Light sterile neutrinos and lepton-number-violating kaon decays in effective field theory

Observation of lepton number violation would represent a groundbreaking discovery with profound consequences for fundamental physics and as such, it has motivated an extensive experimental program searching for neutrinoless double beta decay. However, the violation of lepton number can be also tested by a variety of other observables. We focus on the possibilities of probing this fundamental symmetry within the framework of the Standard Model Effective Field Theory (SMEFT) beyond the minimal dimension-5. Specifically, we study the bounds on ∆L = 2 dimension-7 effective operators beyond the electron flavor imposed by all relevant low-energy observables and confront them with derived high-energy collider limits. We also discuss how the synergy of the analyzed multi-frontier observables can play a crucial role in distinguishing among different dimension-7 SMEFT operators.

Section: Lepton Number Violatingmentioning

confidence: 97%

Probing lepton number violation: a comprehensive survey of dimension-7 SMEFT

Fridell,

Gráf,

Harz

et al. 2024

Long-lived neutral fermions at the DUNE near detector

Günther,

de Vries,

Dreiner

et al. 2024

At the Deep Underground Neutrino Experiment (DUNE), a proton beam hits a fixed target leading to large production rates of mesons. These mesons can decay and potentially provide a source of long-lived neutral fermions. Examples of such long-lived fermions are heavy neutral leptons which can mix with the standard-model active neutrinos, and the bino-like lightest neutralino in R-parity-violating supersymmetry. We show that the Standard Model Effective Field Theory extended with right-handed singlet neutrinos can simultaneously describe heavy neutral leptons and bino-like neutralinos in a unified manner. We use the effective-field-theory framework to determine the sensitivity reach of the DUNE near detector in probing various scenarios of long-lived neutral fermions.

Bounding the Dimension-5 Seesaw Portal with non-pointing photon searches

Duarte,

Jones-Pérez,

Manrique-Chavil

2024

The addition of d = 5 operators to the Seesaw model leads to the Dimension-5 Seesaw Portal. Here, two new operators provide interactions for the heavy sterile neutrinos. In particular, the Higgs boson can have a large branching ratio into two heavy neutrinos, meaning that these states can be searched for at the LHC. Moreover, the heavy neutrinos can now decay dominantly into light neutrinos and photons. If the heavy neutrinos are long-lived, then searches for delayed, non-pointing photons can constrain the model. In this work, we carry out a detailed recast of an ATLAS search for such displaced photons, triggered by a charged lepton produced in association to the Higgs, placing bounds on the branching ratio for Higgs decay into two heavy neutrinos as low as 2%.