When proton–proton collisions turn strange

Protons colliding into other protons at CERN’s Large Hadron Collider (LHC) flushed out the Higgs boson. But for studying quark–gluon plasma (QGP)—the hot, dense soup of unconfined quarks and gluons that briefly filled the universe a few microseconds after the Big Bang—those collisions were supposed to be irrelevant. They are even used as QGP-absent baselines to compare with when investigating heavy-ion collisions that do produce QGP. However, in 2010 the CMS collaboration spotted something unexpected in proton–proton collisions. In the debris of rare, so-called high-multiplicity events—that is, events that produce an unusually high number of charged particles—the researchers discovered spatial correlations reminiscent of those attributed to QGP formation in heavy-ion collisions (see the article by Barbara Jacak and Peter Steinberg, Physics Today, May 2010, page 39 ). The ALICE and ATLAS collaborations soon corroborated the discovery. In addition, ALICE researchers found that in proton–lead ion collisions, the relative yield of particles that contain strange quarks increases with multiplicity. Strangeness enhancement is another hallmark of QGP formation. Now the ALICE collaboration reports that in high-multiplicity proton–proton collisions, such as the one shown here, strangeness is similarly enhanced. The finding is the latest entry in a growing, though not yet conclusive, list of evidence that QGP can form even in proton–proton collisions. Now that the LHC is operating at higher energies, the high-multiplicity collisions are both more frequent and extend to higher multiplicities. Researchers hope to decisively establish whether QGP can indeed be created in proton–proton collisions. And if that’s the case, physicists could probe the properties of QGP in a far simpler system than the one produced in heavy-ion collisions. (J. Adam et al., ALICE collaboration, Nat. Phys. 13, 535, 2017 . Image courtesy of CERN.)