great mysteries of modern physics is why antimatter did not destroy the
universe at the beginning of time. To explain it, physicists suppose there must
be some difference between matter and antimatter – apart from electric charge.
Whatever that difference is, it’s not in their magnetism, it seems.
at CERN in Switzerland have made the most precise measurement ever of the
magnetic moment of an anti-proton – a number that measures how a particle
reacts to magnetic force – and found it to be exactly the same as that of the
proton but with opposite sign. The work is described in Nature.
observations find a complete symmetry between matter and antimatter, which is
why the universe should not actually exist,” says 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
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 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.
the mystery, physicists have been playing spot the difference between matter
and antimatter – searching for some discrepancy that might explain why matter
came to dominate. So far
they’ve performed extremely precise measurements for all sort of properties:
mass, electric charge and so on, but no difference has yet been found. Last year,
scientists at CERN’s Antihydrogen Laser PHysics Apparatus (ALPHA) experiment
probed an atom of anti-hydrogen with light for the first time, again finding no
difference when compared with an atom of hydrogen.
property was known only to middling accuracy compared to the others – the
magnetic moment of the antiproton. Ten years ago, Stefan Ulmer and his team at
BASE collaboration set themselves the task of trying to measure it. First they
had to develop a way to directly measure the magnetic moment of the regular
proton. They did this by trapping individual protons in a magnetic field, and
driving quantum jumps in its spin using another magnetic field. This
measurement was itself a groundbreaking achievement reported in Nature in 2014.
had to perform the same measurement on antiprotons – a task made doubly
difficult by the fact that antiprotons will immediately annihilate on contact
with any matter. To do it, the team used the coldest and longest-lived
antimatter ever created. After
creating the antiprotons in 2015, the team was able to store them for more than
a year inside a special chamber about the size and shape of a can of Pringles. Since no
physical container can hold antimatter, physicists use magnetic and electric
fields to contain the material in devices called Penning traps.
antimatter lifetime is limited by imperfections in the traps – little
instabilities allow the antimatter to leak through. But by using
a combination of two traps, the BASE team made the most perfect antimatter
chamber ever – holding the antiprotons for 405 days. This stable
storage allowed them to run their magnetic moment measurement on the
antiprotons. The result gave a value for the antiproton magnetic moment of
−2.7928473441 μN. (μN is a constant called the nuclear magneton.) Apart from
the minus sign, this is identical to the previous measurement for the proton.
measurement is precise to nine significant digits, the equivalent of measuring
the circumference of the Earth to within a few centimeters, and 350 times more
precise than any previous measurement.
is the culmination of many years of continuous research and development, and
the successful completion of one of the most difficult measurements ever
performed in a Penning trap instrument,” says Ulmer.
universe’s greatest game of spot the difference goes on. The next hotly
anticipated experiment is over at ALPHA, where CERN scientists are studying the
effect of gravity of antimatter – trying to answer the question of whether
antimatter might fall ‘up’.