Higgs Detection at The Compact Muon Solenoid Detector at the Large Hadron Collider
The Compact Muon Solenoid (CMS) is a 14 kiloton detector designed to search for new physics at an unprecedented distance scale of 10-19 m at the CERN Large Hadron Collider (LHC). The detector consists of 220 square meters of silicon pixels and strips (80 million channels) for precision charged particle tracking, 75k lead-tungsten crystals for precision electron and photon measurements, a highly segmented 1000-ton brass hadron-calorimeter plus a quartz-fiber forward calorimeter to measure jets from quark and gluon scattering and energy balance, all surrounded by precision muon chambers embedded in the return yoke of the magnet. Overall, the detector features nearly complete solid-angle coverage and can precisely measure electrons, photons, muons, jets and missing energy over a large range of particle energies. These broad capabilities of the CMS detector allow the exploration of electroweak symmetry breaking and will enable the potential discovery of physics beyond the Standard Model.
The CMS trigger and data acquisition systems are responsible for ensuring that physics-enriched data samples with potentially interesting events are recorded with high efficiency and good quality. The experiment has a two-level trigger system, unlike most other hadron collider experiments that have more traditional three-level systems. The first physical level is hardware-based and is called the “Level-1 Trigger” (L1) while the second physical level is software-based and is called the “High-Level Trigger” (HLT). L1 uses information from the calorimeters and muon detectors and is designed to select, in less than 1 ms, the most interesting events starting from a total input (collision) rate of about 40 MHz. The HLT processor farm further decreases the event rate from around 100 kHz to around 400 Hz, before data storage. The trigger system, therefore, has to provide a high selectivity of ~10-5 with respect to the active LHC bunch crossings while ensuring that the ability to select rare, exotic events is preserved. Boston University has played a leading role in the coordination of the CMS Trigger effort with Bose serving as the CMS Trigger Coordinator (2014-2016) and as the CMS Deputy Trigger Coordinator (2011-2013). Group members were involved in the design and commissioning of the CMS HLT and continued to spearhead the effor thereaftert. Many of the BU personnel resident at CERN served as HLT on-call experts during the LHC Run 1 (Avetisyan, Carerra, Fantasia and Sperka) and Run 2 (Avetisyan, Rankin, Richardson).
The collision rate at the LHC also presents enormous technical challenges on the design of readout electronics due to the intense radiation environment and the high speed at which millions of channels of data must be processed. The Boston University group has a leadership role in calorimeter electronics and related software (Rohlf and Sulak). The group has designed and built the data concentrator, a sophisticated piece of digital electronics based on modern field programmable gate arrays (FPGAs) to read out the hadron calorimeter. Group members have also led the custom design of the 18-channel hybrid photo-diode used to convert scintillation light from the calorimeter into electrical signals. We have also designed the electronics to feed calorimeter signals into the muon trigger to greatly reduce the backgrounds in the online event selection. Members of the BU CMS team have also helped design the two hadronic forward calorimeters which are positioned at either end of the CMS detector, to pick up the myriad particles coming out of the collision region at shallow angles relative to the beam line. Group members also play a key role in the day-to-day operation of the calorimeter.
The Boston University collaborators (Bose, Rohlf, and Sulak, their postdocs Girgis and Pinna, and their grad students, with their engineers Eric Hazen and Shouxiang Wu) are reaping the physics benefits of our investment in detector hardware, trigger and algorithm development. Group members are making significant contributions to physics analyses involving searches for new particles and the hunt for the Higgs Boson. These analyses include (among many others) searches for heavy gauge bosons and exotic WZ resonances. We are also playing a leading role in the study of diboson production; the latter is critical for increasing the sensitivity of our Higgs searches and thereby help understand the mechanism of electroweak symmetry breaking.
Bose has served as the co-convener of the Electroweak Diboson group (2011), co-convener of the Resonances group within the Beyond Two Generations physics group (2012-2014), and co-convener of the Beyond Two Generations group (2016-2017). She is currently serving a two-year term as CMS Physics Co-coordinator (2017-2019). Sulak serves on the Advisory Board of the Hadron Calorimeter, and manages some 15 undergraduates as Director of the BU/CERN/DOE Internship Program.