Constraining the top electroweak sector of the SMEFT through $Z$ associated top pair and single top production at the HL-LHC

Abstract

We study the processes $pp\to t\bar{t}Z$ and $pp\to tZj$ in the framework of Standard Model Effective Field Theory (SMEFT), employing conventional cut-and-count as well as machine learning techniques to take advantage of kinematic information in complex final states involving multiple leptons and $b$ jets. We explore the projected sensitivity for two SMEFT operators, ${\mathcal{O}}_{tZ}$ and ${\mathcal{O}}_{tW}$, that induce electroweak dipole moment interactions for top quarks, through direct searches in these electroweak top production processes at the HL-LHC. New physics modifications to dominant backgrounds are also considered. We show that the new physics sensitivity can be enhanced through a combination of differential distributions for relevant kinematic observables and machine learning techniques. Searches in $t\bar{t}Z$ and $tZj$ production result in stronger constraints on ${\mathcal{C}}_{tZ}$ and ${\mathcal{C}}_{tW}$, respectively. At the HL-LHC, ${\mathcal{C}}_{tZ}$ can be probed up to $-0.41\lesssim {\mathcal{C}}_{tZ}\lesssim 0.47$ through searches in the $pp\to t\bar{t}Z+tWZ\to 3\ell +2b+\ge 2j$ channel while ${\mathcal{C}}_{tW}$ can be probed up to $-0.14\lesssim {\mathcal{C}}_{tW}<0.11$ from searches in the $pp\to tZj+t\bar{t}Z+tWZ\to 3\ell +1b+1/2j$ channel, at $95\%$ CL.

Comments: 19 pages, 9 figures, 8 tables