The Standard Model of particle physics, a triumph of scientific inquiry, has long served as our most accurate description of the fundamental forces and particles that govern the universe. It elegantly explains the behavior of quarks, leptons, and the force-carrying bosons, providing a framework that has withstood decades of rigorous experimental scrutiny. However, the Standard Model, for all its successes, is not without its limitations. It fails to account for phenomena such as dark matter, dark energy, neutrino masses, and the profound imbalance between matter and antimatter observed in the cosmos. These unanswered questions hint at a deeper, more comprehensive theory yet to be uncovered, a tantalizing prospect for physicists worldwide.
One of the most enigmatic particles within the Standard Model is the Higgs boson, famously discovered at the Large Hadron Collider (LHC) in 2012. This elusive boson is responsible for imbuing fundamental particles with mass through the Higgs field. While its discovery was a monumental achievement, the exploration of its properties is far from over. Physicists are keen to probe its interactions with other particles and search for deviations from the Standard Model’s predictions. Any such deviation could be a crack in the edifice of our current understanding, opening a window into new physics.
A particularly exciting avenue of research revolves around the concept of “lepton flavor violation.” In the Standard Model, leptons, a class of fundamental particles that include electrons, muons, and taus, are strictly conserved in terms of their flavor. This means an electron will always remain an electron, and a muon will always remain a muon. However, theoretical extensions to the Standard Model suggest that this conservation law might be violated under certain extreme conditions, leading to processes where one lepton flavor can transform into another.
The possibility of lepton flavor violating (LFV) decays of the Higgs boson is a particularly compelling area of investigation. Imagine the Higgs boson, the very particle that gives mass, undergoing a decay where it transforms into a particle of one lepton flavor and its antiparticle of another. This would be a direct violation of the Standard Model’s predictions and a smoking gun for new physics. Such an observation would necessitate a radical rethinking of our fundamental understanding of particles and forces.
A recent theoretical exploration, published in the prestigious European Physical Journal C, delves into precisely this scenario by examining LFV decays of the Higgs boson within a specific theoretical framework known as the NB-LSSM. This model is an extension of the Minimal Supersymmetric Standard Model (MSSM), which itself is a popular candidate for physics beyond the Standard Model, incorporating a symmetry called supersymmetry. The NB-LSSM introduces additional particles and interactions, offering new pathways for phenomena not seen in the Standard Model.
The NB-LSSM hypothesizes a rich spectrum of new particles, including additional Higgs bosons and superpartners for the known particles. This intricate web of new constituents provides fertile ground for LFV processes. The authors of this study meticulously analyze how the Higgs boson could decay into lepton pairs of different flavors, such as a Higgs decaying into an electron and a muon, or into a muon and a tau. These are precisely the kinds of rare events that future experiments are designed to detect.
The theoretical calculations presented in the paper are complex, involving quantum field theory and intricate mathematical formalisms. The researchers employ sophisticated tools to estimate the probabilities, or branching ratios, of these hypothetical LFV Higgs decays. These probabilities are expected to be extremely small, making their detection a formidable experimental challenge. However, even minuscule signals can be significant in the realm of high-energy physics, as they point towards profound underlying phenomena.
One of the key aspects of the NB-LSSM is its introduction of additional scalar bosons, which are particles with zero intrinsic angular momentum, similar to the Higgs boson. These new scalars can mediate interactions between different lepton flavors. If these mediating particles are sufficiently light and interact strongly enough, they can significantly enhance the rates of LFV Higgs decays, making them potentially observable at the LHC or future colliders.
The study specifically focuses on the decay of the Standard Model Higgs boson into a pair of leptons from different generations, for instance, a Higgs decaying into an electron and a muon ($\text{H} \rightarrow \text{e}\mu$). The branching ratio, a measure of the probability of this specific decay occurring relative to all other possible Higgs decays, is calculated under various parameter choices within the NB-LSSM. The results indicate that these branching ratios, while small, can reach values that might be within the reach of next-generation experiments.
Furthermore, the research explores other LFV Higgs decay channels, such as those involving tau leptons. The tau lepton is the heaviest of the charged leptons and decays much more rapidly than electrons or muons. Detecting a Higgs decay into a tau and another lepton, like a Higgs decaying into a tau and an electron ($\text{H} \rightarrow \tau\text{e}$), would also be a powerful indicator of new physics. The NB-LSSM provides a framework where such decays could occur.
The implications of observing LFV Higgs decays would be revolutionary. It would unequivocally demonstrate that lepton flavor is not an absolute conservation law, as understood in the Standard Model. This would provide strong evidence for the existence of new particles and forces beyond our current Standard Model framework. The specific pattern of LFV decays observed could then be used to constrain the parameters of extension theories like the NB-LSSM, helping physicists to pinpoint the nature of this new physics.
The NB-LSSM, with its rich particle content, offers a compelling explanation for why neutrino masses are so small, a phenomenon that the Standard Model cannot easily accommodate. The interactions of neutrinos with the hypothetical heavy particles in the NB-LSSM can naturally generate the tiny masses observed for neutrinos. This ability to explain multiple “hints” of new physics makes such extended theories particularly attractive to the particle physics community.
The paper also discusses the potential of future colliders, like the proposed Future Circular Collider (FCC) or the Super Charm-Tau Factory, to search for these rare Higgs decays. These accelerators are being designed with unprecedented energy and precision, aiming to explore the energy frontier and discover new particles. The sensitivity of these future machines could be sufficient to either discover LFV Higgs decays or set stringent limits on their occurrence, further guiding theoretical investigations.
The beauty of theoretical physics lies in its ability to predict phenomena that can then be tested by experiment. The NB-LSSM, as explored in this research, provides a concrete theoretical scaffold for LFV Higgs decays. The very act of calculating these decay rates and comparing them to potential experimental reach is a vital step in the ongoing quest to understand the universe at its most fundamental level.
In conclusion, the quest to understand the universe’s fundamental building blocks is an ongoing narrative. The exploration of lepton flavor violating decays of the Higgs boson within theoretical frameworks like the NB-LSSM represents a cutting-edge frontier in this pursuit. The potential discovery of such phenomena at future colliders would not just be another scientific achievement; it would herald a new era in particle physics, fundamentally reshaping our understanding of the cosmos and our place within it. The subtle whispers of new physics are becoming louder, and the Higgs boson may very well be the messenger we need.
Subject of Research: Lepton flavor violating (LFV) decays of the Higgs boson.
Article Title: Lepton flavor violating decays of Higgs boson in the NB-LSSM.
Article References: Guo, C., Dong, XX., Zhao, SM. et al. Lepton flavor violating decays of Higgs boson in the NB-LSSM. Eur. Phys. J. C 85, 1106 (2025). https://doi.org/10.1140/epjc/s10052-025-14750-5
Image Credits: AI Generated
DOI: https://doi.org/10.1140/epjc/s10052-025-14750-5
Keywords: Higgs boson decays, Lepton flavor violation, NB-LSSM, New physics, Particle physics, Supersymmetry.
