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A. A. Usmani

Publications and source records attributed to A. A. Usmani.

At least 37 records · Page 2Linked to original sources
arXiv2012

The (2+1)-dimensional charged gravastars

This is a continuation and generalization of our earlier work on {\it gravastar} in (2+1) anti-de Sitter space-time to 2+1 dimensional solution of charged gravastar. Morphologically this gravastar contains three regions namely: (i) charged interior, (ii) charged shell and (iii) electrovacuum exterior. We have studied different characteristics in terms of Length and Energy, Entropy, and Junction conditions of the spherical charged distribution. It is shown that the present model of charged gravastar is non-singular and represents itself an alternative of Black Hole.

physics.gen-ph↗
arXiv2012

Galactic rotation curves inspired by a noncommutative-geometry background

This paper discusses the observed at rotation curves of galaxies in the context of noncommutative geometry. The energy density of such a geometry is diffused throughout a region due to the uncertainty encoded in the coordinate commutator. This intrinsic property appears to be sufficient for producing stable circular orbits, as well as attractive gravity, without the need for dark matter.

gr-qc↗
arXiv2012

The (2+1)-dimensional gravastars

We propose a new model of a gravastar in (2+1) anti-de Sitter space-time. This new three dimensional configuration has three different regions with different equations of state: [I] Interior: 0 \leq r < r_1, ρ= -p ; [II] Shell: r_1 \leq r < r_2, ρ= p; [III] Exterior: r_2 < r, ρ= p =0. The outer region of this gravastar corresponds to the exterior (2+1) anti-de Sitter space-time, popularly known as the BTZ space-time. Like BTZ model, Λis taken to be negative, which at the junction turns out to be positive as required by stability of gravastar and mathematical consistency. After investigating the Interior space-time, Shell and Exterior space-time we have highlighted different physical features in terms of Length and Energy, Entropy, and Junction conditions of the spherical distribution. It is shown that the present model of charge-free gravastar in connection to the exterior (2+1) anti-de Sitter space-time or the BTZ space-time is stable and non-singular.

gr-qc↗
arXiv2010

Time variable $Λ$ and the accelerating Universe

We perform a deductive study of accelerating Universe and focus on the importance of variable time-dependent $Λ$ in the Einstein's field equations under the phenomenological assumption, $Λ=αH^2$ for the full physical range of $α$. The relevance of variable $Λ$ with regard to various key issues like dark matter, dark energy, geometry of the field, age of the Universe, deceleration parameter and barotropic equation of state has been trivially addressed. The deceleration parameter and the barotropic equation of state parameter obey a straight line relationship for a flat Universe described by Friedmann and Raychaudhuri equations. Both the parameters are found identical for $α= 1$.

gr-qc↗
arXiv2010

Thin-shell wormholes from charged black holes in generalized dilaton-axion gravity

This paper discusses a new type of thin-shell wormhole constructed by applying the cut-and-paste technique to two copies of a charged black hole in generalized dilaton-axion gravity, which was inspired by low-energy string theory. After analyzing various aspects of this thin-shell wormhole, we discuss its stability to linearized spherically symmetric perturbations.

gr-qc↗
arXiv2009

Features of galactic halo in a brane world model and observational constraints

Several aspects of the 4d imprint of the 5d bulk Weyl radiation are investigated within a recently proposed model solution. It is shown that the solution has a number of physically interesting properties. The constraints on the model imposed by combined measurements of rotation curve and lensing are discussed. A brief comparison with a well known scalar field model is also given.

gr-qc↗
arXiv2008

Galactic rotation curves and brane world models

In the present investigation flat rotational curves of the galaxies are considered under the framework of brane-world models where the 4d effective Einstein equation has extra terms which arise from the embedding of the 3-brane in the $5d$ bulk. It has been shown here that these long range bulk gravitational degrees of freedom can act as a mechanism to yield the observed galactic rotation curves without the need for dark matter. The present model has the advantage that the observed rotation curves result solely from well-established non-local effects of gravitation, such as dark radiation and dark pressure under a direct use of the condition of flat rotation curves and does not invoke any exotic matter field.

astro-ph↗
arXiv2008

The dark energy equation of state

We perform a study of cosmic evolution with an equation of state parameter $ω(t)=ω_0+ω_1(t\dot H/H)$ by selecting a phenomenological $Λ$ model of the form, $\dotΛ\sim H^3$. This simple proposition explains both linearly expanding and inflationary Universes with a single set of equations. We notice that the inflation leads to a scaling in the equation of state parameter, $ω(t)$, and hence in equation of state. In this approach, one of its two parameters have been pin pointed and the other have been delineated. It has been possible to show a connection between dark energy and Higgs-Boson.

gr-qc↗
arXiv2006

Behaviour of the potentials due to strangeness degree of freedom in $_{ΛΛ}^6$He hypernucleus

Fully correlated study of $_{ΛΛ} ^6$He hypernucleus has been performed with two- and three- baryon potentials. For the S=-2 sector, various simulations of Nijmegen $ΛΛ$ potential models have been used. We investigate the role of every strength of the strange sector potentials on the energy breakdown and present a crystal clear understanding of their interplay. Consistency between $_Λ^5$He and $_{ΛΛ}^6$He depends on the $ΛN$ space-exchange strength only. Investigation limits the strength of simulated Nijmegen $ΛΛ$ potential models. The study is a step forward to determine all the strengths, to reslove A=5 anomaly and to search for $_{ΛΛ}^4$H in an authentic way.

nucl-th↗
arXiv2006

Behaviour of the $ΛN$ and $ΛNN$ potential strengths in the $_Λ^5$He hypernucleus

Variational study of $_Λ^5$He hypernucleus is presented using a realistic Hamiltonian and a fully correlated wave function including $ΛN$ space-exchange correlation. Behaviour of the $Λ$-separation energy ($B_Λ$) with two- and three- baryon potential strengths is thoroughly investigated. Solutions of the potential strengths giving experimental $B_Λ$ are presented.

nucl-th↗
arXiv2004

$ΛN$ space-exchange correlation effects in $_Λ^5$He hypernucleus

A complete realistic study of $_Λ^5$He hypernucleus is presented using a realistic Hamiltonian and wave function. This study takes into account all relevant dynamical correlations along with $ΛN$ space-exchange correlation (SEC). We also compute $ΛNN$ force and the corelation induced by this force. The SEC affects the central repulsive $λN$ correlation significantly at $r \le 2.0$ fm, specially at its peak and in its vicinity. SEC significantly affects energy breakdown of the hypernucleus, $Λ$-seperation energy, nuclear core (NC) polarization and density profiles. A lage NC polarization is found with and without SEC, respectively. The SEC effect is relatively large in two-pion exchange component of $ΛNN$ force. Therefore, any attempt to pin down the strength of this force with no SEC would be flawed.

nucl-th↗
arXiv2004

QCD motivated three-body force and light nuclei

The need for the three-body forces (3BF) for reproduction of physical properties of light nuclei has been evident for some time. These 3BF are doing quite well for light nuclei, but there seems to be scope for other 3BF which may arise from other physical considerations not included so far. Here we discuss a QCD motivated new 3BF which should be included in studies of nuclei, in particular the light one3s. We perform variational Monte Carlo calculations including this new 3BF and discuss improvements brought in thereby for light nuclei.

nucl-th↗
arXiv2000

Final state interactions in $^{4}$He(e,e$^\prime$p)$^{3}$H at large proton energy

At large proton energy, the final state interactions of the knocked out nucleon in $\eep$ reactions off nuclear targets can be described within the eikonal approximation, treating the spectator particles as a collection of fixed scattering centers. We use a generalization of this approach, suitable to take into account the possible occurrence of color transparency, to carry out an accurate calculation of the missing momentum distribution of the process $^{4}He(e,e^\prime p)^{3}H$. The pattern of final state interaction effect is analyzed for different kinematical setups in the domain corresponding to $2 \le Q^2 \le 20$ (GeV/c)$^2$.

nucl-th↗
arXiv1996

On the missing momentum dependence of the color transparency effects in (e,e'p) scattering

We explore the missing momentum dependence of the CT effects in quasielastic (e,e'p) scattering. We develop the coupled-channel multiple-scattering theory (CCMST) description of final-state interaction including both the coherent and incoherent rescatterings of the ejectile state. We demonstrate that the contribution of the off-diagonal incoherent rescattering does not vanish at low Q^{2} which is a novel correction to the conventional Glauber theory evaluation of nuclear transparency. We comment on the nontrivial impact of this correction on the onset of CT. The sensitivity of the onset of CT to the 3q-nucleon reggeon amplitudes is discussed for the first time. We present numerical results for nuclear transparency as a function of the missing momentum for exclusive (e,e'p) reaction in the kinematical region of Q^{2} \le 40 GeV^{2} and p_{m} \le 250$ MeV/c. Our evaluations show that at Q^{2}\sim 10 CT effects are substantial only in antiparallel kinematics at p_{m,z}\sim -250 MeV/c. The effect is enhanced on light nuclei and could be observed in a high precision experiment.

nucl-th↗
arXiv1996

Glauber theory of initial- and final-state interactions in (p,2p) scattering

We develop the Glauber theory description of initial- and final-state interactions (IFSI) in quasielastic A(p,2p) scattering. We study the IFSI-distortion effects both for the inclusive and exclusive conditions. In inclusive reaction the important new effect is an interaction between the two sets of the trajectories which enter the calculation of IFSI-distorted one-body density matrix for inclusive (p,2p) scattering and are connected with incoherent elastic rescatterings of the initial and final protons on spectator nucleons. We demonstrate that IFSI-distortions of the missing momentum distribution are large over the whole range of missing momentum both for inclusive and exclusive reactions and affect in a crucial way the interpretation of the BNL data on (p,2p) scattering. Our numerical results show that in the region of missing momentum $p_{m}\lsim 100-150$ MeV/c the incoherent IFSI increase nuclear transparency by 5-10\%. The incoherent IFSI become dominant at $p_{m}\gsim 200$ MeV/c.

nucl-th↗
arXiv1995

The longitudinal asymmetry of the $(e,e'p)$ missing momentum distribution as a signal of color transparency

We use multiple scattering theory to evaluate the $Q^{2}$ dependence of the forward-backward asymmetry, $A_{z}$, of the missing momentum distribution in quasielstic $A(e,e'p)$ scattering, which is expected to be affected by color transparency. The novel features of our analysis are a consistent treatment of the structure of the ejectile state formed after absorption of the virtual photon by the struck proton and a careful evaluation of the background asymmetry induced by the nonvanishing real part of the proton-nucleon scattering amplitude. We find that the absolute magnitude of $A_{z}$ is dominated by this background at the $Q^{2}$'s attainable at the Continuous Electron Beam Accelerator Facility. However, the $Q^{2}$ dependence of the asymmetry is sensitive to the onset of color transparency, whose observation in high statistics experiments appears to be feasible, particularly using light targets such as $^{12}C$ or $^{16}O$.

nucl-th↗
arXiv1995

The three-baryon $ΛNN$ potential

Using a three-baryon $ΛNN$ potential having a spin-independent dispersive and a two-pion exchange components, realistic variational Monte Carlo calculations have been performed for the $Λ$-seperation energy for $^{5}_Λ$He. A relationship between strengths associated with both pieces of $ΛNN$ force giving exact experimental $Λ$-seperation energy has been obtained which then combined with our previous study on $^{17}_{\,\,Λ}$O hypernucleus determines a unique and acceptable set of strength parameters for $ΛNN$ potential. The $ΛNN$ force is found important for the core polarization.

nucl-th↗