Elementary Particle Physics Group

Theme #1: Higgs boson properties measurements

The process H → ZZ* → 4l is the „golden channel“ for the discovery and accurate measurement of Higgs boson properties. The analysis relies on the complete reconstruction of four leptons resulting in a narrow four-lepton mass peak in presence of a small continuum background mainly coming from ZZ* → 4l process. The ultimate goal is to perform outstanding measurements of Higgs boson properties as fine deviations from Standard Model predictions could be a sign of new physics beyond the Standard Model. The analysis will use data collected during the entire Run 3 LHC cycle and data that will be collected during the Run 3 LHC cycle. According to the current LHC schedule, this can be achieved in time with the project termination, expected in December 2024. To achieve this goal, the precision in the reconstruction of leptons and measurement of their transversal momentum down to the very low values (pT < 10 GeV) is crucial as well as the development of new methods for improving identification and isolation of electrons and muons measured by the CMS detector. This being said, it is necessary to develop and implement new modern machine learning methods that will optimally differentiate background electrons and muons in the analysis from the signal electrons and muons. The special attention should be devoted to optimizing machine learning model hyperparameters to minimize background events by removing non-isolated electrons and muons coming from decays of heavy-flavor mesons, mis-reconstructed jets (usually originating from light-flavor quarks) and electrons from γ conversions.

Theme #2: ECAL Trigger improvements

The prerequisite for any successful analysis is the flawless operation of the CMS detector to gather high-quality data. The group will play an important role in ensuring the flawless operation of the CMS detector through several activities. During LS2 it is necessary to develop and optimize ECAL trigger spike killer algorithm and measure Level 1 (L1) trigger efficiency. Anomalous signals detected in the ECAL Barrel part of the detector may come from direct ionization within the photodiode on individual crystals. This creates false isolated signals with high apparent energy. These “spikes” can cause high trigger rates at both levels, L1 and HLT. Therefore, it is extremely important to remove such signals. On average, one spike with ET > 3 GeV is observed per 370 minimum bias triggers in CMS at √s = 7 TeV. If untreated, 60% of the EM trigger candidates, above an EG threshold of 12 GeV, would be caused by spikes. At high luminosity, these would be the dominant component of the 100 kHz CMS L1 trigger rate bandwidth.

The ECAL crystal response varies under irradiation due to the formation of color centers that absorb the light and reduce the transparency of the PbWO4 crystals. The damage is partially recovered in a few hours at room temperature. The recovery during LHC cycles is monitored by laser light injected, during the LHC orbit gaps, into each crystal, through optical quartz fibers. The complete cycle takes ∼45 min. The laser light is also injected into PN diodes to follow the laser pulse to pulse variations with an accuracy of 0.1%. ECAL signals are compared and normalized, event by event, to the reference PN diodes. The resulting transparency corrections are ready for prompt reconstruction in less than 48 hours. The resulting transparency corrections need to be validated and, if everything is fine, loaded into the CMS detector to ensure optimal data quality. The group has already been responsible for the corrections validation and upload into the CMS detector. The goal is to develop new algorithms to optimize the whole process and ensure smooth data taking during the Run 3. The described task has a high level of responsibility and high visibility in the CMS collaboration.