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A. Misra

Publications and source records attributed to A. Misra.

30 records · Page 2Linked to original sources
arXiv2000

Field-Dependent BRS Tranformations and Correct Prescription for 1/(n.k)^p-Type Singularities in Axial Gauges

The axial-gauge boson propagator contains 1/(n.k)^p-type singularities. These singularities have generally been treated by inventing prescriptions for them. We propose an alternative procedere for treating these singularities in the path-integral formalism using the known way of treating the 1/k^{2n}-type singularities in Lorentz-type gauges. For this purpose we use a finite field-dependent BRS transformation that inerpolates between the Lorentz and axial-type gauges. We arrive at the ε-dependent tree propagator in axial-type gauges.

hep-th↗
arXiv2000

Derivation of O(q^4) Effective Lagrangian in the Presence of External Fields Directly Within Heavy Baryon Chiral Perturbation Theory

This is a progress report on the extension of the analysis of hep-ph/9909498 to constructing a complete list of O(q^4) terms in the presence of external fields, directly within Heavy Baryon Chiral Perturbation Theory (HBChPT) {\it without having to first construct the relativistic BChPT Lagrangian and then carry out the ${1\over\rm m}$-reduction}. In addition to a phase rule to implement all symmetries including charge conjugation invariance directly at the nonrelativistic level, generalized Jacobi identities, curvature relation (that relates the commutator of two covariant derivatives to a linear combination of the traceless and isosinglet field strengths and the commutator of two axial vector building blocks), Schouten's identity and the relationship between the antisymmetrized covariant derivative - axial-(building block) vector commutator and another traceless field strength, are used to ensure linear independence of the terms and their low energy coupling constants. We first construct O($q^4$) terms for off-shell nucleons, and then perform the on-shell reduction, again within HBChPT.

hep-ph↗
arXiv1999

Correct Treatment of 1/(η\cdot k)^p Singularities in the Axial Gauge Propagator

The propagators in axial-type, light-cone and planar gauges contain 1/(η\cdot k)^p-type singularities. These singularities have generally been treated by inventing prescriptions for them. In this work, we propose an alternative procedure for treating these singularities in the path integral formalism using the known way of treating the singularities in Lorentz gauges. To this end, we use a finite field-dependent BRS transformation that interpolates between Lorentz-type and the axial-type gauges. We arrive at the $ε$-dependent tree propagator in the axial-type gauges. We examine the singularity structure of the propagator and find that the axial gauge propagator so constructed has {\it no} spurious poles (for real $k$). It however has a complicated structure in a small region near $η\cdot k=0$. We show how this complicated structure can effectively be replaced by a much simpler propagator.

hep-th↗
arXiv1999

2-Nucleon 1-Loop Corrections to Pion Double Charge Exchange within Heavy Baryon Chiral Perturbation Theory

One-loop corrections at the two-nucleon level to Pion Double Charge exchange (DCX) scattering off a nuclear target at threshold, are calculated within the framework of Heavy Baryon Chiral Perturbation Theory (HBChPT). An estimate for the (2-nucleon) 1-loop correction is obtained in the static limit and using an impulse approximation. We find a small (1.6%) increase relative to the leading order tree graphs.

nucl-th↗
arXiv1999

Relating Green's Functions in Axial and Lorentz Gauges using Finite Field-Dependent BRS Transformations

We use finite field-dependent BRS transformations (FFBRS) to connect the Green functions in a set of two otherwise unrelated gauge choices. We choose the Lorentz and the axial gauges as examples. We show how the Green functions in axial gauge can be written as a series in terms of those in Lorentz gauges. Our method also applies to operator Green's functions. We show that this process involves another set of related FFBRS transfomations that is derivable from infinitesimal FBRS. We suggest possible applications.

hep-th↗
arXiv1999

Derivation of O(q^4) Effective Pion-Nucleon Lagrangian Within Heavy Baryon Chiral Perturbation Theory

We construct a complete list of O(q^4) terms directly within Heavy Baryon Chiral Perturbation Theory (HBChPT) in the absence of external fields and assuming isospin symmetry. In addition to a phase rule recently developed (NPA646, 343, '99), a variety of algebraic identities and reparameterization invariance, are used to ensure linear independence of the terms and their low energy coupling constants. We first construct O(q^4) terms terms for off-shell nucleons, and then perform the on-shell reduction, again within HBChPT. We discuss an application of the O(q^4) terms to the evaluation of the O(q^4) operator insertions in the "contact graph" of pion double charge exchange.

hep-ph↗
arXiv1999

A Derivation of the Correct Treatment of 1/(eta.k)^p Singularities in Axial Gauges

We use the earlier results on the correlations of axial gauge Green's functions and the Lorentz gauge Green's functions obtained via finite field-dependent BRS transformations to study the question of the correct treatment of 1/(eta\cdot k)^p- type singularities in the axial gauge boson propagator. We show how the known treatment of the 1/(k^2)^n-type singularity in the Lorentz-type gauges can be used to write down the axial propagator via field transformation. We examine the singularity structure of the latter and find that the axial propagator so constructed has $no$ spurious poles, but a complex structure near $η\cdot k=0$.

hep-th↗
arXiv1999

Length and time scale divergences at the magnetization-reversal transition in the Ising model

The divergences of both the length and time scales, at the magnetization- reversal transition in Ising model under a pulsed field, have been studied in the linearized limit of the mean field theory. Both length and time scales are shown to diverge at the transition point and it has been checked that the nature of the time scale divergence agrees well with the result obtained from the numerical solution of the mean field equation of motion. Similar growths in length and time scales are also observed, as one approaches the transition point, using Monte Carlo simulations. However, these are not of the same nature as the mean field case. Nucleation theory provides a qualitative argument which explains the nature of the time scale growth. To study the nature of growth of the characteristic length scale, we have looked at the cluster size distribution of the reversed spin domains and defined a pseudo-correlation length which has been observed to grow at the phase boundary of the transition.

cond-mat.stat-mech↗
arXiv1998

Derivation of the Effective Pion-Nucleon Lagrangian within Heavy Baryon Chiral Perturbation Theory

We develop a method for constructing the Heavy Baryon Chiral Perturbation Theory Lagrangian (L_{HBChPT}), to a given chiral order, within HBChPT. We work within SU(2) theory, with only the pion field interacting with the nucleon. The main difficulties, which are solved, are to develop techniques for implementing charge conjugation invariance, and for taking the nucleon on shell, both within the nonrelativistic formalism. We obtain complete lists of independent terms in L_{HBChPT} through O(q^3) for off- shell nucleons. Then, eliminating equation-of-motion (eom) terms at the relativistic and nonrelativistic level (both within HBChPT), we obtain L_{HBCHPT} for on-shell nucleons, through O(q^3). The extension of the method (to obtain on-shell L_{HBChPT} within HBChPT) to higher orders is also discussed.

nucl-th↗
arXiv1998

Dynamic Magnetization-Reversal Transition in the Ising Model

We report the results of mean field and the Monte Carlo study of the dynamic magnetization-reversal transition in the Ising model, brought about by the application of an external field pulse applied in opposition to the existing order before the application of the pulse. The transition occurs at a temperature T below the static critical temperature T_c without any external field. The transition occurs when the system, perturbed by the external field pulse competing with the existing order, jumps from one minimum of free energy to the other after the withdrawal of the pulse. The parameters controlling the transition are the strength h_p and the duration Delta t of the pulse. In the mean field case, approximate analytical expression is obtained for the phase boundary which agrees well with that obtained numerically in the small Delta t and large T limit. The order parameter of the transition has been identified and is observed to vary continuously near the transition. The order parameter exponent beta was estimated both for the mean field (beta =1) and the Monte Carlo beta = 0.90 \pm 0.02 in two dimension) cases. The transition shows a "critical slowing-down" type behaviour near the phase boundary with diverging relaxation time. The divergence was found to be logarithmic in the mean field case and exponential in the Monte Carlo case. The finite size scaling technique was employed to estimate the correlation length exponent nu (= 1.5 \pm 0.3 in two dimension) in the Monte Carlo case.

cond-mat.stat-mech↗
arXiv1997

Spin-Reversal Transition in Ising Model under Pulsed Field

In this communication we report the existence of a dynamic ``spin-reversal'' transition in an Ising system perturbed by a pulsed external magnetic field. The transition is achieved by tuning the strength ($h_p$) and/or the duration ($Δt$) of the pulse which is applied in a direction opposite to the existing order. We have studied this transition in the kinetic Ising Model in two dimension using Monte Carlo technique, and solved numerically the mean field equation of motion. The transition is essentially dynamic in nature and it takes the system from one ordered equilibrium phase to another by means of the growth of opposite spin domains (in the kinetic Ising case) induced during the period when the pulsed field is applied.

cond-mat↗