Search for a scalar partner of the top quark in the all-hadronic $t\bar{t}$ plus missing transverse momentum final state at $\sqrt{s}=13$ TeV with the ATLAS detector

Abstract

A search for direct pair production of scalar partners of the top quark (top squarks or scalar third-generation up-type leptoquarks) in the all-hadronic $t\bar{t}$ plus missing transverse momentum final state is presented. The analysis of $139\,\mathrm{fb}^{-1}$ of $\sqrt{s}=13$ TeV proton-proton collision data collected using the ATLAS detector at the LHC yields no significant excess over the Standard Model background expectation. To interpret the results, a supersymmetric model is used where the top squark decays via $\tilde{t}\to t^{(*)}\tilde{\chi}_{1}^0$, with $t^{(*)}$ denoting an on-shell (off-shell) top quark and $\tilde{\chi}_1^0$ the lightest neutralino. Three specific event selections are optimised for the following scenarios. In the scenario where $m_{\tilde{t}}>m_t+m_{\tilde{\chi}_1^0}$, top squark masses are excluded in the range 400–1250 GeV for $\tilde{\chi}_1^0$ masses below 200 GeV at 95% confidence level. In the situation where $m_{\tilde{t}}\sim m_t+m_{\tilde{\chi}_1^0}$, top squark masses in the range 300-630 GeV for $\tilde{\chi}_1^0$ masses are excluded, while in the case where $m_{\tilde{t}}<m_W+m_b+m_{\tilde{\chi}_1^0}$ (with $m_{\tilde{t}}-m_{\tilde{\chi}_1^0}\geq 5$ GeV), considered for the first time in an ATLAS all-hadronic search, top squark masses in the range 300-660 GeV are excluded. Limits are also set for scalar third-generation up-type leptoquarks, excluding leptoquarks with masses below 1240 GeV when considering only leptoquark decays into a top quark and neutrino.

Baptiste Ravina

Alexander von Humboldt Fellow

I’m an experimental particle physicist looking for new physics at the ATLAS experiment with machine learning.