255 research outputs found
Limiting two-Higgs-doublet models
We update the constraints on two-Higgs-doublet models (2HDMs) focusing on the
parameter space relevant to explain the present muon anomaly, , in four different types of models, type I, II, "lepton specific" (or
X) and "flipped" (or Y). We show that the strong constraints provided by the
electroweak precision data on the mass of the pseudoscalar Higgs, whose
contribution may account for , are evaded in regions where the
charged scalar is degenerate with the heavy neutral one and the mixing angles
and satisfy the Standard Model limit . We combine theoretical constraints from vacuum stability and
perturbativity with direct and indirect bounds arising from collider and
~physics. Possible future constraints from the electron are also
considered. If the 126 GeV resonance discovered at the LHC is interpreted as
the light CP-even Higgs boson of the 2HDM, we find that only models of type X
can satisfy all the considered theoretical and experimental constraints.Comment: 18 pages, 3 figure
Radiative Corrections to W and Quark Propagators in the Resonance Region
We discuss radiative corrections to W and quark propagators in the resonance
region, |s-M^2| \lsim M*Gamma. We show that conventional mass renormalization,
when applied to photonic or gluonic corrections, leads in next to leading order
(NLO) to contributions proportional to [M*Gamma/(s-M^2)]^n, (n=1,2...), i.e. to
a non-convergent series in the resonance region, a difficulty that affects all
unstable particles coupled to massless quanta. A solution of this problem,
based on the concepts of pole mass and width, is presented. It elucidates the
issue of renormalization of amplitudes involving unstable particles and
automatically circumvents the problem of apparent on-shell singularities. The
roles of the Fried-Yennie gauge and the Pinch Technique prescription are
discussed. Because of special properties of the photonic and gluonic
contributions, and in contrast with the Z case, the gauge dependence of the
conventional on-shell definition of mass is unbounded in NLO. The evaluations
of the width in the conventional and pole formulations are compared and shown
to agree in NLO but not beyond.Comment: 19 pages, 7 figures, LaTeX (uses epsfig). Slight rewording of the
abstract and one of the sentences of the text. Minor misprints corrected. To
appear in Phys. Rev.
Observations on the radiative corrections to pion beta-decay
We find that, in the local V-A theory, the radiative corrections to pion
beta-decay involving the weak vector current, when evaluated in the current
algebra (CA) formulation in which quarks are the fundamental underlying fields,
show a small difference with the more elementary calculations based directly on
the pion fields. We show that this difference arises from a specific
short-distance effect that depends on the algebra satisfied by the weak and
electromagnetic currents. On the other hand, we present a simple theoretical
argument that concludes that this difference does not occur when the CA
formulation is compared with the chiral perturbation theory (chiPT) approach.
Comparisons with previous studies, and with a more recent calculation based on
chiPT, are included. We also briefly review the important differences between
the results in the local V-A theory and the Standard Model.Comment: 5 pages, 1 figure. V2: two paragraphs have been added in Section III.
Final version on PR
Elimination of Threshold Singularities in the Relation Between On-Shell and Pole Widths
In a previous communication by two of us, Phys. Rev. Lett. 81, 1373 (1998),
the gauge-dependent deviations of the on-shell mass and total decay width from
their gauge-independent pole counterparts were investigated at leading order
for the Higgs boson of the Standard Model. In the case of the widths, the
deviation was found to diverge at unphysical thresholds, m_H = 2 root{xi_V} m_V
(V = W,Z), in the R_xi gauge. In this Brief Report, we demonstrate that these
unphysical threshold singularities are properly eliminated if a recently
proposed definition of wave-function renormalization for unstable particles is
invoked.Comment: 8 pages (Latex), 1 figure (Postscript
QED Corrections to the Scattering of Solar Neutrinos and Electrons
We discuss recent calculations of the O(alpha) QED corrections to the recoil
electron energy spectrum in neutrino electron scattering, and to the spectrum
of the combined energy of the recoil electron and a possible accompanying
photon emitted in the scattering process. We then examine the role of these
corrections in the interpretation of precise measurements from solar neutrino
electron scattering experiments.Comment: (16 Pages, 4 Figures) Presented at the Symposium in Honor of
Professor Alberto Sirlin's 70th Birthday: ``50 Years of Precision Electroweak
Physics'', New York University, October 27-28, 200
Tracking down the elusive charginos / neutralinos through leptons at the Large Hadron Collider
An unconstrained minimal supersymmetric standard model with the superpartners
of the strongly interacting particles very heavy (close to the kinematic reach
of the LHC or even beyond it) and a relatively light electroweak sector is
considered. Using the event generator Pythia it is shown that the 1-jet
(tagged) + 2 and 2-jets (tagged) + 1 signals with a reasonably hard
\etslash spectrum either by themselves or in combination with the
conventional signal, which is known to be of rather modest size with a
soft \etslash spectrum, may appreciably extend the reach of
chargino-neutralino search at the LHC with 10 fb of integrated
luminosity. This is especially so if the lighter chargino and the second
lightest neutralino decays via two body leptonic modes with large BRs. The
theoretical motivation of this scenario, yielding large values of the
fine-tuning parameters but consistent with various indirect constraints
including the dark matter relic density, is briefly discussed. It is shown that
in the minimal supergravity (mSUGRA) model with an universal scalar mass at the
GUT scale, the signals involving -jets are not viable. Theoretically
well-motivated variations of these boundary conditions are, however, adequate
for reviving these signals.Comment: 24 pages, 3 figures, only cosmetic changes have been made and new
references have been incorporate
Width and Partial Widths of Unstable Particles in the Light of the Nielsen Identities
Fundamental properties of unstable particles, including mass, width, and
partial widths, are examined on the basis of the Nielsen identities (NI) that
describe the gauge dependence of Green functions. In particular, we prove that
the pole residues and associated definitions of branching ratios and partial
widths are gauge independent to all orders. A simpler, previously discussed
definition of branching ratios and partial widths is found to be gauge
independent through next-to-next-to-leading order. It is then explained how it
may be modified in order to extend the gauge independence to all orders. We
also show that the physical scattering amplitude is the most general
combination of self-energy, vertex, and box contributions that is gauge
independent for arbitrary s, discuss the analytical properties of the NI
functions, and exhibit explicitly their one-loop expressions in the Z-gamma
sector of the Standard Model.Comment: 20 pages (Latex); minor changes included, accepted for publication in
Phys. Rev.
Low-Energy Supersymmetry Breaking from String Flux Compactifications: Benchmark Scenarios
Soft supersymmetry breaking terms were recently derived for type IIB string
flux compactifications with all moduli stabilised. Depending on the choice of
the discrete input parameters of the compactification such as fluxes and ranks
of hidden gauge groups, the string scale was found to have any value between
the TeV and GUT scales. We study the phenomenological implications of these
compactifications at low energy. Three realistic scenarios can be identified
depending on whether the Standard Model lies on D3 or D7 branes and on the
value of the string scale. For the MSSM on D7 branes and the string scale
between 10^12 GeV and 10^17 GeV we find that the LSP is a neutralino, while for
lower scales it is the stop. At the GUT scale the results of the fluxed MSSM
are reproduced, but now with all moduli stabilised. For the MSSM on D3 branes
we identify two realistic scenarios. The first one corresponds to an
intermediate string scale version of split supersymmetry. The second is a
stringy mSUGRA scenario. This requires tuning of the flux parameters to obtain
the GUT scale. Phenomenological constraints from dark matter, (g-2)_mu and
BR(b->s gamma) are considered for the three scenarios. We provide benchmark
points with the MSSM spectrum, making the models suitable for a detailed
phenomenological analysis.Comment: 29 pages, 12 figures, reference adde
Neutralino Dark Matter in Mirage Mediation
We study the phenomenology of neutralino dark matter (DM) in mirage mediation
scenario of supersymmetry breaking which results from the moduli stabilization
in some string/brane models. Depending upon the model parameters, especially
the anomaly to modulus mediation ratio determined by the moduli stabilization
mechanism, the nature of the lightest supersymmetric particle (LSP) changes
from Bino-like neutralino to Higgsino-like one via Bino-Higgsino mixing region.
For the Bino-like LSP, the standard thermal production mechanism can give a
right amount of relic DM density through the stop/stau-neutralino
coannihilation or the pseudo-scalar Higgs resonance process. We also examine
the prospect of direct and indirect DM detection in various parameter regions
of mirage mediation. Neutralino DM in galactic halo might be detected by near
future direct detection experiments in the case of Bino-Higgsino mixed LSP. The
gamma ray flux from Galactic Center might be detectable also if the DM density
profile takes a cuspy shape.Comment: One reference adde
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