Belle II could probe new physics via
A new study proposes using the Belle II experiment to search for long-lived heavy neutral leptons, predicted by extensions of the Standard Model, by
A theoretical study suggests the Belle II experiment could be uniquely sensitive to new physics involving heavy neutral leptons. The search would look for displaced vertices where these particles decay into pairs of electrons or muons.
The existence of right-handed neutrinos is a leading explanation for the small masses of ordinary neutrinos. Their corresponding mass eigenstates are known as heavy neutral leptons, or HNLs. Researchers have now analyzed the potential to discover GeV-scale HNLs at the Belle II electron-positron collider, using a framework called the Standard Model effective field theory with added right-handed neutrinos (νSMEFT).
The search for a displaced vertex
The proposed search strategy focuses on HNLs that are long-lived. These particles would travel a measurable distance inside the Belle II detector before decaying. Their leptonic decays would produce a distinct signature: a displaced vertex from which two charged leptons, like an electron-positron or muon-antimuon pair, emerge. This signature helps distinguish a potential signal from ordinary Standard Model background processes.
Probing effective field theory operators
The analysis estimates Belle II's sensitivity to the energy scale of new physics associated with various νSMEFT operators. The study considered three broad classes of operators: four-fermion operators, Higgs-current operators, and dipole operators. For most of these operators, the authors find that the displaced dilepton search could probe large regions of parameter space currently unexplored by other experimental searches.
Advantages over other experiments
The Belle II experiment offers a clean environment with precise vertex detection capabilities, making it well-suited for this type of search. The study accounts for realistic detection efficiencies and estimated backgrounds at the facility. The findings indicate that Belle II has the potential to constrain new physics scales in a way that complements searches at other facilities, such as those at the Large Hadron Collider or dedicated beam-dump experiments.
The research was detailed in a paper submitted to the arXiv preprint server on September 3, 2026. The authors conclude that a dedicated search for displaced dilepton vertices at Belle II represents a promising avenue to test the νSMEFT framework and potentially uncover evidence for heavy neutral leptons.
