Search for emerging jets in $pp$ collisions at $\sqrt{s} = 13.6$ TeV with the ATLAS experiment

Abstract: A search for emerging jets is presented using 51.8 fb$^{-1}$ of proton-proton collision data at $\sqrt{s} = 13.6$ TeV, collected by the ATLAS experiment during 2022 and 2023. The search explores a hypothetical dark sector featuring 'dark quarks', which are charged under a confining gauge group and couple to the Standard Model via a new mediator particle. These dark quarks undergo showering and hadronization within the dark sector, forming long-lived dark mesons that decay back into Standard Model particles. This results in jets which contain multiple displaced vertices known as emerging jets. The analysis targets events with pairs of emerging jets, produced either through a vector mediator, $Z'$, in the $s$-channel, or a scalar mediator, $\Phi$, in the $t$-channel. No significant excess over the Standard Model background is observed. Assuming a dark pion proper decay length between 5 and 50 mm, $Z'$ mediator masses between 600 and 2550 GeV are excluded for quark and dark quark coupling values of 0.01 and 0.1, respectively. For a quark-dark quark coupling of $0.1$, $\Phi$ mediator masses between 600 and 1375 GeV are excluded. These results provide the first direct constraints on emerging jet pair production via a $Z'$ mediator, and represent the first search to investigate emerging jet production via $t$-channel exchange of a scalar mediator.