JHEP05(2017)036
direct detection predictions, the model has to be extended. For example, the mediator
(Higgs) sector can be enlarged with new scalars [
69–71]. Non-linear Higgs portals [72] and
high-dimensional operators in models with composite Higgs [
73] have been considered as
well. One can also extend the dark sector to incl ude new parti c l e s charged under the SM
gauge group, such as a doublet, a triplet, or a top-partner (see , e.g., [
74–77]), or even
consider multicomponent dark matte r scenarios [
71, 78–81]. More complex scenarios have
also been analysed, where both the dark matter and mediator sectors are enlarged [
82], for
example, adding new portals related to neutrino physics [
83–86]. There i s also the possi-
bility that the dark matter is a singlet-fermion, in which case the Higgs-portal int er ac ti on s
occur at the non-renormalizable le vel. Finally, one can consider changing the natur e of the
DM candidate, see for example refs. [
87, 88].
The goal of thi s paper is to consider and examine the mos t economical modific at i on
of the convent i onal SHP model that could e sc ape the present and future searches, thus
offering a viable (slightly modified) Higgs-port al sc enar i o if a positive detection does not
occur. The model consists of the addi ti on of a second singlet scalar i n the dark sector,
which opens up new anni h i l ati on and coannihilation channels (previous work in thi s line
has been c ar ri e d out in ref. [
89]). We should stress that our solution is not unique: for
example, this model has a simplicity simi l ar to the secluded-dark-matter scenario [
90], but
it works in a different way.
The article is organised as fol l ows. The model is introduced in section
2, where we
explain how the cor r ec t relic abundance can be achieved for large regions of the parameter
space. In section
2.2, we describe the various experimental constraints to which the model
is subject, and explain the way we have evaluated the m. They include bounds from direct
and indirect DM detection, the lifetime of the extra particle and the invisibl e decay of the
Higgs boson. In section
3 we perform a scan in the parameter space, explicitly showing that
our model is viable for any DM mass above 50 GeV, thereby reop e ni ng the Higgs portal for
scalar DM. In section
4 we discuss the interpre tat i on of this model in terms of an Effective
Field Theory. Finally, the conclusions of our study are presented in section
5. The appendix
is devoted to the calculation of the relevant radiative corrections for DM processes.
2 The extended singlet -scal ar Higgs portal (ESHP)
The modification of the conventional SHP model that we consider consists simply of ex-
tending the DM sector with the addition of a second scal ar . Denoting S
1
, S
2
the two scalar
particles, and imposing a global Z
2
symmetry (S
1
→ −S
1
, S
2
→ −S
2
) in order to guarantee
the stability of the lightest one, the most general renormalizable Lagrangian reads
L
ESHP
= L
SM
+
1
2
X
i=1,2
(∂
µ
S
i
)
2
− m
2
i
S
2
i
−
1
12
λ
i4
S
4
i
−
1
6
λ
13
S
1
S
3
2
−
1
6
λ
31
S
3
1
S
2
−
1
4
λ
22
S
2
1
S
2
2
−
1
2
λ
1
S
2
1
|H|
2
−
1
2
λ
2
S
2
2
|H|
2
− λ
12
S
1
S
2
|H|
2
−
v
2
2
, (2.1)
where the subscript ESHP stands for “extended singlet-scalar Higgs portal”. The terms in
the second line describe the DM/SM interactions, which occur through the Higgs sector.
– 4 –