Belle II Detector

Tutorial 1
1/11
Rishabh Singh Sisodia IIT BHU
Belle II Detector

B factory
2/11
B-factory, or 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. Tauons and D mesons are also produced at B-factories.
Three "next generation" B-factories were to be built in the 2010s and 2020s:
• SuperB near Rome in Italy (SuperB was canceled);
• SuperPEP-II, an upgrade to the PEP-II accelerator (never acted upon);
• Belle II, an upgrade to Belle (started taking data in 2018)
• Quark content of B mesons.

Introduction
3/11
The Belle experiment was a particle physics experiment conducted by the Belle
Collaboration, at the High Energy Accelerator Research Organization (KEK)
in Tsukuba, Japan. The experiment ran from 1999 to 2010.
The Belle II experiment is an upgrade of Belle that was approved in June 2010. It started
operation in 2018. Belle II is located at SuperKEKB (an upgraded KEKB accelerator).

Terminology
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1) Luminosity : Luminosity is a measure of how efficiently a particle
accelerator produces collision events. The luminosity of the accelerator
determines the rate at which these collisions take place.
2) Particle Identification: The goal of the particle identification or PID is to
identify a type of a particle associated with a track using responses from
different systems.
3) Trigger System: A trigger is a system that uses simple criteria to rapidly
decide which events in a particle detector to keep when only a small
fraction of the total can be recorded. Trigger systems are necessary due to
real-world limitations in data storage capacity and rates.
4) Time Resolution: The time resolution is defined by how accurately
the time at which a particle crossed the detector can be determined.
5) Spatial Resolution: This is the measure of the accuracy to which the
position of the particle trajectory may be localized.

Belle II Detector
5/11
Image source: https://www.phy.olemiss.edu/HEP/belle2/index.html

Vertex Detector (VXD)
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The Belle II vertex detector is comprised of two sub detectors:
1) The Silicon Pixel Detector (PXD)
2) The Silicon Vertex Detector (SVD)
• The PXD makes up the inner two layers at 𝑟 = 14 mm and 𝑟 = 22 mm radii.
• The SVD constitutes the four outer layers of the VXD, with double-sided silicon strip ladders
at radii of 39 mm, 80 mm, 104 mm, and 135 mm, respectively.
Image source: M. Friedl et al, The Belle II Silicon Vertex Detector, Physics Procedia 37 ( 2012 ) 867 – 873

The Silicon Pixel Detector (PXD)
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The pixel detector contains 65 million pixels, allowing
it to track the paths of particles emerging from the
Collision with extreme accuracy.
This is based on Depleted field effect transistors
(DEPFET) technology.
Each layer is spilt into segments having little silicon sensors. When a charged particle passes
through it gives enough energy for electrons to be ejected from the silicon atoms, creating
electron-hole pairs. Each pixel uses an electric current to collect these charges on the
surface as a small electric signal. Knowing which pixels have been touched allows us to
deduce the particle's trajectory.
However, being so close to the collision means that the number of particles passing
through is huge; the rate of particles received 8cm from the beam line will be around 10
million particles per square centimeter per second.
Image source: https://cms.cern/detector/identifying-tracks/silicon-pixels

Silicon Vertex Detector (SVD)
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The SVD is made of Double-Sided Silicon micro-strip Detectors (DSSDs).
A silicon strip detector is an arrangement of strip like shaped implants acting as charge
collecting electrodes.
The innermost SVD layer has small rectangular sensors .
The other three layers are composed of two types of sensors:
a) large rectangular sensors (named barrel sensors)
b) slanted sensors with a trapezoidal shape, which improves acceptance and precision for
forward boosted particles.
Image source: https://web.infn.it/Belle-II/index.php/detector/svd

Central Drift Chamber (CDC)
9/11
 The CDC reconstructs the trajectories of charged particles to precisely determine their
momenta and provides particle identification in the low momentum region. In addition, it
also generates trigger information for charged particles.
 The Belle-II CDC is cylindrical a large volume gas drift chamber with small drift cells with
14336 sense wires, sense wires arranged in 56 layers, either in axial orientation (aligned
with the solenoidal magnetic field) or stereo (skewed with respect to the axial wires).
 The chamber has smaller drift cells than the one used in Belle to be operate at high event
rates.
 The chamber gas is comprised of a He-C2H6 50:50 mixture.
 By combining information from axial and stereo layers it is possible to reconstruct a full 3D
helix track.

Principle of Drift Chamber
10/11
 The drift chamber works on ionization.
 As a charged particle passes through the gas, it ionizes the gas atoms.
 These atoms then feel the force of an electric potential and drift towards wires that
register a signal.
 Knowledge of the drift time (time from ionization to signal) and the drift velocity in the gas
allows one to determine the position of the incoming particle.
 Stacking several drift chambers on top of one another allows one to resolve a track left by
the particle, which can give information such as incoming angle.
 If there is an applied magnetic field, the track will curve and one can then determine the
momentum of the particle.

Plan for Next Tutorial
11/11
 Particle identification system (TOP and ARICH)
 Electromagnetic Calorimeter (ECL)
 KL- Muon Detector (KLM)
 Trigger System

Belle II Detector

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