2018 Tau Reconstruction Comparison at the HLT ( CMS DP-2018/035)

The high level trigger (HLT) reconstructed hadronically decaying tau leptons (τ_h) with a cone-based reconstruction algorithm throughout Run I and the first portion of Run II. In 2018, the Hadron Plus Strips (HPS) τ_h reconstruction algorithm, which is currently used for offline analysis, was introduced to the HLT to test its performance. In early June 2018, many of the τ_h trigger paths switched from the cone-based to the HPS reconstruction algorithm after showing simultaneous rate reductions and efficiency gains.

The cone-based τ_h algorithm uses a signal cone ranging from ΔR = 0.08 to 0.12 which contains the τ decay products. Charged hadrons, photons, and neutral hadrons within the cone are included in the reconstructed τ_h candidate.

The HPS τ_h algorithm reconstructs the various decay modes of hadronically decaying tau leptons[1]. The charged hadrons and photons within the signal cone are combined together in multiple combinations and ranked based on their consistency with a genuine τ_h decay. The highest ranked combination is selected as the reconstructed τ_h candidate.
[1] “Performance of reconstruction and identification of tau leptons in their decays to hadrons and tau neutrino in LHC Run-2” CMS Collaboration, CMS-PAS-TAU-16-002

The performance of the two τ_h reconstruction methods, cone-based versus HPS, is assessed by comparing the rate and selection efficiency for the associated trigger paths.

The performance is presented for two types of triggers: μτ_h triggers and τ_hτ_h triggers. The μτ_h trigger requires both objects to be isolated and has a pT requirement of 20 GeV for the μ and 27 GeV for the τ_h. The τ_hτ_h trigger requires both τ_h are isolated and have pT > 35 GeV. To maintain a reasonable rate, the cone-based trigger has a relatively tight isolation requirement on both τ_h unless both have pT > 40 GeV in which case the isolation requirements are relaxed. In comparison, the HPS trigger is able to maintain a consistently looser isolation working point across the full pT spectrum.

Rate measurements: The rate associated with each trigger path is measured during the start of a recent LHC fill with average PU = 50.

Efficiency measurements: Trigger efficiency is measured using tag and probe in Z -> ττ -> μτ_h events. 2018 CMS data is used corresponding to 14.3 fb-1. An isolated muon which passes one of the single muon HLT paths is used as the tag. A τ_h candidate, the probe, is required to have pT > 20 GeV, |η| < 2.1, to pass offline isolation and identification criteria, and to pass anti-muon and anti-electron discriminants designed to suppress Z -> μμ/ee events The contribution of jets faking τ_h are suppressed in the measurement with:

μ-τ_h pairs with same charge are subtracted from the μ-τ_h pairs with opposite charge which helps eliminate the contributions from QCD multijet processes. The final selection yields approximately 90% genuine τ_h purity. The τ_hτ_h trigger efficiency is measured with dedicated μτ_h triggers where the τ_h reconstruction and selection is identical to that used in the associated τ_hτ_h paths.

μτh Trigger Performance The τh leg efficiency of the μ(20)τh(27) trigger is shown for both τh reconstruction algorithms. The HPS path has a sharper turn-on compared to the cone-based path which is visible at the pT = 27 GeV threshold. The associated rate at average PU= 50 for the μτh triggers are 5.2 Hz for the cone-based algorithm and 4.6 Hz for the HPS algorithm. The efficiency of both paths is relatively comparable. [Get pdf version] [Get png version with x linear scale] [Get pdf version with x linear scale] Contact: Tyler Ruggles

τhτh Trigger Performance The per-leg efficiency of the τh(35)τh(35) trigger is shown for both τh reconstruction algorithms. The curves compare the selection of the highest efficiency τhτh triggers with an affordable rate. Measured at average PU = 50, the rate of the cone-based algorithm is 50 Hz and the HPS algorithm is 39 Hz, a 20% reduction for the HPS triggers. The HPS algorithm shows large benefits for the τhτh trigger path including a large rate reduction and a simultaneous efficiency increase. [Get pdf version] [Get png version with x linear scale] [Get pdf version with x linear scale] Contact: Tyler Ruggles

-- ElisabettaGallo - 2018-07-03