Measurement of $J/\psi$ meson production in association with a $W^\pm$ boson in $pp$ data at 13 TeV collected by the ATLAS Experiment

This dissertation presents a measurement of the production cross-section of $J/\psi$ mesons in association with a $W^\pm$ boson in $\sqrt{s} = 13~\textrm{TeV}$ $pp$ collisions by the ATLAS experiment, using $139~\textrm{fb}^{-1}$ of integrated luminosity produced by the Large Hadron Collider at CERN. Events are selected with $W^\pm \rightarrow \ell^\pm \nu$ and $J/\psi \rightarrow \mu^+\mu^-$. The ratio of the prompt $J/\psi + W^{\pm}$ cross- section to the inclusive $W^\pm$ cross-section is presented for $J/\psi$ transverse momenta in the range $8-200~\textrm{GeV}$ and rapidity $\left|y_{J/\psi}\right| < 2.1$, and is presented as a differential measurement as a function of $J/\psi$ transverse momenta and rapidity. Comparisons are made with theoretical predictions for single-parton and double-parton production events.