CERN finds four new X particles – how big a deal is this?


Physicists at CERN have discovered a family of new 'exotic' particles. Cathal O’Connell assesses what it means.


The LHCb particle detector magnet.
CERN/SPL CERN/SPL
What’s just happened?

Physicists at CERN have discovered four new “tetraquark” particles – unusual arrangements of four fundamental particles called quarks. The new particles are highly unstable, decaying almost immediately into other particles.

These are not new fundamental particles heralding a new era of physics (like the unexpected new one recently hinted at) but rather are new combinations of previously known particles in the standard model of particle physics (See the Cosmos primer on particle physics for an explanation).

The new particles are called “exotic” because they are made of four quarks. Quarks usually group together in twos and threes.

Physicists at the Large Hadron Collider beauty (LHCb) collaboration made the discoveries by monitoring the decays of B mesons (so-called beauty particles) formed in CERN’s Large Hadron Collider.

The results have not yet been peer reviewed, but have been published in two papers (here and here) on the arXiv.org pre-print server, where physicists share their results prior to peer review.

Tetra-what?

At the deepest level, we are all made of quarks. Quarks are the fundamental particles that gang together to form protons and neutrons. They come in six “flavours”: up, down, top, bottom, strange and charm.

Quarks are most stable in groups of three. And so the physicist who proposed them, Murray Gell-Mann, coined their name from a quote in James Joyce’s novel Finnegan’s Wake, in which a publican dreams of a drunken seagull ordering beer by demanding “three quarks for Muster Mark”, instead of “three quarts for Mister Mark!”

But in the past two years, scientists have discovered that quarks can sometimes form other, temporary alliances – a bit like politicians making an unlikely crossbench deal.

In 2014 scientists at CERN discovered the first four-quark particle, called a tetraquark. Last year, the same team also discovered a five-member variety. None of these particles exist for more than a fraction of a second before decaying.

Discovery, or confirmation?

One of the new tetraquarks had been glimpsed before, though not with enough certainty to be called a “discovery”.

Now LHCb has confirmed its existence with a significant of over five sigma (meaning there’s a less than 1 in 3.5 million chance the signal is a chance fluctuation in the data).

The placeholder name of the particle, X(4140), seems like something off a car number plate, but actually comes from its mass measured in megaelectronvolts (4140 MeV). This makes it about four times heavier than the proton.

The other three particles are dubbed with equally catchy names, X(4274), X(4500) and X(4700). None of these three had been detected before – so they are new discoveries.

What are they made of?

All four are made of the same gang of quarks (one charm, one anti-charm, one strange and one anti-strange) but differ in the energy states of their constituents. In this sense, the four are really just versions of the same particle.

Besides their masses, the physicists were able to measure each particle’s quantum numbers, which describe their subatomic properties.

How big of a deal is this?

The new particles are interesting because they’re the first four-quark particles found that are made of only the heavy quarks (strange and charm). Studying them could help us understand how these big quarks interact.

But because these tetraquark particles are transient, they likely have a very minor role in the makeup of the universe.

With LHC now revved up to a collision energy of 13 TeV, and generating one billion proton collisions per second. That unprecedented deluge of data can only lead to more quirks and varieties of quarks in future.

Expect more tetraquarks.

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Cathal O'Connell is a science writer based in Melbourne.
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