This spring, at a gathering of Syracuse University’s quark physics group, Ivan Polyakov announced that helium had uncovered the fingerprints of a semi-mythical particle.
“We said, ‘This is impossible. What mistake are you making?’” recalled Sheldon Stone, the group’s leader.
Polyakov went distant and double-checked his investigation of information from the Large Hadron Collider quality (LHCb) experimentation the Syracuse radical is portion of. The grounds held. It showed that a peculiar acceptable of 4 cardinal particles called quarks tin signifier a choky clique, contrary to the content of astir theorists. The LHCb collaboration reported the find of the composite particle, dubbed the double-charm tetraquark, astatine a league successful July and successful two papers posted earlier this period that are present undergoing adjacent review.
The unexpected find of the double-charm tetraquark highlights an uncomfortable truth. While physicists cognize the nonstop equation that defines the beardown force—the cardinal unit that binds quarks unneurotic to marque the protons and neutrons successful the hearts of atoms, arsenic good arsenic different composite particles similar tetraquarks—they tin seldom lick this strange, endlessly iterative equation, truthful they conflict to foretell the beardown force’s effects.
The tetraquark present presents theorists with a coagulated people against which to trial their mathematical machinery for approximating the beardown force. Honing their approximations represents physicists’ main anticipation for knowing however quarks behave wrong and extracurricular atoms—and for teasing isolated the effects of quarks from subtle signs of caller cardinal particles that physicists are pursuing.
The bizarre happening astir quarks is that physicists tin attack them astatine 2 levels of complexity. In the 1960s, grappling with a zoo of recently discovered composite particles, they developed the cartoonish “quark model,” which simply says that quarks glom unneurotic successful complementary sets of 3 to marque the proton, the neutron, and different baryons, portion pairs of quarks marque up assorted types of meson particles.
Gradually, a deeper mentation known arsenic quantum chromodynamics (QCD) emerged. It painted the proton arsenic a seething wide of quarks roped unneurotic by tangled strings of “gluon” particles, the carriers of the beardown force. Experiments person confirmed galore aspects of QCD, but nary known mathematical techniques tin systematically unravel the theory’s cardinal equation.
Somehow, the quark exemplary tin basal successful for the acold much analyzable truth, astatine slightest erstwhile it comes to the menagerie of baryons and mesons discovered successful the 20th century. But the exemplary failed to anticipate the fleeting tetraquarks and five-quark “pentaquarks” that started showing up successful the 2000s. These exotic particles surely stem from QCD, but for astir 20 years, theorists person been stumped arsenic to how.
“We conscionable don’t cognize the signifier yet, which is embarrassing,” said Eric Braaten, a particle theorist astatine Ohio State University.
The newest tetraquark sharpens the mystery.
It showed up successful the debris of astir 200 collisions astatine the LHCb experiment, wherever protons smash into each different 40 cardinal times each second, giving quarks uncountable opportunities to cavort successful each the ways quality permits. Quarks travel successful six “flavors” of masses, with heavier quarks appearing much rarely. Each of those 200-odd collisions generated capable vigor to marque 2 charm-flavored quarks, which measurement much than the lightweight quarks that comprise protons but little than the gigantic “beauty” quarks that are LHCb’s main quarry. The middleweight charm quarks besides got adjacent capable to pull each different and enactment successful 2 lightweight antiquarks. Polyakov’s investigation suggested that the 4 quarks banded unneurotic for a glorious 12 sextillionths of a 2nd earlier an vigor fluctuation conjured up 2 other quarks and the radical disintegrated into 3 mesons.