in a recreation of the primordial soup that existed just moments after the Big Bang.

By smashing billions of lead ions together using the Large Hadron Collider,

researchers were able to find just about 100 of these mysterious particles,

but that could be the start of figuring out what exactly this “X” particle is made of.

That’s why it’s just called the “X” particle; it’s not like it broke up with a physicist.

There are a few different ideas on what it could be but no one is sure yet.

Figuring it out could tell us more about what the universe was like a split second after it formed

and help us better understand the protons and neutrons that make up the nuclei of atoms.

You know, life’s little questions.

To understand the mystery, we’re going to have to talk about fundamental particles called quarks.

Don’t panic, it’ll be easy.

Some particles like protons and neutrons are made up of three quarks.

It’s also possible to have particles made of just two quarks, or more specifically a quark and antiquark pair.

These particles are called mesons.

Binding quarks together inside mesons, protons, and neutrons are gluons, which carry the strong force.

And here’s some shocking news: The strong force is really strong.

So strong that an isolated quark has never been observed because they seem to be locked inside protons and neutrons

in all but the most extreme conditions.

Now we come to the mystery of the “X” particle:

We think it’s made up of four quarks but nobody has been able to figure out yet how they’re arranged.

The two leading ideas suggest either the four quarks are tightly bound up in a teeny space

less than a third of a femtometer across,

or they could be paired up in two mesons that form something like a molecule that's loosely bound

and as large as five femtometers.

For reference, we think a proton has a radius of about 0.84 femtometers.

This is where the Large Hadron Collider comes in.

The LHC was not the first particle accelerator to spot the “X” particle;

it was actually first spotted almost two decades ago by the BELLE collaboration in Japan.

But until now, all the experiments that have spotted it were either smashing together electrons and positrons,

protons and antiprotons, or protons and protons.

Scientists think the key to unraveling the “X” particle’s secrets could be recreating conditions like those just after the Big Bang.

That isn’t easy to do by smashing protons together, even at very high energies.

Now you may be wondering why the LHC is different because as you may know, the LHC also smashes protons together.

That is famously how it found the Higgs Boson, often known as “the God Particle,”

but don’t call it that in front of a physicist because it turns out most of them hate that name.

However, for part of each year, the LHC stops smashing protons together and instead sends something much heavier

hurtling around it’s enormous ring: lead.

When two lead ions collide inside the LHC, hundreds of protons and neutrons are slammed together in an instant

and heat up to over 2 trillion degrees.

That’s 100,000 times the temperature inside the core of the Sun.

That’s so hot I didn’t have to say if it’s Celsius, Kelvin, or Fahrenheit because it doesn’t really matter.

Above that threshold, the quarks and gluons normally bound up inside the protons and neutrons

can break their bonds and create what’s called a quark-gluon plasma.

We think the entire universe was in a quark-gluon plasma state for a few millionths of a second after the Big Bang,

allowing exotic matter like “X” particles to form and decay again until things started cooling down

and particles like protons and neutrons formed.

When researchers at the LHC started looking for “X” particles produced by lead ion collisions,

they weren’t sure if recreating this primeval particle soup would help them or hinder them

because it would also generate tons of other particles that might mask their presence.

There’s also the problem that “X” particles decay so fast that the particle itself can’t be detected,

only what it decays into.

Using machine learning algorithms to sniff out the telltale signs of decaying “X” particles,

they were able to identify about 100.

Just 100 from more than 13 billion collisions.

Doesn’t sound like very many, and it isn’t,

but with this method “X” particles were detected about 10 times more frequently than in proton-proton collisions.

Spotting more “X” particles in conditions similar to how they formed in nature can finally tell us what they’re made of.

If they decay very quickly, that would point to them being loosely bound meson molecules.

If they decay still really quickly but slightly less so, then they’re probably made of four tightly bound quarks.

Unfortunately, the 100 seen by the LHC aren’t enough to definitively point to one possibility or the other.

But now that we know how to spot them in a quark-gluon plasma,

scientists are ready to hunt for a lot more.

The LHC has been undergoing upgrades since 2018 but it’s gearing up for another run.

Heavy ion smashing is scheduled for November of 2022, and after scientists have a chance to sift through the data,

they may see enough “X” particles to finally solve the mystery of their makeup

and give them a name once and for all.

I hope they name it Julian. That’s a good name.

If you enjoyed this quick dip into the warm waters of fundamental particles, I’ve got great news.

I just made a three-part deep dive on our other channel Seeker+ all about the Higgs Boson.

I think it makes understanding particle physics surprisingly approachable and I’m so proud of it,

so do me a favor and go check it out.

In the meantime though, place your bets. So do you think this “X” particle is a tetraquark or two mesons?

Let us know in the comments section and be sure to subscribe to Seeker.

Until next time, thank you so much for watching!