Estimating the efficiency turn-on curve for a constant-threshold trigger without a calibration dataset

Many particle physics experiments use constant threshold triggers, where the trigger threshold is in an online variable that can be calculated quickly by the trigger module. Offline data analysis then calculates a more precise offline variable for the same quantity, for example the event energy. The efficiency curve is a step function in the online variable, but not in the offline variable. One typically obtains the shape of the efficiency curve in the offline variable by way of a calibration dataset, where the true rate of events at each value of the offline variable is measured once and compared to the rate observed in the physics dataset. For triggers with a fixed threshold condition, it is sometimes possible to obtain the trigger efficiency curve without use of a calibration dataset. This is useful to verify stability of a calibration over time when calibration data cannot be taken often enough. It also makes it possible to use datasets for which no calibration is available. This paper describes the method and the conditions that must be met for it to be applicable.