1,219 research outputs found
The evolution of tides and tidal dissipation over the past 21,000 years
The 120 m sea-level drop during the Last Glacial Maximum (LGM; 18�22 kyr BP) had a profound impact on the global tides and lead to an increased tidal dissipation rate, especially in the North Atlantic. Here, we present new simulations of the evolution of the global tides from the LGM to present for the dominating diurnal and semidiurnal constituents. The simulations are undertaken in time slices spanning 500�1000 years. Due to uncertainties in the location of the grounding line of the Antarctic ice sheets during the last glacial, simulations are carried out for two different grounding line scenarios. Our results replicate previously reported enhancements in dissipation and amplitudes of the semidiurnal tide during LGM and subsequent deglaciation, and they provide a detailed picture of the large global changes in M2 tidal dynamics occurring over the deglaciation period. We show that Antarctic ice dynamics and the associated grounding line location have a large influence on global semidiurnal tides, whereas the diurnal tides mainly experience regional changes and are not impacted by grounding line shifts in Antarctica
Instantons of Type IIB Supergravity in Ten Dimensions
A family of SO(10) symmetric instanton solutions in Type IIB supergravity is
developed. The instanton of least action is a candidate for the low-energy,
semiclassical approximation to the {D=--1} brane. Unlike a previously published
solution,[GGP] this admits an interpretation as a tunneling amplitude between
perturbatively degenerate asymptotic states, but with action twice that found
previously. A number of associated issues are discussed such as the relation
between the magnetic and electric pictures, an inversion symmetry of the
dilaton and the metric, the topology of the background, and some
properties of the solution in an "instanton frame" corresponding to a
Lagrangian in which the dilaton's kinetic energy vanishes.Comment: 15 pages, no figures; Version 2 has revised sections IV and V.
Earlier equations are essentially unchanged, but interpretation changed, on
advice of counse
Exact results for hydrogen recombination on dust grain surfaces
The recombination of hydrogen in the interstellar medium, taking place on
surfaces of microscopic dust grains, is an essential process in the evolution
of chemical complexity in interstellar clouds. The H_2 formation process has
been studied theoretically, and in recent years also by laboratory experiments.
The experimental results were analyzed using a rate equation model. The
parameters of the surface, that are relevant to H_2 formation, were obtained
and used in order to calculate the recombination rate under interstellar
conditions. However, it turned out that due to the microscopic size of the dust
grains and the low density of H atoms, the rate equations may not always apply.
A master equation approach that provides a good description of the H_2
formation process was proposed. It takes into account both the discrete nature
of the H atoms and the fluctuations in the number of atoms on a grain. In this
paper we present a comprehensive analysis of the H_2 formation process, under
steady state conditions, using an exact solution of the master equation. This
solution provides an exact result for the hydrogen recombination rate and its
dependence on the flux, the surface temperature and the grain size. The results
are compared with those obtained from the rate equations. The relevant length
scales in the problem are identified and the parameter space is divided into
two domains. One domain, characterized by first order kinetics, exhibits high
efficiency of H_2 formation. In the other domain, characterized by second order
kinetics, the efficiency of H_2 formation is low. In each of these domains we
identify the range of parameters in which, the rate equations do not account
correctly for the recombination rate. and the master equation is needed.Comment: 23 pages + 8 figure
Predicted modulated differential rates for direct WIMP searches at low energy transfers
The differential event rate for direct detection of dark matter, both the
time averaged and the modulated one due to the motion of the Earth, are
discussed. The calculations focus on relatively light cold dark matter
candidates (WIMP) and low energy transfers. It is shown that for sufficiently
light WIMPs the extraction of relatively large nucleon cross sections is
possible. Furthermore for some WIMP masses the modulation amplitude may change
sign, meaning that, in such a case, the maximum rate may occur six months later
than naively expected. This effect can be exploited to yield information about
the mass of the dark matter candidate, if and when the observation of the
modulation of the event rate is established.Comment: 16 pages, 22 figures; references adde
Dilaton stabilization by massive fermion matter
The study started in a former work about the Dilaton mean field stabilization
thanks to the effective potential generated by the existence of massive
fermions, is here extended. Three loop corrections are evaluated in addition to
the previously calculated two loop terms. The results indicate that the Dilaton
vacuum field tend to be fixed at a high value close to the Planck scale, in
accordance with the need for predicting Einstein gravity from string theory.
The mass of the Dilaton is evaluated to be also a high value close to the
Planck mass, which implies the absence of Dilaton scalar signals in modern
cosmological observations. These properties arise when the fermion mass is
chosen to be either at a lower bound corresponding to the top quark mass, or
alternatively, at a very much higher value assumed to be in the grand
unification energy range. One of the three 3-loop terms is exactly evaluated in
terms of Master integrals. The other two graphs are however evaluated in their
leading logarithm correction in the perturbative expansion. The calculation of
the non leading logarithmic contribution and the inclusion of higher loops
terms could made more precise the numerical estimates of the vacuum field value
and masses, but seemingly are expected not to change the qualitative behavior
obtained. The validity of the here employed Yukawa model approximation is
argued for small value of the fermion masses with respect to the Planck one. A
correction to the two loop calculation done in the previous work is here
underlined.Comment: 18 pages, 5 figures, the study was extended and corrections on the
former calculations and redaction were done. The paper had been accepted for
publication in "Astrophysics and Space Science
Geometric Transition versus Cascading Solution
We study Vafa's geometric transition and Klebanov - Strassler solution from
various points of view in M-theory. In terms of brane configurations, we show
the detailed equivalences between the two models. In some limits, both models
have an alternative realization as fourfolds in M-theory with appropriate
G-fluxes turned on. We discuss some aspects of the fourfolds including how to
see the transition and a possible extension to the non-supersymmetric case.Comment: 34 pages, LaTex, 2 figures; v2: Some comments added and references
updated. Final version to appear in JHE
Deep Inelastic Scattering and Gauge/String Duality
We study deep inelastic scattering in gauge theories which have dual string
descriptions. As a function of we find a transition. For small , the
dominant operators in the OPE are the usual ones, of approximate twist two,
corresponding to scattering from weakly interacting partons. For large ,
double-trace operators dominate, corresponding to scattering from entire
hadrons (either the original `valence' hadron or part of a hadron cloud.) At
large we calculate the structure functions. As a function of Bjorken
there are three regimes: of order one, where the scattering produces only
supergravity states; small, where excited strings are produced; and,
exponentially small, where the excited strings are comparable in size to the
AdS space. The last regime requires in principle a full string calculation in
curved spacetime, but the effect of string growth can be simply obtained from
the world-sheet renormalization group.Comment: 52 pages, 10 figure
Dynamics of a large extra dimension inspired hybrid inflation model
In low scale quantum gravity scenarios the fundamental scale of nature can be
as low as TeV, in order to address the naturalness of the electroweak scale. A
number of difficulties arise in constructing specific models; stabilisation of
the radius of the extra dimensions, avoidance of overproduction of Kaluza Klein
modes, achieving successful baryogenesis and production of a close to
scale-invariant spectrum of density perturbations with the correct amplitude.
We examine in detail the dynamics, including radion stabilisation, of a hybrid
inflation model that has been proposed in order to address these difficulties,
where the inflaton is a gauge singlet residing in the bulk. We find that for a
low fundamental scale the phase transition, which in standard four dimensional
hybrid models usually ends inflation, is slow and there is second phase of
inflation lasting for a large number of e-foldings. The density perturbations
on cosmologically interesting scales exit the Hubble radius during this second
phase of inflation, and we find that their amplitude is far smaller than is
required. We find that the duration of the second phase of inflation can be
short, so that cosmologically interesting scales exit the Hubble radius prior
to the phase transition, and the density perturbations have the correct
amplitude, only if the fundamental scale takes an intermediate value. Finally
we comment briefly on the implications of an intermediate fundamental scale for
the production of primordial black holes and baryogenesis.Comment: 9 pages, 2 figures version to appear in Phys. Rev. D, additional
references and minor changes to discussio
Thermal history of the string universe
Thermal history of the string universe based on the Brandenberger and Vafa's
scenario is examined. The analysis thereby provides a theoretical foundation of
the string universe scenario. Especially the picture of the initial oscillating
phase is shown to be natural from the thermodynamical point of view. A new tool
is employed to evaluate the multi state density of the string gas. This
analysis points out that the well-known functional form of the multi state
density is not applicable for the important region , and derives a
correct form of it.Comment: 39 pages, no figures, use revtex.sty, aps.sty, aps10.sty &
preprint.st
Scintillation Counters for the D0 Muon Upgrade
We present the results of an upgrade to the D0 muon system. Scintillating
counters have been added to the existing central D0 muon system to provide
rejection for cosmic ray muons and out-of-time background, and to provide
additional fast timing information for muons in an upgraded Tevatron.
Performance and results from the 1994-1996 Tevatron run are presented.Comment: 30 pages, 25 postscript figure
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