Monte Carlo studies on the estimation of ZZ $\to$ $\ell \ell \nu \nu$ background using Z$\gamma$ $\to$ $\ell \ell \gamma$ events

In the search for Dark Matter (DM) at the LHC, SM particles are produced in association with DM particles, which are invisible as they don*t interact with the detector. Thus, events with large imbalance in transverse momentum are of interest. One such signature is $\ell \ell$ +E${}_T^{miss}$. The dominant background contributing to the search for DM in the $\ell \ell$ + E${}_T^{miss}$ channel is ZZ $\to$ $\ell \ell \nu \nu$. Currently, this background is determined using Monte Carlo simulation, with an uncertainty of about 10%. The goal of this study is to establish a data driven method to estimate this background, and reduce the uncertainty. Using Z$\gamma$ $\to$ $\ell \ell \gamma$, a process with low backgrounds and a high BR $\times$ $\sigma$, it is possible to estimate the ZZ $\to$ $\ell \ell \nu \nu$ contribution. In regions where $p_T$($\gamma$) $\gg$ M${}_Z$, the two processes are kinematically similar. They have the same production mechanisms, but differ due to the photon and Z boson couplings to the quarks being different, as well as the difference in mass (photons are massless, while Z bosons are massive). Introducing a transfer factor R as the ratio $\sigma$(ZZ)/$\sigma$(Z$\gamma$) which is determined from simulation, the contribution of ZZ $\to$ $\ell \ell \nu \nu$ to the background can be estimated from Z$\gamma$ $\to$ $\ell \ell \gamma$ data. The uncertainty on the prediction of R due to theoretical aspects is estimated in this work.