What matters about antimatter

On May 8, 2013

LHCb experiment at 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.

Image: LHCb detector. Credit: Claudia Marcelloni/CERN

About 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.

11 Responses to What matters about antimatter

  1. Very interesting! Thanks and best wishes on your upcoming experiments.

    As a simple pedestrian bystander, I have to suggest an even simpler explanation. If the dense primordial universe was spinning, perhaps the condensation of matter produced only particles whose spin orientation conserved that of the universe’s spin. In this case, only particles of matter would be condensed, correct?

    Likewise, the expanded and dispersed universe is no longer spinning so intensely – as a result particles and antiparticles are both commonly produced.

    • I think you are referring to the paper that John linked to in his comment below? – I hadn’t come across this before. It hypothesizes that CP violation is a feature of spacetime, thanks to the gravitational effects of massive rotating bodies. The paper predicts that if this were true, measurements of CP violation could show a variation in value depending on orientation. All I can say regarding this is that our experimental measurements undergo scrutiny for all manner of dependencies that could signify changes. We study results as a function of time, angle, anything we can think of, and so far all experimental results, from all experiments all over the planet, give a very consistent (single) value for CP violation. We don’t have any evidence that it changes in any direction or with time with our data so far – of course that doesn’t mean that there is no dependence, just that we have not yet detected any. And of course we will continue to test in this way as our measurements become more and more precise. SO – no evidence for your hypothesis yet, but we’ll keep looking!

      • Actually, I don’t think I’m referring to anything quite that complicated.
        This simple suggestion, independent if not original, attempts to explain the preponderance of matter over antimatter – based on an analogy with planetary systems.

        I’m suggesting that in a dense, rapidly rotating primordial universe, as particles of matter initially condensed they would conserve the angular momentum of the universe – in this case producing only particles rather than any antiparticles.

        The analogy is based on planetary systems in which planets are accreted from a rotating protoplanetary disk – all with identical spin and axial orientation.

        In the more recent, relaxed – expanded universe, rotation would no longer be a determining factor. As a result, (virtual) pairs of particles and antiparticles are frequently emitted.

  2. Interesting stuff, Tara. I wish I had a job like that. But I picked up on this:

    Perhaps the full answer lies in new physics mechanisms and particles that we haven’t stumbled on yet, new physics that will revolutionise the way we understand the subatomic universe. That would be exciting.

    Maybe the answer is there already in existing physics. The Bs meson is comprised of a bottom antiquark and a strange quark, so it isn’t really matter or antimatter, it’s both. And it oscillates into its own particle and back in about 18 picoseconds, so again it’s both. Take a look at positronium. It’s an exotic atom, neither matter nor antimatter, but both. Step back from the quarks a moment, draw a 2 x 2 table, and on properties alone, put the electron and the positron, and the antiproton and the proton into the table. Only after you’ve done this should you add the column headings “Matter” and “Antimatter”. Where’s the proton? Look again at that positronium: “it can be regarded as a sort of light hydrogen atom”. Hydrogen is both too! So antimatter isn’t missing, weight for weight, you are 99.95% made of it. Where has the antihydrogen gone? Play a game of tennis. Mixed doubles. No matter how evenly matched they are, one side will win, and you will call the winner matter, merely by convention. See http://arxiv.org/abs/1107.1575 for a possible explanation of the asymmetry you’re seeing.

    • Yes, mesons are made of a quark and an antiquark – but they are also unstable, and decay. So although we can study them in particle collisions, they only exist for a short period of time. Ultimately they leave behind photons, (anti)neutrinos or electrons (muons decay too, and anti-electrons produced will annihilate with any passing electron to give photons), in other words, no large source of quark based antimatter. If you make up your “matter” “antimatter” table now, it will be populated mostly by atoms, which are all composed of matter (protons are composed of quarks). Antimatter appears fleetingly in some radioactive decays, cosmic rays, and in some astrophysical processes. You can put positronium in both columns if you like and count it with a weight of a half, but even so, there isnt much of it; there really is much more matter in the universe now than antimatter!

      I hope this makes sense? Your meson accounting certainly applied at the start of the universe, but those particles have all decayed away now. The Bs meson lives for a puny 1.5 x 10^-13 seconds, to give some idea of the timescale involved.

  3. John Duffield,
    Thanks very much for bringing up Positronium – an unstable “exotic atom” composed of an electron and positron orbiting around their common center of mass.

    That this configuration can occur seems to be definitive evidence that electrons and positrons have identical mass.

    This seems to render the ALPHA Collaboration’s experiment to evaluate negative mass, antigravity, etc. moot.

    Please see http://en.wikipedia.org/wiki/Positronium

    • I think there’s a bit of a disconnect between the physics and the reportage, James. I’ve read various original papers where the collaboration concerned plays an absolutely straight bat, but then the press release sensationalises the work, and editors/reporters sensationalise it further. So much so, that to people who know a bit of physics, the reportage sounds like woo. I think the physicists should be giving James Gillies a hard time about this sort of thing. Especially after OPERA, when the CERN press office were happy to promote the FTL neutrinos. Before too long we had people talking about time travel, woo! Then things went pear-shaped, and the CERN press office distanced themselves and let Ereditato and Autiero take all the blame.

      The antigravity thing isn’t quite woo by the way. Sure, antiparticles don’t fall up, but Mark Hadley’s asymmetric Kerr metric relates to gravitomagnetism, and antigravity is really “artificial gravity”. That’s deadly serious. I wish CERN were working on that.

    • … just to note, you can also have exotic states formed of a lepton and antilepton of different masses, eg. muonium (one muon, one positron). No need for the masses of the constituents to be the same! So antimatter experiments like ALPHA who are testing features of antimatter still have an important job to do.

      • Tara,
        Muonium is interesting – thanks, but its Wikipedia entry (your link) states:
        “Due to the mass difference between the antimuon and the electron, muonium (μ+e−) is more similar to atomic hydrogen (p+e−) than positronium (e+e−). Its Bohr radius and ionization energy are within 0.5% of hydrogen, deuterium, and tritium.[4]”

        This statement, along with the positronium entry’s description and illustration of an electron and a positron in a shared circular orbit around their apparently stable common center of mass, clearly indicate that the positron’s mass has been observationally determined to be at least nearly identical to the electron’s, certainly not negative or subject to antigravity.

        I have no qualifications here, but this certainly seems to indicate that the expected findings of the ALPHA experiment have already been observationally confirmed…

        • Additionally, the muonium configuration, with its Bohr radius and ionization energy within 0.5% of hydrogen & its variants, seems to strongly indicate that its electron is orbiting the ostensively much more massive antimuon very much like electrons orbit protons in hydrogen atoms. This also seem to preclude antimuons from having negative mass…

  4. Carry on with the good work, Tara. I guess I think it deserves to be better publicised. Does antimatter fall up? is lame, and The mystery of missing antimatter is tired. It would be really cool if your collaboration came out with the “both” thing and that 2 x 2 fermion table and 99.95% weight by weight, You Are Made Of It. Then you rename antihydrogen to enantiohydrogen and carry on whilst the public feel really happy with CERN that you’re making great progress. Win win.