Hadron Collider Probes of the Quartic Couplings of Gluons to the Photon and $Z$ Boson

Abstract: We explore the experimental sensitivities of measuring the $gg \rightarrow Z \gamma$ process at the LHC to the dimension-8 quartic couplings of gluon pairs to the $Z$ boson and photon, in addition to comparing them with the analogous sensitivities in the $gg \to \gamma \gamma$ process. These processes can both receive contributions from 4 different CP-conserving dimension-8 operators with distinct Lorentz structures that contain a pair of gluon field strengths, $\hat G^a_{\mu \nu}$, and a pair of electroweak SU(2) gauge field strengths, $W^i_{\mu \nu}$, as well as 4 similar operators containing a pair of $\hat G^a_{\mu \nu}$ and a pair of U(1) gauge field strengths, $B_{\mu \nu}$. We calculate the scattering angular distributions for $gg \rightarrow Z \gamma$ and the $Z \to \bar f f$ decay angular distributions for these 4 Lorentz structures, as well as the Standard Model background. We analyze the sensitivity of ATLAS measurements of the $$Z(\to \ell^{+\ell^-,} \bar \nu \nu, \bar q q)\gamma$$ final states with integrated luminosities up to 139 fb$<sup>{-1}$ at $\sqrt{s} = 13$ TeV, showing that they exclude values $\lesssim 2$ TeV for the dimension-8 operator scales, and compare the $Z \gamma$ sensitivity with that of an ATLAS measurement of the $\gamma \gamma$ final state. We present combined $Z \gamma$ and $\gamma \gamma$ constraints on the scales of dimension-8 SMEFT operators and $\gamma \gamma$ constraints on the nonlinearity scale of the Born-Infeld extension of the Standard Model. We also estimate the sensitivities to dimension-8 operators of experiments at possible future proton-proton colliders with centre-of-mass energies of 25, 50 and 100 TeV, and discuss possible measurements of the $Z$ spin and angular correlations.