Neutrino-Oxygen Neutral-Current Elastic Interaction as a Background - PowerPoint PPT Presentation

16th International Conference on Topics in Astroparticle and Underground Physics September 9–13, 2019 / Toyama, Japan Neutrino-Oxygen Neutral-Current Elastic Interaction as a Background in Supernova Relic Neutrino Search Yosuke ASHIDA (Kyoto University) for the T2K Collaboration

� 2 Supernova Relic Neutrinos • Neutrinos from all the past core-collapse supernovae have been accumulated. • Detecting supernova relic neutrinos (SRNs) would provide a lot of information about supernova mechanism, star formation history, heavy nucleosynthesis, etc. • World most sensitive search has been conducted using Super-Kamiokande detector. Inverted ordering normal ordering K. Nakazato et al., Astrophys. Jour., 804, 75 ( 2015 ) .

� 3 Neutral-Current Elastic Background “Neutron tagging” analysis is implemented to obtain higher sensitivity. • Delayed coincidence: e + & γ from n capture • SK: 2.2 MeV gamma-ray by H (efficiency ~20%) • SK-Gd: ~8 MeV gamma-rays by Gd (efficiency ~80% @ 0.1%-Gd load) • Many backgrounds can be reduced by neutron tagging, however, NC remains. • NC at E ν > 200 MeV usually involves nucleon knock-outs (“NCQE”). • This is an irreducible background, so must be measured precisely (current: 100%!). • Supernova relic neutrino (IBD) Atmospheric neutrino (NCQE) H Gd H Gd n n ν ν e or or γ γ p γ γ 16 O e + γ (2.2 MeV) (2.2 MeV) (~8 MeV) (~8 MeV)

� 4 T2K Experiment T2K is a long baseline neutrino experiment. • Beams produced at J-PARC (8 bunch beam structure being separated by 581 ns). • Neutrinos detected at 295 km away Super-Kamiokande. • Flux peak ~630 MeV is close to the atmospheric neutrino flux peak. • Beam timing information can reduce large amount of low energy backgrounds. • So far, T2K has accumulated both neutrino and antineutrino data. • FHC (neutrino mode) 15.12 × 10 20 protons-on-target • RHC (antineutrino mode) 16.51 × 10 20 protons-on-target • Super‐Kamiokande J‐PARC Near Detectors Mt. Noguchi‐Goro 2,924 m Mt. Ikeno‐Yama 1,360 m 1,700 m below sea level Neutrino Beam 295 km

� 5 Previous Result (Run 1–3 FHC) Flux-averaged cross section [Ref.: Phys. Rev. 072012 (2014)] • Large uncertainty Error size is very large: • & Statistical error: +/–25.5% • Result only for neutrinos Systematic error: +41.9/–21.3% •

� 6 Event Simulation T2K Run 1-9 Flux at SK (FHC) -POT] 6 10 ν 1. NEUTRINO FLUX µ 21 /50-MeV/10 ν µ 5 ν 10 30 GeV/c protons are injected onto a graphite target. e • ν e Hadronic interactions are simulated by FLUKA with • 2 Flux [/cm 4 10 reweighing by the NA61/SHINE experiment. 3 10 Transportation and decay are simulated by GEANT3. • 0 2 4 6 8 10 hadrons ( π , K , …) E [GeV] ν 30 GeV/c proton T2K Run 1-9 Flux at SK (RHC) -POT] neutrinos graphite 6 10 ν µ 21 /50-MeV/10 ν FLUKA Simulation 5 10 µ ν + e ν GEANT3/GCALOR External Data (NA61/SHINE) 4 10 e 2 Flux [/cm 3 10 2 10 0 2 4 6 8 10 E [GeV] ν

� 7 Event Simulation (*) A. M. Ankowski et al., Phys. Rev. Lett. 108, 052505 (2012). 2. NEUTRINO INTERACTION + PRIMARY-GAMMA PRODUCTION ν A dedicated generator “NEUT” is used until the final state • interaction (NCQE model = spectral function). NEUT Gamma-ray emission is based on a theoretical calculation (*) . • γ 16 O ν ’ ≤ 10 MeV p 1/2 p 3/2 n GEANT3 s 1/2 ≤ 10 MeV ν γ neutron proton potential 3. SECONDARY INTERACTIONS + DETECTOR RESPONSE Important is “ neutron ” simulations. • Neutrons with <20 MeV: ENDF/B-V nuclear library • Neutrons with >20 MeV: Intra-nuclear cascade model

� 8 Event Reconstruction SK Outer Detector • SK low energy fitter is used to reconstruct events. • Vertex: PMT hit timing information is used. SK Inner Detector • Direction: Cherenkov ring pattern of hit PMTs is used. • Energy: Number of hit PMTs is used. effwall • Fitter performance is checked by various calibrations. Reconstructed vertex • Important variables = { E rec , dwall , effwall , ovaQ , θ C } . dwall Reconstructed direction θ C : Cherenkov opening angle ovaQ = G V2 – G A2 G V : Vertex goodness (quality of reconstructed vertex) G A : Angular badness (quality of reconstructed direction)

� 9 Event Selection 1. Energy window: 4 ≤ E rec < 30 MeV 2. Good spill selection Only for Data 3. Timing cut: dt0 being required to be ±100 ns w.r.t the beam bunch center. 4. Decay-e cut: events having the pre-activity are cut (see backup for the post-activity). 5. Fiducial volume cut: dwall ≥ 200 cm Both for Data and MC 6. Ambient low energy background cut: Optimized { dwall , effwall , ovaQ } for each run to remove beam-unrelated events. 7. CC interaction cut: optimized cut based on the E rec – θ C 2D distribution. s µ FHC Events/0.05- 10 Neutron tagging is not applied in this analysis, while it is in the SRN analysis. 1 1 0 1 2 3 4 5 − dt0 [ s] µ

� 10 Ambient Low Energy Background Cut • Three parameters { dwall , effwall , ovaQ } are optimized based on the figure-of-merit (FOM). • Beam-unrelated events are taken from off-timing data (–500 µs ≤ dt0 ≤ –5 µs) and normalized to the on-timing time scale (495 µs → 200 ns × bunch#). On-timing data Off-timing data dt0 ~ ~ ~ ~ –500 µs –5 µs Optimized dwall for Run 8 Optimized dwall for Run 8 Optimized effwall for Run 8 Optimized effwall for Run 8 Optimized ovaQ for Run 8 Optimized ovaQ for Run 8 300 0.26 dwall [cm] effwall [cm] ovaQ 1000 0.24 280 0.22 800 260 0.2 240 600 0.18 220 0.16 400 200 0.14 200 180 0.12 160 0 0.1 4 4.5 5 5.5 6 6.5 4 4.5 5 5.5 6 6.5 4 4.5 5 5.5 6 6.5 E [MeV] E [MeV] E [MeV] rec rec rec

� 11 Background Rejection Power 4 10 Events/MeV T2K Run 1-9 FHC Off-timing data (before FV cut) 3 10 Off-timing data (after all cuts) MC (before FV cut) 2 10 MC (after all cuts) neutrino events 10 1 beam-unrelated events 1 − 10 2 − 10 5 10 15 20 25 30 E [MeV] rec

� 12 CC Interaction Cut • θ C ~ 34 deg. for µ / θ C ~ 42 deg. for e or single- γ / θ C > 70 deg. for multiple- γ • There are remaining events by CC interaction with decay-e which were not cut by the pre- activity cut. • Cut criteria are optimized based on 2D distributions and FOM. -NCQE (FHC) -NCQE (FHC) CCQE (FHC) CCQE (FHC) ν ν 90 90 [degree] [degree] 3 0.35 80 80 0.3 2.5 70 70 C C θ θ 0.25 60 60 2 50 50 0.2 1.5 40 40 0.15 30 30 1 0.1 20 20 0.5 0.05 10 10 0 0 0 0 5 10 15 20 25 30 5 10 15 20 25 30 E [MeV] E [MeV] rec rec

� 13 Selected Final Samples (FHC) single- γ multiple- γ Events/MeV Events/2.7-degree 50 40 Data (T2K Run1-9 FHC) Data (T2K Run1-9 FHC) -NCQE -NCQE ν ν 35 -NCQE -NCQE ν ν 40 NCother NCother 30 CC CC Beam-unrelated Beam-unrelated 25 30 20 20 15 10 10 5 0 0 5 10 15 20 25 30 0 10 20 30 40 50 60 70 80 90 [degree] E [MeV] θ rec C Event# (Fraction) All nu NCQE nubar NCQE NCother CC Beam-unrelated 238.4 
 178.6 
 4.8 
 42.5 
 8.9 
 3.6 
 MC (100%) (74.9%) (2.0%) (17.8%) (3.7%) (1.5%) T2K Run1-9 FHC Data 204 – – – – –

� 14 Selected Final Samples (RHC) single- γ multiple- γ Events/MeV Events/2.7-degree 18 22 Data (T2K Run1-9 RHC) Data (T2K Run1-9 RHC) -NCQE -NCQE ν ν 20 16 -NCQE -NCQE ν ν 18 NCother NCother 14 CC CC 16 Beam-unrelated Beam-unrelated 12 14 10 12 10 8 8 6 6 4 4 2 2 0 0 5 10 15 20 25 30 0 10 20 30 40 50 60 70 80 90 [degree] E [MeV] θ rec C Event# (Fraction) All nu NCQE nubar NCQE NCother CC Beam-unrelated 94.3 
 17.9 
 56.4 
 15.5 
 2.3 
 2.1 
 MC (100%) (19.0%) (59.9%) (16.5%) (2.5%) (2.2%) T2K Run1-9 RHC Data 97 – – – – –

� 15 Systematic Uncertainty Polarity Type nu NCQE nubar NCQE NCother CC Beam-unrelated Event fraction 74.9% 2.0% 17.8% 3.7% 1.5% Neutrino flux 6.7% 8.6% 7.3% 6.4% - Neutrino interaction 3.0% 3.0% 8.2% 16.5% - Primary- γ production 11.0% 10.6% 6.0% 6.6% - FHC Secondary- γ production 13.5% 13.4% 19.5% 17.6% - Oscillation parameter - - - 4.1% - Detector response 3.4% 3.4% 2.0% 5.2% 3.4% Total error 19.2% 19.7% 23.3% 26.7% 3.4% Event fraction 19.0% 59.9% 16.5% 2.5% 2.2% Neutrino flux 7.0% 6.4% 7.0% 6.5% - Neutrino interaction 3.0% 3.0% 10.8% 38.2% - Primary- γ production 12.2% 11.3% 2.3% 0.5% - RHC Secondary- γ production 13.6% 13.1% 19.3% 21.4% - Oscillation parameter - - - 3.1% - Detector response 3.4% 3.4% 2.0% 5.2% 3.4% Total error 20.1% 19.0% 23.4% 44.7% 3.4%

� 16 Cross Sections Extraction D : Data, M : MC In the case with the observed events and the nominal MC values: f _nu = 0.80 f _nubar = 1.11 • Errors for these scale factors are determined by the toy MC. • All the errors are treated uncorrelated but for the primary- and secondary- γ emission errors. • These are treated fully positive correlated for all the interactions and both modes.