Each week Quanta Magazine explains one of the most important ideas driving modern research. This week, our physics staff writer Charlie Wood describes the difficulties physicists still face in understanding the quantum quirks of the strong force.
| | The Unpredictable Strong Force Continues to Surprise Physicists By CHARLIE WOOD | | After more than a century of slamming particles together, physicists have a pretty good idea of what goes on in the heart of the atom. Electrons buzz in probabilistic clouds around a nucleus of protons and neutrons, each of which contains a trio of bizarre particles known as quarks. The force that holds all the quarks together to make the nucleus is the aptly named strong force. It is the strong force that must be overcome to split the atom. And it is the strong force that binds quarks together so tightly that no quark has ever been spotted solo.
These features of quarks, many of which might be encountered in a high school science class, have been established as experimental facts. And yet from a theoretical perspective, physicists can't really explain any of them.
True, there is a theory of the strong force, and it is a gem of modern physics. It goes by the name of quantum chromodynamics (QCD), with "chromo" referring to an aspect of quarks poetically dubbed "color." Among other things, QCD describes how the strong force intensifies as quarks separate and weakens as they come together, somewhat like an elastic band. That property is precisely the opposite of how more familiar forces like magnetism behave, and its discovery in the 1970s led to Nobel Prizes. Quarks, from a mathematical perspective, were largely demystified.
Yet that mathematics works best when the force between the particles is relatively weak, leaving much to be desired from a broad experimental perspective. QCD's predictions were spectacularly confirmed in collider experiments that smushed quarks close enough together that the strong force between them slackened. But when quarks are given free rein to be themselves, as they are in the nucleus, they pull apart from each other and strain at their confining bonds, and the strong force becomes so strong that pen-and-paper calculations fail. In those circumstances, the quarks form protons, neutrons and a host of other two-quark and three-quark particles known generally as hadrons — but no one can calculate why this occurs.
To learn what quirks quarks are capable of, physicists can really only run brute-force digital simulations (which have made remarkable strides in recent years) or watch particles ricochet in good old-fashioned collider experiments. And so, nearly 60 years after physicists first conceived of the quark, the particle continues to surprise.
What's New and Noteworthy
Just last summer, the LHCb collaboration at the Large Hadron Collider in Europe spotted signs of two hitherto unseen varieties of quark foursomes known as tetraquarks briefly zipping through the collider's underground tunnels. Cataloging the diversity of quark behaviors helps physicists refine their models for simplifying the complexities of the strong force by providing new examples of phenomena the theory must account for.
Tetraquarks were first discovered at the LHC in the summer of 2014, after more than a decade of hints that quarks might form these foursomes as well as ganging up in twos and threes. The discovery fueled a debatethat became heated despite hinging on a seemingly esoteric question: Should four quarks be thought of as a "molecule" of two loosely attracted double-quark hadrons known as mesons, or do they gather in more unusual pairings known as diquarks?
Over the following years, particle physicists amassed evidence of a small menagerie of exotic tetraquarks and five-quark "pentaquarks." One grouping stood out in 2021, a "double charm" tetraquark that lived thousands of times longer than its exotic brethren (clocking in at a Methuselah-like 12 sextillionths of a second). It proved that one variety of quark — the charm quark — could form more resilient pairs than most educated guesses or careful computations had predicted.
Around the same time, researchers developed a new way of sifting through the maelstrom that follows a proton-on-proton collision for evidence of chance encounters between quark composites. These brief rendezvous can reveal whether a given pairing of hadrons attracts or repels, a prediction beyond the reach of QCD. In 2021, physicists used this "femtoscopy" technique to learn what happens when a proton approaches a pairing of "strange" quarks. The insight may improve theories about what happens inside neutron stars.
Just last year, physicists learned that even the quarks in the well-studied helium atom hide secrets. Denuded helium atoms launched the field of nuclear physics in 1909, when Ernest Rutherford (or really, his junior collaborators) fired them at a sheet of gold foil and discovered the nucleus. These days, helium atoms have become the targets for even smaller projectiles. In early 2023, a team fired a stream of electrons at helium nuclei (made of two protons and two neutrons) and were baffled to find that the quark-filled targets ballooned far more than QCD had led them to expect. | | | Big Think featured an article about the history of quarks and why bigger bundles of them were so interesting in 2022, after new tetraquarks and pentaquarks popped up at the LHC. The author, the particle physicist Don Lincoln, participated in the first detection of the heaviest type of quark in 1995. | | | The physicist Derek Leinweber of the University of Adelaide in Australia has generated videos that capture the various behaviors of quarks and the "gluon" particles, carriers of the strong force, that glue them together. Such QCD simulations on supercomputers complement collision experiments as the main ways of understanding quarks. | | | PBS Space Time dives deeply into the concept of quark color and why quarks can't wander off on their own (at least under normal circumstances), in this video. | | | | | | |
