Measurement of the CP properties of Higgs boson interactions with $$\tau $$-leptons with the ATLAS detector (2023)

Abstract A study of the charge conjugation and parity ( $$\textit{CP}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>CP</mml:mi> </mml:math> ) properties of the interaction between the Higgs boson and $$\tau $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>τ</mml:mi> </mml:math> -leptons is presented. The study is based on a measurement of $$\textit{CP}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>CP</mml:mi> </mml:math> -sensitive angular observables defined by the visible decay products of $$\tau $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>τ</mml:mi> </mml:math> -leptons produced in Higgs boson decays. The analysis uses 139 fb $$^{-1}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mrow/> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:math> of proton–proton collision data recorded at a centre-of-mass energy of $$\sqrt{s}= 13$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msqrt> <mml:mi>s</mml:mi> </mml:msqrt> <mml:mo>=</mml:mo> <mml:mn>13</mml:mn> </mml:mrow> </mml:math> TeV with the ATLAS detector at the Large Hadron Collider. Contributions from $$\textit{CP}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>CP</mml:mi> </mml:math> -violating interactions between the Higgs boson and $$\tau $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>τ</mml:mi> </mml:math> -leptons are described by a single mixing angle parameter $$\phi _{\tau }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>ϕ</mml:mi> <mml:mi>τ</mml:mi> </mml:msub> </mml:math> in the generalised Yukawa interaction. Without constraining the $$H\rightarrow \tau \tau $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>H</mml:mi> <mml:mo>→</mml:mo> <mml:mi>τ</mml:mi> <mml:mi>τ</mml:mi> </mml:mrow> </mml:math> signal strength to its expected value under the Standard Model hypothesis, the mixing angle $$\phi _{\tau }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>ϕ</mml:mi> <mml:mi>τ</mml:mi> </mml:msub> </mml:math> is measured to be $$9^{\circ } \pm 16^{\circ }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msup> <mml:mn>9</mml:mn> <mml:mo>∘</mml:mo> </mml:msup> <mml:mo>±</mml:mo> <mml:msup> <mml:mn>16</mml:mn> <mml:mo>∘</mml:mo> </mml:msup> </mml:mrow> </mml:math> , with an expected value of $$0^{\circ } \pm 28^{\circ }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msup> <mml:mn>0</mml:mn> <mml:mo>∘</mml:mo> </mml:msup> <mml:mo>±</mml:mo> <mml:msup> <mml:mn>28</mml:mn> <mml:mo>∘</mml:mo> </mml:msup> </mml:mrow> </mml:math> at the 68% confidence level. The pure $$\textit{CP}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>CP</mml:mi> </mml:math> -odd hypothesis is disfavoured at a level of 3.4 standard deviations. The results are compatible with the predictions for the Higgs boson in the Standard Model.