4,452 research outputs found
How to Generate Four-Fermion Phase Space
We present a scheme for integrating the matrix element of an arbitrary
e^+e^-\to f_1f_2\bar f_3\bar f_4 process over the complete four-fermion phase
space, or its any part, by means of the Monte Carlo technique. The presented
algorithm has been successfully implemented in the KORALW Monte Carlo code.Comment: 16 page
Production of tau tau jj final states at the LHC and the TauSpinner algorithm: the spin-2 case
The TauSpinner algorithm is a tool that allows to modify the physics model of
the Monte Carlo generated samples due to the changed assumptions of event
production dynamics, but without the need of re-generating events. With the
help of weights -lepton production or decay processes can be modified
accordingly to a new physics model. In a recent paper a new version TauSpinner
ver.2.0.0 has been presented which includes a provision for introducing
non-standard states and couplings and study their effects in the
vector-boson-fusion processes by exploiting the spin correlations of
-lepton pair decay products in processes where final states include also
two hard jets. In the present paper we document how this can be achieved taking
as an example the non-standard spin-2 state that couples to Standard Model
particles and tree-level matrix elements with complete helicity information
included for the parton-parton scattering amplitudes into a -lepton pair
and two outgoing partons. This implementation is prepared as the external (user
provided) routine for the TauSpinner algorithm. It exploits amplitudes
generated by MadGraph5 and adopted to the TauSpinner algorithm format.
Consistency tests of the implemented matrix elements, reweighting algorithm and
numerical results for observables sensitive to polarization are
presented.Comment: 17 pages, 6 figures; version published in EPJ
Four-quark final state in W-pair production: Case of signal and background
We discuss theoretical predictions for W-pair production and decay at LEP2
and higher energies in a form suitable for comparison with raw data. We present
a practical framework for calculating uncertainties of predictions given by the
KORALW and grc4f Monte Carlo programs. As an example we use observables in the
decay channel: the total four-quark (four-jet) cross section
and two-quark/jet invariant-mass distribution and cross section, in the case
when the other two may escape detection. Effects of QED bremsstrahlung,
effective couplings, running W and Z widths, Coulomb interaction and the
complete tree level set of diagrams are discussed. We also revisit the question
of technical precision of the new version 1.21 of the KORALW Monte Carlo code
as well as of version 1.2(26) of the grc4f one.
Finally we find predictions of the two programs to have an overall physical
uncertainty of 2%.
As a side result we show, on the example of an invariant mass
distribution, the strong interplay of spin correlations and detector cut-offs
in the case of four-fermion final states.Comment: 26 pages, LaTe
Next Generation Higgs Bosons: Theory, Constraints and Discovery Prospects at the Large Hadron Collider
Particle physics model building within the context of string theory suggests
that further copies of the Higgs boson sector may be expected. Concerns
regarding tree-level flavor changing neutral currents are easiest to allay if
little or no couplings of next generation Higgs bosons are allowed to Standard
Model fermions. We detail the resulting general Higgs potential and mass
spectroscopy in both a Standard Model extension and a supersymmetric extension.
We present the important experimental constraints from meson-meson mixing,
loop-induced decays and LEP2 direct production limits. We
investigate the energy range of valid perturbation theory of these ideas. In
the supersymmetric context we present a class of examples that marginally aids
the fine-tuning problem for parameter space where the lightest Higgs boson mass
is greater than the Standard Model limit of 114 GeV. Finally, we study collider
physics signatures generic to next generation Higgs bosons, with special
emphasis on signal events, and describe the capability of
discovery at the Large Hadron Collider.Comment: 43 pages, 12 figures; v3: minor corrections, published in Physical
Review
Prospects for Observing an Invisibly Decaying Higgs Boson in the t anti-t H Production at the LHC
The prospects for observing an invisibly decaying Higgs boson in the t anti-t
H production at LHC are discussed. An isolated lepton, reconstructed hadronic
top-quark decay, two identified b-jets and large missing transverse energy are
proposed as the final state signature for event selection. Only the Standard
Model backgrounds are taken into account. It is shown that the t anti-t Z, t
anti-t W, b anti-b Z and b anti-b W backgrounds can individually be suppressed
below the signal expectation. The dominant source of background remains the t
anti-t production. The key for observability will be an experimental selection
which allows further suppression of the contributions from the t anti-t events
with one of the top-quarks decaying into a tau lepton. Depending on the details
of the final analysis, an excess of the signal events above the Standard Model
background of about 10% to 100% can be achieved in the mass range m_H= 100-200
GeV.Comment: Final version as accepted by EPJ
MC generator TAUOLA: implementation of Resonance Chiral Theory for two and three meson modes. Comparison with experiment
We present a partial upgrade of the Monte Carlo event generator TAUOLA with
the two and three hadron decay modes using the theoretical models based on
Resonance Chiral Theory. These modes account for 88% of total hadronic width of
the tau meson. First results of the model parameters have been obtained using
BaBar data for three pion mode.Comment: 5 pages, 1 figure, contribution to the Proceedings of the QCD@Work12
Conferenc
Theoretical inputs and errors in the new hadronic currents in TAUOLA
The new hadronic currents implemented in the TAUOLA library are obtained in
the unified and consistent framework of Resonance Chiral Theory: a Lagrangian
approach in which the resonances exchanged in the hadronic tau decays are
active degrees of freedom included in a way that reproduces the low-energy
results of Chiral Perturbation Theory. The short-distance QCD constraints on
the imaginary part of the spin-one correlators yield relations among the
couplings that render the theory predictive.
In this communication, the obtaining of the two- and three-meson form factors
is sketched. One of the criticisms to our framework is that the error may be as
large as 1/3, since it is a realization of the large-N_C limit of QCD in a
meson theory. A number of arguments are given which disfavor that claim
pointing to smaller errors, which would explain the phenomenological success of
our description in these decays. Finally, other minor sources of error and
current improvements of the code are discussed.Comment: 5 pages, no figures, contribution to the Proceedings of the
QCD@Work12 Conferenc
Coherent Exclusive Exponentiation for Precision Monte Carlo Calculations of Fermion Pair Production / Precision Predictions for (Un)stable W+W- Pairs
We present the new Coherent Exclusive Exponentiation (CEEX), in comparison to
the older Exclusive Exponentiation (EEX) and the semi-analytical Inclusive
Exponentiation (IEX), for the process e+e- -> f-bar f + n(gamma),
f=mu,tau,d,u,s,c,b, with validity for centre of mass energies from tau lepton
threshold to 1 TeV. We analyse 2f numerical results at the Z-peak, 189 GeV and
500 GeV. We also present precision calculations of the signal processes e+e- ->
4f in which the double resonant W+W- intermediate state occurs using our
YFSWW3-1.14 MC. Sample 4f Monte Carlo data are explicitly illustrated in
comparison to the literature at LEP2 energies. These comparisons show that a TU
for the signal process cross section of 0.4 percent is valid for the LEP2 200
GeV energy. LC energy results are also shown.Comment: 5 pages, 4 figures, Presented at ICHEP200
Spin effects in tau-lepton pair production at LHC
Question of the proper incorporation of spin effects in tau lepton decays is
often of importance. In present work, case of the Z/gamma to tau+ tau-
production mechanism is studied in detail. As an example, the effects due to
the spin correlations on the potential for the Minimal Supersymmetric Standard
Model (MSSM) Higgs searches in the tau tau decay channel at the Large Hadron
Collider (LHC) are discussed. For these processes, the Standard Model Z/gamma
to tau tau -pair production is a dominant background.
The spin effects in high energy physics reactions, can be implemented up to
certain approximation, independently of the algorithm and matrix elements used
by the production program. Information stored on every generated event can be
sufficient. The algorithm based on such approximation is documented. Question
of the theoretical uncertaintity is discussed only partly.Comment: latex 19 pages, 22 PostScript figures. Corrections of published
version (mainly language) include
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