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Giant Particle Physics Projects Discover New Decay Mode of Sub-Atomic Particle

Combining their experimental data in a collaboration including EPFL, two giant particle physics projects have discovered a new decay mode of a sub-atomic particle, which has been looked for during the past 25 years. This discovery severely constrains physics beyond the Standard Model.

Event display of a candidate B0s particle decaying into two muons in the LHCb detector. (Image: LHCb/CERN)

Combining previous data, the Large Hardron Collider “beauty” (LHCb) and the Compact Muon Solenoid (CMS) experiments at CERN have made the first ever observation of the very rare decay of the Bs0 meson into two muon particles. The data also hints to a similar, but even rarer decay of its “cousin”, the B0 meson, into two muons. These very rare decays are sensitive probes of exotic new physics theories like supersymmetry. This groundbreaking discovery, which involves significant contributions from EPFL among other institutions, is published in Nature.

The Bs0 and B0 particles are mesons: non-elementary, unstable subatomic particles made up of a quark and an antiquark. These are bound together by one of the four fundamental forces, the “strong interaction”. Mesons are produced naturally in the interactions of cosmic rays, or artificially in high-energy collisions such as those performed in particle accelerators like the Large Hadron Collider. In contrast, their decays into a pair of muons is the result of the “weak interaction” or, perhaps, some yet unknown interaction.

In this work, the LHCb and CMS collaborations have, for the first time, analyzed their respective collision data, which were gathered in 2011 and 2012 and published individually in two different papers in 2013. Both sets of data indicated the Bs0 decay, but the significance of each set fell below the historically accepted minimum of 5 sigma, which would allow either LHCb or CMS to claim an actual observation. By combining their data, the two collaborations were easily able to exceed this requirement, reaching 6.2 sigma.

Along the thousands of physicists involved in this work, EPFL’s role spans its entirety. The High Energy Physics Laboratories led by Aurelio Bay, Tatsuya Nakada and Olivier Schneider made significant contributions to the design and construction of the LHCb detector, to the development of the analysis and the internal review within the collaboration that culminated in its seminal 2013 paper, which has been cited already 190 times.

Professor Nakada, who has been recently awarded an honorary doctorate degree by the University of Zurich for his longstanding contribution to the field, was one of the original proponents of the LHCb project and acting as the first spokesperson (project leader) during the approval and construction phase from 1995 until 2008.

In addition, two key members of the project are former PhD students of the Lausanne group: Marc-Olivier Bettler, one of the key physicists who has analysed the LHCb data on this study and is a lead author of the combined paper, and Patrick Koppenburg, whi is the current physics coordinator of the LHCb collaboration.

“The observation of the Bs0 decay is one of the most important results obtained by the LHCb experiment,” says Olivier Schneider. “The measurement is consistent with the predictions of the Standard Model, and it rules out many possibilities of extensions of the Standard Model, which are searched for at the LHC.”

The Standard Model is a theory that best describes the observed phenomena in particle physics at energies accessible so far. A possible extension to the Standard Model is called “supersymmetry”, which predicts that each particle in the Standard Model has a partner particle, which could solve major problems in particle physics such as the mass stability of the Higgs boson. The new result from LHCb and CMS has now squeezed very significantly the parameter space where supersymmetry may exist, focusing future theoretical attention and further experimental searches for new physics.


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