302 research outputs found
Probing Electroweak Top Quark Couplings at Hadron Colliders
We consider QCD t\bar{t}\gamma and t\bar{t}Z production at hadron colliders
as a tool to measure the tt\gamma and ttZ couplings. At the Tevatron it may be
possible to perform a first, albeit not very precise, test of the tt\gamma
vector and axial vector couplings in t\bar{t}\gamma production, provided that
more than 5 fb^{-1} of integrated luminosity are accumulated. The t\bar{t}Z
cross section at the Tevatron is too small to be observable. At the CERN Large
Hadron Collider (LHC) it will be possible to probe the tt\gamma couplings at
the few percent level, which approaches the precision which one hopes to
achieve with a next-generation e^+e^- linear collider. The LHC's capability of
associated QCD t\bar{t}V (V=\gamma, Z) production has the added advantage that
the tt\gamma and ttZ couplings are not entangled. For an integrated luminosity
of 300 fb^{-1}, the ttZ vector (axial vector) coupling can be determined with
an uncertainty of 45-85% (15-20%), whereas the dimension-five dipole form
factors can be measured with a precision of 50-55%. The achievable limits
improve typically by a factor of 2-3 for the luminosity-upgraded (3 ab^{-1})
LHC.Comment: Revtex3, 30 pages, 9 Figures, 6 Table
SUSY QCD one-loop effects in (un)polarized top-pair production at hadron colliders
We study the effects of O(alpha_s) supersymmetric QCD (SQCD) corrections on
the total production rate and kinematic distributions of polarized and
unpolarized top-pair production in pp and p anti-p collisions. At the Fermilab
Tevatron p anti-p collider, top-quark pairs are mainly produced via
quark-antiquark annihilation, q anti-q -> t anti-t, while at the CERN LHC pp
collider gluon-gluon scattering, g g -> t anti-t, dominates. We compute the
complete set of O(alpha_s) SQCD corrections to both production channels and
study their dependence on the parameters of the Minimal Supersymmetric Standard
Model. In particular, we discuss the prospects for observing strong,
loop-induced SUSY effects in top-pair production at the Tevatron Run II and the
LHC.Comment: 56 pages, 29 figures, RevTeX
Soft-gluon resummation for squark and gluino hadroproduction
We consider the resummation of soft gluon emission for squark and gluino
hadroproduction at next-to-leading-logarithmic (NLL) accuracy in the framework
of the minimal supersymmetric standard model. We present analytical results for
squark-squark and squark-gluino production and provide numerical predictions
for all squark and gluino pair-production processes at the Tevatron and at the
LHC. The size of the soft-gluon corrections and the reduction in the scale
uncertainty are most significant for processes involving gluino production. At
the LHC, where the sensitivity to squark and gluino masses ranges up to 3 TeV,
the corrections due to NLL resummation over and above the NLO predictions can
be as high as 35% in the case of gluino-pair production, whereas at the
Tevatron, the NLL corrections are close to 40% for squark-gluino final states
with sparticle masses around 500 GeV.Comment: 31 pages, 7 figure
Electroweak higher-order effects and theoretical uncertainties in deep-inelastic neutrino scattering
A previous calculation of electroweak O(alpha) corrections to deep-inelastic
neutrino scattering, as e.g. measured by NuTeV and NOMAD, is supplemented by
higher-order effects. In detail, we take into account universal two-loop
effects from \Delta\alpha and \Delta\rho as well as higher-order final-state
photon radiation off muons in the structure function approach. Moreover, we
make use of the recently released O(alpha)-improved parton distributions
MRST2004QED and identify the relevant QED factorization scheme, which is DIS
like. As a technical byproduct, we describe slicing and subtraction techniques
for an efficient calculation of a new type of real corrections that are induced
by the generated photon distribution. A numerical discussion of the
higher-order effects suggests that the remaining theoretical uncertainty from
unknown electroweak corrections is dominated by non-universal two-loop effects
and is of the order 0.0003 when translated into a shift in
sin^2\theta_W=1-MW^2/MZ^2. The O(alpha) corrections implicitly included in the
parton distributions lead to a shift of about 0.0004.Comment: 25 pages, latex, 8 postscript figure
Complete fermionic two-loop results for the interdependence
The complete fermionic two-loop contributions to the prediction for the W-boson mass from muon decay in the electroweak Standard Model are evaluated exactly, i.e. no expansion in the top-quark and the Higgs-boson mass is made. The result for the W-boson mass is compared with the previous result of an expansion up to next-to-leading order in the top-quark mass. The predictions are found to agree with each other within about 4 MeV. A simple parameterization of the new result is presented, approximating the full result better than 0.4 MeV for M_H < 1 TeV
Calculation of fermionic two-loop contributions to muon decay
The computation of the correction \Delta r in the W-Z mass correlation, derived from muon decay, is described at the two-loop level in the Standard Model. Technical aspects which become relevant at this level are studied, e.g. gauge-parameter independent mass renormalization, ghost-sector renormalization and the treatment of \gamma_5. Exact results for \Delta r and the W mass prediction including O(\alpha^2) corrections with fermion loops are presented and compared with previous results of a next-to-leading order expansion in the top-quark mass
Quantum effects on Higgs-strahlung events at Linear Colliders within the general 2HDM
The associated production of neutral Higgs bosons with the Z gauge boson is
investigated in the context of the future linear colliders, such as the ILC and
CLIC, within the general two-Higgs-doublet model (2HDM). We compute the
corresponding production cross-sections at one-loop, in full consistency with
the available theoretical and phenomenological constraints. We find that the
wave-function renormalization corrections to the external Higgs fields are the
dominant source of the quantum effects, which turn out to be large and
negative, and located predominantly in the region around \tan\beta=1 and
moderate values of the parameter \lambda_5 (being \lambda_5 < 0). This behavior
can be ultimately traced back to the enhancement potential of the triple Higgs
boson self-couplings, a trademark feature of the 2HDM with no counterpart in
the Higgs sector of the Minimal Supersymmetric Standard Model. The predicted
Higgs-strahlung rates comfortably reach a few tens of femtobarn, which means
barely 10^3 - 10^4 events per 500 inverse femtobarn of integrated luminosity.
Due to their great complementarity, we argue that the combined analysis of the
Higgs-strahlung events and the previously computed one-loop Higgs-pair
production processes could be instrumental to probe the structure of the Higgs
sector at future linac facilities.Comment: LaTeX, 16 pages, 9 Figures, 2 Tables. Extended discussion, references
added, matches published version in Phys. Rev.
Testing Supersymmetry with Lepton Flavor Violating tau and mu decays
In this work the following lepton flavor violating and decays
are studied: , , , , and . We work in a supersymmetric scenario consisting of the minimal
supersymmetric standard model particle content, extended by the addition of
three heavy right handed Majorana neutrinos and their supersymmetric partners,
and where the generation of neutrino masses is done via the seesaw mechanism.
Within this context, a significant lepton flavor mixing is generated in the
slepton sector due to the Yukawa neutrino couplings, which is transmited from
the high to the low energies via the renormalization group equations. This
slepton mixing then generates via loops of supersymmetric particles significant
contributions to the rates of and the correlated decays. We analize here in full detail these rates in terms of the
relevant input parameters, which are the usual minimal supergravity parameters
and the seesaw parameters. For the decays, a full one-loop
analytical computation of all the contributing supersymmetric loops is
presented. This completes and corrects previous computations in the literature.
In the numerical analysis compatibility with the most recent experimental upper
bounds on all these and decays, with the neutrino data, and with
the present lower bounds on the supersymmetric particle masses are required.
Two typical scenarios with degenerate and hierarchical heavy neutrinos are
considered. We will show here that the minimal supergravity and seesaw
parameters do get important restrictions from these and decays in
the hierarchical neutrino case.Comment: Version to appear in Physical Review
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