A second Higgs from the Higgs portal

Abstract: In the Higgs portal framework, the Higgs field generally mixes with the Standard Model singlet leading to the existence of two states, one of which is identified with the 125 GeV scalar observed at the LHC. In this work, we analyse direct and indirect constraints on the second mass eigenstate and the corresponding mixing angle. The existence of the additional scalar can be beneficial as it can stabilise the otherwise-metastable electroweak vacuum. We find parameter regions where all of the bounds, including the stability constraints, are satisfied. We also study prospects for observing the decay of the heavier state into a pair of the 125 GeV Higgs-like scalars.

• The paper rigorously studies a Higgs portal model that introduces a second scalar mixing with the Standard Model Higgs.
• It evaluates experimental constraints from electroweak precision data and LHC searches to delineate allowed parameter spaces.
• The study highlights the potential of the H2 → H1 H1 decay channel as a probe for vacuum stability and new physics in future collider experiments.

A Second Higgs from the Higgs Portal

The paper explores the intriguing theoretical framework of the Higgs portal, which posits the existence of an additional scalar field that interacts with the Standard Model (SM) Higgs field. This interaction gives rise to two mass eigenstates, one being the familiar 125 GeV Higgs boson discovered at CERN's Large Hadron Collider (LHC), and the other being a second Higgs-like scalar. The authors rigorously analyze the direct and indirect experimental constraints on this additional mass eigenstate and the associated mixing angle. Moreover, they examine the parameter space where these constraints, alongside stability considerations of the electroweak vacuum, hold true.

The paper explores the Higgs portal framework, where the SM Higgs bilinear, $H^\dagger H$, couples to a real SM-singlet scalar $s$, leading to the mixing and formation of two Higgs-like states, $H_1$ and $H_2$. This theoretical model is particularly appealing because the presence of the additional scalar can stabilize the otherwise metastable electroweak vacuum—a situation inferred from current Higgs and top quark data.

The stability of the potential and perturbativity are assessed up to the Planck scale, with findings indicating allowed parameter regions for $\lambda_{hs}$ and mixing angles $\sin \theta$. Numerical simulations detail how perturbative constraints align with the demand for vacuum stability, offering insights into viable parameter spaces for $H_2$.

Electroweak precision data and Higgs coupling measurements form a significant part of the constraints analysis. These measurements reveal alterations in SM Higgs boson properties due to the presence of $H_2$, informing bounds on mixing angles and scalar masses. Direct searches for Higgs-like particles at LEP and LHC, as reviewed in the paper, contribute robustly to constraining the model parameters. The authors methodically compile the combined limitations from precision tests, LHC data, and potential new resonance searches across a spectrum of masses.

Importantly, the paper discusses prospects for observing $H_2 \to H_1 H_1$ decay at future LHC runs. This notable decay channel could serve as a vital probe for confirming the theoretical models of beyond-the-SM physics, given sufficient production cross sections in the parameter space that balance constraints from both experimental searches and theoretical predictions.

In conclusion, this comprehensive study ventures into an extension of the SM—the Higgs portal—with implications for both particle physics and cosmology. The paper provides a quantitative exploration to refine searches for new physics at particle colliders. Future developments in experimental precision and novel observational strategies stand to further illuminate the viability of additional Higgs bosons, carrying profound implications for understanding fundamental forces and the universe's evolution. This work broadens the motivation for multifaceted investigations in high-energy physics, a crucial endeavor in advancing our comprehension of the Higgs sector and potential novel interactions therein.