409 research outputs found
High-T QCD and dimensional reduction: measuring the Debye mass
We study the high-temperature phase of SU(2) and SU(3) QCD using lattice
simulations of an effective 3-dimensional SU(N) + adjoint Higgs -theory,
obtained through dimensional reduction. We investigate the phase diagram of the
3D theory, and find that the high-T QCD phase corresponds to the metastable
symmetric phase of the 3D theory. We measure the Debye screening mass m_D with
gauge invariant operators; in particular we determine the O(g^2) and O(g^3)
corrections to m_D. The corrections are seen to be large, modifying the
standard power-counting hierarchy in high temperature QCD.Comment: 3 pages, Latex, 3 figures. Presented by K. Rummukainen at Lattice '9
Gravitational waves from the sound of a first order phase transition
We report on the first three-dimensional numerical simulations of first-order phase transitions in the early Universe to include the cosmic fluid as well as the scalar field order parameter. We calculate the gravitational wave (GW) spectrum resulting from the nucleation, expansion, and collision of bubbles of the low-temperature phase, for phase transition strengths and bubble wall velocities covering many cases of interest. We find that the compression waves in the fluid continue to be a source of GWs long after the bubbles have merged, a new effect not taken properly into account in previous modeling of the GW source. For a wide range of models, the main source of the GWs produced by a phase transition is, therefore, the sound the bubbles make
The Electroweak Phase Transition in a Magnetic Field
We study the finite temperature electroweak phase transition in an external
hypercharge U(1) magnetic field H_Y, using lattice Monte Carlo simulations. For
sufficiently small fields, H_Y/T^2 < 0.3, the magnetic field makes the first
order transition stronger, but it still turns into a crossover for Higgs masses
m_H ~ 80 GeV. For larger fields, we observe a mixed phase analogous to a type I
superconductor, where a single macroscopic tube of the symmetric phase,
parallel to H_Y, penetrates through the broken phase. For the magnetic fields
and Higgs masses studied, we did not see indications of the expected
Ambjorn-Olesen phase, which should be similar to a type II superconductor.Comment: 20 pages, 7 figures. Discussion on lattice results extended. To
appear in Nucl.Phys.
Numerical simulations of necklaces in SU(2) gauge-Higgs field theory
We perform the first numerical simulations of necklaces in a non-Abelian gauge theory. Necklaces are composite classical solutions which can be interpreted as monopoles trapped on strings, rather generic structures in a Grand Unified Theory. We generate necklaces from random initial conditions, modeling a phase transition in the early Universe, and study the evolution. For all cases, we find that the necklace system shows scaling behavior similar to that of a network of ordinary cosmic strings. Furthermore, our simulations indicate that comoving distance between the monopoles or semipoles along the string asymptotes to a constant value at late times. This means that, while the monopole-to-string energy density ratio decreases as the inverse of the scale factor, a horizon-size length of string has a large number of monopoles, significantly affecting the dynamics of string loops. We argue that gravitational wave bounds from millisecond pulsar timing on the string tension in the Nambu-Goto scenario are greatly relaxed.Peer reviewe
Numerical simulations of acoustically generated gravitational waves at a first order phase transition
We present details of numerical simulations of the gravitational radiation produced by a first order thermal phase transition in the early Universe. We confirm that the dominant source of gravitational waves is sound waves generated by the expanding bubbles of the low-temperature phase. We demonstrate that the sound waves have a power spectrum with a power-law form between the scales set by the average bubble separation (which sets the length scale of the fluid flow Lf) and the bubble wall width. The sound waves generate gravitational waves whose power spectrum also has a power-law form, at a rate proportional to Lf and the square of the fluid kinetic energy density. We identify a dimensionless parameter ˜ ΩGW characterizing the efficiency of this “acoustic” gravitational wave production whose value is 8π ˜ ΩGW≃0.8±0.1 across all our simulations. We compare the acoustic gravitational waves with the standard prediction from the envelope approximation. Not only is the power spectrum steeper (apart from an initial transient) but the gravitational wave energy density is generically larger by the ratio of the Hubble time to the phase transition duration, which can be 2 orders of magnitude or more in a typical first order electroweak phase transition
Gauge-invariant strings in the 3d U(1)+Higgs theory
We describe how the strings, which are classical solutions of the continuum
three-dimensional U(1)+Higgs theory, can be studied on the lattice. The effect
of an external magnetic field is also discussed and the first results on the
string free energy are presented. It is shown that the string free energy can
be used as an order parameter when the scalar self-coupling is large and the
transition is continuous.Comment: LATTICE98(higgs); missing author added, no changes to tex
Localisation and mass generation for non-Abelian gauge fields
It has been suggested recently that in the presence of suitably "warped"
extra dimensions, the low-energy limit of pure gauge field theory may contain
massive elementary vector bosons localised on a "brane", but no elementary
Higgs scalars. We provide non-perturbative evidence in favour of this
conjecture through numerical lattice measurements of the static quark-antiquark
force of pure SU(2) gauge theory in three dimensions, of which one is warped.
We consider also warpings leading to massless localised vector bosons, and
again find evidence supporting the perturbative prediction, even though the
gauge coupling diverges far from the brane in this case.Comment: 27 pages; small clarifications adde
First-Order Electroweak Phase Transition in the Standard Model with a Low Cutoff
We study the possibility of a first-order electroweak phase transition (EWPT)
due to a dimension-six operator in the effective Higgs potential. In contrast
with previous attempts to make the EWPT strongly first-order as required by
electroweak baryogenesis, we do not rely on large one-loop thermally generated
cubic Higgs interactions. Instead, we augment the Standard Model (SM) effective
theory with a dimension-six Higgs operator. This addition enables a strong
first-order phase transition to develop even with a Higgs boson mass well above
the current direct limit of 114 GeV. The phi^6 term can be generated for
instance by strong dynamics at the TeV scale or by integrating out heavy
particles like an additional singlet scalar field. We discuss conditions to
comply with electroweak precision constraints, and point out how future
experimental measurements of the Higgs self couplings could test the idea.Comment: 5 pages, 4 figures. v2: corrected typos, improved discussion of the
case lambda<0 and added references. To be published in PR
Vacancy Defects In O-Doped Gan Grown By Molecular-Beam Epitaxy: The Role Of Growth Polarity And Stoichiometry
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