Test of CP invariance in vector-boson fusion production of the Higgs boson in the → Channel in Proton–proton Collisions at with the ATLAS detector

A test of CP invariance in Higgs boson production via vector-boson fusion is performed in the $H\to \tau \tau $ decay channel. This test uses the Optimal Observable method and is carried out using $36.1\phantom{\rule{0.2em}{0ex}}{\mathrm{fb}}^{-1}$ of $\sqrt{s}=13\phantom{\rule{0.2em}{0ex}}\text{TeV}$ proton–proton collision data collected by the ATLAS experiment at the LHC. Contributions from CP-violating interactions between the Higgs boson and electroweak gauge bosons are described by an effective field theory, in which the parameter $\stackrel{˜}{d}$ governs the strength of CP violation. No sign of CP violation is observed in the distributions of the Optimal Observable, and $\stackrel{˜}{d}$ is constrained to the interval [−0.090, 0.035] at the 68% confidence level (CL), compared to an expected interval of $\stackrel{˜}{d}\in \left[-0.035,0.033\right]$ based upon the Standard Model prediction. No constraints can be set on $\stackrel{˜}{d}$ at 95% CL, while an expected 95% CL interval of $\stackrel{˜}{d}\in \left[-0.21,0.15\right]$ for the Standard Model hypothesis was expected.