LHC Startup Research Articles

Track propagation for different detector and magnetic field setups in Acts

Track finding and fitting are among the most complex parts of event reconstruction in high-energy physics, and usually dominate the computing time in a high luminosity environment. A central part of track reconstruction is the transport of a given track parametrisation (i.e. the parameter estimation and associated covariance matrices) through the detector, respecting the magnetic field setup and the traversed detector material. While track propagation in a sparse environment (e.g. tracking detector with layers) can be sufficiently well approximated by considering discrete interactions at several positions, the propagation in a material dense environment (e.g. calorimeters) is better served by a continuous application of material effects. Recently, a common tracking software project (Acts), originally from the Common Tracking code of the ATLAS experiment, has been developed in order to preserve the algorithmic concepts from the LHC start-up era and prepare them for the high luminosity era of the LHC and beyond. The software is designed in an abstract, detector independent way and prepared to allow highly parallelised execution of all involved software modules, including magnetic field access and alignment conditions. Therefore the propagation algorithm needs to be both flexible and adjustable. The implemented solution using a fourth order Runge-Kutta-Nyström integration and its extension with continuous material integration and eventual time propagation is presented and the navigation through different geometry setups involving different environments is demonstrated.

Operational Experience with the Frontier System in CMS

The Frontier framework is used in the CMS experiment at the LHC to deliver conditions data to processing clients worldwide, including calibration, alignment, and configuration information. Each central server at CERN, called a Frontier Launchpad, uses tomcat as a servlet container to establish the communication between clients and the central Oracle database. HTTP-proxy Squid servers, located close to clients, cache the responses to queries in order to provide high performance data access and to reduce the load on the central Oracle database. Each Frontier Launchpad also has its own reverse-proxy Squid for caching. The three central servers have been delivering about 5 million responses every day since the LHC startup, containing about 40 GB data in total, to more than one hundred Squid servers located worldwide, with an average response time on the order of 10 milliseconds. The Squid caches deployed worldwide process many more requests per day, over 700 million, and deliver over 40 TB of data. Several monitoring tools of the tomcat log files, the accesses of the Squids on the central Launchpad servers, and the availability of remote Squids have been developed to guarantee the performance of the service and make the system easily maintainable. Following a brief introduction of the Frontier framework, we describe the performance of this highly reliable and stable system, detail monitoring concerns and their deployment, and discuss the overall operational experience from the first two years of LHC data-taking.

Performance of the ATLAS Inner Detector at the LHC

Since the LHC startup in 2009, the ATLAS inner tracking system has played a central role in many ATLAS physics analyses. Rapid improvements in the calibration and alignment of the detector allowed it to reach nearly the nominal performance in the timespan of a few months. The tracking performance proved to be stable as the LHC luminosity increased by five orders of magnitude during the 2010 proton run, while the performance was only slightly degraded in the extremely dense heavy ion collisions.

Streamlined calibrations of the ATLAS precision muon chambers for initial LHC running

The ATLAS Muon Spectrometer is designed to measure the momentum of muons with a resolution of dp/p=3% at 100GeV and 10% at 1TeV. For this task, the spectrometer employs 355,000 Monitored Drift Tubes (MDTs) arrayed in 1200 chambers. Calibration (RT) functions convert drift time measurements into tube-centered impact parameters for track segment reconstruction. RT functions depend on MDT environmental parameters and so must be appropriately calibrated for local chamber conditions. We report on the creation and application of a gas monitor system based calibration program for muon track reconstruction in the LHC startup phase.

Design, implementation and first measurements with the Medipix2-MXR detector at the Compact Muon Solenoid experiment

The Medipix detector is the first device dedicated to measuring mixed-field radiation in the CMS (Compact Muon Solenoid) experiment cavern and able to distinguish between different particle types. Medipix2-MXR chips bump bonded to silicon sensors with various neutron conversion layers developed by the IEAP CTU in Prague were successfully installed for the 2008 LHC start-up in the CMS experimental and services caverns to measure the flux of various particle types, in particular neutrons. They have operated almost continuously during the 2010 run period, and the results shown here are from the proton run between the beginning of July and the end of October 2010. Clear signals are seen and different particle types have been observed during regular LHC luminosity running, and an agreement in the measured flux is found with the simulations. These initial results are promising, and indicate that these devices have the potential for further and future LHC and high energy physics applications as radiation monitoring devices for mixed field environments, including neutron flux monitoring. Further extensions are foreseen in the near future to increase the performance of the detector and its coverage for monitoring in CMS.

Commissioning of the LHCb preshower detector with cosmic rays and first LHC collisions

The LHCb calorimeter system consists of a Scintillating Pad Detector (SPD), a PreShower detector (PS) followed by electromagnetic (ECAL) and hadronic (HCAL) calorimeters. The LHCb preshower detector provides the longitudinal segmentation of the electromagnetic shower detection. It is located immediately upstream from the ECAL, with one-to-one correspondence between ECAL towers, SPD and preshower cells. The PS detector is part of the first level of the LHCb trigger system (called Level-0). It is used in conjunction with the SPD, ECAL and HCAL to search for clusters of 2 × 2 cells and to identify e, γ, π°, hadron of highest ET. This contribution discusses the requirements for the synchronisation and calibration of the PS detector, as well as the methods that were used to determine the first settings before the LHC start-up. The methods used to perform the fine synchronisation and calibration once LHC collisions were available are introduced, and the precisions that were obtained are given.

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.

The ALICE online-offline framework for the extraction of conditions data

A Large Ion Collider Experiment (ALICE) is the dedicated heavy-ion experiment at the CERN LHC and will take data with a bandwidth of up to 1.25 GB/s. It consists of 18 subdetectors that interact with five online systems (CTP, DAQ, DCS, ECS, and HLT). Data recorded is read out by DAQ in a raw data stream produced by the subdetectors. In addition the subdetectors produce conditions data derived from the raw data, i.e. calibration and alignment information, which have to be available from the beginning of the reconstruction and therefore cannot be included in the raw data. The extraction of the conditions data is steered by a system called Shuttle. It provides the link between data produced by the subdetectors in the online systems and a dedicated procedure per subdetector, called preprocessor, that runs in the Shuttle system. The preprocessor performs merging, consolidation, and reformatting of the data. Finally, it stores the data in the Grid Offline Conditions DataBase (OCDB) so that they are available for the Offline reconstruction. The reconstruction of a given run is initiated automatically once the raw data is successfully exported to the Grid storage and the run has been processed in the Shuttle framework. These proceedings introduce the Shuttle system. The performance of the system during the ALICE cosmics commissioning and LHC startup is described.

Calibration of the barrel muon drift tubes system in CMS

In this report, results on the calibration process of the Drift Tubes (DT) system of the Compact Muon Solenoid experiment are presented. The full commissioning of the calibration procedure has been deployed in year 2008 with the CMS Computing, Software and Analysis challenge (CSA08), which has tested the full work-flow needed for CMS data-taking during the LHC start-up operations. In autumn 2008, the same Calibration work-flow was applied to a high statistics cosmic ray muon data taking period: the Cosmic Run At Four Tesla (CRAFT) period. The accurate measurement of the main calibration conditions corresponding to the Time Pedestals and the Drift Velocity provide the necessary space-time relationship used in the first stage of the muon local reconstruction.

The status of the simulation project for the ATLAS experiment in view of the LHC startup

The Simulation suite for ATLAS is in a mature phase, ready to cope with the challenge of the 2009 data. The simulation framework, which is integrated to the ATLAS framework (Athena) offers a set of pre-configured applications for the full simulation of ATLAS, combined test beam setups, cosmic ray setups and old standalone test-beams. Each detector component has been carefully described in detail and monitored for performance. The few pieces of the apparatus (forward and very forward detectors), inert material and services (toroid supports, support rails, detector feet) that are still missing are about to be integrated in the current simulation suite. Detailed descriptions of the ideal and real geometries for each ATLAS subcomponent allow optimization studies and validation. Small scale productions are monitored daily through a set of tests for different samples of physics events, and large scale productions on the Grid verify the robustness of the implementation as well as possible errors only visible on large statistics. The conditions used in the simulation process are now stored in the output file as metadata, and can be utilized to process the data properly. A fast shower simulation suite has also been developed in ATLAS and performance comparisons are part of the overall evaluation.

Advancement in networks for HEP community

The key role of networks has been brought into focus as a result of the worldwide-distributed computing model adopted by the four LHC experiments, as a necessary response to the unprecedented data volumes and computational needs of the LHC physics program. As we approach LHC startup and the era of LHC physics, the focus has increased as the experiments develop the tools and methods needed to distribute, process, access and cooperatively analyze datasets with aggregate volumes of Petabytes of simulated data even now, rising to many Petabytes of real and simulated data during the first years of LHC operation.

The integration of the ALICE trigger system with sub-detectors

The ALICE Trigger electronics (TRG) has been installed in the experimental cavern and tested with each of the detectors, both individually (‘standalone’ mode) and in ‘global’ runs, i.e. those involving other detectors. Global runs were performed with cosmic ray triggers, and also during the LHC startup period in September 2008. In this paper the status of the trigger system will be reviewed, in particular describing recent extensions to the system.

An overview of RPCs at the LHC startup

This issue of the RPC workshop occurs in a particular historical moment which is characterized by a number of RPC systems already in data taking or in phase of advanced commissioning. In particular the startup of the Cern LHC, foreseen this year, will be a crucial test for the trigger and the time-of-flight RPC systems that have been developed for both trigger and time-of-flight by most of the LHC experiments. This offers the opportunity to evaluate the results achieved by very large systems. This introductory talk will stress some of the main achievements and problems of the last two years. The choice of the topics is somewhat arbitrary and apologies are due for the relevant results not mentioned here.

An overview of the status of the LHCb RICH detectors

The LHCb experiment will make precision measurements of CP violation and rare b-hadron decays. Efficient particle identification with high purity over a wide momentum range is vital to these aims. The experiment employs two ring-imaging Cherenkov (RICH) detectors with three radiators, silica aerogel, C 4F 10 and CF 4, to cover the momentum range from around 1–100 GeV/ c. The RICH system employs a number of innovative techniques, both in hardware and software. A total of 484 custom-built pixel Hybrid Photon Detectors (HPDs) will be used to measure the spatial positions of Cherenkov photons with wavelengths in the range 200–600 nm, covering an active area of around 3.3 m 2. The production of the HPDs has now been completed and the tube quality, including the photo-cathode quantum efficiency, far exceeds expectations. The installation of RICH 1 is almost complete; RICH 2 has been installed and aligned, and commissioning is almost complete. Reconstruction studies incorporating realistic backgrounds indicate excellent kaon efficiencies of around 97% and pion misidentification probabilities of around 6%, averaged over the full momentum range. This paper provides a general overview of the status of the LHCb RICH project, with emphasis on the readiness for LHC start-up and for physics.

Readiness of CMS simulation towards LHC startup

The CMS experiment has used detector simulation software in its conceptual as well as technical design. With the detector construction near its completion, the role of simulation has changed toward understanding collision data to be collected by CMS in near future. CMS simulation software is becoming a data driven, realistic and accurate Monte Carlo programme. The software architecture is described with some detail of the framework as well as detector specific components. Performance issues are discussed as well.

Grid computing for UK particle physics

Over the last few years, UK research centres have provided significant computing resources for many high-energy physics collaborations under the guidance of the GridPP project. This paper reviews recent progress in the Grid deployment and operations area including findings from recent experiment and infrastructure service challenges. These results are discussed in the context of how GridPP is dealing with the important areas of networking, data storage and user education and support. Throughout, the paper offers feedback on observed successes and problems with the gLite middleware, grid usage and the experiment specific software which is required to make the Grid usable. The paper ends with a discussion of the decisions being taken to ensure a stable service and project through LHC startup until GridPP funding ends in 2011, including a brief discussion on monitoring and meeting the strict WLCG availability targets.

Experience running a distributed Tier-2 in Spain for the ATLAS experiment

The main role of the Tier-2s is to provide computing resources for production of physics simulated events and distributed data analysis. The Spanish ATLAS Tier-2 is geographically distributed among three HEP institutes: IFAE (Barcelona), IFIC (Valencia) and UAM (Madrid). Currently it has a computing power of 430 kSI2K CPU, a disk storage capacity of 87 TB and a network bandwidth, connecting the three sites and the nearest Tier-1 (PIC), of 1 Gb/s. These resources will be increased according to the ATLAS Computing Model with time in parallel to those of all ATLAS Tier-2s. Since 2002, it has been participating into the different Data Challenge exercises. Currently, it is achieving around 1.5% of the whole ATLAS collaboration production in the framework of the Computing System Commissioning exercise. A distributed data management is also arising as an important issue in the daily activities of the Tier-2. The distribution in three sites has shown to be useful due to an increasing service redundancy, a faster solution of problems, the share of computing expertise and know-how. Experience gained running the distributed Tier-2 in order to be ready at the LHC start-up will be presented.

CMS event display and data quality monitoring at LHC start-up

The event display and data quality monitoring visualisation systems are especially crucial for commissioning CMS in the imminent CMS physics run at the LHC. They have already proved invaluable for the CMS magnet test and cosmic challenge. We describe how these systems are used to navigate and filter the immense amounts of complex event data from the CMS detector and prepare clear and flexible views of the salient features to the shift crews and offline users. These allow shift staff and experts to navigate from a top-level general view to very specific monitoring elements in real time to help validate data quality and ascertain causes of problems. We describe how events may be accessed in the higher level trigger filter farm, at the CERN Tier-0 centre, and in offsite centres to help ensure good data quality at all points in the data processing workflow. Emphasis has been placed on deployment issues in order to ensure that experts and general users may use the visualization systems at CERN, in remote operations and monitoring centres offsite, and from their own desktops.

US CMS Tier-2 computing

The CMS computing model relies heavily on the use of ‘Tier-2’ computing centers. At LHC startup, the typical Tier-2 center in the United States will have 1 MSpecInt2K of CPU resources, 200 TB of disk for data storage, and a WAN connection of 10 Gbit/s. These centers will be the primary sites for the production of large-scale simulation samples and for the hosting of experiment data for user analysis ? an interesting mix of experiment-controlled and user-controlled tasks. As a result, there are a wide range of services that must be deployed and commissioned at these centers, which are responsible for tasks such as dataset transfer, management of datasets, hosting of jobs submitted through Grid interfaces, and several varieties of monitoring. We discuss the development of the seven CMS Tier-2 computing centers in the United States, with a focus on recent operational performance and preparations for the start of data-taking in 2008.

Database architecture for the calibration of ATLAS monitored drift tube chambers

The size and complexity of LHC experiments raise unprecedented challenges not only in terms of detector design, construction and operation, but also in terms of software models and data persistency. One of the more challenging tasks is the calibration of the 375000 Monitored Drift Tubes, that will be used as precision tracking detectors in the Muon Spectrometer of the ATLAS experiment. A high rate of muon tracks is needed to reach the design average resolution of 80 microns. In this context, data suitable for MDT calibration will be extracted from the second level trigger and then streamed to three remote Tier-2 Calibration Centers. The Calibration sites will also need the ATLAS conditions data that are relevant for the calculation of MDT calibrations: either the appropriate tables of the Conditions Database will be replicated at the remote sites via ORACLE streams, or the remote sites will directly access these tables from the nearest Tier-1. At each centre, the computation of the actual calibration constants will be performed in several steps, including strict validation and data quality checks. All information produced at every stage of the calibration procedure will be stored in local ORACLE Calibration databases that will be replicated to a central database located at CERN using ORACLE streams: this will allow each Calibration site to access the data produced by the others and to eventually provide back-up should one site become unavailable for any reason. The validated calibration constants will be extracted from the CERN Calibration DB and stored into the ATLAS Conditions database for subsequent use in reconstruction and data analysis. This paper reviews the complex chain of databases envisaged to support the MDT Calibration and describes the actual status of the implementation and the tests that are being performed to ensure a smooth operation at the LHC start-up at the end of this year.