B factory

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As a B-factory (ger .: B-factory , rarely: beauty factory ) refers to a specific particle accelerator , specializing in the production of B mesons specializes.

principle

For this purpose, particles (usually electrons and positrons ) are accelerated on circular orbits running in opposite directions to an energy of exactly 10.580  GeV in the center of gravity of the particles. At this energy a resonance occurs, since it corresponds to the mass of the ϒ (4S) , an excited state of the connection of a b-quark with an anti-b-quark. It is thanks to a welcome coincidence that this energy is just 0.021 GeV above that required for the production of two B mesons. The decay of the ϒ (4S) into two B mesons (more precisely: B u, d mesons, since the mass of the B c, s mesons is already too high) is therefore strongly preferred.

An experimental advantage also proves to be the possibility of being able to “switch off” the production of the B mesons by reducing the collision energy relatively small. This makes it possible to study the background of other, unwanted particles, which is inevitable in experiments in particle physics .

Relation to the time-dependent CP asymmetry

Since the energy of the resonance is only slightly above the rest energy of the two B mesons, they hardly get any kinetic energy , so they are almost at rest. To observe interesting effects, such as time-dependent CP asymmetries , it is often necessary that the B mesons move quickly. They get a so-called Lorentz boost by choosing the two colliding beam energies asymmetrically. As a result, the center of gravity of the system is not at rest, so the B mesons move with it in the desired direction, making it possible to measure decay times by reconstructing the decay location. This is of crucial importance for the measurement of time-dependent CP asymmetries in the system of the neutral B mesons in the case of decays into CP eigenstates , since they are in an entangled state due to their generation by both B mesons :

The zero point in time is thus given by the decay of a B meson into a non- CP eigenstate. Since the decay into a CP eigenstate may have taken place beforehand, negative time differences are also measured in the experiment .

Precision measurements of this kind at B-factories showed that the observable CP violation can be traced back to a complex phase of the CKM matrix . Because of this confirmation of their prediction, Makoto Kobayashi and Toshihide Maskawa were awarded the 2008 Nobel Prize in Physics .

Concrete B factories

Examples of B-factories are the PEP-II accelerator at the Stanford Linear Accelerator Center (SLAC) in Stanford ( USA ), which is used for the BaBar experiment , and the KEKB accelerator at the KEK in Tsukuba ( Japan ), which is used for the Belle experiment is working. Both B factories have ceased operations (PEP II in 2008, KEKB in 2010).

The SuperB factory SuperKEKB at KEK is under construction in Japan, where the successor to the Belle experiment, Belle II , started taking data in April 2018.

Individual evidence

  1. ^ Electrons and Positrons Collide for the first time in the SuperKEKB Accelerator. (No longer available online.) KEK Inter-University Research Institute Corporation, High Energy Accelerator Research Organization, April 26, 2018, archived from the original on July 16, 2018 ; accessed on July 16, 2018 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot / www.kek.jp

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