Daya Bay review of sterile neutrino search and
Daya Bay review of sterile neutrino search and Isotope-dependent fluxes Ming-chung Chu The Chinese University of Hong Kong, Hong Kong On behalf of the Daya Bay Collaboration Partial support: CUHK VC Discretionary Fund, RGC CUHK 3/CRF/10 R 15 th Rencontres du Vietnam August 4 – 10, 2019, Quy Nhon, Vietnam 1
Sterile Neutrinos - right-handed neutrinos (no weak interactions) in many Beyond Standard Model theories - May explain some experimental anomalies: LSND, Mini. Boo. NE (ms ~ e. V) - Dark matter candidate (ms ~ ke. V) - May alleviate tension in Hubble parameter between Planck and local Mini. Boo. NE measurement (ms ~ e. V) LSND ΛCDM + sterile @all data local 2 3. 8σ excess of in beam PRD 64, 112007 (2001). 4. 5σ excess of e in beam ΛCDM@CMB PRL 112, 051302 (2014). ar. Xiv: 1805. 12028 v 2
Daya Bay review of sterile neutrino search and Isotope-dependent fluxes • The Daya Bay Reactor Neutrino Experiment • Constraining sterile neutrino mixing • Absolute reactor antineutrino flux, spectrum, and fuel evolution 3
The Daya Bay Reactor Neutrino Experiment F. P. An et al. , Daya Bay Collaboration, NIM A 811, 133 (2016); PRD 95, 072006 (2017). 4
Reactor expt. : a clean way to measure 13 ee - - Reactor: abundant, free, pure source of e - disappearance of e at small L depends only on 13 Near-far configuration Near detectors: e flux and spectrum for normalization 13 only 12 only 12 and 13 Far detectors: near oscillation maximum for best sensitivity Relative measurement: cancel out most systematics L 5
Near/far Configuration Minimize systematic uncertainties: reactor-related: cancelled by near-far ratio detector-related: use ‘identical’ detectors, careful calibration RFar RNear = e detection ratio LNear LFar 1/r 2 Parameter Reaction cross section Number of protons Detection efficiency Reactor power Energy released per fission CHOOZ Combined 2 NFar NNear Far Near Psurv(LFar) Psurv(LNear) number of detector efficiency protons CHOOZ error Survival prob. sin 2(2 13) 0. 6 % Near/far configuration Cancelled out Reduced to ~ 0. 03% Reduced to ~ 0. 2% Reduced to ~ 0. 04% Cancelled out 2. 7 % ~ 0. 21% 1. 9 % 0. 8 % 1. 5 % 0. 7 % 6
Daya Bay (China) ~40 km 7
Daya Bay Experiment - Top five most powerful nuclear plants (17. 4 GWth) → large number of e (3 x 1021/s) - Adjacent mountains shield cosmic rays Far Hall (EH 3) 860 m. w. e. , Target: 80 t <L> ~ 1580 m Ling Ao Near Hall (EH 2) 265 m. w. e. , Target: 40 t <L> ~ 560 m Daya Bay Near Hall (EH 1) 265 m. w. e. , Target: 40 t <L> ~ 510 m 8
Daya Bay detectors RPC : muon veto Water pool: muon veto + shielding from environmental radiations (2. 5 m water) 8 functionally identical anti-neutrino detectors (AD) to suppress systematic uncertainties Calibration units 192 8” PMTs 5 m Top and bottom reflectors: more light, more uniform detector response 5 m 9
Antineutrino detection e detected via inverse beta-decay (IBD): Prompt Signal visible photons in liq. scintillator e p e+ + n (prompt signal) ~180 s + p D + (2. 2 Me. V) + Gd Gd* ~30 s for 0. 1% Gd (delayed signal) Delayed Signal n. H n. Gd + ’s (8 Me. V) Powerful background rejection! E Te+ + Tn + (mn - mp) + me+ Te+ + 1. 8 Me. V 10
The Daya Bay Collaboration 42 Institutes, ~ 203 collaborators from China, USA, Hong Kong, Taiwan, Chile, Czech Republic and Russia 11
Interior of an AD 12
AD Installation - Near Hall 13
AD Installation - Far Hall 14
Oscillation results Pee ≈ 1 – sin 22 13 sin 2( mee 2 L/4 E ) – sin 22 12 cos 4 13 sin 2( m 212 L/4 E ) • Far/near relative measurement, 1958 days of data • Oscillation parameters measured with rate + spectral distortion • Both consistent with neutrino oscillation interpretation F. P. An et al. , Daya Bay Collaboration, PRL 121, 241805 (2018). 15
Oscillation results PRL 121, 241805 (2018). sin 22 13 = 0. 0856 0. 0029 N. H. 16 I. H.
Constraining sterile neutrino mixing F. P. An et al. , Daya Bay Collaboration, PRL 117, 151802 (2016); PRL 113, 141802 (2014). Daya Bay and MINOS Collaborations, PRL 117, 151801 (2016). 17
3+1 Neutrino Oscillations Simplest extension: e s 1 =U 2 3 4 Standard 3ν mixing (NH) 18 Pee ≈ 1 – sin 22 13 sin 2( m 231 L/E ) – sin 22 14 sin 2( m 241 L/E ) for Daya Bay baselines 3+1ν mixing Leff/E
Search for a light sterile neutrino PRL 117, 151802 (2016) Daya Bay IBD data (217 days of 6 ADs + 404 days of 8 ADs): Full 4 -flavor oscillation formula for Pee; free variables: sin 22 13, sin 22 14, | m 241| Method A: predict prompt energy spectrum at far hall from measured spectra in near halls. Minimize Energy bin Covariance matrix Weighting factors calculated from baselines (sys. + stat. ) and reactor power profiles Method B: fit spectra from all ADs simultaneously using a binned log likelihood constructed with nuisance parameters for various systematics. Reactor e flux constrained using Huber-Mueller with enlarged uncertainties. 19
Search for a light sterile neutrino Results: Minimum 24 /NDF = 129. 1/145 Method A: Minimum 23 /NDF = 134. 7/147 PRL 117, 151802 (2016) p-value (using MC) = 0. 41 Minimum 24 /NDF = 179. 74/205 p-value (using MC) = 0. 42 Method B: 2 Minimum 3 /NDF = 183. 87/207 No apparent signature for sterile neutrino mixing is observed. Setting constraints on = (| m 241| , sin 22 14): 1. Feldman-Cousins – using large no. of pseudo-experiments to determine C. L. according to 2 2. CLs method p-value for 4 Exclusion region at C. L. if p-value for 3 CLs 1 - 20
Search for a light sterile neutrino • Sterile neutrino: additional oscillation mode: PRL 117, 151802 (2016). Pee ≈ 1 – sin 22 13 sin 2( m 231 L/E ) – sin 22 14 sin 2( m 241 L/E ) for Daya Bay baselines • 3 expt. halls multiple baselines – Relative measurement at EH 1 (~350 m), EH 2 (~500 m), EH 3 (~1600 m) – Unique sensitivity at 10 -3 e. V 2 < Δm 241 < 0. 1 e. V 2 • most stringent limit on sin 22 14 for 2 x 10 -4 e. V 2 < Δm 241 < 0. 2 e. V 2 Excluded 21
Daya Bay + Bugey-3 + MINOS Updated results with MINOS/MINOS+ will be released soon! - Constrain → e by combining constraints on sin 22 14 from e disappearance in Daya Bay and Bugey with constraints on sin 2 24 from disappearance in MINOS - Set constraints over 6 orders of magnitude in m 241. - Exclude Mini. Boo. NE and LSND parameters for m 241 < 0. 8 e. V 2. 22
Better Limits to come Expect ~2 improvement by 2020 23
Absolute reactor antineutrino flux, spectrum, and fuel evolution F. P. An et al. , Daya Bay Collaboration, PRL 116, 061801 (2016); Chinese Physics C 41(1), 13002 (2017); PRL 118, 251801 (2017); ar. Xiv: 1808. 10836 v 1; ar. Xiv: 1904. 07812 v 1. 24
Reactor antineutrino flux ar. Xiv: 1808. 10836; PRL 118, 251801 (2017); Chin. Phys. C 41, 0130002 (2017). Data/Prediction (Huber+Mueller) • Precise measurement of reactor antineutrino flux using 2. 2 M inverse beta decay (IBD) events collected with the Daya Bay near detectors in 1230 days • IBD yield = (5. 91 0. 09)x 10 -43 cm 2/fission • Measured antineutrino yield = 0. 952 0. 014 0. 023 of Huber-Mueller model prediction: confirm Reactor Antineutrino Anomaly Mean fission fractions: 235 U 238 U 239 Pu 241 Pu 0. 564 0. 076 0. 304 0. 056 25
Reactor antineutrino spectrum • • ar. Xiv: 1904. 07812 (2019). 1958 days of data, 3. 5 M IBD events Measured prompt spectrum vs. Huber+Mueller: Global discrepancy at 5. 3 σ Local deviation in 4 -6 Me. V region: 6. 3 σ Extracted a generic observable reactor antineutrino spectrum by removing the detector response 26
Reactor antineutrino flux evolution Effective fission fraction for ith isotope changes in time as fuel evolves: fi, r(t) (fission fraction for ith isotope in reactor r) and Wth, r(t) (thermal power) obtained from reactor data, validated with MC. ( pr = survival probability Lr = baseline Er = average energy per fission f(t) = i i. Fi(t) also evolves IBD yield ith isotope PRL 118, 251801 (2017) 27
Reactor antineutrino flux and spectrum evolution PRL 118, 251801 (2017). f(t) = i i. Fi(t) also evolves IBD yield ith isotope Best fit of f(t) = i i. Fi(t) to get i Favors: overestimation of 235 U yield Slope differs from theory by 3. 1 Sterile only same fractional flux deficit for all isotopes: (d f/d. F 239)/< f> = theory incompatible with data at 2. 6 28
Reactor antineutrino spectrum evolution Sj = observed IBD per fission in jth energy bin PRL 118, 251801 (2017). - First observation of change in IBD spectrum with F 239 at 5. 1 - Shape ~ theory - Demonstration of neutrino monitoring of reactors 29
235 U and 239 Pu Spectra ar. Xiv: 1904. 07812(2019) Fuel evolution allows to extract 235 U and 239 Pu spectra - Ordered by 239 Pu fission fraction into 20 data groups - Fit the 235 U and 239 Pu spectra, as two dominant components - Not sensitive to 238 U and 241 Pu Compare spectra, yield with Huber-Mueller: - Similar excess in 4— 6 Me. V - Significance of local deviations: • 4σ for 235 U • 1. 2σ for 239 Pu - IBD yield comparison: 235 U: 0. 92 ± 0. 023(exp. )± 0. 021(model) 239 Pu: 0. 99 ± 0. 057(exp. ) ± 0. 025(model) 30 1958 days data
Summary • Daya Bay Expt. ₋ Most precision measurement of : 3. 4% : 2. 8% • Direct search for sterile neutrino mixing - fit Daya Bay spectra with additional oscillation due to sin 22 14 - most stringent limit on sin 22 14 for 2 x 10 -4 e. V 2 < Δm 241 < 0. 2 e. V 2 • Reactor antineutrino flux, spectrum and evolution ₋ Flux : consistent with previous short baseline expt. s, 0. 952± 0. 014± 0. 023 of Huber-Mueller ₋ Prompt Spectrum: 6. 3 deviation from prediction in [4, 6] Me. V ₋ Evolution observed. Favors 235 wrong; disfavors sterile neutrino mixing at 2. 6 ₋ 235 U and 239 Pu spectra extracted • Will continue till 2020 31
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