Investigation of the time resolution of an ultra-compact sampling calorimeter (RADiCAL) module at EM shower maximum over the energy range 25 GeV ≤ E ≤ 150 GeV

The RADiCAL Collaboration is conducting R&D on high performance electromagnetic (EM) calorimetry to address the challenges expected in future collider experiments under conditions of high luminosity and/or high irradiation: FCC-ee/hh, 10 TeV pCM, muon collider and fixed target/forward physics environments. Under development is a sampling calorimeter approach, known as RADiCAL modules, based on scintillation and wavelength-shifting (WLS) technologies and photosensor readout, including SiPM and SiPM-like technology. The modules discussed here consist of alternating layers of very dense (W) absorber and scintillating crystal (LYSO:Ce) plates, assembled to a depth of $25 \mathrm{X}_{0}$. The scintillation signals produced by EM showers in the region of EM shower maximum (shower max) are transmitted to SiPM located at the upstream and downstream ends of the modules via quartz capillaries which penetrate the full length of the module. The capillaries contain DSB1 organic plastic WLS filaments, or LuAG:Ce ceramic WLS filaments, positioned within the region of shower max, where the shower energy deposition is greatest. The capillaries are filled and fused with quartz rod elsewhere. The wavelength shifted light from the spatially-localized shower max region is then propagated to the photosensors. This paper presents the results of measurements of the time resolution of a RADiCAL module over the energy range $25 \mathrm{GeV} \leq \mathrm{E} \leq 150 \mathrm{GeV}$ using the H 2 electron beam at CERN. When using the DSB1 wavelength shifter, the data indicate an energy dependence of the time resolution that follows the functional form: $\sigma_{t}=a / \sqrt{ } E \oplus b$, where $a=256 \sqrt{ } \mathrm{GeV}$ ps and $b=17.5 \mathrm{ps}$. And for DSB1, the time resolution measured at the highest electron beam energy for which data was currently recorded ($\mathrm{E}=150 \mathrm{GeV}$) was found to be $\sigma_{\mathrm{t}}=27 \mathrm{ps} \pm 2 \mathrm{ps}$. For the LuAG:Ce wavelength shifter, the corresponding measurent was $\sigma_{\mathrm{t}}=30 \mathrm{ps} \pm 3 \mathrm{ps}$ at $\mathrm{E}=150 \mathrm{GeV} / \mathrm{c}$. Analysis details will be presented.