Two Rare Beauties

May 14th 2015

Summary: Evidence has been obtained for two extremely rare particle decays. These decays have important implications for the search for new particles.

Image: Two peaks are observed by fitting the data (black points) corresponding to two extremely rare particle decays.

(Credit LHCb & CMS)

Scientists from two experiments at CERN’s Large Hadron Collider (LHC) have combined their data for the first time, allowing them to see evidence for two extremely rare particle decays. The exciting announcement is the climax of three decades of work and has important implications for the search for new particles and phenomena when the LHC restarts in 2015.

In a paper published in Nature today (13th May 2015), the LHCb and CMS collaborations described a joint analysis of the data that has enabled them to observe a particle called a Bs meson decaying into two muons. By pooling their data, they have also been able to see the first evidence for the even rarer decay of a Bd meson.

These decays are predicted to be extremely rare in the Standard Model, the current theory of particle physics, but can be given a boost by beyond the Standard Model theories like supersymmetry with more than one Higgs boson. Thousands of physicists at the LHC are engaged in a wide-ranging search for such ‘new physics’ that might explain some of the biggest mysteries in science, such as the nature of dark matter and why the universe is made of matter and not antimatter.

The Standard Model predicts the probability for Bs and Bd mesons to decay to two muons to be about four in a billion and one in ten billion, respectively. The combined results from LHCb and CMS are in excellent agreement with the Standard Model, and rule out a number of new physics models. Professor Val Gibson from the University of Cambridge said of the result

‘It is amazing that the Standard Model of particle physics can predict the rate (4 in a billion) at which we observe something so rare. It throws down the gauntlet for theories beyond the Standard Model to predict new phenomena while at the same time satisfying these very rare events.’

This is the first time that the LHCb and CMS experiments have analysed their data together. This is not an easy thing to achieve, thanks to different detector technology, systematic effects to account for and different analytical techniques, not to mention that the two teams are often in direct competition with each other. However, this new result shows the commitment of the collaborations to fully exploit the data and search for new physics wherever it may be hiding.

Even though the result is in agreement with the Standard Model the search for new physics is far from over, and there is still a good chance that important discoveries could emerge from these rare particle decays. Dr Marc-Olivier Bettler from CERN, who led the analysis said

‘The LHC will restart very soon with increased energy, doubling the production rate for these rare phenomena. We will be able to collect such rare decays twice as fast as before.’

If the rare Bd decay occurs at the rate predicted by the Standard Model, the new data will allow physicists to conclusively discover it and there is the tantalising possibility that the decays may turn out to be even rarer than predicted by the Standard Model, providing evidence for new physics theories.

Prof. Guy Wilkinson from the University of Oxford and LHCb Spoksperson said

“The data from the first period of operations of the LHC has produced an impressive array of interesting physics results - LHCb alone has now published nearly three hundred physics papers - and this is only the start of this long term project. At the end of this decade we will upgrade the experiment to further improve our potential for uncovering new physics phenomena.“

Whatever the future may hold, measurements of Bs and Bd decays into two muons will help target the hunt for new physics, as the LHC experiments leave no stone unturned in their search for answers to some of science’s greatest mysteries.


Prof Val Gibson, Val.GibsonATNOSPAM

Dr Mark-Olivier Bettler, Mark.OlivierATNOSPAM


The UK participation in the international LHCb experiment is from eleven institutes.

University of Birmingham, University of Bristol, University of Cambridge, University of Edinburgh, University of Glasgow, Imperial College London, University of Liverpool, University of Manchester, University of Oxford, STFC Rutherford Appleton Laboratory, University of Warwick

UK participation in the experiment is funded by the Science and Technology Facillities Council (STFC), with contributions from the participating institutes, the Royal Society and European Union.