241 research outputs found
Flavour Violating Axions
I review the physics case for flavour violating axions. In particular, I
argue that relaxing the assumption of the universality of the Peccei-Quinn
current opens up new pathways, including: the relaxation of the Supernova bound
on the axion mass, a possible connection with the Standard Model flavour puzzle
and the experimental opportunity of discovering the axion via flavoured axion
searches.Comment: 9 pages, 2 figures. Talk given at the Workshop `Flavour changing and
conserving processes' 29-31 August 2019 (Capri Island, Italy). v2: references
adde
Pati-Salam Axion
I discuss the implementation of the Peccei-Quinn mechanism in a minimal
realization of the Pati-Salam partial unification scheme. The axion mass is
shown to be related to the Pati-Salam breaking scale and it is predicted via a
two-loop renormalization group analysis to be in the window eV, as a function of a sliding Left-Right symmetry
breaking scale. This parameter space will be fully covered by the late phases
of the axion Dark Matter experiments ABRACADABRA and CASPEr-Electric. A
Left-Right symmetry breaking scenario as low as 20 TeV is obtained for a
Pati-Salam breaking of the order of the reduced Planck mass.Comment: 33 pages, 3 figures. Minor corrections. Matches version to appear in
JHE
Unification scale vs. electroweak-triplet mass in the SU(5) + 24_F model at three loops
It was shown recently that the original SU(5) theory of Georgi and Glashow,
augmented with an adjoint fermionic multiplet 24_F, can be made compatible both
with neutrino masses and gauge coupling unification. In particular, the model
predicts that either electroweak-triplet states are light, within the reach of
the Large Hadron Collider (LHC), or proton decay will become accessible at the
next generation of megaton-scale facilities. In this paper, we present the
computation of the correlation function between the electroweak-triplet masses
and the unification scale at the next-to-next-to-leading-order (NNLO). Such an
accuracy on the theory side is necessary in order to settle the convergence of
the perturbative expansion and to match the experimental precision on the
determination of the electroweak gauge couplings at the Z-boson mass scale.Comment: 16 pages, 7 figures. One reference added. To appear in Phys. Rev.
On the gauge dependence of the Standard Model vacuum instability scale
After reviewing the calculation of the Standard Model one-loop effective
potential in a class of linear gauges, we discuss the physical observables
entering the vacuum stability analysis. While the electroweak-vacuum-stability
bound on the Higgs boson mass can be formally proven to be gauge independent,
the field value at which the effective potential turns negative (the so-called
instability scale) is a gauge dependent quantity. By varying the gauge-fixing
scheme and the gauge-fixing parameters in their perturbative domain, we find an
irreducible theoretical uncertainty of at least two orders of magnitude on the
scale at which the Standard Model vacuum becomes unstable.Comment: 24 pages, 4 figures. One reference added. To appear in JHE
Accidental SO(10) axion from gauged flavour
An accidental U(1) Peccei-Quinn (PQ) symmetry automatically arises in a class
of SO(10) unified theories upon gauging the SU(3) flavour group. The PQ
symmetry is protected by the center of
up to effective operators of canonical
dimension six. However, high-scale contributions to the axion potential posing
a PQ quality problem arise only at . In the pre-inflationary PQ breaking
scenario the axion mass window is predicted to be eV, where the lower end is bounded by the seesaw scale and the upper
end by iso-curvature fluctuations. A high-quality axion, that is immune to the
PQ quality problem, is obtained for eV. We finally offer a
general perspective on the PQ quality problem in grand unified theories.Comment: 26 pages, 2 figures; To appear in JHE
Aspects of symmetry breaking in Grand Unified Theories
We reconsider the issue of spontaneous symmetry breaking in SO(10) grand
unified theories. The emphasis is put on the quest for the minimal Higgs sector
leading to a phenomenologically viable breaking to the standard model gauge
group. Longstanding results claimed that nonsupersymmetric SO(10) models with
just the adjoint representation triggering the first stage of the breaking
cannot provide a successful gauge unification. The main result of this thesis
is the observation that this no-go is an artifact of the tree level potential
and that quantum corrections opens in a natural way the vacuum patterns
favoured by gauge coupling unification. An analogous no-go, preventing the
breaking of SO(10) at the renormalizable level with representations up to the
adjoint, holds in the supersymmetric case as well. In this respect we show that
a possible way-out is provided by considering the flipped SO(10) embedding of
the hypercharge. Finally, the case is made for the hunting of the minimal
SO(10) theory.Comment: Ph.D. Thesis (SISSA, Trieste), 227 pages, 33 figure
Stability of the electroweak ground state in the Standard Model and its extensions
We review the formalism by which the tunnelling probability of an unstable
ground state can be computed in quantum field theory, with special reference to
the Standard Model of electroweak interactions. We describe in some detail the
approximations implicitly adopted in such calculation. Particular attention is
devoted to the role of scale invariance, and to the different implications of
scale-invariance violations due to quantum effects and possible new degrees of
freedom. We show that new interactions characterized by a new energy scale,
close to the Planck mass, do not invalidate the main conclusions about the
stability of the Standard Model ground state derived in absence of such terms.Comment: 12 pages, 5 figures. To appear in Physics Letters
Seesaw Scale in the Minimal Renormalizable SO(10) Grand Unification
Simple SO(10) Higgs models with the adjoint representation triggering the
grand-unified symmetry breaking, discarded a long ago due to inherent
tree-level tachyonic instabilities in the physically interesting scenarios,
have been recently brought back to life by quantum effects. In this work we
focus on the variant with 45_H+126_H in the Higgs sector and show that there
are several regions in the parameter space of this model that can support
stable unifying configurations with the B-L breaking scale as high as 10^14
GeV, well above the previous generic estimates based on the minimal survival
hypothesis. This admits for a renormalizable implementation of the canonical
seesaw and makes the simplest potentially realistic scenario of this kind a
good candidate for a minimal SO(10) grand unification. Last, but not least,
this setting is likely to be extensively testable at future large-volume
facilities such as Hyper-Kamiokande.Comment: 21 pages, 9 figures, 5 table
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