The L1 Trigger of the CMS Electromagnetic Calorimeter: Implementing a new timing feature for LHC Run 3

A map of the energy deposited in the CMS Electromagnetic calorimeter (ECAL) is provided by its electronics to the CMS Level-one trigger (L1), that selects LHC collisions to be read out, at a 40MHz rate. The on-detector electronics estimate the amplitudes of the signals used to reconstruct the energy deposits with a digital finite impulse response (FIR) filter. The application-specific integrated circuit (ASIC) that implements the filter can provide a second estimate using a second set of weights, a feature which was not used during Runs 1 and 2 of the LHC. Both weight sets are configurable. We will demonstrate, using Monte-Carlo simulation and CMS data, that this double amplitude estimate can be exploited to discriminate between in-time and out-of-time signals at L1. The potential gain is two-fold. Firstly it can provide improved rejection of direct signals arising in the APD photodetectors (termed "spikes"). Secondly, it can be configured to tag out-of-time signals at L1, and therefore be sensitive to physics signatures such as long lived particles. After having shown the potential of using two sets of weights, we will give the prospects for deploying the feature during Run 3 of the LHC, that will start in 2022.