Determination of mass hierarchy
with reactor neutrino experiment
Yoshitaro Takaesu
KIAS/KNRC
In collaboration with S.F. Ge, N. Okamura and K. Hagiwara
arXiv: 1210.8141

Introduction
is observed.

Reactor Experiments
can determine the Neutrino Mass Hierarchy .

The sensitivity is studiedof future Medium Baseline Reactor Experiments
for determining the Neutrino Mass Hierarchy

Reactor neutrino experiment
Reactor

Detector

Inside Detector
Inverse Beta Decay (IBD)PMTEnergy Distribution@ Detector

nearfarHow to distinguish Mass Hierarchy ?

Mass Hierarchy difference
Max:vanish:MH difference inspectrum

Advantages of Reactor Experiment• Can determine MH independently from CP phase and mattereffects.
•Need only smaller detector than other experiments (e.g., LBL,atmospheric).

•Free neutrino source with adjustable baseline lengthAnalysis methods for MH
determination• Fourier analysis
hep-ph/0612022 J.G. Learned et al.
arXiv: 0807.3203 L. Zhan et al.
arXiv: 0901.2976 L. Zhan et al.
arXiv: 1208.1551 X. Qian et al.
arXiv: 1208.1991 E. Ciuffoli et al.

•analysis
hep-ph/030601 S. Choubey et.al.
arXiv: 0810.2580 M. Batygov et.al.
arXiv: 1011.1646 P. Ghoshal et.al.

Fourier Analysis• has sensitivity for small• Don’t need accurate knowledge of.
orflux.

analysis

Penalty term

Data is fitted with the theoretical prediction
assuming NH or IH.

Fitting parameters are,

analysis – MH determination
MH is determined as the hierarchy which gives smallerEx)

MH is NH.

Sensitivity is estimated byWe claim that MH is determined withsignificane.

acc. MHrej. MH.

Sensitivity for MH determination
Typical sensitivity curve as a function of Baseline length L

Max Sensitivity
36 km (Max MH diff.)Effect ofL = 30km

L = 50km

Baseline Length L should be long enough.

Pull Factor

Effect of Energy Resolution
Evergy Resolution
affects the sensitivity
significantly.

a=0a = 6%

How good should the energy resolution be ?
Not enough for RENO50 class of detector (5kton).
3% ?

Sensitivity for MH determination
20GW 5kton 5yrs
Current Resolution

NHOptimal L ~ 50 km

Systematic part of Resolution
b=0
Current Resolution

20GW 5kton 5yrs

b = 1%• Sensitivity is reduced by 40%.
• Optimal L is shortenedby a few km.

a = 2%

a = 3%

Parameter measurement @ L ~ 50 km
1%

L = 50km

1%

1%

can be measured accurately @ L ~ 50 km~ 0.5% accuracy
Parameter measurement is not sensitive to the Energy Resolution.

Fluctuation of Sensitivity
So far, we have discussed on the sensitivity
using the “typical data set”.
In the real experiments, event number in each bin,fluctuates from the “typical event number”,.
20GW 5kton 5yrsThen,followsGaussian distribution.

a = 3%

2% level of energy resolution is desired.

a = 2%0413,

Expected Probability

L = 50 km

How to achieve 2% energy resolution ?
Increase the photo-electron (p.e.)detected by PMT.
~ 200 pe/MeV~ 2000 pe/MeVEx)• Increase PMT coverage• Increase QE of PMT• More transparent LSp.e.

By Channgen Yang @ NuMass 2013

Multi-reactor interference
Baseline difference should be < 500m.

L1

L2|L1 – L2|0m
500 m

1 km
1.5 km2 km
L1
arXiv: 1302.0624

Multi-reactor interference
Because of this effect, DayaBayII has changed its location.

Multi-reactor interferenceRENO
1.3 kmReactor site

RENO

RENO50200 km

50 km

The angle to RENO50 may
affect the sensitivity.

Summary
We have discussed the sensitivity ofMedium Baseline Reactor Experiments
for MH determination.

For 20GW 5kton 5yrs exposure,
50km optimal Baseline Lengtha < 3% of Energy Resolution is required.
b < 1%

0.5% accuracy is achieved for