LHC Phenomenology of an Extended Standard Model with a Real Scalar Singlet

Abstract: Gauge singlet extensions of the Standard Model (SM) scalar sector may help remedy its theoretical and phenomenological shortcomings while solving outstanding problems in cosmology. Depending on the symmetries of the scalar potential, such extensions may provide a viable candidate for the observed relic density of cold dark matter or a strong first order electroweak phase transition needed for electroweak baryogenesis. Using the simplest extension of the SM scalar sector with one real singlet field, we analyze the generic implications of a singlet-extended scalar sector for Higgs boson phenomenology at the Large Hadron Collider (LHC). We consider two broad scenarios: one in which the neutral SM Higgs and singlet mix and the other in which no mixing occurs and the singlet can be a dark matter particle. For the first scenario, we analyze constraints from electroweak precision observables and their implications for LHC Higgs phenomenology. For models in which the singlet is stable, we determine the conditions under which it can yield the observed relic density, compute the cross sections for direct detection in recoil experiments, and discuss the corresponding signatures at the LHC.

• The paper presents a detailed analysis of an extended Standard Model with a real scalar singlet and its impact on LHC Higgs phenomenology.
• It examines how singlet-Higgs mixing modifies decay channels and relaxes constraints from precision electroweak observables.
• The study also evaluates conditions for the singlet to serve as a viable dark matter candidate through relic density calculations.

Overview of the Extended Standard Model with a Real Scalar Singlet

The paper under discussion presents a detailed examination of the phenomenology associated with an extension of the Standard Model (SM) that incorporates a real scalar singlet. This extension serves multiple theoretical and practical purposes, including addressing the shortcomings of the SM related to the electroweak phase transition and cold dark matter (DM) relic abundance. The authors explore the implications of such an extension on Higgs boson searches at the Large Hadron Collider (LHC), focusing on two scenarios: one involving mixing between the SM Higgs and the singlet, and another where the singlet functions as a dark matter candidate.

Motivation and Theoretical Framework

The SM, despite its successes, falls short in explaining key cosmological and particle physics observations, such as the nature of dark matter and the baryon asymmetry of the Universe. A well-motivated extension is the inclusion of a gauge singlet scalar field, potentially resolving these issues. The scalar singlet could either mix with the SM Higgs boson or remain stable if protected by a discrete symmetry like $\mathbb{Z}_2$. These scenarios have implications for electroweak baryogenesis and the nature of dark matter.

The authors derive the potential for a single real scalar singlet extension, defined by parameters governing its self-interaction and interaction with the Higgs doublet. They provide comprehensive formulas for the masses and mixing of the Higgs fields and their coupling to SM particles.

Higgs Mixing and Phenomenological Implications

In the first scenario, the paper examines the effects of singlet-Higgs mixing on Higgs phenomenology, particularly focusing on the LHC's ability to detect such a mixed state. The mixing introduces changes in the Higgs boson coupling structure and branching ratios, potentially altering expected discovery channels. The presence of additional singlet scalars can relax the traditional tensions between SM Higgs mass predictions and precision electroweak constraints, allowing for scenarios where the Higgs boson mass can exceed the anticipated range while remaining consistent with experimental data.

The analysis underscores the importance of considering electroweak precision observables (EWPOs) when assessing the viability of mixed scalar models. The constraints from EWPOs in this extended framework allow for regions where the Higgs discovery potential at the LHC could be altered, either by reducing its couplings to SM matter or by opening exotic decay channels.

Dark Matter Candidate Scenario

The alternative scenario focuses on the singlet as a stable dark matter particle. The authors calculate the relic abundance of the singlet scalar, accounting for its annihilation cross-sections into SM particles through Higgs mediation. This provides a stringent set of conditions under which the singlet's parameters yield the observed relic density, posing challenges and opportunities for direct detection experiments.

LHC Searches and Detection Prospects

The authors highlight the profound impact of the singlet on Higgs searches at the LHC. In scenarios where mixing with the Higgs occurs, additional decay channels, like the invisible decay to dark matter singlets, could significantly affect the discovery potential through conventional channels. The possibility of detecting these states through exotic signatures such as multi-jet or multiple gauge boson channels is considered, emphasizing a shift in search strategy.

In the dark matter context, direct detection experiments play a crucial role in constraining the parameter space, offering complementary insights into the nature of the scalar sector and its interactions with the SM. These experiments may confirm the presence of scalar singlets through nuclear recoil data, bridging collider and non-collider search domains.

Conclusion and Future Outlook

The extension of the Standard Model with a real scalar singlet offers promising avenues for addressing critical gaps in our understanding of fundamental and cosmological phenomena. The potential to reconcile precision data with viable Higgs phenomenology and dark matter parameters indicates a fertile ground for further investigation. The work illuminates paths for future experimental collaboration between LHC findings and direct detection efforts. Continued advances in theoretical and experimental techniques may refine our understanding of the scalar sector's role, shaping the future landscape of particle physics research.