The performance of the ATLAS missing transverse momentum high-level trigger in 2015 pp collisions at $13$ TeV

The performance of the ATLAS missing transverse momentum (${E_\text{T}^\text{miss}}$) high-level trigger during 2015 operation is presented. In 2015, the Large Hadron Collider operated at a higher centre-of-mass energy and shorter bunch spacing ($\sqrt{s} = 13$ TeV and $25$ ns, respectively) than in previous operation. In future operation, the Large Hadron Collider will operate at even higher instantaneous luminosity ($\mathcal{O}(10^{34} \text{ cm$^{-2}$ s$^{-1}$}$) and produce a higher average number of interactions per bunch crossing, $\langle \mu \rangle$. These operating conditions will pose significant challenges to the ${E_\text{T}^\text{miss}}$ trigger efficiency and rate. An overview of the new algorithms implemented to address these challenges, and of the existing algorithms is given. An integrated luminosity of $1.4 \text{ fb$^{-1}$}$ with $\langle \mu \rangle = 14$ was collected from pp collisions of the Large Hadron Collider by the ATLAS detector during October and November 2015 and was used to study the efficiency, correlation with offline reconstruction, and rates of the trigger algorithms. The performance was found to be satisfactory. From these studies, recommendations for future operating specifications of the trigger were made.