Speaker
Adam Davis
(University of Cincinnati (US))
Description
The LHCb tracking system consists of a Vertex Locator around the interaction point, a tracking station with four layers of silicon strip detectors in front of the magnet, and three tracking stations, using either straw-tubes or silicon strip detectors, behind the magnet. This system allows to reconstruct charged particles with a high efficiency (typically > 95% for particles with momentum > 5 GeV) and an excellent momentum resolution (0.5% for particles with momentum < 20 GeV). The high momentum resolution results in very narrow mass peaks, leading to a very good signal-to-background ratio in such key channels as $B_s \to \mu^+ \mu^-$. Furthermore an optimal decay time resolution is an essential element in the studies of time dependent CP violation.
For Run II a novel reconstruction strategy was adopted, allowing to run the same track reconstruction in the software trigger as offline. This convergence was possible due to a staged approach in the track reconstruction and a large reduction in the processing time, without any loss in the key performance numbers like impact parameter or decay time resolution. Having a unified track reconstruction greatly benefits all physics analyses due to the perfect alignment between their online and offline selection.
In this talk, we will give an overview of the track reconstruction in LHCb, review its performance in Run II of the LHC and highlight the challenges and improvements.
A similar scheme is planned to be used in the LHCb upgrade foreseen for 2020. At that time LHCb will run at an instantaneous luminosity of $2\times 10^{33} \mathrm{cm}^{-2} \mathrm{s}^{-1}$ with a fully software based trigger with a read-out of the detector at a rate of 40 MHz. A full new tracking system is being developed: a vertex detector based on silicon pixel sensors, a new silicon micro-strip detector with a high granularity and the scintillating fibre tracker. The new tighter time constraint in the trigger, where only about 13 ms are available per event, combined with a higher luminosity by a factor 5 represent a big challenge for the track reconstruction. A new track finding strategy has been considered and new computing approaches, partly based on GPUs, are under study. We will present the new strategy and the new fast track reconstruction, including the performance and the highlights of the improvements with respect to the current tracking system of LHCb.
