As a lesson in how much we know and know about the universe, let’s think about protons. We know that these tiny, positively charged particles exist within the nucleus of every atom and make up most of the ordinary matter in the universe. I know to make It is the first element on the periodic table and the fuel that makes stars shine. We know that the proton itself is made up of a trio of even smaller particles called quarks. I learned this in part by building a huge multi-billion dollar machine that collides protons at nearly the speed of light. Protons are the protagonists of the creation of the universe and of particle physics, yet we struggle to know just how big protons are.
After half a century of effort, by the turn of the millennium, physicists thought they were close to an answer. Using two of his ultra-precise measurements, each of which probes a proton’s charge, the researchers pinpointed the particle’s radius to about 0.877 femtometers (a femtometer is one trillionth of his millimeter). However, in 2010, a new, more accurate charge technique suggested that the proton radius was about 4% smaller.
Most physicists believe this ‘proton radius puzzle’ has been solved in 2019, and painstaking follow-up work has convincingly settled on a lower value for the particle’s size. . (Whether this discrepancy is due to experimental error or an as-yet-unknown physics signpost is debatable.) Now there is a new, completely independent method for measuring proton size. , that is, a method that uses neutrinos rather than charges is being considered by researchers. , that too. Their findings were Nature in February.
This switch is important because protons, like everything in the quantum realm, are hazy clouds of probability rather than concrete objects with well-defined boundaries. There is no physical membrane that indicates where protons start and end. Instead, there are maelstroms of shape-shifting quarks that allow physicists to map the distribution of quarks to estimate proton size. But quarks also have their own slippery stochastic properties. The answer Quark reveals depends on what exactly is being asked. Getting answers to both the proton’s “charge radius” and its “neutrino radius” is a powerful cross-check of the proton’s size.
The 2019 puzzle-solving Charge Radius results emerged primarily from work at the Thomas Jefferson National Accelerator Facility (Jefferson Labs) in Newport News, Virginia. There, physicists hit proton-packed hydrogen with an electron beam. By tracking how each beam’s electrons bounce off the hydrogen molecule’s protons, physicists were able to sketch step-by-step the charge distribution of a single proton, and thus its size. For electrically neutral neutrinos, this sketch changes considerably. Neutrinos are extremely elusive, with trillions of neutrinos passing through your hand every second without interacting with you. Neutrinos are useful for measuring other small things.
“I always imagine this as another way of looking at it… It’s very difficult to see our universe from the perspective of neutrinos because neutrinos are very difficult to measure,” Toronto. Cai co-led the Main His Injector Neutrino Experiment (MINERvA) to study his vA interactions at the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois. be wasted. (Antineutrinos are the antimatter counterparts of neutrinos. MINERvA uses both, but it is easier for antineutrinos to measure the consequences of their interaction with protons.)
The proton’s three quarks are divided into two types or flavors: two “up” quarks and one “down” quark. When Fermilab’s small, powerful antineutrinos collided with the proton, one of its up quarks changed to a down quark, converting the proton into a neutron (which has the opposite quark configuration to the proton). Based on this new neutron position, physicists worked backwards to figure out where the shape-shifting up quark was at the moment of transformation. This gave us clues about the distribution within the proton.
“What excites me about this measurement is that we can now do things with neutrinos that were previously only possible with electrons,” says Cai.
The team’s measurement of the proton radius was 0.73 femtometers, even smaller than the charge radius of 0.84 femtometers. In both cases, it is about 10,000 times smaller than a hydrogen atom.
To be clear, this apparent 13% contraction does not hurt the charge radius measurements, nor is it as shocking as it seems. The two measurements are complementary and work together to give a complete picture of the tiny proton. Because they measure different distributions of matter, the discrepancy does not challenge our understanding of protons in the same way as the previous 4% contraction. Instead, it adds to that understanding.
“What makes this measurement really interesting is not whether it matches the electronic measurement of the electromagnetic proton radius, but the fact that it didn’t have to match at all,” says Deborah, MINERvA co-spokesperson. says Harris. experiment. This is because the way neutrinos interact with up and down quarks is very different from how quarks interact with electrons. Instead of electromagnetic interactions, neutrinos interact via another force called the weak force. (But don’t let the name fool you. The weak force is very powerful at subatomic distances!)
In fact, the so-called “weakly charged” radius of the proton and its charge radius match each other within uncertainty, and it is not particularly surprising that the radii are similar. Electron scattering experiments measure where electrons touch quark charges, and neutrino experiments measure where antineutrinos change the flavor of quarks. These two regions should be close to each other, as they both depend on the position of the quark-changing clouds. But perhaps even more influential than our current understanding of the proton’s structure are the implications of new research for how neutrinos might be used in the future.
“This new measurement is more important than ours because it addresses a new type of interaction, perhaps even more so,” said physicist at North Carolina Agricultural Technology State University and author of Charge says Ashot Gasparian, one of the Radius research at the Jefferson Institute. These interactions are important in understanding other big questions in physics besides the proton radius, he says. For example, the mystery of how neutrinos acquire their minimal mass.
