SuperKEKB

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 15th 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 (10.57 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 the 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 3016m 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 8×1035 cm−2s−1, this is 40 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. 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. "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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