BaBar experiment

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BaBar is the name for an experiment in high energy physics .

It is set up at the Stanford Linear Accelerator Center near Stanford University in California . An essential aim of the experiment is to find out about the cause of the so-called CP violation , which is an essential prerequisite for the almost complete absence of antimatter in the universe ( baryon asymmetry ). The observable CP violation, a violation of the symmetry of charge and parity , is investigated in the decay of B mesons and compared with the predictions of the Standard Model of particle physics . The CP violation shows up in the different behavior of B and anti-B mesons.

The BaBar collaboration consists of more than 600 physicists and engineers from 72 institutes in 12 countries. The experiment was named after " B a nd B b ar " so and , with the latter for the anti-B is and is referred to in English as B-bar.

BaBar and the electron - positron - storage ring PEP-II including pre-accelerators represent a so-called B-factory . The B-factory at SLAC was established in 1993 as an initiative of US President Bill Clinton . The data taking of the Babar experiment began in May 1999. Almost at the same time as the Belle competition experiment at KEKB in Japan ( KEK ), after a short start-up time in summer 2002, it was possible to detect CP violation in the system of neutral mesons with a significance of more than 5σ . To date (August 2016) no contradiction to the predictions of the Standard Model has been found. CP violation was detected for the first (and only before BaBar) time in the system of neutral kaons .

The BaBar detector is an (almost) 4π particle detector with a typical shell structure. A silicon vertex tracker inside ensures a vertex resolution of better than 60 µm. A multi-wire drift chamber reconstructs impulses of charged particles.

A special feature is an imaging Cherenkov detector with a new design that requires very little material in the active area of ​​the detector. In this way, pions and kaons can be distinguished from one another very efficiently in all momentum ranges.

Further out, a CsI (Tl) calorimeter reconstructs the energies of neutral particles.

The detector is surrounded by muon chambers. Another special feature of the B-Factory is the asymmetrical energy of the two electron (9.0 GeV) and positron beams (3.1 GeV) which are brought to collision inside the detector. The result is that the center of gravity system moves relative to the detector. This enables the reconstruction of different B decay vertices and thus the determination of the difference in the decay times of the B mesons generated in pairs, which is a prerequisite for measuring time-dependent CP asymmetries in a B factory.

The data acquisition was successful and was discontinued in April 2008. PEP-II was able to deliver an integrated luminosity of approx. 557 fb −1 , with a maximum instantaneous luminosity of 1.2 · 10 34 cm −2 s −1 being achieved. Relevant for B physics was the data taking on the Υ (4S) resonance, in which an integrated luminosity of 424 fb −1 was recorded. So far (as of February 2013) the results of the data analyzes have appeared in more than 500 publications in refereed journals. The analysis of the BaBar data continues even after the data collection has ended. In 2012, direct evidence of the T injury was obtained.

Web links

Individual evidence

  1. ^ B. Aubert et al .: Measurement of the CP-Violating Asymmetry Amplitude sin2β . In: Phys. Rev. Lett 89, 201802 (2002) arxiv : hep-ex / 0207042 .
  2. ↑ JP Lees et al .: Time-Integrated Luminosity Recorded by the BABAR Detector at the PEP-II e + e- Collider , (2013), arxiv : 1301.2703 .
  3. ↑ List of BaBar publications
  4. ↑ JP Lees et al. a. Observation of Time-Reversal Violation in the B0 Meson System , Phys. Rev. Lett., Volume 109, 2012, p. 211801
  5. ↑ Dirk Eidemüller time asymmetry directly demonstrated for the first time , Pro Physik, November 2012