Chiral magnetic materials can host skyrmions and hence are of interest for numerous spintronic applications including low-power storage. To reach this goal, room temperature operation and a thin film form are needed. We have studied the chiral magnetism in a β-Mn structured CoZn sputtered film, with a Curie temperature of ≈ 130 °C, by means of polarized neutron reflectometry (PNR) in order to reveal its magnetic field-temperature phase diagram. In bulk this material has a helimagnetic ground state. Here we use PNR data to show that the helimagnetism survives in a CoZn film only 100 nm thick, roughly half a helical period. Given the subtlety of differentiating the helical state from a uniform state with reduced interfacial magnetism, we have developed and employed a Bayesian approach that uses model selection to analyze our PNR data and to map out the field-temperature phase diagram of our thin film.