Belle II experiment

from Wikipedia, the free encyclopedia

Coordinates: 36 ° 9 ′ 28 ″  N , 140 ° 4 ′ 30 ″  E

The opened Belle II detector before the installation of the inner detectors for track reconstruction.

The Belle II experiment is located at the Japanese Research Center for Particle Physics KEK and, like the previous Belle experiment , deals with B physics . In contrast to experiments such as ATLAS and CMS at the LHC at CERN , the Belle II experiment does not work in the high-energy sector, but in the high-precision sector. This means that new physics beyond the standard model is not primarily sought through the direct generation of new particles at high energies , but rather through the exact measurement of rare processes, in Belle II primarily through the investigation of the decays of B mesons . To achieve this, the super is KEKB - accelerator at an energy of 10,580  GeV in the center-of- operated what the mass of the Y (4S) corresponds to resonance. For this reason, SuperKEKB and Belle II are also referred to as B-Fabrik .

history

The Japanese government decided at the end of 2009 to expand the Belle experiment and the KEKB accelerator into Belle II and SuperKEKB. One of the reasons for this was the successful operation of the Belle experiment, with the culmination of the discovery of oscillations of B mesons and the measurement of time-dependent CP violation . This discovery led to the award of the 2008 Nobel Prize in Physics to Makoto Kobayashi , the former director of the INPS (Institute of Particle and Nuclear Studies) department at KEK, as well as Yōichirō Nambu and Toshihide Masukawa .

In February 2016, the first particle beams circulated in SuperKEKB. The first collision data was collected from April to July 2018, albeit with a low collision rate and still without the innermost lane detectors.

On March 25, 2019, the almost complete detector (only half of the pixel detector is installed) was able to record the first collisions of the actual physics program. The installation of the full pixel detector is planned for 2020.

SuperKEKB

SuperKEKB works with the same center of gravity energies as KEKB, but should achieve a collision rate that is 40 times higher. This should achieve a total of 50 a b −1 integrated luminosity within a few years , 50 times more than with KEKB (0.99 from −1 ). The collision rate is increased on the one hand by increasing the flow of electrons and positrons in the beam tubes, on the other hand by the strong focus of the rays at the point of interaction to 10 µm in the horizontal and 50 nm in the vertical direction. This configuration is known as the nano-beam configuration.

Physics program

The planned studies largely correspond to the program of the previous experiment, but will achieve significantly better measurement accuracy. In addition, many sizes are to be measured for the first time. In contrast to KEKB, the energies of the particles are reduced from 8 GeV to 7 GeV in the high-energy ring (HER) for electrons, or increased from 3.5 GeV to 4 GeV in the low-energy ring (LER) for positrons. This will reduce the boost from 0.43 to 0.28. In order to be able to continue investigating the time-dependent CP violation in spite of this change, Belle II, in contrast to Belle, is equipped with a pixel detector in the immediate vicinity of the interaction point. The pixel detector is the German contribution to the Belle II detector.

Detector structure

While the calorimeter was largely taken over by Belle, the other elements of the detector were rebuilt. The innermost part is a two-layer pixel detector based on DePFET technology. The detector consists of about eight million pixels , with each pixel only about 50 × 75 µm 2 in size. Data are read out with a repetition frequency of 50 kHz, which leads to a very high amount of data to be processed of more than 20 gigabytes per second; the data is processed in real time using ASIC , FPGA and optical technology (for data transfer with high bandwidth). Vertex coordinates of traces from the decay of B mesons can thus be determined to an accuracy of 25 µm, which is about a factor of 2 more precise than in the Belle experiment. The pixel detector is enclosed by a four-layer strip detector (Double Sided Silicon Strip Detector), which, like the pixel detector and the drift chamber further out, is used for track reconstruction. Cherenkov radiation , which is produced in two systems, is used for particle identification : The end caps use a RICH system, in the central area the photons are guided in quartz blocks and the flight time to the end of the blocks is measured. The position or the time of flight of the photons makes it possible to calculate the emission angle of the radiation and thereby determine the type of particle. The already mentioned calorimeter for determining the energies of the generated particles is located around these detectors . This is enclosed by a superconducting solenoid magnet , which generates an approximately homogeneous magnetic field of 1.5  T in the direction of the beam, which is required to determine the impulses of the charged particles. The outermost part is made up of detectors that are specially designed to measure muons and long-lived kaons (K L ).

Web links

Individual evidence

  1. ↑ Congratulations to SuperKEKB for "first turns". March 2, 2016, accessed on November 23, 2017 .
  2. ↑ First collisions at Belle II. April 25, 2018, accessed July 10, 2018 .
  3. ^ Kick-off of the Belle II Phase 3 Physics Run. March 25, 2019, accessed March 26, 2019 .
  4. ↑ B-factory goes into series production. March 25, 2019, accessed April 5, 2019 .
  5. ↑ SuperKEKB luminosity projection. Retrieved November 23, 2017 .
  6. ↑ SuperKEKB Project. Retrieved November 23, 2017 .
  7. ↑ SuperKEKB collider. Retrieved November 19, 2018 .
  8. ^ Belle II: search for physics beyond LHC. Retrieved November 23, 2017 .
  9. ^ The Belle II Detector. Retrieved November 23, 2017 .