Search for decays of the Higgs boson into pair-produced pseudoscalar particles decaying into $τ^+τ^-τ^+τ^-$ using $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

Abstract

A search for a pair of low-mass pseudoscalars $a$ that promptly decay into $τ$-leptons is presented using 140 fb$^{-1}$ of proton-proton collision data at $13$ TeV centre-of-mass energy recorded with the ATLAS detector at the Large Hadron Collider. The result is used to place constraints on exotic decays of the Higgs boson into four $τ$-leptons, $H\to aa\to τ^+τ^-τ^+τ^-$. This search focuses on events with either one or two $τ$-leptons decaying into hadrons and neutrinos, and the remaining three or two $τ$-leptons decaying into either electron or muon and neutrinos. No significant excess is observed above the expected Standard Model background and upper limits at the 95% confidence level on $\mathcal{B}(H\rightarrow aa\rightarrow τ^+τ^-τ^+τ^-)$ are set ranging from 0.06 to 0.23, depending on the mass $m_a$ ranging from 15 to 60 GeV.

Figures (3)

• Figure 1: Post-fit visible mass, $m_{\text{vis}}^{\text{3-body}}$, distribution in the $1\ell2\tau_{\text{had}}$ (left) and $\ell e\tau_{\text{had}} / \ell \mu\tau_{\text{had}}$ (right) validation regions. The data (points) are compared with the background prediction, with the post-fit total uncertainty shown as a hashed band. The vertical dashed line at $m_{\text{vis}}^{\text{3-body}} = 110\,\text{Ge V}\xspace$ indicates the upper bound of the regions. The agreement between data and prediction in the high-mass sideband ($m_{\text{vis}}^{\text{3-body}} > 110\,\text{Ge V}\xspace$) validates the modelling of the fake/non-prompt background.
• Figure 2: Post-fit yield in the $2\ell2\tau_{\text{had}}$ and $3\ell1\tau_{\text{had}}$ signal regions from the background-only fit. The data (points) are compared with the background prediction, with the background total uncertainty shown as a hashed band. No data events were observed in the $2\ell2\tau_{\text{had}}$ SR. The expected number of signal events is shown for the $m_a=45\,\text{Ge V}\xspace$ hypothesis assuming $\mathcal{B}(H\to aa\to 4\tau)=10\%$ and the inclusive Higgs boson cross-section.
• Figure 3: The expected and observed 95% CL upper limits on the branching ratio of $H \to aa \to \tau^+\tau^-\tau^+\tau^-$ in the mass range $15<m_a<60\,\text{Ge V}\xspace$.