Discovery of Ultra-Rare Decay at LHC

July 24th 2013

Summary: The LHC experiments LHCb and CMS  have announced the observation of one of the rarest processess in fundamental physics, concluding a search that has lasted almost 30 years.

Image: Results of the LHCb and CMS experiments and the combination of these. By combining the results the threshold to announce discovery is passed. .(Credit LHCb and CMS)

This morning at the EPS conference in Stockholm physicists from CERN, the European particle physics lab, announced the observation of one of the rarest processes in fundamental physics, concluding a search that has lasted almost 30 years. This observation was only possible by combining the results of two CERN experiments, CMS and LHCb.

The result is a stunning success for the Standard Model of particle physics and yet another blow for those hoping for signs of new physics from CERN’s Large Hadron Collider (LHC).

The LHCb and CMS experiments at the LHC have made the first definitive observation of a particle called a Bs meson decaying into two muons, confirming a tentative observation made by LHCb last autumn. The discovery has far-reaching implications for the search for new particles and forces of nature.

Beyond the Standard Model

There are many reasons to believe that the current Standard Model of particle physics is an incomplete description of nature at the fundamental level. Despite its excellent agreement with almost every experimental measurement to date, it has several gaping holes. It fails to describe the force of gravity and has no explanation for the enigmatic dark matter and dark energy that are thought to make up 95% of the Universe. The theory also requires a large amount of “fine-tuning” to match experimental observations, leaving it looking suspiciously like the laws of physics have been orchestrated in a very unnatural way to produce the Universe we live in.

In the last few decades a number of theories have been proposed that solve some of the Standard Model’s problems. One particularly popular idea is supersymmetry (SUSY for short), with posits the existence of a slew of new fundamental particles, each one a mirror image of the particles of the Standard Model. SUSY has many attractive features: it provides a neat explanation for dark matter and unifies the strengths of the three forces of the Standard Model. However, the main reason that physicists were first attracted to it is that it is aesthetically pleasing or “natural” – in other words it doesn’t require the same awkward fine-tuning as the Standard Model.

On the hunt

The decay observed at LHCb and CMS is predicted to be extremely rare in the Standard Model, with a Bs meson only decaying into two muons about 3 times in every billion. However, if ideas like SUSY are correct than the chances of the decay can be significantly increased or even suppressed.

Finding particle decays this rare makes hunting for a needle in a haystack seem easy. Hundreds of millions of collisions take place every second at the LHC, each one producing hundreds of new particles that leave electrical signals in the giant detectors. Physicists from LHCb and CMS trawled through two years worth of data, searching untold trillions of collisions for signs of two muons coming from a Bs meson. The pressure to be the first to find evidence of this rare process was intense, as Dr. Marc-Olivier Bettler, a Cambridge physicist working on the LHCb team told me

“It is a very strange type of race. To avoid bias, we don’t allow ourselves to look at the data until the last minute. So it’s a bit like running blindfolded – you can’t see the landscape around you or your competitors, even though you know that they’re there, so you have no idea if you are doing well or not! You only find out after you cross the finish line.”

However, ultimately the race ended in a draw. Neither LHCb nor CMS alone had enough data to announce a formal discovery, each turning up just a handful of likely candidates.  But when their results were formally combined the signal passed the all-important “five sigma” level, above which a discovery can be declared.

Standard Model Stands Firm

However, in a blow for supporters of SUSY, LHCb and CMS observed the decay occurring at exactly the rate predicted by the Standard Model – approximately 3 times in every billion. This is yet another triumph for the Standard Model and kills off a number of the most popular SUSY theories.

Professor Val Gibson, leader of the Cambridge particle physics group and member of the LHCb experiment explained that

"Measurements of this very rare decay significantly squeeze the places new physics can hide. It is the dedication of our students and post-docs that make such measurements possible. The UK LHCb team are now looking forward to the LHC returning at even higher energy and to an upgrade to the experiment so that we can investigate why new physics is so shy."

This result is certainly not the end of the road for ideas like supersymmetry, which has many different versions, so many in fact that it is almost always possible to contort it so that it agrees with experimental data. However, combined with the recent discovery of the Higgs boson, whose mass is larger than predicted by many SUSY theories, this new result may force SUSY into such baroque configuration that the original motivation - a natural description of nature - is lost.

As particle theorist Nima Arkani-Hamed and colleagues wrote in a recent paper (

“… while natural supersymmetric theories may still turn out to describe physics… these models are rather elaborate. Many of these theories are actually just as fine-tuned as more conventional versions of supersymmetry, but the tuning is more hidden.”

This new result from CERN is yet another demonstration of the fantastic accuracy of the Standard Model as well as the incredible precision that is now being achieved by experiments at the LHC, allowing physicists to uncover ever-rarer particles and phenomena. It also provides a stringent test for beyond the Standard Model physics and adds weight to the argument that theorists may have to abandon the long-cherished principle of naturalness. If ideas like supersymmetry are to survive the onslaught of high precision tests made by the LHC experiments, we may have to accept that we live in a spookily fine-tuned Universe.


Prof. Valerie Gibson, University of Cambridge, Gibson ATNOSPAM


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.