67 research outputs found
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
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
Boson Stars from Self-Interacting Dark Matter
We study the possibility that self-interacting bosonic dark matter forms
star-like objects. We study both the case of attractive and repulsive
self-interactions, and we focus particularly in the parameter phase space where
self-interactions can solve well standing problems of the collisionless dark
matter paradigm. We find the mass radius relations for these dark matter
bosonic stars, their density profile as well as the maximum mass they can
support.Comment: 12 pages, 6 figures; references adde
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
Classical Nonrelativistic Effective Field Theory and the Role of Gravitational Interactions
Coherent oscillation of axions or axion-like particles may give rise to
long-lived clumps, called axion stars, because of the attractive gravitational
force or its self-interaction. Such a kind of configuration has been
extensively studied in the context of oscillons without the effect of gravity,
and its stability can be understood by an approximate conservation of particle
number in a non-relativistic effective field theory (EFT). We extend this
analysis to the case with gravity to discuss the longevity of axion stars and
clarify the EFT expansion scheme in terms of gradient energy and Newton's
constant. Our EFT is useful to calculate the axion star configuration and its
classical lifetime without any ad hoc assumption. In addition, we derive a
simple stability condition against small perturbations. Finally, we discuss the
consistency of other non-relativistic effective field theories proposed in the
literature.Comment: 37 pages, 3 figure
Probing Bosonic Overdensities with Optomechanical Sensing
Previous work has shown that optomechanical force sensing can be used for
efficient detection of ultralight (sub-eV) dark matter candidates. We propose
to extend the reach of this method to the search for ultralight dark matter in
gravitationally-bound configurations in the Milky Way. We consider three
scenarios, each strongly-motivated by previous studies: boson stars traveling
in the galaxy with virial velocity; a bosonic halo centered around the Sun (a
`solar halo'); and a bosonic halo centered around the Earth. For each case, we
consider bound states composed of either scalar particles with a Yukawa
coupling, or vector particles coupled to baryon minus lepton number charge.
Accounting for all experimental constraints on coupling strength, we estimate
the sensitivity reach of an optomechanical sensor search. We conclude that,
although boson star encounters with Earth would be too infrequent to be
detected in the relevant parameter space, current optomechanical force sensing
technologies provide promising search capabilities for solar or Earth-bound
halos.Comment: 18 pages, 1 figur
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