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Aug. 4 () –
Physicists have experimentally extracted force from the strong force, a quantity that strongly supports theories about how almost all mass or ordinary matter in the universe is generated.
This quantity, known as the strong force coupling, describes the force with which two bodies interact or “couple” under this force. The strong force coupling varies with the distance between the particles affected by the force.
Before this research, conducted by scientists at the Thomas Jefferson National Accelerator Facility, theories disagreed about how the coupling of strong forces over long distances should behave: some predicted that it should increase with distance, others that it should decrease, and others that it should decrease with distance. they should become constant.
Using data from the Jefferson Lab, physicists were able to determine the strong force coupling at the largest distances yet. Their results, which provide experimental support for the theoretical predictions, They recently appeared on the cover of Particles magazine.
Ninety-nine percent of the interactions of ordinary mass come from phenomena associated with the strong force, one of the four fundamental forces of particle physics, which binds smaller particles called quarks into larger particles called protons and neutrons that make up the nucleus of atoms of ordinary matter.
At smaller distances between quarks, the strong-force coupling is small, and physicists can solve it with a standard iterative method. However, at larger distances, the strong force coupling becomes so large that the iterative method no longer works.
“This is both a curse and a blessing,” he said. it’s a statement Alexandre Deur, a staff scientist at the Jefferson Lab and co-author of the paper. “While we have to use more complicated techniques to calculate this quantity, its absolute value triggers a number of very important emergent phenomena.”
Despite the challenge of not being able to use the iterative method, Deur and his team they extracted a strong coupling of forces at the largest distances between the affected bodies.
They extracted this value from a handful of Jefferson Lab experiments that were actually designed to study something else entirely: the spin of protons and neutrons.
These experiments fired Jefferson Lab’s polarized electron beam at polarized proton or neutron targets. Over several years of subsequent data analysis, the researchers realized that they could combine the information gathered about the proton and the neutron to extract a strong force coupling at larger distances.
“Only Jefferson Lab’s high-performance polarized electron beam, in combination with developments in polarized targets and detection systems, allowed us to obtain such data,” Chen said.
They found that as the distance between affected bodies increases, the strong-force coupling grows rapidly before leveling off and becoming constant.
“There are some theories that predicted this should be the case, but this is the first experimental time that we’ve actually seen this,” Chen said. “This gives us details about how the strong force actually works, at the scale of the quarks that make up protons and neutrons.”
These experiments were done about 10 years ago, when the Jefferson Lab electron beam was capable of providing electrons with an energy of up to 6 GeV (it is now capable of providing up to 12 GeV). The lowest energy electron beam was required to examine the strong force at these larger distances: a lower energy probe allows access to longer time scales and thus larger distances between affected particles.
Similarly, a higher energy probe is essential to bring closer views of shorter time scales and smaller distances between particles. Laboratories with higher energy beams, such as CERN, Fermilab, and the SLAC Accelerator, have already examined the coupling of strong forces on these smaller space-time scales, when this value is relatively small.
The magnified view offered by the higher energy rays has shown that the mass of a quark is small, only a few MeV. At least, that’s your textbook dough. But when the lower energy quarks are probed, their mass effectively grows to 300 MeV.
This is because quarks build up a cloud of gluons, the particle that carries the strong force, as they travel great distances. The mass-generating effect of this cloud accounts for most of the mass of the universe; without this extra mass, the mass of textbook quarks can only represent about 1% of the mass of protons and neutrons. The other 99% comes from this acquired mass.
Similarly, one theory posits that gluons are massless at short distances but effectively gain mass as they travel farther. Leveling out the coupling of strong forces over long distances supports this theory.
“If the gluons remained massless over long distances, the strong coupling of forces would continue to grow unchecked,” Deur said. “Our measurements show that the strong force coupling becomes constant as the distance probed increases, which is a sign that the gluons have acquired mass through the same mechanism that gives 99% of the mass to the proton and neutron.”
This means that the coupling of strong forces over long distances is important to understand this mass-generating mechanism. These results also help verify new ways to solve equations for quantum chromodynamics (QCD), the accepted theory describing the strong force.