Vector-like leptons: Higgs decays and collider phenomenology (1312.5329v1)

Abstract: We study the impact of heavy vector-like leptons on several observables in collider and low-energy physics. These states, present in many well-motivated extensions of the Standard Model, can induce lepton flavour violation and non-standard Higgs decays. We study these effects in an effective model inspired by the composite Higgs scenario. After deriving bounds on the mass and production cross-section of the vector-like states using recent LHC data on multilepton searches, we discuss the modification of the Higgs decays to dilepton, diphoton and $Z\gamma$ final states as well as low-energy observables like radiative lepton decays, the anomalous magnetic moment of the muon and the electric dipole moment of the electron. We find several interesting correlations. In particular, we show that branching fractions of lepton flavour-violating Higgs decays at an observable level are prohibited due to the strong bounds on the radiative lepton decays.

Overview of Vector-like Leptons: Higgs Decays and Collider Phenomenology

The paper "Vector-like leptons: Higgs decays and collider phenomenology" by Falkowski, Straub, and Vicente presents a comprehensive analysis of the theoretical implications and experimental observations related to the presence of heavy vector-like leptons. These leptons are a common feature in extensions of the Standard Model (SM), particularly in frameworks like the composite Higgs model and models incorporating extra dimensions. This work explores their potential influence on lepton flavor violation (LFV) and non-standard Higgs decays, with implications for collider signals and low-energy physics.

Theoretical Framework

Vector-like leptons are introduced in this paper through an effective model inspired by the composite Higgs scenario. These particles differ from the SM leptons in their mixing mechanism, gaining mass through interaction with massive vector-like states. The model integrates multiple generations of both chiral and vector-like leptons and denotes their mass generation via Dirac mass terms and Yukawa interactions. The resulting mixing parameters and hierarchies among lepton masses lead to distinct phenomenological predictions.

Experimental Constraints and Predictions

The paper establishes bounds on vector-like lepton masses and production cross-sections, utilizing data from the LHC multilepton searches. The authors highlight how these particles, once produced, predominantly decay into multilepton final states. This decay behavior places constraints on their mass and production through reinterpretation of existing search results, as dedicated searches for vector-like leptons remain absent.

A critical aspect of the paper is the investigation into non-standard Higgs decays induced by vector-like leptons. While the LHC data generally supports SM-like Higgs properties, deviations remain a viable avenue for new physics. Specifically, the paper explores lepton flavor-violating Higgs decays, such as $h \to e_i e_j$, positing that vector-like leptons could introduce observable effects in these transitions. However, the strong constraints from radiative lepton decays, like $\mu \to e \gamma$, restrict the branching fractions of LFV-Higgs decays, underscoring an experimental barrier to observation in current collider settings.

Furthermore, the paper investigates Higgs decays involving photons. It suggests that the presence of vector-like leptons can significantly alter the diphoton decay rate $R(h \to \gamma\gamma)$, potentially offering an observable deviation from SM predictions. These modifications arise due to contributions from heavy fermion loops, which can notably suppress or enhance decay rates.

Correlations and Phenomenological Insights

The authors also delve into correlations between observables. Notably, they identify relationships between LFV Higgs decays and radiative lepton decays, providing key predictions that can be experimentally tested. The anomalous magnetic moment of the muon and the electron EDM are also influenced by vector-like leptons, leading to potential new physics signals. The paper highlights correlations between these dipole moments and corrections to Higgs decay rates, providing a framework to navigate future experimental efforts that might address these discrepancies.

Conclusions and Future Directions

In synthesizing their findings, Falkowski et al. demonstrate that vector-like leptons can influence various Higgs decay pathways, while existing experimental constraints severely limit observable LFV in Higgs decays. If future collider experiments were to observe such decays, the paper suggests the necessity to consider other NP scenarios beyond vector-like leptons. The potential deviation in $h \to \gamma\gamma$ decays remains a promising signpost for new physics, warranting further investigation.

Looking forward, this research positions vector-like leptons as candidates for unveiling new interactions in particle physics. While present limits stem from lack of dedicated search strategies and stringent LFV constraints, advancing technology and analytical techniques may improve sensitivity and allow for definitive tests of the predicted phenomena. The paper thus provides a theoretical foundation and impetus for continued exploration in this domain.