Test of lepton flavor universality and real time event selection on GPUs at LHCb
by
Grand amphithéâtre
LPSC
In recent years, tensions between measurements and Standard Model predictions in the decays of b-hadrons, have hinted at the possible violation of lepton universality, specifically in observables in b→sll and b→clυ transitions. Among them is the ratio of branching fractions R(D*)τμ = B→ D*τυ/ B→ D*μυ. I will discuss the first measurement of the the ratio R(D*)eμ = B→ D*eυ / B→ D*μυ at the LHCb experiment at CERN, which which will lead to a combined measurement of all three lepton species. The LHCb experiment is currently being upgraded for Run 3 of the LHC to record more statistics and therefore reduce the uncertainties of observables testing lepton flavor universality. After this upgrade, LHCb will run without a hardware level trigger in 2021, resulting in the complete detector being read out at the full bunch-crossing rate of 30 MHz and a maximum data rate of 40 Tbit/s. Events of interest are selected with a software-only trigger in two stages. This allows unprecedented flexibility for trigger selections but at the same time poses a significant computing challenge. In this seminar, I will also present the ``Allen'' project, capable of processing the full first trigger stage, High Level Trigger 1 (HLT1), on about 500 state of the art graphics processing units (GPUs). During HLT1, a sub-set of the full offline track reconstruction for charged particles is run to select particles of interest based on single or two-track selections. Allen enables the exploitation of LHCb's broad physics program in Run 3. As the first complete high-rate GPU trigger, it has the potential to significantly impact the trigger systems of other HL-LHC experiments, as well as experiments at future facilities and colliders.