Two decades of new physics have resulted in the discovery of a surprisingly complex pathway for creating strange matter within atoms.
Strange matter is matter that contains subatomic particles known as strange quarks. “Weird” here means far from our daily lives. Strange matter appears to emerge only in truly extreme situations, such as collisions of high-energy particles and possibly the very dense and pressurized cores of neutron stars. Examining the details of the emergence of strange matter is part of a broader effort by nuclear physicists to understand the basics of how elementary particles form. In this particular case, a group of researchers turned their attention to one of the strangest substances called lambda particles.
“This data is the first to study lambda. [atomic] Co-author of the study and associate lab director of the Physical Sciences and Engineering Division at Argonne National Laboratory, Kauter Hafidi, said:
Hadrons are subatomic particles made of quarks and subject to strong forces. This is the force that binds quarks together to create larger particles such as protons and neutrons and holds those protons and neutrons within the nucleus. Lambda particles are baryons. In other words, he, an up quark, a down quark, and a strange quark, is a kind of hadron made up of three quarks. Most of the quarks are either up- or down-variations, says Lamiaa El Fassi, lead author of the new study and associate professor of experimental nuclear physics at Mississippi State University. Strange quarks are heavier and rarer beasts than up and down quarks, and the particles they form are correspondingly much less stable and tend to collapse very quickly.
Daniel Brandenburg, assistant professor of physics at Ohio State University, says the rare and slippery nature of strange quarks is what makes them so attractive to researchers. “Our simple picture of protons and neutrons is that they contain up and down quarks,” he says. “He said one of the reasons why strange quarks are interesting, at least in this simple picture, is that they don’t exist in the first place. You have to create them somehow.”
Lambda particles have been studied before, but in the new paper, the researchers used a special process called semi-inclusive deep inelastic scattering to create lambda particles inside the nucleus. This involves firing an electron beam at the nucleus. This transfers energy to the quarks and neutrons within the proton, stimulating lambda production.
But despite these diligent efforts, the esoteric laws of quantum mechanics dictate that, once again, electrons do not directly interact with quarks. Instead, the colliding electrons emit “virtual” photons. This is because they are almost non-existent. These photons are reabsorbed by quarks almost as fast as they are emitted. The resulting energetic kicks pinball the quarks into the nucleus, where they combine with other quarks to create lambdas and other “composite” particles.
This particle alchemy took place in 2004 at the Thomas Jefferson National Accelerator Facility. At the time, Elle Fassi was using the dataset to do another study, but eventually decided to look for evidence of lambda particles in the dataset as well. It took more than a decade of effort to elicit the faint signal of lambda decay (the particles are too short-lived to be detected directly). “It’s a long journey,” says El Fassi.she and her colleagues reported their findings to the journal Physical review letter.
By studying the energy and momentum of the particles produced by decaying lambdas, El Fassi and her colleagues can pinpoint exactly what happened to the freed quarks roaming the nucleus. I was. Interactions with other subatomic particles absorbed the quarks’ energy to varying degrees, resulting in momentum changes as they linked with other quarks to form hadrons.
Most surprisingly, researchers found differences in the production of high-energy and low-energy lambda particles. This suggests that these particles may form in unexpected ways. Instead of a virtual photon he collided with one quark and freed it to find two new quarks to combine, as theorists have long assumed, the virtual photon is a quark known as a diquark. Sometimes it seemed to interact with the pair. Likely composed of the mundane up and down quarks that are very abundant in the nucleus, this die quark then goes in search of a third quark and eventually combines with the strange quark. When this happens, the result is a lambda particle. The discovery doesn’t just reveal how these strange and unusual particles form, says Brandenberg. A particle’s final energy and momentum contain information about what it encounters on its journey through the nucleus, so they can also help reveal what’s going on in the atom’s hidden heart.
However, not all physicists are convinced that this di-quark hypothesis reflects how lambdas actually form. There is another model that could explain the energy and momentum patterns the researchers observed, says Jen-Chieh Peng, a professor of nuclear physics at his University of Illinois at Urbana in Champaign. For example, the pattern of momentum transfer between particles that researchers attribute to diquark dynamics, he says, could be the result of one quark picking up two quarks separately. This means that the original “quark-by-quark” notion of how tripartite particles such as lambda form is correct. “Their data are interesting, but I think their interpretation is very off the mark,” he says.
Better measurements will settle the debate in the near future. El Fassi says the Jefferson Lab’s electron beam is now twice as powerful as his in 2004, and a new hadronization experiment is planned for next year. An electron-ion collider, a particle accelerator currently planned at Brookhaven National Laboratory, would also be a powerful new tool for similar experiments, says Brandenberg.
“Because we’re still building it,” he says.
