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The BelleII experiment is a
particle physics
Particle physics or high energy physics is the study of fundamental particles and forces that constitute matter and radiation. The fundamental particles in the universe are classified in the Standard Model as fermions (matter particles) an ...
experiment designed to study the properties of
B meson
In particle physics, B mesons are mesons composed of a bottom antiquark and either an up (), down (), strange () or charm quark (). The combination of a bottom antiquark and a top quark is not thought to be possible because of the top quark' ...
s (heavy particles containing a
beauty quark) and other particles. BelleII is the successor to the
Belle experiment
The Belle experiment was a particle physics experiment conducted by the Belle Collaboration, an international collaboration of more than 400 physicists and engineers, at the High Energy Accelerator Research Organisation (KEK) in Tsukuba, Ibarak ...
, and commissioned at the
SuperKEKB
SuperKEKB is a particle collider located at KEK (''High Energy Accelerator Research Organisation'') in Tsukuba, Ibaraki Prefecture, Japan. SuperKEKB collides electrons with positrons at the centre-of-momentum energy close to the mass of the Υ(4S) ...
accelerator complex at
KEK
, known as KEK, is a Japanese organization whose purpose is to operate the largest particle physics laboratory in Japan, situated in Tsukuba, Ibaraki prefecture. It was established in 1997. The term "KEK" is also used to refer to the laboratory ...
in
Tsukuba
is a city located in Ibaraki Prefecture, Japan. , the city had an estimated population of 244,528 in 108,669 households and a population density of 862 persons per km². The percentage of the population aged over 65 was 20.3%. The total ar ...
,
Ibaraki prefecture
is a Prefectures of Japan, prefecture of Japan located in the Kantō region of Honshu. Ibaraki Prefecture has a population of 2,871,199 (1 June 2019) and has a geographic area of . Ibaraki Prefecture borders Fukushima Prefecture to the north, ...
,
Japan
Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...
. The BelleII detector was "rolled in" (moved into the collision point of SuperKEKB) in April 2017. BelleII started taking data in early 2018.
Over its running period, BelleII is expected to collect around 50 times more data than its predecessor mostly due to a 40-fold increase in
an instantaneous luminosity provided by
SuperKEKB
SuperKEKB is a particle collider located at KEK (''High Energy Accelerator Research Organisation'') in Tsukuba, Ibaraki Prefecture, Japan. SuperKEKB collides electrons with positrons at the centre-of-momentum energy close to the mass of the Υ(4S) ...
as compared to the previous
KEKB KEKB may refer to:
* KEKB (accelerator)
* KEKB (FM)
KEKB is a radio station serving Grand Junction, Colorado and its vicinity with a country music format. This station broadcasts on FM frequency 99.9 MHz and is under ownership of Townsquare ...
accelerator.
Physics program
Many interesting analyses of the
Belle and
BaBar
Babar ( ur, ), also variously spelled as Baber, Babur, and Babor is a male given name of Pashto, and Persian language, Persian origin, and a popular male given name in Pakistan. It is generally taken in reference to the Persian language, Persian ...
experiments were limited by statistical uncertainties, which was the main motivation to build a new generation of B-factory - Belle II.
The target dataset is 50
ab−1 at BelleII
compared to 988
fb−1 (with 711fb
−1 at the
Υ(4S) energy) at Belle. The dataset of good runs from Belle II before Long shutdown 1 was 424
fb−1 (with 363fb
−1 at the
Υ(4S) energy.)
This immense dataset would allow studies of rare physics processes, which were out of reach for the previous e e experiments and improve precision on the already measured physics observables.
The physics program of Belle II includes the studies of the following particles or processes:
*
B-meson
In particle physics, B mesons are mesons composed of a bottom antiquark and either an up (), down (), strange () or charm quark (). The combination of a bottom antiquark and a top quark is not thought to be possible because of the top quark's sh ...
*
Charm meson
*
Bottomonium
In particle physics, quarkonium (from quark and -onium, pl. quarkonia) is a flavorless meson whose constituents are a heavy quark and its own antiquark, making it both a neutral particle and its own antiparticle.
Light quarks
Light quarks ( up ...
*
Charmonium
In particle physics, quarkonium (from quark and -onium, pl. quarkonia) is a flavorless meson whose constituents are a heavy quark and its own antiquark, making it both a neutral particle and its own antiparticle.
Light quarks
Light quarks ( up ...
*
Tau-lepton
*
Dark sector
''Dark Sector'', stylized as ''darkSector'' is a third-person shooter video game developed by Digital Extremes for the Xbox 360, PlayStation 3 and Microsoft Windows.
The game is set in the fictional Eastern Bloc country of Lasria, and centers o ...
* Low-multiplicity processes
The majority of the Belle II dataset will be recorded at
Upsilon(4S) center-of-mass energy, while a small portion of it will be taken at Upsilon(5S) and as energy scans.
Detector composition
Belle II is a general purpose high-energy particle physics detector with almost full solid angle coverage. It has a cylindrical shape to cover the e e collisions happening on the central axis of the detector. The detector is asymmetric in beam direction, because the initial energy of the electron beam is larger than the positron beam. Much of the original Belle detector has been upgraded
to cope with the higher instantaneous luminosity provided by the
SuperKEKB
SuperKEKB is a particle collider located at KEK (''High Energy Accelerator Research Organisation'') in Tsukuba, Ibaraki Prefecture, Japan. SuperKEKB collides electrons with positrons at the centre-of-momentum energy close to the mass of the Υ(4S) ...
accelerator.
The main components are the following, from the innermost to the outermost systems.:
*
Beryllium
Beryllium is a chemical element with the symbol Be and atomic number 4. It is a steel-gray, strong, lightweight and brittle alkaline earth metal. It is a divalent element that occurs naturally only in combination with other elements to form mi ...
beam pipe at a minimum radius of 10 mm from the beam
* PiXel Detector (PXD) is composed from two cylindrical layers of depleted
field effect transistor
The field-effect transistor (FET) is a type of transistor that uses an electric field to control the flow of current in a semiconductor. FETs (JFETs or MOSFETs) are devices with three terminals: ''source'', ''gate'', and ''drain''. FETs control ...
(DEPFET) pixels, provide as precise position measurement of the charged particle trajectories. First layer is only 14 mm away from the beam. So far, only two ladders of the second PXD layer have been installed.
* Silicon Vertex Detector (SVD) - 8 layers of
silicon strip sensors arranged in cylindrical barrel and an inclined endcap towards electron direction for better coverage. The SVD and PXD detectors sometimes are referred as VerteX Detector (VXD). The total VXD size has been tuned to cover most of the typical
travel distance.
* Central Drift Chamber (CDC) is a
wire chamber
A wire chamber or multi-wire proportional chamber is a type of proportional counter that detects charged particles and photons and can give positional information on their trajectory, by tracking the trails of gaseous ionization. was located via ...
central tracking system, which provides the measurement of momentum and charge of the charged particles produced by the collisions as well as
particle type identification (PID) via their energy depositions.
*
Aerogel
Aerogels are a class of synthetic porous ultralight material derived from a gel, in which the liquid component for the gel has been replaced with a gas, without significant collapse of the gel structure. The result is a solid with extremely low ...
ring-imaging Cherenkov detector The ring-imaging Cherenkov, or RICH, detector is a device for identifying the type of an electrically charged subatomic particle of known momentum, that traverses a transparent refractive medium, by measurement of the presence and characteristics ...
(aRICH) provides PID information about the charged particle going into the forward direction.
* Time Of Propagation (TOP) counter consisting of quartz bars utilising
totally internally reflected Cherenkov photons and measuring the time of propagation, which is used for PID.
* Electromagnetic
calorimeter
A calorimeter is an object used for calorimetry, or the process of measuring the heat of chemical reactions or physical changes as well as heat capacity. Differential scanning calorimeters, isothermal micro calorimeters, titration calorimete ...
(ECL) a highly segmented array of thallium-doped caesium iodide
CsI(
Tl) crystals assembled in a projective geometry to measure energies of the neutral final state particles such as photons,
and neutrons, as well as PID.
* Superconducting solenoid, which provides a 1.5 tesla magnetic field, which bends the trajectories of the final state charged particles to measure their charge and momentum.
*
and
Muon
A muon ( ; from the Greek letter mu (μ) used to represent it) is an elementary particle similar to the electron, with an electric charge of −1 '' e'' and a spin of , but with a much greater mass. It is classified as a lepton. As wi ...
detector (KLM) used to identify
energy depositions as well as for
muon
A muon ( ; from the Greek letter mu (μ) used to represent it) is an elementary particle similar to the electron, with an electric charge of −1 '' e'' and a spin of , but with a much greater mass. It is classified as a lepton. As wi ...
PID.
Timeline
The Belle II experiment data taking is separated into three phases:
* Phase I — completed Feb–June 2016: SuperKEKB commissioning to characterize the beam environment;
* Phase II — started early 2018, running without the nominal inner silicon-based VXD tracking system to measure the beam-induced background radiation in the innermost tracking system. During this phase the VXD subdetector has been replaced by Beam Exorcism for a Stable ExperimenT II or BEAST II system which was used to test various tracking technologies and a total integrated luminosity of 500 pb
has been collected;
* Phase III — started 2019: data taking with the complete Belle II detector;
On November 22, 2018, the Belle II detector was completed with the installation of the VerteX Detector (VXD). On March 25, 2019, the first collisions of the actual physics program were detected.
On 15 June 2020, the
SuperKEKB
SuperKEKB is a particle collider located at KEK (''High Energy Accelerator Research Organisation'') in Tsukuba, Ibaraki Prefecture, Japan. SuperKEKB collides electrons with positrons at the centre-of-momentum energy close to the mass of the Υ(4S) ...
reached an instantaneous luminosity of — surpassing the
LHC
The Large Hadron Collider (LHC) is the world's largest and highest-energy particle collider. It was built by the European Organization for Nuclear Research (CERN) between 1998 and 2008 in collaboration with over 10,000 scientists and hundre ...
's record of set with proton–proton collisions in 2018. A few days later, SuperKEKB pushed the luminosity record to . In June 2022 the luminosity record was nearly doubled to .
Scientific personnel
The Belle II experiment is being governed by Belle II Collaboration, which is an international worldwide scientific community.
The Belle II Collaboration has designed, produced, assembled and is currently operating the Belle II experiment. The collaboration handles the collision data recorded at the experiment, performs the data analysis and delivers the results in form of scientific journal articles, conference talks, etc.
As on May 30, 2019, it included 970 members from 112 institutes and 26 countries around the globe.
Experiment software
In October 2021 the Software development team within the Belle II Collaboration has published Belle II Analysis Software Framework or basf2 as
open-source software
Open-source software (OSS) is computer software that is released under a license in which the copyright holder grants users the rights to use, study, change, and distribute the software and its source code to anyone and for any purpose. Op ...
on
GitHub
GitHub, Inc. () is an Internet hosting service for software development and version control using Git. It provides the distributed version control of Git plus access control, bug tracking, software feature requests, task management, continuous ...
.
This is the main package used to simulate, reconstruct and analyse the recorded collision events at the Belle II experiment and there are several other separate satellite packages, used for
DAQ, computation of the systematic uncertainties, etc.
The
backend of the reconstruction and analysis libraries are written in
C++
C++ (pronounced "C plus plus") is a high-level general-purpose programming language created by Danish computer scientist Bjarne Stroustrup as an extension of the C programming language, or "C with Classes". The language has expanded significan ...
,
while the analysis steering and
facade are implemented in
Python
Python may refer to:
Snakes
* Pythonidae, a family of nonvenomous snakes found in Africa, Asia, and Australia
** ''Python'' (genus), a genus of Pythonidae found in Africa and Asia
* Python (mythology), a mythical serpent
Computing
* Python (pro ...
language.
To coordinate the software development, the Belle II Collaboration uses industrial collaboration tools such as
Atlassian Jira,
Confluence
In geography, a confluence (also: ''conflux'') occurs where two or more flowing bodies of water join to form a single channel. A confluence can occur in several configurations: at the point where a tributary joins a larger river (main stem); o ...
and
git
Git () is a distributed version control system: tracking changes in any set of files, usually used for coordinating work among programmers collaboratively developing source code during software development. Its goals include speed, data inte ...
-based
BitBucket
Bitbucket is a Git (software), Git-based source code repository (version control), repository shared web hosting service, hosting service owned by Atlassian. Bitbucket offers both commercial plans and free accounts with an unlimited number of p ...
service.
See also
*
B-factory In particle physics, a B-factory, or sometimes a beauty factory, is a particle collider experiment designed to produce and detect a large number of B mesons so that their properties and behavior can be measured with small statistical uncertainty. T ...
*
– oscillation
*
LHCb experiment
The LHCb (Large Hadron Collider beauty) experiment is one of eight particle physics detector experiments collecting data at the Large Hadron Collider at CERN. LHCb is a specialized b-physics experiment, designed primarily to measure the paramete ...
Image gallery
References
External links
BelleII public webpageBelle{{nbspII collaboration webpageGerman Belle II webpageA Search for New Physics - The Belle II ExperimentInside Japan's big physics - Belle IIRecord for SuperKEKB-Belle IIexperiment on
INSPIRE-HEP INSPIRE-HEP is an open access digital library for the field of high energy physics (HEP). It is the successor of the Stanford Physics Information Retrieval System (SPIRES) database, the main literature database for high energy physics since the 1970 ...
*Record fo
SuperKEKBproject on
INSPIRE-HEP INSPIRE-HEP is an open access digital library for the field of high energy physics (HEP). It is the successor of the Stanford Physics Information Retrieval System (SPIRES) database, the main literature database for high energy physics since the 1970 ...
Particle experiments
B physics