8,920 research outputs found
Minimal Gauge Invariant Classes of Tree Diagrams in Gauge Theories
We describe the explicit construction of groves, the smallest gauge invariant
classes of tree Feynman diagrams in gauge theories. The construction is valid
for gauge theories with any number of group factors which may be mixed. It
requires no summation over a complete gauge group multiplet of external matter
fields. The method is therefore suitable for defining gauge invariant classes
of Feynman diagrams for processes with many observed final state particles in
the standard model and its extensions.Comment: 13 pages, RevTeX (EPS figures
CompHEP 4.4 - Automatic Computations from Lagrangians to Events
We present a new version of the CompHEP program (version 4.4). We describe
shortly new issues implemented in this version, namely, simplification of quark
flavor combinatorics for the evaluation of hadronic processes, Les Houches
Accord based CompHEP-PYTHIA interface, processing the color configurations of
events, implementation of MSSM, symbolical and numerical batch modes, etc. We
discuss how the CompHEP program is used for preparing event generators for
various physical processes. We mention a few concrete physics examples for
CompHEP based generators prepared for the LHC and Tevatron.Comment: The paper has been presented on IX International Workshop on Advanced
Computing and Analysis Techniques in Physics Research December 1-5, 2003.
KEK, Japan. 10 pages, 2 figure
Hidden Grassmann structure in the XXZ model V: sine-Gordon model
We study one-point functions of the sine-Gordon model on a cylinder. Our
approach is based on a fermionic description of the space of descendent fields,
developed in our previous works for conformal field theory and the sine-Gordon
model on the plane. In the present paper we make an essential addition by
giving a connection between various primary fields in terms of yet another kind
of fermions. The one-point functions of primary fields and descendants are
expressed in terms of a single function defined via the data from the
thermodynamic Bethe Ansatz equations.Comment: 36 pages. Some corrections are done in latest version, especially in
the subsection 10.
Quantum model of interacting ``strings'' on the square lattice
The model which is the generalization of the one-dimensional XY-spin chain
for the case of the two-dimensional square lattice is considered. The subspace
of the ``string'' states is studied. The solution to the eigenvalue problem is
obtained for the single ``string'' in cases of the ``string'' with fixed ends
and ``string'' of types (1,1) and (1,2) living on the torus. The latter case
has the features of a self-interacting system and looks not to be integrable
while the previous two cases are equivalent to the free-fermion model.Comment: LaTeX, 33 pages, 16 figure
W+2jets production at Tevatron -- VECBOS and CompHEP comparison
Results of calculation of all subprocesses in proton-antiproton collisions
which contribute to the W+2jets final state are presented at Tevatron energy.
The calculation has been carried out by means of the CompHEP software package.
A detail comparison with VECBOS generator results for cross sections and
various distributions shows an agreement at the level of Monte-Carlo accuracy.
Therefore the additional independent check of VECBOS generator has been done.
In complement to the VECBOS generator a new generator based on CompHEP allows
to study individual subprocesses like or . The last
point is important, for instance, for study part of the background
for single top or Standard Model Higgs signal at Tevatron.Comment: 8 pages including 4 figures as a gzipped postscript fil
Prospects of mass measurements for neutral MSSM Higgs bosons in the intense-coupling regime at a Linear Collider
We analyze the prospects for detecting the three neutral Higgs bosons of the
Minimal Supersymmetric extension of the Standard Model in the intense-coupling
regime at e+e- colliders. Due to the small mass differences between the Higgs
states in this regime and their relative large total decay widths, the
discrimination between the particles is challenging at the LHC and in some
cases even impossible. We propose to use the missing mass technique in the
Higgs-strahlung process in e+e- collisions to distinguish between the two
CP-even Higgs eigenstates h and H, relying on their b b-bar decay in the
b,b-bar,l+,l- event sample. Ah and AH associated production is then studied in
the 4b-jet event sample to probe the CP-odd A boson. At collider energies
sqrt(s) = 300 GeV and an integrated luminosity of 500 fb-1, accuracies in the
mass measurement of the CP-even Higgs bosons are expected to range from 100 to
300 MeV, while for the CP-odd A boson, accuracies of less than 500 MeV can be
obtained.Comment: 12 pages, 15 Postscript figure
Single top quarks at the Fermilab Tevatron
We present a calculation of the single top quark cross section for
proton-antiproton interactions with sqrt(s) = 1.8 TeV at the Fermilab Tevatron
collider. We examine the effects of top mass, parton distribution functions,
QCD scale, and collision energy, on each of the component production
mechanisms, and study the kinematic distributions for standard model
electroweak production. At the upgraded Tevatron with sqrt(s) = 2.0 TeV and
high luminosity, it will be possible to test the nature of the Wtb coupling
using single top production. We estimate the sensitivity to measure the single
top cross section, and thus to directly measure V_tb and the top quark partial
width. We show what happens to the V_tb measurement when an anomalous (V+A)
component is added to the Wtb coupling, and how the top quark polarization
affects the kinematic distributions.Comment: 31 pages including 11 figure
Simplification of Flavour Combinatorics in the Evaluation of Hadronic Processes
A serious computational problem in the evaluation of hadronic collision
processes is connected with the large number of partonic subprocesses included
in the calculation. These are from the quark and gluon content of the initial
hadrons, and from CKM quark mixing. For example, there are 180 subprocesses
which contribute to the +2jets process, and 292 subprocesses in +3jets
production at the LHC, even when quarks from only the first two generations are
taken into account.
We propose a simple modification of the rules for evaluation of cross
sections and distributions, which avoids multiplication of channels from the
mixture of quark states. The method is based on a unitary rotation of down
quarks, thus, transporting the mixing matrix elements from vertices of Feynman
diagrams to the parton distribution functions (PDF). As a result, one can
calculate cross sections with significantly fewer subprocesses. For the example
mentioned above, with the new rules, one need evaluate only 21 and 33
subprocesses respectively. The matrix elements of the subprocesses are
calculated without quark mixing but with a modified PDF convolution which
depends on the quark mixing angle, and on the topologies of gauge invariant
classes of diagrams. The proposed method has been incorporated into the CompHEP
program and checked with various examples.Comment: 10 pages (standard LaTeX code), 3 figures, 2 table
Infrared singularities in Landau gauge Yang-Mills theory
We present a more detailed picture of the infrared regime of Landau gauge
Yang-Mills theory. This is done within a novel framework that allows one to
take into account the influence of finite scales within an infrared power
counting analysis. We find that there are two qualitatively different infrared
fixed points of the full system of Dyson-Schwinger equations. The first extends
the known scaling solution, where the ghost dynamics is dominant and gluon
propagation is strongly suppressed. It features in addition to the strong
divergences of gluonic vertex functions in the previously considered uniform
scaling limit, when all external momenta tend to zero, also weaker kinematic
divergences, when only some of the external momenta vanish. The second solution
represents the recently proposed decoupling scenario where the gluons become
massive and the ghosts remain bare. In this case we find that none of the
vertex functions is enhanced, so that the infrared dynamics is entirely
suppressed. Our analysis also provides a strict argument why the Landau gauge
gluon dressing function cannot be infrared divergent.Comment: 29 pages, 25 figures; published versio
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