Search for new physics effects in νν¯γ production at a tera-Z factory

Rare decays of the Z boson provide a sensitive probe for physics beyond the standard model. This study investigates the e + e − → Z → ν ν ¯ γ process within the context of the Tera-Z programmes at future colliders such as the FCC-ee and CEPC. The SM predicts a one-loop branching ratio of 7.16 × 10 − 10 for Z → ν ν ¯ γ , a value four times smaller than the current experimental limit from the LEP. To explore this window for new physics, we parameterize anomalous Z ν ν ¯ γ interactions using an Effective Field Theory framework, considering both dimension-6 and dimension-8 operators. A detailed simulation is performed by generating signal and background events with MadGraph, modeling particle showers with Pythia, and simulating detector effects with Delphes. The analysis employs key kinematic variables—including the photon energy ( E γ ), missing transverse energy ( E ̸ T ), and the missing transverse energy significance ( S E ̸ T ) to isolate the signal. The results yield upper limits on the anomalous couplings, from which we infer branching ratios for Z → ν ν ¯ γ on the order of 10 − 9 . This represents a significant improvement of several orders of magnitude over the LEP sensitivity. Consequently, this study demonstrates the unique potential of the Tera-Z runs not only to test the SM loop-level predictions with unprecedented precision but also to tightly constrain or reveal new anomalous interactions.