Dr. Harry Cliff, a Physicist working on the LHCb experiment and the first Science Museum Fellow of Modern Science, writes about a new discovery at CERN for our blog. A new Science Museum exhibition about the Large Hadron Collider will open in November 2013, showcasing particle detectors and the stories of scientific discoveries.
There were high hopes that the world’s most powerful particle collider would find evidence for the theory of supersymmetry, which postulates that every member of the known bestiary of sub-atomic particles has a related but much more massive “super-partner”. The theory is considered more elegant than the current Standard Model of particles and forces and is particularly appealing as some of these supersymmetric particles, or “sparticles”, could account for the “dark matter” that sculpts the structure of the visible universe.
But the experiment I work on at the Large Hadron Collider (LHC) has spotted of one of the rarest particle decays ever seen in nature, a result that poses a serious challenge to supporters of “new physics” theories like supersymmetry.
View of the LHCb cavern. Image credit: CERN
Results presented at the Hadron Collider Physics conference in Kyoto early this morning show the first convincing evidence for a particle called a Bs meson decaying into two muons. The decay was seen by my colleagues at the LHC beauty (LHCb) experiment, a gigantic particle detector on the 27km LHC ring at CERN, near Geneva.
This process is predicted to be very rare in the Standard Model, but if ideas like supersymmetry are correct then it could be much more common. However, the decay seems to be just as rare as the Standard Model predicted.
As we sat sharing a coffee at the Cavendish lab in Cambridge, Dr Marc-Olivier Bettler, a member of the international team who produced the result, told me it puts “strong constraints” on supersymmetry.
Rarer than winning the lottery
The LHC has been smashing protons into each other at close to the speed of light almost non-stop since November 2009. Each collision creates a shower of new particles, and occasionally a Bs meson is produced. The LHCb detector was built to study exotic these exotic particles.
Dr Bettler and his colleagues churned through hundreds of trillions of collisions produced by the LHC in search of the decay. The huge amount of data recorded by the LHCb experiment was processed using a world-wide network of computer processors known as the Grid. In the end they turned up just a handful of likely candidates.
Their results show that the chance of a Bs meson converting to two muons is about one in 300 million. That’s thirty times less likely than winning the jackpot on the lottery with a single ticket.
New physics hiding
Finding evidence of the decay is a triumph for LHCb, but will be a big disappointment for theorists who have spent many years working on supersymmetry. Prof. Val Gibson, leader of the LHCb group at the University of Cambridge said “this key result is putting our supersymmetry theory colleagues in a spin”. The result also makes it much less likely that the other main LHC experiments, ATLAS and CMS, will discover signs of supersymmetric particles any time soon. “If new physics is present then it is hiding very well behind the Standard Model” said Dr Bettler.
Even though it may be less thrilling than discovering new particles or forces of nature, these extremely precise measurements are crucial to improving our understanding of the Universe. “This result is important because it tells us what new physics isn’t.” Dr Bettler certainly didn’t find the outcome disappointing, describing his reaction at seeing the results for the first time two weeks ago as “wow! I was very excited. It has been a very exciting two weeks, that’s for sure.”
Visitors to the Science Museum will have a chance to get up close and personal with the LHC at a new exhibition opening in November 2013. The exhibition will showcase real pieces of the LHC, including an intricate particle detector from the heart of the LHCb experiment.