987 research outputs found
A simple shower and matching algorithm
We present a simple formalism for parton-shower Markov chains. As a first
step towards more complete uncertainty bands, we incorporate a comprehensive
exploration of the ambiguities inherent in such calculations. To reduce this
uncertainty, we then introduce a matching formalism which allows a generated
event sample to simultaneously reproduce any infrared safe distribution
calculated at leading or next-to-leading order in perturbation theory, up to
sub-leading corrections. To enable a more universal definition of perturbative
calculations, we also propose a more general definition of the hadronization
cutoff. Finally, we present an implementation of some of these ideas for
final-state gluon showers, in a code dubbed VINCIA.Comment: 32 pages, 6 figure
Cronin Effect in Hadron Production off Nuclei
Recent data from RHIC for high- hadrons in gold-gold collisions raised
again the long standing problem of quantitatively understanding the Cronin
effect, i.e. nuclear enhancement of high- hadrons due to multiple
interactions in nuclear matter. In nucleus-nucleus collisions this effect has
to be reliably calculated as baseline for a signal of new physics in high-
hadron production. The only possibility to test models is to compare with
available data for collisions, however, all existing models for the Cronin
effect rely on a fit to the data to be explained. We develop a phenomenological
description based on the light-cone QCD-dipole approach which allows to explain
available data without fitting to them and to provide predictions for
collisions at RHIC and LHC. We point out that the mechanism causing Cronin
effect drastically changes between the energies of fixed target experiments and
RHIC-LHC. High- hadrons are produced incoherently on different nucleons at
low energies, whereas the production amplitudes interfere if the energy is
sufficiently high.Comment: the final version to appear in Phys. Rev. Let
Constrained MC for QCD evolution with rapidity ordering and minimum kT
With the imminent start of LHC experiments, development of phenomenological
tools, and in particular the Monte Carlo programs and algorithms, becomes
urgent. A new algorithm for the generation of a parton shower initiated by the
single initial hadron beam is presented. The new algorithm is of the class of
the so called ``constrained MC'' type algorithm (an alternative to the backward
evolution MC algorithm), in which the energy and the type of the parton at the
end of the parton shower are constrained (predefined). The complete kinematics
configurations with explicitly constructed four momenta are generated and
tested. Evolution time is identical with rapidity and minimum transverse
momentum is used as an infrared cut-off. All terms of the leading-logarithmic
approximation in the DGLAP evolution are properly accounted for. In addition,
the essential improvements towards the so-called CCFM/BFKL models are also
properly implemented. The resulting parton distributions are cross-checked up
to the 0.1% precision level with the help of a multitude of comparisons with
other MC and non-MC programs. We regard these tests as an important asset to be
exploited at the time when the presented MC will enter as a building block in a
larger MC program for W/Z production process at LHC.Comment: Submitted to Computer Physics Communication
Magnetic Fluctuations and Turbulence in the Venus Magnetosheath and Wake
Recent research has shown that distinct physical regions in the Venusian
induced magnetosphere are recognizable from the variations of strength and of
wave/fluctuation activity of the magnetic field. In this paper the statistical
properties of magnetic fluctuations are investigated in the Venusian
magnetosheath, terminator, and wake regions. The latter two regions were not
visited by previous missions. We found 1/f fluctuations in the magnetosheath,
large-scale structures near the terminator and more developed turbulence
further downstream in the wake. Location independent short-tailed non-Gaussian
statistics was observed.Comment: 16 pages, 4 figure
Antenna subtraction with hadronic initial states
The antenna subtraction method for the computation of higher order
corrections to jet observables and exclusive cross sections at collider
experiments is extended to include hadronic initial states. In addition to the
already known antenna subtraction with both radiators in the final state
(final-final antennae), we introduce antenna subtractions with one or two
radiators in the initial state (initial-final or initial-initial antennae). For
those, we derive the phase space factorization and discuss the allowed phase
space mappings at NLO and NNLO. We present integrated forms for all antenna
functions relevant to NLO calculations, and describe the construction of the
full antenna subtraction terms at NLO on two examples. The extension of the
formalism to NNLO is outlined.Comment: 33 pages, 3 figure
Next-to-Leading Order Hard Scattering Using Fully Unintegrated Parton Distribution Functions
We calculate the next-to-leading order fully unintegrated hard scattering
coefficient for unpolarized gluon-induced deep inelastic scattering using the
logical framework of parton correlation functions developed in previous work.
In our approach, exact four-momentum conservation is maintained throughout the
calculation. Hence, all non-perturbative functions, like parton distribution
functions, depend on all components of parton four-momentum. In contrast to the
usual collinear factorization approach where the hard scattering coefficient
involves generalized functions (such as Dirac -functions), the fully
unintegrated hard scattering coefficient is an ordinary function. Gluon-induced
deep inelastic scattering provides a simple illustration of the application of
the fully unintegrated factorization formalism with a non-trivial hard
scattering coefficient, applied to a phenomenologically interesting case.
Furthermore, the gluon-induced process allows for a parameterization of the
fully unintegrated gluon distribution function.Comment: 22 pages, Typos Fixed, Reference Added, Minor Clarification Adde
Higgs Hadroproduction at Large Feynman x
We propose a novel mechanism for the production of the Higgs boson in
inclusive hadronic collisions, which utilizes the presence of heavy quarks in
the proton wave function. In these inclusive reactions the Higgs boson acquires
the momenta of both the heavy quark and antiquark and thus carries 80% or more
of the projectile's momentum. We predict that the cross section for the inclusive production of the Standard Model
Higgs coming from intrinsic bottom Fock states is of order 150 fb at LHC
energies, peaking in the region of . Our estimates indicate that
the corresponding cross section coming from gluon-gluon fusion at
is relatively negligible and therefore the peak from intrinsic bottom should be
clearly visible for experiments with forward detection capabilities. The
predicted cross section for the production of the Standard Model Higgs coming
from intrinsic heavy quark Fock states in the proton is sufficiently large that
detection at the Tevatron and the LHC may be possible.Comment: 23 pages, 5 figure
Diffractive Higgs Production at the LHC
We use diffractive parton distributions obtained from fits to the diffractive
structure function measured at HERA to predict cross sections for single
diffractive Higgs production at the LHC. The dominant background processes are
also considered. Although some 5% - 15% of Higgs events are predicted to be
diffractive in this model, the ratio of signal to background is not
significantly improved.Comment: 14 pages, LaTeX, incl. 6 postscript figures, uses epsf.st
Intermittent turbulence, noisy fluctuations and wavy structures in the Venusian magnetosheath and wake
Recent research has shown that distinct physical regions in the Venusian
induced magnetosphere are recognizable from the variations of strength of the
magnetic field and its wave/fluctuation activity. In this paper the statistical
properties of magnetic fluctuations are investigated in the Venusian
magnetosheath and wake regions. The main goal is to identify the characteristic
scaling features of fluctuations along Venus Express (VEX) trajectory and to
understand the specific circumstances of the occurrence of different types of
scalings. For the latter task we also use the results of measurements from the
previous missions to Venus. Our main result is that the changing character of
physical interactions between the solar wind and the planetary obstacle is
leading to different types of spectral scaling in the near-Venusian space.
Noisy fluctuations are observed in the magnetosheath, wavy structures near the
terminator and in the nightside near-planet wake. Multi-scale turbulence is
observed at the magnetosheath boundary layer and near the quasi-parallel bow
shock. Magnetosheath boundary layer turbulence is associated with an average
magnetic field which is nearly aligned with the Sun-Venus line. Noisy magnetic
fluctuations are well described with the Gaussian statistics. Both
magnetosheath boundary layer and near shock turbulence statistics exhibit
non-Gaussian features and intermittency over small spatio-temporal scales. The
occurrence of turbulence near magnetosheath boundaries can be responsible for
the local heating of plasma observed by previous missions
New hominin remains and revised context from the earliest Homo erectus locality in East Turkana, Kenya
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