148 research outputs found

### Exponentially spread dynamical Yukawa couplings from non-perturbative chiral symmetry breaking in the dark sector

We propose a new paradigm for generating exponentially spread standard model
Yukawa couplings from a new $U(1)_F$ gauge symmetry in the dark sector. Chiral
symmetry is spontaneously broken among dark fermions that obtain non-vanishing
masses from a non-perturbative solution to the mass gap equation. The necessary
ingredient for this mechanism to work is the existence of higher derivative
terms in the dark $U(1)_F$ theory, or equivalently the existence of Lee-Wick
ghosts, that (i) allow for a non-perturbative solution to the mass gap equation
in the weak coupling regime of the Abelian theory; (ii) induce exponential
dependence of the generated masses on dark fermion $U(1)_F$ quantum numbers.
The generated flavor and chiral symmetry breaking in the dark sector is
transferred to the standard model Yukawa couplings at one loop level via Higgs
portal type scalar messenger fields. The latter carry quantum numbers of
squarks and sleptons. A new intriguing phenomenology is predicted that could be
potentially tested at the LHC, provided the characteristic mass scale of the
messenger sector is accessible at the LHC as is suggested by naturalness
arguments.Comment: Text improved, new equations and references added, version to appear
in Phys.Rev.D, 12 pages, 2 figure

### Testing PVLAS axions with resonant photon splitting

The photon splitting gamma -> gamma gamma in a time-independent and
inhomogeneous magnetized background is considered when neutral and ultralight
spin-0 particles are coupled to two-photons. Depending on the inhomogeneity
scale of the external field, resonant photon splitting can occur. If an optical
laser crosses a magnetic field of few Tesla with typical inhomogeneity scale of
the order of the meter, a potentially observable rate of photon splittings is
expected for the PVLAS range of couplings and masses.Comment: 7 pages, 2 included eps figures, two references added, minor typos
correcte

### Testing Effective Yukawa Couplings in Higgs Searches at the Tevatron and LHC

We explore the possibility that, while the Higgs mechanism provides masses to
the weak-gauge bosons at the electroweak scale as in the standard model,
fermion masses are generated by an unknown mechanism at a higher energy scale.
At low energies, the standard model can then be regarded as an effective field
theory, where fermion masses explicitly break the electroweak SU(2)_L \times
U(1)_Y gauge symmetry. If \Lambda is the renormalization scale where the
renormalized Yukawa couplings vanish, then at energies lower than \Lambda,
effective Yukawa couplings will be radiatively induced by nonzero fermion
masses. In this scenario, Higgs-boson decays into photons and weak gauge-bosons
pairs are in general quite enhanced for a light Higgs. However, depending on
\Lambda, a substantial decay rate into b \bar{b} can arise, that can be of the
same order as, or larger than, the enhanced H\to gamma gamma rate. A new
framework for Higgs searches at hadron colliders is outlined, vector-boson
fusion becoming the dominant production mechanism at the CERN LHC, with an
important role also played by the WH/ZH associated production. A detailed
analysis of the Higgs branching fractions and their implications in Higgs
searches is provided, versus the energy scale \Lambda.Comment: 35 pages, 10 figures, 5 tables, Fig.[10] corrected, 1 new reference
adde

### g-2 of the muon in SUSY Models with Gauge Multiplets in the Bulk of Extra-Dimensions

We analyze the supersymmetric contributions to the anomalous magnetic moment
of the muon (a_\mu) in the framework of pure and gaugino-assisted anomaly
mediation models, and gaugino mediation models. In the last two models the
gauge multiplets propagate in the higher dimensional bulk, providing a natural
mechanism for solving the problem of negative squared slepton masses present in
the pure anomaly mediation models. In the light of the new BNL results for
a_\mu, we found that the pure and gaugino-assisted anomaly mediation models are
almost excluded by the BNL constraints at 2\sigma level when combined with CLEO
constraints on b->sg at 90 % of C.L. In contrast, the gaugino mediation models
provide extensive regions in the SUSY parameter space where both of these
constraints are satisfied.Comment: 12 pages, 4 figures, added discussion on neutralino-nucleon cross
section, minor change

### Higgs boson plus photon production at the LHC: a clean probe of the b-quark parton densities

Higgs boson production in association with a high pT photon at the CERN Large
Hadron Collider is analyzed, in the framework of the MSSM model, for the
heavier neutral Higgs bosons. The request of an additional photon in the
exclusive Higgs boson final state selects b-quark pairs among the possible
initial partonic states, since gluon-gluon initial states are not allowed by
C-parity conservation. Hence, the measurement of cross sections for neutral
Higgs boson plus photon production can provide a clean probe of the b-quark
density in the proton as well as of the b-quark Yukawa coupling. The
suppression of the production rates by the b-quark electromagnetic coupling can
be compensated by the enhanced Higgs boson Yukawa coupling to b's in the large
tan(beta) regime. The Higgs boson decay into a tau-lepton pair is considered,
and irreducible backgrounds with corresponding signal significances are
evaluated.Comment: 10 pages, 4 figures, a few comments and 3 references added at the
end. To appear in Physical Review

### Implications of the effective axial-vector coupling of gluon on top-quark charge asymmetry at the LHC

We study different top quark charge asymmetries and the variation of $t\bar
t$ total cross section induced by the effective axial-vector coupling of gluon
in the LHC experiments. We show that rapidity cut-dependent asymmetries are
more sensitive to the new physics than the independent ones. We also study the
dependence of the asymmetries and variations of total $t\bar t$ cross sections
on the invariant mass of $t\bar t$ system and show that it would be necessary
to measure those quantities as functions of $m_{tt}$ at the LHC. In the context
of considered new physics scenario, 7 TeV LHC has enough sensitivity either to
confirm the Tevatron top asymmetry anomaly or to rule it out. In the latter
case the LHC is able to put stringent constraint on the new physics scale
$\Lambda$ in this framework.Comment: few small changes in the text, Fig. 2 corrected, same as published
version, 12 pages, 9 figure

### On the dynamical breaking of chiral symmetry: a new mechanism

We consider a U(1) gauge theory, minimally coupled to a massless Dirac field,
where a higher-derivative term is added to the pure gauge sector, as in the
Lee-Wick models. We find that this term can trigger chiral symmetry breaking at
low energy in the weak coupling regime. Then, the fermion field acquires a mass
that turns out to be a function of both the energy scale associated to the
higher-derivative term and the gauge coupling. The dependence of the fermion
mass on the gauge coupling is non-perturbative. Extensions to SU(N) gauge
theories and fermion-scalar interactions are also analyzed, as well as to
theories with massive gauge fields. A few implications of these results in the
framework of quark-mass generation are discussed.Comment: 15 pages 2 figures, a few comments and 4 references added. To appear
in Physical Review

### Dark-Photon searches via Higgs-boson production at the LHC

Dark photons $\bar \gamma$ mediating long-range forces in a dark sector are
predicted by various new physics scenarios, and are being intensively searched
for in experiments. We extend a previous study of a new discovery process for
dark photons proceedings via Higgs-boson production at the LHC. Thanks to the
non-decoupling properties of the Higgs boson, BR($H\to \gamma\bar \gamma$)
values up to a few percent are possible for a massless dark photon, even for
heavy dark-sector scenarios. The corresponding signature consists (for a Higgs
boson at rest) of a striking monochromatic photon with energy $E_{\gamma}=
m_H/2$, and similar amount of missing energy. We perform a model independent
analysis at the LHC of both the gluon-fusion and VBF Higgs production
mechanisms at 14 TeV, including parton-shower effects, and updating our
previous parton-level analysis at 8 TeV in the gluon-fusion channel by a more
realistic background modeling. We find that a $5\sigma$ sensitivity can be
reached in the gluon-fusion channel for BR($H\to \gamma\bar
\gamma)\simeq\,$0.1% with an integrated luminosity of $L\simeq 300\, {\rm
fb}^{-1}$. The corresponding VBF reach is instead restricted to 1%. Such decay
rates can be naturally obtained in dark-photon scenarios arising from unbroken
$U(1)_F$ models explaining the origin and hierarchy of the Yukawa couplings,
strongly motivating the search for this exotic Higgs decay at the LHC.Comment: 8 pages, 3 figure

### FCNC decays of SM fermions into a dark photon

We analyze a new class of FCNC processes, the $f \to f^{\prime} \,
\bar{\gamma}$ decays of a fermion $f$ into a lighter (same-charge) fermion
$f^{\prime}$ plus a {\it massless} neutral vector boson, a {\it dark photon}
$\bar{\gamma}$. A massless dark photon does not interact at tree level with
observable fields, and the $f \!\to\! f^{\prime} \, \bar{\gamma}$ decay
presents a characteristic signature where the final fermion $f^{\prime}$ is
balanced by a {\it massless invisible} system. Models recently proposed to
explain the exponential spread in the standard-model Yukawa couplings can
indeed foresee an extra unbroken {\it dark} $U(1)$ gauge group, and the
possibility to couple on-shell dark photons to standard-model fermions via
one-loop magnetic-dipole kind of FCNC interactions. The latter are suppressed
by the characteristic scale related to the mass of heavy messengers, connecting
the standard model particles to the dark sector. We compute the corresponding
decay rates for the top, bottom, and charm decays ($t\to c\, \bar{\gamma},u\,
\bar{\gamma}$, $\;b\to s\, \bar{\gamma},d\, \bar{\gamma}$, and $c\to u
\bar{\gamma}$), and for the charged-lepton decays ($\tau \to \mu\,
\bar{\gamma}, e\, \bar{\gamma}$, and $\mu \to e \bar{\gamma}$) in terms of
model parameters. We find that large branching ratios for both quark and lepton
decays are allowed in case the messenger masses are in the discovery range of
the LHC. Implications of these new decay channels at present and future
collider experiments are briefly discussed.Comment: 44 pages, 9 figures, BBbar constraints and new references included,
same version as the published on

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