76 research outputs found
MadEvent: Automatic Event Generation with MadGraph
We present a new multi-channel integration method and its implementation in
the multi-purpose event generator MadEvent, which is based on MadGraph. Given a
process, MadGraph automatically identifies all the relevant subprocesses,
generates both the amplitudes and the mappings needed for an efficient
integration over the phase space, and passes them to MadEvent. As a result, a
process-specific, stand-alone code is produced that allows the user to
calculate cross sections and produce unweighted events in a standard output
format. Several examples are given for processes that are relevant for physics
studies at present and forthcoming colliders.Comment: 11 pages, MadGraph home page at http://madgraph.physics.uiuc.ed
Color-flow decomposition of QCD amplitudes
We introduce a new color decomposition for multi-parton amplitudes in QCD,
free of fundamental-representation matrices and structure constants. This
decomposition has a physical interpretation in terms of the flow of color,
which makes it ideal for merging with shower Monte-Carlo programs. The
color-flow decomposition allows for very efficient evaluation of amplitudes
with many quarks and gluons, many times faster than the standard color
decomposition based on fundamental-representation matrices. This will increase
the speed of event generators for multi-jet processes, which are the principal
backgrounds to signals of new physics at colliders.Comment: 23 pages, 11 figures, version to appear on Phys. Rev.
Color-dressed recursive relations for multi-parton amplitudes
Remarkable progress inspired by twistors has lead to very simple analytic
expressions and to new recursive relations for multi-parton color-ordered
amplitudes. We show how such relations can be extended to include color and
present the corresponding color-dressed formulation for the Berends-Giele, BCF
and a new kind of CSW recursive relations. A detailed comparison of the
numerical efficiency of the different approaches to the calculation of
multi-parton cross sections is performed.Comment: 31 pages, 4 figures, 6 table
The heavy quark search at the LHC
We explore further the discovery potential for heavy quarks at the LHC, with
emphasis on the and of a sequential fourth family associated with
electroweak symmetry breaking. We consider QCD multijets, ,
and single backgrounds using event generation based on
improved matrix elements and low sensitivity to the modeling of initial state
radiation. We exploit a jet mass technique for the identification of
hadronically decaying 's and 's, to be used in the reconstruction of the
or mass. This along with other aspects of event selection can reduce
backgrounds to very manageable levels. It even allows a search for both
and in the absence of -tagging, of interest for the early running of
the LHC. A heavy quark mass of order 600 GeV is motivated by the connection to
electroweak symmetry breaking, but our analysis is relevant for any new heavy
quarks with weak decay modes.Comment: 12 pages, 7 figure
Higgs Boson Production in Association with Three Jets
The scattering amplitudes for Higgs + 5 partons are computed, with the Higgs
boson produced via gluon fusion in the large top-quark mass limit. A
parton-level analysis of Higgs + 3 jet production via gluon fusion and via
weak-boson fusion is presented, and the effectiveness of a central-jet veto is
analysed.Comment: 26 pages, 4 Postscript figures, uses JHEP3.cl
t' at the LHC: the physics of discovery
A search for a fourth family at the LHC is presently a low priority, but we
argue that an effective search can be conducted early with only a few inverse
femtobarns of data. We discuss a method based on invariant masses of single
jets for identifying the 's originating from heavy quark decays. This can
significantly increase signal to background in the reconstruction of the
mass. We also study the various types of physics that can impact the background
estimate, most notably higher order effects, initial state radiation, and
models of the underlying event.Comment: 16 pages, 12 figures, small improvements, version to appear in JHE
Search for a Standard Model Higgs Boson in CMS via Vector Boson Fusion in the H->WW->l\nu l\nu Channel
We present the potential for discovering the Standard Model Higgs boson
produced by the vector-boson fusion mechanism. We considered the decay of Higgs
bosons into the W+W- final state, with both W-bosons subsequently decaying
leptonically. The main background is ttbar with one or more jets produced. This
study is based on a full simulation of the CMS detector, and up-to-date
reconstruction codes. The result is that a signal of 5 sigma significance can
be obtained with an integrated luminosity of 12-72 1/fb for Higgs boson masses
between 130-200 GeV. In addition, the major background can be measured directly
to 7% from the data with an integrated luminosity of 30 1/fb. In this study, we
also suggested a method to obtain information in Higgs mass using the
transverse mass distributions.Comment: 26 pages, 22 figure
MadGraph/MadEvent v4: The New Web Generation
We present the latest developments of the MadGraph/MadEvent Monte Carlo event
generator and several applications to hadron collider physics. In the current
version events at the parton, hadron and detector level can be generated
directly from a web interface, for arbitrary processes in the Standard Model
and in several physics scenarios beyond it (HEFT, MSSM, 2HDM). The most
important additions are: a new framework for implementing user-defined new
physics models; a standalone running mode for creating and testing matrix
elements; generation of events corresponding to different processes, such as
signal(s) and backgrounds, in the same run; two platforms for data analysis,
where events are accessible at the parton, hadron and detector level; and the
generation of inclusive multi-jet samples by combining parton-level events with
parton showers. To illustrate the new capabilities of the package some
applications to hadron collider physics are presented:
1) Higgs search in pp \to H \to W^+W^-: signal and backgrounds.
2) Higgs CP properties: pp \to H jj$in the HEFT.
3) Spin of a new resonance from lepton angular distributions.
4) Single-top and Higgs associated production in a generic 2HDM.
5) Comparison of strong SUSY pair production at the SPS points.
6) Inclusive W+jets matched samples: comparison with the Tevatron data.Comment: 38 pages, 15 figure
PHASE, a Monte Carlo event generator for six-fermion physics at the LHC
PHASE is a new event generator dedicated to the study of Standard Model
processes with six fermions in the final state at the LHC. The code is intended
for analyses of vector boson scattering, Higgs search, three gauge boson
production, and top physics. This first version of the program describes final
states characterized by the presence of one neutrino, , at
O(). PHASE is based on a new iterative-adaptive multichannel
technique, and employs exact leading order matrix elements. The code can
generate unweighted events for any subset of all available final states. The
produced parton-level events carry full information on their colour and flavour
structure, enabling the evolution of the partons into fully hadronised final
states. An interface to hadronization packages is provided via the Les Houches
Protocol.Comment: 27 pages, Latex, 6 figure
QCD radiation in the production of heavy colored particles at the LHC
We present a study of effects of QCD radiation in the production of heavy
colored states, employing inclusive multi-jet samples obtained by matching
matrix elements and parton showers. We discuss several examples showing that
matched samples are in general not only more accurate than a parton shower
alone, but also often indispensable to make reliable predictions of beyond the
Standard Model signals.Comment: 26 pages, 9 figure
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