Cross-section measurements of the Higgs boson decaying into a pair of τ-leptons in proton-proton collisions √s at =13 TeV with the ATLAS detector

A measurement of production cross sections of the Higgs boson in proton-proton collisions is presented in the H→ττ decay channel. The analysis is performed using 36.1 fb⁻¹ of data recorded by the ATLAS experiment at the Large Hadron Collider at a center-of-mass energy of √s=13 TeV. All combinations of leptonic (τ→ ℓvv with ℓ= e, μ) and hadronic (τ→hadrons v) τ decays are considered. The H→ττ signal over the expected background from other Standard Model processes is established with an observed (expected) significance of 4.4 (4.1) standard deviations. Combined with results obtained using data taken at 7 and 8 TeV center-of-mass energies, the observed (expected) significance amounts to 6.4 (5.4) standard deviations and constitutes an observation of H→ττ decays. Using the data taken at √s=13 TeV, the total cross section in the H→ττ decay channel is measured to be 3.77^+0.60_−0.59 (stat)  ^+ 0.87_−0.74 (syst) pb, for a Higgs boson of mass 125 GeV assuming the relative contributions of its production modes as predicted by the Standard Model. Total cross sections in the H→ττ decay channel are determined separately for vector-boson-fusion production and gluon-gluon-fusion production to be σ ^VBF _H→ττ = 0.28 ± 0.09 (stat) ^+0.11_−0.09 (syst) pb and σ ^ggF _H→ττ = 3.1 ± 10 (stat) ^+ 1.6_−1.3 (syst) pb, respectively. Similarly, results of a fit are reported in the framework of simplified template cross sections. All measurements are in agreement with Standard Model expectations.