The STEREO experiment, a search for sterile at ILL

Aur´elieBonhomme IRFU, CEA-Saclay

on behalf of the Stereo collaboration

supported by

Pheniics Fest - May 31, 2017 Physics motivations Neutrino physics What’s the status of today ?

Neutrino status in the : Historically introduced as massless Electrically neutral Fermion, existing in three families (flavors)

Neutrino physics construction: 1930: Predicted by Pauli but ”undetectable” ! 1934: Theory of 1956: First experimental detection of neutrinos ! (Cowan and Reines) ... until today: development from experimental anomalies Neutrinos have mass and oscillate !

Aur´elieBonhomme - Stereo collaboration Status of the Stereo experiment 1 / 12 Physics motivations Neutrino physics Masses and oscillations

m2

m 2 3x3 matrix, parameters 3 experimentally measured

Δm 2 « atmospheric » ν ν 32 e m1 ~ 2. 10-3 eV 2 ν ν μ = UPMNS m2 ν ν τ m3 2 m2 2 Δm21 « solar » -5 2 2 ~ 7. 10 eV m1 mass states → propagation ? flavour states 0 → interaction Normal hierarchy for illustration

→ After propagation, neutrinos have a given probability to be detected in an other flavour state.

2 ∆m2L In most of the cases: P”appearance” = 1 - P”disappearance” ∝ sin (a E ) ≡ Neutrino oscillations

Aur´elieBonhomme - Stereo collaboration Status of the Stereo experiment 2 / 12 Physics motivations Motivation The Reactor Antineutrino Anomaly (RAA) Phys.Rev.D83:073006,2011

ν ? e ν ν ν ν m1 m2 m3 ν ? e μ ? ντ PRODUCTION PROPAGATION DETECTION (flavours) (masses) (flavours)

1.1 3 ~ 6 % deficit 1 w e n , 0.9 d 4 ? e r p

) solar anomaly

● P 0.8

Sterile X E N (

neutrino ? / 0.7 atmospheric

S reactor anomaly

B anomaly O 0.6 ? ● Wrong N prediction ? 0.5 0.4 −1 0 1 2 3 4 5 6 10 10 10 10 10 10 10 10 Reactor to detector distance (m)

 2 2  2 2 ∆mnew [eV ]L[km] Pνe →νe (Eνe ,L) = 1 - sin (2θnew ) sin 1.27 Eνe [MeV ]

Aur´elieBonhomme - Stereo collaboration Status of the Stereo experiment 3 / 12 Physics motivations Motivation A light at the eV scale ?

m2 Introducing sterile neutrinos at the eV2 mass-scale → U is now a 4x4 matrix m 2 PMNS 4 2 2 Mainly two new parameters of interest: sin (2θnew ) and∆ mnew )

2 10 (eV 2 Δm «short baseline» 2 new

new m

~ 1 eV 2 Δ

1 Accesible m 2 3 parameter space

2 Δm32 « atmospheric » ~ 2. 10-3 eV 2 10-1 RAA contour @ 95 % CL RAA contour @ 99 % CL Best Fit Exp. contour @ 95 % - Shape Stereo(Stereo Proposal Proposal ) Exp. contour @ 95 % - Norm+Shape (Stereo Proposal ) m 2 2 10-1 1 2 sin2(2θ ) Δm21 « solar » new ~ 7. 10-5 eV 2 m 2 1 → Stereo will look for oscillations in this parameter space region ? 0 Normal hierarchy for illustration Aur´elieBonhomme - Stereo collaboration Status of the Stereo experiment 4 / 12 Detector The Stereo detector Designed to probe the Reactor Antineutrino Anomaly region

X Segmented detector filled with liquid X Close to the compact ILL nuclear core in Grenoble: 9m < Lcore < 11m

In case of discovery: Toward a new physics ! Measure of oscillation parameters. Otherwise: Exclusion of one region of the parameter-space In any cases: First measurement of the antineutrino spectra associated to 235U

Aur´elieBonhomme - Stereo collaboration Status of the Stereo experiment 5 / 12 Detector The Stereo detector Designed to probe the Reactor Antineutrino Anomaly region

What are we looking for ? Acrylic buffers PMT Mineral oil Remember:

2 2 ∆m L Gamma-catcher P ∝ 1 - sin (a ) ------E External crown scintillator liquid Target (no Gd) ------6 cells

scintillator liquid (Gd loaded) Counts L

Without oscillation

Center cell with oscillation

Border cell with oscillation

1 2 3 4 5 6 7 Evisible [MeV] E

Aur´elieBonhomme - Stereo collaboration Status of the Stereo experiment 6 / 12 Detector

Water channel 15 mwe overburden

93 tons moved on air cushions

Autumn 2016

Aur´elieBonhomme - Stereo collaboration Status of the Stereo experiment 6 / 12 Preliminary analysis Data analysis A challenging measurement !

10 meters from reactor... → Radiations (, gammas...) Ground-level experiment... → Cosmic background

↓

SHIELDING 7 ] 10 ↓ -1 First raw measured spectra 106

∼ kHz of triggers Rate [day

105 ↓ 104 DATA ANALYSIS 103

↓ Reactor ON 102 Reactor OFF 10 Expected neutrino signal Neutrino signal ∼ few hundreds per day 1 2 4 6 8 10 12 Reconstructed energy [MeV]

Aur´elieBonhomme - Stereo collaboration Status of the Stereo experiment 7 / 12 Preliminary analysis Shielding against background

µ-metal Borated polyethylene

µ-Veto

Lead

Soft iron

B4C

Lead Support structure

Aur´elieBonhomme - Stereo collaboration Status of the Stereo experiment 7 / 12 Preliminary analysis Shielding against background

Detector Borated polyethylene

µ-Veto

Lead

Soft iron

B4C

Lead Support structure

Aur´elieBonhomme - Stereo collaboration Status of the Stereo experiment 7 / 12 Preliminary analysis Data analysis A challenging measurement !

10 meters from reactor... → Radiations (neutron, gammas...) Ground-level experiment... → Cosmic background

↓

SHIELDING 7 ] 10 ↓ -1 First raw measured spectra 106

∼ kHz of triggers Rate [day

105 ↓ 104 DATA ANALYSIS 103

↓ Reactor ON 102 Reactor OFF 10 Expected neutrino signal Neutrino signal ∼ few hundreds per day 1 2 4 6 8 10 12 Reconstructed energy [MeV]

Aur´elieBonhomme - Stereo collaboration Status of the Stereo experiment 7 / 12 Preliminary analysis Data analysis Identify a neutrino candidate

Inverse Beta Decay in the liquid: e- + νe e γ n p γ

No event 100μs before No event 100μs after

No event between prompt and delayed

t in coincidence prompt delayed

Energy deposit and annihilation of e+ Capture on Gd and γ cascade

Aur´elieBonhomme - Stereo collaboration Status of the Stereo experiment 8 / 12 Preliminary analysis Background identification Stopping

Non tagged-muons that stop and decay in top layer of the detector: High collected charge asymmetry between photomultipliers Low correlation time (µ lifetime ∼ 2.2 µs)

Qcell

μ Q1 Q2 Q3 Q4 40 s]

µ 40 35 T [

∆ 35 30 30 25 25 20 e 20

15 15

10 10

5 5

0 0 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Most Prob. Val. High limit QMAX / Qcell

Q4 =QMAX (QMAX > Qcell /4 ) Aur´elieBonhomme - Stereo collaboration Status of the Stereo experiment 9 / 12 Preliminary analysis Background identification

Fast neutrons that mimic the neutrino signal: Pusle Shape Discrimination (PSD) Correlation with atmospheric pressure

t tail charge - Qtail total charge - Qtot ) -1 PSD ON 22 tot 0.5 PSD OFF / Q 120 20 tail 0.45 Q STEREO PRELIMINARY 18 Rate (day MARCH 2017 100 νe 0.4 16 STEREO PRELIMINARY MARCH 2017 0.35 14 80 12 0.3 nuclear recoil population 10 60

0.25 8 40 0.2 6 4 0.15 20 electronic recoil population 2 0.1 0 1 2 3 4 5 6 7 8 9 0 Event energy [MeV] 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 Q / Q tail tot

Aur´elieBonhomme - Stereo collaboration Status of the Stereo experiment 10 / 12 Preliminary analysis Preliminary results and next steps

Evolution of the correlated rate in the STEREO detector

) 1600 -1 Correlated 1400 Accidental Rate (day

1200 STEREO PRELIMINARY MARCH 2017 1000

800

600

400 Reactor OFF Reactor ON 200

0 22/12 29/12 05/01 12/01 19/01 26/01 02/02 09/02 Time Rates after preliminary IBD selection, and simple topological cut. Software dead time and pressure correction applied.

Next steps: Cuts optimization (deposited energy topologies) Full energy reconstruction → spectra Data taking: 2017: + 95 days reactor ON (sept.-dec.), 2018: + ∼ 150 days ON

Aur´elieBonhomme - Stereo collaboration Status of the Stereo experiment 11 / 12 Preliminary analysis Conclusions !

Neutrinos have mass and oscillate A three-families framework compiles most of the experimental data...... but is challenged by several anomalies.

The Reactor Antineutrino Anomaly revealed a antineutrino deficit close to reactors It can be explained by the existence of light sterile neutrinos ... but is challenged by several anomalies.

The Stereo detector is currently measuring the antineutrino spectrum at ILL (Grenoble) to answer this question !

THANK YOU !

Aur´elieBonhomme - Stereo collaboration Status of the Stereo experiment 12 / 12