17,521 research outputs found
Charged analogue of Finch-Skea stars
We present solutions to the Einstein-Maxwell system of equations in
spherically symmetric gravitational fields for static interior spacetimes with
a specified form of the electric field intensity. The condition of pressure
isotropy yields three category of solutions. The first category is expressible
in terms of elementary functions and does not have an uncharged limit. The
second category is given in terms of Bessel functions of half-integer order.
These charged solutions satisfy a barotropic equation of state and contain
Finch-Skea uncharged stars. The third category is obtained in terms of modified
Bessel functions of half-integer order and does not have an uncharged limit.
The physical features of the charged analogue of the Finch-Skea stars are
studied in detail. In particular the condition of causality is satisfied and
the speed of sound does not exceed the speed of light. The physical analysis
indicates that this analogue is a realistic model for static charged
relativistic perfect fluid spheres.Comment: 17 pages, To appear in Int. J. Mod. Phys.
New ways to access the transverse spin content of the nucleon
We first describe a new way to access the chiral odd transversity parton
distribution in the proton through the photoproduction of lepton pairs. The
basic ingredient is the interference of the usual Bethe-Heitler or Drell-Yan
amplitudes with the amplitude of a process, where the photon couples to quarks
through its chiral-odd distribution amplitude, which is normalized to the
magnetic susceptibility of the QCD vacuum. We also show how the chiral-odd
transversity generalized parton distributions (GPDs) of the nucleon can be
accessed experimentally through the exclusive electro - or photoproduction
process of a meson pair with a large invariant mass and when the final nucleon
has a small transverse momentum. We calculate perturbatively the scattering
amplitude at leading order, both in the high energy domain which may be
accessed in electron-ion colliders and in the medium energy range. Estimated
rates are encouraging.Comment: 6 pages, Proceedings of Spin 2010, 19th International Spin Physics
Symposium September 27 - October 2, 2010, J\"ulich, German
Threshold Resummation for W-Boson Production at RHIC
We study the resummation of large logarithmic perturbative corrections to the
partonic cross sections relevant for the process pp -> W^+- X at the BNL
Relativistic Heavy Ion Collider (RHIC). At RHIC, polarized protons are
available, and spin asymmetries for this process will be used for precise
measurements of the up and down quark and anti-quark distributions in the
proton. The corrections arise near the threshold for the partonic reaction and
are associated with soft-gluon emission. We perform the resummation to
next-to-leading logarithmic accuracy, for the rapidity-differential cross
section. We find that resummation leads to relatively moderate effects on the
cross sections and spin asymmetries.Comment: 25 pages, 15 figures as eps files. One reference added and typo
correcte
Probing CP violation with the electric dipole moment of atomic mercury
The electric dipole moment of atomic Hg induced by the nuclear Schiff
moment and tensor-pseudotensor electron-nucleus interactions has been
calculated. For this, we have developed and employed a novel method based on
the relativistic coupled-cluster theory. The results of our theoretical
calculations combined with the latest experimental result of Hg
electric dipole moment, provide new bounds on the T reversal or CP violation
parameters , the tensor-pseudotensor coupling constant
and . This is the most accurate
calculation of these parameters to date. We highlight the the crucial role of
electron correlation effects in their interplay with the P,T violating
interactions. Our results demonstrate substantial changes in the results of
earlier calculations of these parameters which can be attributed to the more
accurate inclusion of important correlation effects in the present work.Comment: 4 pages and 1 figur
Exact solutions for semirelativistic problems with non-local potentials
It is shown that exact solutions may be found for the energy eigenvalue
problem generated by the class of semirelativistic Hamiltonians of the form H =
sqrt{m^2+p^2} + hat{V}, where hat{V} is a non-local potential with a separable
kernel of the form V(r,r') = - sum_{i=1}^n v_i f_i(r)g_i(r'). Explicit examples
in one and three dimensions are discussed, including the Yamaguchi and Gauss
potentials. The results are used to obtain lower bounds for the energy of the
corresponding N-boson problem, with upper bounds provided by the use of a
Gaussian trial function.Comment: 13 pages, 3 figure
Quasi-simultaneous five-frequency VLBA observations of PKS 0528+134
We present results of Very Long Baseline Array (VLBA) observations of PKS
0528+134 at five frequencies (2.3, 5.0, 8.4, 15.4, and 22.2 GHz). These
quasi-simultaneous data enable us to study the spectral distribution of Very
Long Baseline Interferometer (VLBI) components for the first time in this
highly variable source, from which the central compact core is identified. Our
observations indicate that there are two bendings for the jet motion at parsec
scale. We provide an approximate spatial fit to the curved jet trajectory using
the Steffen et al. (\cite{Steffen95}) helical model. We further investigate the
proper motions of three jet components, which all show superluminal motion. At
high frequencies (15.4 and 22.2 GHz) we detected a new component, which is
estimated to be related to a radio burst peaking at about 2000.Comment: 11 pages, 5 figures, 3 table
Quenching Dynamics of a quantum XY spin-1/2 chain in presence of a transverse field
We study the quantum dynamics of a one-dimensional spin-1/2 anisotropic XY
model in a transverse field when the transverse field or the anisotropic
interaction is quenched at a slow but uniform rate. The two quenching schemes
are called transverse and anisotropic quenching respectively. Our emphasis in
this paper is on the anisotropic quenching scheme and we compare the results
with those of the other scheme. In the process of anisotropic quenching, the
system crosses all the quantum critical lines of the phase diagram where the
relaxation time diverges. The evolution is non-adiabatic in the time interval
when the parameters are close to their critical values, and is adiabatic
otherwise. The density of defects produced due to non-adiabatic transitions is
calculated by mapping the many-particle system to an equivalent Landau-Zener
problem and is generally found to vary as , where is the
characteristic time scale of quenching, a scenario that supports the
Kibble-Zurek mechanism. Interestingly, in the case of anisotropic quenching,
there exists an additional non-adiabatic transition, in comparison to the
transverse quenching case, with the corresponding probability peaking at an
incommensurate value of the wave vector. In the special case in which the
system passes through a multi-critical point, the defect density is found to
vary as . The von Neumann entropy of the final state is shown to
maximize at a quenching rate around which the ordering of the final state
changes from antiferromagnetic to ferromagnetic.Comment: 8 pages, 6 figure
Phase transition in a directed traffic flow network
The generic feature of traffic in a network of flowing electronic data
packets is a phase transition from a stationary free-flow phase to a
continuously growing congested non-stationary phase. In the most simple network
of directed oriented square lattice we have been able to observe all crucial
features of such flow systems having non-trivial critical behavior near the
critical point of transition. The network here is in the shape of a square
lattice and data packets are randomly posted with a rate at one side of
the lattice. Each packet executes a directed diffusive motion towards the
opposite boundary where it is delivered. Packets accumulated at a particular
node form a queue and a maximum of such packets randomly jump out of this
node at every time step to its neighbors on a first-in-first-out (FIFO) basis.
The phase transition occurs at . The distribution of travel times
through the system is found to have a log-normal behavior and the
power-spectrum of the load time-series shows like noise similar to the
scenario of Internet traffic.Comment: Six pages, seven figure
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