| dc.description | The discovery of the Higgs boson in the 8 TeV run of the LHC [1, 2] marks one of the
most important milestones in particle physics. Its mass is already known rather precisely:
mh = 125.09 ± 0.21 (stat.) ±0.11 (syst.) GeV [3], and the signal strength of various LHC
searches has been found consistent with the SM predictions. While this completes the
Standard Model (SM) particle-wise, several questions still remain open, for example: (i) Is
it possible to include the SM in a grand unified theory where all gauge forces unify? (ii) Is
there a particle physics explanation of the observed dark matter relic density? (iii) What
causes the hierarchy in the fermion mass spectrum and why are neutrinos so much lighter
than the other fermions? What causes the observed mixing patterns in the fermion sector?
(iv) What stabilizes the Higgs mass at the electroweak scale?
Supersymmetric model address several of these questions and consequently the search for
supersymmetry (SUSY) is among the main priorities of the LHC collaborations. Up to now
no significant sign for physics beyond SM has been found. The combination of the Higgs
discovery with the (yet) unsuccessful searches has led to the introduction of a model class
called ‘natural SUSY’ [4–15]. Here, the basic idea is to give electroweak-scale masses only
to those SUSY particles giving a sizeable contribution to the mass of the Higgs boson, such
that a too large tuning of parameters is avoided. All other particle masses are taken at the
multi-TeV scale. In particular, masses of the order of a few hundred GeV up to about one
TeV are assigned to the higgsinos (the partners of the Higgs bosons), the lightest stop (the
partner of the top-quark) and, if the latter is mainly a left-stop, also to the light sbottom In
addition the gluino and the heavier stop masses should also be close to at most a few TeV.
Neutrino oscillation experiments confirm that at least two neutrinos have a non-zero mass.
The exact mass generation mechanism for these particles is unknown, and both the SM and
the MSSM remain agnostic on this topic. Although many ways to generate neutrino mass
exist, perhaps the most popular one is the seesaw mechanism [16–21]. The main problem
with the usual seesaw mechanisms lies on the difficulty in testing its validity. In general, if
Yukawa couplings are sizeable, the seesaw relations require Majorana neutrino masses to be
very large, such that the new heavy states cannot be produced at colliders. In contrast, if
one requires the masses to be light, then the Yukawas need to be small, making production
cross-sections and decay rates to vanish. A possible way out of this dilemma lies on what
3
is called the inverse seesaw [22], which is based on having specific structures on the mass
matrix (generally motivated by symmetry arguments) to generate small neutrino masses.
This, at the same time, allows Yukawa couplings to be large, and sterile masses to be light.
We consider here a supersymmetric model where neutrino data are explained via a minimal
inverse seesaw scenario where the gauge-singlet neutrinos have masses in the range
O(keV) to O(100 GeV). We explore this with a parametrization built for the standard seesaw,
and go to the limit where the inverse seesaw emerges, such that Yukawas and mixings
become sizeable. Although non-SUSY versions of this scenario can solve the dark matter
and matter-antimatter asymmetry problems [23–25], we shall make no claim on these issues
in our model.
In view of the naturalness arguments, we further assume that the higgsinos have masses of
O(100 GeV), whereas the gaugino masses lie at the multi-TeV scale (see [26] for an example
of such a scenario). In addition, we assume all squarks are heavy enough such that LHC
bounds are avoided, and play no role in the phenomenology within this work1. In contrast
we allow for fairly light sleptons and investigate the extent to which current LHC data can
constrain such scenarios.
This paper is organized as follows: in the next section we present the model. Section
III summarizes the numerical tools used and gives an overview of the LHC analysis used
for these investigations. In Section IV we present our findings for the two generic scenarios
which differ in the nature of the lighest supersymmetric particle (LSP): a Higgsino LSP
and a sneutrino LSP. In Section V we draw our conclusions. Appendices A and B give the
complete formulae for the neutrino and sneutrino masses. | |
