6,073 research outputs found
Constraining the dipole moments of the top quark
We investigate the direct and indirect bounds on dipole operators involving
the top quark. A careful analysis shows that the experimental upper limit on
the neutron electric dipole moment strongly constrains the chromo-electric
dipole of the top. We improve previous bounds by two orders of magnitude. This
has significant implications for new physics models and it also means that CP
violation in top pair production mediated by dipole operators will not be
accessible at the LHC. The CP conserving chromo-magnetic dipole moments are
constrained by recent measurements of the t\bar t spectrum by the ATLAS
collaboration. We also update the indirect constraints on electric and magnetic
dipole moments from radiative b -> s transitions, finding that they can be
considerably larger than their colored counterparts.Comment: 6 pages, 3 figures; extended and updated discussion of constraints on
top EDM and MDM from rare B decays; typos correcte
SN 1996cr: Confirmation of a Luminous Type IIn Supernova in the Circinus Galaxy
We have recently confirmed SN 1996cr as a late-time type IIn supernova (SN)
via VLT spectroscopy and isolated its explosion date to ~1 yr using archival
optical imaging. We briefly touch upon here the wealth of optical, X-ray, and
radio archival observations available for this enigmatic source. Due to its
relative proximity (3.8 +/-0.6 Mpc), SN 1996cr ranks among the brightest X-ray
and radio SNe ever detected and, as such, may offer powerful insights into the
structure and composition of type IIn SNe. We also find that SN 1996cr is
matched to GRB 4B 960202 at a 2-3 sigma confidence level, making it perhaps the
third GRB to be significantly associated with a type II SN. We speculate on
whether SN 1996cr could be an off-axis or ``failed'' GRB.Comment: 3 pages, 1 figure, poster proceeding for "Supernova 1987A: 20 Years
After: Supernovae and Gamma-Ray Bursters" AIP, New York, eds. S. Immler, K.W.
Weiler, and R. McCra
Probing QCD approach to thermal equilibrium with ultrahigh energy cosmic rays
The Pierre Auger Collaboration has reported an excess in the number of muons
of a few tens of percent over expectations computed using extrapolation of
hadronic interaction models tuned to accommodate LHC data. Very recently, we
proposed an explanation for the muon excess assuming the formation of a
deconfined quark matter (fireball) state in central collisions of
ultrarelativistic cosmic rays with air nuclei. At the first stage of its
evolution the fireball contains gluons as well as and quarks. The very
high baryochemical potential inhibits gluons from fragmenting into
and , and so they fragment predominantly into pairs. In
the hadronization which follows this leads to the strong suppression of pions
and hence photons, but allows heavy hadrons to be emitted carrying away
strangeness. In this manner, the extreme imbalance of hadron to photon content
provides a way to enhance the muon content of the air shower. In this
communication we study theoretical systematics from hadronic interaction models
used to describe the cascades of secondary particles produced in the fireball
explosion. We study the predictions of one of the leading LHC-tuned models
QGSJET II-04 considered in the Auger analysis.Comment: 7 pages LaTeX, 6 .pdf figure
Why Groups Are Politically Active: An Incentive-Theoretical Approach
This is the author accepted manuscript. The final version is available from SAGE Publications via the DOI in this recordPolitical activity is conventionally considered a constitutive feature of interest groups, underpinning an impressive literature on the strategies groups employ to exercise political influence. Whether and how intensely voluntary membership groups engage in political activities to start with, however, is rarely examined. We present a new incentive-theoretical perspective on group political activity, considering both member demands and leadership constraints. We argue that investments in political activities (one way of generating collective incentives) as a means to prevent member exit are more or less important depending on a group’s composition. Simultaneously, the extent to which leaders are incentivized to cater to members’ demands when trying to balance these against conflicting demands, depends on communication channels between leaders and members and the importance of membership fees. Applying Bayesian ordered logit models to data from two group surveys supports our perspective and stresses the importance of considering how intra-organizational dynamics steer groups’ external activities
Electroweak Bremsstrahlung in Dark Matter Annihilation
A conservative upper bound on the total dark matter (DM) annihilation rate
can be obtained by constraining the appearance rate of the annihilation
products which are hardest to detect. The production of neutrinos, via the
process , has thus been used to set a strong
general bound on the dark matter annihilation rate. However, Standard Model
radiative corrections to this process will inevitably produce photons which may
be easier to detect. We present an explicit calculation of the branching ratios
for the electroweak bremsstrahlung processes and
. These modes inevitably lead to electromagnetic
showers and further constraints on the DM annihilation cross-section. In
addition to annihilation, our calculations are also applicable to the case of
dark matter decay.Comment: 7 pages, 4 figures. New appendix with an extensive discussion of
Majorana fermions and helicity suppression
The Migdal Effect and Photon Bremsstrahlung in effective field theories of dark matter direct detection and coherent elastic neutrino-nucleus scattering
Dark matter direct detection experiments have limited sensitivity to light
dark matter (below a few GeV), due to the challenges of lowering energy
thresholds for the detection of nuclear recoil to below
. While impressive progress has been made on this
front, light dark matter remains the least constrained region of dark-matter
parameter space. It has been shown that both ionization and excitation due to
the Migdal effect and coherently-emitted photon bremsstrahlung from the
recoiling atom can provide observable channels for light dark matter that would
otherwise have been missed owing to the resulting nuclear recoil falling below
the detector threshold. In this paper we extend previous work by calculating
the Migdal effect and photon bremmstrahlung rates for a general set of
interaction types, including those that are momentum-independent or -dependent,
spin-independent or -dependent, as well as examining the rates for a variety of
target materials, allowing us to place new experimental limits on some of these
interaction types. Additionally, we include a calculation of these effects
induced by the coherent scattering on nuclei of solar or atmospheric neutrinos.
We demonstrate that the Migdal effect dominates over the bremsstrahlung effect
for all targets considered for interactions induced by either dark matter or
neutrinos. This reduces photon bremsstrahlung to irrelevancy for future direct
detection experiments.Comment: 17 pages, 6 figure
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