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Commentary: Reasons to be jolly about 2008

'TIS the season to be jolly, though after a year in which the much-anticipated Large Hadron Collider at CERN has been delayed, you might think particle physicists don't have much to be jolly about. Two very different developments, however, have made 2008 particularly exciting for them.

The first shows that with a good theory and hard work, we can unlock nature's deepest mysteries. The story began in the 1960s, when it was proposed that protons and neutrons are made up of constituents called quarks. Experiments then demonstrated that protons did contain point-like components. These bounced around inside protons as if they were free. So why didn't they get knocked out of protons when physicists smashed them about in accelerators?

An answer came in the 1970s, when a fascinating property of quantum chromodynamics, the theory of the strong interaction between quarks, was discovered. According to QCD, the attraction between quarks gets weaker the closer they are, and vice versa. Experiments confirmed the idea over very small distances, but at the scale of protons, the forces became too strong to calculate with available mathematics. This was frustrating because in principle QCD should explain all the properties of protons and neutrons, including their masses.

Physicists have been working for more than three decades to describe the theory on a finite lattice of points in space and time, so that it could be crunched by a supercomputer. Now, a European collaboration using this method has reported a remarkable result.

They found that QCD predicts the masses of the lightest strongly interacting particles, with an accuracy of 1 to 2 per cent. This may not seem spectacular until you realise that quarks only contribute a very small fraction of the total mass of the particles they make up. Most of the mass comes from the quantum fields holding the quarks together. A theory drawn up to explain an obscure puzzle about elementary particles has now allowed us to calculate from first principles the properties of a whole range of other particles, including the neutron, which is responsible for more of the mass of the Earth than any other particle.

The second reason to be jolly is a set of puzzling satellite results. While looking at the cosmic rays zooming through our galaxy, one satellite saw more high-energy positrons (anti-electrons) than expected, while another saw an excess of high-energy electrons.

One explanation is that particles and antiparticles of dark matter (thought to make up most of the mass of our galaxy, but yet to be discovered) collide and annihilate each other, throwing out these positrons and electrons. Unfortunately, the abundance and energies of the positrons and electrons don't match what would be expected from the favoured dark matter candidate - a particle called the neutralino. Undaunted, theorists have come up with alternative dark matter ideas for these excess particles ranging from the baroque to the bizarre, excited that they could finally represent direct evidence for a new world of physics.

The dark matter explanation for the excess particles smells wrong to me, not least because there's a simpler explanation - a nearby pulsar could be spewing them out. But what makes these observations worth celebrating is that young theorists can now experience the thrill of ambulance chasing: creating new wild theories to explain puzzling data. I haven't done it since the 1980s - the last time particle physics had enough data to be regularly puzzling.

Young theorists can now experience the thrill of creating wild new theories

Both developments show the promise of cutting-edge physics. Most scientific theories we come up with are wrong - if they weren't, there would be no challenge. But if we persevere when we get a theory that works, the sky is the limit.

  • From issue 2687 of New Scientist magazine, page 74.



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