The Large Hadron Collider at CERN, most commonly known as the LHC, is used to be known for colliding very energetic protons in order to push forward the frontier of our knowledge in particle physics. The LHC machine has however not only been built to collide protons, and beams of other particles can also be injected.
The accelerator can indeed also be filled with beams of heavy ions that are then accelerator before colliding at very high energy. We are considering, at the LHC, collisions of lead nuclei that are more than 200 times more massive than protons and whose electric charge is 82 times larger.
Colliding two of such beams allows physicists to recreate conditions that are similar to those of the very first moments of the life of the Universe, at the time where particles were part of a huge cosmic soup (more information here). This state of matter is known as a plasma of quarks and gluons which are the elementary particles all atomic nuclei are made of (see here for more information).
[image credits: CERN]
In 2012, another type of collisions occurred for the first time at the LHC. One of the colliding beams has been taken to be a beam of heavy lead ions, whilst the other beam was a proton beam. Very surprisingly, data was showing the signs that mini big-bangs occurred, something characteristic from lead-lead collisions but not expected in asymmetric proton-lead collisions. The picture of such a collision is shown above, as recorded by the ALICE detector.
Since last week, LHC proton-lead collisions have restarted, at a higher energy than before. The key point is that the collision energy is now equal to the one of the lead-lead collisions of 2015, so that physicists will have a reference point to compare the results with.
This is exciting since lead-lead collisions have opened the doors of many particle physics puzzles that are still unresolved today. Asymmetric collisions could bring additional information allowing us to solve those puzzles. The future will tell!
More information is available from the ALICE website at CERN.
https://www.facebook.com/ALICE.EXPERIMENT/posts/1120492898019655
and
https://cds.cern.ch/record/2231887