180,356 research outputs found
Bose-Einstein Condensation Temperature of a Homogeneous Weakly Interacting Bose Gas : PIMC study
Using a finite-temperature Path Integral Monte Carlo simulation (PIMC) method
and finite-size scaling, we have investigated the interaction-induced shift of
the phase transition temperature for Bose-Einstein condensation of homogeneous
weakly interacting Bose gases in three dimensions, which is given by a proposed
analytical expression , where
is the critical temperature for an ideal gas, is the s-wave
scattering length, and is the number density. We have used smaller number
densities and more time slices than in the previous PIMC simulations [Gruter
{\it et al.}, Phys. Rev. Lett. {\bf 79}, 3549 (1997)] in order to understand
the difference in the value of the coefficient between their results
and the (apparently) other reliable results in the literature. Our results show
that depends strongly on the
interaction strength while the previous PIMC results are
considerably flatter and smaller than our results. We obtain = 1.32
0.14, in agreement with results from recent Monte Carlo methods of
three-dimensional O(2) scalar field theory and variational
perturbation theory
Spin correlations and velocity-scaling in color-octet NRQCD matrix elements
We compute spin-dependent decay matrix elements for S-wave charmonium and
bottomonium in lattice nonrelativistic quantum chromodynamics (NRQCD).
Particular emphasis is placed upon the color-octet matrix elements, since the
corresponding production matrix elements are expected to appear in the dominant
contributions to the production cross sections at large transverse momenta. We
use three slightly different versions of the heavy-quark lattice Green's
functions in order to minimize the contributions that scale as powers of the
ultraviolet cutoff. The lattice matrix elements that we calculate obey the
hierarchy that is suggested by the velocity-scaling rules of NRQCD.Comment: 25 pages, 2 figures, 2 tables, further discussion of effective
lattice cutoffs and uncertainties, additional minor revisions, version to be
published in Phys. Rev.
Cancellation of soft and collinear divergences in noncommutative QED
In this paper, we investigate the behavior of non-commutative IR divergences
and will also discuss their cancellation in the physical cross sections. The
commutative IR (soft) divergences existing in the non-planar diagrams will be
examined in order to prove an all order cancellation of these divergences using
the Weinberg's method. In non-commutative QED, collinear divergences due to
triple photon splitting vertex, were encountered, which are shown to be
canceled out by the non-commutative version of KLN theorem. This guarantees
that there is no mixing between the Collinear, soft and non-commutative IR
divergences
Perturbative matching of staggered four-fermion operators with hypercubic fat links
We calculate the one-loop matching coefficients between continuum and lattice
four-fermion operators for lattice operators constructed using staggered
fermions and improved by the use of fattened links. In particular, we consider
hypercubic fat links and SU(3) projected Fat-7 links, and their mean-field
improved versions. We calculate only current-current diagrams, so that our
results apply for operators whose flavor structure does not allow
``eye-diagrams''. We present general formulae, based on two independent
approaches, and give numerical results for the cases in which the operators
have the taste (staggered flavor) of the pseudo-Goldstone pion. We find that
the one-loop corrections are reduced down to the 10-20% level, resolving the
problem of large perturbative corrections for staggered fermion calculations of
matrix elements.Comment: 37 pages, no figure, 20 table
Double charmonium production at B-factories within light cone formalism
This paper is devoted to the study of the processes e^+e^- \to J/\Psi \eta_c,
J/\Psi \eta_c', \psi' \eta_c, \psi' \eta_c' within light cone formalism. It is
shown that if one disregards the contribution of higher fock states, the
twist-3 distribution amplitudes needed in the calculation can be unambiguously
determined from the twist-2 distribution amplitudes and equations of motion.
Using models of the twist-2 distribution amplitudes the cross sections of the
processes under study have been calculated. The results of the calculation are
in agreement with Belle and BaBar experiments. It is also shown that
relativistic and radiative corrections to the cross sections play crucial role
in the achievement of the agreement between the theory and experiments. The
comparison of the results of this paper with the results obtained in other
papers has been carried out. In particular, it is shown that the results of
papers where relativistic and radiative corrections were calculated within
NRQCD are overestimated by a factor of ~1.5.Comment: 14 pages, 1 figur
Relations Between Low-lying Quantum Wave Functions and Solutions of the Hamilton-Jacobi Equation
We discuss a new relation between the low lying Schroedinger wave function of
a particle in a one-dimentional potential V and the solution of the
corresponding Hamilton-Jacobi equation with -V as its potential. The function V
is , and can have several minina (V=0). We assume the problem to be
characterized by a small anhamornicity parameter and a much smaller
quantum tunneling parameter between these different minima.
Expanding either the wave function or its energy as a formal double power
series in and , we show how the coefficients of
in such an expansion can be expressed in terms of definite
integrals, with leading order term determined by the classical solution of the
Hamilton-Jacobi equation. A detailed analysis is given for the particular
example of quartic potential .Comment: LaTex, 48 pages, no figur
The Ground State of the Pseudogap in Cuprate Superconductors
We present studies of the electronic structure of La2-xBaxCuO4, a system
where the superconductivity is strongly suppressed as static spin and charge
orders or "stripes" develop near the doping level of x=1/8. Using
angle-resolved photoemission and scanning tunneling microscopy, we detect an
energy gap at the Fermi surface with magnitude consistent with d-wave symmetry
and with linear density of states, vanishing only at four nodal points, even
when superconductivity disappears at x=1/8. Thus, the non-superconducting,
"striped" state at x=1/8 is consistent with a phase incoherent d-wave
superconductor whose Cooper pairs form spin/charge ordered structures instead
of becoming superconducting.Comment: This is the author's version of the wor
Ionospheric refraction effects on orbit determination using the orbit determination error analysis system
The influence of ionospheric refraction on orbit determination was studied through the use of the Orbit Determination Error Analysis System (ODEAS). The results of a study of the orbital state estimate errors due to the ionospheric refraction corrections, particularly for measurements involving spacecraft-to-spacecraft tracking links, are presented. In current operational practice at the Goddard Space Flight Center (GSFC) Flight Dynamics Facility (FDF), the ionospheric refraction effects on the tracking measurements are modeled in the Goddard Trajectory Determination System (GTDS) using the Bent ionospheric model. While GTDS has the capability of incorporating the ionospheric refraction effects for measurements involving ground-to-spacecraft tracking links, such as those generated by the Ground Spaceflight Tracking and Data Network (GSTDN), it does not have the capability to incorporate the refraction effects for spacecraft-to-spacecraft tracking links for measurements generated by the Tracking and Data Relay Satellite System (TDRSS). The lack of this particular capability in GTDS raised some concern about the achievable accuracy of the estimated orbit for certain classes of spacecraft missions that require high-precision orbits. Using an enhanced research version of GTDS, some efforts have already been made to assess the importance of the spacecraft-to-spacecraft ionospheric refraction corrections in an orbit determination process. While these studies were performed using simulated data or real tracking data in definitive orbit determination modes, the study results presented here were obtained by means of covariance analysis simulating the weighted least-squares method used in orbit determination
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