A
bstract
Integrating out a heavy scalar can cause the Higgs trilinear coupling to deviate from its Standard Model value: a good example is provided by an SU(2) quadruplet. Constraints on the full ...theory, however, can limit the size of the deviation. We show that the bounded-from-below conditions for the Standard Model extended by an SU(2) quadruplet strongly constrain the ℤ
2
-breaking Higgs portal and can bound the Higgs trilinear coupling close to its Standard Model value. For TeV-scale quadruplet masses in models with custodial symmetry violation, these constraints can be a few times stronger than constraints from electroweak precision measurements. For the custodial quadruplet, these are the strongest theoretical constraints available.
A
bstract
We consider a scale-invariant inverse seesaw model with dynamical breaking of gauge symmetry and lepton number. In some regions of the parameter space, the Majoron — the pseudo-Goldstone of ...lepton number breaking — is a viable dark matter candidate. The bound on the Majoron decay rate implies a very large dilaton vacuum expectation value, which also results in a suppression of other dark matter couplings. Because of that, the observed dark matter relic abundance can only be matched via the freeze-in mechanism. The scalar field which gives mass to heavy neutrinos can play the role of the inflaton, resulting in a tensor-to-scalar ratio
r
≲ 0
.
01 for metric inflation and
r
≲ 0
.
21 for Palatini gravity.
A
bstract
We consider classically scale-invariant theories with non-minimally coupled scalar fields, where the Planck mass and the hierarchy of physical scales are dynamically generated. The ...classical theories possess a fixed point, where scale invariance is spontaneously broken. In these theories, however, the Planck mass becomes unstable in the presence of explicit sources of scale invariance breaking, such as non-relativistic matter and cosmological constant terms. We quantify the constraints on such classical models from Big Bang Nucleosynthesis that lead to an upper bound on the non-minimal coupling and require trans-Planckian field values. We show that quantum corrections to the scalar potential can stabilise the fixed point close to the minimum of the Coleman-Weinberg potential. The time-averaged motion of the evolving fixed point is strongly suppressed, thus the limits on the evolving gravitational constant from Big Bang Nucleosynthesis and other measurements do not presently constrain this class of theories. Field oscillations around the fixed point, if not damped, contribute to the dark matter density of the Universe.
The goal of this report is to summarize the current situation and discuss possible search strategies for charged scalars, in non-supersymmetric extensions of the Standard Model at the LHC. Such ...scalars appear in Multi-Higgs-Doublet models, in particular in the popular Two-Higgs-Doublet model, allowing for charged and additional neutral Higgs bosons. These models have the attractive property that electroweak precision observables are automatically in agreement with the Standard Model at the tree level. For the most popular version of this framework, Model II, a discovery of a charged Higgs boson remains challenging, since the parameter space is becoming very constrained, and the QCD background is very high. We also briefly comment on models with dark matter which constrain the corresponding charged scalars that occur in these models. The stakes of a possible discovery of an extended scalar sector are very high, and these searches should be pursued in all conceivable channels, at the LHC and at future colliders.
A
bstract
We reconsider complex scalar singlet dark matter stabilised by a ℤ
3
symmetry. We refine the stability bounds on the potential and use constraints from unitarity on scattering at finite ...energy to place a stronger lower limit on the direct detection cross section. In addition, we improve the treatment of the thermal freeze-out by including the evolution of the dark matter temperature and its feedback onto relic abundance. In the regions where the freeze-out is dominated by resonant or semi-annihilation, the dark matter decouples kinetically from the plasma very early, around the onset of the chemical decoupling. This results in a modification of the required coupling to the Higgs, which turns out to be at most few per cent in the semi-annihilation region, thus giving credence to the standard approach to the relic density calculation in this regime. In contrast, for dark matter mass just below the Higgs resonance, the modification of the Higgs invisible width and direct and indirect detection signals can be up to a factor 6.7. The model is then currently allowed at 56.8 GeV to 58.4 GeV (depending on the details of early kinetic decoupling) ≲
M
S
≲ 62.8 GeV and at
M
S
≳ 122 GeV if the freeze-out is dominated by semi-annihilation. We show that the whole large semi-annihilation region will be probed by the near-future measurements at the XENONnT experiment.
The evidence for a new neutral scalar particle from the 750 GeV diphoton excess, and the absence of any other signal of new physics at the LHC so far, suggests the existence of new coloured scalars. ...To study this possibility, we propose a supersymmetry inspired simplified model, extending the Standard Model with a singlet scalar and with heavy scalar fields carrying both colour and electric charges – new scalar quarks. To allow the latter to decay, and to generate the dark matter of the Universe, we also add a neutral fermion to the particle content. We show that this model provides a two-parameter fit to the observed diphoton excess consistently with cosmology, while the allowed parameter space is bounded by the consistency of the model. In the context of our simplified model this implies the existence of other supersymmetric particles accessible at the LHC, rendering this scenario falsifiable.
We study inert charged Higgs boson
H
±
production and decays at LHC experiments in the context of constrained scalar dark matter model (CSDMM). In the CSDMM the mass spectrum of the inert doublet and ...singlet scalars is predicted from the GUT scale initial conditions via RGE evolution. We compute the cross sections of processes
pp
→
H
+
H
−
,
H
±
S
i
0
,
where
S
i
0
are neutral scalar particles, at the LHC experiments. We show that for light
H
±
the first process may receive a sizable contribution from the top quark mediated 1-loop diagram with Higgs boson in
s
-channel. In a significant fraction of the parameter space
H
±
are long-lived because their decays to predominantly singlet scalar dark matter (DM) and next-to-lightest (NL) scalar,
H
±
→
S
DM, NL
f f
′, are suppressed by the small singlet-doublet mixing angle and by the moderate mass difference
. The experimentally measurable displaced vertex in
H
±
decays to leptons and/or jets and missing energy allows one to discover the
H
+
H
−
signal over the huge
W
+
W
−
background. If, however,
H
±
are short-lived, the subsequent decays
necessarily produce additional displaced vertices that allow to reconstruct the full
H
±
decay chain. We propose benchmark points for studies of this scenario at the LHC.
We calculate the running of low-energy neutrino parameters from the bottom–up, parameterizing the unknown seesaw parameters in terms of the dominance matrix
R. We find significant running only if the
...R matrix is non-trivial and the light-neutrino masses are moderately degenerate. If the light-neutrino masses are very hierarchical, the quark–lepton complementarity relation
θ
c
+
θ
12
=
π
/
4
is quite stable, but
θ
13
,
23
may run beyond their likely future experimental errors. The running of the oscillation phase
δ is enhanced by the smallness of
θ
13
, and jumps in the mixing angles occur in cases where the light-neutrino mass eigenstates cross.
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