525 research outputs found

### One-loop Electroweak and QCD corrections to vector boson scattering into top pairs and application to ILC

We calculate the electroweak and QCD corrections to W-W+ -> tt and ZZ -> tt.
We also consider the interplay of these corrections with the effect of
anomalous interactions that affect the massive weak bosons and the top. The
results at the VV level fusion are convoluted with the help of the effective
vector boson approximation to give predictions for a high energy e+e- collider.Comment: 19 pages, 27 figure

### Threshold Behaviour in Gauge Boson Pair Production at LEP 2

We discuss the form of the amplitude for gauge boson pair production at or
near threshold.We show that in the case of W-pair production at LEP2 near
threshold only one anomalous electromagnetic coupling can contribute. This
anomalous coupling is CP violating and contributes to the electric dipole
moment of the $W$. Since this coupling is likely to be small, it is important
to look for ZZgamma couplings in Zgamma production. These couplings are not
suppressed at the W-threshold

### One-loop corrections, uncertainties and approximations in neutralino annihilations: Examples

The extracted value of the relic density has reached the few per-cent level
precision. One can therefore no longer content oneself with calculations of
this observable where the annihilation processes are computed at tree-level,
especially in supersymmetry where radiative corrections are usually large.
Implementing full one-loop corrections to all annihilation processes that would
be needed in a scan over parameters is a daunting task. On the other hand one
may ask whether the bulk of the corrections are taken into account through
effective couplings of the neutralino that improve the tree-level calculation
and would be easy to implement. We address this issue by concentrating in this
first study on the neutralino coupling to i) fermions and sfermions and ii) Z.
After constructing the effective couplings we compare their efficiency compared
to the full one-loop calculation and comment on the failures and success of the
approach. As a bonus we point out that large non decoupling effects of heavy
sfermions could in principle be measured in the annihilation process, a point
of interest in view of the latest limit on the squark masses from the LHC. We
also comment on the scheme dependencies of the one-loop corrected results

### Double Higgs Production at the Linear Colliders and the Probing of the Higgs Self-Coupling

We study double Higgs production in the $e^+ e^-$ and $\gamma \gamma$ modes
of the linear collider. It is also shown how one can probe the scalar potential
in these reactions. We discuss the effective longitudinal $W$ approximation in
$\gamma \gamma$ processes and the $W_L W_L$ luminosities in the two modes of a
high-energy linear collider. A generalised non-linear gauge-fixing condition,
which is particularly useful for tree-level calculations of electroweak
processes for the laser induced collider, is presented. Its connection with the
background-field approach to gauge fixing is given.Comment: 60 pages, LateX, uses epsf, needs FEYNMAN.tex, 26 figs. (eqs. 4.1 and
4.10 changed), full paper (including figs) is also available via www at
http://lapphp0.in2p3.fr/preplapp/psth/doublehiggs.ps.gz or via anonymous ftp
at ftp://lapphp0.in2p3.fr/pub/preprints-theorie/doublehiggsf.u

### Automatised full one-loop renormalisation of the MSSM I: The Higgs sector, the issue of tan(beta) and gauge invariance

We give an extensive description of the renormalisation of the Higgs sector
of the minimal supersymmetric model in SloopS. SloopS is an automatised code
for the computation of one-loop processes in the MSSM. In this paper, the first
in a series, we study in detail the non gauge invariance of some definitions of
tan(beta). We rely on a general non-linear gauge fixing constraint to make the
gauge parameter dependence of different schemes for tan(beta) at one-loop
explicit. In so doing, we update, within these general gauges, an important
Ward-Slavnov-Taylor identity on the mixing between the pseudo-scalar Higgs,
A^0, and the Z^0. We then compare the tan(beta) scheme dependence of a few
observables. We find that the best tan(beta) scheme is the one based on the
decay A^0 -> tau^+ tau^- because of its gauge invariance, being unambiguously
defined from a physical observable, and because it is numerically stable. The
oft used DRbar scheme performs almost as well on the last count, but is usually
defined from non-gauge invariant quantities in the Higgs sector. The use of the
heavier scalar Higgs mass in lieu of tan(beta) though related to a physical
parameter induces too large radiative corrections in many instances and is
therefore not recommended.Comment: 34 pages, 1 figure, typos corrected, accepted for publication in
Phys. Rev.

### Self-annihilation of the neutralino dark matter into two photons or a Z and a photon in the MSSM

We revisit the one-loop calculation of the annihilation of a pair of the
lightest neutralinos into a pair of photons, a pair of gluons and also a Z
photon final state. For the latter we have identified a new contribution that
may not always be negligible. For all three processes we have conducted a tuned
comparison with previous calculations for some characteristic scenarios. The
approach to the very heavy higgsino and wino is studied and we argue how the
full one-loop calculation should be matched into a more complete treatment that
was presented recently for these extreme regimes. We also give a short
description of the code that we exploited for the automatic calculation of
one-loop cross sections in the MSSM that could apply both for observables at
the colliders and for astrophysics or relic density calculations. In particular
the automatic treatment of zero Gram determinants which appear in the latter
applications is outlined. We also point out how generalised non-linear gauge
fixing constraints can be exploited.Comment: 20 pages, 5 figure

### Invisible Decays of the Supersymmetric Higgs and Dark Matter

We discuss effects of the light sparticles on decays of the lightest Higgs in
a supersymmetric model with nonuniversal gaugino masses at the high scale,
focusing on the `invisible' decays into neutralinos. These can impact
significanlty the discovery possibilities of the lightest Higgs at the LHC. We
show that due to these decays, there exist regions of the $M_2-\mu$ space where
the B.R. $(h \to \gamma \gamma)$ becomes dangerously low even after imposing
the LEP constraints on the sparticle masses, implying a possible preclusion of
its discovery in the $\gamma \gamma$ channel. We find that there exist regions
in the parameter space with acceptable relic density and where the ratio ${B.R.
(h \to \gamma \gamma)_{SUSY} \over B.R. (h \to \gamma \gamma)_{SM}}$ falls
below 0.6, implying loss of signal in the $\gamma \gamma$ channel. These
regions correspond to $\tilde \chi_1^+, \tilde \chi_2^0$ masses which should be
accessible already at the Tevatron. Further we find that considerations of
relic density put lower limit on the U(1) gaugino mass parameter $M_1$
independently of $\mu, \tan \beta$ and $m_0$.Comment: 17 pages, 11 figures, Talk presented at Appi2002, Accelerator and
Particle Physics Institute, Appi, Iwate, Japan, February 13--16 200

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