While physicists know the exact equation that defines the strong force—the fundamental force that binds quarks together—they can rarely solve this endlessly iterative equation, so they struggle to predict the strong force’s effects. Via QuantaMagazine
Somehow, the quark model can stand in for the far more complicated truth, at least when it comes to the menagerie of baryons and mesons discovered in the 20th century. But the model
For a tetraquark, that’s an eternity. Previous tetraquarks have contained quarks paired with their equally massive opposing antiquarks, and they tended to puff into nothingness thousands of times faster. The new tetraquark’s formation and subsequent stability surprised Stone’s group, which expected charm quarks to attract each other even more weakly than the quark-antiquark pairs that bind more ephemeral tetraquarks. The tetraquarks' tenacity is a fresh clue to the strong force enigma.
The same was true of “lattice QCD” computer simulations, a powerful approach to approximating QCD. These simulations capture the richness of the theory by analyzing quarks and gluons interacting at points on a fine grid instead of throughout a smooth space. All lattice QCD simulations agreed that the heaviest quarks could make tetraquarks. But when researchers swapped in charm quarks, most simulations found that double-charm tetraquarks couldn’t form.
For lattice QCD practitioners, the new tetraquark highlights the problem that key details about the midsize quarks may be getting lost between their lattice points. Lightweight quarks can zip around enough to allow their movement to be captured even against a coarse grid. And researchers can deal with heavy, more stationary quarks by pinning them to one spot. But charm quarks inhabit an awkward middle ground, and researchers think they’ll need to zoom in to better discern their behavior.