Large Hadron Collider Startup Research Articles

Fundamental cavity impedance and longitudinal coupled-bunch instabilities at the High Luminosity Large Hadron Collider

The interaction between beam dynamics and the radio frequency (rf) station in circular colliders is complex and can lead to longitudinal coupled-bunch instabilities at high beam currents. The excitation of the cavity higher order modes is traditionally damped using passive devices. But the wakefield developed at the cavity fundamental frequency falls in the frequency range of the rf power system and can, in theory, be compensated by modulating the generator drive. Such a regulation is the responsibility of the low-level rf (llrf) system that measures the cavity field (or beam current) and generates the rf power drive. The Large Hadron Collider (LHC) rf was designed for the nominal LHC parameter of 0.55 A DC beam current. At 7 TeV the synchrotron radiation damping time is 13 hours. Damping of the instability growth rates due to the cavity fundamental (400.789 MHz) can only come from the synchrotron tune spread (Landau damping) and will be very small (time constant in the order of 0.1 s). In this work, the ability of the present llrf compensation to prevent coupled-bunch instabilities with the planned high luminosity LHC (HiLumi LHC) doubling of the beam current to 1.1 A DC is investigated. The paper conclusions are based on the measured performances of the present llrf system. Models of the rf and llrf systems were developed at the LHC start-up. Following comparisons with measurements, the system was parametrized using these models. The parametric model then provides a more realistic estimation of the instability growth rates than an ideal model of the rf blocks. With this modeling approach, the key rf settings can be varied around their set value allowing for a sensitivity analysis (growth rate sensitivity to rf and llrf parameters). Finally, preliminary measurements from the LHC at 0.44 A DC are presented to support the conclusions of this work.

RECONSTRUCTING THE K0/S - MESON FROM THE FIRST DATA OF CMS EXPERIMENT

The Large Hadron Collider startup took place on November 2009 in CERN, at center-of-mass energies of 900 GeV for the collidingprotons. The results of processing the first data accumulated by CMS experiment are presented. For processing and analyzing thetrack collections as well as the collection of identified particles have been used. The distributions of charged pions over theirtransverse momentum and pseudorapidity are built. К0s−meson mass is reconstructed in channel of К0/s decay to two charged pions. Comparison with results of computer simulation of proton-proton interactions in CMS detector is also done. It is shown that reconstruction of events is adequate.

The ALICE data quality monitoring system

ALICE (A Large Ion Collider Experiment) is the heavy-ion detector designed to study the physics of strongly interacting matter and the quark-gluon plasma at the CERN Large Hadron Collider (LHC). The online Data Quality Monitoring (DQM) is a key element of the Data Acquisition's software chain. It provide shifters with precise and complete information to quickly identify and overcome problems, and as a consequence to ensure acquisition of high quality data. DQM typically involves the online gathering, the analysis by user-defined algorithms and the visualization of monitored data.This paper describes the final design of ALICE'S DQM framework called AMORE (Automatic MOnitoRing Environment), as well as its latest and coming features like the integration with the offline analysis and reconstruction framework, a better use of multi-core processors by a parallelization effort, and its interface with the eLogBook. The concurrent collection and analysis of data in an online environment requires the framework to be highly efficient, robust and scalable. We will describe what has been implemented to achieve these goals and the procedures we follow to ensure appropriate robustness and performance.We finally review the wide range of usages people make of this framework, from the basic monitoring of a single sub-detector to the most complex ones within the High Level Trigger farm or using the Prompt Reconstruction and we describe the various ways of accessing the monitoring results. We conclude with our experience, before and after the LHC startup, when monitoring the data quality in a challenging environment.

Calibration of the ATLAS Muon Spectrometer Precision Chambers for Initial LHC Beam Collisions

ATLAS is a general purpose experiment at the Large Hadron Collider (LHC) at CERN which began operation in September, 2009. ATLAS has a large muon spectrometer which is designed to measure muon momentum with a resolution ranging from 3% at 100 GeV/c to 10% at 1 TeV for a pseudorapidity range of 0-2.7. In this paper, we describe the derivation and validation of calibration constants for the Monitored Drift Tubes (MDTs) in the ATLAS muon spectrometer for use with initial LHC collisions. At LHC start-up, the luminosity was not sufficient for ordinary calibrations with collision muons, and so alternative methods were developed. The time-to-space (RT) functions were obtained from the University of Michigan gas monitor chamber and the ( t 0 s) were derived from LHC beamsplash events from 2009 and 2010. We have validated these calibration constants by performing muon track reconstruction using 900 GeV collision data from December, 2009 and 7 TeV data from 2010. We find that these calibration constants yield an MDT chamber hit resolution of 142 μ m and a residual width of 91 μ m.

The ALICE data quality monitoring

ALICE is one of the four experiments installed at the CERN Large Hadron Collider (LHC), especially designed for the study of heavy-ion collisions. The online Data Quality Monitoring (DQM) is an important part of the data acquisition (DAQ) software. It involves the online gathering, the analysis by user-defined algorithms and the visualization of monitored data. This paper presents the final design, as well as the latest and coming features, of the ALICE's specific DQM software called AMORE (Automatic MonitoRing Environment). It describes the challenges we faced during its implementation, including the performances issues, and how we tested and handled them, in particular by using a scalable and robust publish-subscribe architecture.We also review the on-going and increasing adoption of this tool amongst the ALICE collaboration and the measures taken to develop, in synergy with their respective teams, efficient monitoring modules for the sub-detectors. The related packaging and release procedure needed by such a distributed framework is also described. We finally overview the wide range of usages people make of this framework, and we review our own experience, before and during the LHC start-up, when monitoring the data quality on both the sub-detectors and the DAQ side in a real-world and challenging environment.

Commissioning and initial experience with the ALICE on-line

ALICE (A Large Ion Collider Experiment) is the heavy-ion detector designed to study the physics of strongly interacting matter and the quark-gluon plasma at the CERN Large Hadron Collider (LHC). A large bandwidth and flexible Data Acquisition System (DAQ) has been designed and deployed to collect sufficient statistics in the short running time available per year for heavy ions and to accommodate very different requirements originated from the 18 sub-detectors. This paper will present the large scale tests conducted to assess the standalone DAQ performances, the interfaces with the other online systems and the extensive commissioning performed in order to be fully prepared for physics data taking. It will review the experience accumulated since May 2007 during the standalone commissioning of the main detectors and the global cosmic runs and the lessons learned from this exposure on the "battle field". It will also discuss the test protocol followed to integrate and validate each sub-detector with the online systems and it will conclude with the first results of the LHC injection tests and startup in September 2008. Several papers of the same conference present in more details some elements of the ALICE DAQ system.

Commissioning of the ATLAS High Level Trigger with single beam and cosmic rays

ATLAS is one of the two general-purpose detectors at the Large Hadron Collider (LHC). The trigger system is responsible for making the online selection of interesting collision events. At the LHC design luminosity of 1034 cm−2s−1 it will need to achieve a rejection factor of the order of 10−7 against random proton-proton interactions, while selecting with high efficiency events that are needed for physics analyses. After a first processing level using custom electronics based on FPGAs and ASICs, the trigger selection is made by software running on two processor farms, containing a total of around two thousand multi-core machines. This system is known as the High Level Trigger (HLT). To reduce the network data traffic and the processing time to manageable levels, the HLT uses seeded, step-wise reconstruction, aiming at the earliest possible rejection of background events. The recent LHC startup and short single-beam run provided a "stress test" of the system and some initial calibration data. Following this period, ATLAS continued to collect cosmic-ray events for detector alignment and calibration purposes. After giving an overview of the trigger design and its innovative features, this paper focuses on the experience gained from operating the ATLAS trigger with single LHC beams and cosmic-rays.

Commissioning of the ATLAS High-Level Trigger With Single LHC Beam and Cosmic Rays

ATLAS is one of the two general-purpose detectors at the Large Hadron Collider (LHC). Using fast reconstruction algorithms, its trigger system needs to efficiently reject a huge rate of background events and still select potentially interesting ones with good efficiency. After a fist processing level using custom electronics, the trigger selection is made by software running on two processor farms, designed to have a total of around two thousand multi-core machines in a system called High Level Trigger (HLT). The recent LHC startup and short single-beam run provided a “stress test” of the trigger. Following this period, ATLAS continued to collect cosmic-ray events for detector alignment and calibration purposes. These running periods allowed testing the HLT in different running conditions. This paper focuses on the experience gained in running the trigger in the fast-changing environment of the detector commissioning.

The muon high level trigger of the ATLAS experiment

The ATLAS experiment at CERN's Large Hadron Collider (LHC) has been designed and built for new discoveries in High Energy Physics as well as for precision measurements of Standard Model parameters. To satisfy the limited data acquisition and recording capability, at the LHC project luminosity, the ATLAS trigger system must select a very small rate of physically interesting events (~200 Hz) among about 40 million events per second. In the case of events containing muons, as described in this work, the first hardware-based level (Level-1) starts from coincidence of hits in the Muon Spectrometer trigger chambers to select Regions of Interest (RoI) where muons produce significant activity. Such RoIs are used as seeds for the two subsequent trigger levels (Level-2 and Event Filter), running on dedicated online farms, which constitute the High Level Trigger (HLT). This seeding strategy is crucial to drastically reduce the total processing time. Within the Muon HLT, few algorithms are implemented in different steps according to predefined sequences of Feature Extraction (FEX) and Hypothesis (HYPO) algorithms, whose goal is to validate the previously selected muon objects. The ATLAS muon trigger system, thanks to its particular design and to the peculiar structure of the Muon Spectrometer, is able to provide muon stand-alone event trigger decisions, that can be furtherly refined by exploiting the muon information coming from the other ATLAS subdetectors. Muon HLT algorithms are described here in terms of working functionality and performance both on simulated and real data, including non-standard trigger configurations (like cosmic data and LHC start-up scenarios).

Commissioning of the ATLAS high-level trigger with single beam and cosmic rays

ATLAS is one of the two general-purpose detectors at the Large Hadron Collider (LHC). Using fast reconstruction algorithms, its trigger system needs to efficiently reject a huge rate of background events and still select potentially interesting ones with good efficiency. After a first processing level using custom electronics, the trigger selection is made by software running on two processor farms, designed to have a total of around two thousand multi-core machines. This system is known as the High Level Trigger (HLT). To reduce the network data traffic and the processing time to manageable levels, the HLT uses seeded, step-wise reconstruction, aiming at the earliest possible rejection of background events.The recent LHC startup and short single-beam run provided a “stress test” of the trigger. Following this period, ATLAS continued to collect cosmic-ray events for detector alignment and calibration purposes. These running periods allowed strict tests of the HLT reconstruction and selection algorithms as well as of its configuration and monitoring systems. This facilitated the commissioning of several tracking, muon-finding, and calorimetry algorithms under different running conditions.After an overview of the trigger design, this paper focuses on the experience gained in running the trigger in the fast-changing environment of the detector commissioning. It will emphasize the commissioning of the HLT algorithms, monitoring and configuration.

CMS Silicon Strip Tracker Commissioning and Preliminary Performances

The Silicon Strip Tracker (SST) of the Compact Muon Solenoid (CMS) Experiment at the CERN Large Hadron Collider (LHC) is the world's largest silicon microstrip detector. The SST is composed of four subdetectors which were integrated into the Tracker Support Tube (TST) at the CERN Tracker Integration Facility (TIF) in March 2007. After the insertion of the TST into CMS in December 2007, the first data taking of CMS with the SST included took place in July 2008 without magnetic field and one month later with a magnetic field of 3.8 Tesla. The SST was ready for the LHC start-up of September 10, 2008. The main aspects of the SST commissioning and a description of the problems encountered and solved so far are summarized here together with some preliminary performance evaluation.

CERN database services for the LHC computing grid

Physics meta-data stored in relational databases play a crucial role in the Large Hadron Collider (LHC) experiments and also in the operation of the Worldwide LHC Computing Grid (WLCG) services. A large proportion of non-event data such as detector conditions, calibration, geometry and production bookkeeping relies heavily on databases. Also, the core Grid services that catalogue and distribute LHC data cannot operate without a reliable database infrastructure at CERN and elsewhere. The Physics Services and Support group at CERN provides database services for the physics community. With an installed base of several TB-sized database clusters, the service is designed to accommodate growth for data processing generated by the LHC experiments and LCG services. During the last year, the physics database services went through a major preparation phase for LHC start-up and are now fully based on Oracle clusters on Intel/Linux. Over 100 database server nodes are deployed today in some 15 clusters serving almost 2 million database sessions per week. This paper will detail the architecture currently deployed in production and the results achieved in the areas of high availability, consolidation and scalability. Service evolution plans for the LHC start-up will also be discussed.

The CMS Calorimeter in 2007: Performance and Physics Goals

Thirteen years after the decision to build a lead tungstate based electromagnetic calorimeter for the CMS experiment at the Large Hadron Collider (LHC) at CERN the detector is now in its final assembly and installation phase for the LHC startup in early 2008. It is made of 75848 lead tungstate crystals, 61200 of 133 cubic centimeter for the barrel and 14648 of 189 cubic centimeter for the endcaps, for a total weight of 90 tons of crystals. This challenging project aims to achieve an extreme precision in photons and electrons energy measurement. General motivations, main technical challenges, measured performances, and the actual status of the project will be discussed.

Prospects for ϕ meson production in pp collisions at the ALICE experiment

The ALICE experiment at the CERN Large Hadron Collider (LHC) will allow the study of resonance production in nucleus–nucleus and proton–proton collisions. This paper presents results based on physics performance studies to discuss prospects in ALICE for ϕ(1020) meson production in pp interactions during the LHC startup.