New Scientist - 29.02.2020

46 | New Scientist | 29 February 2020

The most basic definition

of an antimatter particle is

that it is the same as a matter

particle, except that it has

the opposite charge. So the

familiar electron, for example,

with a negative charge -1,

has an antimatter equivalent

called a positron that has a

charge of +1.

A complicating factor

is that “charge” doesn’t

just mean the familiar,

everyday electric charge.

Three fundamental forces

are covered by particle

physicists’ standard model:

electromagnetism, and the

strong and weak nuclear

forces, which govern the

interactions of the quarks

within protons and neutrons

and processes such as

radioactive beta decay,

respectively. Each of

these forces has a charge

associated with it, and

antimatter particles have

opposite values for these

charges, too.

Not every particle has

an antimatter equivalent,

either. Particles called

bosons transmit influences

rather than respond to

them, and these tend to

be their own antiparticles.

These include photons

and the mass-giving

Higgs boson. And to date,

no one has been able to

establish whether neutrinos,

the most elusive of matter

particles, and their partner

antineutrinos are different,

or the same thing.

What is antimatter?

mysterious entity that makes up most of the

gravitating matter in the universe. The Large

Hadron Collider (LHC) at CERN was partly

conceived to make heavy dark-matter particles

in its high-energy collisions. But it has found

diddly-squat besides the Higgs boson, the

mass-giving particle discovered in 2012.

The machine is currently on sabbatical until

2021, undergoing an upgrade in the number of

collisions it can produce. When it returns, the

precise measurements of rare processes that

LHCb specialises in will offer a chance to solve

the antimatter and dark matter problems –

not by manufacturing new particles, but

by measuring their ghostly influence on

already-known ones.

This is the physics equivalent of reaching up

a hand to have a rummage around on a shelf

you can’t actually see. “With some of these

measurements, we can access mass scales for

hypothetical particles two or three orders of

magnitude beyond the reach of the LHC for

producing them directly,” says Gersabeck.

“It’s a very, very powerful search tool to cover

a huge lot of ground.”

The hope is that these heavier particles could

be sources of CP violation, in effect repeating

the trick that solved the kaon problem. But

there is no guarantee. “We’ve got a few hints

here or there of things that might be going

on, but nothing firm,” says Gersabeck.

A few kilometres due west of LHCb,

Malbrunot, Hangst and others are betting on

a different approach. Rather than searching

for new physics at very high energies, says

Hangst, “we decide to look really carefully

at things we think we understand, and see

if maybe we’ve overlooked something”.

Hidden down a side road on CERN’s main

site, Building 393 seems an implausible

portal to a parallel world of matter. Its grey,

corrugated metal walls, roll-up lorry delivery

bay and asphalt car park surroundings give it

a shabby late 20th-century industrial estate

chic. Only a sign above the entrance saying

“ANTIMATTER FACTORY” gives the game away.

It expresses an aspiration only now

becoming reality: to solve the antimatter

mystery by making large quantities of whole

anti-atoms. All the differences between matter

and antimatter come about because they

have opposite charges, so the idea is to cancel

those differences by taking oppositely charged

antimatter particles and making neutral atoms

out of them. An atom of antihydrogen, the

simplest imaginable anti-atom, should work

exactly like a conventional hydrogen atom.

If it doesn’t, nature’s most profound

symmetry is broken: CPT symmetry. This adds

antimatter different? Are some types of

antimatter more different than others, and

why should this be? Are there other types of

antimatter, corresponding to new types of

matter we haven’t discovered yet?” asks Tara

Shears at the University of Liverpool, UK, and

LHCb. Answering those questions is the key to

working out why we are here now.

Theorists later found that CP violation among

kaons could be explained if three heavier,

unknown particles were disrupting them. All

three of these particles have since been found –

the bottom, charm and top quarks – and their

presence, along with CP violation, is now a

mainstay of the standard model.

Those particles also provide new sources of

CP violation. In the intervening half-century,

we have measured all the main predicted

sources, with LHCb measuring the last, among

charm quarks, just last year. “Now the whole

thing looks completely understood, and all is

well,” says Gersabeck.

There is just one teensy problem. Patterns

in the cosmic microwave background, the

leftover radiation from the big bang, combined

with calculations of the number of galaxies that

must exist, tell us the early matter-antimatter

imbalance we need to explain today’s matter

domination. It is tiny – about one part in a

billion. But the CP violation we have found so

far doesn’t account even for a billionth of that.

Wishful thinking

Perhaps there are unknown sources of

CP violation. In 2017, the LHCb experiment

saw an unexpected hint of the effect among

heavier versions of protons and neutrons.

If the elusive particles known as neutrinos are

their own antiparticles, that would also allow

extremely rare processes to supply an extra

source of asymmetry.

But to expect such small, unconfirmed

effects to account for the current huge

mismatch seems like wishful thinking. Here

the antimatter problem meets a wider malaise

in fundamental physics. “At the moment, we’re

at quite a curious place because we’ve found

all the particles of the standard model and

it looks like the standard model can explain

everything, including CP violation,” says

Gersabeck. “But at the same time, we know

it’s not right and there is other stuff out there.”

Stuff like dark matter, for example, the