Published: 09 May 2017

In a paper published on 24 April 2017 in Nature Physics, the ALICE collaboration reports observing that proton collisions sometimes present similar patterns to those seen in collisions of heavy nuclei. This behaviour was spotted through observation of so-called strange hadrons in a class of proton collisions characterized by the creation of more particles than the average, using the 7 TeV proton collision data from LHC run 1.

Strange hadrons are well-known particles - which have been given names such as Kaon, Lambda, Xi and Omega - all containing at least one strange quark. The observed ‘enhanced production of strange particles’ is a familiar feature of quark-gluon plasma, a very hot and dense state of matter that existed just a few millionths of a second after the Big Bang, and is commonly created in collisions of heavy nuclei. But this is the first time ever that this phenomenon – the enhanced production of strange particles - is unambiguously observed in the rare proton collisions in which many particles are created. This result is likely to challenge existing theoretical models that do not predict an increase of strange particles in these events.

“We are very excited about this discovery,” said Federico Antinori, Spokesperson of the ALICE collaboration. “We are again learning a lot about this primordial state of matter. Being able to isolate the quark-gluon-plasma-like phenomena in a smaller and simpler system, such as the collision between two protons, opens up an entirely new dimension for the study of the properties of the fundamental state that our universe emerged from.”

The study of the quark-gluon plasma provides a way to investigate the properties of strong interaction, one of the four known fundamental forces. This state of matter is produced at very high temperature and energy density, when ordinary matter undergoes a transition to a phase in which quarks and gluons become ‘free’ and are thus no longer confined within hadrons. These conditions can be obtained at the Large Hadron Collider by colliding heavy nuclei at high energy. Unlike the up and down quarks, which are those that compose normal matter, the heavier strange quarks are not brought into the reaction by the colliding nuclei. Therefore, any strange quarks or antiquarks observed in experiments have been "freshly" made from the kinetic energy of the colliding nuclei. This is why an abundance of strange quarks is considered as a signature for the quark-gluon plasma state. This phenomenon may now have been observed within proton collisions as well.

The new results also show that the production rate of these strange hadrons increases with the ‘multiplicity’ – the number of particles produced in a given collision – faster than that of other particles generated in the same collision, While the structure of the proton does not include strange quarks, data also show that the higher the number of strange quarks contained in the induced hadron, the stronger is the increase of its production rate. No dependence on the collision energy or the mass of the generated particles is observed, demonstrating that the observed phenomenon is related to the strange quark content of the particles produced.