Abstract
The CMS experiment at the CERN LHC has been designed with a 2-level trigger system. The Level 1 Trigger (L1) is implemented on custom-designed electronics. The High Level Trigger (HLT) is a streamlined version of the CMS offline reconstruction software running on a computer farm. Being able to identify b-quark jets (b-tagging) at trigger level will play a crucial role during the Run II data taking to ensure the top quark, beyond the Standard Model and Higgs boson physics program of the experiment. It will help to significantly reduce the trigger output rate which will increase due to the higher instantaneous luminosity and higher cross sections at 13 TeV. B-tagging algorithms based on the identification of tracks displaced from the primary proton-proton collision and/or on the reconstruction of secondary vertices have been successfully used during Run I. We will present their design and performance with an emphasis on the dedicated aspects of track and primary vertex reconstruction, as well as the improvements foreseen to meet the challenges of the Run II data taking.
Highlights
2. b-jet identification: principles and algorithms Many channels studied in the Compact Muon Solenoid (CMS) physics program involve b-jets
The High Level Trigger (HLT) is a streamlined version of the CMS offline reconstruction software running on a cluster of commercial rack-mounted computers which consists of about 13000 processors
Application of the b-tagging algorithm: Combined Secondary Vertex A CSV discriminant is computed for each jet based on the information of the pixel primary vertex, and the regional Combinatorial Track Finder (CTF) tracks
Summary
B-tagging algorithms exploit the properties of the B-hadrons such as their large decay lifetimes (cτ ≈ 450 μm) or the presence of leptons in the final state to separate them from light hadrons (made of u,d,s-quarks). The offline b-jet identification efficiency is about 70% for a misidentification probability for light jets of about 1% for jets with a transverse momentum pT between 80 and 120 GeV/c This algorithm has been deployed in an online version at the High Level Trigger during Run I. Application of the b-tagging algorithm: Combined Secondary Vertex A CSV discriminant is computed for each jet based on the information of the pixel primary vertex, and the regional CTF tracks. The second goal is to improve the performance of the online version of the CSV b-tagging algorithm This was achieved thanks to the smart iterative tracking reconstruction and to a better resolution on the position of the primary vertex. Application of the b-tagging algorithm: Combined Secondary Vertex A CSV discriminant is computed for each jet based on the information of the primary vertex and the regional iterative tracks. The mean processing time is reduced by about 15% and the tail at high values is strongly reduced
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