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Abstract

In the context of models with modified Higgs sectors, we examine different possible causes
for a systematic absence of new physics from current searches at the Large Hadron Collider
(LHC). First, having in mind the lack of clear signs for supersymmetry, we consider theories
based on classical scale invariance as an alternative approach towards addressing the
gauge hierarchy problem. Within the aforementioned framework, we present a systematic
analysis of simple Standard Model (SM) extensions. As a result, we identify the minimal
model, which allows for radiative electroweak symmetry breaking, is consistent with all
current experimental constraints and does not reintroduce any fine-tuning issues. Next,
given that very light scalar particles as predicted by several well-motivated beyond-the-
SM theories have not yet been detected either, we propose a novel search strategy based
on exotic Higgs decays into three of these light scalars. We show that said processes can
be abundant enough to be observable at the LHC or future lepton colliders in exciting
new signatures containing six SM fermions in the final state. In particular, we demonstrate
the three-body channel's capabilities in probing scenarios which are inaccessible
to existing searches exclusively relying on the related two-body Higgs decay. Our last
study is motivated by the fact that new physics may manifest itself only through effects
which are out of the LHC's reach due to the large amounts of hadronic background. As
an example, we consider quark-avor-violating Higgs decays involving bottom quarks and
evaluate the expected sensitivity of direct searches at the proposed International Linear
Collider (ILC). In doing so, we identify hadronic SM Higgs decays with mistagged jets as
the dominant background processes and find that a dedicated analysis is anticipated to
explore branching ratios down to the per-mill level.