2,770 research outputs found
Axions and ALPs: a very short introduction
The QCD axion was originally predicted as a dynamical solution to the strong
CP problem. Axion like particles (ALPs) are also a generic prediction of many
high energy physics models including string theory. Theoretical models for
axions are reviewed, giving a generic multi-axion action with couplings to the
standard model. The couplings and masses of these axions can span many orders
of magnitude, and cosmology leads us to consider several distinct populations
of axions behaving as coherent condensates, or relativistic particles. Light,
stable axions are a mainstay dark matter candidate. Axion cosmology and
calculation of the relic density are reviewed. A very brief survey is given of
the phenomenology of axions arising from their direct couplings to the standard
model, and their distinctive gravitational interactions.Comment: This article is a longer version of material contributed to the 13th
Patras Workshop on Axions, WIMPs and WISPs, Thessaloniki, May 15 to 19, 201
Axion Cosmology
1. Introduction 2. Models: the QCD axion; the strong CP problem; PQWW, KSVZ,
DFSZ; anomalies, instantons and the potential; couplings; axions in string
theory 3. Production and I.C.'s: SSB and non-perturbative physics; the axion
field during inflation and PQ SSB; cosmological populations - decay of parent,
topological defects, thermal production, vacuum realignment 4. The Cosmological
Field: action; background evolution; misalignment for QCD axion and ALPs;
cosmological perturbation theory - i.c.'s, early time treatment, axion sound
speed and Jeans scale, transfer functions and WDM; the Schrodinger picture;
simualting axions; BEC 5. CMB and LSS: Primary anisotropies; matter power;
combined constraints; Isocurvature and inflation 6. Galaxy Formation; halo mass
function; high-z and the EOR; density profiles; the CDM small-scale crises 7.
Accelerated expansion: the c.c. problem; axion inflation (natural and
monodromy) 8. Gravitational interactions with black holes and pulsars 9.
Non-gravitational interactions: stellar astrophysics; LSW; vacuum
birefringence; axion forces; direct detection with ADMX and CASPEr; Axion
decays; dark radiation; astrophysical magnetic fields; cosmological
birefringence 10. Conclusions A Theta vacua of gauge theories B EFT for
cosmologists C Friedmann equations D Cosmological fluids E Bayes Theorem and
priors F Degeneracies and sampling G Sheth-Tormen HMFComment: v2 greatly extended: 111 pages, 38 figures. Accepted for publication
in Physics Report
An ultralight pseudoscalar boson
Using a fundamental discrete symmetry, , we construct a
two-axion model with the QCD axion solving the strong- problem, and an
ultralight axion (ULA) with providing
the dominant form of dark matter (DM). The ULA is light enough to be detectable
in cosmology from its imprints on structure formation, and may resolve the
small-scale problems of cold DM. The necessary relative DM abundances occur
without fine tuning in constructions with decay constants , and . An example model
achieving this has , and we construct a range of other possibilities. We
compute the ULA couplings to the Standard Model, and discuss prospects for
direct detection. The QCD axion may be detectable in standard experiments
through the and couplings. In the simplest
models, however, the ULA has identically zero coupling to both of
QCD and of electromagnetism due to vanishing
electromagnetic and color anomalies. The ULA couples to fermions with strength
. This coupling causes spin precession of nucleons and
electrons with respect to the DM wind with period months. Current limits
do not exclude the predicted coupling strength, and our model is within reach
of the CASPEr-Wind experiment, using nuclear magnetic resonance.Comment: 14 pages, 3 figures. v2 numerical error on N corrected, conclusions
unchanged. Typos and notation corrected. Matches version published in PR
Ultra-Light Scalar Fields and the Growth of Structure in the Universe
Ultra-light scalar fields, with masses of between m=10^{-33} eV and
m=10^{-22} eV, can affect the growth of structure in the Universe. We identify
the different regimes in the evolution of ultra-light scalar fields, how they
affect the expansion rate of the universe and how they affect the growth rate
of cosmological perturbations. We find a number of interesting effects, discuss
how they might arise in realistic scenarios of the early universe and comment
on how they might be observed.Comment: 12 pages, 11 figure
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