ATLAS L1 trigger
08 Aug 2017



The ATLAS Level-1 calorimeter trigger selects events based on transverse energy flow in the electromagnetic and hadronic calorimeters

ATLAS L1Calo C-side racks

​ATLAS L1Calo C-side racks as in February 2008

Eric Eisenhandler, QMUL


The RAL group are involved in the ATLAS Level-1 Calorimeter Trigger (L1Calo) project and the proposed ATLAS Global Trigger project.

Level-1 Calorimeter Trigger

The L1Calo system is a vital part of online event selection at ATLAS, providing the first level trigger decision for all calorimeter based decisions: electrons, taus, jets and missing energy.
The L1Calo project is a collaboration between UK groups from Birmingham, Cambridge, QMUL and RAL, and non-UK groups from Germany, Poland, Sweden and USA.

LHC Runs 1 and 2

The original system has three main subsystems.
The PreProcessor converts analogue calorimeter signals to digital, associates the rather broad pulses with the correct proton-proton bunch crossing, and does a final calibration in transverse energy before sending digital data streams to the two algorithmic trigger processors.
The cluster processor identifies and counts electrons, photons and taus.
The jet/energy-sum processor looks for jets and also sums missing and total transverse energy.

LHC Run 3

The Phase-I upgrade will allow the ATLAS experiment to efficiently trigger and record data at instantaneous luminosities that are up to three times that of the original LHC design while maintaining trigger thresholds close to those used in the initial run of the LHC.
The original L1Calo system will be replaced with new Level-1 calorimeter feature extraction processors to allow finer granularity data from the Liquid Argon (LAr) Calorimeter to be used to improve electron, photon, and tau selection; more sophisticated and larger-area algorithms to be used to improve jet selection; and improved pile-up corrections to be used for missing momentum reconstruction. The finer granularity data will be optically transmitted from new dedicated LAr Calorimeter hardware.

LHC Run 4

The electronics installed for Phase-I will provide input to the new Global Trigger described below.

Global Trigger

The Phase-II upgrade will allow the ATLAS experiment to efficiently trigger and record data at instantaneous luminosities up to ten times the original LHC design whilst lowering trigger thresholds back down to those used at the start of LHC Run 1.
The ATLAS Global Trigger system proposed for LHC Run 4 builds on the information from the calorimeter and muon trigger systems, along with full granularity information from the calorimeters, to provide refined event selection in the face of intense HL-LHC backgrounds.
The Global Trigger project is a collaboration between the same UK groups as for L1Calo and groups from Argentina, Germany, Holland, Poland and USA.

RAL responsibilities

We have played and continue to play a pivotal role in the L1Calo project, from the original Run 1 system through to the proposed upgrades for Run 4, and are playing a lead role in the proposed Global Trigger project for Run 4.

LHC Run 1

We bore sole responsibility for all the 'common' (used in multiple subsystems) modules in the original L1Calo trigger system, namely the Common Merger Module (CMM), Timing Control Module, VME Mount Module, and the ReadOut Driver (ROD).
We designed and provided implementations for the module services controlling the full system and all the readout system, along with most of the firmware running on the merger module.
We hosted the overall international integration centre for the original L1Calo trigger system and then led the commissioning at CERN and provided oversight of the running trigger system throughout Run 1.

LHC Run 2 (ATLAS Phase-0 upgrade)

For Run 2 the original RAL CMM was replaced with the CMX module from Michigan State University to allow transmission of trigger objects to the new L1Topo module from Mainz, and the original ASIC digitising modules on the Heidelberg PreProcessor system were upgraded to use FPGAs.
We updated the ROD firmware to match the changes in data coming from the CMX and the upgraded PreProcessor system, significantly extending the options for reading out the latter.
Again we led the recommissioning at CERN and now provide oversight of the running trigger system.

LHC Run 3 (ATLAS Phase-I upgrade)

For Run 3 we are responsible, with Birmingham, for the eFEX modules which will identify electron, photon and tau candidates using the high granularity 'supercell' information from LAr.
In addition we have defined the L1Calo standard ATCA backplane usage and communication protocols, along with a custom board shape to better address the demanding data I/O requirements of the trigger, with input of around 2 Tb/s per module and we are defining the data format for readout of the eFEX into FELIX.
We are coordinating the Surface Test Facility at CERN which will be used for integration and commissioning of the upgraded L1Calo system.
We provide the international Project Engineer for all the Phase-I L1Calo upgrades.

LHC Run 4 (ATLAS Phase-II upgrade)

Having played an extensive part in defining the ATLAS Phase-II trigger architecture we are now leading the Level-0 Calorimeter Trigger subproject and the Global Trigger subproject.

Management responsibilities

Members of the RAL group have held major management responsibilities within ATLAS Trigger:
L1Calo Project Leader
L1Calo Deputy Project Leader
L1Calo Run Coordinator
L1Calo Project Engineer
L1Calo Phase-I Engineering Coordinator
L1Calo UK Phase-I Project Engineer
Phase-I TDAQ IDR editors
Phase-I TDAQ TDR editors
Phase-II Global Trigger co-coordinator
Phase-II Level-0 Calorimeter Trigger coordinator
Phase-II TDAQ IDR editors
TDAQ Upgrade Coordinator
TDIB Chair
Contact: Sankey, David (STFC,RAL,PPD)