The CMS trigger system

TL;DR

This paper details the design, implementation, and performance of the CMS trigger system during LHC Run 1, which efficiently filters high-rate collision data to identify events of potential physics interest.

Contribution

It introduces a two-level trigger system with hardware and software components, optimized for high data rates and adaptable to LHC luminosity conditions.

Findings

Trigger system successfully reduced data rate from GHz to hundreds of Hz.

Adaptive thresholds maintained high efficiency across different luminosities.

System performance met the requirements for physics analyses during Run 1.

Abstract

This paper describes the CMS trigger system and its performance during Run 1 of the LHC. The trigger system consists of two levels designed to select events of potential physics interest from a GHz (MHz) interaction rate of proton-proton (heavy ion) collisions. The first level of the trigger is implemented in hardware, and selects events containing detector signals consistent with an electron, photon, muon, tau lepton, jet, or missing transverse energy. A programmable menu of up to 128 object-based algorithms is used to select events for subsequent processing. The trigger thresholds are adjusted to the LHC instantaneous luminosity during data taking in order to restrict the output rate to 100 kHz, the upper limit imposed by the CMS readout electronics. The second level, implemented in software, further refines the purity of the output stream, selecting an average rate of 400 Hz for offline event storage. The objectives, strategy and performance of the trigger system during the LHC Run 1 are described.