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-$p_T$ hadrons in gold-gold collisions raised again the long standing problem of quantitatively understanding the Cronin effect, i.e. nuclear enhancement of high-$p_T$ 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-$p_T$ hadron production. The only possibility to test models is to compare with available data for $pA$ 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 $pA$ 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-$p_T$ 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 $\delta$-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 ${d \sigma/d x_F}(p \bar p \to H X)$ 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 $x_F \sim 0.9$. Our estimates indicate that the corresponding cross section coming from gluon-gluon fusion at $x_F = 0.9$ 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