57 research outputs found

### Doublet-Triplet Fermionic Dark Matter

We extend the Standard Model (SM) by adding a pair of fermionic
SU(2)-doublets with opposite hypercharge and a fermionic SU(2)-triplet with
zero hypercharge. We impose a discrete Z_2-symmetry that distinguishes the SM
fermions from the new ones. Then, gauge invariance allows for two
renormalizable Yukawa couplings between the new fermions and the SM Higgs
field, as well as for direct masses for the doublet (M_D) and the triplet
(M_T). After electroweak symmetry breaking, this model contains, in addition to
SM particles, two charged Dirac fermions and a set of three neutral Majorana
fermions, the lightest of which contributes to Dark Matter (DM). We consider a
case where the lightest neutral fermion is an equal admixture of the two
doublets with mass M_D close to the Z-boson mass. This state remains stable
under radiative corrections thanks to a custodial SU(2)-symmetry and is
consistent with the experimental data from oblique electroweak corrections.
Moreover, the amplitudes relevant to spin-dependent or independent nucleus-DM
particle scattering cross section both vanish at tree level. They arise at one
loop at a level that may be observed in near future DM direct detection
experiments. For Yukawa couplings comparable to the top-quark, the DM particle
relic abundance is consistent with observation, not relying on co-annihilation
or resonant effects and has a mass at the electroweak scale. Furthermore, the
heavier fermions decay to the DM particle and to electroweak gauge bosons
making this model easily testable at the LHC. In the regime of interest, the
charged fermions suppress the Higgs decays to diphoton by 45-75 % relative to
SM prediction.Comment: 40 pages, v2: discussion and references on dark matter direct
detection expanded, matches published version, v3: formulae in Appendix A
correcte

### Anatomy of the Higgs boson decay into two photons in the unitary gauge

In this work, we review and clarify computational issues about the W-gauge
boson one-loop contribution to the H -> gamma gamma decay amplitude, in the
unitary gauge and in the Standard Model. We find that highly divergent
integrals depend upon the choice of shifting momenta with arbitrary vectors.
One particular combination of these arbitrary vectors reduces the superficial
divergency down to a logarithmic one. The remaining ambiguity is then fixed by
exploiting gauge invariance and the Goldstone Boson Equivalence Theorem. Our
method is strictly realised in four-dimensions. The result for the amplitude
agrees with the "famous" one obtained using dimensional regularisation (DR) in
the limit d-> 4, where d is the number of spatial dimensions in Euclidean
space. At the exact equality d=4, a three-sphere surface term appears that
renders the Ward Identities and the equivalence theorem inconsistent. We also
examined a recently proposed four-dimensional regularisation scheme and found
agreement with the DR outcome.Comment: 1+20 pages, 2 figures, v2 few references added, Appendix A expanded,
v3 matches published versio

### Radiative Light Dark Matter

We present a Peccei-Quinn (PQ)-symmetric two-Higgs doublet model that
naturally predicts a fermionic singlet dark matter in the mass range 10 keV-1
GeV. The origin of the smallness of the mass of this light singlet fermion
arises predominantly at the one-loop level, upon soft or spontaneous breakdown
of the PQ symmetry via a complex scalar field in a fashion similar to the
so-called Dine-Fischler-Sredniki-Zhitnitsky axion model. The mass generation of
this fermionic Radiative Light Dark Matter (RLDM) requires the existence of two
heavy vector-like SU(2) isodoublets, which are not charged under the PQ
symmetry. We show how the RLDM can be produced via the freeze-in mechanism,
thus accounting for the missing matter in the Universe. Finally, we briefly
discuss possible theoretical and phenomenological implications of the RLDM
model for the strong CP problem and the CERN Large Hadron Collider (LHC).Comment: 17 pages, v2: typos corrected, matches published versio

### The Higgs Penguin and its Applications : An overview

We review the effective Lagrangian of the Higgs penguin in the Standard Model
and its minimal supersymmetric extension (MSSM). As a master application of the
Higgs penguin, we discuss in some detail the B-meson decays into a
lepton-antilepton pair. Furthermore, we explain how this can probe the Higgs
sector of the MSSM provided that some of these decays are seen at Tevatron Run
II and B-factories. Finally, we present a complete list of observables where
the Higgs penguin could be strongly involved.Comment: 22 pages, 6 figures, Invited review article to appear in Mod. Phys.
Lett. A, v2: Table 1 updated, comments and references adde

### Complete One-Loop MSSM Predictions for B --> lepton lepton' at the Tevatron and LHC

During the last few years the Tevatron has dramatically improved the bounds
on rare B-meson decays into two leptons. In the case of B_s --> mu+ mu-, the
current bound is only ten times greater than the Standard Model expectation.
Sensitivity to this decay is one of the benchmark goals for LHCb performance
and physics. The Higgs penguin dominates this rate in the region of large
tan(beta) of the MSSM. This is not necessarily the case in the region of low
tan(beta), since box and Z-penguin diagrams may contribute at a comparable
rate. In this article, we compute the complete one-loop MSSM contribution to B
--> l+l'- for l,l' = e, mu. We study the predictions for general values of
tan(beta) with arbitrary flavour mixing parameters. We discuss the possibility
of both enhancing and suppressing the branching ratios relative to their
Standard Model expectations. In particular, we find that there are
"cancellation regions" in parameter space where the branching ratio is
suppressed well below the Standard Model expectation, making it effectively
invisible to the LHC.Comment: 30 pages, 4 figures; v.3: corrected factors of (2 pi) in (2.11),
(3.1), (A.11), (A.13-14

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