The Belle II
Experiment– Quest for New Physics –Purpose of the Belle II ExperimentParticle Physics: Searching for the fundamental law of the universe
From the time of ancient Greek philosophers, people have continued to question the origin of the matter that forms the world. The Earthwhere we live and the matter that makes up the universe have diversity, which seems infinite. However, all matter is composed of “elementary
particles”. Particle physics, which examines the ultimate constituents of matter has progressed greatly in the 20th century; the exploration has
lead to the discoveries of the atom, the nucleus, quarks and other fundamental particles. The laws that rule the microscopic world of elementary particles are important for understanding the early high energy stages of the birth of the universe.

Antiparticles disappeared from the Universe
It is known that “antiparticle” counterparts exist for all elementary particles.
Antiparticles have almost the same properties as particles except that theelectric charge is opposite for antiparticles. When a particle and an antiparticle
meet they annihilate and turn into energy. Antimatter composed of antiparticles has not been discovered in the universe.
When the high energy state, which existed immediately after the creationof the universe, is artificially created using an accelerator, particle-antiparticle
pairs are inevitably produced. In other words, immediately after the creationof the universe equal numbers of particles and antiparticles produced fromthe high energy state of the Big Bang existed.
The antiparticles paired up with particles and annihilated. For an unknown
reason, only particles survived in the universe. Why did the anti-particles disappear? This is one of the mysteries that the Belle II experiment attempts tosolve.

When particles and antiparticles meet, they annihilate.

Beyond “the Standard Model” of Particle Physics

10 –9 m10 –10 m

10 –14 m

10 –15 m

10 –18 m

water molecule

oxygen atom

nucleusnucleon

Quark

Experiments’ results have been very well explained by the Standard Model, which was theoretically established in the 1970s. However, we know that mysteries that cannot be solved by the Standard
model still remain.
For example, the Standard Model cannot explain the disappearance of antiparticles from the universe. It cannot explain the natureof either the dark matter or dark energy, which are considered to be
the major constituent of the universe. To answer these questions, weneed to explore new laws of physics beyond the Standard Model.
Fermions

QuarksThe matter that surrounds us is made up of atoms. The atoms are
made up of a nucleus and electrons, and the nucleus is composedof protons and neutrons. Furthermore, the protons and the neutrons
have been found to be composite particles consisting of quarks.
In the current theory of elementary particles, the quarks which
form protons and neutrons, and the leptons, which are from thesame family as the electrons and the neutrinos, are thought to be
the elementary particles which make up matter. There are six typesof quarks and leptons that can be arranged in three generations.
There are also the elementary particles that transfer “weak force”,“electromagnetic force” and “strong force”, and the Higgs boson thatgives masses to particles. The behavior of the elementary particles is
described by the theory known as “the Standard Model".

Gauge bosons

I

II

III

up

charm

top

Strong force

gluon

Electromagnetic force
down

stragebottomelectron neutrino

mu neutrino

tau neutrino

electron

muon

tau

neutronLeptons

photonWeak force

Quark

W bosonprotonElectron

All matter is made of elementary particles.

01 B e l l e I I E x p e r i m e n t Q u e s t fo r N e w Phy si c s

Higgs

Particles of the Standard Model

Z bosonBelle II Experiment andSuperKEKB accelerator
The Belle II experiment, which is an international
particle physics collaboration at KEK in Tsukuba, Japan,
focuses on the investigation of particles called B mesons, which are produced in collisions of accelerator
particle beams. The B meson is about 5.6 times heavier
than the proton and decays into various other particles
in about 1.5 picoseconds (1 picosecond is one trillionthof a second). The Belle II detector, which examines the
B meson decays, has been significantly upgraded fromthe Belle experiment, which completed operation in
2010, and uses state-of-the-art technology.
The Belle II experiment investigates particleantiparticle symmetry breaking and extremely rare
phenomena, which are thought to have occurred in
the early universe, by collecting 50 times more data
than in the previous Belle experiment. Analyzing thehuge Belle II dataset may reveal unknown particles andthe nature of the forces that govern them. This allows
us to discover new laws of physics and unveil the mystery behind the disappearance of antimatter from theuniverse.
Introduction to the Belle II experiment :https://www.youtube.com/watch?v=vnrQcF-jmpM

Circular accelerator :Blue : electron beam pipe
Red : positron beam pipe

Belle II detector

Linear accelerator

Positron damping ring

SuperKEKB Accelerator

Collision point

SuperKEKB is a collider accelerator with a circumference of 3 km. Electrons and positrons are accelerated toenergies of 7 billion and 4 billion electron volts, respectively, in a linear accelerator and then are injected into two
circular storage rings and circulated in opposite directions.
When the electrons and positrons accelerated close to
the speed of light are brought into collision in the centerof the Belle II detector, a large number of various particle
reactions, which are thought to have taken place in theearly universe, are generated. The accelerator energy is set
to produce a large number of B meson/anti B meson pairs
in the collisions of the two beams. Hence the accelerator
facility is called a Super B Factory (a factory producing alarge number of B mesons).
The KEKB accelerator, the accelerator that preceded
SuperKEKB, achieved the world record for “luminosity”,
which is a figure of merit for accelerator performance
related to the number of particle reactions that are produced. In the SuperKEKB accelerator, a luminosity 40
times greater than in the KEKB accelerator is achieved byintroducing start of the art technology, such as nanobeams, which makes it possible for the accelerated electronsand positrons to be focused to 20 μm (0.02 mm) in widthand 100 nm (0.0001 mm) in height, and provide the BelleII detector with an integrated luminosity 50 times larger
than KEKB.

B e l l e I I E x p e r i m e n t Q u e s t fo r N e w Phy si c s

02From Belle to Belle II
Belle started operation

Belle completed operation

2008

2000

2010

The Novel prize in physics

Dr. Makoto Kobayashi and Dr. Toshihide Maskawa

Pursuing a Solution to the Remaining
Mystery

Achievement of the Belle Experiment

1000

800

B
–
B

2010 Spring

Discovery of B→D*τν decay (decay with missing energy)

2008 Autumn Measurement of the forward backward assymetry in B→K*II decay
2008 Spring

Discovery of new particles tha may be tetraquark atates2007 Spring

Discovery of D meson mixing

2006 Summer Evidence of the missing energy decay B→τν

(/fb)

400
350
300
250
200
150
100
50
0

Various measurements were carried out by the Belle Experimentand almost all of them can be explained by the Kobayashi-Maskawa
theory. On the other hand, we have come to the understanding that
CP violation in the Kobayashi-Maskawa theory is too small to explain
the number of surviving matter particles in the universe. The Belleexperiment also found indications that suggest imperfections in theStandard Model of particle physics. In order to confirm these, a verylarge amount of additional data well in excess that accumulated by
the Belle Experiment is required.

Integrated Luminosity

Number of Events

The Belle experiment, which preceded the Belle II, operated from1999 to 2010 and accumulated approximately eight hundred million
B meson and anti B meson pairs. One of its most important achievements was the validation of the CP violation hypothesis that was thebasis for the 2008 Nobel Prize in Physics for Dr. Makoto Kobayashi and
Dr. Toshihide Maskawa.
CP means that the laws of physics remain unchanged whenparticle and antiparticle are exchanged and mirror-imaged. In 1964,
a measurement at an accelerator in the United States showed that
this symmetry is violated in K meson decay. However, the underlying
mechanism for CP violation remained a mystery for a long time.
The Kobayashi-Maskawa theory, which explains how the CP violation arises, predicted that a large CP violation would occur in the
decays of B mesons, particles containing bottom quarks that belong
to the third generation. The Belle experiment was constructed in order to verify this theory.
In 2001, the Belle experiment and the BABAR experiment, which
ran at the same time in the U.S., finally observed large CP violation in
the B meson sector and confirmed the prediction of the KobayashiMaskawa theory.

600

2005 Summer Observation of the new quark process b→dγ decay

2004 Summer Evidence of direct CP violation in B→Kπ

400

200

2003 Autumn Discovery of new particle X(3872)(new type meson?)2003 Summer Measurement of CP
violation in the b→s penguin processsuggesting physics

2002 Summer Observation of new decay B→K+–2002 Spring

Evidence of CP violation in B→ππ2001 Summer Discovery of CP violation in B meson sysytem

-6

-4

-2

0

2

4

6

Δt (ps)Assymmetry observed in the decay time distributions of B andanti-B mesons demonstrates the existence of large CP violation

03 B e l l e I I E x p e r i m e n t Q u e s t fo r N e w Phy si c s

0

1/19981/20001/2002

1/20041/20061/20081/2010

History of the major results and accumulated luminosity of the Belleexperiment1/20122024

2018
Belle II starts operation !

The integrated luminosity of Belle II
will be 50 times larger than KEKB.

2020

20XiscXover a new

ay d
m
I
I
e
l
l
e
B
ysics ??
h
p
f
o
law

Searching for Hidden Laws of Physics
in the Early Universe

Instantaneous Ultra-High Energy State
Appeared in B Meson Decay

The Belle II experiment searches for unknown laws of physics thatwere important immediately after the Big Bang. In the high energy
state of the early universe, it is believed that unknown laws of physics with CP violation that go beyond the Kobayashi-Maskawa theory
were important. Such new physics may lead to new particles andphysics phenomena such as supersymmetric particles and the existence of extra dimensions. New physics phenomena will be searched
for using ultra-precise measurements with 50 times more data thanin the Belle experiment.

One of the keys to finding new laws of physics is penguin decays.
In these decays the bottom quark in the B mesons goes through anintermediate state, in which it is instantly turned into another particle.
Using this characteristic, the effects of an unknown heavy particle canbe investigated; the Heisenberg uncertainty principle of quantum
mechanics allows a ultra-high energy state that may have existed in
the early universe to develop in the intermediate state of a penguin
decay.
Penguin decays are rare, occurring once in several tens of thousands of B mesons. However the Belle experiment proved that suchmeasurements are possible. Precise measurement of the propertiesof penguin decays are made possible by the large amount of datafrom the Belle II experiment and allow us to investigate the effect of
unknown laws of physics.
* It is called penguin decay because the diagram for the reaction resembles
a penguin.

History of the Universe

Earth’s formation

Formation of Stars

Formation of nuclei

Quark Soup
Inflation

BIG
BANG

Formation of atoms

KEKB
SuperKEKBWbB0Big Bang

d

10-43

10-38

10-33 10-13

10-6

1sec

3min

0.3 milion yr

a bilion yr

4.5 bilion years ago

13.8 bilion yr

t

g

s
s

s

d

Effect of particles
beyond the SM
f

g~

~
bR

d
(d RR
) 23

~
sR

g

K0

Feynman diagrams of the penguin decay for the standard model anda supersymmetry modelB e l l e I I E x p e r i m e n t Q u e s t fo r N e w Phy si c s

04

Belle II Detector
Measuring vertex points of particle decays

Vertex Detector (VXD)
The difference or asymmetry between the beam energiesof the electrons and positrons in the SuperKEKB accelerators
allows each produced B meson to travel about a hundred
micrometers before it decays. The distance traveled enables
a measurement of the decay time of the B meson.
The VXD ( VerteX Detector) consists of two types of semiconductor sensors (PXD: Pixel type semiconductor detector,
SVD: Silicon Vertex Detector equipped with strip sensors)for measuring the decay position of the B-meson. Position
information for charged particles is obtained from electricalsignals produced as they pass through a silicon sensor. After
upgrading the Belle detector to Belle II, the inner diameterof the beam pipe was reduced from 3 cm to 2 cm, and the
PXD (one pixel size is around 50 square microns) is mounted
nearby the beam pipe. These changes help to improve theresolution on the vertex measurement. The SVD sensor area
is also increased, from 88 mm radius to 135 mm in Belle II; thisgives an improvement of 30% in the detection efficiency for
Ks mesons decaying to pairs of π mesons.

Capturing the charged particle trajectory andmeasuring momentum

Central Drift Chamber (CDC)
The Central Drift Chamber of Belle II is a gaseous radiation detector, which is strung with many thin metal wires(56,576) inside a cylindrical volume with a 1.1 m radius. Whena charged particle passes inside the gas of the chamber, anelectric signal is detected in the nearest wire through an ionization process. The CDC can provide a precise momentum
determination for each reconstructed track and information
on particle identification using the signal size. The CDC iscarefully designed to minimize the chamber material andavoid multiple scattering: by using unplated aluminum field
wires and a helium based gas mixture.

05 B e l l e I I E x p e r i m e n t Q u e s t fo r N e w Phy si c s

Identifying electrons and measuring photons

Electromagnetic Calorimeter (ECL)The ECL is a detector consists of Thallium doped Cesium
Iodide crystals, which measures energies of photons andelectrons produced in interactions. Particles, which makeelectromagnetic interactions such as electrons or photons,
lose energy and emit light proportional to the energy wheninteracting inside the crystals. The energies of particles aremeasured by counting the amount of emitted light (photons) with photodiodes attached to the crystals. The flight
direction of particles are estimated from the position of theinteractions in the crystals. The ECL consists of 8736 crystalsof dimensions 5.5x5.5x30 cm3 surrounding the interaction
point. The Belle II ECL is one of the biggest total absorption
type calorimeters with homogeneous crystals, and weighs
about 43 tons in total.

Bending tracks of charged particles

Superconducting Solenoid Magnet
The Belle II detector is equipped with a superconducting
solenoid magnet just outside the ECL. It generates a magnetic field with a strength of 1.5 Tesla in the inside volume ofthe detector. A charged particle detected by CDC displays
a spiral-shape trajectory, being bent by Lorentz force dueto the magnetic field. We measure the momentum of theparticle from the radius of curvature. The coil of the superconducting solenoid magnet is constructed from a special
electric wire, which is made of a Titanium-Niobium alloy. The
wire becomes superconducting after being cooled down to-268°C and then a huge electric current of as much as 4160
amperes can flow without resistance in its narrow 3 mm x 3
mm cross section.

Identification of kaons and pions

Time of Propagation(TOP) Counter,
Aerogel RICH(ARICH) Counter© Ray. Hori / KEK

Identification of muons and detection ofneutral K L0 mesons
0L

K and Muon Detector (KLM)

This detector is used to detect muons, which penetrateeasily through most materials, and K 0L mesons, which areelectrically neutral and cannot be detected until they interact with matter. The detectors are placed in the gaps of theiron Belle II structure. For the barrel part, we use a detector,
with a gas mixture composed of argon and other gases thatfills the region between two glass plates. For the endcap partand the inner two layers of the barrel with high background
fluxes, scintillator strips and silicon photomultipliers are used.

When a fast charged particle passes through a transparent medium (radiator), Cherenkov light is emitted at a certain
angle (Cherenkov angle). The Cherenkov angle depends onvelocity of an incident particle and, therefore, measurementof its velocity allows us to identify particle mass (=particle
species) by combining information on the momentum provided from the CDC. Based on this technique, two new devices called TOP and ARICH perform precise particle identification, to distinguish pions and kaons (two types of charged
particles).
TOP counter consists of 2.5 m long x 45 cm wide quartz
plates 2 cm thick instrumented with micro-channel plate
photomultipliers (MCP-PMTs). Cherenkov light generated bya charged particle travels inside the quartz plate and reaches
the edge of the plate, where it is recorded by MCP-PMTs. Tiny
difference of Cherenkov angles between a pion and a kaon,
after bouncing at the internal faces in the quartz, results indifferences in time of propagation of the Cherenkov light.
The other device, the ARICH, uses silica aerogel, the lightest solid material in the world, as the Cherenkov radiator. The
Cherenkov photons are measured with newly developed
position -sensitive sensor "hybrid avalanche photo-detector(HAPD)", placed 20 cm downstream from a radiator. An arrayof HAPDs provides efficient detection of Cherenkov photons
together with accurate position information for each photon.
Using these measurements, the Cherenkov angle is reconstructed to identify a charged particle. Both the TOP and
ARICH enable us to detect a pion with 97% efficiency while
reducing the kaon misidentification probability to a low level.

B e l l e I I E x p e r i m e n t Q u e s t fo r N e w Phy si c s

06

Sub-detectors for the Belle II experiment
VXD (Vertex Detector)

A part of the pixel detector located closest to the collision point. The
surface of each sensor in the center contains nearly 400,000 pixels.

Assembly of silicon vertex detector (SVD).

CDC (Central Drift Chamber)

Installing one of the about 57,000 wires in the chamber.

Inserting the central drift chamber (CDC) into the Belle II structure.
The CDC is moved using the brown pillar, which passes through its center.

TOP (Time of Propagation) counter

Assembling a module of the TOP detector. The glued quartz bar isgently lifted and lowered into the support support structure.

Laser light reflects many times inside a quartz bar in the TOP before
reaching an array of photodetectors at the end of the bar.

ARICH (Aerogel RICH) counter

An aerogel block (white block on the left) and photo-detectors (onthe right) in a test experiment.

07 B e l l e I I E x p e r i m e n t Q u e s t fo r N e w Phy si c s

The aerogel blocks on the left and the photo-detectors on the rightare now being attached.

ECL (Electromagnetic Calorimeter)

8736 CsI(Tl) crystals readout by photodiodes are placed in a cylindricalarray around the collision point.

Lifting the endcap electromagnetic calorimeter before attaching it to
the BelleII detector.

KLM (K L0 and Muon Detector)

Part of the KLM detector. Bar-shaped plastic scintillators that emit light in response to the passage of charged particles are combined to form a single plate.

The installation of the KLM detector is shown. Yellow plate with scintillator is being inserted into the light blue structure.

First collisions at Belle II
Electron-positron collisions in the SuperKEKB accelerator were ﬁrst observed by the BelleII detector on April 26, 2018. This was an important milestone for the Belle II experiment,
which enabled us to start collecting physics data.

Event display of the Belle II detector showing the
decays of particles produced in an electron-positron
collision.

Researchers in the Belle II control room celebrating the observation of thefirst collision events.

B e l l e I I E x p e r i m e n t Q u e s t fo r N e w Phy si c s

08

Data processing using the latest information technology
A large number of reactions are observed by the Belle II detector. It is very important to select and record those data, and to build
and maintain the environment to perform the data analysis.

Trigger
The trigger is a system to select interesting events in realtime.
The trigger system is necessary to select B -anti B events because
there are many background events, which are generated by beamgas interactions, for example. The trigger system is always reading
special data sent from the detectors and send a trigger signal to the
DAQ within 4.5 microseconds when an interesting event occurs.

Data Acquisition System (DAQ)
The data acquisition system collects data from sub-detectorsof Belle II. The DAQ system also performs preprocessing of data torecord essential events effectively.
The data for each event are first readout by about 200 COPPER
modules separated into small parts. The event builder system gathers data from COPPERs and reconstructs the event data. The highlevel trigger system consisting of hundreds of computers then analyzes the data and selects essential events for more detailed physics
analysis.
The DAQ system can perform these processes and record
data at 30kHz, reducing the input data of 30 Gbytes/sec to about 1
Gbyte/sec.

Trigger System

Computing System
Analysis servers#tracks,
Detected energy,#hits etc.
àLevel1 Trigger Decision

Fast signals for trigger

Collision Event

GlobalDecision Logic

Level1 Trigger
Trigger logic
circuits

Start taking data

Storage system

Belle II Detector~200 COPPERs

Data Acquisition System

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Data Acquisition system

Computing System
The Belle II experiment is expected to produce
tens of peta-bytes (the equivalent of about 2 million
DVDs) of real and simulated data per year at the peakof its operation. In order to process this large amountof data and provide physics results in a timely fashion,
we adopt existing distributed computing technologies such as the “GRID” and “cloud”, connecting thecomputing resources of collaborating institutes in theworld via high speed networks.
Among them, the computing system in KEK plays
a key role for the success of the Belle II experiment.
Now, it has a roughly 10,000 cores of CPU, 13 PB of
disk space and a 70 PB tape storage system and has
started operation.

KEK : main center
GRID site

GRID site
Non-GRID site

Non-GRID site
GRID site

Distributed computing scheme of the Belle II experiments.

KEK central computing system

International Collaborative Experiment / Belle II Collaboration
The Belle II experiment is an international collaboration,
which involves about 1000 scientists from 26 countries/regions and includes about 117 universities/research institutes
as of November 2019.
Scientists gather three times a year for general collaboration meetings. A wide variety of video conferences areheld by scientists from all over the world on a daily basis. The
development of the Belle II sub-detectors, data collection
and physics analysis are carried out within this international
collaboration.

B e l l e I I E x p e r i m e n t Q u e s t fo r N e w Phy si c s

10Belle II collaboration

http://belle2.jp/
Twitter : https://twitter.com/belle2collab
Facebook :https://ja-jp.facebook.com/belle2collab/

Inter-University Research Institute Corporation

High Energy Accelerator Research Organization (KEK)
Institute of Particle and Nuclear Studies

1-1 Oho, Tsukuba, Ibaraki, 305-0801 Japan https://www.kek.jp/en/

2019. 11