5 October 2019 | New Scientist | 45a collection of quarks can attain stability
only if all colours are equally represented,
an infinite number of colours means baryons
with infinite numbers of quarks.
This had consequences. Every quark has
a quantum property called spin. Multiply
the number of quarks and, crudely speaking,
you multiply the amount of maximum spin.
In extreme cases, when all the quarks have
their spins aligned, the resulting baryon has
so much spin that ’t Hooft’s model struggles
to describe it. That’s true not only of particles
in ’t Hooft’s imaginary, infinite colour world,
but for some particles in the real world, such
as the unusual delta++ baryon, which consists
of three up quarks with aligned spins.
The resolution to this has come from an
unlikely place: string theory, a framework to
unify the relativistic physics of the very large
with the quantum physics of the very small.
In the early 2000s, string theorists started
noticing that their equations allowed quarks
to do something bizarre. Under certain
circumstances, they could take on a fraction of
their usual spin. This was something that had
never been seen in experiments, or predicted
by QCD. It seemed like another mathematical
monster. Then a few years ago, people began
to see that QCD could describe quarks with
fractional spin too.
Now, the quark story might be about to
change far more substantially. Last year, Zohar
Komargodski at the Weizmann Institute of
Science in Israel saw a possible way to bring
all of the disparate quark ideas together: using
the infinite colour model of ’t Hooft, but giving
the quarks freedom to take on fractional spins.
Physicists admit that his work shows ingenuity
and skill – but it is also extremely complex.
“I would like to understand it a little better
myself,” says Georgi.many collections of quarks haven’t yet been
calculated using full-on QCD. Instead they
have been done using less sophisticated
models that don’t account for every interaction
a quark might have. “Our knowledge of QCD
is a bit like trying to grasp what an elephant
looks like by feeling some small part,” says
physicist Howard Georgi at Harvard University.
“One approach may describe the trunk without
difficulty but do a really bad job on the ears.”
This isn’t a new problem. As far back as
the 1970s, physicist Gerard ’t Hooft was
searching for a way to make QCD more
tractable. He made a bold compromise on
accuracy, essentially discarding the parts
of the QCD equations that described colour.
This made for a tremendous simplification,
says Van de Water, allowing you to do
calculations on the back of an envelope.
When ’t Hooft tried it, he found that
it reproduced the properties of mesons
with surprising accuracy. “That was pretty
exciting,” says Georgi.
But Gell-Mann’s monsters were about to
bite. Setting the colour term aside freed quarks
from needing to have three colours. Instead,
they could have any number of colours you
liked – even an infinite number. And because“ An infinite number
of colours means
an infinite number
of quarks. This has
consequences”
It was a moment of celebration, but much
remained unclear. One of the major mysteries
was why certain combinations of quarks
flourished and others didn’t. You could, for
example, pair a quark with its antiparticle to
form a meson, or stick three quarks together
to form a baryon, such as a proton or neutron.
But you couldn’t easily produce a composite
particle made of four or five quarks (see
“Quarky quirks”, left), or ever get a quark on
its own. Why was this?
The answer lies in a remarkable property
of quarks known as colour charge, which bears
no relation to the colours we think of in daily
life. “Colour is something we’ve just picked
to name it because it comes in threes,” says
Freya Blekman at the Free University of
Brussels in Belgium. Quarks of these different
colours – called red, green and blue – can sit
together because their colour charges cancel
out, by analogy with the way different colours
of light blend together to make white. Through
the same logic, a quark and an antiquark
could sit together assuming they had colour
charges of red and anti-red. This also explains
why single quarks don’t fall out of atoms in
detectors: without their colour partners they
are too unstable. “Quarks are always team
players,” says Blekman.
By the end of the 1970s, we finally had
what is still the most complete description of
quarks and the force that binds them together:
quantum chromodynamics (QCD), named
for the colour charge that quarks possess.
QCD isn’t perfect. For one thing, using it
to calculate the most complex physics can
be incredibly time-consuming. “A calculation
can take us three years from start to finish,”
says Ruth Van de Water at the Fermi National
Accelerator Laboratory near Chicago.
This is why, she says, the properties of
duu
dd
uNature’s lego bricks
If we dig down deep enough, almost all matter in the universe is made of quarks. They come in six “flavours”, only two of which (up and down) are found in ordinary matterMolecules AtomsNeutronsProtons QuarksApproximate massElectrical charge uUp
3.9 x 10-30 kg+⅔ cCharm
2.3 x 10-27 kg+⅔ tTo p
3.1 x 10-25 kg+⅔dDown
8.3 x 10-30 kg-⅓ sStrange
1.7 x 10-28 kg-⅓ bBottom
7.5 x 10-27 kg-⅓>