First Result of NEOSNeutrino Experiment for Oscillation at Short baseline

Kim Siyeon

Chung-Ang University

Seoul, Korea

On behalf of NEOS Collaboration

July 15, 2017 ICRC 2017, Busan 1

First Result of NEOS

• NEOS Publication• Y. J. Ko et al., Sterile neutrino search at the NEOS experiment, Phys. Rev. Lett. 118(12),

121802 (2017)

• Y. J. Ko et al., Comparison of fast neutrino rates for the NEOS experiment, J. Korean Phys. Soc. 69(11), 1651 (2016)

• B. R. Kim et al., Pulse shape discrimination capability of metal-loaded organic liquid scintillators for a short-baseline reactor neutrino experiment, Phys. Scripta 90(5), 055302 (2015)

• B. R. Kim et al., Development and mass production of a mixture of LAB- and DIN-based gadolinium loaded liquid scintillator for the NEOS experiment, J. Radioanal. Nucl. Chem. 310(1), 311 (2016)

• More papers on analysis and instruments are in progress.

• Collaborators: 20 in 7 institutes• (1) Joo, K., Kim, B.R., (2) Ko, Y.J., Jang, C.H., Jeon, (3) E. J., Kim, Y., Lee, J.,

Lee, J.Y., Oh, Y., Park, H.K., Park, K.S., (4) Han B., Sun, K.M., (5) Park, H., (6) Kim, H., Lee, J., (7) Kim, H.S., Kim, J., and Seo, K.M.

July 15, 2017 ICRC 2017, Busan 2

NEOS: Motivation and Purpose

• Reactor Anomalies

• Flux deficit

• 5-MeV excess

July 15, 2017 ICRC 2017, Busan 3

RENO

Mention et al, 2011

NEOS: Motivation and Purpose

• Reactor Anomalies

• Flux deficit

• 5-MeV excess

July 15, 2017 ICRC 2017, Busan 4

• To scan the possibility of the existen

ce of a sterile neutrino of the mass

~1 eV

• To analyze the spectral shape

(not the absolute rate) of react

or antineutrino

NEOS baseline 24 m

• Single detector needs

- an accurate reference reactor a

ntineutrino flux/spectrum

- understanding the detector resp

onse is crucial, especially energ

y-charge relation

Experiment Site:

July 15, 2017 ICRC 2017, Busan 5

Habit Nuclear Power Plant in Younggwang, Korea

- 2.8-GWth commercial reactor

- Core size: 3.1-m diameter and 3.8-m height

- Low enriched Uranium fuel (4.6% 235U)

Detector in Tendon Gallery

- ~24-m baseline and ~20-m.w.e overburden

ICRC 2017

1 2 3 4 5 6✦

Sep '15 Nov '15 Jan '16 Mar '16 May '16

DA

Q L

ivetim

e (

%)

100

80

60

40

20

Neutrino runs Test/Calibration

•46 days’ reactor off data + 180 day’s reactor on data + calibration

- stable reactor operation, ~90% DAQ efficiency

Livetime + IBD event rate

July 15, 2017 ICRC 2017, Busan 6

Sep '15 Nov '15 Jan '16 Mar '16 May '16

DA

Q L

ivetim

e (

%)

100

80

60

40

20

Neutrino runs Test/Calibration

•46 days’ reactor off data + 180 day’s reactor on data + calibration

- stable reactor operation, ~90% DAQ efficiency

July 15, 2017 ICRC 2017, Busan 7

Livetime + IBD event rate

Active Reactor Core

• 177 fuel rods, low enriched (4.65%) uranium-235 (LEU) fuel

• Refueling by changing 1/3 of fuel rods for each burn-up cycle (~1.5 year)

• Active core size: 3.1 m (φ), 3.8 m (h)

10000 15000

Burn-up [MTU/MWD]0 5000

Fis

sio

nF

ractio

n

0

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1 U-238

July 15, 2017 ICRC 2017, Busan 8

Mean 23.64

Std Dev 0.8146

26L [m]

22 23 24 25

p.d

.f.

(20 m.w.e) 0

overburden

0.1

0.2

0.3

0.4Mean S

td Dev

23.64

0.8146

Detector Location / Baseline

~8 m

NEOS Detector

Active Core

Tendon Gallery

Entrance / Exit

Neutrino travel distance distribution:

- Beta decays uniformly generated in cyli

ndrical core

- IBD interaction points uniformly distribut

ed in target

- Distance-weighted effect: 1/L2

July 15, 2017 ICRC 2017, Busan 9

July 15, 2017 ICRC 2017, Busan 10

Construction

• Homogeneous LS target

— 1008 L volume

(R 51.5, H 121) cm

— LAB+UG-F (9:1)

— 0.5% Gd loaded for high neutr

on capture efficiency

— 38 8″ PMT in mineral oil buffer

• Shieldings

— 10 cm B-PE (n), 10 cm Pb (γ)

— active muon counter

• Data AcQuisition

— 500 MS/s FADC (waveform)

— 62.5 MS/s SADC (μ veto)

• Source calibration through chi

mney

Linear Alkyl

Benzene

5T PTFE reflectors

Detector Specification

July 15, 2017 ICRC 2017, Busan 11

IBD Reconstruction

Selection rules developed for best S/N ratio:

• Prompt energy: 1-10 MeV

• Delayed energy: 4-10 MeV

• Time coincidence: 1-30 μs

• Multiplicity: no event before (after) 30 (150)

μs from the prompt/delayed pair

• Muon veto: 150 μs

• Pulse shape discrimination

30 40 50 60

Neutron Capture Time [µs]0 10 20

Counts

/day/0

.8µ

s

0

50

100

Reactor ON

Reactor OFF

7 8 9

Delayed Energy [MeV]4 5 6

Counts

/day/0

.1M

eV

0

20

60Reactor ON

Reactor OFF

40

4 5 6 7 8 9

Recontructed Energy [MeV]1 2 3

Co

un

ts/s

ec/1

0ke

V

10−2

10−3

10−4

10−1

1 Target Single Events

All: Reactor OFF

All: Reactor ON

Muon veto cut:Reactor OFF

Muon veto cut:Reactor ON

July 15, 2017 ICRC 2017, Busan 12

S1+S2+background

S2

Pulse Shape Discrimination

• IBD events vs. fast neutron’s scattering events

•Qtail/Qtot or meantime of waveform.

•LAB + DIN (UG-F) mixed LS.

•More than 70 % of IBD background could be reduced after PSD.

5

p ( t )psd

−10 −5 0 10 15 20E

ve

nts

/da

y

10−1

1

10

102 Reactor OFF

Reactor ON

Fast n

e, γ

July 15, 2017 ICRC 2017, Busan 13

Prompt Energy Spectrum

01 2 3 4 5 6

Prompt Energy [MeV]

• Signal / Noise ~ 22

• vs H-M: 5 MeV bump, not so practical to do the oscillation analysis

• vs Daya Bay: bump not totally disappeared, fission fraction difference

• Systematic uncertainties for the shape analysis:

reference spectrum > energy recon. >> background fluc., S2 cut efficiency …

7 ⋅ ⋅ 10

Events

/day/1

00

ke

V

1.0

100.9

Systematic total

20

30

40

50

60Data signal (Sneos=ON-OFF)

Data background (OFF)

MC no oscillation (Φhm⋅σ )Vogel

MC no oscillation (Sdyb)

1 2 3 4 5 6 7 10

Prompt Energy [MeV]

Da

ta/P

red

iction

0.9

1.0

1.1

Sneos / MC(Φhm⋅σ )

Systematic totalVogel

4 5

Prompt Energy [MeV]1 2 3 6 7 10

Data

/Pre

dic

tion

1.1 S / MC(S )dybneos

July 15, 2017 ICRC 2017, Busan 14

• c2 minima are found at

(sin22θ, Δm2)=(0.04, 1.3 eV2),

(0.05, 1.73 eV2)

with Δ c2 = c2 3n− c2 4n = 6.5.

• p-value ~ 22%.

• No strong sign of active-to-sterile

n oscillation.

Prompt Energy [MeV]

1 2

(2.32 eV2, 0.142)

3 4 5 6 7 ⋅ ⋅ 10

Da

ta/P

red

ictio

n

0.9

6000

1.0

4000

(1.73 eV2, 0.050)

1.1NEOS/Daya Bay

Systematic total(c)

July 15, 2017 ICRC 2017, Busan 153ν 4ν

χ2 -χ2

00 2 4 6 8 10 12 14

En

trie

s(a

.u.)

2000

8000

10000

MC pseudo-experiment

no sterile neutrino.

χ2 dist. (NDF=2)

Active-to-Sterile Oscillation

• Exclusion limits

• Raster scan with c2 distribution

• Exclude Δm2 < 3 eV2,

• Rule out models, including RAA (reactor antine

utrino anomaly) and recent global analyses.

July 15, 2017 ICRC 2017, Busan 16

Active-to-Sterile Oscillation

Thank you.

NEOS Collaboration

July 15, 2017 ICRC 2017, Busan 17

Backup

July 15, 2017 ICRC 2017, Busan 18

Non-Linear Charge Response to Energy

• Signal quenched at low energy

• Cerenkov light important at higher energy

Source position

(Φ) target center

(P) PMT

(O) outside

(Ch) calibraion chimney

4 5 6 7 8

True Gamma Energy [MeV]

1 2 3

Cha

rge/E

ne

rgy [

pC

/Me

V]

450

500

550

PoBe(Ch)

137Cs(Φ)

40K(P)

60Co(Φ)

n-H(Φ)

Tl(O)208

PoBe(Ch)

NEOS Preliminary@ Neutrino 2016

July 15, 2017 ICRC 2017, Busan 19

NEOS status•NEOS is over due to regular maintenance of the tendon gallery (once every 5

years)

•No strong evidence of oscillation from active-to-(~1 eV) sterile mixing.

•Tendon gallery is a nice place for reactor neutrino experiment, but

•Severe escaping γ effect with a small volume detecter without γ catcher.

•Measurement of 1 whole burn-up cycle should help to understand the spectrum

in detail.

•Rate analysis, Absolute spectrum,

235U

P1239Pu

238U241Pu

NEOS

P2

21July 15, 2017 ICRC 2017, Busan 20

Dedicated room for neutrino experiments

ground

level

active core

experimental

room (proposed)

NEOS

@ tendon

gallery

Proposed

new room

core diameter 312.4 cm 365 cm

core height 381 cm

thermal power 2815 MW 3983 MW

distance to core 24 m ~18 m *

overburden >20

muon rate ~0.2 cm-2 min-1 *

rectangular space

(WxDxH m3)4 x 2.5 x

3.3

8 x 5 x 4 *

# of IBD events

to be detected (

NEOS detector)

~2000

per day

~5000

per day *

July 15, 2017 ICRC 2017, Busan 21