ICRR, Univ. of Tokyo

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Takaaki Kajita

ICRR, Univ. of Tokyo

National Taiwan University Dec 22, 2015

1 Atmospheric Neutrino Oscillations

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Outline

• Introduction: Kamiokande - the starting point of my research -

• Discovery of neutrino oscillations: History

• Discovery of neutrino oscillations

• Recent results

• Future

– Appendix

• Summary

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Introduction:

Kamiokande

- the starting point of my research –

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Proton decay experiments (1980’s)

Grand Unified Theories (in the 1970’s)  t _p =10 ^30±2 years

NUSEX (130ton)

Frejus (700ton) Kamiokande

(1000ton) IMB

(3300ton)

These experiments

observed many contained

atmospheric neutrino

events (background for

proton decay).

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Kamiokande

1983 (Kamiokande construction)

electronics

Water system

3kton water Cherenkov detector

(fiducial mass ~ 1kton)

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Kamiokande construction team (Spring 1983)

Y. Totsuka M. Koshiba

(2002 Nobel prize in physics)

T. Kifune M. Takita

M. Nakahata TK

K. Arisaka

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Discovery of neutrino oscillations: History

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Thesis (1986)

I got PhD in March 1986 based on a search for proton decay.

Of course, I did not find any evidence for proton decay…

I felt that the analysis software, including the particle identification

(electron-like or muon-like, PID) for the multi Cherenkov-ring events, was not good enough to extract all the

information that Kamiokande recorded.

Therefore, as soon as I submitted my thesis, I started a work (i.e., my

personal project) to improve the

software.

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Particle Identification

One of them was a new particle identification (PID) software for multi Cherenkov- ring events. Namely, I

designed that the PID can

identify if a Cherenkov ring of a multi Cherenkov-ring event is a non-showering (muon- like) or showering (electron- like) whenever possible.

The simplest application of the PID was on single

Cherenkov-ring events….

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Particle identification (PID): electron or muon ?

muon-like event

electron-like

event Kamiokande

e: electromagnetic shower, multiple Coulomb scattering

m: propagate almost straightly, loose energy by ionization loss

Particle types are identified using the difference

in the event pattern (maximum likelihood method)

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11

• The PID was applied to the atmospheric neutrino Monte Carlo simulation events. It worked well for them.

• Then, the new PID was applied to the real atmospheric neutrino events.

• The result was strange. The number of m -like events was much fewer than expected.

• At first, I thought that our Monte Carlo simulation might be too simple and the “Monte Carlo detector” simulation did not reproduce the “real detector”.

• I wanted to identify what is different between the real events and the Monte Carlo simulation. I decided to scan the real events.

• Immediately, I found that the PID results for the data were correct! (I had a strong confidence with my eye, since I already scanned many, many Monte Carlo and data events, since the beginning of the

Kamiokande experiments in 1983.)

• Something might be happening in neutrinos. However, I thought that it is much more likely that I made some mistake somewhere in the Monte Carlo simulation, data reduction, and/or event reconstruction….

• We started various studies in late 1986.

Atmospheric Neutrino Oscillations

A strange result…

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Atmosphere

Production of atmospheric neutrinos

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n _m over n _e ratio of the beam

( n _m + n _m )/( n _e + n _e )

n

_m

/n

_e

ratio is calculated to an accuracy of about 2% below ～5GeV.

HKKM11

M. Honda et al., PRD 83, 123001 (2011)

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First result on the m /e ratio (1988)

Kamiokande

Data Prediction e-like

(～CC n _e )

93 88.5 m-like

(～CC n _m )

85 144.0

Paper conclusion: “We are unable to explain the data as the result of

systematic detector effects or

uncertainties in the atmospheric neutrino fluxes. Some as-yet-unaccoundted-for

physics such as neutrino oscillations might explain the data.”

K. Hirata et al (Kamiokande）Phys.Lett.B 205 (1988) 416.

After more than 1 year of studies, we concluded that the muon deficit

cannot be due to any major problem in the data analysis nor in the

Monte Carlo simulation.

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Neutrino oscillations

If neutrinos have masses, neutrinos change their flavor (type) from one flavor (type) to the other. For example, oscillations could occur between n _m and n _t .

Probability:

n _m to remain n _m

Probability:

n _m to n _t

Wikipedia

If neutrino mass is smaller, the oscillation length (L/E) gets longer.

L is the neutrino flight length (km),

E is the neutrino energy (GeV).

Maki,

Nakagawa, Sakata

Pontecorvo

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Results from IMB on small m /e

IMB experiment, which was another large water Ch. detector also reported smaller ( m /e) in 1991 and 1992.

D. Casper et al., PRL 66 (1991) 2561.

R. Becker-Szendy, PRD 46 (1992) 3720.

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After the first result on the m /e ratio …

• Although it was clear that the small m/e ratio implied something unexpected, the physics behind this result was unknown. (We recognized that neutrino oscillation was a possibility as we wrote in the paper.)

– Was the result due to neutrino oscillations?

– If so, n _m  n _e or n _m  n _t ? – Some other physics?

17

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Cosmic ray

Detector

n _m n _t

oscillation

Atmosphere

Down-going

Up-going

What will happen if the moun deficit is due to neutrino oscillations

One should observe a deficit of upward going n _m ’s (=muons) !

Detect down-going

and up-going n

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Angular correlation

(CC n _e events)

(CC n _m events)

Lepton momentum (MeV/c)

n

lepton Nucleon

(M _N =

1GeV/c ² )

q

Events with their energy larger than ~1GeV need to be observed to study the zenith angle dependence

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cos q _zenith

Some features of the beam (2)

Up-going Down

Up/down flux ratio is very close to 1.0 and accurately calculated (1% or

better) above a few GeV.

＠Kamioka (Japan)

Zenith angle

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After the first result on the m /e ratio …

• Although it was clear that the small m/e ratio implied something unexpected, the physics behind this result was unknown. (We

recognized that neutrino oscillation was a possibility as we wrote in the paper.)

– Was the result due to neutrino oscillations?

– If so, n _m  n _e or n _m  n _t ? – Some other physics?

• We thought that we should study multi-GeV neutrino events.

• Therefore we started the data reduction work for partially-

contained multi-GeV neutrino events, ~1 week after the submission of the 1988 paper.

 Kamiokande was not big enough. It took almost 6 years to get some meaningful results.

21 Experimental neutrino programme

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Zenith angle distribution for multi-GeV events (1994)

multi-GeV events

Deficit of

upward-going m -like events

Not high enough statistics to conclude …

Much higher statics required (= much larger detector required)

Kamiokande PLB 335, 237 (1994)

) 9 . 2 ( 58

.

0

^_⁰₀^._.¹³₁₁

 Down 

Up

39 . 0

30 .

38 0

. 1 ^ _ Down 

Up

Up-going Down

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Discovery of Neutrino Oscillations

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50,000 ton water Cherenkov detector (22,500 ton fiducial volume)

１０００ｍ underground

11200 PMTs (Inner detector) 1900 PMTs (Outer detector)

３９ｍ

４２ｍ

Super-Kamiokade detector

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Beginning of the Super-Kamiokade collaboration between USA and Japan

Y. Suzuki

W. Kropp H. Sobel TK Y. Totsuka

K. Nishikawa A. Suzuki

J. Stone J. Arafune

(ICRR director)

K. Nakamura

@ Institute for Cosmic Ray Research,

(Probably) 1991

or 1992

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Water filling in Super-Kamiokande

Jan. 1996

Kamiokande

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Super-K detector construction

Aug. 1995

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Fully automated analysis

FC (fully contained)

n

・One of the limitation of the Kamiokande’s analysis was the necessity of the event scanning for all data and Monte Carlo events, due to no

satisfactory ring identification software.

Multi

Cherenkov ring event

Hough transformation

+ maximum likelihood

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Various types of atmospheric n events (1)

FC (fully contained)

Single

Cherenkov ring muon-like event

Color: timing

Size: pulse height Outer detector (no signal)

Multi

Cherenkov ring event

n

En ~1GeV E n ~a few GeV

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Various types of atmospheric n events (3)

Upward going muon

ν

・ almost pure CC n

_m

Signal in the outer detector

PC n

(partially contained)

・97% CC n

_m

En ~10 GeV En ~100 GeV

All these events are used in the analysis.  Collaborative work of many (young) people!

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Evidence for neutrino oscillations

(Super-Kamiokande

@Neutrino ’98)

Super-Kamiokande concluded that the observed zenith angle

dependent deficit (and the other supporting data)

gave evidence for neutrino oscillations.

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Soudan-2 MACRO

Results from the other atmospheric neutrino experiments

These experiments observed atmospheric neutrinos and confirmed

neutrino oscillations

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Resent results

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Data updates

Super-K

@Neutrino98 Kamiokande

(1994)

135 events 531 events

Number of events plotted:

Super-K (2015)

No oscillation

5485 events

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n _m  n _t allowed parameter region

Y. Itow (SK) nu2012

Super-K (1998)

Super-K (2012)

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~1 /1 0, 00 0, 00 0 of t he e le ct ron m as s

n ₂ n ₃

n _m n _t

q

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Really oscillations

It was very nice to see that approximately half of the long traveling n _m ’s disappear. However, we wanted to really confirm neutrino “oscillations”.

Down- going

Up- going

P rob abili ty ( n m r em ain n m )

1 10 100 1000 L(km) for 1GeV neutrinos

We wanted to observe this dip to confirm neutrino “oscillations”.

A dip is seen around L/E = 500 km/GeV.  Really oscillations !!

Super-K, PRL 93, 101801 (2004)

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Detecting CC n _t events

If the oscillations are n _m n _t , we should observe n _t

interactions

n _t n _t

t _hadrons

hadrons

Example:

n

_t

event (MC)

37

n _m

We wanted to observe these events. The serious analysis started in ~2001.

37

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Up-going Down-going

Zenith angle distribution and fit results

t-appearance signal at 3.8

Fitted number of t events 180.1±44.3(stat) +17.8 / -15.2(syst) Expected number of t events 120.2+34.2/-34.8(syst)

SK PRL 110(2013)181802 See also, SK PRL 97(2006)171801

From the other side of the Earth

From above

n _t -signal

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Future

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Present status:

2 flavors to 3 flavors: summary

parameters 3 range

sin ² q ₁₂ 0.267 – 0.344 Solar (SNO, Super-K etc), KamLAND sin ² q ₂₃ 0.342 – 0.667 Atmospheric (Super-K etc), Long

baseline (MINOS, T2K etc) sin ² q ₁₃ 0.0156 –

0.0299

Long baseline (T2K, MINOS, etc), Reactor (Daya Bay, RENO, D-Chooz) Dm ₁₂ ² (7.00 – 8.09)

×10 ^-5 eV ²

Solar (SNO, Super-K etc), KamLAND

|Dm _{13 or 23} ² | (2.24 – 2.70)

×10 ^-3 eV ²

Long baseline (MINOS, T2K etc) and Atmospheric (Super-K)

arXiv: 1209.3023v3

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Unknowns

q ₁₃

Mass hierarchy ?

n ₃

n ₂ n ₁

n _e n _m n _t or

) (

)

( n _  n _  P n _  n _

P _?

CP violation ?

Is the mass pattern of neutrinos similar to those of quarks and charged leptons?

Baryon asymmetry of the Universe?

q

₁₃

is not very small

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Future experiments

20kton Liq. Sci.

JUNO

RENO-50

Reactor exp’s Atmospheric n exp’s

PINGU

KM3NeT/ ORCA

INO

Hyper-K

LBL n exp’s

LBNF/DUNE

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Oscillation probabilities

N o rm al hi er ar ch y In ve rt ed hi er ar ch y n ₃

n ₃

Neutrino Energy (GeV) Neutrino Energy (GeV)

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J-PARC

• Cavity : 48m(W) x 54m(H) x 250m(L) x 2

• Water volume :

– Total : 0.496x2 = 0.99 Mton

– Fiducial volume = 0.56 Mton ( 25x SK )

• Photo-detectors :

– ID : ~99,000 20” PMTs, 20% photo-coverage – OD : ~25,000 8” PMTs, same coverage as SK

•750 kW (assumed) 2.5 degree off-axis

beam from J-PARC

295km baseline length and

Atmospheric neutrinos

Hyper-K, PTEP (2015)

Hyper-Kamiokande

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Hyper-K’s sensitivity to mass hierarchy

MH determination with Atmospheric neutrinos

Hyper-K 10 years

Hyper-Kamiokande status and plan

 proto-collaboration has been formed

 240 people from 13 countries

 R&D funds have been granted in several countries

 Selected as one of the 25 top priority future projects by

Science Council of Japan in 2014

 But was not included in the

MEXT (Japanese funding agency) roadmap in 2014  must wait for the next round (2017)

 If the construction begins in 2018,

experiment ~2025

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CP violation (LBNF/DUNE and J-PARC/Hyper-Kamiokande)

Plot by M. Shiozawa

K. Abe et al., arXiv: 1502.05199

M.Thomson, 2

^nd

International meeting for Large Neutrino Infrastructure, April 2015

CP violation sensitivity

(MH assumed to be known)

Hyper-K slightly better due to larger statistics

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CP phase measurement

K. Abe et al., arXiv: 1502.05199

Measurement of d _CP

(MH assumed to be known)

Hyper-Kamiokande

(90%CL)

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Appendix:

coming back to the “signal”

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p→νK ⁺ p→e ⁺ π ⁰

560kton

22kton

Year

Proton decays: estimated limit

2010 2020 2030 2040

Liq. Ar

34kton (p n K ⁺ )

Sensitivity (90%CL)  Water Ch. much better for pep ⁰ . Water Ch. can reach 10 ³⁵ years.

 LAr better for p n K ⁺ after many years….

About 35 years ago, proton decay experiments began to search for

proton decays with the lifetime of ~10 ³⁰ years…

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Summary

• Unexpected muon-neutrino deficit in the atmospheric neutrino flux was observed in Kamiokande (1988).

• Subsequently, Super-Kamiokande discovered atmospheric neutrino oscillations (1998).

• I feel that I have been extremely lucky, because I have been involved in the excitement of this discovery from the

beginning.

• The discovery of non-zero neutrino masses opened a window to study physics at a very high energy scale,

ICRR, Univ. of Tokyo

Takaaki Kajita