SEARCH FOR STERILE NEUTRINOS IN DANSS EXPERIMENT M. SHIRCHENKO, N. - - PowerPoint PPT Presentation

• M. SHIRCHENKO, N. SKROBOVA ON BEHALF OF DANSS COLLABORATION (ITEP, JINR)

SEARCH FOR STERILE NEUTRINOS IN DANSS EXPERIMENT

DANSS

Solid state scintillator aimed to detection of the industrial power reactor antineutrinos

JINR ITEP KNPP

285 km 370 km

• Applied physics
• Measurement of the actual reactor power (Nν)
• Deducing the actual fuel composition (Eν)
• On-line reactor monitoring
• Nonproliferation of nuclear technology (prevent unauthorized extraction of 239Pu)
• Fundamental physics
• Search for short-distance neutrino oscillations

!

"" #$ % = 1 − )*+#(2Θ/))*+#(1.27 345 # 67# %[4]

:;

$<[=67]

)

34>"?

#

= 2.3 67# )*+# 2A>"? = 0.14

MOTIVATION

Kalinin Nuclear Power Plant

• 3 GW WWER-1000 reactor

(core: h=3.7m, d=3.2m)

• ~ 50 m.w.e. shielding (µ flux

reduction by factor of ~6)

• Antineutrino flux ~ 5×1013

" #e/cm2/s at a distance of 11 m from reactor core

DETECTOR SITE & KEY FEATURES

• Close position
• Under the reactor (H!)
• Movability
• Solid (sectioning)

DETECTION METHOD

Inversed beta-decay in plastic scintillator interlayed with Gd

ELEMENTS

LIFT AND SHIELDING

Sensitive volume: 1 m3= 1.1 ton Total mass: 16 tons (with shielding) Composition: 2500 strips combined into 50 modules which are read out by small PMTs Lifting system: allow a distance varies from 10.7 to 12.7 m to the reactor core 3 positions / week @ 4 min

ACTIVE MUON VETO

Recent veto system misses only ~2.5%

• f all µ events detected by DANSS

8 plates 3x50x200 cm3 24 plates 3x50x150 cm3 8 plates 5x25x80 cm3

IBD CRITERIA

• Three independent DAQ systems & 2 hard triggers:
• Trigger 1: pair of pulses, 2µs < ∆" < 80 µs, at least one EPMT > 0.5 MeV for

prompt and delayed pulses both, read 100 QDC channels, 100 waveforms (62.5 MS/s)

• Trigger 2: ΣEPMT > 0.7MeV, read 2590 waveforms (125 MS/s) from 50 PMT,

2500 MPPC and 40 µ-veto detectors, look for correlated pairs offline

• Trigger veto

CALIBRATION SYSTEM

Teflon capillary

5 10 15 20 25 30 35 40 1 10 100

N = 5281 / 30 min X3Z3 & Y3Z3

248Cm

Time (Prompt – Delayed), μs

Prompt - delayed time distribution

CALIBRATION VIA COSMIC MUONS

SLOW CONTROL

• Gamma (inside/outside)
• Neutron (inside/outside)
• Temperature
• Muon rate
• Etc…

BACKGROUND MONITORING

• HPGe and “MuMeter” measurement
• Four NaI (3’x3’): 1 inside + 3 outside the DANSS shield

no (ON – OFF) visible difference

• Three 3He neutron counters: 1 inside +2 outside the shield

CH 2

1000 2000 3000 4000 100 200 300 400 500 600 700 800 900 1000 3He + n g t + p

BG (g)

Φn = 0.57 outside (OFF) Φn = 300.4

• utside (ON)

Φn = 0.03 inside (OFF) Φn = 0.04 inside (ON) Φn = 6.0 plein-air

500 1000 1500 2000 2500 0.01 0.1 1 10 100 1000 10000 6500 7000 7500

40K 60Co 137Cs 56Fe(n,g)

SE DE E, keV Count rate, cnt / keV / hour

A336 @ KNPP Lab @ JINR

45 90 135 180 225 270 315 0 2 4 6 8 101214

Z = 30o Z = 45o

DANSS TIMING

2012: DANSSino (prototype) operation in the 3th KNPP reactor unit 2013: DANSSino (prototype) operation in the 4th KNPP reactor unit 2014: Installation of lifting system in the 4th KNPP reactor unit 2015: DANSS construction, passive shielding and muon veto installation December 2015 – April 2016: DAQ systems debugging, testing measurements April 2016: Start of measurements along with spectrometer movement May 2016 - September 2016: Stop of detector movement during maintenance, improving experimental setup October 2016 – March 2018: Data taking April – May 2018: Detector maintenance May 2018: Data taking

REACTOR POWER VS IBD COUNT

Positioning with respect to the reactor core: 10.7m (Up), 11.7m (Middle), 12.7m (Down) Detector movement: 3 times per week (Down -> Up -> Middle -> Down)

ANTINEUTRINO CR DEPENDENCE ON DISTANCE FROM THE REACTOR

Reasonoble agreement with 1/R2 dependence

DANSS COLLABORATION

SEARCH FOR STERILE NEUTRINOS IN DANSS EXPERIMENT

Energy scale

20 30 40 50 60 70 80 90

N p.e.

µ

1.5 2 2.5 3 3.5 4 4.5 5

Deposited energy, MeV

0.5 − 0.5

Deviation [%]

Cm

Experiment MC

Cosmic muons:

• response is linear with energy, σ is 15% worse than MC
• MC is scaled by corresponding factor and describes the resolution for

radioactive sources well Radioactive sources:

• energy spectrum of the delayed signals measured with the 248Cm neutron

source

• energy scale is in a good agreement with MC
• M. Shirchenko, N. Skrobova | Search for sterile neutrinos at the DANSS experiment

PPNS 2018

Event building and background

s µ Time, 5 10 15 20 25 30 35 40 45 50 Events

3

10

4

10

All events after cuts Accidental background IBD events s µ Cut at 2

Building Pairs

Neutron candidate: 3.5-15 MeV total energy (PMT+SiPM), SiPM multiplicity > 3 Search positron 50 µs backwards from neutron Positron candidate: 1-20 MeV in continuous ionization cluster No other signals in the vicinity of IBD signal

Accidental coincidence background

Fake one of the IBD products by uncorrelated triggers Background events from data: search for a positron candidate where it can not be present — 50 µs intervals far away from neutron candidate (5, 10, 15 etc ms) Enlarge statistics for accidentals by searches in numerous non-overlapping intervals Accidental rate is smaller but comparable to IBD rate Mathematically strict procedure, does not increase statistical error Cuts for the accidental coincidence exactly the same as for physics events Selection of cuts to reduce accidental contribution → smaller statistical error

• M. Shirchenko, N. Skrobova | Search for sterile neutrinos at the DANSS experiment

PPNS 2018

Positron spectrum

Positron energy, MeV 1 2 3 4 5 6 7 8 9 Events / day / 0.25 MeV 50 100 150 200 250 300 350 400

11 / day ± Top: 4910 11 / day ± Middle: 4101 8 / day ± Bottom: 3490

3 detector positions Pure positron kinetic energy (annihilation photons not included) About 5000 neutrino events/day in detector fiducial volume of 78% (‘Up’ position closest to the reactor)

• M. Shirchenko, N. Skrobova | Search for sterile neutrinos at the DANSS experiment

PPNS 2018

Spectrum evolution

1 2 3 4 5 6 7 8 Positron energy, MeV 0.75 0.8 0.85 0.9 0.95 1 Events/(day*0.25 MeV) DANSS data Monte Carlo

Normalized ratio before shutdown / after shutdown

Ratio of positron energy spectra collected during the last 3 months of the reactor campaign in 2017 and during the 2nd − 4th months from the beginning of the next campaign. The average 239Pu fission fractions for these periods are 37.7% and 27.1% correspondingly. The stars are the Huber-Mueller MC model prediction.

• M. Shirchenko, N. Skrobova | Search for sterile neutrinos at the DANSS experiment

PPNS 2018

Comparison of reactor power and DANSS rate

31/1031/1202/0302/0502/0701/0931/1031/1202/03

Date day/month

200 400 600 800 1000 1200 1400 1600 1800

10 ×

Accumulated events

Up Middle Down Total

Statistics accumulation

On power graph: Points at different positions equalized by 1/r 2 Normalization by 12 points in November-December 2016 Adjacent reactor fluxes subtracted (0.6% at Up position) Spectrum dependence on fuel composition is included Statistics @100% power, ∼222 days after QA (oct 16 - oct 17)

• M. Shirchenko, N. Skrobova | Search for sterile neutrinos at the DANSS experiment

PPNS 2018

Search for sterile neutrinos

For every ∆m2 and sin2(2θ) e+ spectrum was calculated for Up and Down detector positions taking into account:

• reactor core size
• reactor burning profile (provided by NPP)
• detector energy response including tails (obtained from cosmic muon calibration

and GEANT4 MC simulation identical to data analysis) Ratio of Down/Up spectra was calculated and compared with experiment (independent on ν spectrum, detector efficiency, and many other problems!)

1 2 3 4 5 6 7 8 9 10

Energy, MeV

500 1000 1500 2000 2500 3000 3500 4000

Arbitrary units Reconstructed energy

+

e

1 2 3 4 5 6 7

Positron energy, MeV

0.64 0.66 0.68 0.7 0.72 0.74 0.76 0.78

spectra ratio (Down/Up)

+

e (NDF = 24) = 35

ν 3 2

χ RAA best point = 0.14 θ 2

2

, sin

2

= 2.3 eV

2

m ∆ = 83

ν 4 2

χ

Most plausible parameter set from Reactor and Galium anomalies is excluded!

• M. Shirchenko, N. Skrobova | Search for sterile neutrinos at the DANSS experiment

PPNS 2018

Preliminary results

Exclusion region was calculated using Gaussian CLs method (X.Qian et al. NIMA, 827, 63 (2016)). CLs method is more conservative than usual Confidence Interval method Systematics studies include variations in: Burning profile in reactor core Energy resolution ±10% Level of cosmics background 0.5% Energy intervals used in fit (1.5-6)MeV Systematics is small A large fraction of allowed parameter region is excluded by preliminary DANSS results using only ratio of e+ spectrum at different L (independent

• n ν spectrum, detector efficiency,. . . )
• DANSS plans to collect more data and

to include into analysis all available data

• Detector calibration and systematics studies will be continued
• M. Shirchenko, N. Skrobova | Search for sterile neutrinos at the DANSS experiment

PPNS 2018

Best point

∆m2 = 1.4 eV2, sin2 2θ = 0.05, χ2

4ν = 21.9, χ2 3ν = 35, ∆χ2 = 13.1 1 2 3 4 5 6 7

Positron energy, MeV

0.64 0.66 0.68 0.7 0.72 0.74 0.76 0.78

spectra ratio (Down/Up)

+

e

Significance will be estimated using Feldman and Cousins method with systematic uncertainties

• M. Shirchenko, N. Skrobova | Search for sterile neutrinos at the DANSS experiment

PPNS 2018

Summary

DANSS records about 5000 antineutrino events per day with cosmic background <3% DANSS counting rate consistent with reactor power within ∼2% During reactor shutdown it is consistent with 0 after subtraction of ∼3% cosmic background and 0.6% flux from adjacent reactors Antineutrino spectrum and counting rate dependence on fuel composition is clearly observed Preliminary DANSS analysis based on 662 thousand IBD events excludes a large and the most interesting fraction of available parameter space for sterile neutrino using only ratio of e+ spectra at two distances (with no dependence on ˜ νe spectrum and detector efficiency!) Significance of the best fit point will be evaluated using more elaborated methods and more statistics We plan to collect more data, to improve MC for perfect description of detector response, to refine detector calibration, to continue systematic studies, to include all available statistics into analysis.

Thank you!

The work is partially supported by the State Corporation «RosAtom» through the state contracts Н.4х.44.90.13.1119 and Н.4х.44.9Б.16.1006 and by Russian Science Foundation, grant 17-12-01145

• M. Shirchenko, N. Skrobova | Search for sterile neutrinos at the DANSS experiment

PPNS 2018