High Level Trigger Selection Research Articles

ATLAS High Level Trigger within the multi-threaded software framework AthenaMT

Athena is the software framework used in the ATLAS experiment throughout the data processing path, from the software trigger system through offline event reconstruction to physics analysis. The shift from high-power single-core CPUs to multi-core systems in the computing market means that the throughput capabilities of the framework have become limited by the available memory per process. For Run 2 of the Large Hadron Collider (LHC), ATLAS has exploited a multi-process forking approach with the copy-on-write mechanism to reduce the memory use. To better match the increasing CPU core count and, therefore, the decreasing available memory per core, a multi-threaded framework, AthenaMT, has been designed and is now being implemented. The ATLAS High Level Trigger (HLT) system has been remodelled to fit the new framework and to rely on common solutions between online and offline software to a greater extent than in Run 2. We present the implementation of the new HLT system within the AthenaMT framework, which is going to be used in ATLAS data-taking during Run 3 (2021 onwards) of the LHC. We also report on interfacing the new framework to the current ATLAS Trigger and Data Acquisition (TDAQ) system, which aims to bring increased flexibility whilst needing minimal modifications to the current system. In addition, we show some details of architectural choices which were made to run the HLT selection inside the ATLAS online dataflow, such as the handling of the event loop, returning of the trigger decision and handling of errors.

Electrons and photons at High Level Trigger in CMS for Run II

The CMS experiment has been designed with a 2-level trigger system. The first level is implemented using custom-designed electronics. The second level is the so-called High Level Trigger (HLT), a streamlined version of the CMS offline reconstruction software running on a computer farm. For Run II of the Large Hadron Collider, the increase in center-of-mass energy and luminosity will raise the event rate to a level challenging for the HLT algorithms. New approaches have been studied to keep the HLT output rate manageable while maintaining thresholds low enough to cover physics analyses. The strategy mainly relies on porting online the ingredients that have been successfully applied in the offline reconstruction, thus allowing to move HLT selection closer to offline cuts. Improvements in HLT electron and photon definitions will be presented, focusing in particular on: updated clustering algorithm and the energy calibration procedure, new Particle-Flow-based isolation approach and pileup mitigation techniques, and the electron-dedicated track fitting algorithm based on Gaussian Sum Filter.

The ATLAS online High Level Trigger framework: Experience reusing offline software components in the ATLAS trigger

Event selection in the ATLAS High Level Trigger is accomplished to a large extent by reusing software components and event selection algorithms developed and tested in an offline environment. Many of these offline software modules are not specifically designed to run in a heavily multi-threaded online data flow environment. The ATLAS High Level Trigger (HLT) framework based on the GAUDI and ATLAS ATHENA frameworks, forms the interface layer, which allows the execution of the HLT selection and monitoring code within the online run control and data flow software. While such an approach provides a unified environment for trigger event selection across all of ATLAS, it also poses strict requirements on the reused software components in terms of performance, memory usage and stability. Experience of running the HLT selection software in the different environments and especially on large multi-node trigger farms has been gained in several commissioning periods using preloaded Monte Carlo events, in data taking periods with cosmic events and in a short period with proton beams from LHC. The contribution discusses the architectural aspects of the HLT framework, its performance and its software environment within the ATLAS computing, trigger and data flow projects. Emphasis is also put on the architectural implications for the software by the use of multi-core processors in the computing farms and the experiences gained with multi-threading and multi-process technologies.

Commissioning of the CMS High-Level Trigger with cosmic rays

The CMS High-Level Trigger (HLT) is responsible for ensuring that data samples with potentially interesting events are recorded with high efficiency and good quality. This paper gives an overview of the HLT and focuses on its commissioning using cosmic rays. The selection of triggers that were deployed is presented and the online grouping of triggered events into streams and primary datasets is discussed. Tools for online and offline data quality monitoring for the HLT are described, and the operational performance of the muon HLT algorithms is reviewed. The average time taken for the HLT selection and its dependence on detector and operating conditions are presented. The HLT performed reliably and helped provide a large dataset. This dataset has proven to be invaluable for understanding the performance of the trigger and the CMS experiment as a whole.

J/ψ→μ+μ− reconstruction in the CMS experiment

In this paper the J/ψ → μ+μ− reconstruction performance in the CMS experiment at the LHC is studied in detail by using Bs → J/ψ φ → μμKK events. The reconstruction efficiencies of J/ψ mesons and their decay muons are obtained as a function of the transverse momentum pT and the pseudo-rapidity η. We also study the muon trigger efficiency for this channel with the planned Level-1 trigger and High Level Trigger selection criteria. It was observed that the muon reconstruction efficiency decreases when the two decay muons have a small or large 3D angular separation, which further affects the overall J/ψ reconstruction efficiency.

Implementation and performance of the high level trigger electron and photon selection for the ATLAS experiment at the LHC

The ATLAS experiment at the Large Hadron Collider (LHC) will face the challenge of efficiently selecting interesting candidate events in pp collisions at 14 TeV center of mass energy, while rejecting the enormous number of background events, stemming from an interaction rate of up to 10/sup 9/ Hz. The First Level trigger will reduce this rate to around O(100 kHz). Subsequently, the High Level Trigger (HLT), which is comprised of the Second Level trigger and the Event Filter, will need to further reduce this rate by a factor of O(10/sup 3/). The HLT selection is software based and will be implemented on commercial CPUs, using a common framework built on the standard ATLAS object oriented software architecture. In this paper an overview of the current implementation of the selection for electrons and photons in the HLT is given. The performance of this implementation has been evaluated using Monte Carlo simulations in terms of the efficiency for the signal channels, rate expected for the selection, data access and manipulation times, and algorithm execution times. Besides the efficiency and rate estimates, some physics examples will be discussed, showing that the triggers are well adapted for the physics programme envisaged at LHC. The electron and photon trigger software is also being exercised at the ATLAS 2004 Combined Test Beam, where components from all ATLAS subdetectors are taking data together along the the H8 SPS extraction line at CERN; from these tests a validation of the selection architecture chosen in a real on-line environment is expected.

The CMS High-Level Trigger Selection

The CMS High-Level Trigger (HLT) is designed to reduce the Level-1 accept trigger rate of 100 kHz to a final output rate of 100 Hz, at which selected events are written to permanent storage. In order to satisfy the very high performance requirements of CMS, the HLT will consist of a farm of commercial processors performing the online event filtering. The flexibility provided by a fully programmable environment implies that algorithmic changes can be easily introduced to improve the selection of various physics channels, as well as to deal with unforeseen experimental conditions. The filter farm software includes all major features of the offline reconstruction code. In this report, the selection algorithms and physic performance of the CMS HLT are presented.

High Level Trigger for the [formula omitted] channel in fully hadronic decay at LHC with the CMS detector

One of the most promising channels to discover a “light” Higgs boson (mass of 120 − 130 GeV / c 2 ) with the CMS detector at LHC is the t t ¯ H channel. In this study we investigate the possibility of selecting this channel from the QCD multijets background. We present a High Level Trigger selection for the t t ¯ H decaying in fully hadronic final states. The basic idea is to use a fast b-tagging algorithm after the Level-1 trigger selection based on the number of jets and energy cuts. The main background source considered is direct QCD multijet production with pile up at low luminosity ( 2 × 10 33 cm −2 s −1 ) scenario.

The CMS trigger system

The CMS trigger system must reduce an input data rate from the LHC bunch-crossing frequency of 40 MHz to a rate which will be written to permanent storage. A detailed study has recently been made of the performance of this system. This paper presents key elements of the results obtained and gives details of a draft “trigger table” for the Level-1 Trigger and the High-Level Trigger selection at a “start-up” luminosity of 2× 1033 cm – 2s – 1. High efficiencies for most physics objects are attainable with a selection that remains inclusive and avoids detailed topological or other requirements on the event.

CMS high-level trigger selection

The CMS High-Level Trigger (HLT) is based on sets of dedicated commercial processors, whose goal is to reduce the Level-1 trigger rate of 100 kHz to ≈100 Hz, a value compatible with permanent storage of data. In this report the results from the recently published DAQ TDR are presented. The offline code with little modifications can be used in the trigger chain soon after Level 1 by making use of regional and conditional analyses. For the benchmark channels considered, the signal efficiencies and background rejection factors, obtained within ≈500 ms of a recent CPU (1 GHz processor), are competitive with full offline analyses done without any timing limitation. PACS: CMS Compact muon solenoid experiment – HLT High-Level Trigger

A readout buffer prototype for ATLAS high-level triggers

Readout buffers are critical components in the dataflow chain of the ATLAS trigger/data-acquisition system. At up to 75 kHz, after each Level-1 trigger accept signal, these devices receive and store digitized data from groups of front-end electronic channels. Several readout buffers are grouped to form a readout buffer complex that acts as a data server for the high-level trigger selection algorithms and for the final data-collection system. This paper describes a functional prototype of a readout buffer based on a custom-made PCI mezzanine card that is designed to accept input data at up to 160 MB/s, to store up to 8 MB of data, and to distribute data chunks at the desired request rate. We describe the hardware of the card that is based on an Intel 1960 processor and complex programmable logic devices. We present the integration of several of these cards in a readout buffer complex. We measure various performance figures and discuss to which extent these can fulfil ATLAS needs.