Prescales

Prescales are factors applied to the selection of data to reduce the rate at which events or samples are recorded. In high-energy physics experiments, prescales are used with triggers to control data flow from detectors. A prescale factor p means that only one in every p events that satisfy a given trigger is kept for further analysis. For example, a prescale of 10 records roughly 10% of such events. Prescales may be constant or vary with time, detector conditions, or instantaneous luminosity; they are often adjusted to fit bandwidth or storage limits. When multiple triggers with different prescales are used, the effective data-taking luminosity is computed by combining the prescales, and analysts must account for the varying selection efficiency.

Implementation can be deterministic or stochastic. Some systems forward only events whose run- or event-number modulo p equals a fixed value; others use a random selector to keep the first random fraction of events to reduce potential biases from time-dependent effects.

In electronics and digital systems, a prescaler is a circuit that divides the input frequency by a preset integer, producing a slower clock or event stream. This is common in timer/counter circuits and is chosen to match the needed timing resolution or data rate.

Implications for analysis include the need to record the prescale factor for each trigger path and run so that effective luminosities, event weights, and cross sections can be properly calculated. Prescales must be handled consistently across data sets and Monte Carlo simulations.