Physicists say they've found evidence in data from Europe's Large Hadron Collider for three never-before-seen combinations of quarks, just as the world's largest particle-smasher is beginning a new round of high-energy experiments.
The three exotic types of particles – which include two four-quark combinations, known as tetraquarks, plus a five-quark unit called a pentaquark – are totally consistent with the Standard Model, the decades-old theory that describes the structure of atoms.
In contrast, scientists hope that the LHC's current run will turn up evidence of physics that goes beyond the Standard Model to explain the nature of mysterious phenomena such as dark matter. Such evidence could point to new arrays of subatomic particles, or even extra dimensions in our Universe.
The LHC had been shut down for three years to upgrade its systems to handle unprecedented energy levels. That shutdown ended in April, and since then, scientists and engineers at the CERN research center on the French-Swiss border have been getting ready for today's resumption of scientific operations.
CERN's control center was abuzz as the LHC began its third run of data collection and analysis.
"It's a magic moment now," CERN Director-General Fabiola Gianotti said during today's webcast. "We just had collisions at an unprecedented energy, 13.6 tera-electronvolts, and this opens a new era of exploration at CERN."
Gianotti said the LHC's scientists expect to collect as much data during this third run as they collected over the course of 13 years during the collider's previous two runs. "This, of course, will increase our opportunities for discovery or for understanding the fundamental laws of the Universe," she said.
The 27-kilometer-round (17-mile-round) ring of superconducting magnets and its particle detectors are due to be in operation around the clock for nearly four years during Run 3.
Today's start of the run comes 10 years and a day after LHC physicists announced their biggest discovery to date: evidence for the existence of the Higgs boson, a subatomic particle that helps explain the phenomenon of mass.
The three new types of subatomic particles, described today during a CERN seminar, aren't quite Higgs-level revelations. But they do suggest that the LHC is hot on the trail to discover still more previously unseen building blocks of the Universe.
The Large Hadron Collider smashes protons together at velocities close to the speed of light to study combinations of quarks that are known as hadrons.
"The more analyses we perform, the more kinds of exotic hadrons we find," Niels Tuning, physics coordinator for the collider's LHCb detector, said in a news release.
"We're witnessing a period of discovery similar to the 1950s, when a 'Particle Zoo' of hadrons started being discovered and ultimately led to the quark model of conventional hadrons in the 1960s. We're creating 'Particle Zoo 2.0'."
LHCb spokesperson Chris Parkes said studying new combinations of quarks "will help theorists develop a unified model of exotic hadrons, the exact nature of which is largely unknown".
Most hadrons aren't so exotic. Protons and neutrons, for instance, are made up of three quarks bound together. (In fact, the origin of the word "quark" goes back to a line from Finnegan's Wake by James Joyce: "Three quarks for Muster Mark!") Pions are two-quark combinations.
Four-quark and five-quark combinations are much rarer, and are thought to exist for only an instant before decaying into different types of particles.
Quarks come in six different "flavors": up and down, top and bottom, charm and strange.
The LHCb team analyzed the decays of negatively charged B mesons and saw evidence for the existence of a pentaquark consisting of a charm quark and a charm antiquark, plus an up, down and strange quark. It's the first pentaquark known to include a strange quark.
The two newly identified tetraquarks include a "doubly electrically charged" combination of four quarks: a charm quark, a strange antiquark, an up quark, and a down antiquark.
That tetraquark was spotted in combination with its neutral counterpart, which has a charm quark, a strange antiquark, an up antiquark, and a down quark. CERN says this is the first time a pair of tetraquarks has been observed together.
Some theoretical models visualize exotic hadrons as single units of tightly bound quarks. Others see them as pairs of standard hadrons that are loosely bound together, similar to the way that atoms are bound together to form molecules.
"Only time and more studies of exotic hadrons will tell if these particles are one, the other or both," CERN says.