1,466 research outputs found
Recent advances in the understanding of mid-ocean ridge tectonics and volcanism using swath-mapping tools
New swath mapping sonar systems have revealed the structure of the mid-ocean ridge (MOR) and sunounding deep ocean floor with unprecedent clarity. These images show that the MOR is partitioned into segments by a variety of offsets such as transform faults, overlapping spreading centers and very fine scale discontinuities which are barely detectable. The smallest scale segments are the fundamental building blocks for creation of new oceanic crust. They are only 2-20 km long and are active, distinct units for only =100-10,000 years. At fast spreading centers, the axial neovolcanic zone is a persistent 300-400 m high elevation produced by the bouyancy of hot rock and magma; it is not a volcanic construction (most of the time and in most plac e ~ )so there is almost no vestige of it off-axis. Along ~60%of its length, the spreading center is characterized by an axial summit caldera produced by volcanic inflation and collapse. The size and shape of the axial high are very sensitive indicators of a relatively steady and robust magma supply at fast-spreading ndges, and have been used successfuliy to predict the location of magma chambers and to forecast recent volcanic emptions including one witnessed from ALVIN in March-Apnl 1991. At intermediate spreading rates, the axial region cools sufficiently for a volcanic constructional edifice to develop episodically and for normal faulting to occur along an axial graben. Under these conditions axial volcanoes are split in two by the axial graben and rernnants can be found on the flanks of spreading centers. At slow-spreading ridges, the magma budget is relatively starved as indicated by a persistent axial nft valley and a highly discontinuous neovolcanic zone, and strong asymmetry in profiles along and across the strike of the ridge
Saxion Emission from SN1987A
We study the possibility of emission of the saxion, a superpartner of the
axion, from SN1987A. The fact that the observed neutrino pulse from SN1987A is
in excellent agreement with the current theory of supernovae places a strong
bound on the energy loss into any non-standard model channel, therefore
enabling bounds to be placed on the decay constant, f_a, of a light saxion. The
low-energy coupling of the saxion, which couples at high energies to the QCD
gauge field strength, is expected to be enhanced from QCD scaling, making it
interesting to investigate if the saxion could place stronger bounds on f_a
than the axion itself. Moreover, since the properties of the saxion are
determined by f_a, a constraint on this parameter can be translated into a
constraint on the supersymmetry breaking scale. We find that the bound on f_a
from saxion emission is comparable with the one derived from axion emission due
to a cancellation of leading-order terms in the soft-radiation expansion.Comment: 18 pages, 2 figures; minor changes, typos corrected, version to
appear in JHE
Common gauge origin of discrete symmetries in observable sector and hidden sector
An extra Abelian gauge symmetry is motivated in many new physics models in
both supersymmetric and nonsupersymmetric cases. Such a new gauge symmetry may
interact with both the observable sector and the hidden sector. We
systematically investigate the most general residual discrete symmetries in
both sectors from a common Abelian gauge symmetry. Those discrete symmetries
can ensure the stability of the proton and the dark matter candidate. A hidden
sector dark matter candidate (lightest U-parity particle or LUP) interacts with
the standard model fields through the gauge boson Z', which may selectively
couple to quarks or leptons only. We make a comment on the implications of the
discrete symmetry and the leptonically coupling dark matter candidate, which
has been highlighted recently due to the possibility of the simultaneous
explanation of the DAMA and the PAMELA results. We also show how to construct
the most general U(1) charges for a given discrete symmetry, and discuss the
relation between the U(1) gauge symmetry and R-parity.Comment: Version to appear in JHE
Dark Matter attempts for CoGeNT and DAMA
Recently, the CoGeNT collaboration presented a positive signal for an annual
modulation in their data set. In light of the long standing annual modulation
signal in DAMA/LIBRA, we analyze the compatibility of both of these signal
within the hypothesis of dark matter (DM) scattering on nuclei, taking into
account existing experimental constraints. We consider the cases of elastic and
inelastic scattering with either spin-dependent or spin-independent coupling to
nucleons. We allow for isospin violating interactions as well as for light
mediators. We find that there is some tension between the size of the
modulation signal and the time-integrated event excess in CoGeNT, making it
difficult to explain both simultaneously. Moreover, within the wide range of DM
interaction models considered, we do not find a simultaneous explanation of
CoGeNT and DAMA/LIBRA compatible with constraints from other experiments.
However, in certain cases part of the data can be made consistent. For example,
the modulation signal from CoGeNT becomes consistent with the total rate and
with limits from other DM searches at 90% CL (but not with the DAMA/LIBRA
signal) if DM scattering is inelastic spin-independent with just the right
couplings to protons and neutrons to reduce the scattering rate on xenon.
Conversely the DAMA/LIBRA signal (but not CoGeNT) can be explained by
spin-dependent inelastic DM scattering.Comment: 20 pages, 9 figure
Characterising WIMPs at a future Linear Collider
We investigate the prospects for detecting and measuring the parameters of
WIMP dark matter in a model independent way at the International Linear
Collider. The signal under study is direct WIMP pair production with associated
initial state radiation . The analysis
accounts for the beam energy spectrum of the ILC and the dominant machine
induced backgrounds. The influence of the detector parameters are incorporated
by full simulation and event reconstruction within the framework of the ILD
detector concept. We show that by using polarised beams, the detection
potential is significantly increased by reduction of the dominant SM background
of radiative neutrino production . The
dominant sources of systematic uncertainty are the precision of the
polarisation measurement and the shape of the beam energy spectrum. With an
integrated luminosity of 500 fb the helicity structure of the interaction
involved can be inferred, and the masses and cross-sections can be measured
with a relative accuracy of the order of 1 %.Comment: 20 pages, 11 figure
Naturalness and Higgs Decays in the MSSM with a Singlet
The simplest extension of the supersymmetric standard model - the addition of
one singlet superfield - can have a profound impact on the Higgs and its
decays. We perform a general operator analysis of this scenario, focusing on
the phenomenologically distinct scenarios that can arise, and not restricting
the scope to the narrow framework of the NMSSM. We reexamine decays to four b
quarks and four tau's, finding that they are still generally viable, but at the
edge of LEP limits. We find a broad set of Higgs decay modes, some new,
including those with four gluon final states, as well as more general six and
eight parton final states. We find the phenomenology of these scenarios is
dramatically impacted by operators typically ignored, specifically those
arising from D-terms in the hidden sector, and those arising from weak-scale
colored fields. In addition to sensitivity of m_Z, there are potential tunings
of other aspects of the spectrum. In spite of this, these models can be very
natural, with light stops and a Higgs as light as 82 GeV. These scenarios
motivate further analyses of LEP data as well as studies of the detection
capabilities of future colliders to the new decay channels presented.Comment: 3 figures, 1 appendix; version to appear in JHEP; typos fixed and
additional references and acknowledgements adde
Thermal Unparticles: A New Form of Energy Density in the Universe
Unparticle \U with scaling dimension d_\U has peculiar thermal properties
due to its unique phase space structure. We find that the equation of state
parameter \omega_\U, the ratio of pressure to energy density, is given by
1/(2d_\U +1) providing a new form of energy in our universe. In an expanding
universe, the unparticle energy density \rho_\U(T) evolves dramatically
differently from that for photons. For d_\U >1, even if \rho_\U(T_D) at a
high decoupling temperature is very small, it is possible to have a large
relic density \rho_\U(T^0_\gamma) at present photon temperature ,
large enough to play the role of dark matter. We calculate and
\rho_\U(T^0_\gamma) using photon-unparticle interactions for illustration.Comment: 5 pages; v3, journal version
Breakdown of correspondence in chaotic systems: Ehrenfest versus localization times
Breakdown of quantum-classical correspondence is studied on an experimentally
realizable example of one-dimensional periodically driven system. Two relevant
time scales are identified in this system: the short Ehrenfest time t_h and the
typically much longer localization time scale T_L. It is shown that
surprisingly weak modification of the Hamiltonian may eliminate the more
dramatic symptoms of localization without effecting the more subtle but
ubiquitous and rapid loss of correspondence at t_h.Comment: 4 pages, 5 figures, replaced with a version submitted to PR
Prospects For Identifying Dark Matter With CoGeNT
It has previously been shown that the excess of events reported by the CoGeNT
collaboration could be generated by elastically scattering dark matter
particles with a mass of approximately 5-15 GeV. This mass range is very
similar to that required to generate the annual modulation observed by
DAMA/LIBRA and the gamma rays from the region surrounding the Galactic Center
identified within the data of the Fermi Gamma Ray Space Telescope. To
confidently conclude that CoGeNT's excess is the result of dark matter,
however, further data will likely be needed. In this paper, we make projections
for the first full year of CoGeNT data, and for its planned upgrade. Not only
will this body of data more accurately constrain the spectrum of nuclear recoil
events, and corresponding dark matter parameter space, but will also make it
possible to identify seasonal variations in the rate. In particular, if the
CoGeNT excess is the product of dark matter, then one year of CoGeNT data will
likely reveal an annual modulation with a significance of 2-3. The
planned CoGeNT upgrade will not only detect such an annual modulation with high
significance, but will be capable of measuring the energy spectrum of the
modulation amplitude. These measurements will be essential to irrefutably
confirming a dark matter origin of these events.Comment: 6 pages, 6 figure
Dirac gaugino as leptophilic dark matter
We investigate the leptophilic properties of Dirac gauginos in an
R--symmetric N=2 supersymmetric model with extended gauge and Higgs sectors.
The annihilation of Dirac gauginos to leptons requires no chirality flip in the
final states so that it is not suppressed as in the Majorana case. This implies
that it can be sizable enough to explain the positron excess observed by the
PAMELA experiment with moderate or no boost factors. When squark masses are
heavy, the annihilation of Dirac gauginos to hadrons is controlled by their
Higgsino fraction and is driven by the and final states.
Moreover, at variance with the Majorana case, Dirac gauginos with a
non-vanishing higgsino fraction can also have a vector coupling with the
gauge boson leading to a sizable spin--independent scattering cross section off
nuclei. Saturating the current antiproton limit, we show that Dirac gauginos
can leave a signal in direct detection experiments at the level of the
sensitivity of dark matter searches at present and in the near future.Comment: 24 pages, 10 figures, typos corrected, final version published on
JCA
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