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Real-time precision tests of lepton universality

Periodic Reporting for period 3 - RECEPT (Real-time precision tests of lepton universality)

Reporting period: 2020-06-01 to 2021-11-30

The Standard Model (SM) of particle physics is a remarkably successful description of nature at microscopic scales, some of whose predictions have been verified to better than one part in a billion. Six quarks, six leptons, four force carriers and the Higgs boson are all that is needed to describe the microscopic universe! Nevertheless, the SM is in contradiction with successful theories of the macroscopic universe. For example, the mass of stars and galaxies estimated from their emitted light is in significant disagreement with that predicted from their motion by General Relativity, leading physicists to postulate the existence of so-called "dark" matter. Because of this and other contradictions, physicists believe that the SM is incomplete, and that a more fundamental theory is needed.

The scientific objectives of RECEPT are to find particles or forces beyond the SM which can give a hint of this more fundamental theory. Even if no new particles or forces are found, the RECEPT measurements will narrow down the range of possible theories beyond the SM. In either case, humanity will gain a more complete understanding of the world around us, and of the relationship between the smallest and largest building blocks of our reality. In order to enable these fundamental advances, RECEPT researchers will have to develop new and uniquely efficient ways of processing their data in real time. This work goes beyond state-of-the-art in both research and industry, and the new algorithms developed by the RECEPT team may therefore have practical benefits for real-time data processing in society.

One of the SM's most precise predictions is called “lepton universality” : that the six leptons couple with equal strength to the force carriers (photon and the W/Z bosons) of the electroweak force. RECEPT uses the LHCb experiment, based at the Large Hadron Collider (LHC) at CERN, to make the world’s most precise tests of lepton universality by studying the decays of SM particles called beauty mesons. By measuring the rate at which these mesons decay into final states containing electrons or muons, RECEPT researchers are able to indirectly search for the presence of particles and forces beyond the SM, which may break this universality.
In the first phase of RECEPT, LHCb data is being used to measure lepton universality between electrons and muons in decays of particles containing a “bottom” quark. In order to pave the way for these lepton universality tests, RECEPT researchers developed a method for measuring the efficiency of detecting single electrons in LHCb data, similarly to techniques which already existed for muons. The RECEPT team is now in the final stages of two lepton universality measurements, one of which involves abundant transitions between "beauty" and "charm" quarks, and the other much rarer transitions between "beauty" and "strange" quarks. Previous results from both LHCb and other experiments indicated potential deviations from the Standard Model in these processes. The new measurements from the RECEPT team will have a much greater precision, and could therefore provide conclusive evidence of processes beyond the SM.

RECEPT’s researchers also play a crucial role in the upgrade of LHCb, which will increase the data volume 100 times, allowing much more precise SM tests and searches for new particles and forces. In order to take full advantage of this, LHCb will have to process 5 Terabytes of data each second, not only tagging individual LHC collisions as "interesting" but finding the trajectories of particles produced in such collisions and inferring their fundamental physical properties in real time. Such a data volume is equivalent to around 5% of today's global internet traffic, and must be processed in a data centre located close to LHCb, using only around 2000 computer servers. The RECEPT team has developed high-performance algorithms for both CPU and GPU computing architectures which have been shown to be able to meet this challenge. A particular highlight has been the development of "Allen", a complete framework for high-throughput processing on GPUs. Allen has been reviewed and judged as a viable alternative to the baseline CPU processing for the LHCb upgrade, and the RECEPT team is now waiting to find out if LHCb will decide to use it.
At the project midpoint, RECEPT has progressed beyond state of the art in two areas: the understanding of electron reconstruction in LHCb, and the real-time reconstruction of the upgraded LHCb detector.

The method used to measure the efficiency with which single electrons can be reconstructed directly in LHCb data had previously been applied to other charged particles, but this is the first time it has been shown to work for electrons. The better than percent level precision shows that the measurements planned by RECEPT over the course of the project will not be limited by our knowledge of electron reconstruction.

The contributions of the RECEPT team to the real-time reconstruction of the upgraded LHCb detector have involved multiple novel algorithms and techniques which go significantly beyond the state of the art. The charged particle reconstruction algorithms developed by RECEPT researchers have both significantly better physics performance and are significantly faster than state of the art algorithms. Allen is the first feature-complete high-throughput GPU trigger in high-energy physics which has been declared viable for use in production.

RECEPT researchers are currently finalizing measurements of lepton universality in the decays of beauty hadrons, which are expected to be published during 2020. Regardless of the processing technology chosen, RECEPT researchers will deliver their components of LHCb's real-time processing in 2021. This will include not only the high-speed reconstruction algorithms but also the machinery which allows the physical properties of particles inferred through these reconstruction algorithms to be saved for further analysis. The RECEPT team will use this new real-time processing and upgraded LHCb to test lepton universality in kaon decays.
Allen throughput on different GPU cards
Efficiency of LHCb electron reconstruction in data and simulation (from LHCb-DP-2019-003)
Mass distribution of electron probe candidates in data (from LHCb-DP-2019-003)
Allen throughput as a function of the event complexity