Results from the Super-Kamiokande Experiment

Results from the Super-Kamiokande Experiment September 30th, 2025 aSchool ofPhysics & Astronomy, University ofGlasgow +Correspondence: [email protected] L. N. Machado a,+, on behalf of the Super-Kamiokande Collaboration 1 Lucas N Machado (University of Glasgow) The Super-Kamiokande Experiment The Super-Kamiokande (SK) Neutrino Detector is a50 kton water Cherenkov located in the Kamioka mine in Japan, overburden with 1000 m of rock. In operation since April 1996. 41 m 39 m Inner detector: currently has around 11,000 20-inch PhotoMultipliers Tubes (PMTs). Outer detector: water layer ~2m thick, with ~1,885 8-inch PMTs, facing the outside of the detector. 22.5 kton fiducial volume Overburden ~1 km rock: cosmic ray muons reduced to 1/100,000 2 The Super-Kamiokande Collaboration The Super-Kamiokande Collaboration consists of approximately 240 members from 54 institutions. 3 SK-V, 2019-2020 11,129 ID PMTs (refurbished for Gd and with Hyper-K PMTs) History of Super-Kamiokande 1996 2002 2006 2008 2019 SK-I, 1996-2001 11,146 ID PMTs (with 40% coverage) SK-II, 2002-2005 5,182 ID PMTs (with 19% coverage + FPR) SK-III, 2006-2008 11,129 ID PMTs (again, 40% coverage) SK-IV, 2008-2018 11,129 ID PMTs (upgraded electronics) 4 40 tons Gd2(SO4)3*8H2O 0.03% Gd Super-Kamiokande with Gadolinium (SK-Gd) Beacom,Vagins PRL.93, (2004) 171101 Nuclear Inst. and Methods inPhysics Research, A 1027(2022) 166248 ~8 MeV ΔT0.03% Gd ~60 µs Improve Super-Kamiokande’s sensitivity electron anti-neutrinos by adding water-soluble gadolinium (Gd) salt to the water in the detector. July/August 2020 13 tons Gd2(SO4)3*8H2O 0.01% Gd June/July 2022 Isotope neutron capture cross section 157Gd 255,000 barns 155Gd 61,000 barns H 0.3 barn Nuclear Inst. and Methods inPhysics Research, A 1065 (2024) 169480 SK-VI: SK-VII/ SK-VIII: 5 ΔT ~200 µs 2.2 MeV Super-Kamiokande –Physics Goals Super-Kamiokande is unique in covering neutrinos from MeV solar energies to TeV cosmic rays, while probing rare processes like nucleon decay. T2K long-baseline program •Astrophysics Solar neutrinos (~15 events/day) Supernova burst neutrinos (~10,000 events galactic SN) Diffuse Supernova Neutrino Background (few events/year, relic of all past stars) Pre-supernova neutrinos (hours early warning signal) •Neutrino Oscillations Atmospheric neutrinos (flux from ~100 MeV to >10 TeV) Accelerator, T2K (controlled beam, precision θ₁₃, δCP studies) •Rare Searches Nucleon Decay (lifetime sensitivity >10³⁴ years) Exotic/astrophysical neutrinos (test for dark matter, GW/GRB/AGN coincidence, new physics) Jash, Abhik. (2018) 6 Atmospheric Neutrinos Cosmic rays (mainly protons) interact with nuclei in the atmosphere, producing particle hadronic showers (e.g., pions, kaons). These decay into neutrinos, called atmospheric neutrinos. •Energy range few MeV to several TeV. •Super-K observes ~8 events/day Travel length of atmospheric neutrinos varies 15 km to ~13000 km. Zenith angle describes different atmospheric neutrino baselines -> matter effects induced by passage through Earth. Oscillation probabilities for atmospheric neutrinos as a function of the cosine of the zenith angle and the neutrino energy Eν. 7 Atmospheric neutrinos as probe of neutrino oscillations: •𝜈𝜇 disappearance: Δm232, sin2 θ23 •𝜈𝑒 appearance: 𝛿𝐶𝑃, θ23 octant, mass ordering Phys.Rev.D83:123001,2011 Atmospheric Neutrinos Cosmic rays (mainly protons) interact with nuclei in the atmosphere, producing particle hadronic showers (e.g., pions, kaons). These decay into neutrinos, called atmospheric neutrinos. •Energy range few MeV to several TeV. •Super-K observes ~8 events/day Travel length of atmospheric neutrinos varies 15 km to ~13000 km. Zenith angle describes different atmospheric neutrino baselines -> matter effects induced by passage through Earth. Oscillation probabilities for atmospheric neutrinos as a function of the cosine of the zenith angle and the neutrino energy Eν. 8 Atmospheric neutrinos as probe of neutrino oscillations: •𝜈𝜇 disappearance: Δm232, sin2 θ23 •𝜈𝑒 appearance: 𝛿𝐶𝑃, θ23 octant, mass ordering Upward-going, NO ordering: enhance 𝜈𝑒 appearance Phys.Rev.D83:123001,2011 Most recent analysis: Phys. Rev. D 109, 072014 (2024): •6,511 days of atmospheric neutrino data (SK-I to SK-V, start until July 2020) •Expanded fiducial volume: cut 100 cm from detector walls (before 200 cm) -> 20% increase in statistics. •Search for neutron captures to improve 𝜈/ ҧ𝜈 separation using Boosted Decision Tree, improved charged current/neutral current separation. 9 Atmospheric Neutrinos in Super-Kamiokande (1) 29 analysis samples: Sub-divided by event topology: (FC/PC,UP-μ), energy range, e/μ–like, and number of rings, number of neutron candidates. Multi-GeV e-like samples are divided into ν-like and ν-like samples to improve sensitivity for mass hierarchy. Best-fit, normal ordering: 𝛿𝐶𝑃~−𝜋/2 sin2θ23~0.45 Δm232 ~2.4×10−3 eV2 Mass ordering: Δχ2 (NO - IO) ~ -5.7 Reject inverted ordering at the 92.3% CL The efficient neutron identification provided by gadolinium allows the search for signals from nuclear power reactors (electron anti-neutrinos from fission products). 16 Reactor Neutrinos Reactor SK-VI Reactor SK-VII Expected reactor events in SK (model Huber/Muller) •Primarily a result of the activity of Japanese nuclear power reactors (small contribution from Korean reactors); •Constrain neutrino oscillation parameters, complementary to solar sector. •Expected ~ 5 events/day in SK. Reactors from Wakasa-Bay: Mihama 3 (146 km), Ohi 3, 4 (179km), Takahama 1, 2 3, 4 (191 km) Two analyses are currently being carried out using two different low energy triggers (SLE and WIT). SLE WIT SK-VI preliminary ON/OFF analysis WIT SK-VI preliminary More details in the contributed talk ”Enhancing Low-Energy Neutrino Sensitivity in Super-Kamiokande with the Wide-band Intelligent Trigger”. Energy spectrum The Diffuse Supernova Neutrino Background (DSNB) is a steady redshifted flux of neutrinos from all past core-collapse supernovae in the history of the Universe. 17 Diffuse Supernova Neutrino Background (1) Φ𝐷𝑆𝑁𝐵(𝐸)∝න𝑅𝑆𝑁(𝑧)𝑑𝐹ഥ 𝜈(𝐸,𝑧) 𝑑𝐸 𝑑𝑡 𝑑𝑧 𝑑𝑧 Detecting it would provide unique insight into stellar evolution and the cosmic supernova rate, but the signal is extremely faint. Super-K has the world’s leading sensitivity. Detection through IBD, main backgrounds: •Reactor neutrinos •Spallation-induced (mainly 9Li) •Atmospheric neutrinos (both CC, NC) Redshift-dependent star formation rate Supernova neutrino emission spectrum Many analysis improvements: •New reduction for atmospheric NCQE interactions event using gamma-ray cut variable (further 90% reduction) •Dedicated machine learning techniques implemented for neutron tagging. Performed both energy-binned analysis and unbinned energy spectrum fitting. 18 Diffuse Supernova Neutrino Background (2) NEUTRINO 2024: Review of Diffuse SN Neutrino Background, M. Harada DSNB search in SK-Gd K. Abe et al, Phys. Rev. D 104, 122002 (2021): 2970.1 days of SK-IV data + M. Harada et al, Phys. Rev. Lett. 951, L27 (2023): 552 days (SK-VI, 0.01% Gd) + Recent studies: 404 days (SK-VII, 0.03% Gd) 956 days of SK-Gd data! •Main channel IBD: ҧ𝜈𝑒+𝑝 → 𝑒++𝑛 •~7.5–30 MeV signal window •Constraints to > 30 MeV side-band 19 Diffuse Supernova Neutrino Background (3) No significant excess observed (min. p-value=0.04). Sensitivity reaches model prediction region for Eν> 14 MeV. SK starting to probe into the predicted DSNB signal region! SK-VI + SK-VII energy spectrum SUPER-KAMIOKANDE PRELIMINARY SUPERKAMIOKANDE PRELIMINARY Competitive SK-Gd with < 1000 days exposure with 3000 days of pure-water period. 20 Diffuse Supernova Neutrino Background (4) Spectrum fitting analysis to extract detection significance: •Total of 6779 days of SK (5823 days pure-water + 956 days with Gd). •Analysis threshold: Eν> 17.3 MeV (to avoid spallation background). •Define main/side-band regions based on Cherenkov angle •Suppress uncertainty of background prediction by fitting IBD-like and non IBD-like events. •Profile likelihood ratio. 2.3σ rejection of a background-only hypothesis (very preliminary). Spectral fitting for a DSNB theoretical model (Horiuchi+09), showing all individual SK phases considered and their combination Best-fit rate: 2.9 events ⋅ year-1 Best-fit flux: 1.4 cm-2 s-1 Compact binary mergers are prime candidates for neutrino–gravitational wave coincidence searches: •Neutron Star–Black Hole (NSBH) mergers: may produce both high-energy and thermal neutrinos, depending on remnant and ejecta. •Binary Neutron Star (BNS) mergers: emit low-energy neutrinos (MeV) within milliseconds of the GW signal. •Black Hole–Black Hole (BBH) mergers: no neutrino emission expected, but some models allow for high-energy neutrino production if matter is present (e.g. accretion disk). Models expect that BNS and NSBH mergers emit thermal MeV neutrinos within < 1 s of merger, with peak luminosity > 10⁵³ erg/s, in the 5 to 30 MeV range. Upper limits (90% confidence level) were obtained for both low and high energy samples: 21 Gravitational Wave Coincidence Searches Super-Kamiokande preliminary Super-Kamiokande preliminary Searches in SK for coincidences with the first period of GW-O4 run (O4a) from May to October 2023. No significant excess is observed. Flux limits: ~103−104 GeV cm−2 Super-Kamiokande is a neutrino observatory running since 1996, with a broad physics program in neutrino astrophysics, neutrino oscillations and rare searches. In 2020, the experiment started the SK-Gd phase, in which gadolinium was added to the water in the detector to increase its sensitivity to electron antineutrinos. 22 Summary Atmospheric neutrinos: •Combined SK-I to V data with expanded fiducial volume (27.2 kton) and improved selections •Preference for normal mass ordering, rejecting inverted ordering at the 92.3% CL; •Analysis in agreement with T2K on mass ordering & δCP (previous talk). •Neutron production modeling validated with early SK-Gd data. Search for Nucleon Decay: •Systematically searches for many different decay modes to probe Grand Unified Theories; •Imposing very strict lifetime limits. Solar Neutrinos: •Full SK-IV (October 2008 to May 2018) data analyzed; •Solar upturn favoured by 2.1σ for SK+SNO; •3.2σ direct evidence of earth matter effects. Reactor Neutrinos: •First searches of reactor neutrino signal in SK, using two low energy triggers; •Expected event rates consistent with data •On/off reactor analysis shows excellent agreement. Diffuse Supernova Neutrino Background: •There is no significant DSNB signal, but SK starting to probe into the predicted DSNB signal region. •2.3σ tension from non-DSNB hypothesis Coincidence with gravitational waves: •GW-O4as search of neutrino counterparts. No significant excess observed. •Flux limits set ~103−104 GeV cm−2.