νμ

Tokai to Kamioka
Long-baseline Neutrino
Oscillation ExperimentTokai to Kamioka

Super-Kamiokande (ICRR, Univ. Tokyo)

νeNeutrino

295km

ν
μ

Neutrino Experimental
Facility at J-PARC(KEK-JAEA, Tokai)

νe

295km

νμ

Anti-neutrino©2013 Google™

T2K Collaboration

t2k-experiment.org
High Energy Accelerator Research Organizationwww.kek.jp
March 2019e
Example of a candidate event ofelectron neutrino appearance

The T2K Experiment to Probe the Mysteries of the
Picture of the Mass and Three Generation Mixing
QuarksνμνeBottom

b

Top

ντ

s

c

Down

Up

d

3rd

t

Strange Charmu

The T2K Neutrino Oscillation Experiment

Leptons

is an experiment to probe the mysteries of neutrinos. A high intensity neutrino beam is produced using a high intensity proton accelerator
and the neutrino facility at J-PARC, which was newly constructed at
Tokai village, Ibaraki prefecture in 2009. The neutrinos then travel over
295 km to the Super-Kamiokande 50 kton water Cherenkov detector,
which is located 1000 m deep within Mt. Ikenoyama at the Kamioka
mine in Gifu prefecture.
We aim for:Tau
Tau neutrino

τ ν
τ
2nd

Muon
Muon neutrino

μ ν
μ

1st
Generaton◦Discovery of a new oscillation mode, νμ → νe oscillation, which iscomplementary to the νμ → ντ neutrino oscillation mode discovered by
the Super-Kamiokande experiment and verified by the K2K experiment◦Consequently, clarification of neutrino oscillation and neutrino
mixing among three generations◦Furthermore, indication of the violation of CP symmetry (thesymmetry between a particle and its anti-particle) in the lepton sector

Electron Electron
neutrinoeνeChargeA diagram of neutrino oscillation.
T2K aims to establish that neutrino
oscillation occurs among the threeneutrino generations (three generation mixing).

Quarks and Leptons are elementary
particles which make up nature.
There are three generations of quarks
and leptons, respectively.

Based on these studies, we are able to probe the following
fundamental questions:

The History of Neutrino Research◦What is the full picture of neutrino mass and three generation
neutrino mixing?
◦What is the origin of the matter dominant Universe?

●1930 Prediction of the existence of neutrinos (W. Pauli)●1956 Discovery of the electron anti-neutrino (F. Reines, C. Cowan)

What are neutrinos?

●1962 Discovery of the muon neutrino (L. Lederman et al.)●1962 Proposal of the theory of neutrino oscillation (J. Maki, M. Nakagawa, S. Sakata)

Neutrinos generated by a supernova explosion are observed for the first time●1987
(M. Koshiba et al., Kamiokande experiment)●1991 It is proven that light neutrinos exist in only three generations (LEP experiment)●1998

Neutrino oscillation is discovered by the observation of atmospheric neutrinos(Y. Totsuka, T. Kajita et al., Super-Kamiokande experiment)●1999

Successful observation of accelerator-produced neutrinos in K2K, the world’s-firstaccelerator-based long-baseline neutrino oscillation experiment●2000 Confirmation of the existence of the tau neutrino (K. Niwa et al., DONUT experiment)●2001

The existence of neutrino oscillation of solar neutrinos is proven (Super-Kamiokande
experiment and SNO experiment in Canada)●2002

The oscillation of electron anti-neutrinos from nuclear reactors is observed (KamLAND
experiment)●2002 M. Koshiba, R. Davis Jr. et al. receive the Nobel Prize in Physics●2004

Neutrino oscillation is established with an accelerator-produced neutrino beam (K2K
experiment)●2013 “Electron neutrino appearance” is discovered (T2K experiment)●2015 T. Kajita and A. McDonald receive Nobel Prize in Physics●2015 K. Nishikawa et al. and the K2K/T2K experiments receive the Breakthrough Prize●2016 A hint of “CP violation” is seen in neutrinos (T2K experiment)

Number of observed neutrino events

SK-I+II+III, 2806 Days

θ

600

Multi-GeV μ -like + PC

0
-1

Data
データ
With neutrino
oscillation
ニュートリノ振動あり
Without
neutrino
ニュートリノ振動なしoscillation

0

What is neutrino oscillation?

Neutrino oscillation is a phenomenon where if one type of neutrino
is produced, a different type may be observed after the neutrino travels.
For example, even if a 100% pure muon neutrino beam is produced byan accelerator, some of the muon neutrinos can be observed as tau orelectron neutrinos after they travel over some distance. The probabilityof observing one neutrino type varies (oscillates) along with the travel
distance. This phenomenon is called “neutrino oscillation”.
Neutrino oscillation only occurs if neutrinos have a finite mass andif there is mixing of neutrinos among the three generations. Since neutrinos are massless particles in the Standard Model of particle physics,
the phenomenon of neutrino oscillation indicates a new physics beyond
the Standard Model. At present, neutrino oscillation is the only method
to investigate both the extremely tiny mass and the three generation
mixing of neutrinos.

Discovery of neutrino oscillation by
Super-Kamiokande

400

200

Neutrinos are elementary particles that have no electric charge. The
mass of a neutrino is less than one millionth that of an electron.
It is known that there are three types (called “generations” or “flavors”) of neutrinos: electron neutrino (νe), muon neutrino (νμ), and tau
neutrino (ντ). For each neutrino there is also an associated anti-neutrino,
which is the anti-particle of the neutrino.
There are many neutrinos around us. Every second, hundreds oftrillions of neutrinos emitted by the Sun pass through our bodies. However, they do not harm us at all.

1

cos θ
Distribution of zenith angles for atmospheric neutrinos produced by the reaction
between cosmic rays and the atmosphere observed by Super-Kamiokande. The
number of muon neutrinos coming from below (through the Earth) was smaller
than the number coming from above (from the atmosphere).

In June 1998, the Super-Kamiokande collaboration reported thediscovery of neutrino oscillation. They reported a difference in the distribution of zenith angles for atmospheric neutrinos, where the numberof muon neutrinos coming from below (through the Earth) was smaller
than the number coming from above (from the atmosphere). This was
because muon neutrinos change into undetectable tau neutrinos due toneutrino oscillation. Therefore, these distributions provided the experimental proof that neutrinos have a finite mass and neutrino oscillation
occurs.
Prof. Takaaki Kajita for the Super-Kamiokande experiment and Prof.
Arthur McDonald for the SNO experiment were awarded the 2015 Nobel
prize in physics for the experimental discovery of neutrino oscillations.

Matter Dominant Universe and Establish a Fullof Neutrinos
Neutrino Beam Production using an
Accelerator

When a high energy proton beam strikes a target, many π mesons(or pions) are produced from interactions between the protons andthe nuclei in the target. Pions are an unstable particle, and decay intoa muon and a muon neutrino after traveling tens of meters. These
resulting muon neutrinos are used as a neutrino beam. The beam createdby an accelerator in this way is a nearly pure beam of muon neutrinos.

Muon Neutrino
π mesonProtonMuon

Atomic nucleus
Diagram of the production of a neutrino beam using an accelerator

Although an accelerator-generated neutrino beam consists of almost
entirely muon neutrinos, the percentage of muon neutrinos observed decreases after the particles travel over a certain distance due to neutrino
oscillation. It is possible to study neutrino oscillations by investigating
the decrease in muon neutrinos or by directly detecting the neutrinos
that have changed into a different type. When studying neutrino oscillation, is also important to measure the distribution of the observed neutrino energies, since the fraction of neutrinos oscillating to other types
depends on the energy of the neutrinos.
In the T2K experiment, a nearly pure muon neutrino beam produced
at J-PARC is sent to Super-Kamiokande. The T2K experiment aimed to
discover νμ→νe oscillation by observing electron neutrino events at theSuper-Kamiokande detector.

Discovery of νμ→νe Oscillation by the T2K
Experiment

The probability that a muon neutrino will change into an electron
neutrino is small in comparison to the probability that it will change
into a tau neutrino, and therefore νμ → νe oscillation had not been
experimentally observed prior to the T2K experiment. The J-PARC
neutrino beamline was newly constructed to produce a high-intensity
neutrino beam and the T2K experiment started taking data in 2010.
In July 2013, the T2K collaboration announced the discoveryof νμ → νe oscillation after observing electron neutrino appearance at
the far detector, Super-Kamiokande. This is decisive evidence thata muon neutrino can change into an electron neutrino in-flight. This
discovery indicates that neutrino oscillation occurs among all threeneutrino generations. As a result of the T2K measurement, neutrino
oscillation research has entered a new phase.

Violation of CP Symmetry in the Neutrino
Sector

If CP symmetry is violated in neutrinos, its effects will appear
as a difference between the probability of muon neutrino to electron
neutrino oscillation compared to the probability of muon anti-neutrino to electron anti-neutrino oscillation. As a consequence, the discovery of νμ→νe oscillation opens the door to search for a “violationof CP symmetry” in the neutrino sector. T2K is aiming to discover
CP violation in neutrinos.
The T2K experimental group consists of an international collaboration of around 500 researchers from inside and outside of Japan working together at J-PARC and Super-Kamiokande.

Neutrinos arriving at
Super-Kamiokande

Neutrino beam
produced at J-PARC

The majority have changed into tau neutrinos.
A small fraction have become electron neutrinos.

Nearly pure muon
neutrino beam

Discovery that muon neutrinos can change to electron neutrinos(A new type of neutrino oscillation observed by the T2K experiment in 2013)
Diagram of “electron neutrino appearance”, oscillation from muon neutrinos to electron
neutrinos, directly observed by the T2K experiment

Number of νe candidate events

Study of Neutrino Oscillations using an
Accelerator

10

T2K RUN1-4 data
Best fit νe signal
Background component

5

0
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500

1000

Reconstructed neutrino energy (MeV)

The neutrino energy distribution for the 28 observed electron neutrino
events. It is clear that the data (black dots) closely match the sum of thenumber of expected background events (green) and the number of
expected electron neutrino appearance events (red).
The probability to reproduce the observed number of events by a statisticalfluctuation of the background is less than 1 in a trillion.

Neutrino beam
produced at J-PARC

Neutrinos arriving at
Super-Kamiokande

Nearly pure muon
neutrino beam

The majority have changed into tau neutrinos.
A small fraction have become electron neutrinos.

Nearly pure muon
anti-neutrino beam

The majority have changed into tau anti-neutrinos.
A small fraction have become electron anti-neutrinos.

The search for “CP symmetry violation” using electron neutrino appearance
events. By comparing νμ→νe oscillation of neutrinos and anti-neutrinos, “CPsymmetry violation” will be explored.

The Experimental Facilities Distributed AcrossOscillation Experiment
In the T2K experiment, the world’s highest intensity neutrino beam is generated
at the J-PARC Neutrino Experimental Facility and shot towards the SuperKamiokande detector. The neutrinos can propagate through the earth as if there wereno iron, concrete, or bedrock, and arrive at Kamioka, 295 km away from J-PARC,
in approximately 1 millisecond. A small portion are then observed by the SuperKamiokande detector.
The neutrino beam is also monitored using a near detector at J-PARC. It is
possible to study “neutrino oscillation”, in which neutrinos change their type in flight,
by comparing the observation at the near detector to the one at Super-Kamiokande.

295km©GoogleTM

Super-KAMIOKANDE

J-PARC
Near Neutrino Detector

Noguchi Goro Peak2,924m
Ikenoyama
1,360m0m above sea level

Neutrinos

1°

1,700m（Adjustable）
θ= 2°〜 2.5°

The center of the neutrino beam
is sent toward the Sea of Japan

Super-KAMIOKANDE
Super-Kamiokande is a massive neutrino detection device operated by the
University of Tokyo, Institute for Cosmic Ray Research. The detector is located1,000m deep underground in the Kamioka mine in the city of Hida in Gifu prefecture.
Super-Kamiokande operation began in April 1996, and even now the detector
continues to observe neutrinos that come from outer space. The tank has a diameterof 39.3 meters, a height of 41.4 meters, and contains 50,000 tons of pure water. It isequipped with 11,129 highly-sensitive photodetectors (photomultiplier tubes), which
detect the Cherenkov radiation light which is generated when the neutrinos interact
with the water (a very rare process). This photodetector information can be used todetermine the type, direction of arrival, and energy of the interacting neutrinos.

A photomultiplier tube used in SuperKamiokande. The diameter is 50 cm −the world’s largest.

A candidate event for the world
pioneering observation of electron
neutrino appearance by the T2K
experiment (May 2010)
Provided by: University of Tokyo, Institute for
Cosmic Ray Research, Kamioka Observatory

Japan for the T2K Long-Baseline Neutrino
Japan Proton Accelerator Research Complex J-PARC
Hadron Experimental Facility

Flux • σ (arbitrary unit)θ =0˚Material and Life Science
Experimental Facility (MLF)

2˚
2.5˚

Main Ring

Circumference 1,568m∆m2atm = 2.0~3.0x10-3eV2

3˚

0

1

2

3

Neutrino BeamEν (GeV)

Neutrino energy distributions for an on-axis beam
(θ=0°) and off-axis beam (θ=2°~3°)

The neutrino beam produced at J-PARC is verybroad. By pointing the central axis of the beamslightly lower than the Super-Kamiokande direction,
the energy of the neutrinos arriving in Kamioka is
reduced and the width of the energy distribution becomes narrower. The J-PARC Neutrino Experimental Facility was the first facility in the world to adopt
this off-axis beam configuration.

3 GeV RCS

Near Neutrino Detector

Circumference 348m

Linac
330m

J-PARC (Japan Proton Accelerator Research Complex) is a general term for the proton
accelerator and surrounding facilities constructed in Tokai village, Ibaraki prefecture during
2001~2009 through the cooperation of the High Energy Accelerator Research Organization(KEK) and the Japan Atomic Energy Agency (JAEA).
At J-PARC, protons are accelerated in a linear accelerator, they are then accelerated to 3
GeV using a synchrotron (RCS), and are finally sent into the Main Ring (MR), where they areaccelerated to 30 GeV. Protons are then extracted from the MR to the neutrino primary beamline, using electromagnets called “kickers”. In the primary beamline, the protons are bent towards the Kamioka direction. The protons then strike a target, which produces a neutrino beamheaded towards Super-Kamiokande.

Near Neutrino Detector
A near neutrino detector is installed in a pit (at the neutrino monitoringbuilding) located 280 meters downstream from the target. The pit is 33.5 meters
deep and 17.5 meters in diameter.
The near detector is composed of two independent detectors, the “on-axis
INGRID detector”, sitting in the center of the beam, and the “ND280 off-axis
detector”, sitting in the direction of Kamioka.
The INGRID detector monitors the stability of the neutrino beam direction
and intensity, while the ND280 detector measures the energy distribution of theneutrino beam and the composition of muon and electron neutrinos in the beam.
The electromagnets surrounding the off-axis detector were manufactured in1979 for the UA1 experiment at the CERN Proton-Antiproton Collider and contributed to the discovery of the W and Z bosons in 1983.

Off-axis
detector

UAI
magnet(0.2T)

On-axisdetector(INGRID)

On-axis detector

Off-axis detector

The J-PARC Neutrino Experimental Facility
Intensity Neutrino Beam in the World
30 GeV protons are extracted from the MR to the neutrino primary beamline using electromagnets called “kickers”. The protons pass through theprimary beamline, which consists of a series of normal-conducting and super-conducting magnets, and has various beam monitors. The protons are bent
to the west and strike a graphite target located in the target station. Many pions are produced when the protons interact with the target. These pions arefocused in the forward direction by a special device called a magnetic horn, and fly into a 100-meter-long tunnel called the “decay volume”. They decay
in flight into pairs of muon neutrinos and muons. All of the particles aside from the neutrinos and a fraction of the muons are then absorbed by the beam
dump and do not leave the experimental facility.

TargetMagnetic horn

The target is inserted into the center of the inner conductor of thefirst magnetic horn by a manipulator. Many pions are produced when
protons interact with the target. The target is made of graphite and iscovered with two sheaths made of graphite (inner) and titanium alloy(outer). Helium gas is circulated in the gap between the target andsheaths for cooling the core graphite rod after heat is generated byproton interactions. During operation, the temperature of the core canrise to approximately 700°C.

The magnetic horn is a device consisting of two layers of cylindricalaluminum conductors. A 2-tesla magnetic field between the inner
and the outer conductors is generated using a pulsed current of up to
320,000 amperes, which focuses pions in the forward direction. The
T2K experiment uses 3 magnetic horns.

Muon monitoring building
Neutrino monitoring building(NM)295 km to Super-Kamiokande⑥ Beam Dump installation⑤ Decay Volume

A stack of graphite blocks used as abeam dump is installed at the mostdownstream end of the beamline. It
absorbs protons that did not interact
in the target, as well as other outgoingparticles. It also intercepts the heat andradiation generated by the beam.

The decay volume is a tunnel surroundedby approximately 6-meter-thick concrete. In
order to cool the heat generated by secondary
particles, water pipes are installed all over theinterior of this tunnel.

② Super-conducting Combined Function

Magnet (SCFM)

A superconducting magnet system is used in the arc
section, where the proton beam extracted from the Main
Ring is sharply bent towards the Kamioka direction.
There are 28 combined function (dipole and quadrupole)magnets called “SCFMs”. Each SCFM is 3.3 meters longwith a maximum dipole field of 2.5 tesla and a maximum
quadrupole field gradient of 18.6 tesla per meter. In order

Produces the Highest⑦ Muon Monitor④ Target Station (TS)The muon monitor is a measurement device for indirectly
observing the direction andstability of the neutrino beam.
It functions by measuring
the muons that are produced
together with the neutrinos.

A massive helium container, which is integrated withthe decay volume and has a volume of 1,500 m3,
sits in the basement of the target station. The threemagnetic horns, the baffle protecting these horns, andother pieces of equipment sit in this helium container
and are securely covered by steel and concrete, which
provide shielding from radiation.

OTR

ESM

SSEM

CT

Beam Monitors
Various beam monitoring devices (CT, ESM, SSEM,
BLM) monitor the intensity, position, profile and lossof the proton beam, and are installed in the primary
beamline. These monitors ensure that the proton beam
extracted from the Main Ring is safely sent to the target.
An OTR monitor is also installed in front of the target
and measures the profile of the proton beam.

Mai
nR
ing
Neutrino Utility building #3(NU3)

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6

ProtonPions
Muons
Neutrinos

Target Station
(TS)

4

5

BLM

Neutrino Facility
at J-PARC

3
2

110m

280m

Neutrino Utility building #2(NU2)

Carry-in building (NC)

Neutrino Utility
building #1(NU1)① Preparation section

1

Distribution of magnetic
field strength in the SCFM

③ Final focus section
to keep the magnets in a superconducting state,
the system is equipped with a helium freezer
that has an absolute temperature of 4.5 degrees
and a cooling capacity of 2 kilowatts.

Normal-conducting magnets in the primary beamline
Normal-conducting magnets are used at the preparation and final focus sectionsof the beamline.

Challenges to discover “Violation of CP Symmetry” in the Neutrino Sector
In order to study the difference between neutrinos and anti-neutrinos, the T2K collaboration began measurements using an anti-neutrino
beam in 2014. Data collected until 2016 indicates that there is a difference between neutrinos and anti-neutrinos with a 90% confidence
level. This is the world’s first indication that “CP symmetry violation” occurs in neutrinos. Since 2016, T2K has improved its measurement
significance and has observed a difference in the probability of neutrino and anti-neutrino oscillations at the 2σ confidence level (95.4%) after
increasing the amount of available data by using a higher intensity beam and new analysis methods.

Nearly pure muon
anti-neutrino beam

The majority have changed into tau anti-neutrinos.
A small fraction have become electron anti-neutrinos.

ObservednumberNeutrino

68

90

Anti-neutrino

139(Note) The reason why the expected number of events differs
between neutrinos and anti-neutrinos even with no “CPsymmetry violation”, is that the probability of interacting withwater is different for neutrinos and anti-neutrinos, even if thesame number of neutrinos and anti-neutrinos pass through theSuper-Kamiokande detector.

T2K Run1-9c Preliminary[ Larger values indicate a lower chance of matching the observed data]
Consistency with experimental data (log likelihood): -2lnL(Assuming no CP
symmetry violation)

The majority have changed into tau neutrinos.
A small fraction have become electron neutrinos.

Nearly pure muon
neutrino beam

30

Expected number

Experimental
data

Neutrinos arriving at
Super-Kamiokande

Neutrino beam
produced at J-PARC

Region in which the T2K experimental
data matches the given value of δCP at the2σ confidence level (95.4%)

Analysis results when theminimum mass is assumed forthe third generation neutrino

NormalInverted25
20
15

Analysis results when the
maximum mass is assumed forthe third generation neutrino

10
5
0
-180˚

-90˚

0˚

+90˚

+180˚
CP phase : δCP

Region where there is a violationof CP symmetry
Point where there is no
violation of CP symmetry

Region where there is a violationof CP symmetry

Point where there is no
violation of CP symmetry

Point where there is no
violation of CP symmetry

The figure on the left shows log likelihood values as a function of
the CP phase (angle) δCP, which indicate the size of any observed “CPsymmetry violation”. Higher log likelihood values indicate that theexperimental data have poorer agreement with the expectation at thatvalue of δCP. The black and red curves show the log likelihood values
for the two different neutrino mass scenarios. The black and red hatchedregions show the allowed 2σ (95.4%) confidence level intervals.
The results of the T2K experiment show that the δ CP values
corresponding to no “violation of CP symmetry” (δ CP=0°, ±180°)
are outside of the 2σ confidence interval. This means that the T2K
data exclude the conservation of CP symmetry in neutrinos at the2σ (95.4%) level.

T2K Upgrade Plan Aimed at Discovering a “Violation of CP Symmetry”
The current results of the T2K experiment are interim results basedon approximately 34% of the initially approved amount of data, but
indicate a 2σ (95.4%) confidence level measurement of “CP symmetry
violation in neutrinos”. J-PARC and the T2K collaboration plan toincrease the intensity of the proton beam to three times its current
level and to accumulate 2.5 times more data than initially approved(approximately 8 times more data than has currently been collected).
With these improvements, T2K aims to verify its measurement of“CP symmetry violation” in the neutrino sector at the 3σ confidence
level (99.7%). As part of this plan, along with upgrading the neutrino
beamline through improvements to the target, magnetic horns and otherequipment, the performance of the near neutrino detector will also be
improved by replacing a portion of the existing detectors with newly
developed detectors.

Neutrino
Beam

Scintillator detector
High-Angle TPCs
TOF counters

Improvement plan for the near neutrino detector (off-axis detector). The
upstream part of the detector will be replaced with newly developed
detectors to allow for more precise measurements of neutrinos.