Universe’s underlying symmetry still baffling

One of the great mysteries of modern physics is why antimatter – particles with reversed electric charge that destroy normal matter on contact – did not annihilate the universe at the beginning of time. To explain it, physicists suppose there must be some miniscule difference, or ‘asymmetry’, between ordinary particles and their mirror images.

Whatever that difference is, it seems it is not in their magnetism. Physicists at CERN in Switzerland made the most precise measurement yet of an antiproton’s ‘magnetic moment’ – how the particle responds to a magnetic force – and found it is perfectly symmetrical with the proton.

This is the latest in a series of extremely precise measurements of antimatter properties, including mass and electric charge, looking for differences from normal matter. So far none has been found. “All of our observations find a complete symmetry between matter and antimatter, which is why the universe should not actually exist,” jokes Christian Smorra, a physicist at CERN’s Baryon-Antibaryon Symmetry Experiment (BASE) collaboration. “An asymmetry must exist here somewhere but we simply do not understand where the difference is.”

Antimatter is notoriously unstable – any contact with regular matter and it annihilates in a burst of pure energy that is the most efficient reaction known to physics. That’s why it was chosen as the fuel to power the starship Enterprise in Star Trek. 

The standard model of particle physics predicts the Big Bang should have produced equal amounts of matter and antimatter – but that’s a combustive mixture that would have annihilated itself, leaving nothing behind to make galaxies or planets or people.

To explain the mystery, physicists have long been searching for some discrepancy to explain why matter came to dominate. 

The antiproton measurement by Stefan Ulmer and the BASE team has been a decade in the making. First they had to develop a way to directly measure the magnetic moment of the regular proton – itself a groundbreaking achievement, reported in Nature in 2014. 

Making the same measurement on antiprotons was doubly difficult. 

Since antimatter would destroy any physical container, physicists used magnetic and electric fields to contain the material in devices called Penning traps.

Usually antimatter’s longevity is limited by trap imperfections that allow leaks. By using a combination of two traps, the BASE team made the most perfect antimatter chamber ever – holding the antiprotons for 405 days and enabling measurement of their magnetic moment.

The result, –2.7928473441 μN 

(μN being a constant called the nuclear magneton), was identical, apart from the minus sign, to the proton measurement.

Their finding, published in Nature, is 350 times more precise than any previous attempt, equivalent to measuring the Earth’s circumference to within a few centimetres. 

The universe’s greatest game of spot the difference thus goes on. The next hotly anticipated experiment is over at ALPHA, where CERN scientists are studying gravity’s effect on antimatter and whether it might fall ‘up’.

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