Subluminal pulses in the surface-scintillator detectors of AugerPrime

In extensive air showers, the signals from the electromagnetic and muonic components typically span a few microseconds in scintillation detectors. Neutrons are the only stable neutral hadrons over the timescale of air showers. They lose energy exclusively through hadronic interactions and quasi-elastic scattering, which results in their high abundance at ground level. These neutrons can produce delayed pulses in scintillation detectors, appearing up to several milliseconds after the primary shower signal. This allows us to probe hadronic interactions in the development of air showers. In this study, we characterize such subluminal pulses using the first measurements from the scintillator surface detectors of the AugerPrime upgrade of the Pierre Auger Observatory.