arXiv subjects

Gautam Rupak

Publications and source records attributed to Gautam Rupak.

At least 55 records · Page 3Linked to original sources

arXiv2008

Viscous damping of r-mode oscillations in compact stars with quark matter

We determine characteristic timescales for the viscous damping of r-mode oscillations in rapidly rotating compact stars that contain quark matter. We present results for the color-flavor-locked (CFL) phase of dense quark matter, in which the up, down and strange quarks are gapped, as well as the normal (ungapped) quark phase. While the ungapped quark phase supports a temperature window between 10^8 K and 5x10^9 K where the r-mode is damped even for rapid rotation, the r-mode in a rapidly rotating pure CFL star is not damped in the temperature range 10^10 K - 10^11 K. Rotating hybrid stars with quark matter cores display an instability window whose width is determined by the amount of quark matter present, and they can have large spin frequencies outside this window. Except at high temperatures T > 10^10 K, the presence of a quark phase allows for larger critical frequencies and smaller spin-periods compared to rotating neutron stars. If low-mass X-ray binaries contain a large amount of ungapped or CFL quark matter, then our estimates of the r-mode instability suggest that there should be a population of rapidly rotating binaries at frequencies greater than 1000 Hz which have not yet been observed.

nucl-th↗

arXiv2007

Shear viscosity of a superfluid Fermi gas in the unitarity limit

We compute the shear viscosity of a superfluid atomic Fermi gas in the unitarity limit. The unitarity limit is characterized by a divergent scattering length between the atoms, and it has been argued that this will result in a very small viscosity. We show that in the low temperature T limit the shear viscosity scales as xi^5/T^5, where the universal parameter 'xi' relates the chemical potential and the Fermi energy, mu=xi E_F. Combined with the high temperature expansions of the viscosity our results suggest that the viscosity has a minimum near the critical temperature T_c. A naive extrapolation indicates that the minimum value of the ratio of viscosity over entropy density is within a factor of ~ 5 of the proposed lower bound hbar/(4πk_B).

cond-mat.other↗

arXiv2006

Polarized fermions in the unitarity limit

We consider a polarized Fermi gas in the unitarity limit. Results are calculated analytically up to next-to-leading order in an expansion about d=4 spatial dimensions. We find a first order transition from superfluid to normal phase. The critical chemical potential asymmetry for this phase transition is delta_mu_c= 2/(mu epsilon)*(1-0.467ε), where epsilon=4-d is the expansion parameter and 'mu' is the average chemical potential of the two fermion species. Stability of the superfluid phase in the presence of supercurrents is also studied.

cond-mat.other↗

arXiv2006

Dimer scattering in the epsilon expansion

The atom-dimer and dimer-dimer scattering lengths are analytically calculated in an expansion around four spatial dimensions for fermions with a large 2-body scattering length 'a'. We find the atom-dimer scattering length a_ad/a = 4/3-2/9 epsilon and the dimer-dimer scattering length a_dd/a = epsilon -0.344 epsilon^2, where epsilon = 4-d and d is the number of spatial dimensions. These ratios a_ad/a = 1.11 and a_dd/a = 0.656 at epsilon = 1 are to be compared with the non-perturbatively calculated numerical results 1.18 and 0.6 respectively. The neutron-deuteron scattering length in the quartet channel a_nD = 4.78 fm is in reasonable agreement with the experimental value 6.35 fm, considering 2-body effective range corrections r_0/a = 0.4 were excluded. The deuteron-deuteron scattering length a_DD = 3.15 fm in the spin-2 channel.

nucl-th↗

arXiv2006

Universality in a 2-component Fermi System at Finite Temperature

Thermodynamic properties of a fermionic system close to the unitarity limit, where the 2-body scattering length |a| approaches infinity is studied at high temperature Boltzmann regime. For dilute systems the virial expansion coefficients in the Boltzmann regime are expected, from general arguments, to be universal. A model independent finite temperature T calculation of the third virial coefficient b3(T) is presented. At the unitarity limit, b3(T) = -0.48 is an universal number. The energy density up to the third virial expansion is derived. These results are relevant for dilute neutron matter and could be tested in current atomic experiments on dilute fermionic gases near the Feshbach resonance.

nucl-th↗

arXiv2005

Chiral Perturbation Theory for Staggered Sea Quarks and Ginsparg-Wilson Valence Quarks

We study lattice QCD with staggered sea and Ginsparg-Wilson valence quarks. The Symanzik effective action for this mixed lattice theory, including the lattice spacing contributions of O(a^2), is derived. Using this effective theory we construct the leading order chiral Lagrangian. The masses and decay constants of pseudoscalars containing two Ginsparg-Wilson valence quarks are computed at one loop order.

hep-lat↗

arXiv2005

A nucleon in a tiny box

We use Chiral Perturbation Theory to compute the nucleon mass-shift due to finite volume and temperature effects. Our results are valid up to next-to-leading order in the "\eps-regime" (mL ~ mβ<< 1) as well as in the "p-regime" (mL ~ mβ>> 1). Based on the two leading orders, we discuss the convergence of the expansion as a function of the lattice size and quark masses. This result can be used to extrapolate lattice results obtained from lattice sizes smaller than the pion cloud, avoiding the numerical simulation of physics under theoretical control. An extraction of the low-energy coefficient c_3 of the chiral Lagrangean from lattice simulations at small volumes and a ``magic'' ratio β=1.22262 L might be possible.

hep-lat↗

arXiv2004

Phase Structure of 2-Flavor Quark Matter: Heterogeneous Superconductors

We analyze the free energy of charge and color neutral 2-flavor quark matter within the BCS approximation. We consider both the homogeneous gapless superconducting phase and the heterogeneous mixed phase where normal and BCS superconducting phases coexist. We calculate the surface tension between normal and superconducting phases and use it to compare the free energies of the gapless and mixed phases. Our calculation, which retains only the leading order gradient contribution to the free energy, indicates that the mixed phase is energetically favored over an interesting range of densities of relevance to 2 flavor quark matter in neutron stars.

nucl-th↗

arXiv2003

Quantum Corrections to Dilute Bose Liquids

It was recently shown (A. Bulgac. Phys. Rev. Lett. {\bf 89}, 050402 (2002)) that an entirely new class of quantum liquids with widely tunable properties could be manufactured from bosons (boselets), fermions (fermilets) and their mixtures (ferbolets) by controlling their interaction properties by the means of a Feshbach resonance. We extend the previous mean--field analysis of these quantum liquids by computing the lowest order quantum corrections to the ground state energy and the depletion of the Bose--Einstein condensate and by estimating higher order corrections as well. We show that the quantum corrections are relatively small and controlled by the diluteness parameter $\sqrt{n|a|^3} \ll 1$, even though strictly speaking in this case there is no low density expansion.

cond-mat.stat-mech↗

arXiv2003

Phase separation in asymmetrical fermion superfluids

Motivated by recent developments on cold atom traps and high density QCD we consider fermionic systems composed of two particle species with different densities. We argue that a mixed phase composed of normal and superfluid components is the energetically favored ground state. We suggest how this phase separation can be used as a probe of fermion superfluidity in atomic traps.

cond-mat↗

arXiv2003

Chiral perturbation theory at O(a^2) for lattice QCD

We construct the chiral effective Lagrangian for two lattice theories: one with Wilson fermions and the other with Wilson sea fermions and Ginsparg-Wilson valence fermions. For each of these theories we construct the Symanzik action through order $a^2$. The chiral Lagrangian is then derived, including terms of order $a^2$, which have not been calculated before. We find that there are only few new terms at this order. Corrections to existing coefficients in the continuum chiral Lagrangian are proportional to $a^2$, and appear in the Lagrangian at order $a^2 p^2$ or higher. Similarly, O(4) symmetry breaking terms enter the Symanzik action at order $a^2$, but contribute to the chiral Lagrangian at order $a^2 p^4$ or higher. We calculate the light meson masses in chiral perturbation theory for both lattice theories. At next-to-leading order, we find that there are no order $a^2$ corrections to the valence-valence meson mass in the mixed theory due to the enhanced chiral symmetry of the valence sector.

hep-lat↗

arXiv2003

Goldstone Bosons in the 3P2 Superfluid Phase of Neutron Matter and Neutrino Emission

At the high densities present in the interior of neutron stars, the neutrons are condensed into the 3P2 superfluid phase. While this condensation has little impact on the equation of state, it can have an important role in determining the low-temperature energy-momentum transport properties. The spontaneous breaking of baryon number by the condensate gives rise to the familiar Goldstone boson, but in addition, the spontaneous breaking of rotational invariance by the condensate gives rise to three Goldstone bosons, in general, one for each broken generator of rotations. These Goldstone bosons, which couple to the Z0, provide a new mechanism for neutrino emission. Using a low-energy effective field theory to describe the dynamics of these Goldstone bosons we estimate the neutrino emissivity of dense neutron matter and show that their annihilation is the dominant energy-loss mechanism over a range of temperatures.

nucl-th↗

arXiv2003

Quartet S-wave p-d scattering in EFT

We present a power counting to include Coulomb effects in the three-nucleon system in a low-energy pionless effective field theory (EFT). With this power counting, the quartet S-wave proton-deuteron elastic scattering amplitude is calculated. The calculation includes next-to-leading order (NLO) Coulomb effects and next-to-next-to-leading order (NNLO) strong interaction effects, with an estimated theoretical error of about 7 %. The EFT results agree with potential model calculations and phase shift analysis of experimental data within the estimated errors.

nucl-th↗

arXiv2002

Low Energy Expansion in the Three Body System to All Orders and the Triton Channel

We extend and systematise the power counting for the three-body system, in the context of the ``pion-less'' Effective Field Theory approach, to all orders in the low-energy expansion. We show that a sub-leading part of the three-body force appears at the third order and delineate how the expansion proceeds at higher orders. After discussing the renormalisation issues in a simple bosonic model, we compute the phase shifts for neutron-deuteron scattering in the doublet S wave (triton) channel and compare our results with phase shift analysis and potential model calculations.

nucl-th↗

arXiv2002

Dilute resonating gases and the third virial coefficient

We study dilute gases with short range interactions and large two-body scattering lengths. At temperatures between the condensation temperature and the scale set by therange of the potential there is a high degree of universality. The first two terms in the expansion of thermodynamic functions in powers of the fugacity $z$, which measures the diluteness of the system, are determined by the scattering length only. The term proportional to $z^3$ depends only on one new parameter describing the three-body physics. We compute the third term of the expansion and show that, for many values of this new parameter, the $z^3$ term may be the dominant one.

cond-mat↗

arXiv2002

Chiral perturbation theory for the Wilson lattice action

We extend chiral perturbation theory to include linear dependence on the lattice spacing $a$ for the Wilson action. The perturbation theory is written as a double expansion in the small quark mass $m_q$ and lattice spacing $a$. We present formulae for the mass and decay constant of a flavor-non-singlet meson in this scheme to order $a$ and $m_q^2$. The extension to the partially quenched theory is also described.

hep-lat↗

arXiv2000

Nucleon Polarisabilities from Compton Scattering on the Deuteron

An analytic calculation of the differential cross section for elastic Compton scattering on the deuteron at photon energies ωin the range of 25-50 MeV is presented to next-to-next-to-leading order, i.e. to an accuracy of \sim 3%. The calculation is model-independent and performed in the low energy nuclear Effective Field Theory without dynamical pions. The iso-scalar, scalar electric and magnetic nucleon polarisabilities α_0 and β_0 enter as free parameters with a theoretical uncertainty of about 20%. Using data at $ω_{Lab}=49 MeV$ we find $α_0=8.4\pm 3.0(exp)\pm 1.7(theor)$, $β_0=8.9\pm 3.9(exp)\pm 1.8(theor)$, each in units of $10^{-4} fm^3$. With the experimental constraint for the iso-scalar Baldin sum rule, $α_0=7.2\pm 2.1(exp)\pm 1.6(theor)$, $β_0=6.9\mp 2.1(exp)\mp 1.6(theor)$. A more accurate result can be achieved by: (i) better experimental data, and (ii) a higher order theoretical calculation including contributions from a couple of so far undetermined four-nucleon-two-photon operators.

nucl-th↗

arXiv1999

Precision Calculation of n+p->d+gamma Cross Section for Big-Bang Nucleosynthesis

An effective field theory calculation of the n+p->d+gamma cross section accurate to 1% for center of mass energy E up to ~1 MeV is presented. At these energies, which are relevant for big-bang nucleosynthesis, isovector magnetic transitions M1 and isovector electric transitions E1 give the dominant contributions. The M1 amplitude is calculated up to next-to-next-to-leading order (NNLO) and the contribution from the associated four-nucleon-one-photon operator is determined from the cold neutron capture rate. The E1 amplitude is calculated up to NNNNLO. The four-nucleon-one-photon operator contribution to E1 is determined from the related deuteron photodisintegration reaction gamma+d->n+p.

nucl-th↗