20th Lomonosov Conference on
Elementary Particle Physics
Moscow State University
August 20, 2021

Five years of sterile neutrino
searches with DANSSMikhail Danilov, LPI (Moscow)
for the DANSS Collaboration

DANSS collected 5.5M antineutrino events in 5 years
x103

Reactor off

Reactor off

Reactor off❖ Total statistics accumulated is 5.5M IBD-events in 5 years
Including 2.46M/2.32M events at Top/Bottom positions
and 3 reactor off periods(5M events in oscillation analysis)2

There are several indications of 4th neutrino
LSND, MiniBoone: νe appearance
SAGE and GALEX νe deficit (GA)
Reactor νe deficit (RAA)

Indication of a sterile neutrino
Δm2 ~ 1 eV2Sin22θ14 ~0.1=> Short range neutrino oscillations

Inverse Beta Decay(IBD) process

Reactor models are based on ILL measurements of 235U, 239Pu, 241Pu
electron spectra.
Recently Kurchatov Inst. Group observed 5.4% smaller e- yields
3for 235U, 239Pu (arXiv:2103.01684v1). This can explain the RAA!

Recent (2018) indications of sterile neutrinos
NEUTRIN0-4: ∆m2~7eV2 sin22θ~0.35! JETP Lett. 109 (2019) no.4, 213; Arxiv:2005.05301
NEUTRINO-4 claimed observation ofsterile neutrinos although significance isonly 3σ and there are concerns aboutvalidity of the analysis:
M.D., N.Skrobova JETP Lett.112,199(2020)
C.Giunti et al. Phys.Lett.B 816(2021)136214

MiniBooNE νe excess of 4.8σ (6σ with LSND)

Phys.Rev.Lett. 121 (2018) no.22, 221801

Searches for sterile neutrinos are very exciting

4DANSS Detector design ( ITEP-JINR Collaboration)
Grooves with fibers
Gd containing
coating 1.6 mg/cm2
0.35%wt

Polystyrenebasedscintillator

JINST 11(2016)no11,P11011

SiPM MPPC S12825-050C
Y11 1.2mm ᴓ WLS fibers
PMT R7600U-300
1 layer = 5 strips = 20 cm
Y-Module

X-Module

10 layers= 20 cm

PMTPMT

100fibers
100 fibers

SiPMs• 2500 scintillator strips with Gd
containing coating for neutron capture• Light collection with 3 WLS fibers• Central fiber read out with individual
SiPM• Side fibers from 50 strips make a bunchof 100 on a PMT cathode = Module• Two-coordinate detector with finesegmentation – spatial information• Multilayer closed passive shielding:electrolytic copper frame ~5 cm,borated polyethylene 8 cm, lead 5 cm,borated polyethylene 8 cm
5• 2-layer active μ-veto on 5 sides

DANSS at Kalinin Nuclear Power PlantWaterCoreh20.3
19.6

DANSS

10.7

DANSS is installed on a movable platform under
3.1 GW WWER-1000 reactor(Core:h=3.7m, =3.1m) at Kalinin NPP.
~50 mwe shielding => μ flux reduction ~6!
No cosmic neutrons!

6.6

forms (125MHz), look for correlated pairs offline.

60.0

Fuel fission fractions: average
start and end of campaign [%]235U 54.1 63.7 44.7239Pu 33.2 26.6 38.9
Detector distance from reactor core 10.9-12.9m
238U
7.3
6.8 7.5(center to center) is changed 2-3 times a week241Pu 5.5
2.8 8.5(for a typical campaign)
Trigger: ΣE(РМТ)>0.5-0.7MeV=>Read 2600 wave

Improvements since last year*(in comparison with our presentations in 2020: M.D. arXiv:2012.10255)❖Median of Landau distribution is used in muon calibration instead of
most probable value → More stable calibration.
SiPM - 18.9p.e./MeV; PMT – 15.3p.e/MeV❖Use of additional 12B production mechanism: µ-+12С-> 12B + vµ
Consistent results in Energy scale calibration with n+12С-> 12B + p❖One more Reactor Off period❖Additional 1.5 million of IBD events7

Accidental coincidence background
Beforesubtraction

After
subtraction

cut depends on Ee+
Accidental
Background❖ Accidental coincidence of 2 uncorrelated signals (e+-like and neutron-like) in a IBD
window [1-50] ms → accidental coincidence background (ACB)❖ ACB spectrum is constructed directly from data applying the same physics cuts as forIBD signal except coincidence time taken outside IBD time window [1-50] ms in numerous
non-overlapping intervals (large statistics is essential to decrease statistical errors ofsubtraction) ➔ No systematic errors❖ ACB rate is 15.4% of IBD rate (Top detector position in [1-50] ms, Ee+: 1.5-6 MeV).
❖ Selection of cuts (e.g. geometric) to reduce ACB  smaller statistical errors
8Subtraction of residual backgrounds

Reactor OFF:73 events/dayin [1.5-6] MeV

Cosmic backgroundrejected by m VETO
(20.2% of n signal)in [1.5-6] MeVFast neutrons❖ 25 ν events/day from neighbor reactors were subtracted❖ Fast neutrons: linearly extrapolate from high energy region and subtract separately frompositron and visible cosmic spectra, CR (fast neutron) = 16 events/day (in 1.5-6 MeVrange)❖ Visible cosmic background (CB) has been directly rejected by VETO,
it is 20.2% of neutrino signal (for top position in [1.5-6 MeV] range)❖ CB of ~1% at Top position due to VETO inefficiency, which was found to be ~5% fromreactor OFF data, was subtracted (41 events/day ).
❖ Additional 16 events/day at low energies observed in reactor off data were subtracted❖ Total background subtracted background is 1.7% for the top detector position. S/B>50!
9Calibration

2500 SiPM gains and X-talks are calibrated every 30-40 min.
All 2550 channels are calibrated every 2 days using cosmic muonsCosmic muonsLinear response

MC smeared12%/√E ⨁ 6%
Module responseto vertical muons(PMT only)

Several calibration sources are used to check the detector response
MC smeared12%/√E ⨁ 4%

MC smeared12%/√E ⨁ 4%

ΔE=0.5%

ΔE=-0.8%

10

Calibrations

Neutron source
248Cm

ΔE=+0.8%
Gd(n,γ)H(n,γ)

12B(µ12C)

Fifrelincascades

Еxpected 29.1 ms

MichelElectrons

12B(n12C)

τ=2.15±0.007µs

Expected 2.13µs

ΔE=-1.2%
SiPM only

τ=29.4±0.6 ms

Еxpected 29.1 ms

τ [µs]❖ Energy scale has been fixed using b-spectrum of 12B, which is similar to positron signal❖ Systematic error on E scale of +/-2% was added due to spread in source responses (<1%)
Hope to reduce this error soon
❖ Energy resolution for calibration sources is still worse than in MC and additional
11smearing of 12%/√E  4% has been added to MC

Positron spectrum of IBD-signal❖ Positron kinetic energy spectra (no annihilation photons) at 3 detector positions❖ ~5000 events/day in detector fiducial volume (78% of full volume)at ‘Top’ position (closest to the reactor).
❖ Background ~1.7% (Top position, E: 1.5-6MeV). Signal/Background >50!

12

Positron spectrum: experiment vs. H-M Model

RENO-Phys.Rev.Lett. 121 (2018)20, 201801❖ In order to reach best agreement with H-M modelin 1-3 MeV region е+ spectrum was scaled by 0.995
and shifted on -25 keV.
The nature of this shift (if it exists!)
is still under investigation.
❖ With such a shift we see a bump in e+ spectrum❖ similar to other experiments.
❖ However, we can not claim its existence yet because of high sensitivity of the shape to13
energy scale and shift.

Positron spectrum dependence on fuel composition is clearly seen

Ratio of e+ spectra 1-4 months
after reactor shutdown and3 months before shutdown

IBD rate dependence on 239Pu fission fraction(dσ/dF239)/σ(F239=0.3) for various Ee+
It agrees with H-M modeland somewhat larger than at DayaBay

Preliminary
Statisticalerrors only

H-M MC

14

Reactor power monitoring❖ DANSS points after all corrections (all backgrounds including
adjacent reactor fluxes (0.6%), fuel composition using H-M model,etc.) and free overall normalization agree with reactor power
measured with several methods❖ Reactor power is measured by the DANSS with neutrino flux with1.5% accuracy in 2 days during 4.5 years,❖ Consistent with statistical fluctuations.
❖Changes in absolute detector efficiency are known with accuracy
better than ~ 1% during 4.5 years!

❖ Relative efficiency is even more stable (<0.2%) because of frequentchanges of detector positions
15

Test statistics

3 position data

2 position data

Nuisance parameters(systematics and efficiency)

χ2(4ν)-χ2(3ν)

k0=1 ƞ0=0
Difference in χ2 between
4ν and 3ν hypotheses
Red - χ2(4ν)<χ2(3ν),
Blue – χ2(4ν)>χ2(3ν),
Dark blue region is excluded at 2σ CL
in case of χ2 distribution with 2 DoF(χ2(4ν)-χ2min)=6.18
This assumption is not valid ➔ we use
Gaussian CLs method to get limits

16

Ratio of positron spectra
Bottom/TopMiddle/√(Top x Bottom)❖ Fit in 1.5-6 MeV range (to be conservative)❖ Using current statistics 2016-2020 (~5 million IBD events)
we see no statistically significant indication of 4ν signal:∆X2=-3.2 (< 1.3s ) for 4ν hypothesis best point ∆m2=1.3 eV2 , sin22θ=0.014❖ RAA has been excluded with ∆X2= 107.

❖ RAA was excluded by DANSS with more than 5s already in 2018
(arXive:1804.04046v1) arXiv:1804.04046v1
17

The DANSS results❖ Exclusion region was calculatedusing Gaussian CLs method (for e+in 1.5-6 MeV to be conservative),❖ σ’s for nuisance parameters**

relative detector efficiencies - 0.2%
additional smearing in energy resolution - 25%energy scale - 2%energy shift - 50 keV
distance to fuel burning profile center - 5 cm
cosmic background - 25%
fast neutron background - 30%❖ New data make limits more smoothin reasonable agreement withsensitivity

PRELIMINARY❖ The most stringent limit reaches
sin22θ < 8x10-3 level.
❖ A very interesting part of 4v
parameters is excluded.
❖ The most probable point ofRAA+GA is excluded at 5σ
confidence level** - G.Mention J.Phys.:Conf.Ser. 408 (2013) 012025

18

The DANSS upgrade
New scintillator strips
Main goal: to reach resolution 13%/√E
18.3
w.r.t. current very modest 34%/√E.
5.3
New geometry:
Strips: 2x5x120 cm, 2-side 4SiPM readout
Structure: 60 layers x 24 strips: 1.7 m3
Setup uses the same shielding and moving
5.1
platform.
WLS fiber positions were optimized for
Gd is in foils between layers.
better uniformity of response
Upgrade will be finished in 2022

Strip tests at p-beamTransverse and longitudinal responses are very uniform
Sum
Left

RightSensitivity
2018

1.5 years of data taking

Longitudinal nonuniformity can be further corrected
More work on SiPM-WLS fiber connection is needed

19

DANSS has measured Pressure and Temperature dependenceof muon flux

Flux dependence on pressure(1hour points)

Flux dependence on effective temperature(1 hour points)

Temperature correlation coefficientagrees well with expectations

Prediction
Positiveterm

Negative term
20

DANSS records about 5 thousand
antineutrino events per day with cosmic
background ~1.7%, S/B>50❑

Summary

5.5 million IBD events were collected in 5 years
Reactor power was measured using anti-ν
rate with statistical error of ~1.5% in two days
during 4.5 years of operation.
❑Relative IBD σ dependence on 239Pu fraction
was measured. It agrees with H-M model❑❑Indication of 5MeV bump, but not conclusive

KNPPДетектор DANSS наPreliminary DANSS analysis based on 5этапеPowerсборки Plant,Kalinin Nuclear
million IBD events excludes a large and theRussia
most interesting fraction of available
We plan:parameter space for sterile neutrino using only
To take data for one more reactor off period
ratio of e+ spectra at 3 distances (with noTo refine detector calibration and energy
dependence on ν spectrum and detector
scale determination in order to reduce
absolute efficiency!)
systematic errors❑

RAA was excluded by DANSS with morethan 5s already in 2018 (arXive:1804.04046v1)

To upgrade detector in 2022

Muon flux dependence on pressure andtemperature was measured

Thank you !

❑❑20Backup slides

22

DANSS has 40% larger F239Pu range: DANSS 0.25-0.39

DB 0.25-0.35

23

Comparison of results❖ Large progress in 90% CL exclusionarea since 2018

❖ Sensitivity plots: relative rate analysis
contributes mostly at low mass region(Phys.Lett. B787 (2018) 56)

24

IBD total rate vs. effective distance❖ IBD intensity follows reasonably the 1 / L2 dependence.
❖ Detector was divided on 3 parts in each position.

25

WFD

PAs

Input
amplifiers

Single pixel
SiPM signal,selftrigger

ADCsFPGAs

Powerand VMEbuffers• Preamplifiers PA in groups of 15 andSiPM power supplies HVDAC for eachgroup inside shielding, current andtemperature sensing• Total 46 Waveform Digitisers WFD in 4
VME crates on the platform• WFD: 64 channels, 125 MHz, 12 bit
dynamic range, signal sum and trigger
generation and distribution (no additionalhardware)• 2 dedicated WFDs for PMTs and μ-vetofor trigger production• Each channel low threshold selftrigger on
SiPM noise for gain calibration• Exceptionally low analog noise ~1/12 p.e.

1bit noiseADCu

PAsHVDAC•

ADCu

Data acquisition systemt, ns

PMT signal~27 MeV,system trigger
High dynamic
range
t, ns

1 pixel2 pixels
3 pix.

4 pixels
22

Sensitivity to fuel evolution
Top – Middle – Bottom datawith and without efficiency and fuel evolution correction

Raw data w/o fuel correction

Fuel-corrected data

27

Comparison of exclusions in 2021 and 202028