The ATLAS High Level Trigger A. Introduction

VACHON, B.M.C. PIN 207385 1 The ATLAS High Level Trigger A. Introduction The Large Hadron Collider (LHC) at CERN will soon become the premier project in high-energy particle physics. The data to be recorded by the ATLAS experiment starting in 2008 is expected to revolutionize our understanding of the smallest constituents of nature and their interactions. Four decades of research have led to the development of the Standard Model of Particle Physics, a theory that has proved extremely successful at describing current experimental measurements over a wide range of energies. We have, however, not yet experimentally observed the existence of the predicted Higgs Boson, which is responsible for the breaking of the symmetry of the electroweak interaction into the more familiar electromagnetic and weak nuclear forces by generating mass for the elementary particles. Even with the discovery of the Higgs boson, the Standard Model seems to provide an incomplete description of nature at energy scales near 1 TeV. The ATLAS experiment is designed to discover the Higgs boson for any masses allowed within current experimental and theoretical constraints, as well as explore physics at energy scales where theories beyond the Standard Model strongly favour observable new physics. Major construction of ATLAS detector components is finished and the installation of these components in the ATLAS experimental area is nearly complete. The current emphasis of the ongoing work is in the areas of integration and commissioning of the detector and data acquisition systems. In addition to offline computing and physics studies, Canadians are playing leading roles in two distinct areas of the ATLAS experiment: the liquid argon calorimeter system and the High Level Trigger system. The ATLAS trigger is a system designed to identify, in real time, potentially interesting interactions out of the billions produced per second. A three-tiered system provides the necessary rejection to select approximately 200 Hz of collision data from the LHC beam crossing rate of 40 MHz. The first level of decision making is achieved using custom-built electronics and reduces the input data rate to approximately 100 kHz. The second and third levels of the trigger system, together referred to as the “High Level Trigger” (HLT), provide the additional three orders of magnitude rejection needed to reduce the data rate to permanent storage to a manageable level. The HLT consists of robust, time-optimized reconstruction algorithms running on commercial off-the-shelf computer components with demanding requirements on network data flow and management. To achieve the performance needed to fulfill the ATLAS physics program, it is estimated that the HLT will require the equivalent of nearly 20,000 computing units (CPU cores with a minimum clock speed of 2 GHz). Events that are not selected by the ATLAS trigger system are discarded, hence the performance of the trigger directly impacts the entire ATLAS physics program. Canadian groups, led by investigators who have joined ATLAS in the past few years, have contributed to the development of significant parts of the HLT. This includes the design and implementation of algorithms that can run in the specialized HLT framework, design and implementation of benchmarks to assess HLT algorithm performance, and the deployment of an HLT testbed farm at McGill which is used for the evaluation of different hardware architecture config- VACHON, B.M.C. PIN 207385 2 urations and the detailed testing of algorithms. A summary of Canadian HLT activities is shown in Table 1. These contributions are strongly analogous to the prototyping and testing phase of detector construction, but do not yet include the capital funding necessary to complete the full system. The ATLAS HLT is not yet a fully funded project, and completing it is essential to the success of the ATLAS physics program. Although the complete HLT system does not need to be running from the first day of ATLAS data-taking since it will take a number of years for the LHC to ramp up to the full design luminosity, a significant increase in HLT capability between now and 2010 is required to avoid compromising the ATLAS physics program. The scope of this grant application is to request capital equipment funding for the purchase of HLT hardware components corresponding to an important fraction of the total system, on par with the significant, and still growing, Canadian contributions to the ATLAS HLT. The Canadian involvement on the ATLAS experiment has been supported via different sources of funding. The Canadian ATLAS group, with critical roles in the ATLAS liquid argon calorimeter system, received about $15M in NSERC funding for capital equipment for current ATLAS detector subsystems. This funding, together with the NSERC-funded ATLAS-Canada operating grant and computing support from CFI and provincial governments, places Canadians in a good position to be major players in ATLAS physics. The contributed levels of Canadian ATLAS funding per author to the initial detector construction were reasonable, although somewhat lower than other participating countries. Thus, additional Canadian capital contributions to ATLAS would not be out of scale. Since 2002, there has been 15 grant eligible researchers joining the Canadian ATLAS group, most of whom are new hires. The majority of these researchers were not part of the initial detector construction, but are now involved in complementary activities in the HLT. This more recent Canadian involvement further diversifies and increases the impact of Canadian contributions to ATLAS, thereby providing yet more opportunities for Canadian groups to take on leadership roles in the analysis of ATLAS data. While the HLT contributions are strongly analogous to detector construction contributions, there is an important difference to the way they are treated within the ATLAS collaboration. Contributions to the HLT, including the capital contributions requested here, do not imply additional contributions to the ATLAS construction common fund. In fact, ATLAS management has agreed that capital contributions to HLT computing hardware at CERN will count towards Canada’s construction common fund. We also note that the ATLAS experiment is a top priority in the recent NSERC Subatomic Physics long range plan, and funds were foreseen for ATLAS HLT capital contributions in the initial five-years (up to 2010) considered in that plan. B. Canadian HLT Involvement Canada’s participation in the High Level Trigger (HLT) has significantly increased over the past 3 years with the addition of new faculty in Alberta, Carleton, McGill and York and well as existing faculty moving to ATLAS at Montreal and Victoria. This has increased the number of faculty working on the HLT to 10, representing about 5 FTE. These are supported by 3 postdocs and 10 graduate students. In addition to this 5 undergraduate students, 3 with NSERC summer VACHON, B.M.C. PIN 207385 3 Investigators Activities ATLAS HLT Fraction Fraction David Asner Carleton Algorithm development/studies 75% 25% Georges Azuelos Montréal Algorithm development/studies 100% 10% Kamal Benslama Regina Data Quality / e-gamma slice 100% 30% Bryan Caron Alberta/TRIUMF remote farms 100% 30% Robert Kowalewski Victoria Algorithm development/studies 75% 60% Robert McPherson Victoria/IPP Monitoring/Data Quality 100% 20% RogerMoore Alberta remotefarms 100% 60% Jim Pinfold Alberta remote farms 100% 50% Steve Robertson McGill/IPP Alg.Devel/DQ/testbed studies 80% 50% Wendy Taylor York Algorithm development/studies 75% 20% Brigitte Vachon McGill Alg.Devel/testbed/management 100% 75% Table 1: ATLAS-Canada investigators currently active in areas relevant to the HLT, along with their fraction of research time spent on ATLAS and the fraction of their ATLAS time dedicated to the HLT. Also listed are their principal areas of activity in the HLT. student scholarships, have worked on the trigger effort to date. We expect the number of graduate students to grow by another 3 next year. ATLAS Canada’s role on the HLT is focused into four overlapping areas. (i) Trigger Algorithms, Performance and Menu (TAPM) Group This group is tasked with the development, tuning and validation of the various trigger reconstruction algorithms. The focus for the Canadian role within this group is built on the existing Canadian expertise in ATLAS calorimetry. ATLAS Canada is currently the dominant leader of the HLT jet slice with Canadians responsible for the software development, maintenance and validation, including the cluster algorithms, as well as the the jet trigger menu configuration for the Event Filter. In addition Canadians are involved in performance, calibration and pile-up studies. The entire ATLAS Jet slice effort is coordinated by B. Vachon who is the ATLAS HLT Jet convener and a member of the TAPM steering committee. Canadians are also working on the data access for the L2 missing ET algorithms. This is of the crucial aspects for this algorithm because the HLT trigger design at both the L2 and EF levels is based on regions of interest whereas missing ET algorithms require access to the entire detector’s calorimetry readout. The expertise gained from these tasks is already enabling Canada to have a direct impact on the ATLAS physics program with Canadians developing trigger criteria for the charged Higgs and top pair production triggers. This synergy between trigger algorithms and physics topics is one of the primary reasons for a strong Canadian participation in the trigger and as our involvement VACHON, B.M.C. PIN 207385 4 in the trigger matures we expect this effort to expand to include other physics topics aligned with the interests of Canadian physicists. As well as these specific contributions to the trigger algorithms Canada is playing a leading role in the technical testing of the trigger executables. One contribution in this regard was the use of Westgrid Linux farm for large scale testing of the monitoring and control framework.

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