New Scientist - USA (2019-10-05)

44 | New Scientist | 5 October 2019

Delve into the mysteries of neutrinos with Melissa Uchida

at New Scientist Live on 11 October

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Beyond strange

The standard model of particle

physics predicts the existence

of six types, or flavours, of quark:

up, down, strange, charm,

bottom and top. We knew that

quarks can change flavour, but

now it seems they might be

capable of a more radical

transformation too.

Benedetta Belfatto at the

Gran Sasso Science Institute in

L’Aquila, Italy, and her colleagues

have analysed experimental

data on flavour change and

found that some quarks seem

to be disappearing. “It means

there is something missing

from our picture,” says Belfatto.

One possibility is that some

quarks are turning into another

quark that we have never seen,

says Ruth Van de Water at the

Fermi National Accelerator

Laboratory near Chicago,

probably heavier than any of 

the six we know of.

Quark nuggets

Dark matter is one of the most

perplexing mysteries in physics.

Making up about 27 per cent

of the universe, it is an invisible

substance that can be felt only

through the gravitational pull it

exerts. But what is it made of?

Quark nuggets, also known

as strangelets, are hypothetical

particles made of up, down and

strange quarks that possess

the right properties to be dark

matter. They would require

immense pressure to form,

but Ariel Zhitnitsky at the

University of British Columbia

in Canada thinks he knows

what could do it: a hypothetical

new field that would span the

universe, creating bubbles

pressurised enough to create

quark nuggets that double as

dark matter.

It may sound like speculation

piled on speculation, but the

potential is exciting. More

controversially, Zhitnitsky

believes that quark nuggets

could be colliding with their

antiparticles inside the sun,

spitting out prodigious amounts

of energy and explaining why

the sun’s outer atmosphere is

as hot as it is.

Pentaquarks

and hexaquarks

In familiar particles like protons

and neutrons, quarks come

in collections of three colours

(see main feature).

There is theoretically a way

for collections of five quarks,

known as pentaquarks, to form,

but no one was sure they could

exist until 2015, when a team

at the Large Hadron Collider near

Geneva, Switzerland, snared

one in the detectors.

Could hexa- or septaquarks

exist? In the first instance, yes,

definitely. Nuclei of deuterium

atoms – isotopes of hydrogen –

are hexaquarks. The rules of

colour confinement would also

allow you to have a septaquark

if you could squeeze five quarks

and two antiquarks into the

same object.

an external observer sees a new phenomenon

emerge that looks nothing like individual

football fans. In particle physics, these

apparitions are known as quasiparticles.

For all the nuance of this picture, however,

it still makes physicists’ lives easier to think

of particles as real objects that exist in the

world. But that wasn’t the case when quarks

were first conceived.

In the 1950s, more particles were turning

up than physicists knew what to do with,

barrelling towards us out of the depths of

space or summoned into existence by particle

colliders. It created a messy zoo of particles

of different masses, charges and sizes that

seemed impossible to corral. The insight

that would resolve the chaos was developed

separately by three researchers: Murray

Gell-Mann, Yuval Ne’eman and George Zweig.

They noticed that many of these particles

obeyed a symmetry, suggesting that they

were all produced by different combinations

of the same core ingredients.

Instead of treating the particles in the zoo

as fundamental entities, Gell-Mann and the

others invented a new set of particles one size

smaller. With these quarks in place (Gell-Mann

coined the name and got most of the credit –

he was the only one to win a Nobel prize for his

work), the mess of particle physics suddenly

snapped into order. “The introduction of the

idea of quarks was revolutionary,” says Tara

Shears at the University of Liverpool, UK. “That

idea of a similarity, or symmetry, in behaviour

hinting at deeper structure is something we

hold very current in research today.”

Mathematical monsters

At first, no one was sure whether quarks

were real particles or just a helpful organising

idea. In a 1972 lecture, Gell-Mann warned his

audience against invoking “fictitious objects

in our models that end up turning into real

monsters that devour us”.

There were two pieces of evidence to suggest

that quarks were more than just monsters in the

mathematics. First, physicists firing electrons at

protons noticed that some bounced off at wide

angles. This suggested that the electrons had

hit something inside the proton – something

like a quark. What’s more, Gell-Mann’s model

indicated that certain combinations of quarks

remained undiscovered. Like the gaps in

Dmitri Mendeleev’s original periodic table of

the elements, this gave the model predictive

power. When the missing particles turned up

as expected, the quark model’s acceptance

was nigh-on guaranteed.

QUARKY

QUIRKS