533 research outputs found
Quartic Gauge Couplings and the Radiation Zero in pp to l nu gamma gamma events at the LHC
We report a study of the process pp to l nu gamma gamma at CERN's Large
Hadron Collider, using a leading order partonic-level event generator
interfaced to the Pythia program for showering and hadronisation and a with a
generic detector simulation. The process is sensitive to possible anomalous
quartic gauge boson couplings of the form W W gamma gamma. It is shown how
unitarity-safe limits may be placed on these anomalous couplings by applying a
binned maximum likelihood fit to the distribution of the two-photon invariant
mass, M(gamma gamma), below a cutoff of 1TeV. Assuming 30fb-1 of integrated
luminosity, the expected limits are two orders of magnitude tighter than those
available from LEP. It is also demonstrated how the Standard Model radiation
zero feature of the qq to W gamma gamma process may be observed in the
difference between the two-photon and charged lepton pseudo-rapidities.Comment: 9 pages, 7 figure
A Positive-Weight Next-to-Leading-Order Monte Carlo for Z Pair Hadroproduction
We present a first application of a previously published method for the
computation of QCD processes that is accurate at next-to-leading order, and
that can be interfaced consistently to standard shower Monte Carlo programs. We
have considered Z pair production in hadron-hadron collisions, a process whose
complexity is sufficient to test the general applicability of the method. We
have interfaced our result to the HERWIG and PYTHIA shower Monte Carlo
programs. Previous work on next-to-leading order corrections in a shower Monte
Carlo (the MC@NLO program) may involve the generation of events with negative
weights, that are avoided with the present method. We have compared our results
with those obtained with MC@NLO, and found remarkable consistency. Our method
can also be used as a standalone, alternative implementation of QCD
corrections, with the advantage of positivity, improved convergence, and
next-to-leading logarithmic accuracy in the region of small transverse momentum
of the radiated parton.Comment: 33 pages, 10 figure
Azimuthal anisotropy of jet quenching at LHC
We analyze the azimuthal anisotropy of jet spectra due to energy loss of hard
partons in quark-gluon plasma, created initially in nuclear overlap zone in
collisions with non-zero impact parameter. The calculations are performed for
semi-central Pb-Pb collisions at LHC energy.Comment: Talk given at Fourth International Conference "Physics and
Astrophysics of Quark-Gluon Plasma", November 26-30, 2001; 4 pages including
4 eps-figure
Report of the QCD Working Group
The activities of the QCD working group concentrated on improving the
understanding and Monte Carlo simulation of multi-jet final states due to hard
QCD processes at LEP, i.e. quark-antiquark plus multi-gluon and/or secondary
quark production, with particular emphasis on four-jet final states and b-quark
mass effects. Specific topics covered are: relevant developments in the main
event generators PYTHIA, HERWIG and ARIADNE; the new multi-jet generator
APACIC++; description and tuning of inclusive (all-flavour) jet rates; quark
mass effects in the three- and four-jet rates; mass, higher-order and
hadronization effects in four-jet angular and shape distributions; b-quark
fragmentation and gluon splitting into b-quarks.Comment: 95 pages, 48 figures, contribution to Proceedings of the LEP2 Monte
Carlo Workshop. References for NLO 4-jet matrix elements adde
Finite-Width Effects in Top Quark Production at Hadron Colliders
Production cross sections for t\bar{t} and t\bar{t}j events at hadron
colliders are calculated, including finite width effects and off resonance
contributions for the entire decay chain, t --> bW --> b\ell\nu, for both top
quarks. Resulting background rates to Higgs search at the CERN LHC are updated
for inclusive H --> WW studies and for H --> \tau\tau and H --> WW decays in
weak boson fusion events. Finite width effects are large, increasing
t\bar{t}(j) rates by 20% or more, after typical cuts which are employed for
top-background rejection.Comment: 32 pages, 11 figures, 7 tables; minor changes, reference added, to be
published in Phys. Rev.
Incorporating next-to-leading order matrix elements for hadronic diboson production in showering event generators
A method for incorporating information from next-to-leading order QCD matrix
elements for hadronic diboson production into showering event generators is
presented. In the hard central region (high jet transverse momentum) where
perturbative QCD is reliable, events are sampled according to the first order
tree level matrix element. In the soft and collinear regions next-to-leading
order corrections are approximated by calculating the differential cross
section across the phase space accessible to the parton shower using the first
order (virtual graphs included) matrix element. The parton shower then provides
an all-orders exclusive description of parton emissions. Events generated in
this way provide a physical result across the entire jet transverse momentum
spectrum, have next-to-leading order normalization everywhere, and have
positive definite event weights. The method is generalizable without
modification to any color singlet production process.Comment: 13 pages, 9 figure
Top Pair Production Beyond Double-Pole Approximation: pp, pp~ --> 6 Fermions and 0, 1 or 2 Additional Partons
Hadron collider cross sections for tt~ production and di-lepton,
single-lepton and all-jet decays with up to 2 additional jets are calculated
using complete LO matrix elements with 6-, 7- and 8-particle final states. The
fixed-width, complex-mass and overall-factor schemes (FWS, CMS & OFS) are
employed and the quality of narrow-width and double-pole approximations (NWA &
DPA) is investigated for inclusive production and suppressed backgrounds to new
particle searches. NWA and DPA cross sections differ by 1% or less. The
inclusion of sub- and non-resonant amplitudes effects a cross section increase
of 5-8% at pp supercolliders, but only minor changes at the Tevatron. On-shell
tt~/Wtb backgrounds for the H --> WW decay in weak boson fusion, the hadronic
\tau decay of a heavy H^\pm and the \phi --> hh --> \tau\tau bb~ radion decay
at the LHC are updated, with corrections ranging from 3% to 30%. FWS and CMS
cross sections are uniformly consistent, but OFS cross sections are up to 6%
smaller for some backgrounds.Comment: 20 pages, 6 tables, 1 figur
Determining the Higgs Boson Self Coupling at Hadron Colliders
Inclusive Standard Model Higgs boson pair production at hadron colliders has
the capability to determine the Higgs boson self-coupling, lambda. We present a
detailed analysis of the gg\to HH\to (W^+W^-)(W^+W^-)\to
(jjl^\pm\nu)(jj{l'}^\pm\nu) and gg\to HH\to (W^+W^-)(W^+W^-)\to
(jjl^\pm\nu)({l'}^\pm\nu {l''}^\mp\nu) (l, {l'}, {l''}=e, \mu) signal channels,
and the relevant background processes, for the CERN Large Hadron Collider, and
a future Very Large Hadron Collider operating at a center-of-mass energy of 200
TeV. We also derive quantitative sensitivity limits for lambda. We find that it
should be possible at the LHC with design luminosity to establish that the
Standard Model Higgs boson has a non-zero self-coupling and that lambda /
lambda_{SM} can be restricted to a range of 0-3.8 at 95% confidence level (CL)
if its mass is between 150 and 200 GeV. At a 200 TeV collider with an
integrated luminosity of 300 fb^{-1}, lambda can be determined with an accuracy
of 8 - 25% at 95% CL in the same mass range.Comment: 28 pages, Revtex3, 9 figures, 3 table
Lepton flavor violating signals of a little Higgs model at the high energy linear colliders
Littlest Higgs model predicts the existence of the doubly charged
scalars , which generally have large flavor changing couplings
to leptons. We calculate the contributions of to the lepton
flavor violating processes and , and compare our numerical results with the current
experimental upper limits on these processes. We find that some of these
processes can give severe constraints on the coupling constant and the
mass parameter . Taking into account the constraints on these free
parameters, we further discuss the possible lepton flavor violating signals of
at the high energy linear collider
experiments. Our numerical results show that the possible signals of
might be detected via the subprocesses in the future experiments.Comment: 16 pages, 7 figures. Discussions and references added, typos
correcte
Focus Point SUSY at the LHC Revisited
The estimation of the backgrounds for gluino signals in focus point
supersymmetry is extended by including the backgrounds from the production of
four third generation quarks in the analysis. We find that these backgrounds
are negligible if one uses the strong selection criteria proposed in the
literature (including this analysis) for heavy gluino searches. Softer
selection criteria often recommended for lighter gluino searches yield
backgrounds which are small but numerically significant. We have also repeated
the more conventional background calculations and compared our results with the
other groups. We find that the size of the total residual background estimated
by different groups using different event generators and hard kinematical cuts
agree approximately. In view of the theoretical uncertainties in the leading
order signal and background cross sections mainly due to the choice of the QCD
scale, the gluino mass reach at the LHC cannot be pinpointed. However,
requiring a signal with tagged b-jets (instead of the standard
choice of ) it is shown that gluino masses close to 2 TeV can be
probed at the LHC for a range of reasonable choices of the QCD scale for an
integrated luminosity of 300 fb.Comment: 17 pages, 4 figures, minor typos correctio
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