Higgs Partner Searches and Dark Matter Phenomenology in a Classically Scale Invariant Higgs Boson Sector

TL;DR

This work analyzes a minimal classically scale invariant extension of the SM with a CP-symmetric complex singlet that radiatively breaks electroweak symmetry and yields a stable pseudoscalar dark matter candidate χ plus an additional CP-even scalar σ. By integrating Planck relic density, LUX direct-detection bounds, and LEP/LHC Higgs searches, the authors map the viable parameter space in terms of the Higgs–singlet mixing angle ω, the σ and χ masses, and the right-handed neutrino sector, presenting unified exclusion plots. The findings indicate that viable regions require small mixing (|sin ω| ≲ 0.2) and TeV-scale χ, with direct-detection data providing stringent constraints and collider data offering complementary limits, thereby making the scenario highly predictive. The results establish concrete experimental expectations for Xenon1T and future colliders, enabling falsification or discovery of this scale-invariant framework linking electroweak breaking, dark matter, and neutrino physics.

Abstract

In a previous work, a classically scale invariant extension of the standard model was proposed, as a potential candidate for resolving the hierarchy problem, by minimally introducing a complex gauge singlet scalar, and generating radiative electroweak symmetry breaking by means of the Coleman- Weinberg Mechanism. Postulating the singlet sector to respect the CP-symmetry, the existence of a stable pseudoscalar dark matter candidate with a mass in the TeV range was demonstrated. More- over, the model predicted the presence of another physical CP-even Higgs boson (with suppressed tree-level couplings), in addition to the 125 GeV scalar discovered by the LHC. The viable region of the parameter space was determined by various theoretical and experimental considerations. In this work, we continue to examine the phenomenological implications of the proposed minimal sce- nario by considering the constraints from the dark matter relic density, as determined by the Planck collaboration, as well as the direct detection bounds from the LUX experiment. Furthermore, we investigate the implications of the collider Higgs searches for the additional Higgs boson. Our results are comprehensively demonstrated in unified exclusion plots, which analyze the viable region of the parameter space from all relevant angles, demonstrating the testability of the proposed scenario.

Figures (12)

• Figure 1: Validity of the narrow-width approximation for the $\sigma$ boson. The panels display the ratio of the boson's total width to its mass as a function of its mass for $m_{\sigma}\leq 1$ TeV. Three values of the mixing angle---motivated by the experimental constraints Farzinnia:2013pga---are selected for illustration in each panel. In the left panel ($M_{N}=300$ GeV), the decay channel of the $\sigma$ scalar to a pair of right-handed neutrinos is kinematically open, whereas in the right panel ($M_{N}=1000$ GeV), such a decay is not permitted. The effect of this non-SM decay mode is, thus, negligible in the entire mass range of interest.
• Figure 2: Left: Theoretical curves of the $\mu$ parameter \ref{['iisVV']} as a function of the $\sigma$ boson mass, for three representative values of the mixing angle, along with the most stringent experimental upper limit from LEP Barate:2003sz and LHC LHCHeavyH Higgs searches at 95% C.L. Right: The experimental exclusion limits in the $\sin\omega-m_{\sigma}$ plane. All colored regions are excluded at 95% C.L., taking into account the electroweak precision tests (dot-dashed), direct measurements of the LHC 125 GeV $h$ Higgs' properties (solid), and the LEP and LHC Higgs searches (dotted).
• Figure 3: Pair-annihilation of the $\chi$ WIMP dark matter into the (dominant) pairs of scalar, fermion, and vector final states. Diagrams in the top three rows illustrate their scattering process in all possible channels with the corresponding mediators.
• Figure 4: Constraint from the dark matter relic abundance in the $\sin\omega-M_{\chi}$ plane, for benchmark values of the right-handed neutrino mass, $M_{N}$ (columns), and the input parameter $\lambda_{m}^{-}$ (rows). The thick (red) band represents the thermal relic density of the cold WIMP pseudoscalar as constrained by the data from the Planck collaboration Ade:2013zuv. The thickness of the line corresponds to the 1$\sigma$ uncertainty quoted by the collaboration. A dependence on the sign of the $\lambda_{m}^{-}$ parameter is negligible. In addition, the experimental exclusion bounds from the electroweak precision tests (dot-dashed) and the direct measurements of the LHC 125 GeV Higgs' properties (solid) at 95% C.L. are displayed, which set the upper limit on the mixing angle. The solid black region, inferred from the stability condition of the one-loop potential \ref{['staboneloop']}, determines the formal lower bound on the WIMP mass, $M_{\chi}$, for each selected value of $M_{N}$. The enumerated thin contours represent the values of the $x_{\text{fo}}$ parameter \ref{['xfo']}, illustrating the validity of the non-relativistic treatment ($x_{\text{fo}}\gg3$), and hence the cold dark matter nature of the pseudoscalar $\chi$. The observed relic density is comfortably accommodated within the allowed region of the model's parameter space.
• Figure 5: Elastic scattering of a dark matter WIMP, $\chi$, off a nucleon, $N$. The process is mediated by the exchange of the $h$ and $\sigma$ scalars.