LHC trigger design [article]

John R Hubbard
1997
Trigger issues at LHC are discussed, based on the physics requirements of the experiments. Typical trigger algorithms are presented, as well as full trigger menus and estimated trigger rates. Trigger architectures under consideration for the LHC experiments are compared using very simple 'paper models' based on average values for all system parameters INTRODUCTION Trigger design and trigger architectures will be discussed in the context of the LHC experiments. These lectures will present a
more » ... will present a 'top-down' analysis of the LHC trigger requirements and design, based on physics requirements. Level-1 trigger algorithms, which are based on specific trigger hardware, will be described and compared. Higher-level trigger algorithms, based on commercial switching networks and processor farms, will be presented, as well as the expected algorithm execution times. Full trigger menus and expected trigger rates will also be presented. Trigger architectures and implementations under consideration for the LHC experiments will be compared using very simple 'paper models' based on average values for all system parameters. Most of the examples presented in these talks will be based on the ATLAS trigger. I would like to acknowledge the many contributions from my colleagues in the ATLAS trigger/DAQ working groups. Nonetheless, I take full responsibility for the arguments presented here, which do not always reflect the views of the ATLAS community. PHYSICS REQUIREMENTS FOR LHC TRIGGERS The first step in determining a trigger strategy is to review the physics requirements of the system. This lecture is not meant as a 'physics' lecture. The objective is to review the physics goals to determine which specific trigger algorithms are needed. Inclusive triggers must also be included to provide some coverage for unexpected new physics. A catalog of physics processes for the generalpurpose LHC detectors, ATLAS and CMS, including Higgs decays, SUSY particles, gauge bosons, heavy vector bosons, top quarks, and B physics, will be presented. The physics objectives of the specialized detectors, LHC-B and ALICE, will also be discussed. LHC machine characteristics The LHC ring is shown in Fig. 1 , with the emplacement of the four LHC experiments -ATLAS, CMS, LHC-B, and ALICE. The interaction energy will be 7 TeV per proton (14 TeV for p-p interactions, 1 PeV for Pb-Pb interactions). The bunch-crossing interval is 25 ns for p-p collisions (40 MHz bunch-crossing rate) and 125 ns for Pb-Pb collisions [1]. The LHC will have a nominal luminosity of 10 34 /cm 2 /s for p-p collisions. The average number of inelastic interactions per bunch crossing will be about 23 at this luminosity. Initial operation will be at lower luminosity (d10 33 /cm 2 /s), with an average of 2.3 minimum-bias interactions per bunch crossing (3.3 interactions including the interaction responsible for the trigger) [2]. The radius of each of the intersecting proton beams will be about 16 Pm at collision; the length of the interaction region will be about 5 cm (rms). During the run, the luminosity will degrade as the intensity falls. Each of the experiments will be responsible for monitoring the luminosity and the collision region during the run and during initial beam tuning.
doi:10.5170/cern-1997-008.117 fatcat:tz4m4qepmbeb3gbgqfcsmpfpli