be completely determined by the direction of the vorticity and the chirality of fermions. These features are exemplified by shock propagation in dense neutrino matter in the hydrodynamic regime.

hep-th↗

arXiv2016

Exponential reduction of finite volume effects with twisted boundary conditions

Flavor-twisted boundary conditions can be used for exponential reduction of finite volume artifacts in flavor-averaged observables in lattice QCD calculations with $SU(N_f)$ light quark flavor symmetry. Finite volume artifact reduction arises from destructive interference effects in a manner closely related to the phase averaging which leads to large $N_c$ volume independence. With a particular choice of flavor-twisted boundary conditions, finite volume artifacts for flavor-singlet observables in a hypercubic spacetime volume are reduced to the size of finite volume artifacts in a spacetime volume with periodic boundary conditions that is four times larger.

hep-lat↗

arXiv2015

Low Energy Effective Theory of QCD at High Isospin Chemical Potential

The goal of this paper is to arrive at a low energy effective theory of QCD with two massless flavors of quarks at very high isospin density and zero baryon density. In a seminal paper by Son and Stephanov in the year 2001, it was conjectured that the low energy dynamics of QCD with two light flavors at asymptotically high isospin density was described by that of a pure Yang-Mills effective Lagrangian. Since the existence of a first order deconfinement phase transition with increasing temperature is a feature of every pure SU(N) Yang-Mills theory with N greater than or equal to 3, the regime considered in this paper is also expected to exhibit a first order deconfinement phase transition with increasing temperature. However, the low energy constants(LEC) of this pure Yang-Mills theory have not been calculated till date. We calculate the LEC s for this effective theory which in turn enables us to calculate the critical temperature of the deconfinement transition as a function of the isospin chemical potential $μ_I$.The scaling of the critical temperature as a function of the isospin chemical potential that we find in this paper can be compared with future lattice calculations at finite isospin density.

hep-ph↗

arXiv2015

Deconfinement Transition at High Isospin Chemical Potential and Low Temperature

We consider QCD with two degenerate flavors of light quarks(up and down) at asymptotically high isospin chemical potential with zero baryon chemical potential and calculate for the first time a quantitative expression for the critical temperature of the deconfinement transition in this regime. At high isospin chemical potential and sufficiently low temperatures this theory becomes equivalent to a pure Yang-Mills theory and accordingly has a first order deconfinement phase transition. Although this was conjectured in a seminal paper by Son and Stephanov in the year 2001, the critical temperature of this deconfinement phase transition was not computed. This paper computes the energy scale associated with this transition as a function of the isospin chemical potential by relating the parameters of the equivalent Yang-Mills theory to those of the underlying theory. We also relate the equation of state in one strongly interacting regime of QCD namely at finite isospin density to that in pure Yang-Mills, with the latter being amenable to straightforward numerical calculation. Our results for the critical temperature of deconfinement transition can be compared with future lattice calculations.

hep-ph↗

arXiv2015

Anisotropic Propagator for the Goldstone Modes in Color-flavor Locked Phase in the Presence of a Magnetic Field

We consider the phase diagram of QCD at very high baryon density and at zero temperature in the presence of a strong magnetic field. The state of matter at such high densities and low temperatures is believed to be a phase known as the color-flavor locked phase which breaks color and electromagnetic gauge invariance leaving a linear combination of them unbroken. Of the 9 quarks (three flavors and three colors), five are neutral under this unbroken generator and four are oppositely charged. In the presence of a magnetic field corresponding to the unbroken generator however, the properties of the condensate changes and a new phase known as the magnetic color flavor locked (MCFL)phase is realized. This phase breaks some of the color-flavor symmetry of the Lagrangian spontaneously, giving rise to 6 Goldstone modes, 5 of which are pseudo Goldstone modes. These Goldstone modes are composed of excitations that correspond to both neutral quarks and charged quarks. Hence it is natural to expect that the propagators of these Goldstone modes get affected in the presence of a magnetic field and their speed becomes considerably anisotropic. Although this anisotropy is self-evident from symmetry arguments, it has not been quantified yet. We calculate this anisotropy in the speed of the Goldstone modes using an NJL model type of interaction between the quarks and comment on the impact of such anisotropic modes on the transport properties of the MCFL phase.

hep-ph↗

arXiv2015

Center symmetry and Hagedorn spectrum

This paper explores the conjecture that large Nc gauge theories have a Hagedorn spectrum, if, and only if, they are confining and posses an explicit or emergent center symmetry. Evidence in support of this conjecture is presented. Many classes of large Nc gauge theories are considered. In all cases, we find that theories for which there exists a strong plausibility argument for a Hagedorn spectrum at large Nc are also believed to be confining and possess either an explicit center symmetric or have a strong plausibility argument for the existence of an emergent center symmetry at large Nc . Conversely, all theories we considered which are believed not to have a Hagedorn spectrum at large Nc, either were known not to be confining or else were believed to lack an emergent center symmetry. This is consistent with expectations based on the conjecture.

hep-th↗

arXiv2014

Massive and massless modes of the triplet phase of neutron matter

Neutron matter at densities of the order of the nuclear saturation density is believed to have neutrons paired in the 3P2 channel. We study the low lying modes of this phase and find two massless modes (angulons), resulting from the spontaneous breaking of rotational symmetry as well as three other, gapped modes. We compute their masses at arbitrary temperatures.

nucl-th↗

arXiv2014

Large Nc gauge theory with quarks in high representations

This paper explores a novel tractable regime for ultraviolet-complete quantum field theories -- the large Nc limit of non-abelian gauge theories with quarks in high dimensional representations (scaling with Nc faster than Nc^2 ), such as quarks with 'a' fundamental indices with a greater than or equal to 3. A smooth and nontrivial Nc limit can be obtained if g^2 Nc^(a-1) is held fixed instead of the standard t Hooft coupling g^2 Nc as Nc goes to infinity where g is the gauge coupling. SU(Nc) gauge theories in 3+1 dimensions are not asymptotically free at large Nc when they contain quarks in representations whose dimensions scale faster than Nc^2 and hence are not ultraviolet complete. However, in lower space-time dimensions (2+1, 1+1), for any Nc, renormalization group flow for such theories always has a stable ultraviolet fixed point at g = 0; the theory is thus ultraviolet complete. For the case of massless quarks, the theory has an infrared fixed point. For massive quarks, the theory is confining. The confining scale is parametrically of the order Nc^((2-a)/(4-d)) and is driven to zero at large Nc for theories with a > 2 and d < 4 where d is the space time dimension.

hep-th↗

arXiv2013

Neutrino emissivity from Goldstone boson decay in magnetized neutron matter

Neutron matter at densities somewhat above nuclear densities is believed to be superfluid due to the condensation of neutron pairs in the 3 P2 channel. This condensate breaks rotational symmetry spontaneously and leads to the existence of Goldstone bosons (angulons). We show that the coupling to magnetic fields mediated by the magnetic moment of the neutron makes angulons massive and capable of decaying into a neutrino-antineutrino pair. We compute the rate for this process and argue they become competitive with other cooling processes for temperatures around 10^7 K as long as the interior magnetic field of the star is in the B=10^15 G range or above.

nucl-th↗

arXiv2013

A new crystalline phase in magnetar crusts

We show that ions at the low densities and high magnetic fields relevant to the outer crust of magnetars form a novel crystalline phase where ions are strongly coupled along the magnetic field and loosely coupled in the transverse direction. The underlying cause is the anisotropic screening of the Coulomb force by electrons in the presence of a strongly quantizing magnetic field which leads to Friedel oscillations in the ion-ion potential. In particular, the Friedel oscillations are much longer-ranged in the direction of the magnetic field than is the case in the absence of magnetic fields, a factor that has been neglected in previous studies. These "Friedel crystals" have very anisotropic elastic moduli, with potentially interesting implications for the Quasi-periodic Oscillations seen in the X-ray flux of magnetars during their giant flares. We find the minimum energy configuration of ions taking into account these anisotropic effects and find that, depending on the density, temperature and magnetic field strength, different lattice structures (fcc, hcp or bcc oriented in different ways in relation to the magnetic field) are favored.

astro-ph.HE↗

arXiv2013

Friedel crystals and the outer crust of magnetars

The strong magnetic fields found on the surface of magnetars are known to have profound effects in the physics of atoms in magnetar envelopes. We argue that the Friedel oscillations in the Coulomb force between the ions due to electron shielding can, for certain values of the parameters, be the dominant effect determining the crystal structure in the outer crust of magnetars. We estimate the densities and magnetic fields for which this occurs, compute some of the elastic moduli and lattice phonon dispersion relations in this "Friedel crystal" phase.

astro-ph.HE↗

arXiv2012

Thermodynamics of nuclear condensates and phase transitions in white dwarfs

We study the thermodynamics of helium at densities relevant for white dwarf physics. We find evidence that, as the temperature is increased, there is first a first order transition between two superconducting phases followed by a second order transition to the normal state. These transitions occur, for realistic densities, at temperatures below the crystallization temperature and the crystalline state is likely to remain as the true ground state of the system. The calculations are performed with a screening but non-dynamical electron background and we comment on the impact of this and other approximations to our result.

astro-ph.HE↗

arXiv2011

Stable vortex loops in two-species BECs

We consider the creation of stable, stationary closed vortex loops, analogue to the vortons and superconducting cosmic strings, in cold atom BEC's. We explore the parameter region where these solutions are likely to exist and comment on methods to create them experimentally.

cond-mat.quant-gas↗