This spring, at a meeting of Syracuse University’s quark physics group, Ivan Polyakov announced that he 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 away and double-checked his analysis of data from the Large Hadron Collider beauty (LHCb) experiment, which the Syracuse group is part of. The evidence held. It showed that a particular set of four fundamental particles called quarks can form a tight clique, contrary to the belief of most theorists. The LHCb collaboration reported the discovery of the composite particle, dubbed the double-charm tetraquark, at a conference in July and in two papers posted earlier this month that are now undergoing peer review.
The unexpected discovery of the double-charm tetraquark highlights an uncomfortable truth. While physicists know the exact equation that defines the strong force — the fundamental force that binds quarks together to make the protons and neutrons in the hearts of atoms, as well as other composite particles like tetraquarks — they can rarely solve this strange, endlessly iterative equation, so they struggle to predict the strong force’s effects.
The tetraquark now presents theorists with a solid target against which to test their mathematical machinery for approximating the strong force. Honing their approximations represents physicists’ main hope for understanding how quarks behave inside and outside atoms — and for teasing apart the effects of quarks from subtle signs of new fundamental particles that physicists are pursuing.
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