Speaker
Description
Symmetry-protected low-scale type-I seesaw models can explain the observed pattern of neutrino masses while predicting collider-accessible heavy neutral leptons (HNLs) that appear as pseudo-Dirac pairs. In such scenarios, lepton-number (LN) violation is strongly suppressed at the amplitude level by the approximate LN-like symmetry. However, a realistic treatment of HNL production, propagation, and decay, including finite lifetimes, neutrino-antineutrino oscillations, and decoherence effects, can substantially modify this expectation. These effects govern the transition between LN-conserving and LN-violating signatures and introduce a critical mass-splitting scale above which decoherence enhances observable LN violation. While oscillation effects are especially relevant for long-lived HNLs, they also impact prompt collider searches. We compare the sensitivity of LN-violating and LN-blind search channels at hadron colliders, identify the parameter regions in which LN-violating searches remain competitive, and emphasize the need to reinterpret existing searches beyond the Dirac and Majorana limits.
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