29 February 2020 | New Scientist | 47frequency of a “hyperfine” transition between
two positron states in antihydrogen, showing
agreement with the hydrogen value to a couple
of parts in a trillion. That might sound like
conclusive evidence of nothing doing, but
the hydrogen transition has been measured
to 1000 times better precision, meaning
there is still plenty of room for discrepancy.
Just last week, the team published a
measurement of the even tinier “Lamb shift”
in antihydrogen. This effect is caused by
energy fluctuations in the quantum vacuum,
and should be very sensitive to any signs
of unknown physics. Again, there was
no measurable difference compared with
hydrogen – but any definitive statement would
require much more sensitive measurements.
In late 2018, however, the Antiproton
Decelerator was switched off to be hooked
up to a new machine, the Extra Low Energy
Antiproton ring, or ELENA. Long-term, ELENA
will enable antiprotons to be slowed even
more, increasing by between 10 and 100 times
the number that experiments can play with.
For Hangst, it was a blow. “They shut us down
at the worst possible time,” he says. “We were
just getting really mature with all of this.”
The beefed-up machine should be turned on
again early in 2021. With it will come not just
better measurements of antihydrogen, but also
the answer to an even bigger question – one
that could render all the previous discussion
redundant. Does antimatter fall down, or up?
Again, there are strong suppositions. “A huge
majority of theoretical physicists, perhaps
too huge to be right, believes antimatter falls
the same way as matter,” says CERN theorist
Dragan Hajduković. If it turns out that it
instead falls up, everything is to play for.
“It would really turn everything on its head
from the instant of the big bang,” says Hangst.
For a start, it means that anyone aiming to
explain matter’s dominance in our universe
through the breaking of fundamental
symmetries might be chasing shadows.
If matter and antimatter fall in opposite
directions, they probably also repel each
other gravitationally. In that case, they could
have chased each other away to opposite
ends of the universe before having a chance
to annihilate each other. The option that
antimatter is just hiding, now lurking beyond
the horizon of our observable universe, is
back on the table. “You could potentially
imagine that matter and antimatter early
on have completely separated because of
some antigravity,” says Malbrunot.
The electromagnetic force is far more potent
than gravity on small scales, so to measuretime reversal, or “T”, symmetry to the CP mix.
If particles swap charges and their orientation
in both space and time – if the universe is
completely mirrored – then the laws of physics
should work the same way. This assumption
lies at the heart of relativity and the quantum
field theories underlying the standard model.
“If we find any difference, that would have
dramatic impacts on physics,” says Malbrunot.
The problem is working with antiparticles,
with their penchant for going up in smoke.
To stand a chance, the Antimatter Factory is
doing the opposite of what CERN is famous for:
slowing particles down. A dedicated machine,
the Antiproton Decelerator, is fed antiprotons
and calms them so they can begin to create
stable unions with positrons, and so form
stable antihydrogen atoms.
A predecessor to the Antiproton Decelerator
at CERN, known as LEAR, first manufactured
antihydrogen atoms in 1995, but only in
small quantities, and for very short times,
and jiggling about too much to do precise
measurements on them. Malbrunot’s
experiment, ASACUSA, has been trying to
solve these problems by making a de-excited
antihydrogen beam that can be investigated
by tickling it gently in flight with laser light.
The collaboration reported the first tentative
signs of success in 2014 – and just recently,
more certain signs. “Right now, we are aboutto show we have succeeded in forming
antihydrogen in this new way,” says Malbrunot.
Meanwhile, Hangst’s ALPHA experiment
has stolen a march. Its approach is to cool
antihydrogen atoms to within a whisker of
absolute zero and hold them in suspended
animation. Its best effort is trapping more than
1000 of them at once. “At every step of this,
people said that this would never work,” says
Hangst. “You would never make antihydrogen,
if you made it, you would never trap it. If you
trapped it, you would never have enough.
And now we have all those things, but all that
has taken about 30 years, and it’s only really
worked in the last three.”
In 2018, the ALPHA collaboration published
its first comparative measurement of the“ If we find any
difference
between atoms
and anti-atoms
that would have
dramatic impacts
on physics”
CERN’s Antimatter
Factory is a portal
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