SuperKEKB[1] 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) resonance making it a second-generation B-factory for the Belle II experiment. The accelerator is an upgrade to the KEKB accelerator, providing approximately 40 times higher luminosity,[2] due mostly to superconducting quadrupole focusing magnets.[1] The accelerator achieved "first turns" (first circulation of electron and positron beams) in February 2016.[3] First collisions occurred on 26 April 2018.[4] At 20:34 on 15 June 2020, SuperKEKB achieved the world’s highest instantaneous luminosity for a colliding-beam accelerator, setting a record of 2.22×1034 cm−2s−1.[5]

Description

The SuperKEKB design reuses many components from KEKB.[1] Under normal operation, SuperKEKB collides electrons at 7 GeV with positrons at 4 GeV[2] (compared to KEKB at 8 GeV and 3.5 GeV respectively). The centre-of-momentum energy of the collisions is therefore at the mass of the Υ(4S) resonance (10.58 GeV/c2).[6] The accelerator will also perform short runs at energies of other Υ resonances, in order to obtain samples of other B mesons and baryons.[1] The asymmetry in the beam energy provides a relativistic Lorentz boost to the B meson particles produced in the collision. The direction of the higher-energy beam determines the 'forward' direction, and that affects the design of much of the Belle II detector.

As with KEKB, SuperKEKB consists of two storage rings: one for the high-energy electron beam (the High Energy Ring, HER) and one for the lower energy positron beam (the Low Energy Ring, LER). The accelerator has a circumference of 3016 m with four straight sections and experimental halls in the centre of each, named "Tsukuba", "Oho", "Fuji", and "Nikko".[2] The Belle II experiment is located at the single interaction point in Tsukuba Hall.[7]

Luminosity

The target luminosity for SuperKEKB is 6.5×1035 cm−2s−1, this is 30 times larger than the luminosity at KEKB.[8] The improvement is mostly due to a so-called 'nano-beam' scheme, originally proposed[9] for the cancelled[10] SuperB experiment. In the nano-beam scheme at SuperKEKB, the beams are squeezed in the vertical direction and the crossing angle is increased, which reduces the area of the crossing.[1] The luminosity is further increased by a factor of two, due to a higher beam current than KEKB.[1] The focus and crossing angle is achieved by two new superconducting quadrupole magnets at the interaction point[1] that were installed in February 2017.[11]

On June 15, 2020, SuperKEKB set a new world record for the highest instantaneous luminosity for a colliding-beam accelerator: 2.22×1034 cm−2s−1. (On June 21, 2020, SuperKEKB broke its own record and achieved an instantaneous luminosity of 2.40×1034 cm−2s−1.) The previous world record of 2.14×1034 cm−2s−1 was achieved by LHC in 2018.[12][13]

See also

References

  1. 1 2 3 4 5 6 7 Ohnishi, Yukiyoshi; Abe, Tetsuo; Adachi, Toshikazu; Akai, Kazunori; Arimoto, Yasushi; Ebihara, Kiyokazu; Egawa, Kazumi; Flanagan, John; Fukuma, Hitoshi (1 January 2013). "Accelerator design at SuperKEKB". Progress of Theoretical and Experimental Physics. 2013 (3): 3A011. Bibcode:2013PTEP.2013cA011O. doi:10.1093/ptep/pts083.
  2. 1 2 3 "SuperKEKB". www-superkekb.kek.jp. Retrieved 21 May 2017.
  3. ↑ "Congratulations to SuperKEKB for "first turns" | CERN". home.cern. Retrieved 21 May 2017.
  4. ↑ "Electrons and Positrons Collide for the first time in the SuperKEKB Accelerator". kek.jp. Retrieved 26 April 2018.
  5. ↑ "SuperKEKB collider achieves the world's highest luminosity". kek.jp. Retrieved 26 June 2020.
  6. ↑ Patrignani, C.; Group, Particle Data (2016). "Review of Particle Physics". Chinese Physics C. 40 (10): 100001. Bibcode:2016ChPhC..40j0001P. doi:10.1088/1674-1137/40/10/100001. ISSN 1674-1137.
  7. ↑ "Belle II Experiment". belle2.jp. Retrieved 22 May 2017.
  8. ↑ "SuperKEKB". www-superkekb.kek.jp. Retrieved 23 January 2021.
  9. ↑ SuperB Collaboration (4 September 2007). "SuperB: A High-Luminosity Asymmetric e+ e- Super Flavor Factory. Conceptual Design Report". arXiv:0709.0451 [hep-ex].
  10. ↑ "Italy cancels €1bn SuperB collider - physicsworld.com". physicsworld.com. Retrieved 28 May 2017.
  11. ↑ "Belle II Experiment on Twitter". Twitter. Retrieved 28 May 2017.
  12. ↑ "KEK reclaims luminosity record". CERN Courier. 30 June 2020. Retrieved 8 September 2020.
  13. ↑ "SuperKEKB collider achieves the world's highest luminosity". Interactions. Retrieved 8 September 2020.
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