182 research outputs found
Goldstone Fermion Dark Matter
We propose that the fermionic superpartner of a weak-scale Goldstone boson
can be a natural WIMP candidate. The p-wave annihilation of this `Goldstone
fermion' into pairs of Goldstone bosons automatically generates the correct
relic abundance, whereas the XENON100 direct detection bounds are evaded due to
suppressed couplings to the Standard Model. Further, it is able to avoid
indirect detection constraints because the relevant s-wave annihilations are
small. The interactions of the Goldstone supermultiplet can induce non-standard
Higgs decays and novel collider phenomenology.Comment: 25 pages, 6 figures. References added, minor typos corrected.
Submitted to JHE
The M33 Globular Cluster System with PAndAS Data: The Last Outer Halo Cluster?
We use CFHT/MegaCam data to search for outer halo star clusters in M33 as
part of the Pan-Andromeda Archaeological Survey (PAndAS). This work extends
previous studies out to a projected radius of 50 kpc and covers over 40 square
degrees. We find only one new unambiguous star cluster in addition to the five
previously known in the M33 outer halo (10 kpc <= r <= 50 kpc). Although we
identify 2440 cluster candidates of various degrees of confidence from our
objective image search procedure, almost all of these are likely background
contaminants, mostly faint unresolved galaxies. We measure the luminosity,
color and structural parameters of the new cluster in addition to the five
previously-known outer halo clusters. At a projected radius of 22 kpc, the new
cluster is slightly smaller, fainter and redder than all but one of the other
outer halo clusters, and has g' ~ 19.9, (g'-i') ~ 0.6, concentration parameter
c ~ 1.0, a core radius r_c ~ 3.5 pc, and a half-light radius r_h ~ 5.5 pc. For
M33 to have so few outer halo clusters compared to M31 suggests either tidal
stripping of M33's outer halo clusters by M31, or a very different, much calmer
accretion history of M33.Comment: 37 pages, 9 figures. Accepted by the Astrophysical Journa
Stealth Supersymmetry
We present a broad class of supersymmetric models that preserve R-parity but
lack missing energy signatures. These models have new light particles with
weak-scale supersymmetric masses that feel SUSY breaking only through couplings
to the MSSM. This small SUSY breaking leads to nearly degenerate fermion/boson
pairs, with small mass splittings and hence small phase space for decays
carrying away invisible energy. The simplest scenario has low-scale SUSY
breaking, with missing energy only from soft gravitinos. This scenario is
natural, lacks artificial tunings to produce a squeezed spectrum, and is
consistent with gauge coupling unification. The resulting collider signals will
be jet-rich events containing false resonances that could resemble signatures
of R-parity violation. We discuss several concrete examples of the general
idea, and emphasize gamma + jet + jet resonances, displaced vertices, and very
large numbers of b-jets as three possible discovery modes.Comment: 12 pages, 4 figure
Diffusion entropy analysis on the scaling behavior of financial markets
In this paper the diffusion entropy technique is applied to investigate the
scaling behavior of financial markets. The scaling behaviors of four
representative stock markets, Dow Jones Industrial Average, Standard&Poor 500,
Heng Seng Index, and Shang Hai Stock Synthetic Index, are almost the same; with
the scale-invariance exponents all in the interval . These
results provide a strong evidence of the existence of long-rang correlation in
financial time series, thus several variance-based methods are restricted for
detecting the scale-invariance properties of financial markets. In addition, a
parsimonious percolation model for stock markets is proposed, of which the
scaling behavior agrees with the real-life markets well.Comment: 5 pages, 3 figure
HST/ACS color-magnitude diagrams of candidate intermediate-age M 31 globular clusters. The role of blue horizontal branches
We present deep (V ~ 28.0) BV photometry obtained with the wide field channel
of the Advanced Camera for Surveys on board HST for four M31 globular clusters
that were identified as candidate intermediate-age (age ~ 1-9 Gyr) by various
authors, based on their integrated spectra and/or broad/intermediate-band
colors. Two of them (B292 and B350) display an obvious blue horizontal branch,
indicating that they are as old as the oldest Galactic globulars. On the other
hand, for the other two (B058 and B337), which display red horizontal branches,
it was not possible either to confirm or disconfirm the age estimate from
integrated spectra. The analysis of the distribution in the spectral indices
Mg2 and H_beta of the M31 and Milky Way clusters whose horizontal branch can be
classified as red or blue based on existing CMDs, strongly suggests that
classical age diagnostics from integrated spectra may be significantly
influenced by the HB morphology of the clusters and can lead to erroneous
age-classifications. We also provide the CMD for another two clusters that fall
into the field of the main targets, B336, an old and metal-poor globular with a
significant population of RR-Lyrae variables, and the newly discovered B531, a
cluster with a very red red giant branch.Comment: Accepted for publication in Astronomy and Astrophysics; 13 pages, 13
figures and 7 tables. Some figures have been decreased in quality, an higher
resolution version is available at http://www.bo.astro.it/M31/hstcatalog
Thick disks and halos of spiral galaxies M 81, NGC 55 and NGC 300
By using images from the HST/WFPC2/ACS archive, we have analyzed the spatial
distribution of the AGB and RGB stars along the galactocentric radius of nearby
spiral galaxies M~81, NGC 300 and NGC 55. Using color-magnitude diagrams and
stellar luminosity functions, we gauge the stellar contents of the surroundings
of three galaxies. The red giant population (RGB) identified at large
galactocentric radii yields a distance of Mpc for M~81,
Mpc for NGC 55, and Mpc for NGC 300, and a mean
stellar metallicity of 0.65, 1.25, and 0.87. We find that there are
two number density gradients of RGB stars along the radius, which correspond to
the thick disk and halo components of the galaxies. We confirm the presence of
metallicity gradient of evolved stars at these galaxies, based on the
systematic changes of the color distribution of red giant stars. These results
imply that thick disk might be a general feature of the spiral galaxies, and
endorse a further investigation of the outer stellar edges of nearby spirals,
which is critical in constraining the origin and evolution of galaxies.Comment: 17 pages, 2 tables, 13 figures, accepted to A&
Reduced branching ratio for H -> AA -> 4 tau from A - eta_b mixing
Models with an extended Higgs sector, as the NMSSM, allow for scenarios where
the Standard Model-like CP-even Higgs boson H decays dominantly as H -> AA -> 4
tau where A is a light CP-odd Higgs boson. Tight constraints on this scenario
in the form of lower bounds on M_H have recently been published by the ALEPH
group. We show that, due to A - eta_b mixing, the branching ratio H -> AA -> 4
tau is strongly reduced for M_A in the range 9 - 10.5 GeV. This is the range of
M_A in which the tension between the observed eta_b(1S) mass and its prediction
based on QCD can be resolved due to mixing, and which is thus still consistent
with a light CP-even Higgs boson H satisfying LEP constraints with a mass well
below 114 GeV. This result is practically independent from the coupling of A to
b quarks.Comment: 11 pages, 2 Figures, reference adde
Dark Matter from Minimal Flavor Violation
We consider theories of flavored dark matter, in which the dark matter
particle is part of a multiplet transforming nontrivially under the flavor
group of the Standard Model in a manner consistent with the principle of
Minimal Flavor Violation (MFV). MFV automatically leads to the stability of the
lightest state for a large number of flavor multiplets. If neutral, this
particle is an excellent dark matter candidate. Furthermore, MFV implies
specific patterns of mass splittings among the flavors of dark matter and
governs the structure of the couplings between dark matter and ordinary
particles, leading to a rich and predictive cosmology and phenomenology. We
present an illustrative phenomenological study of an effective theory of a
flavor SU(3)_Q triplet, gauge singlet scalar.Comment: 10 pages, 2 figures; v2: references added, minor changes to collider
analysis, conclusions unchange
A Stealth Supersymmetry Sampler
The LHC has strongly constrained models of supersymmetry with traditional
missing energy signatures. We present a variety of models that realize the
concept of Stealth Supersymmetry, i.e. models with R-parity in which one or
more nearly-supersymmetric particles (a "stealth sector") lead to collider
signatures with only a small amount of missing energy. The simplest realization
involves low-scale supersymmetry breaking, with an R-odd particle decaying to
its superpartner and a soft gravitino. We clarify the stealth mechanism and its
differences from compressed supersymmetry and explain the requirements for
stealth models with high-scale supersymmetry breaking, in which the soft
invisible particle is not a gravitino. We also discuss new and distinctive
classes of stealth models that couple through a baryon portal or Z' gauge
interactions. Finally, we present updated limits on stealth supersymmetry in
light of current LHC searches.Comment: 45 pages, 16 figure
- …