Matti Heikinheimo

Towards Completing the Standard Model: Vacuum Stability, EWSB and Dark Matter

We study the standard model (SM) in its full perturbative validity range between

Physical Naturalness and Dynamical Breaking of Classical Scale Invariance

\Lambda_QCD

The Dawn of FIMP Dark Matter: A Review of Models and Constraints

and the

Unnatural Origin of Fermion Masses for Technicolor

U(1)_Y

Dark photons and resonant monophoton signatures in Higgs boson decays at the LHC

Landau pole, assuming that a yet unknown gravitational theory in the UV does not introduce additional particle thresholds, as suggested by the tiny cosmological constant and the absence of new stabilising physics at the EW scale. We find that, due to dimensional transmutation, the SM Higgs potential has a global minimum at 10^26 GeV, invalidating the SM as a phenomenologically acceptable model in this energy range. We show that extending the classically scale invariant SM with one complex singlet scalar S allows us to: (i) stabilise the SM Higgs potential; (ii) induce a scale in the singlet sector via dimensional transmutation that generates the negative SM Higgs mass term via the Higgs portal; (iii) provide a stable CP-odd singlet as the thermal relic dark matter due to CP-conservation of the scalar potential; (iv) provide a degree of freedom that can act as an inflaton in the form of the CP-even singlet. The logarithmic behaviour of dimensional transmutation allows one to accommodate the large hierarchy between the electroweak scale and the Landau pole, while understanding the latter requires a new non-perturbative view on the SM.

Dark-photon searches via Higgs-boson production at the LHC

We propose a model of a confining dark sector, dark technicolor, that communicates with the Standard Model (SM) through the Higgs portal. In this model electroweak (EW) symmetry breaking and dark matter (DM) share a common origin, and the EW scale is generated dynamically. Our motivation to suggest this model is the absence of evidence for new physics from recent Large Hadron Collider (LHC) data. Although the conclusion is far from certain at this point, this lack of evidence may suggest that no mechanism exists at the EW scale to stabilize the Higgs mass against radiative corrections from ultraviolet (UV) physics. The usual reaction to this puzzling situation is to conclude that the stabilizing new physics is either hidden from us by accident, or that it appears at energies that are currently inaccessible, such that nature is indeed fine-tuned. In order to re-examine the arguments that have led to this dichotomy, we review the concept of naturalness in effective field theories, discussing in particular the role of quadratic divergences in relation to different energy scales. This leads us to suggest classical scale invariance as a guideline for model building, implying that explicit mass scales are absent in the underlying theory.

Higgs-boson production in association with a dark photon in e + e collisions

We present an overview of scenarios where the observed Dark Matter (DM) abundance consists of Feebly Interacting Massive Particles (FIMPs), produced nonthermally by the so-called freeze-in mechanism. In contrast to the usual freeze-out scenario, frozen-in FIMP DM interacts very weakly with the particles in the visible sector and never attained thermal equilibrium with the baryon–photon fluid in the early Universe. Instead of being determined by its annihilation strength, the DM abundance depends on the decay and annihilation strengths of particles in equilibrium with the baryon–photon fluid, as well as couplings in the DM sector. This makes frozen-in DM very difficult but not impossible to test. In this review, we present the freeze-in mechanism and its variations considered in the literature (dark freeze-out and reannihilation), compare them to the standard DM freeze-out scenario, discuss several aspects of model building, and pay particular attention to observational properties and general testability o...

Simulations of Galaxy Cluster Collisions with a Dark Plasma Component

We explore the scenario in which the breaking of the electroweak symmetry is due to the simultaneous presence and interplay of a dynamical sector and an unnatural elementary Higgs. We introduce a low energy effective Lagrangian and constrain the various couplings via direct search limits and electroweak and flavor precision tests. We find that the model we study is a viable model of dynamical breaking of the electroweak symmetry.

Anomalous Higgs-boson coupling effects in HW+W− production at the LHC

Motivated by possible massless dark photon solutions to long-standing cosmological and particle physics problems, we explore experimentally allowed scenarios where the Higgs boson coupling H is enhanced. Correspondingly, large rates are possible for the H ! decay, giving rise to one monochromatic photon with E ’ mH=2 (i.e., more than twice the photon energy in the rare standard-model decay H ! Z ! ), and similar amount of missing energy. We perform a model-independent study of this exotic resonant mono-photon signature at the LHC, featuring a distinctive E T peak around 60 GeV, and + = ET transverse invariant mass ruled by mH. At parton level, we nd a 5 sensitivity of the present LHC dataset for a H ! branching fraction of 0:5%. Such large branching fractions can be naturally obtained in dark U(1)F models explaining the origin and hierarchy of the standard model Yukawa couplings. We urge the LHC experiments to search for this new exotic resonance in the present dataset, and in future LHC runs.

Dark-photon searches via ZH production at e(+) e(-) colliders

Dark photons mediating long-range forces in a dark sector are predicted by various new physics scenarios, and are being intensively searched for in experiments. We extend a previous study of a new discovery process for dark photons proceedings via Higgs-boson production at the LHC. Thanks to the non-decoupling properties of the Higgs boson, BR(H ! ) values up to a few percent are possible for a massless dark photon, even for heavy dark-sector scenarios. The corresponding signature consists (for a Higgs boson at rest) of a striking monochromatic photon with energy E = mH=2, and similar amount of missing energy. We perform a model independent analysis at the LHC of both the gluon-fusion and VBF Higgs production mechanisms at 14 TeV, including parton-shower eects, and updating our previous parton-level analysis at 8 TeV in the gluon-fusion channel by a more realistic background modeling. We nd that a 5 sensitivity can be reached in the gluon-fusion channel for BR(H! )’ 0.1% with an integrated luminosity of L’ 300 fb 1 . The corresponding VBF reach is instead restricted to 1%. Such decay rates can be naturally obtained in dark-photon scenarios arising from unbroken U(1)F models explaining the origin and hierarchy of the Yukawa couplings, strongly motivating the search for this exotic Higgs decay at the LHC.