1,099 research outputs found
b-Initiated processes at the LHC: a reappraisal
Several key processes at the LHC in the standard model and beyond that
involve quarks, such as single-top, Higgs, and weak vector boson associated
production, can be described in QCD either in a 4-flavor or 5-flavor scheme. In
the former, quarks appear only in the final state and are typically
considered massive. In 5-flavor schemes, calculations include quarks in the
initial state, are simpler and allow the resummation of possibly large initial
state logarithms of the type into the
parton distribution function (PDF), being the typical scale of the
hard process. In this work we critically reconsider the rationale for using
5-flavor improved schemes at the LHC. Our motivation stems from the observation
that the effects of initial state logs are rarely very large in hadron
collisions: 4-flavor computations are pertubatively well behaved and a
substantial agreement between predictions in the two schemes is found. We
identify two distinct reasons that explain this behaviour, i.e., the
resummation of the initial state logarithms into the -PDF is relevant only
at large Bjorken and the possibly large ratios 's are
always accompanied by universal phase space suppression factors. Our study
paves the way to using both schemes for the same process so to exploit their
complementary advantages for different observables, such as employing a
5-flavor scheme to accurately predict the total cross section at NNLO and the
corresponding 4-flavor computation at NLO for fully exclusive studies.Comment: Fixed typo in Eq. (A.10) and few typos in Eq. (C.2) and (C.3
The holographic fluid dual to vacuum Einstein gravity
We present an algorithm for systematically reconstructing a solution of the
(d+2)-dimensional vacuum Einstein equations from a (d+1)-dimensional fluid,
extending the non-relativistic hydrodynamic expansion of Bredberg et al in
arXiv:1101.2451 to arbitrary order. The fluid satisfies equations of motion
which are the incompressible Navier-Stokes equations, corrected by specific
higher derivative terms. The uniqueness and regularity of this solution is
established to all orders and explicit results are given for the bulk metric
and the stress tensor of the dual fluid through fifth order in the hydrodynamic
expansion. We establish the validity of a relativistic hydrodynamic description
for the dual fluid, which has the unusual property of having a vanishing
equilibrium energy density. The gravitational results are used to identify
transport coefficients of the dual fluid, which also obeys an interesting and
exact constraint on its stress tensor. We propose novel Lagrangian models which
realise key properties of the holographic fluid.Comment: 31 pages; v2: references added and minor improvements, published
versio
Higher Curvature Gravity and the Holographic fluid dual to flat spacetime
Recent works have demonstrated that one can construct a (d+2) dimensional
solution of the vacuum Einstein equations that is dual to a (d+1) dimensional
fluid satisfying the incompressible Navier-Stokes equations. In one important
example, the fluid lives on a fixed timelike surface in the flat Rindler
spacetime associated with an accelerated observer. In this paper, we show that
the shear viscosity to entropy density ratio of the fluid takes the universal
value 1/4\pi in a wide class of higher curvature generalizations to Einstein
gravity. Unlike the fluid dual to asymptotically anti-de Sitter spacetimes,
here the choice of gravitational dynamics only affects the second order
transport coefficients. We explicitly calculate these in five-dimensional
Einstein-Gauss-Bonnet gravity and discuss the implications of our results.Comment: 13 pages; v2: modified abstract, added references; v3: added
clarifying comments, modified discussio
IFNβ Protects Neurons from Damage in a Murine Model of HIV-1 Associated Brain Injury.
Infection with human immunodeficiency virus-1 (HIV-1) causes brain injury. Type I interferons (IFNα/β) are critical mediators of any anti-viral immune response and IFNβ has been implicated in the temporary control of lentiviral infection in the brain. Here we show that transgenic mice expressing HIV-1 envelope glycoprotein 120 in their central nervous system (HIVgp120tg) mount a transient IFNβ response and provide evidence that IFNβ confers neuronal protection against HIVgp120 toxicity. In cerebrocortical cell cultures, neuroprotection by IFNβ against gp120 toxicity is dependent on IFNα receptor 1 (IFNAR1) and the β-chemokine CCL4, as IFNAR1 deficiency and neutralizing antibodies against CCL4, respectively, abolish the neuroprotective effects. We find in vivo that IFNβ mRNA is significantly increased in HIVgp120tg brains at 1.5, but not 3 or 6 months of age. However, a four-week intranasal IFNβ treatment of HIVgp120tg mice starting at 3.5 months of age increases expression of CCL4 and concomitantly protects neuronal dendrites and pre-synaptic terminals in cortex and hippocampus from gp120-induced damage. Moreover, in vivo and in vitro data suggests astrocytes are a major source of IFNβ-induced CCL4. Altogether, our results suggest exogenous IFNβ as a neuroprotective factor that has potential to ameliorate in vivo HIVgp120-induced brain injury
Optimal control models of the goal-oriented human locomotion
In recent papers it has been suggested that human locomotion may be modeled
as an inverse optimal control problem. In this paradigm, the trajectories are
assumed to be solutions of an optimal control problem that has to be
determined. We discuss the modeling of both the dynamical system and the cost
to be minimized, and we analyze the corresponding optimal synthesis. The main
results describe the asymptotic behavior of the optimal trajectories as the
target point goes to infinity
Population-Level Benefits from Providing Effective HIV Prevention Means to Pregnant Women in High Prevalence Settings
Background:HIV prevalence among pregnant women in Southern Africa is extremely high. Epidemiological studies suggest that pregnancy increases the risk of HIV sexual acquisition and that HIV infections acquired during pregnancy carry higher risk of mother-to-child transmission (MTCT). We analyze the potential benefits from extending the availability of effective microbicide to pregnant women (in addition to non-pregnant women) in a wide-scale intervention.Methods and Findings:A transmission dynamic model was designed to assess the impact of microbicide use in high HIV prevalence settings and to estimate proportions of new HIV infections, infections acquired during pregnancy, and MTCT prevented over 10 years. Our analysis suggests that consistent use of microbicide with 70% efficacy by 60% of non-pregnant women may prevent approximately 40% and 15% of new infections in women and men respectively over 10 years, assuming no additional increase in HIV risk to either partner during pregnancy (RRHIV/preg = 1). It may also prevent 8-15% MTCT depending on the increase in MTCT risk when HIV is acquired during pregnancy compared to before pregnancy (RRMTCT/preg). Extending the microbicides use during pregnancy may improve the effectiveness of the intervention by 10% (RRHIV/preg = 1) to 25% (RRHIV/preg = 2) and reduce the number of HIV infections acquired during pregnancy by 40% to 70% in different scenarios. It may add between 6% (RRHIV/preg = 1, RRMTCT/preg = 1) and 25% (RRHIV/preg = 2, RRMTCT/preg = 4) to the reduction in the residual MTCT.Conclusion:Providing safe and effective microbicide to pregnant women in the context of wide-scale interventions would be desirable as it would increase the effectiveness of the intervention and significantly reduce the number of HIV infections acquired during pregnancy. The projected benefits from covering pregnant women by the HIV prevention programs is more substantial in communities in which the sexual risk during pregnancy is elevated. © 2013 Dimitrov et al
Transport coefficients, membrane couplings and universality at extremality
We present an efficient method for computing the zero frequency limit of
transport coefficients in strongly coupled field theories described
holographically by higher derivative gravity theories. Hydrodynamic parameters
such as shear viscosity and conductivity can be obtained by computing residues
of poles of the off-shell lagrangian density. We clarify in which sense these
coefficients can be thought of as effective couplings at the horizon, and
present analytic, Wald-like formulae for the shear viscosity and conductivity
in a large class of general higher derivative lagrangians. We show how to apply
our methods to systems at zero temperature but finite chemical potential. Our
results imply that such theories satisfy universally in the
Einstein-Maxwell sector. Likewise, the zero frequency limit of the real part of
the conductivity for such systems is shown to be universally zero, and we
conjecture that higher derivative corrections in this sector do not modify this
result to all orders in perturbation theory.Comment: 29 pages, v2: Small text changes for clarity, typos correcte
Using Bayesian mark-recapture modelling to quantify the strength and duration of post-release effects in reintroduced populations
Translocated animals often suffer elevated mortality during some acclimation period after release. Such post-release effects must be accounted for when estimating normal survival rates and therefore predicting population persistence. The standard approach for doing this is to nominate a fixed acclimation period, and either i) exclude survival data over that period, or ii) use model selection criteria to test whether survival differs over that period. We present a more flexible approach where the acclimation period is treated as unknown and is estimated simultaneously with the pre- and post-acclimation survival probabilities. We illustrate this approach using survival data for six reintroduced populations involving three New Zealand forest bird species. Analyses of the complete data sets (22–73 surveys conducted over 4–14 years) indicated that significant post-release effects occurred in at least one sex in five of the six populations, with 30–84% mortality attributable to post-release effects over acclimation periods ranging from 1 to 9 months. When we applied the approach to just the first year of data for each population, the estimated normal survival rates were consistent with those obtained from the complete data sets, and always at least as accurate as our previous approach of excluding data up to the next breeding season after translocation. The flexible approach therefore appears to be effective for accounting for post-release effects in survival estimation, and is beneficial in quantifying both the strength and duration of those effects so that pre- and post-release management strategies are better informed
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