R. Yohay

The Standard Model (SM) of particle physics is a thoroughly well-tested theory of all electroweak scale phenomena to date—the most recent test being the discovery of the long-predicted Higgs boson. However, there is much evidence to suggest that it is incomplete. On the formal side, the mass of the Higgs particle itself is perplexingly small, while the largest theoretical energy scale of the SM is 17 orders of magnitude larger, a puzzle known as the hierarchy problem. On the experimental side, among other things, the SM lacks a candidate for dark matter, which is now known to play an indispensable role in astrophysical large scale structure formation. Many theories that address the fundamental puzzles of the SM predict the existence of multiple Higgs bosons, including a very light boson that couples preferentially to third-generation fermions. Observation of this new light boson, either directly produced or in the decay of the recently discovered SM Higgs, would provide an unmistakable sign of new dynamics in nature. The research supported by this award used data collected by the Compact Muon Solenoid (CMS) detector at the Large Hadron Collider (LHC) to explore boosted di-tau signatures of a new light boson as a tool in the quest to understand the shortcomings of the SM. With a proton-proton collision energy of 13 trillion electron-volts (TeV) and peak instantaneous proton luminosity of over 10^34 collisions per square centimeter per second (cm^−2 s^−1), the LHC is the only facility in the world that could enable this line of research. The research sought to address the hierarchy problem and other shortcomings of the SM through a mixture of LHC data analysis, maintenance of the CMS detector to ensure high quality data, and contributions to the Phase 2 upgrade detector for the upcoming High Luminosity phase of the LHC (HL-LHC).

The replacement of the existing endcap calorimeter in the Compact Muon Solenoid (CMS) detector for the high-luminosity LHC (HL-LHC), scheduled for 2027, will be a high granularity calorimeter. It will provide detailed position, energy, and timing information on electromagnetic and hadronic showers in the immense pileup of the HL-LHC. The High Granularity Calorimeter (HGCAL) will use 120-, 200-, and 300-μm-thick silicon (Si) pad sensors as the main active material and will sustain 1 MeV neutron equivalent fluences up to about 1016 neq cm−2. In order to address the performance degradation of the Si detectors caused by the intense radiation environment, irradiation campaigns of test diode samples from 8-inch and 6-inch wafers were performed in two reactors. Characterization of the electrical and charge collection properties after irradiation involved both bulk polarities for the three sensor thicknesses. Since the Si sensors will be operated at −30oC to reduce increasing bulk leakage current with fluence, the charge collection investigation of 30 irradiated samples was carried out with the infrared-TCT setup at −30oC. TCAD simulation results at the lower fluences are in close agreement with the experimental results and provide predictions of sensor performance for the lower fluence regions not covered by the experimental study. All investigated sensors display 60% or higher charge collection efficiency at their respective highest lifetime fluences when operated at 800 V, and display above 90% at the lowest fluence, at 600 V. The collected charge close to the fluence of 1016 neq cm−2 exceeds 1 fC at voltages beyond 800 V.

The 2022 Snowmass Community Summer Study consisted of two years of engagement with the particle physics community to craft a US particle physics strategy for the next decade, culminating in the 2022 Snowmass Book. This Report of the Topical Group on Calorimetry forms the basis for a small section of the Snowmass Book dealing with the community's vision for calorimetry going forward. It is the distillation of ideas put forth in public meetings and numerous white papers already freely available. The Report describes particle flow and dual readout approaches, integration of precision timing, and trends in calorimetric materials, and highlights key research directions for the next decade.

The projected proton beam intensity of the High Luminosity Large Hadron Collider (HL-LHC), slated to begin operation in 2026, will result in between 140 and 200 concurrent proton-proton interactions per 25 ns bunch crossing. The scientific program of the HL-LHC, which includes precision Higgs coupling measurements, measurements of vector boson scattering, and searches for new heavy or exotic particles, will benefit greatly from the enormous HL-LHC dataset. However, particle reconstruction and correct assignment to primary interaction vertices presents a formidable challenge to the LHC detectors that must be overcome in order to reap that benefit. Time tagging of minimum ionizing particles (MIPs) produced in LHC collisions with a resolution of 30 ps provides further discrimination of interaction vertices in the same 25 ns bunch crossing beyond spatial tracking algorithms. The Compact Muon Solenoid (CMS) Collaboration is pursuing two technologies to provide MIP time tagging for the HL-LHC detector upgrade: LYSO:Ce crystals read out by silicon photomultipliers (SiPMs) for low radiation areas and silicon low gain avalanche detectors (LGADs) for high radiation areas. This talk will motivate the need for a dedicated timing layer in the CMS upgrade, describe the two technologies and their performance, and present simulations showing the improvements in reconstructed observables afforded by four dimensional tracking.

The CMS experiment at CERN will undergo significant improvements during the Phase-II Upgrade to operate with a 10-fold increase in luminosity and the associated event pileup of the High Luminosity LHC (HL-LHC). The forward calorimetry will be exposed to very high radiation levels and the CMS collaboration is designing a new calorimeter, the High Granularity Calorimeter (CE), to replace the existing endcap calorimeters. It will have higher transverse and longitudinal segmentation for both electromagnetic (CE-E) and hadronic (CE-H) sections to facilitate particle-flow reconstruction. The fine structure of showers can be measured and used to enhance particle identification, whilst still achieving good energy resolution. The CE-E, and a large fraction of CE-H, will be based on hexagonal silicon sensors produced from 8-inch wafers, each with several hundreds of individual cells of 0.5 – 1 cm2 cell size. The remainder of the CE-H will be based on highly-segmented scintillators read out with SiPMs. The overview of the CE project presented in this paper is focused on the silicon sensors covering motivation, engineering design, expected performance and the current status of prototypes, from lab measurements to beam tests.

While most semiconductor materials are susceptible to radiation damage, we report here an observation of enhancements in the conductivity of undoped composite amorphous/nanocrystalline silicon thin films after irradiation with high-energy protons. When a series of films for which the nanocrystal concentration is varied were irradiated with 16-MeV protons with fluences from $2\ifmmode\times\else\texttimes\fi{}{10}^{13}$ to ${10}^{15}\phantom{\rule{0.16em}{0ex}}\mathrm{protons}/\mathrm{c}{\mathrm{m}}^{2}$, the dark conductivity following irradiation is increased by up to a factor of 10. Unlike the persistent photoconductivity effect observed in amorphous semiconductors, this enhancement is permanent and is not removed by annealing. Various mechanisms are tested to explain this effect, but none are able to fully account for our observations.