What matters about antimatter

LHCb detector. Credit: Claudia Marcelloni/CERN

Just like the dog that didn’t bark in the night time, the absence of antimatter in the universe worries us. Why there isn’t more of it is one of the biggest mysteries in particle physics, and one which my experiment (LHCb, at Cern’s Large Hadron Collider) was built to explore. On April 24 this year the LHCb experiment unveiled its latest findings. I want to explain here why these results matter, why they are a triumph, and why, despite them, we are little nearer that precious understanding of why and how this has happened.

Antimatter was a major player in the early universe, so much so that we think it formed half of it at the Big Bang. Antimatter fundamental particles annihilated their matter equivalents, releasing photons of light which were sufficiently energetic to transform back into antimatter-matter particle pairs again, which then annihilated … and so on. Particle life really was nasty, brutish and short. The rapidly cooling universe quickly put a stop to this cycle after a matter of seconds. What’s important is that, at the point when the cycle stopped, there must have been slightly more matter than antimatter. That tiny difference in the nature of antimatter is the reason the universe evolved into the matter dominated one we live in, and the reason for that tiny difference is an absolute mystery to us. Antimatter definitely matters, even if we’re not sure quite how.

Nothing in our theory of particle physics suggested that antimatter should be anything other than an identical, oppositely charged version of normal matter. The observation that it sometimes wasn’t, in 1964, led to a Nobel prize and a fix to the theory. The fix, which describes how the behaviour of matter and antimatter quarks can differ, is simple (one number), and powerful. It predicted the existence of a further generation of quarks at a time when only two were known.  And, remarkably, every measurement we’ve made of matter-antimatter quark differences, in experiments worldwide, is consistent with our theory.  This is wonderful, but in the absence of any deeper explanation, can also be frustrating.

The picture isn’t yet complete because the theory hasn’t been tested to its utmost. The LHCb experiment hopes to fill in some of the remaining gaps by studying matter and antimatter behaviour in particles that it has been impossible to investigate with much precision or in sufficient number up to now.  Our latest result is the first ever observation of matter-antimatter differences in Bs mesons (a particle composed of a bottom antiquark and strange quark). In principle the measurement is simple: you isolate samples of Bs mesons, based on the experimental signatures they leave behind in the LHCb detectors, and then count how many of these are antimatter, how many matter. The difference in number between the two types is the measurement. It comes out at around 25% in our case.

It sounds simple but, as always, the devil is in the detail. In this case the detail is a fantastically challenging data analysis which no previous experiment could meet. Approximately 70 trillion proton-proton collisions supplied by the Large Hadron Collider must first be whittled down to extract just over a thousand Bs mesons used to make the measurement. To achieve this we use detectors that can locate particles to within a hundredth of a millimeter, within millionths of a second. This precision allows physicists to reconstruct the very short flight distance characteristic of a Bs meson, on a timescale that allows this signature to be recognised in real time and retained for later study. Other detectors inside LHCb provide definitive identification and the ability to distinguish between matter and antimatter. The data analysis is complex and reliant on an extremely good understanding of detector response that takes time and care to develop and master. In short, the experimental measurement is a triumph. And this is an important, textbook measurement which the experiment will be remembered for.

We found 25% more matter than antimatter. This is consistent with our predictions. That amazing fix in our theory holds. It’s what we expected to see, and we saw it. So just what does that ultimately tell us about the nature of antimatter?

Unfortunately, not much more than we already knew. Our theory might match data, but if you wind back the clock and ask how much antimatter it predicts to be present at the very start of the universe, it falls very short. Forget forming half a universe, the amount of antimatter predicted by our theory is nearer just one galaxy’s worth. What our LHCb result underlines is that quarks alone do not hold the whole answer to the antimatter mystery .

Perhaps the full answer lies in new physics mechanisms and particles that we havent stumbled on yet, new physics that will revolutionise the way we understand the subatomic universe.  That would be exciting. Discovering what lies beyond our current, limited understanding of the subatomic universe is what every particle physicist dreams of doing. And that may yet happen.

Don’t forget, we are not operating at the design performance of the LHC yet – and who knows what clues to antimatter or any new physics we might find when we restart in 2015. Physicists at LHCb and the other LHC experiments will patiently sift that data in search of any glimpses of the unknown. In the meantime we have plenty more data already in hand to analyse and understand. It’s back to work as usual, and thinking harder.


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

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