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

SEARCH FOR STERILE NEUTRINOS IN DANSS EXPERIMENT M. SHIRCHENKO, N. SKROBOVA ON BEHALF OF DANSS COLLABORATION (ITEP, JINR)

DANSS Solid state scintillator aimed to detection of the industrial power reactor antineutrinos KNPP 285 km JINR 370 km ITEP

MOTIVATION Applied physics • o Measurement of the actual reactor power (N ν ) o Deducing the actual fuel composition (E ν ) o On-line reactor monitoring o Nonproliferation of nuclear technology (prevent unauthorized extraction of 239 Pu) • Fundamental physics o Search for short-distance neutrino oscillations # 67 # %[4] #$ % = 1 − )*+ # (2Θ / ))*+ # (1.27 34 5 ! ) "" : ; $ < [=67] )*+ # 2A >"? = 0.14 # = 2.3 67 # 34 >"?

DETECTOR SITE & KEY FEATURES 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 × 10 13 # e /cm 2 /s at a distance of 11 m " from reactor core 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 1 m 3 = 1.1 ton volume: 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 8 plates 3x50x200 cm 3 24 plates 3x50x150 cm 3 8 plates 5x25x80 cm 3 Recent veto system misses only ~2.5% of all µ events detected by DANSS

IBD CRITERIA • Three independent DAQ systems & 2 hard triggers: o Trigger 1: pair of pulses, 2µs < ∆" < 80 µs, at least one E PMT > 0.5 MeV for prompt and delayed pulses both, read 100 QDC channels, 100 waveforms (62.5 MS/s) o Trigger 2: ΣE PMT > 0.7MeV, read 2590 waveforms (125 MS/s) from 50 PMT, 2500 MPPC and 40 µ-veto detectors, look for correlated pairs offline o Trigger veto

CALIBRATION SYSTEM Teflon capillary Prompt - delayed time distribution 100 248 Cm 10 N = 5281 / 30 min X3Z3 & Y3Z3 1 0 5 10 15 20 25 30 35 40 Time (Prompt – Delayed), μs

CALIBRATION VIA COSMIC MUONS

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

BACKGROUND MONITORING Count rate, cnt / keV / hour 0 10000 60 Co 137 Cs 40 K 1000 45 315 DE SE 56 Fe ( n , g ) 100 0 2 4 6 8 101214 10 A336 @ KNPP 1 Lab @ JINR 90 270 0.1 Z = 45 o 0.01 E, keV 0 500 1000 1500 2000 2500 6500 7000 7500 135 Z = 30 o 225 HPGe and “MuMeter” measurement 180 • Four NaI (3’x3’): 1 inside + 3 outside the DANSS shield • no (ON – OFF) visible difference Three 3 He neutron counters: 1 inside +2 outside the shield • 4000 Φn = 0.57 outside (OFF) Φn = 300.4 outside (ON) 3000 Φn = 0.03 inside (OFF) 3 He + n g t + p 2000 Φn = 0.04 inside (ON) BG ( g ) Φn = 1000 6.0 plein-air 0 0 100 200 300 400 500 600 700 800 900 1000 CH 2

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/R 2 dependence

SEARCH FOR STERILE NEUTRINOS IN DANSS EXPERIMENT DANSS COLLABORATION

Energy scale 90 N p.e. µ Cm 80 Experiment MC 70 60 50 40 30 20 Deviation [%] 1.5 2 2.5 3 3.5 4 4.5 5 0.5 0 − 0.5 1.5 2 2.5 3 3.5 4 4.5 5 Deposited energy, MeV 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 248 Cm 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 Events Building Pairs 4 10 Neutron candidate: 3.5-15 MeV total energy (PMT+SiPM), SiPM multiplicity > 3 Search positron 50 µ s backwards from neutron All events after cuts Accidental background Positron candidate: 1-20 MeV in continuous IBD events µ Cut at 2 s ionization cluster 3 10 0 5 10 15 20 25 30 35 40 45 50 µ Time, s 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 µ s 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 Events / day / 0.25 MeV 400 ± Top: 4910 11 / day ± Middle: 4101 11 / day 350 ± Bottom: 3490 8 / day 300 250 200 150 100 50 0 1 2 3 4 5 6 7 8 9 Positron energy, MeV 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 Normalized ratio before shutdown / after shutdown 1 Events/(day*0.25 MeV) 0.95 0.9 0.85 DANSS data Monte Carlo 0.8 0.75 1 2 3 4 5 6 7 8 Positron energy, MeV Ratio of positron energy spectra collected during the last 3 months of the reactor campaign in 2017 and during the 2 nd − 4 th months from the beginning of the next campaign. The average 239 Pu 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 On power graph: Statistics accumulation × 3 10 Accumulated events Points at different positions equalized by 1 / r 2 1800 1600 Up Normalization 1400 Middle Down by 12 points in November-December 2016 1200 Total 1000 Adjacent reactor 800 fluxes subtracted (0.6% at Up position) 600 400 Spectrum 200 dependence on fuel composition is included 0 31/1031/1202/0302/0502/0701/0931/1031/1202/03 Date day/month 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 ∆ m 2 and sin 2 (2 θ ) 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!) Arbitrary units spectra ratio (Down/Up) 0.78 4000 RAA best point ∆ 2 2 2 θ m = 2.3 eV , sin 2 = 0.14 0.76 3500 χ 2 = 83 ν 4 3000 0.74 Reconstructed + e energy 2500 0.72 2000 0.7 1500 0.68 χ 2 = 35 + ν 3 e 1000 0.66 (NDF = 24) 500 0.64 0 0 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 Energy, MeV Positron energy, MeV 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 CL s method (X.Qian et al. NIMA, 827, 63 (2016)). CL s 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 on 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