The CMS trigger system

TL;DR

The CMS trigger system addresses the challenge of harvesting rare, high-value physics events from an overwhelming collision rate by employing a two-tier architecture: a fast hardware-based L1 trigger and a flexible software-based HLT. The L1 system builds object candidates from calorimeter and muon subsystems, with regional and global calorimeter triggers, plus multiple muon processing paths, all feeding a programmable GT that shapes the L1 output. The HLT refines event selection using offline-like reconstruction (tracking, PF jets, MET, b-tagging) within CPU-budgeted paths, enabling complex signatures such as Higgs decays, top quark processes, SUSY searches, and exotic scenarios. Across Run 1, the trigger menu evolved to cope with increasing luminosity and pileup, incorporating PF-based jet and MET triggers, robust spike suppression, ECAL transparency corrections, and heavy-ion specializations, yielding high data-quality efficiency and enabling a rich physics program. The work demonstrates the practical viability and scalability of a two-level trigger system for a large, multi-purpose detector in a high-rate environment, with extensive performance characterizations and data-driven validations. The trigger system thus provided essential infrastructure for CMS’s Run 1 discoveries and measurements, shaping trigger strategies for future LHC runs.

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.

Figures (76)

• Figure 1: Integrated (top) and peak (bottom) proton-proton luminosities as a function of time for calendar years 2010--2012. The 2010 integrated (instantaneous) luminosity is multiplied by a factor of 100 (10). In the lower plot, $1\text{\,Hz/nb}\xspace$ corresponds to $10^{33}\,\text{cm}^\text{$-$2}\,\text{s}^\text{$-$1}\xspace$.
• Figure 2: Overview of the CMS L1 trigger system. Data from the forward (HF) and barrel (HCAL) hadronic calorimeters, and from the electromagnetic calorimeter (ECAL), are processed first regionally (RCT) and then globally (GCT). Energy deposits (hits) from the resistive-plate chambers (RPC), cathode strip chambers (CSC), and drift tubes (DT) are processed either via a pattern comparator or via a system of segment- and track-finders and sent onwards to a global muon trigger (GMT). The information from the GCT and GMT is combined in a global trigger (GT), which makes the final trigger decision. This decision is sent to the tracker (TRK), ECAL, HCAL or muon systems (MU) via the trigger, timing and control (TTC) system. The data acquisition system (DAQ) reads data from various subsystems for offline storage. MIP stands for minimum-ionizing particle.
• Figure 3: Block diagram of the regional calorimeter trigger (RCT) system showing the data flow through the different cards in a RCT crate. At the top is the input from the calorimeters; at the bottom is the data transmitted to the global calorimeter trigger (GCT). Data exchanged on the backplane is shown as arrows between cards. Data from neighboring towers come via the backplane, but may come over cables from adjoining crates.
• Figure 4: A schematic of the global calorimeter trigger (GCT) system, showing the data flow through the various component cards.
• Figure 5: Neutral pion (left) and $\eta$ (right) invariant mass peaks reconstructed in the barrel with 2012 data. The spectra are fitted with a combination of a double (single) Gaussian for the signal and a 4th (2nd) order polynomial for the background. The entire 2012 data set is used, using special online $\pi^0/\eta$ calibration streams. The sample size is determined by the rate of this calibration stream. Signal over background (S/B) and the fitted resolution are indicated on the plots. The fitted peak positions are not exactly at the nominal $\pi^0/\eta$ mass values mainly due to the effects of selective readout and leakage outside the $3{\times}3$ clusters used in the mass reconstruction; however, the absolute mass values are not used in the inter-calibration.