3,441 research outputs found
Top Quarks at Photon Colliders
A review of results on top quark physics expected at the Photon Linear
Colliders is presented.Comment: 12 pages, 10 figures, talk given at the International Workshop on
High Energy Photon Colliders, 14-17 June 2000, Hamburg, German
General Analysis of Single Top Production and W Helicity in Top Decay
We provide a framework for the analysis of the W boson helicity in the decay
of the top quark that is based on a general effective tbW coupling. Four
independent coupling coefficients can be uniquely determined by the fractions
of longitudinal and transverse W boson polarizations as well as the single top
production rates for the t-channel and the s-channel processes. The knowledge
of these coefficients can be used to discriminate models of electroweak
symmetry breaking.Comment: Typo on Eqs. 5 and 6 correcte
Integrable models and quantum spin ladders: comparison between theory and experiment for the strong coupling ladder compounds
(abbreviated) This article considers recent advances in the investigation of
the thermal and magnetic properties of integrable spin ladder models and their
applicability to the physics of real compounds. The ground state properties of
the integrable two-leg spin-1/2 and the mixed spin-(1/2,1) ladder models at
zero temperature are analyzed by means of the Thermodynamic Bethe Ansatz.
Solving the TBA equations yields exact results for the critical fields and
critical behaviour. The thermal and magnetic properties of the models are
investigated in terms of the recently introduced High Temperature Expansion
method, which is discussed in detail. It is shown that in the strong coupling
limit the integrable spin-1/2 ladder model exhibits three quantum phases: (i) a
gapped phase in the regime , (ii) a fully polarised phase for
, and (iii) a Luttinger liquid magnetic phase in the regime
. The critical behaviour in the vicinity of the critical
points is of the Pokrovsky-Talapov type. The temperature-dependent thermal and
magnetic properties are directly evaluated from the exact free energy
expression and compared to known experimental results for a range of strong
coupling ladder compounds. Similar analysis of the mixed spin-(1/2,1) ladder
model reveals a rich phase diagram, with a 1/3 and a full saturation
magnetisation plateau within the strong antiferromagnetic rung coupling regime.
For weak rung coupling, the fractional magnetisation plateau is diminished and
a new quantum phase transition occurs. The phase diagram can be directly
deduced from the magnetisation curve obtained from the exact result derived
from the HTE. The thermodynamics of the spin-orbital model with different
single-ion anisotropies is also investigated.Comment: 90 pages, 33 figures, extensive revisio
Radiative Corrections to W^+W^- \to W^+W^- in the Electroweak Standard Model
The cross-section for W^+W^- \to W^+W^- with arbitrarily polarized W bosons
is calculated within the Electroweak Standard Model including the complete
virtual and soft-photonic O(alpha) corrections. We show the numerical
importance of the radiative corrections for the dominating polarized
cross-sections and for the unpolarized cross-section. The numerical accuracy of
the equivalence theorem is investigated in O(alpha) by comparing the
cross-section for purely longitudinal W bosons obtained from the equivalence
theorem and from the direct calculation. We point out that the instability of
the W boson, which is inherent in the one-loop corrections, prevents a
consistent calculation of radiative corrections to the scattering of
on-real-mass-shell longitudinal W bosons beyond O(alpha).Comment: 24 pages, LaTeX, uses axodraw, epsfig. Statement clarifie
Hadron Collider Signatures for New Interactions of Top and Bottom Quarks
One of the main goals for hadron colliders is the study of the properties of
the third generation quarks. We study the signatures for new TeV resonances
that couple to top or bottom quarks both at the Tevatron Run II and at the LHC.
We find that in the simplest production processes of Drell-Yan type at the
Tevatron, the signals are overwhelmed by QCD backgrounds. We also find that it
is possible to study these resonances when they are produced in association
with a pair of heavy quarks or in association with a single top at the LHC.In
particular, with an integrated luminosity of 300 fb at the LHC, it is
possible to probe resonance masses up to around 2 TeV.Comment: 24 pages, 15 figures, Minor corrections, version to appear in Phys.
Rev.
The Higgs sector of the phenomenological MSSM in the light of the Higgs boson discovery
The long awaited discovery of a new light scalar at the LHC opens up a new
era of studies of the Higgs sector in the SM and its extensions. In this paper
we discuss the consequences of the observation of a light Higgs boson with the
mass and rates reported by the ATLAS and CMS collaborations on the parameter
space of the phenomenological MSSM, including also the so far unsuccessful LHC
searches for the heavier Higgs bosons and supersymmetric particle partners in
missing transverse momentum as well as the constraints from B physics and dark
matter. We explore the various regimes of the MSSM Higgs sector depending on
the parameters MA and tan beta and show that only two of them are still allowed
by all present experimental constraints: the decoupling regime where there is
only one light and standard--like Higgs boson and the supersymmetric regime in
which there are light supersymmetric particle partners affecting the decay
properties of the Higgs boson, in particular its di-photon and invisible
decays.Comment: 21 pages, 9 figures v2 - Discussion of the impact of LHC data
extended, scan statistics increased, a few figures added and typos correcte
in NonCommutative Standard Model
We study the top quark decay to b quark and W boson in the NonCommutative
Standard Model (NCSM). The lowest contribution to the decay comes from the
terms quadratic in the matrix describing the noncommutative (NC) effects while
the linear term is seen to identically vanish because of symmetry. The NC
effects are found to be significant only for low values of the NC
characteristic scale.Comment: 11 page Latex file containing 2 eps figures (redrawn). More
discussion included. To appear in PR
Light Stop Decay in the MSSM with Minimal Flavour Violation
In supersymmetric scenarios with a light stop particle and a
small mass difference to the lightest supersymmetric particle (LSP) assumed to
be the lightest neutralino, the flavour changing neutral current decay
can be the dominant decay channel and can
exceed the four-body stop decay for certain parameter values. In the framework
of Minimal Flavour Violation (MFV) this decay is CKM-suppressed, thus inducing
long stop lifetimes. Stop decay length measurements at the LHC can then be
exploited to test models with minimal flavour breaking through Standard Model
Yukawa couplings. The decay width has been given some time ago by an
approximate formula, which takes into account the leading logarithms of the MFV
scale. In this paper we calculate the exact one-loop decay width in the
framework of MFV. The comparison with the approximate result exhibits
deviations of the order of 10% for large MFV scales due to the neglected
non-logarithmic terms in the approximate decay formula. The difference in the
branching ratios is negligible. The large logarithms have to be resummed. The
resummation is performed by the solution of the renormalization group
equations. The comparison of the exact one-loop result and the tree level
flavour changing neutral current decay, which incorporates the resummed
logarithms, demonstrates that the resummation effects are important and should
be taken into account.Comment: 29 page
Supersymmetric effects in top quark decay into polarized W-boson
We investigate the one-loop supersymmetric QCD (SUSY-QCD) and electroweak
(SUSY-EW) corrections to the top quark decay into a b-quark and a longitudinal
or transverse W-boson. The corrections are presented in terms of the
longitudinal ratio \Gamma(t-->W_L b)/\Gamma(t--> W b) and the transverse ratio
\Gamma(t-->W_- b)/\Gamma(t--> W b). In most of the parameter space, both
SUSY-QCD and SUSY-EW corrections to these ratios are found to be less than 1%
in magnitude and they tend to have opposite signs. The corrections to the total
width \Gamma(t-->W b) are also presented for comparison with the existing
results in the literature. We find that our SUSY-EW corrections to the total
width differ significantly from previous studies: the previous studies give a
large correction of more than 10% in magnitude for a large part of the
parameter space while our results reach only few percent at most.Comment: Version in PRD (explanation and refs added
Higgs Boson Theory and Phenomenology
Precision electroweak data presently favors a weakly-coupled Higgs sector as
the mechanism responsible for electroweak symmetry breaking. Low-energy
supersymmetry provides a natural framework for weakly-coupled elementary
scalars. In this review, we summarize the theoretical properties of the
Standard Model (SM) Higgs boson and the Higgs sector of the minimal
supersymmetric extension of the Standard Model (MSSM). We then survey the
phenomenology of the SM and MSSM Higgs bosons at the Tevatron, LHC and a future
e+e- linear collider. We focus on the Higgs discovery potential of present and
future colliders and stress the importance of precision measurements of Higgs
boson properties.Comment: 90 pages, 31 figures. Revised version. To be published in Progress in
Particle and Nuclear Physics. This paper with higher resolution figures can
be found at http://scipp.ucsc.edu/~haber/higgsreview/higgsrev.p
- …