Exclusive $\phi$ Meson Electroproduction with CLAS12

The CLAS12 detector completed in May 2018 its first data taking following the Jefferson Laboratory 12 GeV upgrade. About 3 months of longitudinally polarized electron beam were gathered on a 5 cm long liquid hydrogen target, at beam energies of up to 10.6 GeV and currents up to 75 nA. First rounds of calibration, data quality monitoring, and full reconstruction were achieved. We analyze the exclusive electroproduction of a $\phi$ meson in the Bjorken regime, using its decay into charged Kaons $ep\rightarrow epK^+K^-$. The main goal of this study is the extraction of the longitudinal cross section $t$-slopes as a function of $x_{B}$, which is sensitive to the gluon generalized parton distribution. The analysis strategy consists in two steps: first to establish the approach to the small-size regime by testing model-independent features of the reaction mechanism, such as the $Q^2$-independence of the t-slopes; then in a second step, extracting the gluonic size in the valence region as a function of $x_{B}$. We aim to present preliminary results on the reaction selection using cuts on exclusivity variables, compared to design expectations.