Searches for Sterile neutrinos Charm & Beauty in Physics Lebedev Physical Institute RAS, Moscow, November 10 – 11, 2016 Searches for Sterile neutrinos M.Danilov (LPI, MIPT, NRNU MEPhI)

There are several ~3σ indications of 4th neutrino LSND, MiniBoone: νe appearance SAGE and GALEX νe deficit Reactor νe deficit Indication of a sterile neutrino Δm2 ~ 1 eV2 Sin22θ14 ~ 0.17 => Short range neutrino oscillations

Reactor models do not describe well neutrino spectrum Measurements at one distance are not sufficient! Best strategy – measurements with the same detector at different distances since many systematic effects are cancelled out

Many reactor experiments plan to search for sterile neutrino

Problems: Large background and small counting rate

DANSS advantages DANSS disadvantages Handling is much safer (non-flammable, non-caustic) no restrictions to move the detector very close to the reactor core higher neutrino flux => better sensitivity. High segmentation (2500 strips) => space information better IBD signature => stronger BG suppression. possibility of continuouse calibration with cosmics for every strip Dual readout with SiPM and usual PMT => better control of systematic Detector on movable platform under reactor => better control of systematic low cosmic background DANSS disadvantages Worse energy resolution in comparison with liquid scintillator Large reactor core => smearing of the oscillation pattern Relatively large number of readout channels

View of a Module (under construction). DANSS Design glue WLS- -fiber ○1.2 mm Each scintillator strip is read out individually by a Silicon Photo Multiplier (SiPM) via a WLS fiber. Sensitivity is ~20 p.e./MeV Light attenuation ~20%/m 50 strips are combined into a Module which is also read out by a small PMT (via 2 additional WLS fibers per strip). The frame of a Module is made of radio-pure electrolytic copper and thus shields the sensitive part against insufficiently pure components of front-end electronics placed outside the frame. Gd-containing light-reflecting coating~6mg/cm2 PS SiPMs (MPPC S12825-050C) X-plane Copper frames Y-plane View of a Module (under construction).

Transverse nonuniformity Strip properties Strip response to cosmics during production at different distances to SiPM Strip Near end Transverse nonuniformity Center Far end Drop near strip edges is partially due to poor trigger

DANSS at Kalinin Nuclear Power Plant DANSS is installed on a movable platform under 3GW WWER-1000 reactor (Core:h=3.5m, =3.1m) at Kalinin NPP (~50 mwe shielding => μ flux reduction ~6!) Detector distance from reactor core 10.7-12.7m (center to center) 20.3

Calibration Calibration is extremely important for stable energy scale Shifts in energy scale can imitate oscillations and reduce sensitivity Elaborated system of calibration is foreseen in DANSS 1. Several radioactive sources can be put into 10 special tubes inside every section of 100x100x20 cm 248Cm neutron source provides signals similar to IBD 2. Gd signal will be monitored with 5000 events/day 3. High granularity of DANSS allows cosmic calibration with ~40000 events/strip/day. Cosmics spans E range 2-5 MeV for individual strips with SiPM readout and 10-40MeV for modules of 50 strips read out by one PMT 4. SiPM amplification will be calibrated continuously

Calibration With cosmic muons Response is linear with energy With radioactive sources. 248Cm n source is similar to IBD process IBD process Gd(n,γ) H(n,γ)

Reactor data after accidental and cosmic background subtraction (7 day exposure) (15 day exposure) (5 day exposure) Time between prompt and delayed signals About 5000 neutrino events/day with ~5% cosmic background in detector fiducial volume of 78%

First results First results Ratios of positron cluster energy Ratios of positron cluster energy are shown for 27 days of data taking Many systematic effects including uncertainties in neutrino spectrum and detector efficiency are canceled in the ratio Expected oscillation curves are shown for the most plausible point: ΔM2=2eV2, Sin2(2*θ)=0.17 “Realistic” burning profile is assumed Energy resolution is assumed to be dominated by photoelectron statistic Systematic effects should be studied before making quantitative conclusions on mixing parameter Ratios of positron cluster energy at different distances

Sensitivity estimates (1 year, 95% CL, only shape distortions with L) Systematic effects estimated by changing E scale by 1% and by adding 1% Background (~E-2) at one distance from the core Δm2 With systematic Without systematic sin22θ Most interesting parameter space is well covered Use of e+ spectrum and/or rate information increase sensitivity considerably

In October NEOS Collaboration presented results based on measurements at one distance (24m). Many critical questions about their analysis … Best fit prefers neutrino with dm=0.16eV2 but they give upper limits Paper was withdrawn Measurements at different distances are crucial!

Summary Searches for sterile neutrinos is a very active field No conclusive results so far Combination of different experiments is problematic (systematics) Measurements at different distances are crucial DANSS started data taking in April Neutrino rate is ~5000 events/day with ~5% cosmic background. We had a technical stop to repair a water leak. During this stop shielding was improved. Accidental and cosmic background will be reduced by better shielding. We resumed data taking in October DANSS is sensitive to the most interesting mixing parameter space using only ratios of positron spectrum at different distances Use of e+ spectrum and/or rate information can further increase sensitivity

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