Search for Supersymmetry with a Compressed Mass Spectrum in Events with a Soft $\tau$ Lepton, a Highly Energetic Jet, and Large Missing Transverse Momentum in Proton-Proton Collisions at $\sqrt{s}=13\mathrm{TeV}$

The first search for supersymmetry in events with an experimental signature of one soft, hadronically decaying $\tau$ lepton, one energetic jet from initial-state radiation, and large transverse momentum imbalance is presented. These event signatures are consistent with direct or indirect production of scalar $\tau$ leptons ($\tilde{\tau }$) in supersymmetric models that exhibit coannihilation between the $\tilde{\tau }$ and the lightest neutralino (${\tilde{\chi }}_1^0$), and that could generate the observed relic density of dark matter. The data correspond to an integrated luminosity of $77.2{\mathrm{fb}}^{-1}$ of proton-proton collisions at $\sqrt{s}=13\mathrm{TeV}$ collected with the CMS detector at the LHC in 2016 and 2017. The results are interpreted in a supersymmetric scenario with a small mass difference ($\mathrm{\Delta }m$) between the chargino (${\tilde{\chi }}_1^{\pm }$) or next-to-lightest neutralino (${\tilde{\chi }}_2^0$), and the ${\tilde{\chi }}_1^0$. The mass of the $\tilde{\tau }$ is assumed to be the average of the ${\tilde{\chi }}_1^{\pm }$ and ${\tilde{\chi }}_1^0$ masses. The data are consistent with standard model background predictions. Upper limits at 95% confidence level are set on the sum of the ${\tilde{\chi }}_1^{\pm }$, ${\tilde{\chi }}_2^0$, and $\tilde{\tau }$ production cross sections for $\mathrm{\Delta }m({\tilde{\chi }}_1^{\pm },{\tilde{\chi }}_1^0)=50\mathrm{GeV}$, resulting in a lower limit of 290 GeV on the mass of the ${\tilde{\chi }}_1^{\pm }$, which is the most stringent to date and surpasses the bounds from the LEP experiments.

Figure 1

The $m_{\mathrm{T}}$ distribution for SR events with 2016 plus 2017 data. In the upper panel, the solid colors correspond to the expected background processes, the black dots to the observed data, and the dashed lines to the expected signal from simulation. The lower panel shows the ratio between the observed data and the total expected pre-fit background (BG). The shaded band corresponds to the total pre-fit uncertainty on the BG prediction, while the error bars on the black dots represent the statistical uncertainties on the data yields.

Figure 2

(left) The 95% confidence level (C.L.) upper limits on the SSM1 production cross sections (${\sigma }_{95\%\mathrm{C}.\mathrm{L}.}$) as a function of $m({\tilde{\chi }}_1^{\pm })$. The solid blue line shows the theoretical cross section, and the dashed blue line its uncertainty. The observed limit is shown with the solid black line, while the expected limit is shown with the dashed black line. The green (yellow) band corresponds to the one (two) standard deviation range about the central value of the expected limit. (right) The ratio of the 95% C.L. upper limit on the direct $\tilde{\tau }$ pair production signal cross section in SSM2 to the theoretical cross section, as a function of $m(\tilde{\tau })$ and $\mathrm{\Delta }m(\tilde{\tau },{\tilde{\chi }}_1^0)$.