First Demonstration of a Hybrid Cherenkov and Scintillation Detector in a Proof-of-Principle Axion Search at a Beam Dump
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newmark-dnewmark-phd-physics-2026-thesis.pdf
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Author(s)
Newmark, Darcy A.
Advisor(s)
Conrad, Janet M.
Date Issued
May 2026
Publisher
Massachusetts Institute of Technology
Abstract
This thesis presents the development and first experimental demonstration of a hybrid Cherenkov and scintillation optical detector at a beam dump facility, utilized for a proof-of-principle search for high mass (𝑚_𝑎 ∼ 𝒪(1 MeV)) axion-like particles (ALPs). This research leverages the Coherent CAPTAIN-Mills (CCM) experiment, a 10-ton liquid argon light collection detector located at Los Alamos National Laboratory. The CCM detector is instrumented with 200 photomultiplier tubes (PMTs) to achieve 50% photo-cathode coverage. A key aspect of the detector design is that 80% of the PMTs are coated in wavelength shifting material while the remaining 20% of the PMTs are left uncoated. This wavelength discrimination, along with 2 ns timing resolution, allows for enhanced sensitivity to Cherenkov radiation.
This work includes detailed modeling of ²²Na calibration source data, resulting in the first detailed description of both scintillation and Cherenkov light production and propagation parameters, with systematic uncertainties, in a liquid argon light collection detector. Using this same low-energy ²²Na source, this work demonstrates the first event-by-event separation of Cherenkov radiation from sub-MeV electrons in a high light-yield scintillation detector.
Additionally, this thesis includes the development of a machine learning-based position reconstruction algorithm with 𝒪(5 cm) spatial resolution and an energy reconstruction framework achieving ~ 10% resolution at 1 MeV.
Using both the features of Cherenkov radiation and event reconstruction algorithms developed for this thesis, this work performs a proof-of-principle ALP search at a beam dump. Four discriminating observables, leveraging the enhanced sensitivity of the uncoated PMTs to visible Cherenkov radiation in the early portion of events, directionality of Cherenkov emission, pulse shape differences between 𝒪(1 MeV) electromagnetic final state particles and high energy fast neutrons, and spatial extent of event topologies, are constructed. These are then combined into a likelihood ratio test statistic for powerful rejection of steady state backgrounds in this analysis focusing on ≤ 10 MeV kinetic energy events.
Fitting for this ALP signal hypothesis in a frequentist framework yielded no significant excess of observed events over the expected background distribution. The enhanced background rejection capabilities, however, leveraging both Cherenkov and scintillation signals, enabled exclusion of new regions of the mass–coupling parameter space at the 90% confidence level compared to a previous CCM analysis with greater exposure time.
Finally, the thesis explores the broader utility of liquid argon detectors, investigating neutral current neutrino-argon interactions for supernovae neutrino flux constraints in the DUNE far detector. Additionally, ongoing detector developments targeting an ultra large liquid argon hybrid optical Cherenkov and scintillation detector are discussed as well.
The techniques established in this thesis demonstrate the powerful background rejection capabilities of hybrid optical detectors, providing a road map for next-generation dark sector and neutrino experiments.
MIT Department
Massachusetts Institute of Technology. Department of Physics
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