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The LHC Is Now Blasting Lead Ions at Each Other at Record-Breaking Energies

Heavy ions take over for one month only.
Collision between lead ions seen within the CMS detector Image: CMS/CERN

On Wednesday, engineers at the Large Hadron Collider finished the week-long process of warming up the chain of particle accelerators destined to deliver the experiment's latest record-breaking feat: collisions between beams of heavy lead ions at energies of roughly twice that of any previous collider experiment.

For most of the year, the LHC collides protons. These are the positively charged particles that, together with neutrons, make up atomic nuclei. Put some electrons in orbit around one of these nuclei and you have a proper atom. Strip away those electrons, so that you have just the positively charged core, and the result is an ion.


A lead nucleus comes with a whopping 82 protons, making it one of the heaviest naturally occurring elements on the periodic table (uranium is heaviest at 92, while hydrogen is lightest, with just a single proton). So, you can already see how we're going to get some extreme energies in lead ion collisions.

Heavy ions get the LHC all to themselves for a whole month.

Collision between lead ions seen within the ALICE detector. Image: ALICE/CERN

"It is a tradition to collide ions over one month every year as part of our diverse research programme at the LHC," offered CERN Director General Rolf Heuer in an announcement. "This year however is special as we reach a new energy and will explore matter at an even earlier stage of our universe."

So, yes, that's the idea: With temps in the neighborhood of several trillion degrees, the ion collisions will offer a peek into the universe as it was within its first few millionths of a second of existence. All that was around then was a homogeneous stew of quarks and gluons, particles that come together to form protons in the cooled-off universe we see today (gluons "glue" together quarks).

In this very early period, the quark-gluon universe would have been ruled by the strong force. This is one of the four fundamental forces of physics (together with the weak force, the electromagnetic force, and gravity), and nowadays we know it only within the context of atomic nuclei. With a greater volume of quark-gluon plasma at higher energies (compared to last year's heavy ion run, that is), the LHC will put us a bit closer to understanding a universe consisting mostly of this strongly-interacting medium.

And also understanding what happened right after the quark-gluon medium to eventually lead to, well, us.