Proton decay and light sterile neutrinos

Within the standard model, non-renormalizable operators at dimension six ($d=6$) violate baryon and lepton number by one unit and thus lead to proton decay. Here, we point out that the proton decay mode with a charged pion and missing energy can be a characteristic signature of $d=6$ operators containing a light sterile neutrino, if it is not accompanied by the standard $\pi^0e^+$ final state. We discuss this effect first at the level of effective operators and then provide a concrete model with new physics at the TeV scale, in which the lightness of the active neutrinos and the stability of the proton are related.Comment: 7 pages, 2 figures, published versio

Heavy neutral fermions at the high-luminosity LHC

Long-lived light particles (LLLPs) appear in many extensions of the standard model. LLLPs are usually motivated by the observed small neutrino masses, by dark matter or both. Typical examples for fermionic LLLPs (a.k.a. heavy neutral fermions, HNFs) are sterile neutrinos or the lightest neutralino in R-parity violating supersymmetry. The high luminosity LHC is expected to deliver up to 3/ab of data. Searches for LLLPs in dedicated experiments at the LHC could then probe the parameter space of LLLP models with unprecedented sensitivity. Here, we compare the prospects of several recent experimental proposals, FASER, CODEX-b and MATHUSLA, to search for HNFs and discuss their relative merits.Comment: 21 pages, 6 figures; v2:references and minor comments added, plots update

Neutrinoless double beta decay and QCD running at low energy scales

There is a common belief that the main uncertainties in the theoretical analysis of neutrinoless double beta ($0\nu\beta\beta$) decay originate from the nuclear matrix elements. Here, we uncover another previously overlooked source of potentially large uncertainties stemming from non-perturbative QCD effects. Recently perturbative QCD corrections have been calculated for all dimension 6 and 9 effective operators describing $0\nu\beta\beta$-decay and their importance for a reliable treatment of $0\nu\beta\beta$-decay has been demonstrated. However, these perturbative results are valid at energy scales above $\sim 1$ GeV, while the typical $0\nu\beta\beta$-scale is about $\sim 100$ MeV. In view of this fact we examine the possibility of extrapolating the perturbative results towards sub-GeV non-perturbative scales on the basis of the QCD coupling constant "freezing" behavior using Background Perturbation Theory. Our analysis suggests that such an infrared extrapolation does modify the perturbative results for both short-range and long-range mechanisms of $0\nu\beta\beta$-decay in general only moderately. We also discuss that the tensor$\otimes$tensor effective operator can not appear alone in the low-energy limit of any renormalizable high-scale model and then demonstrate that all five linearly independent combinations of the scalar and tensor operators, that can appear in renormalizable models, are infrared stable.Comment: 7 pages, 4 figure

Minimal 3-loop neutrino mass models and charged lepton flavor violation

We study charged lepton flavor violation for the three most popular 3-loop Majorana neutrino mass models. We call these models "minimal" since their particle content correspond to the minimal sets for which genuine 3-loop models can be constructed. In all the three minimal models the neutrino mass matrix is proportional to some powers of Standard Model lepton masses, providing additional suppression factors on top of the expected loop suppression. To correctly explain neutrino masses, therefore large Yukawa couplings are needed in these models. We calculate charged lepton flavor violating observables and find that the three minimal models survive the current constraints only in very narrow regions of their parameter spaces.Comment: 32+7 pages, 23 figures, 7 table