A Particle Full of Charm

The Xicc++ particle will become a tool to observe the strong force in another light.


By Shelby Traynor

Collage by Alex Hanson using an image representing the new particle observed by LHCb, containing two charm quarks and one up quark. (Image credit: Daniel Dominguez/CERN)

Almost two-hundred metres beneath the France-Switzerland border, physicists at CERN’s Large Hadron Collider have observed a new particle. A charming particle. They’ve been on the look-out for him for some time, and now they’ve got him— Xicc++.  

The name doesn’t exactly roll off the tongue, but this guy is more than meets the eye. He’s heavy. He’s charming. He’s a subatomic particle, and he’s a real catch. CERN’s particle smasher— based near Geneva, Switzerland— spotted the fellow during the Large Hadron Collider beauty experiment, or the LHCb experiment for short.

Xicc++ is the elusive brother of the proton, neutron, and of a number of other composite subatomic particles. These particles are all a part of the same family, because they’re all made up of three quarks. What are quarks, you ask? Well, let’s dive in.

I’ll start with the universe. Most of what you know is made up of quarks— you, your cat, that house plant you keep killing— because quarks make up the familiar particles we all know and love. Again, think protons and neutrons. The fan favourites.

When the atom was discovered, it was thought to be indivisible. When the atom was split to reveal protons, neutrons, and electrons, they were also thought to be indivisible. At this time, leptons and quarks seem to be the fundamental buildings blocks. The smallest it gets. But physicists aren’t strangers to being proved wrong.

There are different kinds of quarks: up, down, strange, charm, top, and bottom. This is a helpful song to learn, if you ever want to become a high school science teacher who’s hip with the kids. Up and down quarks are the most common, and they’re on the lighter end of the scale in terms of mass. Strange, charm, top, and bottom are heavier.  

Our mystery man, Xicc++, is part of the baryon family. A baryon is a particle made up of three quarks. Again, the most famous baryons are protons and neutrons, because they make up most of the mass of the visible matter in the universe. Plants and trees, Britney Spears, the works.

Here’s where it gets tricky, and where Xicc++ stands out from the rest.

A proton is made up of two up quarks and one down quark. Up Up Down. A neutron is made up of two down quarks and an up quark. Down Down Up. In theory, any three kinds of quarks can make up a baryon. Anything is possible, quarks. Go chase your dreams.

Protons and neutrons, though, are pretty light. All baryons observed to this point have been made up of just one heavy quark— strange, charm, top, or bottom— or in the case of the proton and neutron, none at all.

This is where Xicc++ comes in. You can call him sigh-see-see-plus-plus if you like, I’m sure he won’t mind. The Xicc++ particle is made up of two charm quarks and one up quark. Charm Charm Up. A doubly charmed particle, four-times heavier than a proton.

So why does a baryon’s mass matter so darn much?

It’s understood, right now, at this moment, that there are four fundamental forces acting throughout the universe. Gravitation, electromagnetism, the weak force, and the strong force. It’s that last one that applies here.

The strong force binds quarks together to make subatomic particles—protons, neutrons, and some of our lesser known friends lambda, sigma, xi, omega. The list goes on.  In a typical baryon, three quarks rotate around one another almost equally, in a sort of dance. Up Up Down, Down Down Up, dancing around one another. If one quark strays too far out, the strong force will pull it back into line.

But in Xicc++, Charm Charm Up, the charmed quarks are heavier. They aren’t nimble enough to dance. Physicists expect the inside of a Xicc++ particle behaves differently. They suspect the two heavier charm quarks act like suns, while the lighter up quark orbits around them. A tiny solar system.

Guy Wilkinson, a former spokesperson for the beauty experiment, said at the EPS Conference on High Energy Physics in Venice on July 6th:

“In contrast to other baryons, in which the three quarks perform an elaborate dance around each other, a doubly heavy baryon is expected to act like a planetary system, where the two heavy quarks play the role of heavy stars orbiting one around the other, with the lighter quark orbiting around this binary system.”

What Guy is talking about there is the strong force behaving in a way we haven’t quite observed before.

The strong force, and quarks, are theorized to have formed within the second after the Big Bang. In the theoretical blink of an eye. At 10-35 seconds, the strong force separated from the electro-weak force. Quarks and leptons were created. 10-6 seconds and those quarks formed protons and neutrons. 3.4 billion years later, and here we all are.

The Xicc++ particle will become a tool to observe the strong force in another light. If physicists can learn more about this fundamental force, their findings could hint at how we came to be, and how particles came to be, just seconds after the Big Bang.

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