Trigger Decision Research Articles

Performance of the RPC-based ALICE muon trigger system at the LHC

The forward muon spectrometer of ALICE (A Large Ion Collider Experiment) is equipped with a trigger system made of four planes of Resistive Plate Chambers (RPC), arranged in two stations with two planes each, for a total area of about 140 m2. The system provides single and di-muon triggers with suitable transverse momentum selection, optimised for the physics of quarkonia and open heavy flavour.In the first two years of data-taking at the Large Hadron Collider (2010 and 2011) the 72 RPCs were operated in highly saturated avalanche mode in both pp and Pb-Pb collisions. The integrated charge was about 1.3 mC/cm2 on average and 3.5 mC/cm2 for the most exposed detectors.This paper describes two main results. The first result is the determination of the RPC performance, with particular focus on the stability of the main detector parameters such as efficiency, dark current, and dark rate. The second result is the measurement of the muon trigger performance in Pb-Pb collisions at √sNN = 2.76 TeV, in terms of the reliability and stability of the trigger decision logic.

The ATLAS Level-1 Calorimeter Trigger: PreProcessor implementation and performance

The PreProcessor system of the ATLAS Level-1 Calorimeter Trigger (L1Calo) receives about 7200 analogue signals from the electromagnetic and hadronic components of the calorimetric detector system. Lateral division results in cells which are pre-summed to so-called Trigger Towers of size 0.1 × 0.1 along azimuth (ϕ) and pseudorapidity (η). The received calorimeter signals represent deposits of transverse energy.The system consists of 124 individual PreProcessor modules that digitise the input signals for each LHC collision, and provide energy and timing information to the digital processors of the L1Calo system, which identify physics objects forming much of the basis for the full ATLAS first level trigger decision. This paper describes the architecture of the PreProcessor, its hardware realisation, functionality, and performance.

Event-Based State Estimation with Switching Static-Gain Observers

An event-based state estimation problem is considered where the state of a dynamic system is observed from multiple distributed sensors that sporadically transmit their measurements to a remote estimator over a common bus. The common bus allows each sensor to run a copy of the remote estimator and to make the triggering decision based on this estimate: a measurement is transmitted only if its prediction by the estimator deviates by more than a tunable threshold. The event-based estimator is a switching observer that mimics a Luenberger observer with full communication of all measurements. It is proven that the difference between the event-based estimator and its full communication counterpart is bounded. The reduction of average sensor communication rates achieved by using the event-based state estimator for feedback control is demonstrated in experiments on a balancing cube.

Instrumentation of the upgraded ATLAS tracker with a double buffer front-end architecture for track triggering

The Large Hadron Collider will be upgraded to provide instantaneous luminosity L = 5 × 1034 cm−2s−1, leading to excessive rates from the ATLAS Level-1 trigger. A double buffer front-end architecture for the ATLAS tracker replacement is proposed, that will enable the use of track information in trigger decisions within 20 μs in order to reduce the high trigger rates. Analysis of ATLAS simulations have found that using track information will enable the use of single lepton triggers with transverse momentum thresholds of pT ∼ 25 GeV, which will be of great benefit to the future physics programme of ATLAS.

Use of associative memories for L1 triggering in LHC environment

The CDF Associative-Memory device (AM), proven technology developed for the Silicon-Vertex-Trigger at the CDF experiment, is one of the proposed solutions at LHC for track reconstruction at level-1 in the HL-LHC upgrade, for very high-luminosity conditions (hundreds proton-proton collisions every 25 ns, at 5 × 1034 cm−2sec−1). This luminosity requires a drastic revision of the existing trigger strategies. In the framework of the CMS experiment, one of the identified challenges for future upgrades is the capability of using already at L1 the tracker information to trigger events.This strategy requires massive computing power to minimize theonline execution time of complex tracking algorithms and the``combinatorial challenge''.The AM allows to compare the tracker information of each event topre-calculated ``expectations'' (pattern matching) in such ashort time that tracks can contribute to the trigger decision. One ofthe main challenges for the CMS tracker is the latency due to thetracker data distribution to the AM processors. A very parallelizedreadout architecture and a possible layout are discussed.

An Adaptive Handoff Triggering Mechanism for Vehicular Networks

Handoff plays an important role in vehicular networks due to high movement of vehicles. To provide seamless connectivity under Access Points (AP), this paper proposes an adaptive handoff triggering method to minimize communication time for a vehicle with an AP switch (i.e., whether and when to trigger a handoff process). In the proposed method, combined with an improved data transmission rate based trigger, handoff triggering decision is executed based on three different communication methods (called C-Dire, C-Relay and C-ALLRelay) to minimize the transmission delay when a vehicle moves from an AP to another. Transmission delay is derived through considering vehicle mobility and transmission rate diversity. The simulation results show that the proposed method is proven to be adaptive to vehicular networks.

Engineering the ATLAS TAG Browser

ELSSI is a web-based event metadata (TAG) browser and event-level selection service for ATLAS. In this paper, we describe some of the challenges encountered in the process of developing ELSSI, and the software engineering strategies adopted to address those challenges. Approaches to management of access to data, browsing, data rendering, query building, query validation, execution, connection management, and communication with auxiliary services are discussed. We also describe strategies for dealing with data that may vary over time, such as run-dependent trigger decision decoding. Along with examples, we illustrate how programming techniques in multiple languages (PHP, JAVASCRIPT, XML, AJAX, and PL/SQL) have been blended to achieve the required results. Finally, we evaluate features of the ELSSI service in terms of functionality, scalability, and performance.

The ATLAS High Level Trigger Configuration and Steering: Experience with the First 7 TeV Collision Data

In March 2010 ATLAS saw the first proton-proton collisions at a center-of-mass energy of 7 TeV. Within the year, a collision rate of nearly 10 MHz was achieved. At ATLAS, events of potential physics interest are selected by a three-level trigger system, with a final recording rate of about 200 Hz. The first level (LVL1) is implemented in customized hardware, the two levels of the high level trigger (HLT) are software triggers. For the ATLAS physics program more than 500 trigger signatures are defined. The HLT Steering is responsible for testing each signature on each LVLl-accepted event, the test outcome is recorded with the event for later analysis. The steering code also ensures the independence of each signature test and an unbiased trigger decision. To minimize data readout and execution time, cached detector data and once-calculated trigger objects are reused to form the decision. In order to reduce the output rate and further limit the execution time, some signature tests are performed only on an already down scaled fraction of candidate events. For some of these signatures it is important for physics analysts to know the would-be decision on the event fraction that was not analysed due to the down-scaling. For this the HLT-Steering is equipped with a test-after-accept feature. The HLT-Steering receives the setup of the signatures from the trigger configuration system. This system dynamically provides the online setup for the LVL1 and HLT. It also archives the trigger configuration for analysis, which is crucial for understanding trigger efficiencies. We present the performance of the steering and configuration system during the first collisions and the expectations for the LHC phase 1.

ATLAS level-1 calorimeter trigger hardware: initial timing and energy calibration

The ATLAS Level-1 Calorimeter Trigger identifies high-pT objects in the Liquid Argon and Tile Calorimeters with a fixed latency of up to 2.5μs using a hardware-based, pipelined system built with custom electronics. The Preprocessor Module conditions and digitizes about 7200 pre-summed analogue signals from the calorimeters at the LHC bunch-crossing frequency of 40 MHz, and performs bunch-crossing identification (BCID) and deposited energy measurement for each input signal. This information is passed to further processors for object classification and total energy calculation, and the results are used to make the Level-1 trigger decision for the ATLAS detector. The BCID and energy measurement in the trigger depend on precise timing adjustments to achieve correct sampling of the input signal peak. Test pulses from the calorimeters were analysed to derive the initial timing and energy calibration, and first data from the LHC restart in autumn 2009 and early 2010 were used for validation and further optimization. The results from these calibration measurements are presented.

A novel calorimeter trigger concept: The jet trigger of the H1 experiment at HERA

We report on a novel trigger for the liquid argon calorimeter which was installed in the H1 Experiment at HERA. This trigger, called the “Jet Trigger”, was running at level 1 and implemented a real-time cluster algorithm. Within only 800 ns, the Jet Trigger algorithm found local energy maxima in the calorimeter, summed their immediate neighbors, sorted the resulting jets by energy, and applied topological conditions for the final level 1 trigger decision. The Jet Trigger was in operation from the year 2006 until the end of the HERA running in the summer of 2007. With the Jet Trigger it was possible to substantially reduce the thresholds for triggering on electrons and jets, giving access to a largely extended phase space for physical observables which could not have been reached in H1 before. The concepts of the Jet Trigger may be an interesting upgrade option for the LHC experiments.

The central trigger control system of the CMS experiment at CERN

The Large Hadron Collider will deliver up to 32 million physics collisions per second. Thisrate is far too high to be processed by present-day computer farms, let alone stored on disk by the experiments for offline analysis.A fast selection of interesting events must therefore be made. In the CMS experiment,this is implemented in two stages: the Level-1 Trigger of the CMS experiment uses custom-made, fast electronics,while the experiment's high-level trigger is implemented in computer farms. The Level-1 Global Triggerelectronics has to receive signals from the subdetector systems that enter the trigger (mostly from muon detectorsand calorimeters), synchronize them, determine if a pre-set trigger condition is fulfilled, check if thevarious subsystems are ready to accept triggers based on information from the TriggerThrottling System and on calculations of possible dead-times, and finally distribute the trigger decision(``Level-1 Accept'') together with timing signals to the subdetectors over the so-called ``Trigger, Timing and Control'' distribution tree of the experiment. These functions are fulfilled by severalspecialized, custom-made VME modules, most of which are housed in one crate. The overallcontrol is exerted by the central ``Trigger Control System'', which is described in this paper.It consists of one main module and several ancillary boards for input and output functions.

Design of the local trigger board for the Daya Bay reactor neutrino experiment

We have designed a local trigger board for the Daya Bay reactor neutrino experiment, which is aimed to measure the neutrino mixing angle sin 2 2 θ 13 with a precision down to 1% level. The local trigger board processes both the total number of coincident photomultiplier tube (PMT) hits and the PMT energy sum to make trigger decisions. With this design, a high trigger probability is achieved to meet the system requirement. The design of the local trigger board is presented.

Operation and performance of the ALICE silicon pixel detector with proton-proton beams

Abstract The ALICE Silicon Pixel Detector (SPD) forms the two innermost barrel layers of the ALICE inner tracking system (ITS) and is made of hybrid silicon pixel detectors. The cell size is 50 μ m × 425 μ m and the SPD contains a total of about 107 readout cells. The two layers are located at average radii of 3.9 and 7.6 cm from the interaction point. The total material-budget of the SPD per layer is 1.14% X0. In addition of being a pixel detector with a very low material-budget in the sensitive area, the SPD is also capable of providing a fast trigger signal to contribute to the L0 trigger decision. The SPD has been installed in the experiment in 2007. Commissioning of the system was carried out in the following year, followed by several experimental cosmic runs in 2008 and 2009. The SPD was also operational during LHC beam injection tests as well as during the first beam collisions at the end of 2009. Since then the SPD has shown to be a key element of the ALICE experiment in providing data and trigger information. This presentation will review the experience gained during the commissioning of the detector and summarize the performance during the cosmic runs, LHC injection tests and first collisions.

Trigger performance of the ALICE Silicon Pixel Detector

Abstract The ALICE Silicon Pixel Detector (SPD) forms the two innermost layers of the ALICE Inner Tracking System (ITS). It consists of two-layer barrel of hybrid silicon pixel detectors. The SPD contains 1200 pixel readout chips and about 107 pixel cells. The unique feature of the SPD among the vertex detectors of LHC experiments is the prompt trigger capability, called Fast-Or, which allows it to contribute to the experimental first level trigger decision. The SPD data stream includes 1200 digital signals (Fast-Or) promptly asserted by each readout chip on the presence of at least one pixel hit in the matrix. The off-detector Pixel Trigger system elaborates in real time the 1200 Fast-Or signals continuously received from the detector in order to generate a pre-processed signal for the ALICE first level trigger. All the Fast-Or signals are written in the data and available for offline analysis. In this contribution it is presented the methodology used to evaluate the performance of the Fast-Or. The methods are entirely based on offline data analysis. Results are given for a sample of data of proton–proton collisions at s = 7 TeV .

The ALICE Silicon Pixel Detector: commissioning and performance optimization

This paper describes the tests and measurements made during the final commissioning with beams of the ALICE Silicon Pixel Detector (SPD) in the first year of operation and the optimization of its performance.The ALICE Silicon Pixel Detector (SPD) is the innermost detector of the ALICE experiment and therefore plays a key role for vertexing and tracking. It consists of two cylindrical layers of pixel detectors, with a total of ∼ 107 pixels.The detector provides a prompt trigger signal that contributes to the first level trigger decision in ALICE. The trigger signal has been extensively used in the first trigger level of the ALICE experiment for recording data of proton-proton collisions at energies of 900 GeV, 2.36 TeV and 7 TeV.

Design and development of micro-strip stacked module prototypes for tracking at S-LHC

Experience at high luminosity hadron collider experiments shows that tracking information enhances the trigger rejection capabilities while retaining high efficiency for interesting physics events [1]. The design of a tracking based trigger for Super LHC (S-LHC), the high luminosity upgrade of the LHC collider [2], is an extremely challenging task requiring the identification of high-momentum tracks as a part of the Level 1 trigger decision. Simulation studies show that this can be achieved by correlating hits on two closely spaced silicon strip sensors. This paper focuses on the design and development of micro-strip stacked prototype modules and their performance. The prototypes have been built with the silicon sensors and electronics used to equip the present CMS Tracker [3]. Correlation of signals collected from sensors are processed off detector. Preliminary results in terms of signal over noise and tracking performance with cosmic rays will be also shown.

Software development for the P¯ANDA barrel DIRC

The charged particle identification in the barrel region of the P ¯ ANDA detector in the future FAIR facility at GSI is planned with a very thin Cherenkov detector using the DIRC principle. Due to a very compact design of the barrel DIRC with focusing optics, the reconstruction of the Cherenkov angle is quite challenging. In this contribution, the possible reconstruction algorithm of the barrel DIRC will be discussed, with emphasis on the possibility to include the DIRC in the trigger decision and the correction of the chromatic dispersion with fast timing information.

GPUs for fast pattern matching in the RICH of the NA62 experiment

In rare decays experiments an effective online selection is a fundamental part of the data acquisition system (DAQ) in order to reduce both the quantity of data written on tape and the bandwidth requirements for the DAQ system. A multilevel architecture is commonly used to achieve a higher reduction factor, exploiting dedicated custom hardware and flexible software in standard computers. In this paper we discuss the possibility to use commercial video card processors (GPU) to build a fast and effective trigger system, both at hardware and software level. The computing power of the GPUs allows to design a real-time system in which trigger decisions are taken directly in the video processor with a defined maximum latency. This allows building lowest trigger levels based on standard off-the-shelf PCs with CPU and GPU (instead of the commonly adopted solutions based on custom electronics with FPGA or ASICs) with enhanced and high performance computation capabilities, resulting in high rejection power, high efficiency and simpler low level triggers. The ongoing work presented here shows the results achieved in the case of fast pattern matching in the RICH detector of the NA62 at CERN, aiming at measuring the Branching Ratio of the ultra rare decay K + → π + ν ν ¯ , is considered as use case, although the versatility and the customizability of this approach easily allow exporting the concept to different contexts. In particular the application is related to particle identification in the RICH detector of the NA62 experiment, where the rate of events to be analyzed will be around 10 MHz. The results obtained in lab tests are very encouraging to go towards a working prototype. Due to the use of off-the-shelf technology, in continuous development for other purposes (Video Games, image editing,…), the architecture described would be easily exported into other experiments, for building powerful, flexible and fully customizable trigger systems.

Performance of the Low-Jitter High-Gain/Bandwidth Front-End Electronics of the HADES tRPC Wall

A front-end electronics (FEE) chain for accurate time measurements has been developed for the new Resistive Plate Chamber (RPC)-based Time-of-Flight (TOF) wall of the High Acceptance Di-Electron Spectrometer (HADES). The wall covers an area of around 8 m2, divided in 6 sectors. In total, 1122 4-gap timing RPC cells are read-out by 2244 time and charge sensitive channels. The FEE chain consists of 2 custom-made boards: a 4-channel DaughterBOard (DBO) and a 32-channel MotherBOard (MBO). The DBO uses a fast 2 GHz amplifier feeding a dual high-speed discriminator. The time and charge information are encoded, respectively, in the leading edge and the width of an LVDS signal. Each MBO houses up to 8 DBOs providing them regulated voltage supply, threshold values via DACs, test signals and, additionally, routing out a signal proportional to the channel multiplicity needed for a 1st level trigger decision. The MBO delivers LVDS signals to a multi-purpose Trigger Readout Board (TRB) for data acquisition. The FEE allows achieving a system resolution around 75 ps fulfilling comfortably the requirements of the HADES upgrade .

Detector performance of the ALICE silicon pixel detector

The ALICE Silicon Pixel Detector (SPD) forms the two innermost layers of the ALICE Inner Tracking System (ITS). It consists of two barrel layers of hybrid silicon pixel detectors at radii of 39 and 76 mm. The physics targets of the ALICE experiment require that the material budget of the SPD is kept within ≈ 1 % X 0 per layer. This has set some stringent constraints on the design and construction of the SPD. A unique feature of the ALICE SPD is that it is capable of providing a prompt trigger signal, called Fast-OR, which contributes to the L0 trigger decision. The pixel trigger system allows to apply a set of algorithms for the trigger selection, and its output is sent to the Central Trigger Processor (CTP). The detector has been installed in the experiment in summer 2007. During the first injection tests in June 2008 the SPD was able to record the very first sign of life of the LHC by registering secondary particles from the beam dumped upstream the ALICE experiment. In the following months the SPD has participated in the ALICE cosmic campaign to test the integration with all experimental sub-systems and to acquire data for alignment. Since the LHC start-up in November 2009, the SPD has been recording the LHC activities and in particular the first proton–proton collisions in ALICE.