CMS and LHCb experiments reveal new rare particle decay
In an article published today in Nature, CMS and LHCb collaborations describe the first observation of the very rare decay of the B0s particle into two muon particles, which are similar to electrons but heavier. A research team from the Institute of Cosmos Sciences of the University of Barcelona (ICCUB) participate in these experiments of the Large Hadron Collider (LHC). This rare subatomic process happens about four times out of a billion decays, but it has never been seen before.
In an article published today in Nature, CMS and LHCb collaborations describe the first observation of the very rare decay of the B0s particle into two muon particles, which are similar to electrons but heavier. A research team from the Institute of Cosmos Sciences of the University of Barcelona (ICCUB) participate in these experiments of the Large Hadron Collider (LHC). This rare subatomic process happens about four times out of a billion decays, but it has never been seen before.
The analysis is based on data taken at the LHC in 2011 and 2012, when ICCUB research team was designing and implementing two lines of the trigger system to select muon pairs among every proton-proton collision of the LHCb experiment. Moreover, ICCUB research team participated in the definition of the physical phenomena used to distinguish the signal indicating this important decay, as well as in the definition of the control channels that measure process paces.
Eugeni Graugés, ICCUB researcher and head of the UB group that collaborates in the LHCb, explains that “from an experimental perspective, the result has a profound impact: it delimits many possible theories beyond the Standard Model because it hinders validation as the energy regime required is far from the one which has been studied today by means of particle accelerators”.
“From a theoretical perspective, our present knowledge of the Universe is based on two main pillars: the Standard Model and the Cosmological Model, both of them widely studied at ICCUB”, affirms Lluís Garrido, ICCUB director and member of the LHCb experiment. “However, present particle model does not explain dark matter and energy of the current cosmological model, so new theories are being suggested”, says the researcher. “Measurements obtained from the LHCb experiment constitute an important filter to these new models as they must support this remarkable result”, concludes Garrido.
A rare decay
The B0s and B0 are mesons, in other words, non-elementary unstable subatomic particles composed of a quark and an antiquark, bound together by the strong interaction. Such particles are produced only in high-energy collisions —at particle accelerators, or in nature, for example in cosmic-ray interactions.
The two collaborations first released their individual results for B0s meson decay in July 2013. While the results were in excellent agreement, both fell just below the 5 sigma statistical precision historically needed to claim an observation. The combined analysis easily exceeds this requirement, reaching 6.2 sigma. This is the first time that CMS and LHCb have analysed their data together.
This exciting result is a major milestone in a search conducted by many experiments over almost three decades, and has important implications in the search for new particles and phenomena beyond the Standard Model, when the LHC physics run restarts in the next weeks.
More than seven hundred scientists from sixty-nine research centres from seventeen countries participate in the experiment LHCb. In Spain, besides ICCUB research team, a group of researchers from the University of Santiago de Compostela also participates in it. In total, two hundred Spanish scientists and technicians collaborate in LHC experiments, supported by the National Center for Particle, Astroparticle and Nuclear Physics (CPAN).