The irresistible rise of the standard model

Event display of rare Bs decay. Credit: Cern

Physicists don’t take summer holidays. Instead, they go to summer conferences. This year’s conference, in Stockholm, made a splash with the announcement that two experiments at Cern’s Large Hadron Collider have made the first observation of an ultra-rare decay of the Bs meson (a particle composed oft a bottom quark and a strange antiquark). You may wonder how such an esoteric piece of information could be important enough to keep 750 physicists away from the beach. The secret is that this tiny, rare process could unveil the deep nature of the universe, and potentially solve some of the biggest mysteries we have.

In truth, our understanding of particle physics is at a crossroads, which is why this measurement could have such an impact. We have a wonderful theory, the Standard Model, which predicts what we measure in experiments with great success. There’s only one catch. The theory is incomplete, in fact stunningly incomplete when you consider it only describes visible matter which is a mere 5% of the universe. Dark matter and dark energy make up the remainder and are both mysteries. And so, to be fair, are many of the things we can see, like the preponderance of matter over antimatter, gravity and the precise number of fundamental particles and forces.

Theorists have proposed a variety of alternatives that try to address at least some of these shortcomings. One of the most popular theories is supersymmetry (SUSY for short). This extends the Standard Model by introducing a deep symmetry, or connection, between force and matter. In doing this a raft of new fundamental particles appear, one which may form dark matter, and others which can modify the forces to unite into one super-force at very high energies. As an added attraction these new SUSY particles can avoid ugly situations that crop up in the Standard Model, by providing “natural” cancellations in calculations that otherwise require careful fine-tuning to converge on sensible answers.

SUSY makes the universe even more simple and it would be wonderful if it were true. Unfortunately, it’s something of a Scarlet Pimpernel and no sign of any SUSY particle has appeared in our data yet. Our quest to see if SUSY can supplant the Standard Model has switched to looking for signs of the stealthy influence it can exert on the particles we already know about, and that’s where the Bs result comes in.

Bs mesons are created in the Large Hadron Collider, and decay to other particles fractions of second afterwards. By being quick, and clever, we can observe them in our experiments. One very distinctive decay occurs when a Bs meson produces a muon and it’s antimatter twin. This happens incredibly rarely – in the Standard Model a mere three times in every billion Bs meson decays.

The rate can speed up, or even be suppressed if we live in a SUSY universe. SUSY particles provide alternative ways for this decay to happen, and alter the number you would expect to see experimentally. Counting how often this ultra rare decay occurs therefore tells us if the real world matches the Standard Model or not. If it doesn’t we have our first glimpse of new physics in action, be it SUSY or something else. And as we know the Standard Model isn’t the whole story, but don’t yet know what to replace it by, this knowledge is invaluable.

It’s a good plan. However, it is also a slow one. The process is so rare that to tease it out from everything else in the hundreds of trillions of proton-proton collisions produced by the Large Hadron Collider demands a precise detector, a committed workforce, and a lot of patience. It is only now, with all the data collected so far and the combined efforts of two experiments – LHCb and CMS, that the process has finally been observed, almost 30 years since efforts to see it started. Neither experiment has yet collected sufficient data to claim an observation by itself – but when combined together they pass the threshold.

So what did they find? The experiments saw a handful of these decays, a number which is bang on the Standard Model prediction. It’s a fantastic confirmation of the Standard Model, albeit an extremely frustrating one.

You might wonder if this result finally marks the end of the line for supersymmetry, especially if you have been reading headlines that supersymmetry is “maimed”, “in hospital” and generally on the ropes. The answer is: no, not yet. SUSY is the many-headed hydra of particle physics theories – notoriously flexible, and although the result rules out many of the most popular and attractive versions of it, other variants could still exist. It’s Standard Model 1, SUSY 0, but the match isn’t over yet.

The Bs measurement leaves us with a horribly successful Standard Model, and although we’ve learnt more about where new physics isn’t, we still need to find out where it is. To stand a chance of making progress we need more and higher energy data from the Large Hadron Collider. That will come when it restarts in 2015, and in the meantime we will exhaust the data we have for any signs anywhere that the Standard Model might be failing. There will be no more summer holidays for a while.

Tara Shears

Tara Shears

Tara Shears is a particle physicist and professor of physics at the University of Liverpool. She has spent her career investigating the behaviour of fundamental particles and the forces holding them together, and has worked at experiments at CERN, the European centre for particle physics, and at the Fermilab particle physics facility near Chicago, USA. Tara joined the LHCb experiment at CERN’s Large Hadron Collider in 2004, where she works to this day.
Tara Shears

Comment via Facebook

Comment via Disqus


Comment via Google+