[Paper Review] Performance of the ATLAS Minimum Bias Trigger in pp collisions at the LHC

L. Tompkins|arXiv (Cornell University)|Sep 30, 2010

Particle Detector Development and Performance1 references3 citations

TL;DR

This paper evaluates the performance of the ATLAS Minimum Bias Trigger system in proton-proton collisions at 7 TeV, focusing on two complementary triggers: a scintillator-based system (MBTS) in the forward regions and a tracking-based trigger (mbSpTrk) using the Pixel and SCT detectors. The MBTS trigger achieves over 95% efficiency for events with more than 10 tracks at pT > 100 MeV, demonstrating high inclusiveness and low model dependence, enabling precise measurements of charged particle multiplicities in soft QCD interactions.

ABSTRACT

The early physics program at the ATLAS experiment includes measuring the basic properties of proton proton collisions, such as charged particle multiplicities, in order to constrain phenomenological models of soft interactions in the LHC energy regime. An inclusive and well understood trigger is crucial to minimize any possible bias in the event selection. The ATLAS experiment uses two complementary types of minimum bias triggers. A scintillator trigger sensitive to the forward regions of 2.1

Motivation & Objective

• To ensure minimal bias in early LHC physics measurements by implementing a highly efficient, inclusive minimum bias trigger system.
• To characterize the performance of the MBTS scintillator trigger in the forward regions (2.1 < |η| < 3.8) for detecting minimum bias events.
• To validate the efficiency and response uniformity of the MBTS counters using data and Monte Carlo simulations.
• To compare the performance of the MBTS trigger with a complementary tracking-based trigger (mbSpTrk) using inner detector hits.
• To provide a robust, low-bias data collection framework for measuring charged particle multiplicities in p-p collisions at high energies.

Proposed method

• The MBTS trigger uses 16 scintillator counters (2 rings in η, 8 in φ) at |η| ≈ 3.6, read out via wavelength-shifting fibers and thresholded at 0.15 pC to distinguish signal from noise.
• Level 1 triggers use leading-edge discriminators to register hits; the CTP sets global trigger bits (MBTS_X, MBTS_X_X) based on minimum counter thresholds per side.
• The mbSpTrk trigger uses random sampling of colliding bunches at Level 1, with Level 2 requiring at least 4 pixel and 4 SCT clusters from hits in the inner tracking detectors.
• Efficiency is measured using mbSpTrk as a reference trigger, with offline track reconstruction used to define the true event multiplicity.
• Systematic uncertainties are assessed via Monte Carlo simulations, particularly for biases introduced by using mbSpTrk as a reference.
• Counter-to-counter variations in MBTS efficiency are studied by comparing data and Monte Carlo, with edge effects and operating point differences identified as key sources of discrepancy.

Experimental results

Research questions

• RQ1 What is the efficiency of the MBTS trigger for events with varying numbers of charged particles (pT > 100 MeV, |η| < 2.5)?
• RQ2 How do counter-to-counter variations in MBTS efficiency affect the overall trigger performance?
• RQ3 To what extent does the mbSpTrk trigger serve as a reliable reference for measuring MBTS efficiency?
• RQ4 How do beam backgrounds and multiple interaction rates impact trigger efficiency measurements?
• RQ5 How well does the Monte Carlo simulation reproduce the data response of the MBTS counters, particularly at edges and in regions of low η?

Key findings

• The MBTS_1 trigger achieves over 95% efficiency for events with more than 10 tracks (pT > 100 MeV, |η| < 2.5), demonstrating high inclusiveness.
• The MBTS_2 trigger, which requires at least two counters over threshold per side, maintains high efficiency (nearly matching MBTS_1) while being less sensitive to beam-related backgrounds.
• The requirement of coincidence (e.g., MBTS_1_1 and MBTS_4_4) significantly reduces efficiency for low-multiplicity events, as expected.
• Data show counter-to-counter efficiency variations of several percent, attributed to differences in operating points and reduced light collection at counter edges—discrepancies not fully captured in Monte Carlo simulations.
• The systematic bias introduced by using mbSpTrk as a reference trigger is negligible, as confirmed by Monte Carlo studies.
• The overall performance of both triggers enables ATLAS to collect inclusive minimum bias data with minimal model dependence, crucial for studying soft QCD interactions at the LHC.