The width of the Z boson decay provides the tightest bounds on the number of types of these things, and elastic scattering among them is also mediated by the Z boson.
A gas of these non-photon things with a temperature of less than 2 kelvins is thought to permeate the universe. A hypothetical right-handed one of these things may be a significant contributor to dark matter if it is "sterile." The width of the Z boson decay provides the tightest bounds on the number of types of these things, and elastic scattering among them is also mediated by the Z boson. The Standard Model fails to account for one non-gravitational behavior of these leptons, in which their flavors vary over time; that phenomenon, characterized by the PMNS matrix, is called "oscillation." An "anti" version of one of these elementary particles is emitted in beta decay. For 10 points, name these incredibly light uncharged particles that barely interact with anything.
A gas of these non-photon things with a temperature of less than 2 kelvins is thought to permeate the universe.
neutrinos [accept electron neutrinos; accept muon neutrinos; accept tau neutrinos]