Pile-up Events Research Articles

The new CMS DAQ system for LHC operation after 2014 (DAQ2)

The Data Acquisition system of the Compact Muon Solenoid experiment at CERN assembles events at a rate of 100 kHz, transporting event data at an aggregate throughput of 100 GByte/s. We are presenting the design of the 2nd generation DAQ system, including studies of the event builder based on advanced networking technologies such as 10 and 40 Gbit/s Ethernet and 56 Gbit/s FDR Infiniband and exploitation of multicore CPU architectures. By the time the LHC restarts after the 2013/14 shutdown, the current compute nodes, networking, and storage infrastructure will have reached the end of their lifetime. In order to handle higher LHC luminosities and event pileup, a number of sub-detectors will be upgraded, increase the number of readout channels and replace the off-detector readout electronics with a μTCA implementation. The second generation DAQ system, foreseen for 2014, will need to accommodate the readout of both existing and new off-detector electronics and provide an increased throughput capacity. Advances in storage technology could make it feasible to write the output of the event builder to (RAM or SSD) disks and implement the HLT processing entirely file based.

Evolution of the ATLAS distributed computing system during the LHC long shutdown

The ATLAS Distributed Computing project (ADC) was established in 2007 to develop and operate a framework, following the ATLAS computing model, to enable data storage, processing and bookkeeping on top of the Worldwide LHC Computing Grid (WLCG) distributed infrastructure. ADC development has always been driven by operations and this contributed to its success. The system has fulfilled the demanding requirements of ATLAS, daily consolidating worldwide up to 1 PB of data and running more than 1.5 million payloads distributed globally, supporting almost one thousand concurrent distributed analysis users. Comprehensive automation and monitoring minimized the operational manpower required. The flexibility of the system to adjust to operational needs has been important to the success of the ATLAS physics program. The LHC shutdown in 2013-2015 affords an opportunity to improve the system in light of operational experience and scale it to cope with the demanding requirements of 2015 and beyond, most notably a much higher trigger rate and event pileup. We will describe the evolution of the ADC software foreseen during this period. This includes consolidating the existing Production and Distributed Analysis framework (PanDA) and ATLAS Grid Information System (AGIS), together with the development and commissioning of next generation systems for distributed data management (DDM/Rucio) and production (Prodsys-2). We will explain how new technologies such as Cloud Computing and NoSQL databases, which ATLAS investigated as R&D projects in past years, will be integrated in production. Finally, we will describe more fundamental developments such as breaking job-to-data locality by exploiting storage federations and caches, and event level (rather than file or dataset level) workload engines.

An Event Building scenario in the trigger-less PANDA experiment

The PANDA experiment will be running up to 2 × 107 antiproton-proton collisions per second at energies reaching 15 GeV. The lack of simple features distinguishing the interesting events from background, as well as strong pileup of events' data streams make the use of a hardware trigger impossible. As a consequence the whole data stream of about 300 GB/s has to be analyzed online, i.e: tracking, vertex finding, particle identification and event building.The GEM Tracker covers polar angles from 4 to 20 degrees in the forward direction, and can be used for the event building in a large fraction of events. In this work the analysis chain and the implemented algorithms will be presented. Moreover, the event builder prototype based on GEM Tracker data will be presented.

The design and performance of the ATLAS jet trigger

The ATLAS jet trigger is an important element of the event selection process, providing data samples for studies of Standard Model physics and searches for new physics at the LHC. The ATLAS jet trigger system has undergone substantial modifications over the past few years of LHC operations, as experience developed with triggering in a high luminosity and high event pileup environment. In particular, the region-of-interest based strategy has been replaced by a full scan of the calorimeter data at the third trigger level, and by a full scan of the level-1 trigger input at level-2 for some specific trigger chains. Hadronic calibration and cleaning techniques are applied in order to provide improved performance and increased stability in high luminosity data taking conditions. In this note we discuss the implementation and operational aspects of the ATLAS jet trigger during 2011 and 2012 data taking periods at the LHC.

FPGA-based 10-Gbit Ethernet Data Acquisition Interface for the Upgraded Electronics of the ATLAS Liquid Argon Calorimeters

A stepwise upgrade of the LHC is foreseen starting now until the year 2023 to increase the instantaneous luminosity up to five times of its design value. It implies a challenge for the ATLAS experiment coping with the expected event pile-up, especially for the Level-1 calorimeter trigger system. In order to keep the trigger rates within the limited bandwidth new algorithms have to be applied which in turn requires an upgrade of the ATLAS Liquid Argon calorimeter trigger readout electronics. Towards this upgrade, the ATLAS Liquid Argon calorimeter group develops a high-speed data acquisition interface in ATCA standard using commercial hardware instead of complex and expensive in-house developments where possible. This paper gives an overview of the general concepts of the DAQ interface, the engaged technologies and the current status of the development efforts for an FPGA based fast data link with a standard 10 Gbps Ethernet protocol which may also be useful for DAQ systems of other high energy physics experiments.

Efficient Reduction of Piled-Up Events in Gamma-Ray Spectrometry at High Count Rates

When dealing with high count rate γ-ray spectrometry, the pile-up phenomenon is an important obstacle to high detection efficiency and good energy resolution. In traditional pulse processing piled-up events are rejected to preserve good energy resolution at the expense of detection efficiency. This is especially problematic in in-beam γ-ray spectrometry, where searching for coincidences between two different detection signals further accentuates the loss of efficiency. Consequently we want to detect as many events as possible and obtain the best possible energy resolution. We propose a method that reduces the number of piled-up events, using a parallel moving window deconvolution (MWD) block implementation where the shaping time parameter differs for every MWD block. With appropriate analysis logic we get more experimental information through reduced dead time, at the cost of controlled and selectively worsened energy resolution, on an event-by-event basis, thus achieving better overall detection efficiency. We tested our method on real experimental data for different count rates. At 500 kcps count rate the detection efficiency of our method is higher, by a factor of about 37, than the efficiency of a standard digital method commonly used in commercial systems with pile-up rejection.

Concept of Metallic Magnetic Calorimeters for Rare Event Search in the LUMINEU Project

The project LUMINEU is mainly aiming at the search for neutrinoless double-beta decay of the candidate nuclide \(^{100}\)Mo using cryogenic ZnMoO\(_4\) detectors with simultaneous heat and scintillation light detection for radioactive background rejection. It also includes some development for dark matter search using cryogenic Ge detectors with simultaneous heat and ionization detection for background rejection. For both cases, metallic magnetic calorimeters (MMCs) are studied among several thermometer types. In double-beta decay search, the intrinsically fast response of MMCs reading out the light detector may allow for a very fast signal rise time and help to reduce the potential background due to pile-up of two-neutrino double beta decay events. In dark matter search, MMCs reading out the heat channel may improve the energy resolution with respect to the standard NTD Ge thermistor readout and hence the sensitivity of the detectors for low-mass WIMPs.

Scintillation event energy measurement via a pulse model based iterative deconvolution method

This work focuses on event energy measurement, a crucial task of scintillation detection systems. We modeled the scintillation detector as a linear system and treated the energy measurement as a deconvolution problem. We proposed a pulse model based iterative deconvolution (PMID) method, which can process pileup events without detection and is adaptive for different signal pulse shapes. The proposed method was compared with digital gated integrator (DGI) and digital delay-line clipping (DDLC) using real world experimental data. For singles data, the energy resolution (ER) produced by PMID matched that of DGI. For pileups, the PMID method outperformed both DGI and DDLC in ER and counts recovery. The encouraging results suggest that the PMID method has great potentials in applications like photon-counting systems and pulse height spectrometers, in which multiple-event pileups are common.

Characterization of a pnCCD for applications with synchrotron radiation

In this work we study the response of a pnCCD by means of X-ray spectroscopy in the energy range between 6keV and 20keV and by Laue diffraction techniques. The analyses include measurements of characteristic detector parameters like energy resolution, count rate capability and effects of different gain settings. The limit of a single photon counting operation in white beam X-ray diffraction experiments is discussed with regard to the occurrence of pile-up events, for which the energy information about individual photons is lost. In case of monochromatic illumination the pnCCD can be used as a fast conventional CCD with a charge handling capacity (CHC) of about 300,000 electrons per pixel. If the CHC is exceeded, any surplus charge will spill to neighboring pixels perpendicular to the transfer direction due to electrostatic repulsion. The possibilities of increasing the number of storable electrons are investigated for different voltage settings by exposing a single pixel with X-rays generated by a microfocus X-ray source. The pixel binning mode is tested as an alternative approach that enables a pnCCD operation with significantly shorter readout times.

Fast CdTe and CdZnTe Semiconductor Detector Arrays for Spectroscopic X-Ray Imaging

A growing interest has recently been observed in high flux X-ray imaging detectors with energy discrimination or spectroscopy capabilities. In this study we evaluate the performances of energy sensitive CdTe and CdZnTe detectors for X-ray imaging at fluxes of up to 2 107 X/mm2/s in the 20-150 keV energy range. Linear array detectors made from 3 mm thick single CdTe or CdZnTe crystals have 16 pixels with an 800 in pitch. These detectors were coupled to an innovative custom-designed 16-channel fast spectroscopy front-end electronic circuit. For each channel, the signal is continuously digitized and a FPGA controls acquisition and reconstructs the energy spectra on 256 bins for each channel. The detector was tested under X-rays for fluxes in the 105 to 2 × 107 X/mm2/s range. The main problem encountered at such high fluxes is the multiplication of pile-up events that reduces count rate and degrades energy resolution. When a very short shaping time was used, dead time was lower than 50 ns, and an energy resolution of 11 keV full width at half maximum (FWHM) at 2 × 106 X/mm2 s and 20 keV (FWHM) at 107 X/mm2/ s was achieved. We also show imaging results that illustrate the interest of the spectral information provided by each pixel.

The LHCb Upgrade

With the demonstration that LHCb can successfully perform forward precision measurements with event pileup, the operation and trigger strategy evolved significantly during the LHC Run 1 allowing LHCb to collect over 3fb −1 at centre-of-mass energies of 7TeV and 8TeV. Increased bandwidth opened the door for LHCb to extend the physics program. The additional statistics and well managed systematic effects together with the stable trigger and data taking conditions have led to a very large number of world-class measurements and dominance in heavy flavour physics [1], in addition to a reputation of an excellent forward general purpose detector at the LHC. Long Shutdown (LS) 1 (2013–2014) will allow LHCb to fully explore the large statistics collected and prepare LHCb for Run 2 (2015 – 2017). However, even after an additional expected integrated luminosity of 5–6 fb −1 in Run 2, many of the LHCb precision measurements will remain limited by statistics, and some exploratory physics modes will not even be accessible yet. With the need for reconstructing the event topology in order to efficiently trigger on the beauty and the charm hadrons decays, the current 1 MHz readout limit is the main bottle neck to run at higher luminosity and with higher trigger efficiencies. LHCb will therefore undergo a major upgrade in LS 2 ( 2018 – 2019) aimed at collecting an order of magnitude more data by 2028. The upgrade consists of a full readout at the LHC bunch crossing rate ( 40 MHz) with the ultimate flexibility of only a software trigger. In order to increase the instantaneous luminosity up to 2x10 33 cm −2 s −1 , several sub-detector upgrades are also underway to cope with the higher occupancies and radiation dose.

The Design and Performance of the ATLAS jet trigger

The ATLAS jet trigger, in combination with other triggers, provides an important ingredient to studies of Standard Model physics and searches for new physics at the LHC. The ATLAS jet trigger system has undergone substantial modifications over the past few years of LHC operations, as experience developed with triggering in a high luminosity and high event pileup environment. In particular, the region-of-interest (ROI) based strategy has been replaced by a full scan of the calorimeter data at the third trigger level, and by a full scan of the level-1 trigger input at level-2 for some specific trigger chains. Hadronic calibration and cleaning techniques are applied in order to provide improved performance and increased stability in high luminosity data taking conditions. In this presentation we describe the structure and performance of the jet trigger in recent data taking conditions.

Analysis of three digital n/γ discrimination algorithms for liquid scintillation neutron spectrometry

Abstract In mixed neutron/γ radiation fields, particle identification based on different filter algorithms and radiation intensities is an important issue for liquid scintillation neutron spectrometry. We have compared the performance of three algorithms for digital neutron/γ discrimination in a deuterated liquid scintillator. The effects of both digital filters and simulated pile-up events on the performance of these techniques were investigated. The discrimination methods utilized are the digital charge comparison (CC) method, simplified digital charge collection (SDCC) method and pulse gradient analysis (PGA) method. The results show that the SDCC and the CC methods are preferred choices when applied in low count-rates fields, whereas PGA method with a seven-point average running filter is favorable when pile-up signals constitute a large fraction of the detected events. All these discrimination approaches are potentially suitable for developing a compact digital neutron spectrometer in mixed neutron/γ radiation fields for use in various applications.

Missing ET Reconstruction with the CMS Detector

The CMS experiment uses missing ET to both measure processes in the Standard Model and test models of physics beyond the Standard Model. These proceedings show the performance of the missing ET reconstruction evaluated by using 4.6 fb−1 of proton-proton collision data at the center-of-mass energy √s = 7 TeV collected in 2011 with the CMS detector at the Large Hadron Collider. Missing ET was reconstructed based on a particle-flow technique. Jet energy corrections were propagated to missing ET. After anomalous signals and events were addressed, the missing ET spectrum was well reproduced by MC simulation. The multiple proton-proton interactions in a single bunch crossing, pile-up events, degraded the performance of the missing ET reconstruction. Mitigations of this degradation have been developed.

CMS Simulation Software

The CMS simulation, based on the Geant4 toolkit, has been operational within the new CMS software framework for more than four years. The description of the detector including the forward regions has been completed and detailed investigation of detector positioning and material budget has been carried out using collision data. Detailed modeling of detector noise has been performed and validated with the collision data. In view of the high luminosity runs of the Large Hadron Collider, simulation of pile-up events has become a key issue. Challenges have raised from the point of view of providing a realistic luminosity profile and modeling of out-of-time pileup events, as well as computing issues regarding memory footprint and IO access. These will be especially severe in the simulation of collision events for the LHC upgrades; a new pileup simulation architecture has been introduced to cope with these issues.The CMS detector has observed anomalous energy deposit in the calorimeters and there has been a substantial effort to understand these anomalous signal events present in the collision data. Emphasis has also been given to validation of the simulation code including the physics of the underlying models of Geant4. Test beam as well as collision data are used for this purpose. Measurements of mean response, resolution, energy sharing between the electromagnetic and hadron calorimeters, shower shapes for single hadrons are directly compared with predictions from Monte Carlo. A suite of performance analysis tools has been put in place and has been used to drive several optimizations to allow the code to fit the constraints posed by the CMS computing model.

Study of a Fine Grained Threaded Framework Design

Traditionally, HEP experiments exploit the multiple cores in a CPU by having each core process one event. However, future PC designs are expected to use CPUs which double the number of processing cores at the same rate as the cost of memory falls by a factor of two. This effectively means the amount of memory per processing core will remain constant. This is a major challenge for LHC processing frameworks since the LHC is expected to deliver more complex events (e.g. greater pileup events) in the coming years while the LHC experiment's frameworks are already memory constrained. Therefore in the not so distant future we may need to be able to efficiently use multiple cores to process one event. In this presentation we will discuss a design for an HEP processing framework which can allow very fine grained parallelization within one event as well as supporting processing multiple events simultaneously while minimizing the memory footprint of the job. The design is built around the libdispatch framework created by Apple Inc. (a port for Linux is available) whose central concept is the use of task queues. This design also accommodates the reality that not all code will be thread safe and therefore allows one to easily mark modules or sub parts of modules as being thread unsafe. In addition, the design efficiently handles the requirement that events in one run must all be processed before starting to process events from a different run. After explaining the design we will provide measurements from simulating different processing scenarios where the processing times used for the simulation are drawn from processing times measured from actual CMS event processing.

CMS reconstruction improvements for the tracking in large pile-up events

The CMS tracking code is organized in several levels, known as ‘iterative steps’, each optimized to reconstruct a class of particle trajectories, as the ones of particles originating from the primary vertex or displaced tracks from particles resulting from secondary vertices. Each iterative step consists of seeding, pattern recognition and fitting by a Kalman filter, and a final filtering and cleaning. Each subsequent step works on hits not yet associated to a reconstructed particle trajectory. The CMS tracking code underwent a major upgrade deployed in two phases. It was needed to make the reconstruction computing load compatible with the increasing instantaneous luminosity of LHC, resulting in a large number of primary vertices and tracks per bunch crossing. The improvements are described. Among the others, the iterative steps have been reorganized and optimized and an iterative step specialized for the reconstruction of photon conversion has been added. The overall impact on reconstruction performances is discussed and the prospects for future applications are given.

FPGA-Based Pulse Pile-Up Correction With Energy and Timing Recovery

Modern field programmable gate arrays (FPGAs) are capable of performing complex discrete signal processing algorithms with clock rates well above 100 MHz. This, combined with FPGA's low expense, ease of use, and selected dedicated hardware make them an ideal technology for a data acquisition system for a positron emission tomography (PET) scanner. The University of Washington is producing a high-resolution, small-animal PET scanner that utilizes FPGAs as the core of the front-end electronics. For this scanner, functions that are typically performed in dedicated circuits, or offline, are being migrated to the FPGA. This will not only simplify the electronics, but the features of modern FPGAs can be utilized to add significant signal processing power to produce higher quality images. In this paper we report on an all-digital pulse pile-up correction algorithm that has been developed for the FPGA. The pile-up mitigation algorithm will allow the scanner to run at higher count rates without incurring large data losses due to the overlapping of scintillation signals. This correction technique utilizes a reference pulse to extract timing and energy information for most pile-up events. Using pulses acquired from a Zecotech Photonics MAPD-N with an LFS-3 scintillator, we show that good timing and energy information can be achieved in the presence of pile-up utilizing a moderate amount of FPGA resources.

Performances and Ageing Study of Resistive-Anodes Micromegas Detectors for HL-LHC Environment

With the tenfold luminosity increase envisaged at the HL-LHC, the background (photons, neutrons, ...) and the event pile-up probability are expected to increase in proportion in the different experiments, especially in the forward regions like, for instance, the muons chambers of the ATLAS detector. Detectors based on the Micromegas principle should be good alternatives for the detector upgrade in the HL-LHC framework because of a good spatial (<100 \mum) and time (few ns) resolutions, high-rate capability, radiation hardness, good robustness and the possibility to build large areas. The aim of this study is to demonstrate that it is possible to reduce the discharge probability and protect the electronics by using a resistive anode plane in a high flux hadrons environment. Several prototypes of 10x10 cm2, with different pitches (0.5 to 2 mm) and different resistive layers have been tested at CERN (pi+@SPS). Several tests have been performed with a telescope at different voltages to assess the performances of the detectors in terms of position resolution and efficiency. The spark behaviour in these conditions has also been evaluated. Resistive coating has been shown to be a successful method to reduce the effect of sparks on the efficiency of micromegas. A good spatial resolution (~80 \mum) can be reached with a resistive strip coating detector of 1mm pitch and a high efficiency (> 98%) can be achieved with resistive-anode micromegas detector. An X-rays irradiation has been also performed, showing no ageing effect after more than 21 days exposure and an integrated charge of almost 1C.

Effect of nanomechanical and microstructural properties on annealed multilayer Si0·8Ge0·2–Si films

In this study, ultrahigh vacuum chemical vapour deposition was employed to deposit multilayered silicon–germanium (SiGe) films. Subsequently, we take those samples for ex situ thermal treatments in the furnace system (400 and 500°C). The periodic multilayer SiGe with different annealing conditions measured by a commercial nanoindenter observed the slight increase in hardness. The cross-section profile and the microstructure of SiGe multilayer films were characterised by means of atomic force microscopy and transmission electron microscopy. The effect of the thermodynamics of the thin film/substrate system is evidenced by annealing treatment. It is demonstrated that the SiGe multilayer films are more susceptible to plastic deformation while annealing treatments are carried out. The misfit dislocations in the critical pile-up event were observed in the periodical SiGe multilayer that can be relaxed at thermal annealing, thus providing the nanomechanical performance.