In 2017, the Large Hadron Collider (LHC) achieved a groundbreaking milestone in instantaneous luminosity, delivering data at twice the rate it was originally designed for. This presented a new challenge for the CMS detector: efficiently selecting and retaining significant events at such a high rate. To address this, an innovative strategy called data parking was implemented. This strategy maximized the detector’s data-taking capacity at lower luminosity by storing unprocessed data for later analysis. By utilizing this method with a single muon trigger, CMS successfully collected an inclusive sample of approximately 10 billion b hadrons in 2018.
For Run 3, which commenced in 2022, the program was further enhanced with the introduction of an inclusive dimuon trigger, covering the low mass range up to 8.5 GeV. Thanks to advancements in the CMS Tier-0 prompt reconstruction workflow, Run 3 parking data is now reconstructed without delay, utilizing the former Run 2 HLT farm at P5 and European Tier-1 resources.
Last month, at the International Conference on High Energy Physics (ICHEP), CMS presented the first results using its improved trigger and data collection strategy. Using data collected in 2022 and 2023, CMS conducted a search for the rare D0 meson decay into a pair of muons. Rare decays of the charm quark, which are less explored compared to those of the bottom quark, offer a unique opportunity to probe new physics effects beyond the direct reach of current colliders.
In 2023, the LHCb collaboration set an upper limit for B(D0→μ+μ−) at 3.5×10−9 with a 95% confidence level (CL) using Run 2 data. Given that the Standard Model prediction is four orders of magnitude smaller, there is considerable unexplored territory. CMS surpassed the LHCb result, achieving a new sensitivity level of 2.6×10−9 at a 95% CL. The figures below show distributions for a reference decay D0→π+π− and the signal D0→μ+μ−. For details see: https://cds.cern.ch/record/2905689
The announcement was a surprise to physicists from other experiments and theorists alike. No one expected CMS to be able to perform such measurements and deliver the best sensitivity in the world with just a fraction of the Run 3 data. This is only the first result among many that the collaboration is currently working on. Members of our group, Zhangqier Wang and Dmytro Kovalskyi, are leading the effort by being involved in designing and improving data collection, data processing, and data analysis.
Beginning with the 2024 run, the CMS flavor physics program will benefit from an additional data stream known as data scouting. This stream captures events at a very high rate using new high-purity single muon L1 triggers in a reduced format. This format is suitable for reconstructing decays of heavy hadrons, offering performance comparable to standard data processing. Interestingly, in 2024, CMS is collecting data at rates seven times higher than the nominal rates of Run 2, reaching approximately 70% of the HL-LHC nominal trigger rate.