https://lpsc.in2p3.fr/ DocDB/ 0006/000673/001/logo- ILL.png Latest results of the STEREO Experiment a Search for a Sterile Neutrino ~1eV at Short Baseline Adrien Blanchet (CEA Paris-Saclay) Supported by : on behalf of the STEREO collaboration TAUP 2019 The 11th of September
Worldwide Experimental Program DANSS Neutrino 4 Solid PROSPECT NEOS STEREO Research reactor (Pure Research reactor (Pure Power reactor (Mixed 2 U235), Li-loaded U235) Gd-loaded Pu9/U5), Gd-loaded
The STEREO Experiment
STEREO : Experimental Site ILL research facility, Grenoble, France → 10 19 ¯ Research reactor core ~ 58MWth ν e / s • Little damping of the oscillation pattern • Compact core ( ∅ 40cm × 80cm) • Short baseline 9.4 m < L core < 11.2 m • Pure U235 neutrino spectrum : Highly enriched fuel 93 tons moved on air cushions Autumn 2016 Autumn 2016 Challenging Mitigation of the Background • Gamma & Neutron Background from neighboring experiments --> heavy passive + active shielding • Surface-level experiment (15 m.w.e overburden thanks to water channel) 4
STEREO : The Detector • Little damping of the oscillation pattern also thanks to good energy resolution Oscillation analysis based on the relative comparison of the cell spectra. No dependence on the prediction of the reactor spectrum. 5
STEREO : Detection Principle Interaction Channel : Inverse Beta Decay (IBD) IBD tagging via Prompt- Δ T Delayed correlated pairs Prompt Delayed Correlation distance : Detection Efficiency ~ 60% 6
STEREO : Data Taking Phase-I Phase-II ON = 66 days ON = 119 days OFF = 22 days OFF = 211 days • Data taking efficiency: 98.5% • Inefficiencies for neutrino data : • Calibrations: 3.3% • 14% dead-time from selection cuts. 7
Data Analysis
Prompt Signal : E Reconstruction Energy calibration • Weekly calibration with 54Mn sources • Monitoring light leaks between cells using cosmics data • Dedicated algorithm to go from deposited charge (Q) to reconstructed energy (E) parametrized by calibration coefficients (C) and light leaks coefficients (L) : Q j = ∑ M ij E i = ∑ Testing detector's response with C i L ij E i several gamma sources E i = ∑ i i ( M − 1 ) ij Q j 1.04 Data/MC Erec detector AmBe AmBe n-Gd 1.03 AmBe n-H j Zn K Anchoring energy scale with Co Mn Na 1.02 Cs Mn54 gammas 1.01 Counts (a.u.) 1.0 Data Data 1.00 MC MC 0.8 0.99 Cell 1 Cell 1 Cell 1 Cell 1 Cell 1 Cell 1 0.6 Cell 2 Cell 2 Cell 2 Cell 2 Cell 2 Cell 2 0.98 Cell 3 Cell 3 Cell 3 Cell 3 Cell 3 Cell 3 0.4 Cell 4 Cell 4 Cell 4 Cell 4 Cell 4 Cell 4 0.97 Cell 5 Cell 5 Cell 5 Cell 5 Cell 5 Cell 5 0.2 Cell 6 Cell 6 Cell 6 Cell 6 Cell 6 Cell 6 0.96 0 2 4 6 8 10 0.0 9 Nominal Energy (MeV) 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 Reconstructed Energy (MeV)
Remaining Background : Cosmic Induced Using Pulse Shape Discrimination (= Qtail/Qtot) e-recoil signal PSD properties of the LS allow to p-recoil signal discriminate neutrinos from dominant remaining cosmic background τ e τ p < Q tail /Q tot (e) < Q tail /Q tot (p) • PSD distributions are corrected from temperature & evolution of optical properties • ON data fitted using OFF model T ail charge - Qtail • Background model given by OFF data T otal charge - Qtot • Accidentals accurately measured and included in the model Neutrino rates are extracted for each cell and Energy bin Extraction relies on two hypothesis only • Gaussian shape of the neutrino component • Stability of the PSD spectrum shape of correlated background 10
Background Stability Major Background Contribution : Cosmic Induced • Clear correlation between atmospheric pressure and counting rate PSD background shapes independent of environmental conditions • Splitting data in 2 groups : Low and High pressure • Normalization factor = (93.3 ± 0.25)% • Expected for a 10 hPa difference = (93.8 ± 0.3)% High stability of the shape of the background PSD distribution is demontrated for all cells 11
(Absolute-Predictions-less) Oscillation Analysis Data MC Δ i = Δ c = cell ; b = Ebin ≐ D cb − ϕ b M cb ( Δ m 2 14 , θ 14 ) Fully free parameters only constraint : common to all cells Involved systematic uncertainties : Cell-to-Cell Source Uncertainty Study Correlation • Time stability of the energy response with cosmic- Cell-to-cell correlated induced events ± 1.1 % 100 % Energy-Scale • Anchoring of the MC Cell-to-cell uncorrelated ± 0.5 % 0 % • Cell-to-Cell deviations Energy-Scale Normalisation Factor ± 1.2 % 0 % • Neutron capture efficiencies 12
(Absolute-Predictions)-less Oscillation Analysis Statistical inference cross-validated by 2 independent 10 1 Preliminary studies : with Covariance Matrix : χ 2 = ∑ i ∑ Δ i ( V − 1 cov ) ij Δ j j 41 ( eV 2 ) with Nuisance Parameters : 1 ∆ m 2 2 2 i ( ) j ( ) syst . α j χ 2 = ∑ Δ i ∑ + RAA 95% C.L. σ syst . RAA 99% C.L. σ stat . RAA: Best fit i j Stereo: Contours drawn using a Δχ² : Pull Terms Contour (PII) Grenoble Contour PII Covariance Contour (PII) 10 − 1 Saclay contour PII Δ χ 2 = χ 2 ( Δ m 2 10 − 1 1 14 , θ 14 ) − χ 2 sin 2 ( 2 θ ee ) best fit Preliminary • Raster-scan method (best fit search in a given ∆ m ² slice) • Due to detector maintenance phase I & II can be considered as independent measurements . • Preliminary combination is done by summing the two χ² and assuming a standard χ² law for the C.L. • Best-fit value of the RAA rejected at C.L. ~ 99.8% • Sensitivity still dominated by the statistical errors 13
Perspectives
Testing RAA with Pure U5 Reactor Fuel Uncertainty dominated by two components • Neutrino detection efficiency • Mostly depends on neutron event • Topology cut efficiency depends on the spectrum shape of the Gd cascade : Improved description in MC with FIFRELIN (cf. arXiv:1905.11967 + 10M cascades available on zenodo:2653787 ) • MC efficiency corrected for border effects of the Target • Thermal Power estimation • Data/MC neutron detection efficiency Well controlled uncertainties thanks to the ILL redundant and regularly calibrated instrumentation • Current estimate : δ P / P ≃ 1.4 % Stereo experiment has a good potential to check the RAA thanks to the control of the two dominant systematics Current estimate of experimental uncertainty : 2.4 % 15
Testing the Spectrum Shape STEREO Phase-II data only • Predicted Spectrum • Pure U235 Huber Spectrum • + % level corrections in the first 2 energy bins (n-Al capture, off-equilibrium effect, spent fuel) • Quantified comparison with a spectrum prediction requires high precision in the treatment of the energy scale • Residual distortions between measured and simulated reconstructed energies are described by the most δ ( E MC ) general function: E Data = E MC ( 1 + δ ( E MC ) ) • New stringent constraints , complementary to the calibration sources, can be added to the delta function by considering the boron data. 16
Extracting Boron12 Spectrum Boron12 in Data Capture of stopping muon at rest : μ − + 12 C → 12 B 12 B Spectrum Coincidence signal : events/day/100 keV • Prompt (stopping muon) Data • Delayed (beta from Boron 12 decay) − MC 1 10 Boron 12 extracted : • Dominant accidental background is measured on line. • ~ 700 events/day Preliminary • S/B ratio ~ 0.1 to 1 Boron12 in MC − 2 10 Simulating the spectrum : • Beta spectrum prediction using 3 main branches 4 6 8 10 12 14 • Vertices distribution from cosmic muon simulation Reconstructed energy (MeV) • Nitrogen12 background fitted (% level contribution) Systematic Uncertainties : • Weak magnetism corrections • Distribution of µ-capture vertices • Radiative corrections 17
Global Fit of the Energy Scale Point-like energy constraints (calibration sources) Spectrum constraints (Boron 12) S MC ) ( δ ′ � ( E MC ) + δ ( E MC ) S ′ � MC E Data S Data S MC = 1 + δ ( E MC ) + E MC E MC − 1 = δ ( E MC ) 12 STEREO B Spectrum Ratio - 1 1.20 MC B Preliminary 12 MC 0.02 rec / S 1.15 Calib. Points / E 12 B Ratio Data B 1.10 Data 12 Syst. only rec 0.01 S ) = E 1.05 1.00 MC 0.00 rec (E 0.95 δ Preliminary − 0.01 0.90 Neutrino spectrum range 0.85 − 0.02 0.80 0 1 2 3 4 5 6 7 8 9 2 4 6 8 10 12 14 MC E (MeV) MC rec E (MeV) rec δ ( E MC ) Common fit of δ ( E MC ) can be chosen as a polynomial parametrization, or with a non-parametric expression (KDE) δ ( E MC ) At the end, propagate the function on the neutrino spectrum and its uncertainty to infer on the Huber spectrum prediction Stay tuned..! 18
Conclusion and Outlook
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