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A Level-1 Tau Trigger for CMS at HL-LHC

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New detector upgrades increase the chance of Higgs particle selection

The High Luminosity Large Hadron Collider (HL-LHC), which is expected to be operational after 2025, will increase the LHC luminosity by a factor of 10. To achieve this major upgrade, one of the main activities will be to incorporate tracking information in the CMS trigger, thus providing a better chance to see rare processes.

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Fundamental Research

The CMS tracker is the largest silicon detector ever built, accurately tracking particle paths. Tracking is essential for reconstructing objects such as jets, muons, electrons and tau leptons, starting from the raw data from the silicon pixel and strip detectors. Due to the high number of tracks per bunch crossing and the read-out bandwidth limitations that were not predicted at the time of the original construction of the CMS tracker, the tracker needs to be replaced. In addition, high data rates dictate the need for a trigger that can select the potentially interesting events, such as those that will produce the Higgs particle. Within the EU-funded project HLTAUS (A level-1 tau trigger for CMS at HL-LHC), researchers successfully developed an algorithm for level-1 of the trigger that looks for tau-jets. Being the first ever algorithm that employs tracking information in CMS, it efficiently selects tau-jets and feeds them to the higher-level trigger at sufficiently low rates, even when CMS is performing at its peak. The new algorithm is seeded with calorimeter tau objects. The track that is found to be closest to the tau object is called the matching track. An isolation cone is formed around the direction of the matched track, whose size is proportional to the transverse energy of the level-1 tau trigger candidate. If no other track within this isolation cone is found close to the matched track, then the algorithm selects it. Overall, employment of tracking information allows maintaining the same energy thresholds for calorimeter objects and improves the performance for their selection. Project results were disseminated through the project website, workshops, seminars and posters.


Higgs particle, HL-LHC, tracking information, CMS tracker, HLTAUS, tau-jets

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