Primary Vertex Reconstruction for Heterogeneous Architecture at CMS

This note reports on vertex reconstruction at CMS in HL-LHC conditions. The CMS vertex reconstruction is a two-step procedure consisting of vertex finding and fitting. The vertex is found producing clusters of tracks coming from the same interaction vertex using the Deterministic Annealing (DA) algorithm and then an Adaptive Vertex Fit computes the best estimate of the vertex position and the parameters of the associated tracks. Given the high impact on reconstruction time of this procedure (up to 10% of the whole time with HL-LHC conditions), new algorithms have been developed for the two steps mentioned above. The new clustering procedure sorts the tracks in the z coordinate, splits them in blocks of same size (set by default to 512) with a fixed overlap fraction between blocks (set by default to 0.5) and performs independently the DA along all the blocks. The new estimator iteratively estimates the vertex 3D coordinates and errors using the weighted mean of tracks impact point at the beamspot position and uncertainty. The iterations include an outlier rejection to improve the performance.