540 research outputs found

### Top quark and leptonic charge asymmetries for the Tevatron and LHC

We compute, for $t \bar t$ production at the LHC and at the Tevatron, several
charge asymmetries to NLO QCD, including also the electromagnetic and
weak-interaction corrections. We calculate these asymmetries both inclusively
and with additional kinematic cuts and compare our results, where possible,
with recent experimental results and with SM predictions. The $t \bar t$
asymmetries induce also corresponding asymmetries for the charged leptons from
semileptonic top-quark decay. Although these asymmetries are, in the SM,
smaller than the corresponding ones for top quarks, they are expected to be
measurable quite precisely. In fact, measurement of a lepton asymmetry in
$\ell$ + jets events was reported by the D$\emptyset$ and CDF experiments. We
analyze and compute to NLO in the gauge couplings leptonic charge asymmetries
for dileptonic and semileptonic \ttbar events, with and without acceptance
cuts, at the Tevatron and the LHC.Comment: Some references and a note added on comparisons with recent
experimental results. Published in Phys. Rev. D 86, 034026 (2012

### Distributions and correlations for top quark pair production and decay at the Tevatron and LHC.

We investigate a number of observables that are and will be instrumental in
the exploration of \tbart production and decay at the Tevatron and the LHC.
For this analysis we made a computer program that incorporates besides the NLO
QCD corrections to \tbart production and decay also mixed weak-QCD
corrections to the production amplitudes, and that allows for studies of
correlated versus uncorrelated \tbart events. In this paper we analyze and
compute observables mostly for dileptonic \tbart final states to
next-to-leading order in the strong and weak gauge couplings (NLOW), with
selection cuts. We calculate charge asymmetries of the top quark and of
$\ell=e,\mu$ and we compare, where possible, with experimental results. We show
that top-quark spin correlations affect a leptonic pair asymmetry (which has
not been measured so far) by $\sim 7%$. We determine several dileptonic angular
correlations, which reflect \tbart spin correlations, namely the beam,
off-diagonal and helicity correlation, and the opening angle distribution
(defined in the $t$, $\bar t$ rest frames) when selection cuts are applied. Our
NLOW predictions for the beam, off-diagonal, and helicity correlation for the
Tevatron agree with recent measurements by the CDF and D0 experiments. In
addition we make predictions for estimators of these correlations as functions
of \mtt. These estimators may prove useful for the Tevatron and also in the
early rounds of LHC data-analyses, where the event numbers will probably not be
abundant. Furthermore, we compute to NLOW in the gauge couplings two dilepton
angular correlations that are defined in the laboratory frame, for correlated
and uncorrelated \tbart events at the LHC. Finally, based on our SM results,
we identify several observables that allow to search for non-standard parity-
and CP-violating interactions, especially with future LHC data.Comment: Latex, 39 pages, 39 figures, 4 table

### A set of top quark spin correlation and polarization observables for the LHC: Standard Model predictions and new physics contributions

We consider top-antitop quark ($t{\bar t}$) production at the Large Hadron
Collider (LHC) with subsequent decay into dileptonic and lepton plus jets final
states. We present a set of leptonic angular correlations and distributions
with which experiments can probe all the independent coefficient functions of
the top-spin dependent parts of the $t{\bar t}$ production spin density
matrices. We compute these angular correlations and distributions for LHC
center-of-mass energies 8, 13, and 14 TeV within the Standard Model at
next-to-leading order in the QCD coupling including the mixed QCD-weak
corrections and for the transverse top-quark polarization and the $t{\bar t}$
charge asymmetry also the mixed QCD-QED corrections. In addition we analyze and
compute the effects of new interactions on these observables in terms of a
gauge-invariant effective Lagrangian which contains all operators relevant for
hadronic $t{\bar t}$ production up to mass dimension six.Comment: Latex, 35 pages, sevaral comments and 2 tables added, version to be
published in JHE

### Weak interaction corrections to hadronic top quark pair production: contributions from quark-gluon and $b \bar b$ induced reactions

As an addendum to our previous evaluation of the weak-interaction corrections
to hadronic top-quark pair production we determine the leading weak-interaction
contributions due to the subprocesses $b {\bar b} \to t {\bar t}$ and $g q
({\bar q}) \to t {\bar t} q ({\bar q})$. For several distributions in $t {\bar
t}$ production at the LHC we find that these contributions are non-negligible
as compared to the weak corrections from the other partonic subprocesses.Comment: Latex, 9 pages, 4 figure

### Differential decay rates of CP-even and CP-odd Higgs bosons to top and bottom quarks at NNLO QCD

We consider the decay of a neutral Higgs boson of arbitrary CP nature to a
massive quark antiquark pair at next-to-next-to-leading order in perturbative
QCD. Our analysis is made at the differential level using the antenna
subtraction framework. We apply our general set-up to the decays of a CP-even
and CP-odd heavy Higgs boson to a top-quark top-antiquark pair and to the decay
of the 125 GeV Higgs boson to a massive bottom-quark bottom-antiquark pair. In
the latter case we calculate, in particular, the two-jet, three-jet, and
four-jet decay rates and, for two-jet events, the energy distribution of the
leading jet.Comment: Latex, 23 pages, a reference adde

### Cancellation of divergences in unitary gauge calculation of $H \to \gamma \gamma$ process via one W loop, and application

Following the thread of R. Gastmans, S. L. Wu and T. T. Wu, the calculation
in the unitary gauge for the $H \to \gamma \gamma$ process via one W loop is
repeated, without the specific choice of the independent integrated loop
momentum at the beginning. We start from the 'original' definition of each
Feynman diagram, and show that the 4-momentum conservation and the Ward
identity of the W-W-photon vertex can guarantee the cancellation of all terms
among the Feynman diagrams which are to be integrated to give divergences
higher than logarithmic. The remaining terms are to the most logarithmically
divergent, hence is independent from the set of integrated loop momentum. This
way of doing calculation is applied to $H \to \gamma Z$ process via one W loop
in the unitary gauge, the divergences proportional to $M_Z^2/M^3$ including
quadratic ones are all cancelled, and terms proportional to $M_Z^2/M^3$ are
shown to be zero. The way of dealing with the quadratic divergences
proportional to $M_Z^2/M^3$ in $H \to \gamma Z$ has subtle implication on the
employment on the Feynman rules especially when those rules can lead to high
level divergences. So calculation without integration on all the $\delta$
functions until have to is a more proper or maybe necessary way of the
employment of the Feynman rules.Comment: 1 figure, 34 pages (updated

### Central-edge asymmetry as a probe of Higgs-top coupling in $t\bar{t}h$ production at LHC

The Higgs-top coupling plays a central role in the hierarchy problem and the
vacuum stability of the Standard Model (SM). We propose a central-edge
asymmetry ($A_{CE}$) to probe the CP violating Higgs-top coupling in dileptonic
channel of $t\bar{t}h(\to b\bar{b})$ production at the LHC. We demonstrate that
the CP-violating Higgs-top coupling can affect the central-edge asymmetry
through distorting $\Delta y_{\ell^+\ell^-}$ distribution because of the
contribution of new top charge asymmetric term. Since $\Delta y_{\ell^+\ell^-}$
distribution is frame-independent and has a good discrimination even in boosted
regime, we use the jet substructure technique to enhance the observability of
the dileptonic channel of $t\bar{t}h$ production. We find that (1) the
significance of dileptonic channel of $t\bar{t}h$ production can reach
$5\sigma$ for CP phase $\xi=0,\pi/4,\pi/2$ when the luminosity ${\cal
L}=795,993,1276$ fb$^{-1}$ at 14 TeV LHC. (2) the central-edge asymmetry
$A_{CE}$ show a good discrimination power of CP phase of $t\bar{t}h$
interaction, which are -40.26\%, -26.60\%, -9.47\% for $\xi=0$, $\pi/4$, $\pi/2$ respectively and are hardly affected by the event selections. Besides,
by performing the binned-$\chi^2$ analysis of $\Delta y_{\ell^+\ell^-}$
distribution, we find that the scalar and pseudo-scalar interactions can be
distinguished at 95\% C.L. level at 14 TeV HL-LHC.Comment: minor changes, version accepted by PL

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