19 research outputs found
Production of \uptau τ lepton pairs with high p T jets at the LHC and the TauSpinner reweighting algorithm
The TauSpinner algorithm allows to modify the physics of the Monte Carlo
generated samples due to the changed assumptions of event production dynamics,
without re-generating events. To each event it attributes weights: the spin
effects of tau-lepton production or decay, or the production mechanism are
modified. There is no need to repeat the detector response simulation. We
document the extension to 2 to 4 processes in which the matrix elements for the
parton-parton scattering amplitudes into a tau-lepton pair and two outgoing
partons are used. Tree-level matrix elements for the Standard Model processes,
including the Higgs boson production are used. Automatically generated codes by
MadGraph5 have been adapted. Tests of the matrix elements, reweighting
algorithm and numerical results are presented. For averaged tau lepton
polarisation, we perform comparison of 2 to 2 and 2 to 4 matrix elements used
to calculate the spin weight in pp to tau tau j j events. We show, that for
events with tau-lepton pair close to the Z-boson peak, the tau-lepton
polarisation calculated using 2 to 4 matrix elements is very close to the one
calculated using 2 to 2 Born process only. For the m_(tautau) masses above the
Z-boson peak, the effect from including 2 to 4 matrix elements is also
marginal, however when restricting into subprocesses qq,q bar q to tau tau j j
only, it can lead to a 10% difference on the predicted tau-lepton polarisation.
Choice of electroweak scheme can have significant impact. The modification of
the electroweak or strong interaction can be performed with the re-weighting
technique. TauSpinner v.2.0.0, allows to introduce non-standard couplings for
the Higgs boson and study their effects in the vector-boson-fusion. The
discussion is relegated to forthcoming publications.Comment: 33 pages 7 figure
Why do we need higher order fully exclusive Monte Carlo generator for Higgs boson production from heavy quark fusion at LHC?
In this paper we argue that having available higher order fully exclusive
Monte Carlo generator for Higgs boson production from heavy quark fusion will
be mandatory for data analysis at LHC. The H to tau tau channel, a key for
early discovery of the Higgs boson in the MSSM scenario, is discussed. With
simplified example and for mH = 120 GeV we show, that depending on choice among
presently available approaches, used for simulation of Higgs boson production
from b bbar H Yukawa coupling, final acceptance for the signal events being
reconstructed inside mass window may differ by a factor of 3. The spread is
even larger (up to a factor of 10) for other production mechanisms (promising
for some regions of the MSSM parameter space). The complete analysis, which
necessarily will add stringent requirements for background rejection (such as
identification of b-jet or veto on b-jet) and which will require statistical
combination of samples selected with different selection criteria may only
enhance the uncertainty.Comment: 14 pages, 22 figure
Universal Interface of TAUOLA Technical and Physics Documentation
Because of their narrow width, tau decays can be well separated from their
production process. Only spin degrees of freedom connect these two parts of the
physics process of interest for high energy collision experiments. In the
following, we present a Monte Carlo algorithm which is based on that property.
The interface supplements events generated by other programs, with tau decays.
Effects of spin, genuine weak corrections or of new physics may be taken into
account at the time when a tau decay is generated and written into an event
record.Comment: 1+44 pages, 17 eps figure
Associated production as a probe of the polarized gluon distribution
Associated production of and a has recently been proposed
as clean probe of the gluon distribution. The same mechanism can be used to
probe the polarized gluon content of the proton in polarized proton-proton
collisions. We study production at both polarized fixed
target and polarized collider energies.Comment: 16 pages (10 figures available from M.A.D.), MAD/PH/745, SNUTP 93-6,
YUMS 93-
Matching NLO parton shower matrix element with exact phase space: case of W -> l nu (gamma) and gamma^* -> pi^+pi^-(gamma)
The PHOTOS Monte Carlo is often used for simulation of QED effects in decay
of intermediate particles and resonances. Momenta are generated in such a way
that samples of events cover the whole bremsstrahlung phase space. With the
help of selection cuts, experimental acceptance can be then taken into account.
The program is based on an exact multiphoton phase space. Crude matrix element
is obtained by iteration of a universal multidimensional kernel. It ensures
exact distribution in the soft photon region. Algorithm is compatible with
exclusive exponentiation. To evaluate the program's precision, it is necessary
to control the kernel with the help of perturbative results. If available,
kernel is constructed from the exact first order matrix element. This ensures
that all terms necessary for non-leading logarithms are taken into account. In
the present paper we will focus on the W -> l nu and gamma^* -> pi^+ pi^-
decays. The Born level cross sections for both processes approach zero in some
points of the phase space. A process dependent compensating weight is
constructed to incorporate the exact matrix element, but is recommended for use
in tests only. In the hard photon region, where scalar QED is not expected to
be reliable, the compensating weight for gamma^* decay can be large. With
respect to the total rate, the effect remains at the permille level. It is
nonetheless of interest. The terms leading to the effect are analogous to some
terms appearing in QCD. The present paper can be understood either as a
contribution to discussion on how to match two collinear emission chains
resulting from charged sources in a way compatible with the exact and complete
phase space, exclusive exponentiation and the first order matrix element of QED
(scalar QED), or as the practical study of predictions for accelerator
experiments.Comment: 24 page
Hadronic production and the Gottfried Sum Rule
The difference in production rate between and at hadron colliders
is very sensitive to the the difference between up- and down-quark
distributions in the proton. This sensitivity allows for a variety of useful
measurements. We consider the difference in the sea
distributions and the difference in the
polarized parton distribution functions. In both cases we construct an
asymmetry to reduce systematic uncertainties. Although we discuss measurements
at the Tevatron and future hadron colliders, we find that the Brookhaven
Relativistic Heavy Ion Collider (RHIC) is the most appropriate hadron collider
for these measurements.Comment: 19 pages (20 figures available from the authors), MAD/PH/74
Pair production of neutral Higgs bosons at the CERN Large Hadron Collider
We study the hadroproduction of two neutral Higgs bosons in the minimal
supersymmetric extension of the standard model, which provides a handle on the
trilinear Higgs couplings. We include the contributions from quark-antiquark
annihilation at the tree level and those from gluon-gluon fusion, which
proceeds via quark and squark loops. We list compact results for the tree-level
partonic cross sections and the squark loop amplitudes, and we confirm previous
results for the quark loop amplitudes. We quantitatively analyze the hadronic
cross sections at the CERN Large Hadron Collider assuming a favorable
supergravity-inspired scenario.Comment: 22 pages (Latex), 16 figures (Postscript). Discussion of theoretical
uncertainties and background processes added. Accepted for publication in
Phys. Rev.
Measuring Higgs boson couplings at the LHC
For an intermediate mass Higgs boson with SM-like couplings the LHC allows
observation of a variety of decay channels in production by gluon fusion and
weak boson fusion. Cross section ratios provide measurements of various ratios
of Higgs couplings, with accuracies of order 15% for 100 fb^{-1} of data in
each of the two LHC experiments. For Higgs masses above 120 GeV, minimal
assumptions on the Higgs sector allow for an indirect measurement of the total
Higgs boson width with an accuracy of 10 to 20%, and of the H-->WW partial
width with an accuracy of about 10%.Comment: 25 pages, Revtex, 1 figur