Search for direct stau production in events with two hadronic $\tau$-leptons in $\sqrt{s}=13\mathrm{TeV}$ $pp$ collisions with the ATLAS detector

A search for the direct production of the supersymmetric partners of $\tau$-leptons (staus) in final states with two hadronically decaying $\tau$-leptons is presented. The analysis uses a dataset of $pp$ collisions corresponding to an integrated luminosity of $139{\mathrm{fb}}^{-1}$, recorded with the ATLAS detector at the Large Hadron Collider at a center-of-mass energy of 13 TeV. No significant deviation from the expected Standard Model background is observed. Limits are derived in scenarios of direct production of stau pairs with each stau decaying into the stable lightest neutralino and one $\tau$-lepton in simplified models where the two stau mass eigenstates are degenerate. Stau masses from 120 GeV to 390 GeV are excluded at 95% confidence level for a massless lightest neutralino.

Figure 1

A diagram illustrating the pair production of staus and subsequent decay into a two-$\tau$-lepton final state with missing transverse momentum from the neutralinos.

Figure 2

Illustration of the ABCD method for the multijet background determination for SR-lowMass (left) and SR-highMass (right). The control regions A, B, C, and signal region D for the ABCD method described in the text (labeled as CR-A, CR-B, CR-C and SR-lowMass/SR-highMass) are drawn as white boxes. Shown shaded and labeled VR are the regions E and F, which are used to validate the ABCD method and to estimate the systematic uncertainty. The transfer factor $T$ used in the ABCD method is the ratio of number of multijet events in the regions C and B.

Figure 3

The $m_{\mathrm{T}2}$ (top) and $E_{\mathrm{T}}^{\text{miss}}$ (bottom) postfit distributions in the multijet background validation region VR-F (lowMass) at left and VR-F (highMass) at right. The stacked histograms show the contribution of each relevant SM process. The multijet shape is taken from VR-E in the ABCD method and the normalization is determined by the transfer factor $T$ and rescaled by a correction factor determined by the background-only fit to data, described in Sec. 8. The hatched bands represent the combined statistical and systematic uncertainties in the sum of the SM backgrounds shown. For illustration, the distributions from the SUSY reference points (defined in Sec. 3) are also shown as dashed lines.

Figure 4

The prefit $m_{\mathrm{T}2}$ distribution in the $W\mathrm{CR}$. The SM backgrounds other than multijet production are estimated from MC simulation. The multijet contribution is estimated from data using the OS–SS method. The hatched bands represent the combined statistical and systematic uncertainties of the total SM background. For illustration, the distributions from the SUSY reference points defined in Sec. 3 are also shown as dashed lines.

Figure 5

The postfit yields in the $W\mathrm{VR}$, $T\mathrm{VR}$s, $Z\mathrm{VR}$s and $VV\mathrm{VR}$s. The SM backgrounds other than multijet production are estimated from MC simulation. The multijet contribution is negligible and is estimated from data using the ABCD method, using CRs obtained with the same technique used for the SRs, and described in Sec. 6a. The hatched bands represent the combined statistical and systematic uncertainties of the total SM background. The background-only fit to data is used, described in Sec. 8. For illustration, the distributions from the SUSY reference points (defined in Sec. 3) are also shown as dashed lines. The lower panel shows the ratio of data to the SM background estimate.

Figure 6

The postfit $m_{\mathrm{T}2}$ distribution for SR-lowMass (left) and SR-highMass (right). The stacked histograms show the expected SM backgrounds. The multijet contribution is estimated from data using the ABCD method. The contributions of multijet and $W+\text{jets}$ events are scaled with the corresponding normalization factors derived from the background-only fit described in Sec. 8. The hatched bands represent the sum in quadrature of systematic and statistical uncertainties of the total SM background. For illustration, the distributions from the SUSY reference points (defined in Sec. 3) are also shown as dashed lines. The last bin includes the overflow events.

Figure 7

The 95% C.L. exclusion contours for the combined fit of SR-lowMass and SR-highMass for simplified models with (a) combined ${\tilde{\tau }}_{\mathrm{R},\mathrm{L}}^{+}{\tilde{\tau }}_{\mathrm{R},\mathrm{L}}^{-}$ production and (b) ${\tilde{\tau }}_{\mathrm{L}}^{+}{\tilde{\tau }}_{\mathrm{L}}^{-}$ only production. The text provides details of exclusion curves and uncertainty bands.