Abstract

During Run-2 the LHC is providing proton–proton collisions to the ATLAS experiment with high luminosity (up to 2.1•1034 cm−2s−1), placing stringent operational and physical requirements on the ATLAS trigger system in order to reduce the 40 MHz collision rate to a manageable event storage rate of ∼1 kHz, while not rejecting interesting physics events. The first level (Level-1) trigger is the first rate-reducing step in the ATLAS trigger system with a maximum output rate of 100 kHz and decision latency of less than 2.5 µs. An important role is played by its newly commissioned component: the L1 Topological Processor (L1Topo). This innovative system consists of two blades designed in AdvancedTCA form factor, including four individual state-of-the-art processors, and providing high input bandwidth and low latency data processing. Up to 128 topological trigger algorithms can be implemented to select interesting events by applying kinematic and angular requirements on electromagnetic clusters, jets, muons and total energy. This results in a significantly improved background event rejection and improved acceptance of physics signal events, despite the increasing luminosity. This is becoming more and more important for analyses making use of low p T objects, like the Heavy Flavour and Higgs physics programme. An overview of the L1Topo architecture, simulation and performance results during Run-2 is presented.

Highlights

  • ATLAS [1] is one of the main experiments at the Large Hadron Collider (LHC) [2]

  • The first level (Level-1) is a hardware-based trigger which uses a subset of the detector information to reduce the rate of 40 MHz to less than 100 kHz, mostly due to the detector readout capabilities. This is followed by a software-based high level trigger (HLT) which reduces the event rate to around 1 kHz on average

  • New electronics modules were introduced in the real-time data processing path: the Level-1 Topological Processor (L1Topo)

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Summary

INTRODUCTION

ATLAS [1] is one of the main experiments at the Large Hadron Collider (LHC) [2]. The LHC provides proton–proton collisions at a rate of 40 MHz. With the inclusion of L1Topo, for the first time at this stage, it is possible to transfer and analyse the angular and energy information of each trigger object in the event in one system and compute topological angular and kinematic selections providing a largely improved background rejection with minimal or no signal loss. Different categories of topological selections have been implemented in L1Topo according to the needs of the physics analyses and to help the commissioning of new systems This includes angular selections like requirements on the azimuthal angle (∆φ), pseudorapidity (∆η), radial separation (∆R), box acceptance cuts, and energy threshold of objects inside a cone; different physics mass selections like invariant mass, transverse mass and effective mass; and event selections like event hardness (sum of all jets transverse momenta in the event) and corrections to the missing transverse momentum (ETmiss) via look-up tables. L1Topo offers the flexibility to use central and/or forward objects, to combine calorimeter and muon information for exotic signatures such

The Level-1 Topological Processor during Run-2
THE L1TOPO HARDWARE
VALIDATION
PHYSICS PERFORMANCE
Findings
CONCLUSION
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