The great antimatter mystery
It is lucky for us that the infant universe did not behave the way our best cosmic theories would have it. Nearly 14 billion years ago, the big bang forged equal amounts of matter and its nemesis, antimatter. These should have annihilated each other in bursts of pure radiation, leaving a universe filled with light. Instead, it is full of stars and planets and gas - something threw a cosmic spanner in the works.
The stars and galaxies that light up the heavens would not exist today if matter had not won out over antimatter at some very early time in the evolution of
the universe. How and when did this happen? Why is there something rather than nothing? These questions are at the root of our very existence, but as yet
science has no clear answers.
That's not to say we haven't made progress. As in any good detective story physicists have picked up important clues, mainly by creating antimatter and studying what it does. Other evidence comes from neutrinos, those ghostly particles created in radioactive decays. These clues have provided two very promising lines of inquiry and thrown up some controversial results along the way. With the advent of new experiments there is a chance we will have answers very soon.
To create the universe we see today, a preference for matter must have arisen in the early universe. It only needed a minute imbalance, with as few as one extra particle of matter surviving for every 30 million antimatter particles.
That couldn't occur by chance, though. Even this tiny excess is too big to occur as a random fluctuation in the hot, early universe. Nor is the universe likely to have started out with such a finely tuned imbalance (see "Was the universe born lopsided?"). What's more, it definitely doesn't seem to be hiding pockets of antimatter today (see "Where are all the anti-galaxies?"). So how the excess arose during the history of the universe must be encoded somewhere in the basic laws of physics.
Russian physicist Andrei Sakharov was the first to take on this puzzle in 1967. He showed that for there to be more matter than antimatter, three conditions are needed. First, Sakharov argued that no conservation law can forbid reactions which effectively change the balance between particles and antiparticles. This was a bold claim, as such reactions have never been seen experimentally.
To make this possible, Sakharov pointed out that the laws of physics must be slightly different for matter and antimatter, as had been revealed in experiments three years earlier with particles called long-lived kaons. These showed that the weak force, which is best known for its role in radioactive decay, does not act equally on quarks and antiquarks.
Finally, there must have been a period early in the universe's history when the various reactions going on between the different particles and antiparticles and radiation in the primordial plasma started to take place at different rates. This can only happen if they are, for some reason, not in thermal equilibrium. Without these conditions the universe would never have evolved from its initial state of having equal amounts of matter and antimatter to its present highly unbalanced state.
Fast forward to today and Sakharov's conditions remain as relevant as ever. In the intervening years, they have acted as an important guide for our theories of the early universe.
The standard models of cosmology and particle physics suggest that when the universe was less than 10-12 seconds old, particles and their interactions were very different from what they are today. All the fundamental particles were massless and the weak interactions between them were more active. As the universe expanded and cooled, it switched to a more favourable, lower-energy state. Here the particles gained mass and the weak interactions became less active.
This cooler state started off as a tiny bubble that expanded rapidly throughout the early universe. As it did so, the bubble's surface upset the thermal equilibrium of the universe and interacted with the massless particles and antiparticles. Some of them passed through and ended up inside the bubble, while others bounced off.
Interactions at the bubble wall made it more likely for a quark to break through the bubble wall than an antiquark, so inside the bubble there was an excess of quarks, while the antiquarks outside were removed by the more active processes. Today, the bubble is the size of the universe, and because we live inside it we see the excess of quarks as a dominance of matter over antimatter.
It's a lovely, neat picture. The only problem is, it doesn't give the right numbers. When we use the standard model to calculate the amount of matter and antimatter, we get far too small an excess. This is one of the reasons why particle physicists think the standard model is incomplete. Is there a way to fix it?
Perhaps. One of the most promising extensions of the standard model is supersymmetry, which demands many as-yet-unknown particles beyond the reach of existing experiments. As well as explaining the antimatter imbalance, supersymmetry might tell us about the nature of the dark matter that accounts for 90 per cent of the matter in the universe, and why gravity is so puny compared with the other forces.
While theorists embrace supersymmetry, so far we have found no evidence for it in experiments. However, hints of a process that does not fit the standard picture recently came to light. Last month, a team of physicists in Italy, France and Switzerland known as the UTfit collaboration analysed particles called Bs mesons, created in two experiments at the Tevatron accelerator at Fermilab in Batavia, Illinois. Made of a "bottom" antiquark and a "strange" quark, Bs mesons are unstable and decay via the weak force into particles made of lighter quarks and antiquarks.
The UTfit collaboration argues that when they combine all the Bs meson results, they find a small discrepancy that could be evidence for a new interaction outside the standard model that acts differently on quarks and antiquarks, and might possibly be a reason for the excess of quarks in our universe (New Scientist, 18 March, p 10).
It is far too early to say whether this a first hint of supersymmetry. More observations are needed to confirm the UTfit group has indeed found something amiss, and we will still need to discover some supersymmetric particles to provide proof. They might turn up at the Large Hadron Collider (LHC), the world's most powerful accelerator, due to switch on later this year at the CERN laboratory near Geneva, Switzerland. Assuming supersymmetric particles are detected there, we will be able to measure their masses and some of their interactions, but even that won't be enough. Additional experiments will be needed to tell if supersymmetry generated the right excess of matter when the universe was about 10-11 seconds old.
Other planned experiments to study supersymmetry in detail include the International Linear Collider, which will smash electrons and positrons together (New Scientist, 25 August 2006, p 36) and an experiment to study the electromagnetic properties of the neutron.
An alternative way to explain the mystery of the missing antimatter emerged in the mid-1980s. Japanese physicists Masataka Fukugita and Tsutomu Yanagida showed how the matter-antimatter imbalance might have arisen in a scenario known as leptogenesis. If this idea is correct, we owe our existence to neutrinos. If certain scenarios of the early universe are correct, we owe our existence to neutrinos
Neutrinos are the most elusive of all particles in the standard model, and were long thought to be massless. However, a series of beautiful experiments carried out over the past 40 years in the US, Japan, Canada and elsewhere have established that the standard model is wrong: neutrinos do have mass, albeit a very tiny one.
This means they could have played a role in the antimatter imbalance. Adding neutrino masses into the theoretical picture means adjusting the standard model, and the simplest way to do this is to assume the existence of a new type of particle, a kind of very heavy neutrino called a singlet neutrino. These neutrinos are unlike any other particle we know because they do not interact with other particles via the usual forces in nature, so they are probably extremely difficult to detect. Like all fundamental particles, they would have been produced in appreciable quantities in the very early universe. But their interactions would have been too feeble to keep them in thermal equilibrium with the rest of the primordial plasma, in keeping with one of Sakharov's three conditions.
According to the leptogenesis scenario, singlet neutrinos travel freely across the universe until they decay into either neutrinos or antineutrinos. Crucially, according to the theory more antineutrinos can be produced than neutrinos, once again in line with Sakharov's ideas.
Leptogenesis therefore leaves the very early universe with an excess of antineutrinos. At this stage, the standard model predicts that certain reactions could occur in the very high-temperature conditions to convert antineutrinos into matter particles, eventually producing protons and neutrons and leaving the universe devoid of antimatter.
Testing leptogenesis will be tricky, as there is unlikely to be a way to produce singlet neutrinos in the lab and measure their decays. They are likely to be much too heavy and their interactions are dramatically too feeble for us to be able to do that. However, there are ways to test whether the idea is at least possible.
Leptogenesis predicts that the singlet neutrinos can interact with normal neutrinos by swapping Higgs particles - the particles that are thought to give mass to all matter and antimatter particles. From what we know about normal neutrinos and the Higgs particle, we can make inferences about singlet neutrinos. So far, their features appear to match what is needed for leptogenesis, and this provides some circumstantial evidence in support of the idea.
Another test concerns a property called the "lepton number". Electrons and neutrinos belong to the family of particles called leptons and are assigned a lepton number of 1. Their antimatter counterparts have a value of -1. In all the reactions we have measured so far, the lepton number before and after the reaction has stayed the same.
However, the leptogenesis theory predicts that adding singlet neutrinos to the mix makes it possible for regular neutrinos to change into antineutrinos and vice versa. So it fails to conserve lepton number. Particle physicists regularly check their experiments for signs of lepton number violation because this would directly prove Sakharov's first condition, that there is no conservation law in nature protecting the matter-antimatter balance.
So far, only one group claims to have seen a reaction that violates lepton conservation. Hans Klapdor-Kleingrothaus at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, says that his group first saw lepton violation in 2001, in germanium-76 nuclei (New Scientist, 4 September 2004, p 37).
They claim to have observed a reaction called neutrinoless double beta decay. In normal beta decay, a neutron inside a nucleus spontaneously transforms into a proton, producing an electron and an antineutrino in the process. The lepton number is 0 before and afterwards. A few rare radioactive elements go one better and undergo double beta decay, where two neutrons inside the same nucleus change at the same time, spitting out two electrons and two antineutrinos. In the neutrinoless version of double beta decay, there are no antineutrinos, only two electrons. Here the lepton number changes from 0 before the reaction to 2 afterwards.
The Heidelberg group's findings are controversial, though, and while several teams of physicists are attempting to replicate the experiment, none has yet succeeded. Still, many physicists are convinced that leptogenesis is the prime suspect for solving the antimatter puzzle, and so the search for lepton number violation goes on.
For now the mystery has at least two possible answers. It is down to experiments to choose between them, or even eliminate both and send theorists back to the drawing board. If supersymmetry provides the answer we will eventually know it. But if leptogenesis is the right answer, then it is likely to remain forever a plausible yet unproven aspect of cosmology. Like it or not, the universe may never reveal all its secrets.
by Helen Quinn and Yossi Nir