1. 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)

2. 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

4. 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

5. Many reactor experiments plan to search for sterile neutrino

6. Problems: Large background and small counting rate

7. 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

8. 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 S C)
X-plane
Copper
frames
Y-plane
View of a Module (under construction).

9. 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

10. 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 m
(center to center)
20.3

11. 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

12. 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)

13. 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%

14. 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

15. 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

16. 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!

17. 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

18. Backup slides