Data-taking Period Research Articles

The ATLAS Trigger System: Ready for Run-2

The ATLAS trigger system has been used very successfully for the online event selection during the LHC's first run (Run-1) between 2009 and 2013 at centre-of-mass energies (√s) between 900 GeV and 8 TeV. The trigger system consists of a hardware Level-1 (L1) and a software-based high-level trigger (HLT) that reduces the event rate from the design bunch-crossing rate of 40 MHz to an average recording rate of a few hundred Hz. During the next data-taking period (Run-2) starting in early 2015, the LHC will operate at √s = 13 TeV, resulting in roughly five times higher trigger rates.We will review the upgrades to the ATLAS trigger system that have been implemented during the long shutdown and that will allow us to cope with these increased trigger rates while maintaining or even improving our efficiencies to select relevant physics processes. These include changes to the L1 calorimeter trigger, the introduction of new L1 topological trigger modules, improvements in the L1 muon system and the merging of the previous two-level HLT system into a single event-filter farm. Finally, we will summarize the commissioning status of the trigger system in view of the imminent restart of data-taking.

The CMS Condition Database System

The Condition Database plays a key role in the CMS computing infrastructure. The complexity of the detector and the variety of the sub-systems involved are setting tight requirements for handling the Conditions. In the last two years the collaboration has put a substantial effort in the re-design of the Condition Database system, with the aim at improving the scalability and the operability for the data taking starting in 2015. The re-design has focused on simplifying the architecture, using the lessons learned during the operation of the Run I data-taking period (20092013). In the new system the relational features of the database schema are mainly exploited to handle the metadata (Tag and Interval of Validity), allowing for a limited and controlled set of queries. The bulk condition data (Payloads) are stored as unstructured binary data, allowing the storage in a single table with a common layout for all of the condition data types. In this paper, we describe the full architecture of the system, including the services implemented for uploading payloads and the tools for browsing the database. Furthermore, the implementation choices for the core software will be discussed.

Analysis Preservation in ATLAS

Long before data taking, ATLAS established a policy that all analyses need to be preserved. In the initial data-taking period, this has been achieved by various tools and techniques. ATLAS is now reviewing the analysis preservation with the aim of bringing coherence and robustness to the process and with a clearer view of the level of reproducibility that is reasonably achievable. The secondary aim is to reduce the load on the analysts. Once complete, this will serve for our internal preservation needs but also provide a basis for any subsequent sharing of analysis results with external parties.

Calibration and data quality systems of the ATLAS Tile Calorimeter during the LHC Run-I operations

The Tile Calorimeter is the hadronic calorimeter covering the central region of the ATLAS detector at the LHC. It consists of thin steel plates and scintillating tiles. Wavelength shifting fibers coupled to the tiles collect the produced light and are read out by photomultiplier tubes. The calibration scheme of the Tile Calorimeter comprises Cs radioactive source, laser and charge injection systems. Each stage of the signal production of the calorimeter from scintillation light to digitization is monitored and equalized. Description of the different TileCal calibration systems as well as the results on their performance in terms of calibration factors, linearity and stability is given. The data quality procedures and efficiency of the Tile Calorimeter during the LHC Run-1 data-taking period are presented as well.

The COBRA demonstrator at the LNGS underground laboratory

The COBRA demonstrator, a prototype for a large-scale experiment searching for neutrinoless double beta-decay, was built at the underground laboratory Laboratori Nazionali del Gran Sasso (LNGS) in Italy. It consists of an array of 64 monolithic, calorimetric CdZnTe semiconductor detectors with a coplanar-grid design and a total mass of 380g. It is used to investigate the experimental challenges faced when operating CdZnTe detectors in low-background mode, to identify potential background sources and to show the long-term stability of the detectors. The first data-taking period started in 2011 with a subset of the detectors, while the demonstrator was completed in November 2013. To date, more than 250kgd of data have been collected. This paper describes the technical details of the experimental setup and the hardware components.

Data-flow Performance Optimisation on Unreliable Networks: the ATLAS Data-Acquisition Case

The ATLAS detector at CERN records proton-proton collisions delivered by the Large Hadron Collider (LHC). The ATLAS Trigger and Data-Acquisition (TDAQ) system identifies, selects, and stores interesting collision data. These are received from the detector readout electronics at an average rate of 100 kHz. The typical event data size is 1 to 2 MB. Overall, the ATLAS TDAQ system can be seen as a distributed software system executed on a farm of roughly 2000 commodity PCs. The worker nodes are interconnected by an Ethernet network that at the restart of the LHC in 2015 is expected to experience a sustained throughput of several 10 GB/s.A particular type of challenge posed by this system, and by DAQ systems in general, is the inherently burstynature of the data traffic from the readout buffers to the worker nodes. This can cause instantaneous network congestion and therefore performance degradation. The effect is particularly pronounced for unreliable network interconnections, such as Ethernet.In this paper we report on the design of the data-flow software for the 2015-2018 data-taking period of the ATLAS experiment. This software will be responsible for transporting the data across the distributed Data-Acquisition system. We will focus on the strategies employed to manage the network congestion and therefore minimisethe data-collection latency and maximisethe system performance. We will discuss the results of systematic measurements performed on different types of networking hardware. These results highlight the causes of network congestion and the effects on the overall system performance.

Reconstruction and Identification of Tau Leptons in ATLAS

These proceedings describe the reconstruction and identification of tau leptons in the ATLAS experiment at the LHC. Tau leptons play an important role in the measurement of Standard Model processes, in the search for new physics (Supersymmetry, heavy resonances) and in the measurement of Higgs boson properties. Hadronic tau lepton decays are reconstructed calorimeter based and separation against QCD-induced jets is done relying on shapes of energy depositions in the calorimeter as well as tracking based variables. The energy scale of tau leptons is obtained by scaling the measured momentum to the simulated one and is cross-checked in data. The tau trigger makes use of the collimated nature of tau decays and relies on a combination of objects (e.g. tau + muon) to reduce the rates and thresholds. An outlook to the improved tau reconstruction to be used in the next data-taking period of ATLAS concludes the proceedings.

The monitoring and data quality assessment of the ATLAS liquid argon calorimeter

The ATLAS experiment is designed to study the proton-proton (pp) collisions produced at the Large Hadron Collider (LHC) at CERN. Liquid argon (LAr) sampling calorimeters are used for all electromagnetic calorimetry in the pseudo-rapidity region |η| < 3.2, as well as for hadronic calorimetry in the range 1.5 < |η| < 4.9. The electromagnetic calorimeters use lead as passive material and are characterized by an accordion geometry that allows a fast and uniform response without azimuthal gaps. Copper and tungsten were chosen as passive material for the hadronic calorimetry; while a classic parallel-plate geometry was adopted at large polar angles, an innovative design based on cylindrical electrodes with thin liquid argon gaps is employed at low angles, where the particle flux is higher. All detectors are housed in three cryostats maintained at about 88.5 K. The 182,468 cells are read out via front-end boards housed in on-detector crates that also contain monitoring, calibration, trigger and timing boards. In the first three years of LHC operation, approximately 27 fb−1 of pp collision data were collected at centre-of-mass energies of 7-8 TeV. Throughout this period, the calorimeter consistently operated with performances very close to specifications, with high data-taking efficiency. This is in large part due to a sophisticated data monitoring procedure designed to quickly identify issues that would degrade the detector performance, to ensure that only the best quality data are used for physics analysis. After a description of the detector design, main characteristics and operation principles, this paper details the data quality assessment procedures developed during the 2011 and 2012 LHC data-taking periods, when more than 98% of the luminosity recorded by ATLAS had high quality LAr calorimeter data suitable for physics analysis.

Recent Results on SUSY and Exotica Searches at the LHC

A review is presented of searches for Beyond the Standard Model (BSM) physics at the ATLAS and CMS experiments at the Large Hadron Collider (LHC) with emphasis on most recent results. This includes a summary of Supersymmetry searches covering strong, electroweak, and R-parity violating scenarios, and a range of exotica searches including heavy boson searches, extra dimensions, and long-lived particles. The status of BSM physics after the first data-taking period (Run 1) of the LHC and the prospects for the second data-taking period (Run 2), beginning in 2015, are discussed.

Performance of the ATLAS Tile Hadronic Calorimeter at LHC in Run 1 and planned upgrades

The Tile Calorimeter (TileCal) is the central section of the ATLAS hadronic calorimeter at the Large Hadron Collider, a key detector for the measurements of hadrons, jets, tau leptons and missing transverse energy. Scintillation light produced in the tiles is transmitted by wavelength shifting fibres to photomultiplier tubes (PMTs). The resulting electronic signals from approximately 10000 PMTs are digitized before being transferred to off-detector data-acquisition systems. The data quality procedures used during the LHC data-taking and the evolution of the detector status are explained in the presentation. The energy and the time reconstruction performance of the digitized signals is presented and the noise behaviour and its improvement during the detector consolidation in maintenance periods are shown. A set of calibration systems allow monitoring and equalization of the calorimeter channels responses via signal sources that act at every stage of the signal path, from scintillation light to digitized signal. These partially overlapping systems are described in detail, their individual performance is discussed as well as the comparative results from measurements of the evolution of the calorimeter response with time during the full LHC data-taking period. The TileCal upgrade aims at replacing the majority of the on- and off-detector electronics so that all calorimeter signals will be directly digitized and sent to the off-detector electronics in the counting room. To achieve the required reliability, redundancy has been introduced at different levels. For the off-detector electronics a special pre-processor board is being developed, which will take care of the initial trigger processing, while the main data are temporarily stored in the pipeline and de-randomiser memories.

Performance of the ALICE muon trigger RPCs during LHC Run I

ALICE (A Large Ion Collider Experiment) studies the transition of nuclear matter to a deconfined phase known as Quark Gluon Plasma, in ultra-relativistic heavy-ion collisions at the LHC. ALICE is equipped with a muon spectrometer for the detection of quarkonia and heavy flavour particles. The trigger system of the spectrometer consists of 72 RPCs arranged in four detection planes, with a total area of 140 m2. In the first three years of LHC operation, the muon trigger system was fully operational in data-taking in pp, Pb-Pb and p-Pb collisions. The RPC performance and stability throughout the whole data-taking period is presented and discussed, for the parameters such as the efficiency, the dark counting rate, the dark current and the cluster size.

Performance of the ATLAS Liquid Argon Calorimeter after three years of LHC operation and plans for a future upgrade

The ATLAS experiment is a multi-purpose detector built for analyzing LHC collision data. In July 2012, ATLAS announced the discovery of the Higgs boson, the last undiscovered particle in the Standard Model of Particle Physics. The ATLAS Liquid Argon (LAr) Calorimeter played a crucial role in the discovery by providing accurate measurements of Higgs final state objects such as photons, electrons and jets. The LAr detector is a sampling calorimeter consisting of four subsystems: an electromagnetic barrel, electromagnetic endcaps, hadronic endcaps, and forward calorimeters. The purity and temperature of the liquid argon remained well above the required levels throughout the data-taking period. Overall the calorimeter performed very well, with over 99% of data it collected in 2012 proton-proton collisions being suitable for physics analyses. In order to ensure good LAr detector performance at future higher luminosity LHC operation, several upgrades are being planned and implemented.

Upgrades to the ATLAS Level-1 Calorimeter Trigger

In 2015 the Large Hadron Collider will run with increased center-of-mass energy and luminosity. To maintain a high efficiency in selecting interesting collisions for the physics analyses in the next data-taking period, event topology information will be added to the ATLAS Level-1 real time data path and processed by a new Topology Processor (L1Topo). To cope with the luminosity levels foreseen after the 2018 LHC upgrade, a new digital trigger path for the Liquid Argon calorimeters will provide finer granularity and depth segmentation in the electromagnetic layer to new Level-1 feature extractors (FEX) for an improved electron, photon, tau and jet selection. We present the ongoing and future calorimeter trigger upgrades to the ATLAS Level-1 trigger.

Triggers for displaced decays of long-lived neutral particles in the ATLAS detector

A set of three dedicated triggers designed to detect long-lived neutral particles decaying throughout the ATLAS detector to a pair of hadronic jets is described. The efficiencies of the triggers for selecting displaced decays as a function of the decay position are presented for simulated events. The effect of pile-up interactions on the trigger efficiencies and the dependence of the trigger rate on instantaneous luminosity during the 2012 data-taking period at the LHC are discussed.

Measurements ofWγandZγproduction inppcollisions ats=7 TeVwith the ATLAS detector at the LHC

The integrated and differential fiducial cross sections for the production of a W or Z boson in association with a high-energy photon are measured using pp collisions at sqrt{s} = 7 TeV. The analyses use a data sample with an integrated luminosity of 4.6 fb^{-1} collected by the ATLAS detector during the 2011 LHC data-taking period. Events are selected using leptonic decays of the W and Z bosons (W(e nu,mu nu) and Z(e+ e-, mu+ mu-, nu nubar)) with the requirement of an associated isolated photon. The data are used to test the electroweak sector of the Standard Model and search for evidence for new phenomena. The measurements are used to probe the anomalous WWgamma, ZZgamma and Zgammagamma triple-gauge-boson couplings and to search for the production of vector resonances decaying to Zgamma and Wgamma. No deviations from Standard Model predictions are observed and limits are placed on anomalous triple-gauge-boson couplings and on the production of new vector meson resonances.

Observation of Reactor Electron Antineutrino Disappearance at RENO

The Reactor Experiment for Neutrino Oscillation (RENO) has obtained a definitive measurement of the smallest neutrino mixing angle of θ13 by observing the disappearance of electron antineutrinos emitted from a nuclear reactor, excluding the no-oscillation hypothesis at 4.9 σ. From the deficit, the best fit value of sin22θ13 is obtained as 0.113±0.013(stat.)±0.019(syst.) based on a rate-only analysis. Antineutrinos from six 2.8 GWth reactors at the Yonggwang Nuclear Power Plant in Korea, are detected by two identical detectors at 294 m and 1383 m, respectively, from the reactor array center. In the 229 day data-taking period between 11 August 2011 and 26 March 2012, the far (near) detector observed 17102 (154088) electron antineutrino candidate events with a background fraction of 5.5% (2.7%). The ratio of observed to expected numbers of the reactor antineutrinos in the far detector is 0.920±0.009(stat.)±0.014(syst.).

Track and vertex reconstruction in the ATLAS experiment

The track and vertex reconstruction algorithms of the ATLAS Inner Detector have demonstrated excellent performance in the early data from the LHC. However, the rapidly increasing number of interactions per bunch crossing introduces new challenges both in computational aspects and physics performance. The combination of both silicon and gas based detectors provides high precision impact parameter and momentum measurement of charged particles, with high efficiency and small fake rate. Vertex reconstruction is used to identify with high efficiency the hard scattering process and to measure the amount of pile-up interactions, both aspects are crucial for many physics analyses. The performance of track and vertex reconstruction efficiency and resolution achieved in the 2011 and 2012 data-taking period are presented.

Search for supersymmetry in final states with jets, missing transverse momentum and isolated leptons with the ATLAS detector in 5 fb−1of √s= 8 TeV pp collisions

We present an inclusive search for supersymmetry (SUSY) by the ATLAS experiment at the LHC in proton-proton collisions at a center-of-mass energy p s = 8 TeV in final states with jets, missing transverse momentum and one isolated electron or muon. The search is based on the data from the early 2012 data-taking period, corresponding to an integrated luminosity of 5.8 fb 1 . No excess is observed in the data, and limits on supersymmetry are set in the MSUGRA/ CMSSM model.

B-jet triggering in ATLAS

The online event selection is crucial to reject most of the events containing uninteresting background collisions while preserving as much as possible the interesting physics signals. The b-jet selection is part of the trigger strategy of the ATLAS experiment and a set of dedicated triggers is in place from the beginning of the 2011 data-taking period and is contributing to keep the total bandwidth to an acceptable rate. These triggers are used in many physics analyses, especially those with topologies containing more than one b-jet where higher rejection factors are achieved that benefit from requesting this trigger to be fired. An overview of the b-jet trigger menu and the performance on data is presented in this contribution.

File and metadata management for BESIII distributed computing

The BESIII experiment at the Institute of High Energy Physics (IHEP), Beijing, uses the high-luminosity BEPCII e+e− collider to study physics in the π-charm energy region around 3.7 GeV; BEPCII has produced the worlds largest samples of J/ϕ and ϕ’ events to date. An order of magnitude increase in the data sample size over the 2011-2012 data-taking period demanded a move from a very centralized to a distributed computing environment, as well as the development of an efficient file and metadata management system. While BESIII is on a smaller scale than some other HEP experiments, this poses particular challenges for its distributed computing and data management system. These constraints include limited resources and manpower, and low quality of network connections to IHEP. Drawing on the rich experience of the HEP community, a system has been developed which meets these constraints. The design and development of the BESIII distributed data management system, including its integration with other BESIII distributed computing components, such as job management, are presented here.