20 research outputs found
Topography of the hot sphaleron Transitions
By numerical simulations in {\it real time} we provide evidence in favour of
sphaleron like transitions in the hot, symmetric phase of the electroweak
theory. Earlier performed observations of a change in the Chern-Simons number
are supplemented with a measurement of the lowest eigenvalues of the
three-dimensional staggered fermion Dirac operator and observations of the
spatial extension of energy lumps associated with the transition. The
observations corroborate on the interpretation of the change in Chern-Simons
numbers as representing continuum physics, not lattice artifacts. By combining
the various observations it is possible to follow in considerable detail the
time-history of thermal fluctuations of the classical gauge-field
configurations responsible for the change in the Chern-Simons number.Comment: 11 pages. No figures (sorry, but ps files too huge). Latex file.
NBI-HE-92-5
Grand-Canonical Ensemble of Random Surfaces with Four Species of Ising Spins
The grand-canonical ensemble of dynamically triangulated surfaces coupled to
four species of Ising spins (c=2) is simulated on a computer. The effective
string susceptibility exponent for lattices with up to 1000 vertices is found
to be . A specific scenario for models is
conjectured.Comment: LaTeX, 11 pages + 1 postscript figure appended, preprint LPTHE-Orsay
94/1
Screening of Very Intense Magnetic Fields by Chiral Symmetry Breaking
In very intense magnetic fields, T, the breaking of
the strong interaction symmetry arranges itself so that
instead of the neutral field acquiring a vacuum expectation value it
is the charged field that does and the magnetic field is screened.
Details are presented for a magnetic field generated by a current in a wire; we
show that the magnetic field is screened out to a distance from the wire.Comment: 7 pages (using macropackage REVTEX II
On the Absence of an Exponential Bound in Four Dimensional Simplicial Gravity
We have studied a model which has been proposed as a regularisation for four
dimensional quantum gravity. The partition function is constructed by
performing a weighted sum over all triangulations of the four sphere. Using
numerical simulation we find that the number of such triangulations containing
simplices grows faster than exponentially with . This property ensures
that the model has no thermodynamic limit.Comment: 8 pages, 2 figure
Ising-link Quantum Gravity
We define a simplified version of Regge quantum gravity where the link
lengths can take on only two possible values, both always compatible with the
triangle inequalities. This is therefore equivalent to a model of Ising spins
living on the links of a regular lattice with somewhat complicated, yet local
interactions. The measure corresponds to the natural sum over all 2^links
configurations, and numerical simulations can be efficiently implemented by
means of look-up tables. In three dimensions we find a peak in the ``curvature
susceptibility'' which grows with increasing system size. However, the value of
the corresponding critical exponent as well as the behavior of the curvature at
the transition differ from that found by Hamber and Williams for the Regge
theory with continuously varying link lengths.Comment: 11 page
A Lorentzian cure for Euclidean troubles
There is strong evidence coming from Lorentzian dynamical triangulations that
the unboundedness of the gravitational action is no obstacle to the
construction of a well-defined non-perturbative path integral. In a continuum
approach, a similar suppression of the conformal divergence comes about as the
result of a non-trivial path-integral measure.Comment: 3 page
Generalized Penner models to all genera
We give a complete description of the genus expansion of the one-cut solution
to the generalized Penner model. The solution is presented in a form which
allows us in a very straightforward manner to localize critical points and to
investigate the scaling behaviour of the model in the vicinity of these points.
We carry out an analysis of the critical behaviour to all genera addressing all
types of multi-critical points. In certain regions of the coupling constant
space the model must be defined via analytical continuation. We show in detail
how this works for the Penner model. Using analytical continuation it is
possible to reach the fermionic 1-matrix model. We show that the critical
points of the fermionic 1-matrix model can be indexed by an integer, , as it
was the case for the ordinary hermitian 1-matrix model. Furthermore the 'th
multi-critical fermionic model has to all genera the same value of
as the 'th multi-critical hermitian model. However, the
coefficients of the topological expansion need not be the same in the two
cases. We show explicitly how it is possible with a fermionic matrix model to
reach a multi-critical point for which the topological expansion has
alternating signs, but otherwise coincides with the usual Painlev\'{e}
expansion.Comment: 27 pages, PostScrip
Strong Sphalerons and Electroweak Baryogenesis
We analyze the spontaneous baryogenesis and charge transport mechanisms
suggested by Cohen, Kaplan and Nelson for baryon asymmetry generation in
extended versions of electroweak theory. We find that accounting for
non-perturbative chirality-breaking transitions due to strong sphalerons
reduces the baryonic asymmetry by the factor or ,
provided those processes are in thermal equilibrium.Comment: CERN-TH.7080/9
On the Segregation Phenomenon in Complex Langevin Simulation
In the numerical simulation of certain field theoretical models, the complex
Langevin simulation has been believed to fail due to the violation of
ergodicity. We give a detailed analysis of this problem based on a toy model
with one degree of freedom (). We find that the failure is
not due to the defect of complex Langevin simulation itself, but rather to the
way how one treats the singularity appearing in the drift force. An effective
algorithm is proposed by which one can simulate the behaviour of
the expectation value in the small limit.Comment: (20 pages + 8 figures on request). Siegen Si-93-8, Tokuyama TKYM-93-
Non-perturbative Thermodynamics in Matrix String Theory
A study of the thermodynamics in IIA Matrix String Theory is presented. The
free string limit is calculated and seen to exactly reproduce the usual result.
When energies are enough to excite non-perturbative objects like D-particles
and specially membranes, the situation changes because they add a large number
of degrees of freedom that do not appear at low energies. There seems to be a
negative specific heat (even in the Microcanonical Ensemble) that moves the
asymptotic temperature to zero. Besides, the mechanism of interaction and
attachment of open strings to D-particles and D-membranes is analyzed.Comment: 30 pages, 3 figures. Several points are clarified. Final version to
appear in Nucl. Phys. B. Also available at http://condmat1.ciencias.uniovi.e