Standard Model Stands Firm

Dr. Harry Cliff, a Physicist working on the LHCb experiment and the first Science Museum Fellow of Modern Science, writes about a recent discovery at CERN. A new Collider exhibition opens in November 2013, taking a behind-the-scenes look at the famous particle physics laboratory. 

On Friday afternoon, at the EPS conference in Stockholm, two colleagues of mine from CERN stood up to announce that the search for one of the rarest processes in fundamental physics is over. The result is a stunning success for the Standard Model, our current best theory of particles and forces, 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 B_s meson decaying into two muons, confirming a tentative sighting at LHCb (my experiment) last autumn. The discovery has far-reaching implications for the search for new particles and forces of nature.

Beyond the Standard Model

There are a lot of reasons to suspect that the current Standard Model isn’t the end of the story when it comes to the building blocks of our Universe. Despite agreeing with almost every experimental measurement to date, it has several gaping holes. It completely leaves out 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 tinkered with in a very unnatural way to produce the Universe we live in.

In the last few decades a number of theories have been put forward that attempt to solve some of the Standard Model’s problems. One particularly popular idea is supersymmetry (SUSY for short), which proposes 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 (this suggests that they could all be aspects of one unified force, which should definitely be referred to as The Force, if it turns out to exist someday). It would also keep my colleagues in work for decades to come, thanks to a whole new load of super-particles (or sparticles) to discover and study.

However, physicists were first attracted to it because the theory is aesthetically pleasing. Unlike the Standard Model, SUSY doesn’t require any awkward fine-tuning to produce laws of physics that match our experience. This is not a very scientific argument, more a desire amongst physicists for theories to be elegant, but historically it has often been the case that the most beautiful theory turns out to be right one.

On the hunt

The decay observed at LHCb and CMS is predicted to be extremely rare in the Standard Model, with a B_s 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 boosted.

Finding particle decays this rare makes hunting for a needle in a haystack seem like a doddle. 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 B_s meson. The pressure to be the first to find evidence of this rare process was intense, as Dr. Marc-Olivier Bettler, a colleague of mine from Cambridge and member of 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 are formally combined next week it is expected that the number of observed decays will pass the all-important “five sigma” level, above which a discovery can be declared.

Standard Model Stands Firm

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 a 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. We are now looking forward to the LHC returning at even higher energy and to an upgrade of 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. However, combined with the recent discovery of the Higgs boson (whose mass is larger than predicted by many SUSY theories) this new result may only leave us with versions of SUSY that are somewhat inelegant, meaning that the original motivation – a natural description of nature – is lost.

This new result from CERN is yet another demonstration of the fantastic (and somewhat annoying) accuracy of the Standard Model. Incredible precision is now being achieved by experiments at the LHC, allowing physicists to uncover ever-rarer particles and phenomena. 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.