search for ev sterile neutrinos: status of the stereo ...ilham el atmani on behalf stereo...

29/07/2019I. El Atmani - INPC 2019 Glasgow

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Search for eV sterile neutrinos: Status of the STEREO

experiment

Ilham El Atmani

On behalf STEREO Collaboration

CEA, Saclay

Hassan II University, Casablanca

Reactor Antineutrino Anomaly

29/07/2019 I. El Atmani - INPC 2019 Glasgow2

Re-evaluation of the reactor antineutrinos spectrum (RAA). (Mueller et al Phys Rev C83 )

Re-analysis of antineutrinos flux measurements. (Mention et al Phys Rev D83

073006).

Possible explanation:

Error on the flux prediciton ?

New oscillation at short distance ? New oscillation :(4 th neutrino – n sterile):

sin²(2θ) = 0.17 ± 0.04 ; Δm² = 2.3 ± 0.1 eV²

R= 0.940 ± 0.024 ; 3 s

6.5% deficit

3 n model

3 +1 nmodelPredicted

Observed

The very accurate measurements of the flux confirm the initial RAA

Neutrino oscillation spectrum

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Shape

Low Enriched Uranium reactor experiment observes spectral deviations (Double Chooz, Daya Bay , Reno…) Spectral distortion in 4-6 MeV energy region appear.. Exclusion of light ν sterile in the very low region .

Rate

Discrepancy between observed and predicted 235U yieldby Daya Bay ,suggests 235U may be the primary contributor

of the RAA.

Accurate measurement from fission 235U neutrino

STEREO will measure the pure U-235 neutrino fission spectrum

STEREO Experiment @ ILL Grenoble France

Challenging mitigation of the background

ReactorCosmic-rays Neighboring experiments .

Compact core Φ≈ 40 cm, h ≈ 80 cm Located at [9-11] m from reactor core . Transfer channel covers STEREO (15 m.w.e) .

ILL core

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Power: 58.3 MW research reactor. Operated in 3-4 cycles/year each of 50 days. 1019 s-1 pure ne flux

Highly enriched : 93% 235U.

STEREO Experiment : DETECTOR

Goals : Measure the neutrino spectrum relativedistortion between cells, at [9-11] m from the ILL nucleus. Measurement of pure 235U n energy spectrum


20 cm thick acrylic buffers for homogeneous detector response. PMT coupling via oil bath.

Target: 6 identical cells- Gd-loaded (0.2% in mass)- Vtot= 2.2 x 0.9 x 0.9 m3

Gamma-catcher (unloaded):- Vetos ext. background- Captures escaping g’s

Stereo takes Data since

November 2016

Data taking

Liquid Scintillator

Selection cuts


Topology

IBD Coincidence

Energy

e+ n capture

2 µs< Dt <70 µs

DL < 600 mm

Space-time correlation

Epromp ϵ [1.6 -7.1] MeV Edelayed ϵ [4.5-10] MeV

Eprompt in neighboring cells < 1MeVEprompt not neighboring cells < 0.4 MeV

Edelayed in target > 1 MeV

Background rejection (cosmic rays)

Dtm >100 µs , Dtdetector >200 µs

Stopping Muon ( top of the detector)Prompt = m stop

Delayed = Michel e- / e+

Muon

Charge asymmetry per cell:

QPMT_max/ Qcell < 0.5

Detected by (IBD) with a pair of coincidence signals.

e⁺ ionisation+annihilation

En ≃ E e + + 0.78 MeV

Thermalisation+capture

neutron by Gd


Calibration :Energy Reconstruction

Energy calibration

Weekly calibration with 54Mn sources.

Dedicated algorithm to go from deposited charge to reconstructed energy

Evaluation of systematics Use the cosmic-induced n-H peak (Quasi-uniform vertex distribution, similar to the

neutrino distribution)

Time stability

Fit of the n-H peak

Preliminary

Preliminary

Preliminary

I. El Atmani - INPC 2019 Glasgow 8

Neutron detection efficiency : Improved Gd g cascade

Thermalization and n-capture timewell reproduced by improved MC

Published on : arXiv:1905.11967

zenodo/2653787

X Cascade of Gd nuclei not well described in Geant 4

Discrepancy between data and MC corrected by a 3D model

New Gd g cascade from FIFRELINExplain DATA/MC discrepancy

Global cuts efficiency ~(61.4 ± 0.9 )

Dominant uncertainty in efficiency comes from the neutron capture

->Dedicated studies using AmBe neutron source deployed in the detector

29/07/2019

Preliminary

Preliminary


The normalization factor is corrected from the effectivetime 93.3±0.3% and compatible with the expected pressuredependence.

OFF periods background model :

Background Stability

e recoils (e-,e+,g)

p recoil

(fast n)

Improving Background subtraction for Phase II analysiswith more statistics. Test of the PSD shape stability for: Pressure andWater level variations by splitting reactor OFF periodsin low and high Patmos periods . Very stable shape of the PSD distributions for all cellsafter accurate correction of Patmos dependence. Validates background model.


Neutrino candidates are extracted for each cell and each 500 KeV E binUsed also the distributions of accidentals pairs.OFF rescaled to the ON by a global factor « a » fits the ON spectrum + 1 Gaussian for the neutrino component

On periods Background + neutrino extraction

OFF periods Background model

PSD: Late charge of the light pulse / Total charge = Qtail/Qtot

Rejected the cosmic-induced dominantbackground by the Pulse Shape discrimination .

Neutrino rate extraction from PSD spectra

PSD distributions are corrected from temperature & evolution of optical properties.

Compatibility between ON & OFF

S/B ~1 reached .


-Testing the non-oscillation hypothesis : Phase II data only

The null oscillation hypothesis not rejected

Oscillation analysis

Good agreement between Data and model no oscillation x Φi . Φi : eleven free normalization parameters, one for each energy bin i, but common the all 6 cells .

Compare measured and simulated spectra


Due to detector maintenance phase I & II can be considered as independent measurements.

Using Raster Scan approach from : G.J. Feldman, R.D.

Cousins, Phys. Rev. D 57 (1998) 3873).

Generate pseudo- experiments to estimate D2 pdf.

Results are compatible with the null oscillation Hypothesis.

Rejects the best fit of the ★ RAA excluded

at 99.8 % C.L

★(sin²(2θ) = 0.17 ; Δm² = 2.4 eV² )

Oscillation analysis : Exclusion contours

- Phase I + II data


Reference spectrum per fission of U-235

P. Huber, Phys.Rev. C84 (2011) 024617

Available in 50 keV bins.

Conversion of ILL total β-spectrum .

12h irradiation time Pure U-235 spectrum.

Standard reference to test the 5MeV bump

Mean Fission fraction of Pu-239 at ILL : 0.7 %-> Small effect ~0.3%

Predicted Antineutrino Spectrum including all contributions accurate prediction

Corrections applied to Huber’s predicted spectrum

Total antineutrino spectrum

Huber model

Predicted anti neutrino Spectrum

( ) ( )STEREOpred k k corr

k

S E f S E

235

( )( ) ( )th

k k k

kU

W tS E f S E

E

Thermal Power

Mean energy released /fission

Huber x s ( IBD )

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Rel

at. R

esid

ual

s n

to

ON

Time evolution calculated with FISPACT+BESTIOLE

Spent fuel elements are stored in the water channel above STEREO.

Correction associated with the relative to full power spent fuel

Amplification factors due to the changing storage positions Correction applied after reactor stop Nominal cycle (50 days, 55MW)

Cumulative residual neutrino rate History of all elements (spent fuel of the last 17 cycles)

Correction Reactor Prediction : Spent fuel

I. El Atmani - INPC 2019 Glasgow

Preliminary

Preliminary

Monitoring of all spent fuel element

Preliminary

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Comparison of OFF equilibrium between STEREO and (Mention et al PhysRev D83 073006).

Used the ratio of predicted neutrinos rates

Neutrinos spectra reactor ON , available from 12h to 50 days with reference power of 55 MW

Accumulation of long-lived isotopes is estimated by reactor simulation codes. (FISPACT + BESTIOLE)

At ILL: long-lived isotopethat contribute to off-equilibrium do not have time to accumulate in the fuel.

~ 1 % in the first energy bin . Confirmed with each cycle ,

phase I,II STEREO

Correction Reactor Prediction : Off equilibrium effects

I. El Atmani - INPC 2019 Glasgow

Preliminary

Preliminary


Dominated by -decay of 28Al in the first 500 keV E with ~ 9±0.1%

-Capture of neutrons on structural elements:

28Al is abundant in the fuel element, in the heavy

water vessel and in the neutron guides, and Mn56 only with 0.5±0.2 % in the first bin .

Using a complementary MNCP-2.5 +TRIPOLI simulation . Evolution code was used, allowing to take into account the contributions of all beta decays in-core

Correction Reactor Prediction : Neutrinos n-captures Al-28

28Al n spectrum

Ratio of Al+U235 spectrum to U-235 spectrum after 12 irradiation ( BESTIOLE+FISPACT)

28Al n spectrum

Preliminary


STEREO demonstrates its high precision capability:

43.4 k neutrinos detected in phase-II, 65.5 k total. Good control of Prompt and Delayed Signal Extensive measurements (233 days OFF and 185 ON) show a very high stability of the background Major fraction of the initial RAA contour is now rejected . Improved description of the Gd neutron capture gamma cascade with FIFRELIN

arXiv:1905.11967 with cascades on Zenodo

Oscillation analysis ->Upcoming Long Paper

Measurement of the pure 235U neutrino fission spectrum shape :

Spectrum Shape :Ongoing analysis to provide an updated of new reference spectrum from pure 235U fission with large statistics and refined control of the energy scale.

Absolute Rate :STEREO has the potential of accurate measurement thanks to good control of the detection efficiency and the reactor power. Potential accurate rate of the 8% deficit counting out by DayaBay.

Conclusion & Perspectives

Perspectives


Back up


Predicted spectrum Huber 235U spectrum + % level corrections in the first 2 energy bins (n-Al capture, off-equilibrium effect, spent fuel).

agreement with the data up to 6.375 MeV (χ2=14.9/18).

Large deviation observed in the 3 highest energy bins (χ2=33.3/21). Such localized large distortion cannot be explained by varying the 3 parameters of a quadratic model of the energy scale.

Further constraints from upcoming higher statistical accuracy and combination with other pure U235 spectra are required to draw pertinent tests of the spectrum shape.

Spectrum Shape


2 2 22

, ,2

,

( , , )Cells Ebins Cells CellsN N N NNorm U Escale UEscale Cl i i l i l l

Norm U Escale C Escale Ul i l ll i l l

D M m s

s s s s

, , ,( , , ) ( , ) (1 ( ) ( ))NormU Escale C Escale U Escale

l i l i l l iM M Sm s m s m

Relied on the oscillation parameters (Dm2 , sin²(2θ) ) and αset of nuisance parameters have been added on this builtmodel :

Insensitive to absolute rate predictions -> no requiredinput of the spectrum prediction.

Φ : Free normalization parameters for each energy bin i,but common the all 6 cells l .

Depending only to the spectrum shape.

- Formalism to test the different oscillation hypotheses of the parameter space

D l,I : Numbers of measured neutrino candidates.

M l,I : Expected numbers of neutrinos

With :

Complementary analysis besides this method carried out to check ,by integrating all the systematic into an overall covariance matrix.-> Very good agreement

Oscillation analysis : Method

23

Measured neutrino rates corrected for detection efficiency found in perfect agreement with the expected 1/L2 law:

Nn/day = N0 / L2

Rates have been corrected from the detection efficiency

29/07/2019 I. El Atmani - INPC 2019 Glasgow

Measured Neutrino Signal

Experiment Pth

[MW]L[m] Technique Depth

[m.w.e]S/B

NEOS 2800 25 Gd-LS 20 22

DANSS 3000 9-12 Gd-LS 50 ~22

Neutrino-4 90 6-11 Gd-LS 5-10 < 1

STEREO 58.3 9-11 Gd-LS 15 ~ 1

Prospect 85 7-12 Li-LS < 1 > 1

SoLiδ 72 6-11 Li-PS 10 ~3

A Worldwide Challenge

LEU HEU

Example of reactor experiments looking for oscillation at very short distances


search for ev sterile neutrinos: status of the stereo ...ilham el atmani on behalf stereo...

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