523 research outputs found
The distance and neutral environment of the massive stellar cluster Westerlund 1
The goal of this study is to determine a distance to Westerlund 1 independent
of the characteristics of the stellar population and to study its neutral
environment, using observations of atomic hydrogen. The HI observations are
taken from the Southern Galactic Plane Survey to study HI absorption in the
direction of the HII region created by the members of Westerlund 1 and to
investigate its environment as observed in the HI line emission. A Galactic
rotation curve was derived using the recently revised values for the Galactic
centre distance of kpc, and the velocity of the Sun around the
Galactic centre of km s. The newly determined
rotation model leads us to derive a distance of kpc to Westerlund
1, consistent with a location in the Scutum-Crux Arm. Included in this estimate
is a very careful investigation of possible sources of error for the Galactic
rotation curve. We also report on small expanding HI features around the
cluster with a maximum dynamic age of 600,000 years and a larger bubble which
has a minimum dynamic age of 2.5 million years. Additionally we re-calculated
the kinematic distances to nearby HII regions and supernova remnants based on
our new Galaxic rotation curve. We propose that in the early stages of the
development of Wd 1 a large interstellar bubble of diameter about 50 pc was
created by the cluster members. This bubble has a dynamic age similar to the
age of the cluster. Small expanding bubbles, with dynamical ages Myr
are found around Wd 1, which we suggest consist of recombined material lost by
cluster members through their winds.Comment: 8 pages, accepted for publication in A&
Upper and Lower Limits on Neutralino WIMP Mass and Spin--Independent Scattering Cross Section, and Impact of New (g-2)_{mu} Measurement
We derive the allowed ranges of the spin--independent interaction cross
section \sigsip for the elastic scattering of neutralinos on proton for wide
ranges of parameters of the general Minimal Supersymmetric Standard Model. We
investigate the effects of the lower limits on Higgs and superpartner masses
from colliders, as well as the impact of constraints from \bsgamma and the
new measurement of \gmtwo on the upper and lower limits on \sigsip. We
further explore the impact of the neutralino relic density, including
coannihilation, and of theoretical assumptions about the largest allowed values
of the supersymmetric parameters. For , requiring the latter to lie
below 1\tev leads to \sigsip\gsim 10^{-11}\pb at \mchi\sim100\gev and
\sigsip\gsim 10^{-8}\pb at \mchi\sim1\tev. When the supersymmetric
parameters are allowed above 1\tev, for 440\gev \lsim \mchi\lsim 1020 \gev
we derive a {\em parameter--independent lower limit} of \sigsip \gsim 2\times
10^{-12}\pb. (No similar lower limits can be set for nor for
1020\gev\lsim\mchi\lsim2.6\tev.) Requiring \abundchi<0.3 implies a {\em
parameter--independent upper limit} \mchi\lsim2.6\tev. The new \epem--based
measurement of restricts \mchi\lsim 350\gev at CL
and \mchi\lsim515\gev at CL, and implies . The largest
allowed values of \sigsip have already become accessible to recent
experimental searches.Comment: LaTeX, 17 pages, 9 eps figures. Version to appear in JHE
A Simple Way of Calculating Cosmological Relic Density
A simple procedure is presented which leads to a dramatic simplification in
the calculation of the relic density of stable particles in the Universe.Comment: 7 pages in LaTex, no figures; University of Michigan preprint
UM-TH-94-02 (February 1994). Changes: a coefficient in (Eq. 16)
corrected; added Acknowledgements and revised Note Added; plain LaTex only
(no need to use RevTex
Extending preferred axion models via heavy-quark induced early matter domination
We examine the cosmological consequences of the heavy quarks in KSVZ-type
axion models. We find that their presence often causes an early matter
domination phase, altering the evolution of the Universe. This extends the
axion mass into the region where standard cosmology leads to overproduction,
and allows for a greater number of axion models with non-renormalizable terms
to be viable. Quantitatively, we find that decays proceeding through effective
terms of up to dimension 9 () remain consistent with cosmological
constraints, in contrast with the result previously found in the
literature. As a consequence, the heavy quarks can be much heavier and the
axion mass window with the correct relic density for dark matter is extended by
orders of magnitude, down to . This is
achieved without resorting to fine-tuning of the initial misalignment angle,
bolstering the motivation for many future axion haloscope experiments.
Additionally, we explore how these models can be probed through measurements of
the number of relativistic degrees of freedom at recombination.Comment: 24 pages, 6 figure
Exact Cross Sections for the Neutralino WIMP Pair-Annihilation
We derive a full set of exact, analytic expressions for the annihilation of
the lightest neutralino pairs into all two-body tree-level final states in the
framework of minimal supersymmetry. We make no simplifying assumptions about
the neutralino nor about sfermion masses and mixings other than the absence of
explicit CP--violating terms. The expressions should be particularly useful in
computing the neutralino WIMP relic abundance without the usual approximation
of partial wave expansion.Comment: LaTeX, 46 pages, no figures. Several minor typographical errors
correcte
Dark matter production through a non-thermal flavon portal
The Froggatt-Nielsen (FN) mechanism provides an attractive way of generating
the determined fermion mass hierarchy and quark mixing matrix elements in the
Standard Model (SM). Here we extend it by coupling the FN field, the flavon, to
a dark sector containing one or more dark matter particles which are produced
non-thermally sequentially through flavon production. Non-thermal flavon
production occurs efficiently via freeze-in and through field oscillations. We
explore this in the regime of high-scale breaking of the global
group and at the reheating temperature
where the flavon remains out of equilibrium at all times. We identify
phenomenologically acceptable regions of and the flavon mass where the
relic abundance of dark matter and other cosmological constraints are
satisfied. In the case of one-component dark matter we find an effective upper
limit on the FN charges at high , i.e. . In
the multi-component dark sector scenario the dark particle can be the heaviest
dark particle that can be effectively stable at cosmological timescales,
alternatively it can be produced sequentially by decays of the heavier ones.
For scenarios where dark decays occur at intermediate timescales, i.e. , we find that existing searches can effectively probe
interesting regions of parameter space. These searches include indirect probes
on decays such as -ray and neutrino telescopes as well as analyses of
the Cosmic Microwave Background, as well as constraints on small scale
structure formation from the Lyman- forest. We comment on the future
prospects of such probes and place projected sensitivities.Comment: 21 pages, 4 figure
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