Europe's largest particle accelerator, the Large Hadron Collider, is back in action. According to two newly published studies, its latest round of experiments yielded three new "exotic" particles and confirmed the existence of a fourth.

The newly identified particles are considered "exotic" because they contain four quarks, the building blocks of all matter. Particle physicists used to believe all particles were composed of mesons, a quark-antiquark pair, or baryons, three quarks -- but no more than three quarks. A litany of discoveries have shown otherwise.

The exotic particles are named for their reconstructed mass in megaelectronvolts -- a single electronvolt is approximately 160 zeptojoules, a tiny fraction of a joule. The particle X(4140), for example, has a mass of 4,140 megaelectronvolts. Scientists had previously observed X(4140); the latest findings confirm its existence.

Three heavier exotic particles spotted by CERN physicists -- X(4274), X(4500) and X(4700) -- had never been seen before.

"Even though the four particles all contain the same quark composition, they each have a unique internal structure, mass and their own sets of quantum numbers," researchers explained in a news release.

Continued research is necessary to further illuminate the idiosyncrasies of each exotic particle.

The latest findings are detailed in two papers, both published online in the open source journal Arxiv.