131 research outputs found
The influence of D-branes' backreaction upon gravitational interactions between open strings
We argue that gravitational interactions between open strings ending on
D3-branes are largely shaped by the D3-branes' backreaction. To this end we
consider classical open strings coupled to general relativity in Poincare AdS5
backgrounds. We compute the linear gravitational backreaction of a static
string extending up to the Poincare horizon, and deduce the potential energy
between two such strings. If spacetime is non-compact, we find that the
gravitational potential energy between parallel open strings is independent of
the strings' inertial masses and goes like 1/r at large distance r. If the
space transverse to the D3-branes is suitably compactified, a collective mode
of the graviton propagates usual four-dimensional gravity. In that case the
backreaction of the D3-branes induces a correction to the Newtonian potential
energy that violates the equivalence principle. The observed enhancement of the
gravitational attraction is specific to string theory; there is no similar
effect for point-particles.Comment: 28 pages, 7 figures. Typos corrected, minor addition
Comments on black holes I: The possibility of complementarity
We comment on a recent paper of Almheiri, Marolf, Polchinski and Sully who
argue against black hole complementarity based on the claim that an infalling
observer 'burns' as he approaches the horizon. We show that in fact
measurements made by an infalling observer outside the horizon are
statistically identical for the cases of vacuum at the horizon and radiation
emerging from a stretched horizon. This forces us to follow the dynamics all
the way to the horizon, where we need to know the details of Planck scale
physics. We note that in string theory the fuzzball structure of microstates
does not give any place to 'continue through' this Planck regime. AMPS argue
that interactions near the horizon preclude traditional complementarity. But
the conjecture of 'fuzzball complementarity' works in the opposite way: the
infalling quantum is absorbed by the fuzzball surface, and it is the resulting
dynamics that is conjectured to admit a complementary description.Comment: 34 pages, 6 figures, v3: clarifications & references adde
Singlet Portal to the Hidden Sector
Ultraviolet physics typically induces a kinetic mixing between gauge singlets
which is marginal and hence non-decoupling in the infrared. In singlet
extensions of the minimal supersymmetric standard model, e.g. the
next-to-minimal supersymmetric standard model, this furnishes a well motivated
and distinctive portal connecting the visible sector to any hidden sector which
contains a singlet chiral superfield. In the presence of singlet kinetic
mixing, the hidden sector automatically acquires a light mass scale in the
range 0.1 - 100 GeV induced by electroweak symmetry breaking. In theories with
R-parity conservation, superparticles produced at the LHC invariably cascade
decay into hidden sector particles. Since the hidden sector singlet couples to
the visible sector via the Higgs sector, these cascades necessarily produce a
Higgs boson in an order 0.01 - 1 fraction of events. Furthermore,
supersymmetric cascades typically produce highly boosted, low-mass hidden
sector singlets decaying visibly, albeit with displacement, into the heaviest
standard model particles which are kinematically accessible. We study
experimental constraints on this broad class of theories, as well as the role
of singlet kinetic mixing in direct detection of hidden sector dark matter. We
also present related theories in which a hidden sector singlet interacts with
the visible sector through kinetic mixing with right-handed neutrinos.Comment: 12 pages, 5 figure
Low-Energy Signals from Kinetic Mixing with a Warped Abelian Hidden Sector
We investigate the detailed phenomenology of a light Abelian hidden sector in
the Randall-Sundrum framework. Relative to other works with light hidden
sectors, the main new feature is a tower of hidden Kaluza-Klein vectors that
kinetically mix with the Standard Model photon and Z. We investigate the decay
properties of the hidden sector fields in some detail, and develop an approach
for calculating processes initiated on the ultraviolet brane of a warped space
with large injection momentum relative to the infrared scale. Using these
results, we determine the detailed bounds on the light warped hidden sector
from precision electroweak measurements and low-energy experiments. We find
viable regions of parameter space that lead to significant production rates for
several of the hidden Kaluza-Klein vectors in meson factories and fixed-target
experiments. This offers the possibility of exploring the structure of an extra
spacetime dimension with lower-energy probes.Comment: (1+32) Pages, 13 Figures. v2: JHEP version (minor modifications,
results unchanged
Secluded Dark Matter Coupled to a Hidden CFT
Models of secluded dark matter offer a variant on the standard WIMP picture
and can modify our expectations for hidden sector phenomenology and detection.
In this work we extend a minimal model of secluded dark matter, comprised of a
U(1)'-charged dark matter candidate, to include a confining hidden-sector CFT.
This provides a technically natural explanation for the hierarchically small
mediator-scale, with hidden-sector confinement generating m_{gamma'}>0.
Furthermore, the thermal history of the universe can differ markedly from the
WIMP picture due to (i) new annihilation channels, (ii) a (potentially) large
number of hidden-sector degrees of freedom, and (iii) a hidden-sector phase
transition at temperatures T << M_{dm} after freeze out. The mediator allows
both the dark matter and the Standard Model to communicate with the CFT, thus
modifying the low-energy phenomenology and cosmic-ray signals from the secluded
sector.Comment: ~50p, 8 figs; v2 JHEP versio
Visible and Invisible Trends in Black Men's Health: Pitfalls and Promises for Addressing Racial, Ethnic, and Gender Inequities in Health
Over the past two decades, there has been growing interest in improving black men's health and the health disparities affecting them. Yet, the health of black men consistently ranks lowest across nearly all groups in the United States. Evidence on the health and social causes of morbidity and mortality among black men has been narrowly concentrated on public health problems (e.g., violence, prostate cancer, and HIV/AIDS) and determinants of health (e.g., education and male gender socialization). This limited focus omits age-specific leading causes of death and other social determinants of health, such as discrimination, segregation, access to health care, employment, and income. This review discusses the leading causes of death for black men and the associated risk factors, as well as identifies gaps in the literature and presents a racialized and gendered framework to guide efforts to address the persistent inequities in health affecting black men
Brane-World Gravity
The observable universe could be a 1+3-surface (the "brane") embedded in a
1+3+\textit{d}-dimensional spacetime (the "bulk"), with Standard Model
particles and fields trapped on the brane while gravity is free to access the
bulk. At least one of the \textit{d} extra spatial dimensions could be very
large relative to the Planck scale, which lowers the fundamental gravity scale,
possibly even down to the electroweak ( TeV) level. This revolutionary
picture arises in the framework of recent developments in M theory. The
1+10-dimensional M theory encompasses the known 1+9-dimensional superstring
theories, and is widely considered to be a promising potential route to quantum
gravity. At low energies, gravity is localized at the brane and general
relativity is recovered, but at high energies gravity "leaks" into the bulk,
behaving in a truly higher-dimensional way. This introduces significant changes
to gravitational dynamics and perturbations, with interesting and potentially
testable implications for high-energy astrophysics, black holes, and cosmology.
Brane-world models offer a phenomenological way to test some of the novel
predictions and corrections to general relativity that are implied by M theory.
This review analyzes the geometry, dynamics and perturbations of simple
brane-world models for cosmology and astrophysics, mainly focusing on warped
5-dimensional brane-worlds based on the Randall--Sundrum models. We also cover
the simplest brane-world models in which 4-dimensional gravity on the brane is
modified at \emph{low} energies -- the 5-dimensional Dvali--Gabadadze--Porrati
models. Then we discuss co-dimension two branes in 6-dimensional models.Comment: A major update of Living Reviews in Relativity 7:7 (2004)
"Brane-World Gravity", 119 pages, 28 figures, the update contains new
material on RS perturbations, including full numerical solutions of
gravitational waves and scalar perturbations, on DGP models, and also on 6D
models. A published version in Living Reviews in Relativit
An approach for particle sinking velocity measurements in the 3â400 Îźm size range and considerations on the effect of temperature on sinking rates
The flux of organic particles below the mixed layer is one major pathway of carbon from the surface into the deep ocean. The magnitude of this export flux depends on two major processesâremineralization rates and sinking velocities. Here, we present an efficient method to measure sinking velocities of particles in the size range from approximately 3â400 Îźm by means of video microscopy (FlowCAMÂŽ). The method allows rapid measurement and automated analysis of mixed samples and was tested with polystyrene beads, different phytoplankton species, and sediment trap material. Sinking velocities of polystyrene beads were close to theoretical values calculated from Stokesâ Law. Sinking velocities of the investigated phytoplankton species were in reasonable agreement with published literature values and sinking velocities of material collected in sediment trap increased with particle size. Temperature had a strong effect on sinking velocities due to its influence on seawater viscosity and density. An increase in 9 °C led to a measured increase in sinking velocities of ~40 %. According to this temperature effect, an average temperature increase in 2 °C as projected for the sea surface by the end of this century could increase sinking velocities by about 6 % which might have feedbacks on carbon export into the deep ocean
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