View
0
Download
0
Category
Preview:
Citation preview
THE STEREO EXPERIMENTHELENA ALMAZÁN
ON BEHALF OF THE STEREO COLLABORATION MAX-PLANCK-INSTITUT FÜR KERNPHYSIK
HEIDELBERG
10TH OCTOBER - THE STEREO EXPERIMENT - H. ALMAZAN
REACTOR ANTINEUTRINO ANOMALY
2
L [m]
R=N
expN
cal
10 102 1030.70
0.80
0.90
1.00
1.10
1.20
R = 0.934 ± 0.024
Bugey−3Bugey−4Chooz
Daya BayDouble ChoozGosgen
ILLKrasnoyarskNucifer
Palo VerdeRENORovno88
Rovno91SRPRAA:
• deficit at 2.8σ in flux measured by several experiments at different distancies from reactors
Possible explanations
arXiv: 1703.00860, S. Gariazzo et al.
→ sterile neutrino hypothesis?→ wrong prediction? Discrepancy between observed and predicted IBD yields from 235U by Daya Bay. Suggests 235U could be the primary contributor of the RAA
∼ 1 eV sterile neutrino new oscillation angle
and mass splitting need dedicated measurement at short
distances
P⌫̄e!⌫̄e(E⌫̄e , L) = 1� sin2(2✓new)sin2
✓1.27
�m2new[eV
2]L[km]
E⌫̄e [MeV ]
◆
⌫̄e
arXiv: 1704.01082, Daya Bay Collaboration
10TH OCTOBER - THE STEREO EXPERIMENT - H. ALMAZAN
STEREO EXPERIMENT
3
→ spectral shape studies Measurement of a pure 235U energy spectrum → Help to understand Daya Bay results
→ sterile neutrino hypothesis to probe the RAA region by measuring relative distortions of the energy spectrum as a function of the distance [9-11m]
arXi
v: 1
512.
0665
6, C
. Buc
k et
al.
↑rea
ctor↑
segmented detector (6 cells)
∼ 10 m
Cell
1
Cell
2
Cell
3
Cell
4
Cell
5
Cell
6
Independent from predicted energy spectrum
⌫̄e
⌫̄e
10TH OCTOBER - THE STEREO EXPERIMENT - H. ALMAZAN
EXPERIMENTAL SITE - ILL
4
ILL research reactor
Grenoble (France)• nominal power 58.3 MWtherm 1019. s-1
• compact fuel element 40cm ∅
• HEU fuel 93% 235U
Experimental
site• short baseline experiment
8.9m < Lcore < 11.1m
• ground-level experiment
• gamma and neutron
background from neighbour
experiments
⌫̄e
10TH OCTOBER - THE STEREO EXPERIMENT - H. ALMAZAN
STEREO DETECTOR
5
3m
1.5m
PMTs
Gamma-Catcher----------
outer crown+
liquid scintillator(without Gd)
Mineral oil
Acrylic wall2mm thick
Reflective foilVM2000Nylon net
(air gap)
Neutrino Target • 2.2 x 1.5 x 0.9 m3
• Gd-β-diketonate liquid scintillator → good response
Gamma catcher • Outer crown volume
• filled Gd-free liquid scintillator
Buffer • 48 PMTs (8inch) • acrylic buffers • mineral oil (optical coupling)
STEREO detector
10TH OCTOBER - THE STEREO EXPERIMENT - H. ALMAZAN
DETECTION PRINCIPLE
6
p
n
e+ e-
⌫e + p ! e+ + n
Interaction via Inverse Beta Decay IBD
2. Delayed Event neutron capture
• Gd nuclei X
i
E�,i ⇠ 8MeV
E� ⇠ 2.2MeV• H nuclei
1. Prompt Event positron annihilation
Evis,e+ ' E⌫e � 0.8MeV
STEREO detector with shielding
10TH OCTOBER - THE STEREO EXPERIMENT - H. ALMAZAN
DATA TAKING - TIME LINE
7
NovOct Nov Dec Jan Feb Mar Oct2016
Nov Dec Jan Feb Mar2017
Commissioning ON OFF ON OFF ON
2017Apr May Jun Jul Aug Sep Oct
OFF
2018
OFFON ON
Data Taking - Phase I Data Taking - Phase II
Phase I: • Loss of optical coupling between PMTs
and target for one target and on GC cell • Evolving light cross-talks between cells → repaired during summer 2017
Phase II: • Stable conditions ∼ 95% of data taking
time
10TH OCTOBER - THE STEREO EXPERIMENT - H. ALMAZAN
DETECTOR RESPONSE: LIGHT CROSS-TALKS
8
Date [MM/DD/YY]10/31/16 12/31/16 03/02/17
[%]
ij L
0
5
10
15
20 Cell3→Cell2
GC Back→Cell6
GC D19→Cell3 Cell6→Cell5
Date [MM/DD/YY]10/31/17 12/31/17 03/02/18
[%]
ij L
0
5
10
15
20 Cell3→Cell2
GC Back→Cell6 GC D19→Cell3
Cell6→Cell5
liquid slowly infiltrating into acrylic walls detector stability after walls reparation
Evolution of the light cross-talk between cells estimated by muon events in neutrino runs
arXiv
: 180
4.09
052,
N. A
llem
ando
u et
al.
Phase I Phase II
10TH OCTOBER - THE STEREO EXPERIMENT - H. ALMAZAN
0 200 400 600 800 1000 1200 1400Charge [PE]
3−
2−
1−
0
1
2
3
Non
-line
arity
[%]
Charge deposited in cell 6 [PE]50 100 150 200 250
Even
ts /
4 PE
02468
10121416182022
310×
DATA
MC
DETECTOR RESPONSE: CALIBRATION
9
LED system, used to study the single photoelectron and the
PMT-DAQ linearity in the detector target
extensive circulation of radioactive sources Ge68 , Sb124 , Cs137 , Mn54 , Zn65 , Na24 , AmBe
along 3 different calibration systems
arXi
v: 1
804.
0905
2, N
. Alle
man
dou
et a
l.
Cell response for Mn source internal deployment in Cell 6
Deviation from linearity for all the Target PMTs
10TH OCTOBER - THE STEREO EXPERIMENT - H. ALMAZAN
DETECTOR RESPONSE: ENERGY RECONSTRUCTION
10
~Ereco = M�1 · ~Q
arXi
v: 1
804.
0905
2, N
. Alle
man
dou
et a
l.
Charge and energy connection by inverse M matrix
collected photons/MeV from calibration runs in cell i
cross-talk cells j->i measured online
Mij = CjLji
described by
Date [MM/DD/YY]10/31/17 12/31/17 03/02/18
[%]
ij L
0
5
10
15
20 Cell3→Cell2
GC Back→Cell6 GC D19→Cell3
Cell6→Cell5
Charge deposited in cell 6 [PE]50 100 150 200 250
Even
ts /
4 PE
02468
10121416182022
310×
DATA
MC
10TH OCTOBER - THE STEREO EXPERIMENT - H. ALMAZAN
DETECTOR RESPONSE: ENERGY RECONSTRUCTION
11
~Ereco = M�1 · ~Q
arXi
v: 1
804.
0905
2, N
. Alle
man
dou
et a
l.
Charge and energy connection by inverse M matrix
Mij = CjLji
described by
1.00%0.35%
n-H peak n-Gd peak Time Stability Cell to cell deviation
Stability of the reconstructed n-H & n-Gd peaks – probing whole target volume
10TH OCTOBER - THE STEREO EXPERIMENT - H. ALMAZAN
BACKGROUND IN STEREO
12
Reactor Induced Environmental Radioactivity Muon induced
• neutrons • gamma radiation
from n-capture
• Thorium/Uranium (concrete)
• Radon/Argon (air)
• spallation neutrons (in shielding)
• stopping muons
Reactor OFFReactor ON
10TH OCTOBER - THE STEREO EXPERIMENT - H. ALMAZAN
passive shielding
SHIELDING IN STEREO
13
Reactor Induced Environmental Radioactivity Muon induced
• neutrons • gamma radiation
from n-capture
• Thorium/Uranium (concrete)
• Radon/Argon (air)
• spallation neutrons (in shielding)
• stopping muons
10TH OCTOBER - THE STEREO EXPERIMENT - H. ALMAZAN
SHIELDING IN STEREO
14
Reactor Induced Environmental Radioactivity Muon induced
• neutrons • gamma radiation
from n-capture
• Thorium/Uranium (concrete)
• Radon/Argon (air)
• spallation neutrons (in shielding)
• stopping muons
water pool, active veto
10TH OCTOBER - THE STEREO EXPERIMENT - H. ALMAZAN
ANTINEUTRINO SELECTION AND CORRELATED BACKGROUND
15
⌫e + p ! e+ + nIBD coincidence selection Prompt: • 1.6 < Eprompt < 7.1 MeV • Prompt contained in a target cell
and its 4 neighboring cells
Delayed: • 4.5 < Edelayed < 10 MeV
Cosmic rejection: • Muons: Veto 100µs after each
detected muon • Fast neutrons/multiple neutron
captures: Isolation cut of 100µs after and before prompt
• Stopping muons: Cut high asymmetry of the light collection in the vertex cell
e+n capture
Accidental pair: estimated with shifted time windows for the delayed search
Pulse shape discrimination for prompt signal: background estimation for neutron induced reactions
10TH OCTOBER - THE STEREO EXPERIMENT - H. ALMAZAN
PULSE SHAPE DISCRIMINATION
16
Pulse shape discrimination (PSD)
allow us to distinguish background events (neutron induced reactions) with
real positron interactions
But PSD follows temperature changes • seasons and when reactor going on or
off (lasting for several weeks) • A PSD cut does not permit to have at
the same time: a constant neutrino acceptance and a constant background rejection
10TH OCTOBER - THE STEREO EXPERIMENT - H. ALMAZAN
ANTINEUTRINO SIGNAL EXTRACTION: FROM PSD DISTRIBUTION
17
→ Solution: fit PSD distributions to extract neutrino rates
Reactor OFF Reactor ON
Self-consistent method to estimate background under component: • Adapt to PSD variations (temperature sensitivity) • Local rescaling to global norm (pressure sensitivity)
Multi-Gaussian fit for each cell / energy / time bin Aγ/Ap ratio constrained by the OFF model
10TH OCTOBER - THE STEREO EXPERIMENT - H. ALMAZAN
ANTINEUTRINO SIGNAL EXTRACTION: FROM PSD DISTRIBUTION
18
Phase-II: Updated background model with increased statistics (large time binning)
A second component (Ap) for p-recoils, anchored relatively to the first one (Ap) → Possible physical origin: multiple proton recoils (under study)
10TH OCTOBER - THE STEREO EXPERIMENT - H. ALMAZAN
RELATIVE COMPARISON OF NEUTRINO ENERGY DISTRIBUTION
19
Oscillation test using ratio of energy distributions - cell 1 taken as reference • Reduced systematics • Insensitive to absolute flux normalization and predicted spectrum shape
MC takes into account cells differences,
detection efficiencies etc.
• Vi is the covariance matrix of the 5 ratios (common reference for each cell) for the energy bin i • {α} are nuisance parameters to take into account estimated systematics
Pull terms Cell-to-cell correlated Uncorrelated
Energy scale 0.35% 1.00% from energy scale
Normalization - 1.70% from neutron efficiencies arXi
v:18
06.0
2096
, STE
REO
Col
labo
ratio
n
10TH OCTOBER - THE STEREO EXPERIMENT - H. ALMAZAN
NO OSCILLATION HYPOTHESIS TEST
20
• Minimized pull terms stay within ± 1 σ • Non-oscillation hypothesis (H0) can not be rejected: p-value = 34 % for phase-I
Comparison between cells for data
ar
Xiv:
1806
.020
96, S
TERE
O C
olla
bora
tion
10TH OCTOBER - THE STEREO EXPERIMENT - H. ALMAZAN
EXCLUSION CONTOURS
21
• Phase I and II combined results (66 + 47 data days reactor ON)
Considered as two independent measurements:
• Raster scan approach (Δχ2 law simulated in each Δm2 bin)
• Best-fit value of the RAA rejected at 98% C.L.
�2 = �2I(~↵I) + �2
II(~↵II)
★: RAA oscillation best fit ∆m2RAA = 2.3 eV2 - sin2 (2θRAA) = 0.14
arXiv:1806.02096, STEREO Collaboration Talk at Neutrino 2018
10TH OCTOBER - THE STEREO EXPERIMENT - H. ALMAZAN
CONCLUSION
22
• Stereo is now running under stable conditions
• Data taking will continue until end 2019, reaching 300 days of reactor-ON data
• The correlated background understanding improves using reactor-OFF periods
• Exclusion contour obtained using the robust ratio method, rejects the original RAA best fit value is at 98% CL using a ratio comparison insensitive to bias in spectra prediction arXiv:1806.02096, STEREO Collaboration (submitted and accepted at PRL)
• Improved results are coming soon, with a pure 235U spectrum
thanks for your atte
ntion
THE STEREO COLLABORATION
BACK-UP
10TH OCTOBER - THE STEREO EXPERIMENT - H. ALMAZAN
BACKGROUND SPECTRUM
25
Reactor OFF prompt energy spectrum
both prompt and delayed are neutron captures
H(n,γ)
12C(n,n’γ)12C
10TH OCTOBER - THE STEREO EXPERIMENT - H. ALMAZAN
BACKGROUND SPECTRUM
26
10TH OCTOBER - THE STEREO EXPERIMENT - H. ALMAZAN
BACKGROUND SPECTRUM
27
10TH OCTOBER - THE STEREO EXPERIMENT - H. ALMAZAN
SHIELDING
28
10TH OCTOBER - THE STEREO EXPERIMENT - H. ALMAZAN
SHIELDING
29
10TH OCTOBER - THE STEREO EXPERIMENT - H. ALMAZAN
FIT
30
10TH OCTOBER - THE STEREO EXPERIMENT - H. ALMAZAN
SENSITIVITY CONTOUR
31
10TH OCTOBER - THE STEREO EXPERIMENT - H. ALMAZAN
NEUTRON EFFICIENCY STUDIES
32deployment position
norm
alize
d Da
ta/M
C ra
tio
0.92
0.94
0.96
0.98
1.00
1.02
1.04
1.06
Homogeneity of the Cut Efficiency Cell 1Cell 4Cell 6
1.08
-1%
+1%
bottom10 cm
top80 cm
center45 cm
n-Hn-Gd
reconstructed energy [MeV]0 1 2 3 4 5 6 7 8 9 10
entri
es/0
.05
MeV
0
100
200
300
400
500
600
700
800captures in Gd = (88.9 ± 0.2) %
AmBe neutron capture energy spectra
10TH OCTOBER - THE STEREO EXPERIMENT - H. ALMAZAN
SYSTEMATIC UNCERTAINTIES
33
Recommended