CCM200 Liquid Argon Detector

• CCM200 is a liquid argon detector with a hybrid optical design that uses selective TPB coating to distinguish prompt Cherenkov light from delayed scintillation signals.
• It features an optimized PMT layout with 50% photocoverage and nanosecond timing resolution, enhancing sensitivity to MeV-scale neutrinos and beyond-Standard-Model searches.
• Simulation and experimental validations confirm event-by-event Cherenkov identification, offering improved background rejection and directional information for low-energy rare-event studies.

Searching arXiv for the cited CCM papers to ground the article in the referenced literature. {"query":"id:(Aguilar-Arevalo et al., 10 Jul 2025) OR id:(Aguilar-Arevalo et al., 2021)","max_results":5,"sort_by":"relevance"} {"query":"(Aguilar-Arevalo et al., 10 Jul 2025)","max_results":3,"sort_by":"relevance"} CCM200 is a liquid argon detector in the Coherent CAPTAIN-Mills program at Los Alamos National Laboratory, configured for MeV-scale neutrinos and beyond-Standard-Model searches and distinguished by selective wavelength-shifting instrumentation that enables event-by-event identification of Cherenkov radiation from sub-MeV electrons in a high scintillation light-yield medium (Aguilar-Arevalo et al., 10 Jul 2025). The detector is located at the Lujan Center on the Los Alamos Neutron Science Center beamline, follows the CCM120 prototype, and improves physics reach with higher photocoverage, refined timing, and selective wavelength-shifting (Aguilar-Arevalo et al., 10 Jul 2025, Aguilar-Arevalo et al., 2021). In the reported 2025 result, gamma rays from a $^{22}$Na source were used to isolate prompt Cherenkov light with $>5\sigma$ confidence, establishing the first event-by-event observation of Cherenkov photons from sub-MeV electrons in a high-yield scintillator detector (Aguilar-Arevalo et al., 10 Jul 2025).

1. Detector configuration and optical architecture

CCM200 employs an upright cylindrical cryostat, $2.58 \ \mathrm{m}$ in diameter and $2.25 \ \mathrm{m}$ in height, containing $10$ tons of liquid argon. The inner detector fiducial region is $7$ tons, and a $3$-ton optically isolated veto region surrounds it (Aguilar-Arevalo et al., 10 Jul 2025). The inner walls are lined with Mylar reflective foils evaporatively coated with tetraphenyl butadiene, which wavelength-shifts liquid argon ultraviolet scintillation into the visible (Aguilar-Arevalo et al., 10 Jul 2025).

The fiducial volume is instrumented with $200$ 8-inch Hamamatsu R5912-Y002 10-stage cryogenic photomultiplier tubes, providing approximately $50\%$ optical photocoverage. Of these, $160$ ($>5\sigma$0) are TPB-coated and $>5\sigma$1 ($>5\sigma$2) are uncoated (Aguilar-Arevalo et al., 10 Jul 2025). PMT faces on the coated channels are directly TPB-coated, whereas uncoated channels lack a face coating and thus rely on visible photons, such as Cherenkov light, or ultraviolet photons that first wavelength-shift at other surfaces before being detected (Aguilar-Arevalo et al., 10 Jul 2025).

This selective coating pattern is central to the CCM200 concept. The coated channels are optimized for high scintillation-light collection, while the uncoated channels provide enhanced sensitivity to prompt visible Cherenkov photons (Aguilar-Arevalo et al., 10 Jul 2025). A plausible implication is that the detector is not simply a high-yield scintillation instrument, but a deliberately hybrid optical system in which photocathode segmentation by wavelength-shifting treatment functions as a particle-identification handle.

CCM200 also inherits a broader programmatic role from CCM120. The earlier detector was a 10-ton liquid argon scintillation detector at the LANSCE Lujan Center designed to search simultaneously for sterile neutrinos via neutral-current disappearance and for light dark matter produced in the pulsed proton beam dump (Aguilar-Arevalo et al., 2021). Lessons from CCM120 directly informed the CCM200 upgrade, which was engineered to improve sensitivity through increased optical coverage, cleaner liquid argon, enhanced vetoing, and new shielding (Aguilar-Arevalo et al., 2021).

2. Timing structure and the prompt-window method

The event start time is defined by a trigger that requires total charge exceeding $>5\sigma$3 photoelectrons in a sliding $>5\sigma$4 window across PMT digitized waveforms. Digitization uses CAEN V1730/V1730S boards at $>5\sigma$5 with $>5\sigma$6-bit precision, and the detector’s effective timing resolution for the Cherenkov analysis is $>5\sigma$7 (Aguilar-Arevalo et al., 10 Jul 2025).

The analysis defines a prompt Cherenkov-enhanced window, $>5\sigma$8, relative to the event start time $>5\sigma$9, and a subsequent scintillation-enhanced region for $2.58 \ \mathrm{m}$0 (Aguilar-Arevalo et al., 10 Jul 2025). A characteristic small peak appears at $2.58 \ \mathrm{m}$1 on uncoated PMTs for electromagnetic events from the $2.58 \ \mathrm{m}$2Na source, before the reconstructed event start (Aguilar-Arevalo et al., 10 Jul 2025).

The physical basis of the method is the different transport and emission timescales of Cherenkov and scintillation light in liquid argon. Ultraviolet scintillation in liquid argon is produced near $2.58 \ \mathrm{m}$3 and must be wavelength-shifted to be detected efficiently by visible-sensitive PMTs. On uncoated PMTs, such ultraviolet scintillation photons generally cannot be detected without first traveling to a TPB-coated surface, being re-emitted in the visible, and then propagating back, introducing path-length and re-emission delays (Aguilar-Arevalo et al., 10 Jul 2025). By contrast, visible Cherenkov photons produced at the interaction site propagate directly to uncoated PMTs and arrive in the prompt window (Aguilar-Arevalo et al., 10 Jul 2025).

Liquid argon scintillation itself arises from excited $2.58 \ \mathrm{m}$4 dimers decaying via singlet and triplet channels. For lightly ionizing electrons, about $2.58 \ \mathrm{m}$5--$2.58 \ \mathrm{m}$6 of the scintillation yield is fast singlet and $2.58 \ \mathrm{m}$7--$2.58 \ \mathrm{m}$8 is triplet. The singlet decay constant is $2.58 \ \mathrm{m}$9, and the triplet is $2.25 \ \mathrm{m}$0 (Aguilar-Arevalo et al., 10 Jul 2025). These timescales, together with wavelength-shifting-induced propagation delays, permit temporal separation in which visible Cherenkov dominates the prompt window on uncoated PMTs, while scintillation dominates for $2.25 \ \mathrm{m}$1 (Aguilar-Arevalo et al., 10 Jul 2025).

This timing structure differs from the beam timing logic emphasized in CCM120, where time-of-flight separation was used to define a beam region of interest and to isolate prompt neutrino or light-dark-matter signals from later neutron backgrounds (Aguilar-Arevalo et al., 2021). In CCM200, the same general emphasis on nanosecond-scale timing is applied inside individual events to separate optical channels rather than beam species.

3. Cherenkov production in liquid argon

The Cherenkov condition used in the CCM200 analysis is

$2.25 \ \mathrm{m}$2

where $2.25 \ \mathrm{m}$3 and $2.25 \ \mathrm{m}$4 is the refractive index at the photon’s wavelength (Aguilar-Arevalo et al., 10 Jul 2025). The spectral yield per unit path length is

$2.25 \ \mathrm{m}$5

which emphasizes the $2.25 \ \mathrm{m}$6 preference for shorter wavelengths (Aguilar-Arevalo et al., 10 Jul 2025).

The threshold occurs at $2.25 \ \mathrm{m}$7, equivalently

$2.25 \ \mathrm{m}$8

with total-energy threshold

$2.25 \ \mathrm{m}$9

and corresponding kinetic threshold

$10$0

The paper cites $10$1 for visible light in liquid argon (Aguilar-Arevalo et al., 10 Jul 2025). For $10$2 near $10$3 and $10$4, $10$5, so $10$6, consistent with the observed preference for $10$7 in the prompt hit-pair angles (Aguilar-Arevalo et al., 10 Jul 2025).

The paper notes that sub-MeV electrons in liquid argon emit Cherenkov light only above a kinetic threshold of roughly $10$8, and that the detected sample is dominated by electrons with $10$9, produced by Compton scattering of the $7$0Na gamma rays (Aguilar-Arevalo et al., 10 Jul 2025). This places the CCM200 result in a parameter regime where Cherenkov emission is kinematically allowed but sparse, making timing-based isolation essential.

A common misconception is that high scintillation yield necessarily obscures Cherenkov emission on an event-by-event basis. The CCM200 result shows that this is not generally true in liquid argon when selective wavelength-shifting and nanosecond timing are combined (Aguilar-Arevalo et al., 10 Jul 2025). This suggests that the limiting factor is not only total scintillation yield, but the joint structure of emission times, propagation delays, and channel-specific optical sensitivity.

4. Source deployment, data selection, and prompt-hit tagging

The demonstration of sub-MeV Cherenkov identification used a $7$1 $7$2Na source encapsulated in approximately $7$3 of stainless steel, deployed on a stainless steel bayonet through a central flange and positioned at the detector midline (Aguilar-Arevalo et al., 10 Jul 2025). The dominant $7$4 branch, approximately $7$5, emits a $7$6 positron and a $7$7 gamma from $7$8 de-excitation; the positron annihilation yields two $7$9 gammas. The electron-capture branch, approximately $3$0, emits only the $3$1 gamma (Aguilar-Arevalo et al., 10 Jul 2025).

Event reconstruction again uses the sliding $3$2 window with a $3$3 PE threshold to set $3$4 (Aguilar-Arevalo et al., 10 Jul 2025). Data-quality cuts remove high-charge cosmic muons observed within the $3$5 DAQ window, events with less than $3$6 separation from neighboring triggers, and events with reconstructed radial positions more than $3$7 from the detector center, where position is estimated by PMT-weighted charge in the first $3$8 (Aguilar-Arevalo et al., 10 Jul 2025).

After these cuts, the total charge in the first $3$9 shows a lower peak, approximately $200$0 PE, associated with electron capture, and a higher peak, approximately $200$1 PE, associated with the $200$2 branch (Aguilar-Arevalo et al., 10 Jul 2025). Events within $200$3 PE around the $200$4 peak center are selected to increase statistics and reduce contamination, with expected random background contamination after cuts of $200$5 (Aguilar-Arevalo et al., 10 Jul 2025). The dataset used for the early-time study consists of $200$6 selected $200$7Na events (Aguilar-Arevalo et al., 10 Jul 2025).

A simple per-event selection requires at least one hit on uncoated PMTs in $200$8 (Aguilar-Arevalo et al., 10 Jul 2025). In this Cherenkov-enhanced window, the Cherenkov purity reaches $200$9 for events with at least one prompt hit. The selection efficiency with all cuts plus at least one prompt hit is $50\%$0 in data and $50\%$1 in simulation (Aguilar-Arevalo et al., 10 Jul 2025).

Quantity	Value	Context
Selected $50\%$2Na events	$50\%$3	Early-time light study
Prompt window	$50\%$4	Cherenkov-enhanced region
Cherenkov purity	$50\%$5	Events with $50\%$6 prompt hit
Selection efficiency in data	$50\%$7	All cuts plus $50\%$8 prompt hit
Selection efficiency in simulation	$50\%$9	All cuts plus $160$0 prompt hit
Random background contamination	$160$1	After charge and quality cuts

Since uncoated PMTs represent only $160$2 of the total photocathode coverage, this simple requirement already yields a high-purity Cherenkov tag with useful efficiency for sub-MeV electromagnetic identification (Aguilar-Arevalo et al., 10 Jul 2025). The statement refers to photocathode coverage rather than PMT count, and it underscores that selective optical instrumentation, rather than maximal uniform coating, is essential to the CCM200 strategy.

5. Simulation, validation, and statistical demonstration

Optical modeling uses GEANT4 to simulate scintillation and Cherenkov photon production, transport, wavelength shifting at TPB-coated surfaces, PMT response, dark noise, and post-pulsing, with measured background spectra from source-removed data included (Aguilar-Arevalo et al., 10 Jul 2025). Offline reconstruction converts digitized voltage traces to PE-level hits using fits that account for PMT response, dark noise, and post-pulsing time structure (Aguilar-Arevalo et al., 10 Jul 2025).

For a typical uncoated PMT, the summed hit-time distribution in data and simulation agrees within $160$3 as a $160$4 systematic band across the prompt region and beyond out to $160$5 (Aguilar-Arevalo et al., 10 Jul 2025). The early $160$6 peak is reproduced by the Cherenkov component, while the simulation decomposes the total rate into Cherenkov, scintillation, and random-background contributions (Aguilar-Arevalo et al., 10 Jul 2025). The number-of-hits distribution in the prompt window on uncoated PMTs shows that scintillation plus random backgrounds alone are two orders of magnitude lower than the total data rate in the Cherenkov-enhanced window (Aguilar-Arevalo et al., 10 Jul 2025).

Angular validation provides the principal statistical test. For events with at least two prompt hits, the angle between PMT-hit direction vectors, defined from PMT centers to the source location at the origin, shows a clear preference for $160$7 in data, consistent with visible Cherenkov in liquid argon and electron kinetic energies $160$8--$160$9 (Aguilar-Arevalo et al., 10 Jul 2025). The total simulation, including Cherenkov, scintillation, and random backgrounds, matches the data with $>5\sigma$00 for $>5\sigma$01 degrees of freedom (Aguilar-Arevalo et al., 10 Jul 2025). A $>5\sigma$02 test rejects the background-only hypothesis at $>5\sigma$03 confidence, establishing event-by-event Cherenkov identification (Aguilar-Arevalo et al., 10 Jul 2025).

The key empirical signatures are therefore mutually reinforcing: a distinct early-time peak on uncoated PMTs, a prompt-hit multiplicity excess over scintillation-plus-background expectations, and an angular distribution aligned with the visible-index Cherenkov angle in liquid argon (Aguilar-Arevalo et al., 10 Jul 2025). This suggests that CCM200’s result is not a single-observable excess but a coherent optical signature supported by temporal, topological, and simulation-based validation.

6. Programmatic role within the CCM experiment

CCM200 is part of a broader experimental program aimed at MeV-scale electromagnetic final states and coherent scattering signatures at $>5\sigma$04 without electromagnetic activity (Aguilar-Arevalo et al., 10 Jul 2025). Event-by-event Cherenkov identification directly strengthens both categories (Aguilar-Arevalo et al., 10 Jul 2025). The detector therefore occupies a dual role: it is both a low-energy rare-event search instrument and a testbed for hybrid optical discrimination in liquid argon.

The CCM120 predecessor established the experimental setting. It was located $>5\sigma$05 meters from a high-flux neutron/neutrino source, designed to search for sterile neutrinos and light dark matter, and performed an engineering run in Fall 2019 (Aguilar-Arevalo et al., 2021). The lessons learned from that run guided the development and construction of CCM200, which was engineered to improve sensitivity through increased optical coverage, cleaner liquid argon, enhanced vetoing, and new shielding (Aguilar-Arevalo et al., 2021). CCM200 was described as having $>5\sigma$06 8-inch PMTs, twice as many veto PMTs, highly efficient evaporative coated foils, and filtered and recirculated liquid argon (Aguilar-Arevalo et al., 2021).

The broader improvements were motivated by CCM120 limitations. In the earlier configuration, contaminated liquid argon reduced the $>5\sigma$07 attenuation length, and shielding and veto upgrades were introduced to mitigate beam-related gamma backgrounds and improve prompt-window usability (Aguilar-Arevalo et al., 2021). Within that development arc, the 2025 Cherenkov result represents a detector-capability milestone rather than a standalone source-calibration exercise. It demonstrates that the upgraded optical layout can support low-energy topology and particle-identification information in addition to scintillation calorimetry (Aguilar-Arevalo et al., 10 Jul 2025).

7. Scientific significance and prospective applications

The 2025 CCM200 result establishes the first event-by-event observation of Cherenkov light from sub-MeV electrons in a high scintillation-yield liquid argon detector (Aguilar-Arevalo et al., 10 Jul 2025). The physics impact stated in the paper is that hybrid liquid argon detectors can combine excellent energy resolution, from large scintillation yield of approximately $>5\sigma$08 photons/MeV, with directional and particle-identification information from Cherenkov light (Aguilar-Arevalo et al., 10 Jul 2025).

Event-by-event Cherenkov tagging can enhance background rejection in rare-event searches, provide directional information for low-energy electrons such as solar neutrinos, and enable separation of electromagnetic and highly ionizing low-energy signals in the CCM program’s two primary categories (Aguilar-Arevalo et al., 10 Jul 2025). Pure liquid argon is transparent to visible light and does not intrinsically absorb above approximately $>5\sigma$09, allowing ultraviolet photons to propagate and be shifted at instrumented surfaces rather than quenched in bulk dopants; its relatively slow scintillation components and geometry-induced wavelength-shifting delays strengthen temporal separation from Cherenkov (Aguilar-Arevalo et al., 10 Jul 2025). Cryogenic operation also supports future SiPM readout with dramatically reduced dark noise (Aguilar-Arevalo et al., 10 Jul 2025).

The result further shows that even with only $>5\sigma$10 photocathode coverage using uncoated PMTs, simple timing selections provide clean, per-event Cherenkov tags at approximately $>5\sigma$11 efficiency, with scope to improve by incorporating early portions of the scintillation-enhanced region and coated-PMT waveform information (Aguilar-Arevalo et al., 10 Jul 2025). This suggests a broader detector-design principle: in low-energy liquid argon systems, selective wavelength shifting and channel-specific timing may be as consequential as absolute light yield for extracting event topology.

Within the CCM program’s larger objectives, such capabilities are relevant to MeV-scale neutrino measurements, beyond-Standard-Model searches, and any analysis in which discrimination between electromagnetic and non-relativistic energy deposition is limiting (Aguilar-Arevalo et al., 10 Jul 2025). The CCM200 detector is therefore significant both as a specific experimental apparatus and as a concrete realization of hybrid scintillation-Cherenkov detection in liquid argon.