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

Final Report Summary - HLTAUS (A Level-1 Tau Trigger for CMS at HL-LHC)

The following gives a concise description of the project's objectives and accomplishments towards those objectives. A description of the project and the related activities and accomplisments are presented at the project's website:
Project website:

* Publishable Summary
The work conducted in the duration of this project is concerned with preparations of the CMS Experiment towards the proposed upgrades of CERN’s Large Hadron Collider (LHC) for its High Luminosity program (HL-LHC). The upgrades are planned in order to exploit the Physics related to the newly discovered Higgs boson and to extend the discovery potential and sensitivity of the LHC experiments for new Physics. In this phase of operations (Phase-II), CMS plans to make use of tracking information at the first level of the trigger system (L1) for the first time, in order to reduce the amount of the additional inelastic interactions per proton-proton beam crossings from the interaction region (Pileup or PU). This will be achieved by including reconstructed tracks of charged particles at the L1 Trigger system (L1 Track Trigger). This trigger is an integral part of the design of the new silicon Tracker. In parallel, upgrades to the CMS detectors’ readout systems will allow reading out of collision events at a rate of 750 kHz at L1, compared to 100 kHz during Run-1.

The Physics program of HL-LHC requires that CMS retains its capability to efficiently trigger events originating from low-mass Physics processes, like Higgs production at 125 GeV. To maintain the overall Physics acceptance, the trigger thresholds on leptons, photons, taus, jets and missing transverse energy should remain comparable to those used in CMS Run-1, despite the much higher luminosities and challenging PU conditions. In this project, the design of a L1 Trigger was implemented in order to reduce the rate of events containing a tau lepton that decays hadronically (tau-jet). This so called L1 Tau Trigger combines information from the Tracker and the Calorimeter detectors to select events with at least one tau-jet in the final state. Tau-jets appear in many physics process and their distinct properties constitute them significant handles for probing familiar and new Physics.

The first objective of this project was to design, develop and maintain an innovative and high-performance L1 Trigger algorithm for selecting tau-jets, the first ever in CMS to employ tracking information. This L1 Tau Trigger algorithm should be able to efficiently select tau-jets and feed them to the High Level Trigger (HLT) at sufficiently low rates, even at the peak of the HL-LHC instantaneous luminosity. The second objective was the in-depth investigation of the performance of the L1 Tau Trigger algorithm in Physics processes and the assessment of the algorithm's abilities and discriminating power. The third objective of this project was concerned with employing the developed L1 Tau Trigger algorithm in producing Physics output for various Physics processes of interest, and the fourth objective was the dissemination of results to the scientific community and the general public.

The completion of this project has resulted in all work objectives being met. More specifically, a fully developed innovative L1 Tau Trigger algorithm has been designed that performs as requested in the hard collision environments of the HL-LHC. This algorithm can efficiently operate with energy thresholds that retain a high efficiency on important Physics processes, like H0 -> tau+ tau-, even under the extreme CMS Phase-II conditions. The algorithm is seeded with Calorimeter Tau objects, which are first required to be within close proximity of at least one good-quality, high-momentum track. The track that is found to be closest to the Calorimeter Tau object is called the matching track. An isolation cone is then opened around the direction of the matched track, whose size is proportional to the transverse energy of the L1 Tau Trigger candidate. This cone is used to apply track-based isolation criteria to the L1 Tau Trigger candidate. The candidate object is accepted as isolated if no track within this isolation cone is found close to the matched track. The energy and direction of the L1 Tau Trigger object is defined from the original Calorimeter Tau objects. Events with multiple L1 Tau Trigger objects are made more robust against PU by demanding that the taus originate from a common vertex. Moreover, the L1 Tracks in the event are used to select the true primary interaction vertex leading to a reduction of Trigger rates from additional interactions in the events. The employment of tracking information in the design of the L1 Tau Trigger allows to maintain energy thresholds in the desired range, while ensuring that the total L1 Tau Trigger rate remains below allocated bandwidth of 50 kHz, even when the luminosity is such that 200 PU interactions are expected. The summary of the results is presented in the CERN-LHCC-2015-010:“Technical Proposal for the Phase-II Upgrade of the Compact Muon Solenoid” and in the CERN-LHCC-2015-019: "CMS Phase II Upgrade Scope Document".

In parallel to the scientific output, an important objective of this project was the dissemination of the acquired knowledge to the scientific community and the general public. Apart from the documentation of the work in internal and public CMS reports and the presentation of its results in International Conferences and Workshops, numerous and various outreach activities were organized to support this project. These included seminars and talks open to the general public at the Host Organization, on topics related to the CMS activities during LHC and HL-LHC. In addition, Physics posters and demonstrations were specifically prepared for participation of the project team in Science Festivals and similar outreach events. Finally, a website was designed specifically for the project, in order to increase public awareness for the work that has been done while also providing easy access to all outreach-related material.