As Fermilab’s tiny, mighty antineutrinos smashed into a proton, they turned one of its up quarks into a down quark, converting the proton into a neutron (which has the opposite quark configuration of a proton). The proton’s three quarks are separated into two types, or flavors: two “up” quarks and one “down” quark. MINERvA uses both, but measuring the outcome of an interaction with protons is easier with antineutrinos.) (An antineutrino is the antimatter counterpart of a neutrino. Cai led the Main Injector Neutrino Experiment to Study v-A Interactions (MINERvA) collaboration at the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Ill., in the latest hunt for the proton structure using an antineutrino beam-which many of his colleagues thought would be futile. we have a lot of difficulties seeing our universe in terms of these neutrinos because they’re so difficult to measure,” says Tejin Cai, a postdoctoral researcher at York University in Toronto. “I always imagine this as different ways of looking at stuff. This means any interaction that does occur takes place very, very close by, making neutrinos useful in measuring other small things. Neutrinos are so elusive that trillions of them pass through your hand every second without interacting with you. With electrically neutral neutrinos, this sketch changes quite a bit. By tracking how the electrons in each beam ricocheted off the hydrogen molecules’ protons, the physicists could progressively sketch out the electric charge distribution for a single proton-in other words, its size. The puzzle-solving electric charge radius results of 2019 largely emerged from work at the Thomas Jefferson National Accelerator Facility (Jefferson Lab) in Newport News, Va., where physicists shot beams of electrons at proton-packed hydrogen. Getting answers to both, then-the proton’s “electric charge radius” as well as its “neutrino radius”-is a potent cross-check of the proton’s size. But quarks have their own slippery, probabilistic properties, too-any answers they reveal depend on what exactly they are being asked, and querying them via electrical charge is a different sort of question than probing with neutrinos, which have no charge. There is instead a shape-shifting maelstrom of quarks, and physicists can map quark distribution to estimate the proton’s size. There is no physical membrane that delineates where a proton starts and where it ends. This switch matters because a proton, like all things in the quantum realm, is less a concrete object with well-defined boundaries and more a hazy cloud of probabilities. Their findings were published in Nature in February. (Whether the discrepancy was because of experimental errors or the signposting of as-yet-unknown physics remains up for debate.) Now researchers using a new and entirely independent method to measure proton size-one involving neutrinos rather than electrical charge-are weighing in, too. Most physicists considered this “proton radius puzzle” solved in 2019 when painstaking follow-up work convincingly settled on the lower value for the particle’s size as correct. But in 2010 a new, even more precise electric charge technique suggested the proton’s radius was some 4 percent smaller still-a seemingly minuscule discrepancy that nonetheless deviated immensely from theoretical expectations. Using two types of ultraprecise measurement that each probed the proton’s electric charge, researchers pegged the particle’s radius as about 0.877 femtometer (a femtometer is a trillionth of a millimeter). Protons are the workhorses of cosmic creation and particle physics alike-yet, despite all this, we struggle to know simply how big they are.Īfter a half century of effort, by the turn of the millennium physicists thought they were approaching an answer. We know protons themselves are composed of a trio of even smaller particles called quarks-which we learned in part by building gigantic, multibillion-dollar machines to slam protons together at nearly the speed of light. We know a proton paired with an electron makes hydrogen-the first element on the periodic table and the fuel that allows stars to shine. We know these tiny, positively charged particles reside within the nucleus of every atom and constitute most of the ordinary matter in the universe. For a lesson on just how much-and just how little-we really know about the universe, consider protons.
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