541 research outputs found
Present Limits on the Precision of SM Predictions for Jet Energies
We investigate the impact of theoretical uncertainties on the accuracy of
measurements involving hadronic jets. The analysis is performed using events
with a Z boson and a single jet observed in collisions at
= 1.96 TeV in 4.6 of data from the Collider Detector at
Fermilab (CDF). The transverse momenta (\pt) of the jet and the boson should
balance each other due to momentum conservation in the plane transverse to the
direction of the and beams. We evaluate the dependence of the
measured \pt-balance on theoretical uncertainties associated with initial and
final state radiation, choice of renormalization and factorization scales,
parton distribution functions, jet-parton matching, calculations of matrix
elements, and parton showering. We find that the uncertainty caused by parton
showering at large angles is the largest amongst the listed uncertainties. The
proposed method can be re-applied at the LHC experiments to investigate and
evaluate the uncertainties on the predicted jet energies. The distributions
produced at the CDF environment are intended for comparison to those from
modern event generators and new tunes of parton showering.Comment: Submitted to Nucl. Instr. and Meth.
Electroweak and finite width corrections to top quark decays into transverse and longitudinal -bosons
We calculate the electroweak and finite width corrections to the decay of an
unpolarized top quark into a bottom quark and a -gauge boson where the
helicities of the are specified as longitudinal, transverse-plus and
transverse-minus. Together with the corrections these
corrections may become relevant for the determination of the mass of the top
quark through angular decay measurements.Comment: 4 pages, 7 postscript figures adde
NRQCD Analysis of Bottomonium Production at the Tevatron
Recent data from the CDF collaboration on the production of spin-triplet
bottomonium states at the Tevatron p \bar p collider are analyzed within the
NRQCD factorization formalism. The color-singlet matrix elements are determined
from electromagnetic decays and from potential models. The color-octet matrix
elements are determined by fitting the CDF data on the cross sections for
Upsilon(1S), Upsilon(2S), and Upsilon(3S) at large p_T and the fractions of
Upsilon(1S) coming from chi_b(1P) and chi_b(2P). We use the resulting matrix
elements to predict the cross sections at the Tevatron for the spin-singlet
states eta_b(nS) and h_b(nP). We argue that eta_b(1S) should be observable in
Run II through the decay eta_b -> J/psi + J/psi.Comment: 20 pages, 3 figure
On the mechanisms of heavy-quarkonium hadroproduction
We discuss the various mechanisms potentially at work in hadroproduction of
heavy quarkonia in the light of computations of higher-order QCD corrections
both in the Colour-Singlet (CS) and Colour-Octet (CO) channels and the
inclusion of the contribution arising from the s-channel cut in the CS channel.
We also discuss new observables meant to better discriminate between these
different mechanisms.Comment: Invited review talk at 3rd International Conference On Hard And
Electromagnetic Probes Of High-Energy Nuclear Collisions (HP2008), 8-14 June
2008, Illa da Toxa, Galicia, Spain. 11 pages, 21 figures, LaTeX, uses
svjour.cls and svepj.clo (included
Hard diffractive quarkonium hadroproduction at high energies
We present a study of heavy quarkonium production in hard diffractive process
by the Pomeron exchange for Tevatron and LHC energies. The numerical results
are computed using recent experimental determination of the diffractive parton
density functions in Pomeron and are corrected by unitarity corrections through
gap survival probability factor. We give predictions for single as well as
central diffractive ratios. These processes are sensitive to the gluon content
of the Pomeron at small Bjorken-x and may be particularly useful in studying
the small-x physics. They may also be a good place to test the different
available mechanisms for quarkonium production at hadron colliders.Comment: 7 pages, 3 figures, 1 table. Final version to be published in
European Physical Journal
Associated production of light gravitinos in e^+e^- and e^-\gamma collisions
Light gravitino productions in association with a neutralino (selectron) in
e^+e^- (e^-\gamma) collisions are restudied in a scenario that the lightest
supersymmetric particle is a gravitino and the produced neutralino (selectron)
promptly decays into a photon (electron) and a gravitino. We explicitly give
the helicity amplitudes for the production processes by using the effective
goldstino interaction Lagrangian, and present the cross sections with different
collision energies and mass spectra. We also examine selection efficiencies by
kinematical cuts and beam polarizations for the signal and background
processes, and show that the energy and angular distributions of the photon
(electron) can explore the mass of the t-channel exchange particle as well as
the mass of the decaying particle at a future e^+e^- (e^-\gamma) collider.Comment: 12 pages, 12 figures, 4 tables; references added, version to appear
in EPJ
HELAS and MadGraph with goldstinos
Fortran subroutines to calculate helicity amplitudes with goldstinos, which
appear as the longitudinal modes of massive gravitinos in high energy
processes, are added to the HELAS (HELicity Amplitude Subroutines) library.
They are coded in such a way that arbitrary amplitudes with external goldstinos
can be generated automatically by MadGraph, after slight modifications. All the
codes have been tested carefully by making use of the goldstino equivalence
theorem and the gauge invariance of the helicity amplitudes. Hadronic total
cross sections for associated gravitino productions with a gluino and a squark
are also presented.Comment: 9 pages, 6 figures; a footnote and a reference added, version to
appear in EPJ
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
Bulk matter physics and its future at the Large Hadron Collider
Measurements at low transverse momentum will be performed at the LHC for
studying particle production mechanisms in and heavy-ion collisions. Some
of the experimental capabilities for bulk matter physics are presented,
focusing on tracking elements and particle identification. In order to
anticipate the study of baryon production for both colliding systems at
multi-TeV energies, measurements for identified species and recent model
extrapolations are discussed. Several mechanisms are expected to compete for
hadro-production in the low momentum region. For this reason, experimental
observables that could be used for investigating multi-parton interactions and
help understanding the "underlying event" content in the first collisions
at the LHC are also mentioned.Comment: 6 pages, 7 figures. To appear in the proceedings of Hot Quarks 2008,
Estes Park, Colorado, 18-23 August 200
Colour reconnections in Herwig++
We describe the implementation details of the colour reconnection model in
the event generator Herwig++. We study the impact on final-state observables in
detail and confirm the model idea from colour preconfinement on the basis of
studies within the cluster hadronization model. Moreover, we show that the
description of minimum bias and underlying event data at the LHC is improved
with this model and present results of a tune to available data.Comment: 19 pages, 21 figures, 2 tables. Matches with published versio
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