This document discusses the sensitivity of future reactor neutrino experiments to determine the neutrino mass hierarchy. It finds that to achieve a greater than 3-sigma determination within 5 years, an energy resolution of less than 3% for the alpha term and less than 0.5% for the beta term is required. Interference among reactor cores can significantly impact the sensitivity. The experiment would also need an 18 kiloton detector located 50 km from a 16.5 gigawatt reactor complex. With such parameters, neutrino oscillation parameters could be measured to less than 1% accuracy.

1. Yoshitaro Takaesu
KIAS/KNRC
The Sensitivity to Mass Hierarchy
determination of Future Reactor
Neutrino experiments
In collaborationwith S.F. Ge, N. Okamura and K. Hagiwara
JHEP 1305:131,2013

2. Introduction
2

3. Mass Hierarchy
3
NH
IH

4. Reactor neutrino experiment
Detector
Reactors
4
@ Detector

5. Reactor neutrino experiment
Detector
Reactors
5
PMT
Inside Detector (Inverse Beta Decay)

6. MH difference in spectrum
6
NH
IH

7. Sensitivity
7

8. Analysis method
Data
8
MH = NH/IH
Fitting function Parameter constraints

9. Analysis method
We define
Naively, the significance of this
rejection is , but…
NH
IH
Not appropreate
for the MH determination
c.f.) Qian et.al. arXiv: 1210.3651
Evslin et.al. arXiv: 1305.5150
NH is true: tends to be Positive.
IH is true: tends to be Negative.
IH is rejected
NH is rejected
NH is true case
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10. Sensitivity to the Mass hierarchy
The typical sensitivity curve of this experiment
Small flux
10
Vvvv vvv

11. Effects of to the sensitivity
L = 30km
L = 50km
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0%
2.5%
-2.5%
10 20 30 40 50 60 70 80 90km

12. 12

13. Pull Factor
13

14. Sensitivity to the Mass hierarchy
The typical sensitivity curve of this experiment
14

15. Effect of Energy resolution
15

16. Effect of Energy resolution 1
18kton 16.5GW 5yrs
Current value
NH
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17. Effect of Energy Resolution 2
18kton 16.5GW 5yrs
a = 3%a = 2%
b = 0 b = 1% Sensitivity 40%
Imrovementof b term is also important
17
Current value

18. How to achieve better Energy Resolution?
Increase the photo-electron (p.e.)detected by PMT.
p.e.~ 200 pe/MeV ~ 2000 pe/MeV
Ex)
• Increase PMT coverage
• Increase QE of PMT
• More transparent LS
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~ 6% ~ 2%

19. 25m 35%
Energy Resolution (simulation)
J.S. Park (RENO)

20. Interference Effect
20

21. E. Ciuffoli, J. Evslin, X. Zhang: 1302.0624
Y-F. Li, J. Cao, Y. Wang, L. Zhan: 1303.6733
ΔL = L1 – L2
Reactor1
Multi-reactor interference
L1 L2
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Reactor1 + Reactor2

22. multi-cores
Multi-reactor interference
16.5GW
18kton 5yrs
a = 3%, b = 0.5%
All Reactors
50 km
22
250m

23. Parameter Measurement
23

24. Parameter measurement
< 0.5% accuracy
can be measured accurately @ L ~ 50 km
L = 50km
1%
1%
1%
1%
* Effect of energy scale unc. Is not considered.

25. Summary
Future Reactor neutrino Experiment for MH determination
For > 3-sigma determination within 5 years
a < 3%
b < 0.5%
of Energy Resolution is required
with 16.5GW 18kton Reactor Complex.
Interference among reactor cores significantly affects
the sensitivity.
•Energy Scale uncertainty should be controlled very carefully.
arXiv: 0901.2976, 1208.1551, 1308.0591
* Neutrino parameters can be determined with < 1% accuracy.
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