Strong power is a mystery. Gluons bind quarks, one of the two basic building blocks of matter, to the proton and neutron at the center of every atom. As the name suggests, he is the most powerful of the four elemental powers, though he can only exert his powers at sub-atomic distances. Despite their power and importance, strong forces are the most difficult forces to observe in practice, and their behavior is nearly impossible to predict mathematically.
Now, a group of scientists at Brookhaven National Laboratory on Long Island have caught an unexpected and fresh glimpse of a powerful force at work. It was so unexpected that theorists invented new models to explain it. If the theorists are correct, this experiment is the first to measure how a strong force field varies over short distances. The results were announced on January 18th. Nature.
“I don’t think this local variation of the strong force field has been measured before,” says Aihong Tang, one of the Brookhaven physicists who conducted the new study. This will allow scientists to ‘study strong forces from a different perspective’.
To observe the mighty force, nations around the world poured billions of dollars into particle accelerators, breaking atoms apart in violent collisions. Here, quarks and gluons are released for a fraction of a second in a swirling soup of plasma, recombining into rare new particles as the fireball cools.
One of these strange particles, called the phi meson, lies at the heart of these latest puzzling results. Mesons are made up of one quark and one antiquark, while protons and neutrons are made up of three quarks. There are six different “flavors” of quarks, and phi mesons are made up of pairs of quarks and antiquarks with the same flavor called stranges.
Scientists wanted to know if the vortex motion of a soup of quarks and gluons could cause the phi mesons to spin together, like beach balls in a whirlpool, immediately after a collision. This effect, called spin polarization, has been seen in other exotic particles, although it is not taken for granted. By measuring if and how particles are bound to churning quarks and gluons, the researchers learned how strong forces affect the visible matter around us. Expect to get an unparalleled glimpse into what you’re building.
The task is no trivial task, requiring automated software (and a keen-eyed scientist) to identify phi mesons among the thousands of new particles produced with each collision. Karl Slifer, a nuclear physicist at the University of New Hampshire, said: No one was involved in the new experiments. “It’s a huge amount of information just by sifting through it.”
After finding the elusive quarry, scientists saw something completely unexpected. Phi mesons were indeed spinning with a soup of quarks and gluons, but they were far from theoretical predictions.
Here’s how these predictions work: Phi mesons can rotate in one of three directions. If their spins were unaffected by that swirling pool of particles, each of their spin directions would be equally common, and each would appear about a third of the time. Even small deviations from these one-third odds indicate that the spin of the phi meson was influenced by the orbital momentum around it.
What the researchers instead saw was a large deviation from the 1 in 3 probability, 1,000 times larger than traditional models could explain. Usual variables such as spin change interference from electromagnetic fields could not explain such large differences. The team released its preliminary results in 2017, leaving theorists baffled.
“We were really scratching our heads and saying, ‘What’s going on?'” says Xin-Nian Wang, a theoretical physicist at Lawrence Berkeley National Laboratory in California. A paper reporting the results. I couldn’t believe it at the time. It’s too big,” he recalls.
But the equally skeptical Brookhaven team ran the analysis over and over again, arriving at the same seemingly impossible result each time. “And then we realized that it could be something that we as theorists don’t understand,” says Wang.
Theorists have overlooked something big: Electromagnetic fields may not be strong enough to affect the spin of phi mesons, but what about fields produced by strong forces? Created by particles. Strong fields are generated when quarks and gluons move, just as electrons generate electromagnetic fields when they move. Existing models have essentially ignored the potential for strong fields, as their effects are usually irrelevant. Even at typical subatomic distances, the random motions of quarks and gluons that create such fields cancel out and have no effect on the system as a whole.
But at very small scales (think distances shorter than the minimal span of a proton), the cumulative details of all these random motions can actually matter. This is what Wang and his colleagues proposed in a recent preprint study. The movement of the phi meson itself creates a strong force field, small variations of which affect the spin polarization of the meson.
“There is no other theory that can explain the measurements,” says Bedangadas Mohanty, one of the researchers involved in the Brookhaven experiment and a physicist at the National Institute of Science Education and Research in India.
If this idea is correct, the Brookhaven experiment represents the first time physicists have observed such fluctuations in a strong force field. “This is completely new, and I think the impact is probably far-reaching,” he says Wang.
Qun Wang, a theoretical physicist at the University of Science and Technology of China and one of Xin-Nian Wang’s collaborators, first of all describes what happens in the strong interaction of quarks and gluons in the fireball. , says that it “tells us much more information”. – Author of recent preprint papers. Understanding these interactions is, in most respects, the most urgent goal of particle accelerator experiments today.
To test the new hypothesis, the Brookhaven scientists plan to replicate the experiment using another meson called the J/psi meson. If phi mesons can be added to a strong force field, their cousins should also be added, and the spin polarization of J/psi mesons should be similarly affected by the resulting fluctuations.
Even though we are dealing with particle physics, such work is by no means small. Tracking the tiniest movements of ephemeral particles in a trillion-degree Celsius plasma vortex is akin to reconstructing the shortest, brightest candle from ash alone. “You have to appreciate the challenge,” says Mohanty. That everyone tries it proves the basic truth it can bring within reach.
