47 research outputs found
Bounds from Stability and Symmetry Breaking on Parameters in the Two-Higgs-Doublet Potential
The most general -symmetric quartic potential with two
Higgs doublets, subject to an only softly broken discrete symmetry
, is considered. At tree-level, analytic
bounds on the parameters are derived that ensure a stable vacuum, breaking
down to .Comment: from last year, but due to recent interest now on the e-print
archive, 8 pages, no figures, plain late
Universal anisotropic finite-size critical behavior of the two-dimensional Ising model on a strip and of d-dimensional models on films
Anisotropy effects on the finite-size critical behavior of a two-dimensional
Ising model on a general triangular lattice in an infinite-strip geometry with
periodic, antiperiodic, and free boundary conditions (bc) in the finite
direction are investigated. Exact results are obtained for the scaling
functions of the finite-size contributions to the free energy density. With
xi_> the largest and xi_< the smallest bulk correlation length at a given
temperature near criticality, we find that the dependence of these functions on
the ratio xi_ and on the angle parameterizing the orientation of the
correlation volume is of geometric rather than dynamic origin. Since the
scaling functions are independent of the particular microscopic realization of
the anisotropy within the two-dimensional Ising model, our results provide a
limited verification of universality. We explain our observations by
considering finite-size scaling of free energy densities of general weakly
anisotropic models on a d-dimensional film, i.e., in an L x infinity^(d-1)
geometry, with bc in the finite direction that are invariant under a shear
transformation relating the anisotropic and isotropic cases. This allows us to
relate free energy scaling functions in the presence of an anisotropy to those
of the corresponding isotropic system. We interpret our results as a simple and
transparent case of anisotropic universality, where, compared to the isotropic
case, scaling functions depend additionally on the shape and orientation of the
correlation volume. We conjecture that this universality extends to cases where
the geometry and/or the bc are not invariant under the shear transformation and
argue in favor of validity of two-scale factor universality for anisotropic
systems.Comment: 16 pages, 4 figures; ref. [14] correcte
Perturbative Finite-Temperature Results and Pad'e Approximants
Pad'e approximants are used to improve the convergence behavior of
perturbative results in massless scalar and gauge field theories at finite
temperature.Comment: 11 pages, LaTeX, incl. 10 ps-figures; typo in caption of fig.4
corrected; to appear in Phys.Rev.
Fluctuation Pressure of a Membrane Between Walls Through Five Loops
An earlier four-loop calculation of the fluctuation pressure of a fluid
membrane between two infinite walls is extended to five loops. Variational
perturbation theory is used to extract the hard-wall limit from perturbative
results obtained with a smooth potential. Comparison with a structurally
similar quantum mechanics problem of a particle in a box is used for an
alternative way of extracting the membrane pressure and also to estimate the
quality of the results. Our values lie above the best available Monte Carlo
data.Comment: 18 pages, 3 figures, typos in references fixe
Shift of BEC Temperature of Homogenous Weakly Interacting Bose Gas
We report on the computation of the shift of the Bose-Einstein condensation
temperature for a homogenous weakly interacting Bose gas in leading order in
the diluteness parameter a n^(1/3), where `a' is the scattering length and `n'
is the particle density. The perturbative series, which is afflicted by
infrared divergences, is resummed by means of variational perturbation theory.
Using coefficients through seven loops, we arrive at Delta T_c/T_c = 1.27 +/-
0.11 a n^(1/3), which compares favorably with recent Monte-Carlo data.Comment: Talk presented at the 12th International Laser Physics Workshop,
LPHYS'03 (Hamburg, Germany, August 25-29, 2003
Four-Loop Vacuum Energy Beta Function in O(N) Symmetric Scalar Theory
The beta function of the vacuum energy density is computed at the four-loop
level in massive O(N) symmetric phi^4 theory. Dimensional regularization is
used in conjunction with the MSbar scheme and all calculations are in momentum
space in the massive theory. The result is
beta_v = g N/4+g^3 N(N+2)/96+g^4 N(N+2)(N+8)[12 zeta(3)-25]/1296+o(g^5).Comment: 16 pages, latex, no figures. Enlarged and updated reference list,
minor typographical change
Non-universal Critical Quantities from Variational Perturbation Theory and Their Application to the BEC Temperature Shift
For an O(N) symmetric scalar field theory with Euclidean action integral d^3x
[1/2 |nabla phi|^2 + 1/2 r phi^2 + 1/4! u phi^4], where phi = (phi_1,...,phi_N)
is a vector of N real field components, variational perturbation theory through
seven loops is employed for N = 0,1,2,3,4 to compute the renormalized value of
r/(N+2)u^2 at the phase transition. Its exact large-N limit is determined as
well. We also extend an earlier computation of the interaction-induced shift
Delta/Nu for N = 1,2,4 to N = 0,3. For N = 2, the results for the two
quantities are used to compute the second-order shift of the condensation
temperature of a dilute Bose gas, both in the homogenous case and for the wide
limit of a harmonic trap. Our results are in agreement with earlier Monte Carlo
simulations for N = 1,2,4. The appendix contains previously unpublished
numerical seven-loop data provided to us by B.Nickel.Comment: 19 page
Efficient Algorithm for Perturbative Calculation of Multiloop Feynman Integrals
We present an efficient algorithm for calculating multiloop Feynman integrals
perturbatively.Comment: Author Information under
http://www.physik.fu-berlin.de/~kleinert/institution.html . Latest update of
paper also at http://www.physik.fu-berlin.de/~kleinert/29
The Free Energy of Hot Gauge Theories with Fermions Through g^5
We compute the free energy density for gauge theories, with fermions, at
high temperature and zero chemical potential. In the expansion , we determine
and analytically by calculating two- and three-loop diagrams. The
term constitutes the first correction to the term and is for the
non-Abelian case the last power of that can be computed within perturbation
theory. We find that the term receives no contributions from overlapping
double-frequency sums and that vanishes.Comment: 31 pages, 6 figures, LaTeX; minor beautifications, reference list
extended, version to be published in Phys.Rev.
Charge Transport and Quantum Phase Transitions in Singlet Superconductor - Ferromagnet - Singlet Superconductor Junctions
We study the Josephson current, I_J, in a junction consisting of two s-wave
superconductors that are separated by a ferromagnetic barrier possessing a
magnetic and non-magnetic scattering potential, g and Z, respectively. We
discuss the general dependence of I_J on g, Z, and the phase difference \phi
between the two superconductors. Moreover, we compute the critical current, I_c
for given g and Z, and show that it possesses two lines of non-analyticity in
the (g, Z)-plane. We identify those regions in the (g, Z)-plane where the
Josephson current changes sign with increasing temperature without a change in
the relative phase between the two superconductors, i.e., without a transition
between a 0 and \pi state of the junction. Finally, we show that by changing
the relative phase \phi, it is possible to tune the junction through a
first-order quantum phase transition in which the spin polarization of the two
superconductors' combined ground state changes from =0 to =1/2.Comment: final version, published in Phys. Rev.