Outer Detector of Hyper-Kamiokande

Abstract: Hyper-Kamiokande (HK) is the world's largest water Cherenkov ring-imaging detector, planning to start data taking in 2028. The Outer Detector (OD) surrounds the Inner Detector and plays a critical role in rejecting background events entering from outside, particularly cosmic-ray muons. We report on the selection of $8\,\mathrm{cm}$ diameter photomultiplier tubes (PMTs) for the OD, comparing Hamamatsu R14374 and NNVT N2031 candidates, and present the evaluation of cosmic-ray muon background reduction performance using a full detector simulation. Hamamatsu PMTs were adopted for their superior in-water detection efficiency in deep-UV and stability. The cosmic-ray muon reduction inefficiency reaches $O(10<sup>{-6})$ with OD-based cuts alone, and $O(10<sup>{-9})$ is expected when combined with fiducial volume cuts, which is sufficiently negligible for nucleon decay and atmospheric neutrino analyses.

• The paper establishes Hamamatsu R14374 PMTs as the preferred option, achieving a Cherenkov-weighted detection efficiency of 35.8 ± 1.3% versus 27.3 ± 1.0% for the NNVT N2031 and meeting all tested operating requirements.
• The paper uses WCSim and MUSIC simulations with three OD- and ID-based cuts to reduce cosmic-ray muon backgrounds to approximately 3.8 events per day, corresponding to an inefficiency of O(10^-6) against roughly 80 atmospheric neutrino events per day.
• The paper projects that adding a fiducial-volume cut at least 1 m inside the ID wall could improve rejection to O(10^-9), while identifying reconstruction accuracy, in-situ validation, and long-term detector aging as important open issues.

The Outer Detector (OD) of Hyper-Kamiokande (HK) provides the primary defense against cosmic-ray muon backgrounds for a detector scheduled to begin data taking with a roughly 190 kton fiducial volume in 2028. This paper documents two results: the comparative evaluation that led to the adoption of Hamamatsu R14374 photomultiplier tubes (PMTs) over the NNVT N2031 candidate, and a Geant4-based assessment demonstrating that the OD-based reduction of cosmic-ray muon backgrounds reaches an inefficiency of $\mathcal{O}(10^{-6})$, improving to $\mathcal{O}(10^{-9})$ with fiducial volume cuts (2605.11580).

Detector context and background rates

HK is a water Cherenkov ring-imaging detector located at approximately 600 m rock overburden (1,750 m.w.e.). The ID is surrounded on all sides by a 1–2 m thick OD water layer, optically separated from the ID. At this depth, cosmic-ray muons arrive at approximately 50 Hz, or $4.3 \times 10^6$ events per day, and deposit light in both the ID and OD, whereas signal events—atmospheric neutrino interactions at roughly 80 per day, and nucleon decays—produce light only in the ID. This topological distinction is the basis of the OD rejection strategy. The OD is instrumented with approximately 3600 PMTs of 8 cm diameter, each coupled to a $30 \times 30 \times 0.7$ cm$^3$ wavelength-shifting (WLS) plate doped with POPOP in a PMMA substrate, which shifts deep-UV Cherenkov photons to approximately 400 nm and guides them to the photocathode by total internal reflection. Tyvek sheets with 80–90% reflectivity line the OD walls. The mean PMT spacing of approximately 2.5 m is comparable to the approximately 2 m diameter of a typical OD Cherenkov ring, so the design is constrained to maximize photon collection within budget.

PMT selection

Detection efficiency (DE) was measured for the Hamamatsu R14374 and NNVT N2031 at 405, 365, 315, and 275 nm, both standalone and through an identical WLS plate in water, normalized against a calibrated reference PMT. Standalone, the Hamamatsu PMT showed superior deep-UV efficiency, consistent with vendor QE values within $\pm$10%. Through the WLS plate the two PMTs performed comparably, as expected given that POPOP re-emission near 400 nm lies where the QE difference is minimal. Weighting by the Cherenkov spectrum ($1/\lambda^2$), the full-module DE was $35.8 \pm 1.3\%$ for the R14374 versus $27.3 \pm 1.0\%$ for the N2031, a ratio of $1.3 \pm 0.1$.

Stability and operating characteristics were assessed on 7 R14374 and 5 N2031 samples. All 7 Hamamatsu samples satisfied every requirement. The N2031 failed on three counts: 2 of 5 samples required operating voltages (1475 V, 1500 V) above the 1450 V limit; the mean operating voltage of 1383 V exceeded the 1300 V bound; and 2 of 5 samples exhibited dark-rate fluctuations exceeding the $\mathcal{O}(10^{-9})$00.2 kHz requirement. Both PMTs met the requirements for gain stability, dark rate, timing resolution (<3 ns FWHM), and charge resolution. The adoption of the R14374 therefore rests on deep-UV efficiency, voltage compliance, and dark-rate stability rather than on a uniform performance advantage.

Cosmic-ray muon reduction performance

The reduction study used WCSim, the Geant4-based full HK detector simulation incorporating measured R14374 characteristics, with the cosmic-ray muon flux propagated through rock using MUSIC. A bench-top measurement with an actual R14374 + WLS module found the simulation underestimates light yield by approximately 10%, making the reported inefficiencies conservative. The Monte Carlo sample comprised $\mathcal{O}(10^{-9})$1 equivalent muon events, biased toward stopping muons with low OD-hit topologies that are hardest to reject; the authors state this sample was verified to contain all dangerous categories from an unbiased flux sample, though this verification itself depends on the fidelity of the flux model.

Three sequential cuts exploiting OD and ID hit timing and topology were applied:

1. Typical muon reduction: rejection of events with $\mathcal{O}(10^{-9})$220 on-timing OD hits (NHITAC < 20 retained), removing muons with clear OD Cherenkov signatures.
2. Michel electron reduction: a 1 $\mathcal{O}(10^{-9})$3s sliding window scanned back to $\mathcal{O}(10^{-9})$4s from the trigger to catch Michel electrons from stopped muon decays ($\mathcal{O}(10^{-9})$5s); events with $\mathcal{O}(10^{-9})$62 OD hits within 10 m of the true muon entry point are rejected.
3. Low-energy reduction: exclusion of events with on-timing ID NHIT $\mathcal{O}(10^{-9})$7 166, corresponding to roughly 30 MeV visible energy.

After all three cuts the reduction inefficiency is $\mathcal{O}(10^{-9})$8, leaving approximately 3.8 residual muon events per day against approximately 80 atmospheric neutrino events per day—a signal-to-background ratio of approximately 20. False rejection from dark noise was consistent with $\mathcal{O}(10^{-9})$9 events out of $4.3 \times 10^6$0. A further fiducial volume cut of $4.3 \times 10^6$11 m inside the ID wall is expected to reach $4.3 \times 10^6$2 inefficiency, justified by the approximately 30 cm vertex resolution near the wall providing >3$4.3 \times 10^6$3 separation. The authors conclude this residual level is negligible for both nucleon decay searches and atmospheric neutrino analyses.

Limitations and open questions

The performance figures are simulation-based, albeit benchmarked against a bench-top light-yield measurement; no in-situ validation with an instrumented OD exists yet. The Michel-electron cut uses the muon's true ID entry point, which is unavailable in data and must be replaced by a reconstruction-based proxy whose performance is not quantified here. The $4.3 \times 10^6$4 figure additionally relies on the fiducial volume cut and its assumed vertex resolution near the wall. The paper explicitly leaves open the evaluation of long-term (>10 years) performance, including degradation from electronics and PMT malfunctions and aging of the Tyvek reflectors—factors that could erode the quoted efficiencies over the detector's lifetime.

Conclusion

The paper establishes the HK OD design basis: Hamamatsu R14374 PMTs satisfy all specifications with a $4.3 \times 10^6$5 Cherenkov-weighted efficiency advantage over the NNVT N2031, and a three-stage cut chain yields a conservative cosmic-ray muon reduction inefficiency of $4.3 \times 10^6$6 from OD information alone, with $4.3 \times 10^6$7 expected once fiducial volume cuts are applied. The residual background is sufficiently small for the HK nucleon decay and atmospheric neutrino programs, contingent on the simulation benchmarks and the long-term stability studies identified as outstanding work (2605.11580).