The ATLAS trigger menu for early data-taking

TL;DR

This paper describes the development and planning of the ATLAS trigger system for early LHC data-taking, detailing how it adapts to changing beam conditions to efficiently select physics events.

Contribution

It presents the design and expected performance of the ATLAS trigger menu during early data-taking, including adaptation strategies for varying luminosities.

Findings

Trigger system designed to reduce event rate from 40 MHz to 200 Hz.

Expected trigger rates and physics performance simulated for early and nominal conditions.

Trigger menu adaptable to changing beam conditions to optimize physics data collection.

Abstract

The ATLAS trigger system is based on three levels of event selection that select the physics of interest from an initial bunch-crossing rate of 40 MHz. During nominal LHC operations at a luminosity of 10^34 cm^-2 s^-1, decisions must be taken every 25 ns with each bunch crossing containing about 23 interactions. The selections in the three trigger levels must provide sufficient rejection to reduce the rate down to 200 Hz, compatible with the offline computing power and storage capacity. The LHC is expected to begin operations in summer 2008 with a peak luminosity of 10^31 cm^-2 s^-1 with far fewer bunches than nominal running, but quickly ramp up to higher luminosities. Hence, we need to deploy trigger selections that can adapt to the changing beam conditions preserving the interesting physics and detector requirements that may vary with these conditions. We present the status of the preparation of the trigger menu for the early data-taking showing how we plan to deploy the trigger system from the first collision to the nominal luminosity. We also show expected rates and physics performance obtained from simulated data.