62 research outputs found
Lattice field theories with an energy current
We investigate a lattice scalar field theory in the presence of a bias
favouring the establishment of an energy current, as a model for stationary
nonequilibrium processes at low temperature in a non-integrable system. There
is a transition at a finite value of the bias to a gapless modulated phase
which carries a classical current; however, unlike in similar, integrable,
models, quantum effects also allow for a non-zero current at arbitrarily small
bias. The transition is second order in the magnetically disordered phase, but
is pre-empted by a first-order transition in the ferromagnetic case, at least
at the mean-field level.Comment: 19 pages, LaTeX, 3 postscript figure
A study of the XY model by the Monte Carlo method
The massively parallel processor is used to perform Monte Carlo simulations for the two dimensional XY model on lattices of sizes up to 128 x 128. A parallel random number generator was constructed, finite size effects were studied, and run times were compared with those on a CRAY X-MP supercomputer
Stability of D-brane embeddings in nontrivial backgrounds
We propose a new analytical method for determining whether nonsupersymmetric
probe branes embedded in nontrivial backgrounds are perturbatively stable or
not. The method is based on a relationship between zero mass solutions of the
relevant DBI equations of motion and tachyonic solutions. Furthermore, due to
the above relation, the question, of whether a classical solution is stable or
not, can be answered simply by studying the derivatives of that solution with
respect to its integration constants. Finally, we illustrate the efficiency of
this method by applying it to several interesting examples.Comment: 18 pages; introductory material added in Section 2, journal versio
Decay of Ultralight Axion Condensates
Axion particles can form macroscopic condensates, whose size can be galactic
in scale for models with very small axion masses eV, and which
are sometimes referred to under the name of Fuzzy Dark Matter. Many analyses of
these condensates are done in the non-interacting limit, due to the weakness of
the self-interaction coupling of axions. We investigate here how certain
results change upon inclusion of these interactions, finding a decreased
maximum mass and a modified mass-radius relationship. Further, these
condensates are, in general, unstable to decay through number-changing
interactions. We analyze the stability of galaxy-sized condensates of
axion-like particles, and sketch the parameter space of stable configurations
as a function of a binding energy parameter. We find a strong lower bound on
the size of Fuzzy Dark Matter condensates which are stable to decay, with
lifetimes longer than the age of the universe.Comment: 24 pages, 2 figures. v2: Added brief discussion of angular momentum;
extended Appendix A; typos correcte
QCD Axion Star Collapse with the Chiral Potential
In a previous work, we analyzed collapsing axion stars using the low-energy
instanton potential, showing that the total energy is always bounded and that
collapsing axion stars do not form black holes. In this paper, we provide a
proof that the conclusions are unchanged when using instead the more general
chiral potential for QCD axions.Comment: 11 page
Collisions of Dark Matter Axion Stars with Astrophysical Sources
If QCD axions form a large fraction of the total mass of dark matter, then
axion stars could be very abundant in galaxies. As a result, collisions with
each other, and with other astrophysical bodies, can occur. We calculate the
rate and analyze the consequences of three classes of collisions, those
occurring between a dilute axion star and: another dilute axion star, an
ordinary star, or a neutron star. In all cases we attempt to quantify the most
important astrophysical uncertainties; we also pay particular attention to
scenarios in which collisions lead to collapse of otherwise stable axion stars,
and possible subsequent decay through number changing interactions. Collisions
between two axion stars can occur with a high total rate, but the low relative
velocity required for collapse to occur leads to a very low total rate of
collapses. On the other hand, collisions between an axion star and an ordinary
star have a large rate, collisions/year/galaxy, and
for sufficiently heavy axion stars, it is plausible that most or all such
collisions lead to collapse. We identify in this case a parameter space which
has a stable region and a region in which collision triggers collapse, which
depend on the axion number () in the axion star, and a ratio of mass to
radius cubed characterizing the ordinary star (). Finally, we
revisit the calculation of collision rates between axion stars and neutron
stars, improving on previous estimates by taking cylindrical symmetry of the
neutron star distribution into account. Collapse and subsequent decay through
collision processes, if occurring with a significant rate, can affect dark
matter phenomenology and the axion star mass distribution.Comment: 19 pages, 5 figures. v2: References added, typos correcte
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