The ten-meter high and wide, 1,400-ton, Belle II experiment consists of seven detector systems. Their task is to record, read-out and analyze the particles, and their decay products, created when electrons collide with their anti-particles, the positrons.
Here, B mesons, short-lived pairs of quarks with one bottom antiquark each, are created. Their decay period is around a trillionth of a second. The scientists are searching for differences in the decay structures of the B mesons and their anti-particles – a possible indication of the disappearance of antimatter in the universe.
Radiation shielding for the vertex detector
It is crucial to the measurements that the decay points of the B meson pairs created during electron-positron disintegration are precisely determined. The decay tracks are exactly measured by the vertex detector in the core area of the Belle II experiment.
The scientists hope to use BEAST to understand how the particle collisions will later affect the vertex detector. "If the radiation level is too high, the vertex detector may become permanently blind," explains Hans-Günther Moser of the Max Planck Institute for Physics. "The accelerator's particle beam must therefore be adjusted such that as few radioactive background particles as possible hit the pixels in the detector."
The BEAST test detector, named after the fairy tale "Beauty and the Beast (La belle et la bête)", contains only a part of all the components. With regard to the sensor modules used to detect locations to the precise pixel, BEAST covers only around one-tenth of the entire solid angle (360 degrees). "Despite its reduced configuration, BEAST delivers sufficient data to precisely adjust the detectors and the particle beam," explains Moser.
New beam scheme testing in SuperKEKB
In addition, further instruments used exclusively during the test phase are installed in BEAST. Before they collide in BEAST, the electrons and positrons are brought up to speed in the modernized SuperKEKB accelerator.
However, when BEAST testing is carried out, SuperKEKB will not yet be operating at full capacity, as Moser explains: "The accelerator will initially generate 25 B/anti-B meson pairs, similar to its predecessor. During subsequent operations, this will be 40 times as many."
This is possible due to a very much greater luminosity (number of collisions per second and unit area), for which the accelerator will be upgraded with new technologies. These upgrades are collectively referred to as the nano-beam scheme.
A critical moment in BEAST installation was connecting the beam pipe to the quadrupole magnets in the accelerator. They focus the beams to nanometer cross-sections. Here, the vacuum-tight connection developed by DESY, which can be remotely closed and opened again from the outside, is deployed for the first time.
BEAST testing will continue until July 2018. The scientists are hoping to gather as much information as possible to subsequently successfully operate the SuperKEKB/Belle II team. Installation of the vertex detector is planned for summer 2018.
Vertex detector design
The Belle II detector comprises seven different sub-detectors. One of these is the vertex detector. It, in turn, incorporates the inner pixel detector, the development of which was headed by the Max Planck Institute for Physics. It can identify the exact decay location of the B mesons - these decay after travelling 0.1 millimeters in around one-trillionth of a second.
The next layer is represented by the silicon vertex detector, used to reconstruct the trajectories of charged particles created when the B mesons decay. In addition, this instrument provides information on how much energy the particles lose on their way through the detector - and therefore on which particles are involved.
The vertex detector is completed by the central drift chamber, with which the tracks of the charged particles can be calculated, among other things.
What insights will the Belle II detector provide?
With the Belle II detector, scientists are looking for answers to the question of, among other things, why there is matter in the universe, but practically no antimatter. During the Big Bang, both forms of matter were created in equal parts.
When particles meet their anti-particles, they undergo mutual annihilation. Why matter dominates today's universe cannot yet be explained. The MPI for Physics is one of 12 German and 106 international institutes from 25 countries jointly developing and building the Belle II detector, and subsequently analyzing the data.