841 research outputs found
Study of direct versus orbital entry for Mars missions. Volume 3 - Appendix A - Launch vehicle performance and flight mechanics
Titan 3 launch vehicle performance and flight mechanics analyses - direct versus orbital entry for Mars mission
Azimuthal decorrelation of Mueller-Navelet jets at the Tevatron and the LHC
We study the production of Mueller-Navelet jets at hadron colliders in the
Balitsky-Fadin-Kuraev-Lipatov (BFKL) framework. We show that a measurement of
the relative azimuthal angle \Delta\Phi between the jets can provide a good
testing ground for corrections due to next-leading logarithms (NLL). Besides
the well-known azimuthal decorrelation with increasing rapidity interval
\Delta\eta between the jets, we propose to also measure this effect as a
function of R=k_2/k_1, the ratio between the jets transverse momenta. Using
renormalisation-group improved NLL kernel, we obtain predictions for
d\sigma/d\Delta\eta dR d\Delta\Phi. We analyse NLL-scheme and
renormalisation-scale uncertainties, and energy-momentum conservation effects,
in order to motivate a measurement at the Tevatron and the LHC.Comment: 11 pages, 7 figures, energy conservation effects adde
Exclusive vector meson production at HERA from QCD with saturation
Following recent predictions that the geometric scaling properties of deep
inelastic scattering data in inclusive gamma*-p collisions are expected also in
exclusive diffractive processes, we investigate the diffractive production of
vector mesons. Using analytic results in the framework of the BK equation at
non-zero momentum transfer, we extend to the non-forward amplitude a
QCD-inspired forward saturation model including charm, following the
theoretical predictions for the momentum transfer dependence of the saturation
scale. We obtain a good fit to the available HERA data and make predictions for
deeply virtual Compton scattering measurements.Comment: 10 pages, 5 figures, full analysis including the charm contribution
and J/PSI production. Conclusions confirme
Consequences of strong fluctuations on high-energy QCD evolution
We investigate the behaviour of the QCD evolution towards high-energy, in the
diffusive approximation, in the limit where the fluctuation contribution is
large. Our solution for the equivalent stochastic Fisher equation predicts the
amplitude as well as the whole set of correlators in the strong noise limit.
The speed of the front and the diffusion coefficient are obtained. We analyse
the consequences on high-energy evolution in QCD.Comment: 5 pages, 1 figure, more detailed discussions added, version to appear
in Phys. Rev.
Transient growth analysis of the flow past a circular cylinder
We apply direct transient growth analysis in complex geometries to investigate its role in the primary and secondary bifurcation/transition process of the flow past a circular cylinder. The methodology is based on the singular value decomposition of the Navier-Stokes evolution operator linearized about a two-dimensional steady or periodic state which leads to the optimal growth modes. Linearly stable and unstable steady flow at Re=45 and 50 is considered first, where the analysis demonstrates that strong two-dimensional transient growth is observed with energy amplifications of order of 10(3) at U-infinity tau/D approximate to 30. Transient growth at Re=50 promotes the linear instability which ultimately saturates into the well known von-Kaacutermaacuten street. Subsequently we consider the transient growth upon the time-periodic base state corresponding to the von-Kaacutermaacuten street at Re=200 and 300. Depending upon the spanwise wavenumber the flow at these Reynolds numbers are linearly unstable due to the so-called mode A and B instabilities. Once again energy amplifications of order of 10(3) are observed over a time interval of tau/T=2, where T is the time period of the base flow shedding. In all cases the maximum energy of the optimal initial conditions are located within a diameter of the cylinder in contrast to the spatial distribution of the unstable eigenmodes which extend far into the downstream wake. It is therefore reasonable to consider the analysis as presenting an accelerator to the existing modal mechanism. The rapid amplification of the optimal growth modes highlights their importance in the transition process for flow past circular cylinder, particularly when comparing with experimental results where these types of convective instability mechanisms are likely to be activated. The spatial localization, close to the cylinder, of the optimal initial condition may be significant when considering strategies to promote or control shedding
Gaps between jets in hadronic collisions
We propose a model to describe diffractive events in hadron-hadron collisions
where a rapidity gap is surrounded by two jets. The hard color-singlet object
exchanged in the t-channel and responsible for the rapidity gap is described by
the pQCD Balitsky-Fadin-Kuraev-Lipatov Pomeron, including corrections due to
next-to-leading logarithms. We allow the rapidity gap to be smaller than the
inter-jet rapidity interval, and the corresponding soft radiation is modeled
using the HERWIG Monte Carlo. Our model is able to reproduce all Tevatron data,
and allows to estimate the jet-gap-jet cross section at the LHC.Comment: 7 pages, 3 figures, version to appear in PR
Is the abundance of Faecalibacterium prausnitzii relevant to Crohn's disease?
Reports that bacteria within the Firmicutes phylum, especially the species Faecalibacterium prausnitzii, are less abundant in Crohn’s disease (CD) patients and supernatants from cultures of this bacterium are anti-inflammatory prompted the investigation of the possible correlations between the abundance of F.prausnitzii and the response to treatment in patients with gut diseases and healthy controls. In a randomized, double-blind trial, faeces were collected from healthy volunteers, and from patients with active CD, ulcerative colitis (UC) and irritable bowel syndrome before and after treatment. The levels of F. prausnitzii DNA in faecal suspensions were determined by PCR. Treatment by an elemental diet was effective, resulting in decreases in both the Harvey and Bradshaw index (P<0.001) and the concentrations of serum C-reactive protein (P<0.05). The total levels of F. prausnitzii in faecal samples from CD patients at presentation were lower than those in the other groups both before and after the treatment. There was no correlation between F. prausnitzii abundance and the severity of CD before treatment. Clinical improvement unexpectedly correlated with a significant decrease in the abundance of F. prausnitzii, especially the A2-165 subgroup (P<0.05). Our data suggest that a paucity of F. prausnitzii in the gastrointestinal microbial communities is likely to be a minor aetiological factor in CD: recovery following elemental diet is attributed to lower levels of gut flora
JIMWLK evolution in the Gaussian approximation
We demonstrate that the Balitsky-JIMWLK equations describing the high-energy
evolution of the n-point functions of the Wilson lines (the QCD scattering
amplitudes in the eikonal approximation) admit a controlled mean field
approximation of the Gaussian type, for any value of the number of colors Nc.
This approximation is strictly correct in the weak scattering regime at
relatively large transverse momenta, where it reproduces the BFKL dynamics, and
in the strong scattering regime deeply at saturation, where it properly
describes the evolution of the scattering amplitudes towards the respective
black disk limits. The approximation scheme is fully specified by giving the
2-point function (the S-matrix for a color dipole), which in turn can be
related to the solution to the Balitsky-Kovchegov equation, including at finite
Nc. Any higher n-point function with n greater than or equal to 4 can be
computed in terms of the dipole S-matrix by solving a closed system of
evolution equations (a simplified version of the respective Balitsky-JIMWLK
equations) which are local in the transverse coordinates. For simple
configurations of the projectile in the transverse plane, our new results for
the 4-point and the 6-point functions coincide with the high-energy
extrapolations of the respective results in the McLerran-Venugopalan model. One
cornerstone of our construction is a symmetry property of the JIMWLK evolution,
that we notice here for the first time: the fact that, with increasing energy,
a hadron is expanding its longitudinal support symmetrically around the
light-cone. This corresponds to invariance under time reversal for the
scattering amplitudes.Comment: v2: 45 pages, 4 figures, various corrections, section 4.4 updated, to
appear in JHE
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