Enabling Novel LLP Searches through 5D Calorimetry with the Compact Muon Solenoid Experiment at the Large Hadron Collider

Long-lived particles are a compelling direction to search for physics beyond the Standard Model, and implementing dedicated triggers provides an excellent avenue to expand experimental coverage into this challenging parameter space. With the recent upgrades of the Hadron Calorimeter (HCAL) in the Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider (LHC), the HCAL provides new timing capabilities for jets and hadronic tau decays with nearly $4\pi$ coverage, and sensitivity to highly displaced decays that occur within the calorimeter volume. The augmentation of the calorimeter information at the hardware trigger level enables quick identification of long-lived particle (LLP) decays using lower thresholds on energy-based event quantities. Both the depth segmentation and online timing capabilities are utilized in novel HCAL-based hardware-level triggers to identify displaced and delayed LLPs, either decaying inside the calorimeter volume (with sensitivity for decays up to almost 6 m from the collision point) or arriving at a delayed time. These triggers are deployed for Run-3 of the LHC, beginning data-taking in 2022. This two-pronged calorimeter trigger approach leverages the new capabilities of the CMS HCAL to expand the phase space accessible in ongoing LLP searches. In addition, online timing is utilized for detector alignment based on positioning the pulse rising edge, achieving an alignment accuracy of 0.5 ns, considerably higher than previous energy-weighting based approaches. The data collected with the new triggers provides a first look at the capabilities to capture softer events and expand the phase space accessible in LLP searches. A search is conducted for long-lived particles using the CMS experiment's 2022 and 2023 Run-3 datasets, comprising $67 \text{ fb}^{-1}$ from proton-proton collisions at a center of mass energy of $13.6$ TeV. LLPs have unique signatures distinct from Standard Model processes, and this analysis utilizes calorimeter and track-based variables such as shower radial distribution, neutral hadron energy, and trackless jet characteristics to identify LLP jets. The use of HCAL timing, energy, and positional ($x, y, z$) measurements in the trigger and analysis form the basis of 5D calorimetry. A background estimation is performed, and expected signal limits are set for an extended Higgs sector model.