405 research outputs found
Multicanonical Cluster Algorithm and the 2-D 7-State Potts Model
I present a hybrid-like two-step algorithm, which combines a microcanonical
update of a spin system using demons, with a multicanonical demon refresh. The
algorithm is free from the supercritical slowing down that burdens the
canonical methods: the exponential increase of the tunnelling time between the
metastable states in the first-order phase transitions, when the volume of the
system is increased. The demons act as a buffer between the multicanonical heat
bath and the spin system, allowing the spin system to be updated with any
microcanonical demon procedure, including cluster methods. The cluster
algorithm is demonstrated with the 2-dimensional 7-state Potts model, using
volumes up to . The tunnelling time is found to increase as ,
where is the linear dimension of the system.Comment: 14 pages, 8 ps-figures, the flashy one missing to save
space+troubles, sorry, the whole thing available from the author. Preprint
CERN-TH.6654/9
What's new with the electroweak phase transition?
We review the status of non-perturbative lattice studies of the electroweak
phase transition. In the Standard Model, the complete phase diagram has been
reliably determined, and the conclusion is that there is no phase transition at
all for the experimentally allowed Higgs masses. In the Minimal Supersymmetric
Standard Model (MSSM), in contrast, there can be a strong first order
transition allowing for baryogenesis. Finally, we point out possibilities for
future simulations, such as the problem of CP-violation at the MSSM electroweak
phase boundary.Comment: LATTICE98(electroweak), 6 pages. List of references update
A Strong Electroweak Phase Transition up to m_H ~ 105 GeV
Non-perturbative lattice simulations have shown that there is no electroweak
phase transition in the Standard Model for the allowed Higgs masses, m_H \gsim
75 GeV. In the Minimal Supersymmetric Standard Model, in contrast, it has been
proposed that the transition should exist and even be strong enough for
baryogenesis up to m_H ~ 105 GeV, provided that the lightest stop mass is in
the range 100...160 GeV. However, this prediction is based on perturbation
theory, and suffers from a noticeable gauge parameter and renormalization scale
dependence. We have performed large-scale lattice Monte Carlo simulations of
the MSSM electroweak phase transition. Extrapolating the results to the
infinite volume and continuum limits, we find that the transition is in fact
stronger than indicated by 2-loop perturbation theory. This guarantees that the
perturbative Higgs mass bound m_H ~ 105 GeV is a conservative one, allows
slightly larger stop masses (up to ~ 165 GeV), and provides a strong motivation
for further studies of MSSM electroweak baryogenesis.Comment: 4 pages, 3 figure
3D Physics and the Electroweak Phase Transition: Perturbation Theory
We develop a method for the construction of the effective potential at high
temperatures based on the effective field theory approach and renormalization
group. It allows one to sum up the leading logarithms in all orders of
perturbation theory. The method reproduces the known one-loop and two-loop
results in a very simple and economic way and clarifies the issue of the
convergence of the perturbation theory. We also discuss the assumptions being
made for the determination of the critical temperature of the electroweak phase
transition, and analyse different perturbative uncertainties in its
determination. These results are then used for the non-perturbative lattice
Monte Carlo simulations of the EW phase transition in forthcoming paper.Comment: 44 pages, preprint CERN-TH.6973/9
First order thermal phase transition with 126 GeV Higgs mass
We study the strength of the electroweak phase transition in models with two
light Higgs doublets and a light SU(3)_c triplet by means of lattice
simulations in a dimensionally reduced effective theory. In the parameter
region considered the transition on the lattice is significantly stronger than
indicated by a 2-loop perturbative analysis. Within some ultraviolet
uncertainties, the finding applies to MSSM with a Higgs mass m_h approximately
126 GeV and shows that the parameter region useful for electroweak baryogenesis
is enlarged. In particular (even though only dedicated analyses can quantify
the issue), the tension between LHC constraints after the 7 TeV and 8 TeV runs
and frameworks where the electroweak phase transition is driven by light stops,
seems to be relaxed.Comment: Presented at 31st International Symposium on Lattice Field Theory -
LATTICE 201
O(2) symmetry breaking vs. vortex loop percolation
We study with lattice Monte Carlo simulations the relation of global O(2)
symmetry breaking in three dimensions to the properties of a geometrically
defined vortex loop network. We find that different definitions of constructing
a network lead to different results even in the thermodynamic limit, and that
with typical definitions the percolation transition does not coincide with the
thermodynamic phase transition. These results show that geometrically defined
percolation observables need not display universal properties related to the
critical behaviour of the system, and do not in general survive in the field
theory limit.Comment: 14 pages; references added, version to appear in Phys.Lett.
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