leptonnumrerelated

Lepton-number related describes topics tied to lepton number, a quantum number used to track leptons and antiparticles in particle physics. In commonly used conventions, leptons carry L = +1 and antileptons carry L = −1, with the total lepton number in a process equal to the sum of the individual contributions. Lepton-number conservation is a guiding principle in many weak interactions.

In the Standard Model, lepton number is an accidental global symmetry of renormalizable interactions and is conserved in perturbative processes. Nonperturbative electroweak effects, such as sphalerons, can violate lepton and baryon numbers separately while preserving B − L. This means that lepton number can change under high-energy conditions, notably in the early universe. Neutrino oscillations demonstrate flavor change, but do not necessarily require total lepton-number violation if neutrinos are Dirac particles.

Beyond the Standard Model, lepton number can be violated if neutrinos are Majorana particles, via mass terms that relate a neutrino to its antiparticle and change L by two units. The most consequential experimental target is neutrinoless double beta decay (0νββ), whose discovery would prove lepton-number violation and imply Majorana neutrinos. The seesaw mechanism is a leading theoretical framework that generates small neutrino masses and introduces heavy Majorana states, potentially enhancing observable lepton-number-violating effects.

Lepton-number violation has cosmological relevance through leptogenesis, a scenario in which lepton-number-violating decays create a lepton asymmetry that sphalerons can partly convert into the baryon asymmetry of the universe. Experimental searches for lepton-number-violating signals continue in 0νββ experiments and collider studies, while neutrino experiments place increasingly stringent limits on such processes.