Multiboson measurements in CMS and ATLAS

The collection of more than 150 fb$^{-1}$ of data at the Large Hadron Collider (LHC) allows for the exploration of rare processes predicted in the Standard Model (SM) that were previously inaccessible, such as, the production of triple gauge bosons. The discovery of triboson production was announced by the CMS Collaboration in April, 2020 and marks an important achievement in SM physics. The first observation of tribosons ($\mathrm{VVV}$) in leptonic final states is reported. The $\mathrm{VVV}$ process is observed (expected) with a significance of 5.7 (5.9) $\sigma$ and the measured signal strength is 1.02$^{+0.26}_{-0.23}$. The abundance of data also enables the analyses of diboson processes in considerable detail. Two representative precision analyses are discussed, namely, the measurement of ${\rm W}^{+} {\rm W}^{-}$ using data from both ATLAS and CMS experiments and the detailed study of the ${\rm{Z}}\gamma$ process using the ATLAS detector. The study of ${\rm W}^{+} {\rm W}^{-}$ is performed using complementary approaches in ALTAS and CMS. In ATLAS while a zero jet bin is utilized to isolate the ${\rm W}^{+} {\rm W}^{-}$ signal from background processes, in CMS both zero and one jet bins are used. A novel method involving the use of random forest discriminators to suppress backgrounds is also pursued. The ${\rm{Z}}\gamma$ process is studied using the complete Run II dataset of 139 fb$^{-1}$ collected with the ATLAS detector. The fiducial cross section of this process is measured with an unprecedented experimental precision of 2.9%.