arXiv subjects

Johan Alwall

Publications and source records attributed to Johan Alwall.

28 records · Page 2Linked to original sources

arXiv2005

Quark Asymmetries and Intrinsic Charm in Nucleons

We have developed a physical model for the non-perturbative x-shape of parton density functions in the proton, based on Gaussian fluctuations in momenta, and quantum fluctuations of the proton into meson-baryon pairs. The model describes the proton structure function and gives a natural explanation of observed quark asymmetries, such as the difference between the anti-up and anti-down sea quark distributions and between the up and down valence distributions. We find an asymmetry in the momentum distribution of strange and anti-strange quarks in the nucleon, large enough to reduce the NuTeV anomaly to a level which does not give a significant indication of physics beyond the standard model. We also consider charmed fluctuations, and show that they can explain the excess at large x in the EMC F2c data.

hep-ph↗

arXiv2005

Quark Asymmetries in Nucleons

We have developed a physical model for the non-perturbative x-shape of parton density functions in the proton, based on Gaussian fluctuations in momenta, and quantum fluctuations of the proton into meson-baryon pairs. The model describes the proton structure function and gives a natural explanation of observed quark asymmetries, such as the difference between the anti-up and anti-down sea quark distributions and between the up and down valence distributions. We also find an asymmetry in the momentum distribution of strange and anti-strange quarks in the nucleon, large enough to reduce the NuTeV anomaly to a level which does not give a significant indication of physics beyond the standard model.

hep-ph↗

arXiv2005

Quark asymmetries in the proton from a model for parton densities

Based on quantum fluctuations in momentum and of the proton into meson-baryon pairs, we develop a physical model for the non-perturbative x-shape of parton density functions in the proton. The model describes the proton structure function and gives a natural explanation of observed quark asymmetries, such as the difference between the anti-up and anti-down sea quark distributions and between the up and down valence distributions. An asymmetry in the momentum distribution of strange and anti-strange quarks in the nucleon is found to reduce the NuTeV anomaly to a level which does not give a significant indication of physics beyond the standard model.

hep-ph↗

arXiv2005

MATCHIG: A program for matching charged Higgs boson production at hadron colliders

This manual describes how to use the MatCHig code for matching the charged Higgs boson production processes gg->tbH+/- and gb->tH+/-. A negative term, correcting for the double-counting between these processes, is implemented as an external process to PYTHIA, allowing the Monte Carlo generation of matched events. Results from the matching were published in JHEP 0412, 050. The code can be downloaded from http://www.isv.uu.se/thep/MC/matchig/ .

hep-ph↗

arXiv2004

Sea Quark Asymmetries in the Nucleon - A physical model for parton distributions in hadrons

A physical model for parton densities in hadrons, based on Gaussian momentum fluctuations of partons and hadronic baryon-meson fluctuations, is presented. The model has previously been shown to describe proton structure function data, and is now applied to sea quark asymmetries and shown to describe the dbar-ubar asymmetry of the proton. By considering fluctuations involving strange quarks, the model gives an asymmetry between the momentum distributions of s and sbar, which would reduce the significance of the NuTeV anomaly.

hep-ph↗

arXiv2004

Strange quark asymmetry in the nucleon and the NuTeV anomaly

The NuTeV anomaly of a non-universal value of the fundamental parameter sin^2θ_W in the electroweak theory has been interpreted as an indication for new physics beyond the Standard Model. However, the observed quantity depends on a possible asymmetry in the momentum distributions of strange quarks and antiquarks in the nucleon. This asymmetry occurs naturally in a phenomenologically successful physical model for such parton distributions, which reduces the NuTeV result to only about two standard deviations from the Standard Model.

hep-ph↗

arXiv2004

An improved description of charged Higgs boson production

Many extensions of the Standard Model predict the existence of charged Higgs bosons. In order to be able to find those particles, an accurate description of their production is needed. In Monte Carlo simulations of charged Higgs boson production at hadron colliders, the two tree-level processes gb->H+/-t and gg->H+/-tb are used. Since those processes overlap in the collinear region of the phase-space of the outgoing b-quark, care must be taken not to introduce double-counting if both processes are to be used together. In this talk I present a method for matching these processes, developed by Johan Rathsman and myself. The method also allows for investigations of the factorization scale dependence of the processes and a better understanding of which factorization scale to choose to get a reliable description of charged Higgs production.

hep-ph↗

arXiv2004

Interpretation of electron-proton scattering at low Q^2

Low-Q^2 photons do not resolve partons in the proton, which gives problems when applying the deep inelastic scattering formalism, such as an unphysical, negative gluon density extracted from data. Considering instead hadronic fluctuations of the photon, we show that the generalised vector meson dominance model (GVDM) gives a good description of the measured cross section at low Q^2, i.e. reproduces F_2(x,Q^2), using only few parameters with essentially known values. Combining GVDM and parton density functions makes it possible to obtain a good description of F_2 data over the whole range of x and Q^2.

hep-ph↗

arXiv2003

The ppbar -> tbH^+/- Process at the Tevatron in HERWIG and PYTHIA Simulations

Charged Higgs boson production in association with a top quark could be the first indication of the existence of Higgs particles. The Tevatron Run-II started data-taking in April 2001 at a centre-of-mass energy of 1960 GeV and could probe the existence of a charged Higgs boson beyond the current mass limit. We study the ppbar -> tbH^+/- production process with Monte Carlo simulations in HERWIG and PYTHIA, comparing expected cross sections and basic selection variables.

hep-ph↗

arXiv2003

Generalized VDM and F_2 data at low Q^2

The generalised vector meson dominance model (GVDM) gives a good description of F_2 data at very low Q^2. At intermediate Q^2 a GVDM component avoids problems when applying the large-Q^2 DIS formalism, such as a negative gluon distribution in the proton. The fluctuations of the exchanged photon into vector mesons is also a natural part of a model with hadronic fluctuations of the target proton, which gives a good description of the non-perturbative x-shape of the proton's parton density functions at the starting scale Q^2_0 for DGLAP evolution.

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