278 research outputs found
Charge asymmetries of top quarks at hadron colliders revisited
A sizeable difference in the differential production cross section of top-
compared to antitop-quark production, denoted charge asymmetry, has been
observed at the Tevatron. The experimental results seem to exceed the theory
predictions based on the Standard Model by a significant amount and have
triggered a large number of suggestions for "new physics". In the present paper
the Standard Model predictions for Tevatron and LHC experiments are revisited.
This includes a reanalysis of electromagnetic as well as weak corrections,
leading to a shift of the asymmetry by roughly a factor 1.1 when compared to
the results of the first papers on this subject. The impact of cuts on the
transverse momentum of the top-antitop system is studied. Restricting the ttbar
system to a transverse momentum less than 20 GeV leads to an enhancement of the
asymmetries by factors between 1.3 and 1.5, indicating the importance of an
improved understanding of the -momentum distribution. Predictions for
similar measurements at the LHC are presented, demonstrating the sensitivity of
the large rapidity region both to the Standard Model contribution and effects
from "new physics".Comment: 23 pages. Final version to appear in JHE
Electroweak corrections to W-boson pair production at the LHC
Vector-boson pair production ranks among the most important Standard-Model
benchmark processes at the LHC, not only in view of on-going Higgs analyses.
These processes may also help to gain a deeper understanding of the electroweak
interaction in general, and to test the validity of the Standard Model at
highest energies. In this work, the first calculation of the full one-loop
electroweak corrections to on-shell W-boson pair production at hadron colliders
is presented. We discuss the impact of the corrections on the total cross
section as well as on relevant differential distributions. We observe that
corrections due to photon-induced channels can be amazingly large at energies
accessible at the LHC, while radiation of additional massive vector bosons does
not influence the results significantly.Comment: 29 pages, 15 figures, 4 tables; some references and comments on
\gamma\gamma -> WW added; matches version published in JHE
A Tree-Loop Duality Relation at Two Loops and Beyond
The duality relation between one-loop integrals and phase-space integrals,
developed in a previous work, is extended to higher-order loops. The duality
relation is realized by a modification of the customary +i0 prescription of the
Feynman propagators, which compensates for the absence of the multiple-cut
contributions that appear in the Feynman tree theorem. We rederive the duality
theorem at one-loop order in a form that is more suitable for its iterative
extension to higher-loop orders. We explicitly show its application to two- and
three-loop scalar master integrals, and we discuss the structure of the
occurring cuts and the ensuing results in detail.Comment: 20 pages. Few typos corrected, some additional comments included,
Appendix B and one reference added. Final version as published in JHE
SUSY QCD corrections to electroweak gauge boson production with an associated jet at the LHC
We study the stability of the neutral- and charged-current Drell-Yan process
in association with a jet as a standard candle at the LHC under the inclusion
of O(alpha_s) supersymmetric QCD (SQCD) corrections within the MSSM. We include
the decay of the electroweak gauge boson into dileptons, i.e. we consider the
production of charged lepton--anti-lepton or lepton-neutrino final states with
one hard jet. We find that the SQCD corrections are negligible for the
integrated cross section. Only at high lepton transverse momentum can they
induce effects of the percent level.Comment: 21 pages, 9 figures, 2 table
NLO QCD corrections to off-shell top-antitop production with leptonic decays at hadron colliders
We present details of a calculation of the cross section for hadronic
top-antitop production in next-to-leading order (NLO) QCD, including the decays
of the top and antitop into bottom quarks and leptons. This calculation is
based on matrix elements for \nu e e+ \mu- \bar{\nu}_{\mu}b\bar{b} production
and includes all non-resonant diagrams, interferences, and off-shell effects of
the top quarks. Such contributions are formally suppressed by the top-quark
width and turn out to be small in the inclusive cross section. However, they
can be strongly enhanced in exclusive observables that play an important role
in Higgs and new-physics searches. Also non-resonant and off-shell effects due
to the finite W-boson width are investigated in detail, but their impact is
much smaller than naively expected. We also introduce a matching approach to
improve NLO calculations involving intermediate unstable particles. Using a
fixed QCD scale leads to perturbative instabilities in the high-energy tails of
distributions, but an appropriate dynamical scale stabilises NLO predictions.
Numerical results for the total cross section, several distributions, and
asymmetries are presented for Tevatron and the LHC at 7 TeV, 8 TeV, and 14 TeV.Comment: 61 pp. Matches version published in JHEP; one more reference adde
RG-improved single-particle inclusive cross sections and forward-backward asymmetry in production at hadron colliders
We use techniques from soft-collinear effective theory (SCET) to derive
renormalization-group improved predictions for single-particle inclusive (1PI)
observables in top-quark pair production at hadron colliders. In particular, we
study the top-quark transverse-momentum and rapidity distributions, the
forward-backward asymmetry at the Tevatron, and the total cross section at
NLO+NNLL order in resummed perturbation theory and at approximate NNLO in fixed
order. We also perform a detailed analysis of power corrections to the leading
terms in the threshold expansion of the partonic hard-scattering kernels. We
conclude that, although the threshold expansion in 1PI kinematics is
susceptible to numerically significant power corrections, its predictions for
the total cross section are in good agreement with those obtained by
integrating the top-pair invariant-mass distribution in pair invariant-mass
kinematics, as long as a certain set of subleading terms appearing naturally
within the SCET formalism is included.Comment: 55 pages, 14 figures, 6 table
Modelling the nucleon wave function from soft and hard processes
Current light-cone wave functions for the nucleon are unsatisfactory since
they are in conflict with the data of the nucleon's Dirac form factor at large
momentum transfer. Therefore, we attempt a determination of a new wave function
respecting theoretical ideas on its parameterization and satisfying the
following constraints: It should provide a soft Feynman contribution to the
proton's form factor in agreement with data; it should be consistent with
current parameterizations of the valence quark distribution functions and
lastly it should provide an acceptable value for the \jp \to N \bar N decay
width. The latter process is calculated within the modified perturbative
approach to hard exclusive reactions. A simultaneous fit to the three sets of
data leads to a wave function whose -dependent part, the distribution
amplitude, shows the same type of asymmetry as those distribution amplitudes
constrained by QCD sum rules. The asymmetry is however much more moderate as in
those amplitudes. Our distribution amplitude resembles the asymptotic one in
shape but the position of the maximum is somewhat shifted.Comment: 32 pages RevTex + PS-file with 5 figures in uu-encoded, compressed
fil
Photon Radiation with MadDipole
We present the automation of a subtraction method for photon radiation using
the dipole formalism within the MadGraph framework. The subtraction terms are
implemented both in dimensional regularization and mass regularization for
massless and massive cases and non-collinear-safe observables are accounted
for.Comment: 23 pages, 2 figures, minor additions, references added, version
published in JHE
Probing Colored Particles with Photons, Leptons, and Jets
If pairs of new colored particles are produced at the Large Hadron Collider,
determining their quantum numbers, and even discovering them, can be
non-trivial. We suggest that valuable information can be obtained by measuring
the resonant signals of their near-threshold QCD bound states. If the particles
are charged, the resulting signatures include photons and leptons and are
sufficiently rich for unambiguously determining their various quantum numbers,
including the charge, color representation and spin, and obtaining a precise
mass measurement. These signals provide well-motivated benchmark models for
resonance searches in the dijet, photon+jet, diphoton and dilepton channels.
While these measurements require that the lifetime of the new particles be not
too short, the resulting limits, unlike those from direct searches for pair
production above threshold, do not depend on the particles' decay modes. These
limits may be competitive with more direct searches if the particles decay in
an obscure way.Comment: 39 pages, 9 figures; v2: more recent searches include
Physics Opportunities of e+e- Linear Colliders
We describe the anticipated experimental program of an e+e- linear collider
in the energy range 500 GeV -- 1.5 TeV. We begin with a description of current
collider designs and the expected experimental environment. We then discuss
precision studies of the W boson and top quark. Finally, we review the range of
models proposed to explain the physics of electroweak symmetry breaking and
show, for each case, the central role that the linear collider experiments will
play in elucidating this physics. (to appear in Annual Reviews of Nuclear and
Particle Science)Comment: 93 pages, latex + 23 figures; typos corrections + 1 reference adde
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