1,454 research outputs found
Time Dependent Hadronization via HERMES and EMC Data Consistency
Using QCD-inspired time dependent cross sections for pre-hadrons we provide a
combined analysis of available experimental data on hadron attenuation in DIS
off nuclei as measured by HERMES with 12 and 27 GeV and by EMC with 100 and 280
GeV lepton beam energies. We extract the complete four-dimensional evolution of
the pre-hadrons using the JETSET-part of PYTHIA. We find a remarkable
sensitivity of nuclear attenuation data to the details of the time-evolution of
cross sections. Only cross sections evolving linearly in time describe the
available data in a wide kinematical regime. Predictions for experimental
conditions at JLAB (5 and 12 GeV beam energies) are included.Comment: version as publishe
Improved Parton Showers at Large Transverse Momenta
Several methods to improve the parton-shower description of hard processes by
an injection of matrix-element-based information have been presented over the
years. In this article we study (re)weighting schemes for the first/hardest
emission. One objective is to provide a consistent matching of the POWHEG
next-to-leading order generator to the Pythia shower algorithms. Another is to
correct the default behaviour of these showers at large transverse momenta,
based on a comparison with real-emission matrix elements
Hadron formation and attenuation in deep inelastic lepton scattering off nuclei
We investigate hadron formation in deep inelastic lepton scattering on N, Kr
and Xe nuclei in the kinematic regime of the HERMES experiment. The elementary
electron-nucleon interaction is described within the event generator PYTHIA
while a full coupled-channel treatment of the final state interactions is
included by means of a BUU transport model. We find a good agreement with the
measured charged hadron multiplicity ratio for N and Kr targets by
accounting for the deceleration and absorption of the primarily produced
particles as well as for the creation of secondary hadrons in the final state
interactions.Comment: 13 pages, 5 figures; revised and extended version, accepted for
publication in Phys. Lett.
Radial flow afterburner for event generators and the baryon puzzle
A simple afterburner including radial flow to the randomized transverse
momentum obtained from event generators, Pythia and Hijing, has been
implemented to calculate the ratios and compare them with available
data. A coherent trend of qualitative agreement has been obtained in
collisions and in for various centralities. Those results indicate that
the radial flow does play an important role in the so called baryon puzzle
anomaly.Comment: 11 pages, 5 figures. To appear in Journal of Physics
Modified Signals for Supersymmetry in the NMSSM with a Singlino-like LSP
In the framework of the NMSSM with a singlino-like LSP, we study
quantitatively the impact of the additional bino -> singlino cascade on the
efficiencies in several search channels for supersymmetry of the ATLAS and CMS
collaborations. Compared to the MSSM, the additional cascade reduces the
missing transverse energy, but leads to additional jets or leptons. For the
NMSSM benchmark lines which generalize cMSSM benchmark points, the efficiencies
in the most relevant 2/3 jet + missing energy search channels can drop by
factors ~1/3 to ~1/7, and can reduce the present lower bounds on M_{1/2} by as
much as ~0.9 - 0.75 in the NMSSM for large bino-singlino mass differences. The
larger efficiencies in multijet or multilepton search channels are not strong
enough to affect this conclusion. In the fully constrained cNMSSM, sparticle
decay cascades via the lightest stau can lead to signal cross sections in
multilepton and 2tau search channels which are potentially visible at the LHC
with 7 TeV center of mass energy.Comment: 24 pages, 9 Figures, misprint in Table 1 correcre
Open charm production at HERA
Inclusive charm meson production cross sections in the deep inelastic
scattering and photoproduction regimes are compared with QCD leading and
next-to-leading (NLO) calculations. The NLO predictions are significantly below
the data in some parts of the measured kinematic range. Angular distributions
of dijet events with charm show clear evidence for the existence of charm
originating from the incoming photon. The charm fragmentation function is
measured for the first time at HERA. Various fragmentation ratios and the
fragmentation fractions of the low-lying charm hadrons are determined and
compared to previous e+e- results.Comment: 6 pages, 7 figures, to appear in the proceedings of the International
Conference on the Structure and Interactions of the Photon, Frascati (Italy),
7-11 April 200
Using gamma+jets Production to Calibrate the Standard Model Z(nunu)+jets Background to New Physics Processes at the LHC
The irreducible background from Z(nunu)+jets, to beyond the Standard Model
searches at the LHC, can be calibrated using gamma+jets data. The method
utilises the fact that at high vector boson pT, the event kinematics are the
same for the two processes and the cross sections differ mainly due to the
boson-quark couplings. The method relies on a precise prediction from theory of
the Z/gamma cross section ratio at high pT, which should be insensitive to
effects from full event simulation. We study the Z/gamma ratio for final states
involving 1, 2 and 3 hadronic jets, using both the leading-order parton shower
Monte Carlo program Pythia8 and a leading-order matrix element program Gambos.
This enables us both to understand the underlying parton dynamics in both
processes, and to quantify the theoretical systematic uncertainties in the
ratio predictions. Using a typical set of experimental cuts, we estimate the
net theoretical uncertainty in the ratio to be of order 7%, when obtained from
a Monte Carlo program using multiparton matrix-elements for the hard process.
Uncertainties associated with full event simulation are found to be small. The
results indicate that an overall accuracy of the method, excluding statistical
errors, of order 10% should be possible.Comment: 22 pages, 14 figures; Accepted for publication by JHE
PYTHIA 6.4 Physics and Manual
The PYTHIA program can be used to generate high-energy-physics `events', i.e.
sets of outgoing particles produced in the interactions between two incoming
particles. The objective is to provide as accurate as possible a representation
of event properties in a wide range of reactions, within and beyond the
Standard Model, with emphasis on those where strong interactions play a role,
directly or indirectly, and therefore multihadronic final states are produced.
The physics is then not understood well enough to give an exact description;
instead the program has to be based on a combination of analytical results and
various QCD-based models. This physics input is summarized here, for areas such
as hard subprocesses, initial- and final-state parton showers, underlying
events and beam remnants, fragmentation and decays, and much more. Furthermore,
extensive information is provided on all program elements: subroutines and
functions, switches and parameters, and particle and process data. This should
allow the user to tailor the generation task to the topics of interest.Comment: 576 pages, no figures, uses JHEP3.cls. The code and further
information may be found on the PYTHIA web page:
http://www.thep.lu.se/~torbjorn/Pythia.html Changes in version 2: Mistakenly
deleted section heading for "Physics Processes" reinserted, affecting section
numbering. Minor updates to take into account referee comments and new colour
reconnection option
Quaero@H1: An Interface to High-pT HERA Event Data
Distributions from high-pT HERA event data analyzed in a general search for
new physics at H1 have been incorporated into Quaero, an algorithm designed to
automate tests of specific hypotheses with high energy collider data. The use
of Quaero@H1 to search for leptoquarks, R-parity violating supersymmetry, and
excited quarks provides examples to develop intuition for the algorithm's
performance.Comment: Submitted to Eur. Phys. J.
Discovering the constrained NMSSM with tau leptons at the LHC
The constrained Next-to-Minimal Supersymmetric Standard Model (cNMSSM) with
mSugra-like boundary conditions at the GUT scale implies a singlino-like LSP
with a mass just a few GeV below a stau NLSP. Hence, most of the squark/gluino
decay cascades contain two tau leptons. The gluino mass >~ 1.2 TeV is somewhat
larger than the squark masses of >~ 1 TeV. We simulate signal and background
events for such a scenario at the LHC, and propose cuts on the transverse
momenta of two jets, the missing transverse energy and the transverse momentum
of a hadronically decaying tau lepton. This dedicated analysis allows to
improve on the results of generic supersymmetry searches for a large part of
the parameter space of the cNMSSM. The distribution of the effective mass and
the signal rate provide sensitivity to distinguish the cNMSSM from the
constrained Minimal Supersymmetric Standard Model in the stau-coannihilation
region.Comment: 18 pages, 3 Figure
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