The 21st Particle & Nuclei International Conference 1-5 September 2017, IHEP, Beijing, China China National Convention Center Measurements of the reactor antineutrino with solid state scintillation detector Igor Alekseev* for the DANSS collaboration: ITEP(Moscow) + JINR(Dubna) *ITEP (Alikhanov Institute for Theoretical and Experimental Physics)

There are several indications in favor of existence of the 4th neutrino flavor - “sterile” neutrino seen in oscillations Expected parameters (G. Mention et al. Phys. Rev D83 073006 (2011): DANSS: Measure ratio of neutrino spectra at different distance from the reactor core – both spectra are measured in the same experiment with the same detector. No dependence on the theory, absolute detector efficiency or other experiments. Naïve ratio without smearing by reactor and detector sizes and the resolution Igor Alekseev (ITEP)

e+ (n,) Inverse Beta-Decay (IBD) Ee ≈ Eν – 1806 MeV Continuous ionization cluster Fast (prompt) signal Ee ≈ Eν – 1806 MeV Delayed signal Gamma flush in the whole detector e+ (n,) T ~ tens us Prompt Delayed Neutron thermalization and capture Igor Alekseev (ITEP)

Detector design 10 layers = 20 cm X-Module 1 layer = 5 strips = 20 cm Y-Module PMT 100 fibers Scintillation strips 10x40x100 mm3 with Gd-dopped coating Double PMT (groups of 50) and SiPM (individual) readout Strips along X and Y – 3D-picture 2500 strips = 1 m3 of sensitive volume Multilayer closed passive shielding: electrolytic copper frame ~5 cm, borated polyethylene 8 cm, lead 5 cm, borated polyethylene 8 cm 2-layer active μ-veto on 5 sides Igor Alekseev (ITEP)

KNPP - Kalinin Nuclear Power Plant, Russia WWER1000 reactor KNPP - Kalinin Nuclear Power Plant, Russia Below 3.1 GW commercial reactor DANSS on a lifting platform A week cycle of up/middle/down position No flammable or dangerous materials – can be put just after reactor shielding Reactor fuel and body with cooling pond and other reservoirs provide overburden ~50 m w.e. for cosmic background suppression Lifting system allows to change the distance between the centers of the detector and of the reactor core from 10.7 to 12.7 m on-line The top position corresponds to ~15000 IBD events per day for 100% efficiency Cosmic muon suppression measured Igor Alekseev (ITEP)

Specially designed DAQ system PAs HVDAC WFD Input amplifiers ADCs FPGAs Power and VME buffers Single pixel SiPM signal Selftrigger Exceptionally low electronic noise Preamplifiers PA in groups of 15 and SiPM power supplies HVDAC for each group inside shielding, current and temperature sensing STP cables to get through the shielding Total 46 Waveform Digitisers WFD in 4 VME crates on the platform WFD: 64 channels, 125 MHz, 12 bit dynamic range, signal sum, trigger generation and distribution (no additional hardware) 2 dedicated WFDs for PMTs and μ-veto System trigger on certain energy deposit in the whole detector (PMT based) or μ-veto signal Each channel selftrigger on SiPM noise (with decimation) 1 pixel 2 pixels 4 pixels 3 pix. SiPM noise spectrum Igor Alekseev (ITEP)

The crew on board Igor Alekseev (ITEP)

Gd(n,γ) H(n,γ) 248Cm fission – neutron source 22Na – gamma + positron 60Co – 2 x gamma Gd(n,γ) Σ=2.297 MeV Σ=2.506 MeV H(n,γ) µ-data: calibration SiPM linearity and uniformity Average 18.25 p.e./MeV Igor Alekseev (ITEP)

Building Pairs Trigger = digital sum of PMT > 0.7 MeV Total trigger rate ≈ 1 kHz Veto rate ≈ 400 Hz True muon rate ≈ 180 Hz Positron candidate rate ≈ 170 Hz Neutron candidate rate ≈ 30 Hz IBD rate ~ 0.1 Hz IBD event = two time separated triggers: Positron track and annihilation Neutron capture by gadolinium SiPM noise cut: Time window ± 15 ns Single pixel hits require PMT confirmation Building Pairs Positron candidate: 1-20 MeV in continuous ionization cluster Neutron candidate: 3-15 MeV total energy (PMT+SiPM), SiPM multiplicity >3 Search positron 50 µs backwards from neutron Significant background by uncorrelated triggers. Subtract background events: search for a positron candidate where it can not be present – 50 μs intervals 5, 10, 15 ms etc. away from neutron candidate. Use 16 non-overlapping intervals to reduce statistical error. All physics distributions = events - accidental events/16 After subtraction Before subtraction Accidental Background Time between positron and neutron: 2 – 50 µs Thermalization Capture Igor Alekseev (ITEP)

‘Muon’ cut : NO VETO 60 μs before positron Muon Cuts VETO ‘OR’: 2 hits in veto counters veto energy >4MeV energy in strips >20 MeV Two distinct components of muon induced paired events with different spectra: ‘Instantaneous’ – fast neutron ‘Delayed’ – two neutrons from excited nucleus ‘Muon’ cut : NO VETO 60 μs before positron ‘Isolation’ cut : NO any triggers 45 μs before and 80 μs after positron (except neutron) ‘Showering’ cut : NO VETO with energy in strips > 300 MeV for 200 μs before positron ‘Muon’ cut: t > 60 µs Instantaneous component Delayed component τ≈10 µs Igor Alekseev (ITEP)

Positron cluster position: 4 cm from all edges Distance between positron cluster and neutron capture center, 3D case : < 55 cm Positron cluster coordinate X-projection : > 4 cm from edges Comparison of the distributions for the events which passed the muon cut with similar for those accompanied by muons Positron cluster position: 4 cm from all edges Vertical projection of the distance: <40 cm Multiplicity beyond positron cluster: <11 Totally 8 cuts of this kind Reject cosmic background >3 times, but only 15% of the events Prompt energy beyond positron cluster : < 1.8 MeV Igor Alekseev (ITEP)

Reactor OFF Background Spectrum and Fit of Cosmic Fraction 5.5/day 28/day * VT ≤ 1.5/day @1-7 MeV Fast neutron tails: linearly extrapolate from high energy region and subtract separately from positron and visible cosmic spectra Subtract fraction of visible cosmics based on VETO transparency Amount of visible cosmics ~50% of neutrino signal VETO transparency uncertainty: 2.5% from muon count in sensitive volume, missed by veto -- underestimate 5.5% from ‘reactor OFF’ spectra – poor statistics Cosmic background fraction ≤3% of neutrino signal, subtracted 9Li and 8He background estimates: < 2.7/day @90% CL Igor Alekseev (ITEP)

Detector operation Points at different positions equalized by 1/r2 Oct. ‘16 May ‘17 Dec. ‘16 | Jan. ‘17 Normalization by 12 points Low power data in good agreement Points at different positions equalized by 1/r2 Normalization by 12 points in November-December Adjacent reactor flux subtracted (0.45% of full power) Statistics @100% power, ~170 days: 7.8∙105 total 7.0 ∙105 in definite positions 6.75 ∙105 after QA Full detector operation Detector comissioning Summer shutdown for cooling repair 2016 2017 Igor Alekseev (ITEP)

Positron spectrum 3 detector positions cycled weekly 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) Down/Up spectrum ratio does not contradict to straight line with current statistics Igor Alekseev (ITEP)

!!! Preliminary !!! Positron spectrum - compare to the theory MC simulated DANSS response use theoretical antineutrino spectrum by Huber and Mueller Very preliminary – more work on calibration and simulations needed and planned !!! Preliminary !!! Igor Alekseev (ITEP)

Probing for sterile neutrino Gaussian CLs method (X. Qian et al. arXiv: 1407.5052 [hep-ex]) – a conservative estimate Theoretical curves for each Δm2 and sin2(2θ) calculated based on: Model neutrino spectrum from Huber and Mueller Fuel burning profile from NPP Detector size Detector response including tails Systematics studies include variations in: Reactor burning profile Detector energy resolution (+20%/√E) Levels of cosmic backgrounds (veto and fast neutrons) Energy intervals used in fit Combinations of the above The most conservative variant is used Only ratio of positron spectra at different R is used → independent of neutrino spectrum, detector efficiency and so on. Igor Alekseev (ITEP)

NEOS: Phys. Rev. Lett. 118, 121802 (2017) DANSS data already excludes a large fraction of allowed parameter region 3ν: χ2=28 / NDF=27 Igor Alekseev (ITEP)

More results are coming ! DANSS recorded the first data in April 2016 and now takes statistics at full speed of about 5000 antineutrino events per day in the closest position. Fine detector segmentation reduce residual background to less than 1% level. We already have 2 more months of data collected this summer, awaiting the analysis. Plan to double statistics by the end of the year. Preliminary analysis of the portion of data collected till May 2017 already excludes a large and most interesting fraction of available sterile neutrino parameter space, using only the ratio of positron spectrum at two distances (independent on neutrino spectrum and detector efficiency) We took data during 40 days scheduled reactor shutdown this summer and hope for even better background estimates In addition to collection of more data we plan: Improve MC for perfect reflection of detector energy response Refine detector calibration Continue systematic studies Elaborate more analysis methods for better sensitivity More results are coming ! 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 Igor Alekseev (ITEP)

Thank you ! Igor Alekseev (ITEP)

Reactor Reactor vertical burning profile for 100% power during the campaign Fuel contribution during the campaign Reactor center Begin Middle End 235U 69% 58% 47% 238U 7% 239Pu 21% 30% 39% 241Pu 3% 5% Igor Alekseev (ITEP)

Oct 2015 Section #4 Section #1

Front-end electronics Nov 2015 1 of 50 PMTs Front-end electronics

Average number of neutrons from Cm fission = 3.2 Igor Alekseev, ITEP

9Li and 8He background compatible with zero 90% limit: 1.64 * 0.129 * 257.2 / 20 = 2.7 events/day Eshower > 800 MeV 9Li lifetime 257.2 ms Igor Alekseev, ITEP

Accidental background subtraction Before subtraction Fiducial volume – cut 4 cm on each side of the detector for positron coordinate. We took the whole detector for neutron capture flush. Distance between positron and neutron, 2D case: <45 cm Accidental Background After subtraction Distance, cm 92 cm 92 cm Igor Alekseev (ITEP)

Very preliminary Partial statistics Agree with 1/R2 MIDDLE DOWN UP The fiducial volume divided vertically into 3 parts Partial statistics Agree with 1/R2 Igor Alekseev (ITEP)

NEOS uses normalization on Daya Bay – systematic error ? arXiv:1610.05134 [hep-ex] Comparison of our preliminary results to NEOS, Bugey, Daya Bay █ DANSS preliminary 90% CLs ― NEOS 90% CL ― Bugey-3 90% CL ― Daya Bay 90% CLs NEOS uses normalization on Daya Bay – systematic error ? Igor Alekseev (ITEP)

Igor Alekseev (ITEP)