Abstract: We explore long-lived particle (LLP) searches using non-pointing photons at the LHC as a probe for sterile-to-sterile and active-to-sterile transition magnetic dipole moments of sterile neutrinos. We consider heavy sterile neutrinos with masses ranging from a few GeV to several hundreds of GeV. We discuss transition magnetic dipole moments using the Standard Model effective field theory and low-energy effective field theory extended by sterile neutrinos (NRSMEFT and NRLEFT) and also provide a simplified UV-complete model example. LLP searches at the LHC using non-pointing photons will probe sterile-to-sterile dipole moments two orders of magnitude below the current best constraints from LEP, while an unprecedented sensitivity to sterile neutrino mass of about 700 GeV is expected for active-to-sterile dipole moments. For the UV model example with one-loop transition magnetic moments, the searches for charged lepton flavour violating processes in synergy with LLP searches at the LHC can probe new physics at several TeV mass scales and provide valuable insights into the lepton flavour structure of new physics couplings.

Abstract: We present and analyze two minimal extensions of the Standard Model featuring a spontaneously broken global, chiral, and anomaly-free U(1)D symmetry. This breaking generates naturally small Dirac neutrino masses via a seesaw mechanism and yields a physical massless Goldstone boson, the Diracon. Although both models share the same particle content and scalar potential, their distinct symmetry breaking pattern leads to remarkably different phenomenological and cosmological signatures. In the first model, the Diracon couples weakly to charged leptons but right-handed neutrinos can be efficiently produced in the early Universe, resulting in stringent constraints from the effective number of relativistic species, triangle Neff. Conversely, in the second one, right-handed neutrino production is suppressed, and flavour-violating processes such as μ-> eD\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \mu \to e\mathcal{D} $$\end{document} provide the most promising probes. These simple but elegant models showcase the complementarity between cosmological observations and low-energy flavour experiments in the search for physics beyond the Standard Model.

Abstract: We investigate the dimuon Higgs decay h -> mu+mu- in the context of an extended Scotogenic model. The model introduces a singlet complex scalar in addition to the standard Scotogenic scalar doublet and singlet fermions, charged under a dark & Zopf;2 symmetry. By exploring the one-loop contributions, we show that the model allows for sizable deviations in the Higgs dimuon decay rate, quantified by the quotient R μμ= Br(h -> mu+mu-)/Br(h -> mu+mu-)SM. Crucially, these deviations comply with experimental limits, including those on Br(mu+ -> e+gamma) and Br(h -> gamma gamma). Such deviations can be tested and constrained by future precision measurements at the LHC.

Abstract: The particle spectrum of all Majorana neutrino mass models with spontaneous violation of global lepton number include a Goldstone boson, the so-called majoron. The presence of this massless pseudoscalar changes the phenomenology dramatically. In this work we derive general analytical expressions for the 1-loop coupling of the majoron to charged leptons. These can be applied to any model featuring a majoron that have a clear hierarchy of energy scales, required for an expansion in powers of the low-energy scale to be valid. We show how to use our general results by applying them to some example models, finding full agreement with previous results in several popular scenarios and deriving novel ones in other setups.