The strong force is an enigma. Through gluons, it binds together quarks, one of the two basic building blocks of matter, into the protons and neutrons at the center of every atom. True to its name, it is the strongest of the four known fundamental forces, but it only exerts its might across subatomic distances. Despite its power and importance, the strong force is the hardest force to observe in action, and its behavior is nearly impossible to mathematically predict.
Now a group of scientists at Brookhaven National Laboratory on Long Island have caught a fresh, unexpected glimpse of the strong force at work—so unexpected that theorists have invented new models to explain it. If the theorists are right, this experiment is the first measurement of how the strong force field fluctuates over short distances. The results were published on January 18 in Nature.
“This local fluctuation of the strong force field, we don’t think it has ever been measured before,” says Aihong Tang, one of the Brookhaven physicists who conducted the new study. It will allow scientists “to study the strong force from a different perspective.”
In the quest to observe the strong force, countries around the world have poured billions of dollars into particle accelerators that break atoms apart in fiery collisions. Here, quarks and gluons are liberated for tiny fractions of a second in a swirling soup of plasma, then recombine into new, rarely seen particles as the fireball cools.
One of these bizarre particles, called the phi meson, sits at the heart of these latest befuddling results. Unlike protons and neutrons, which are made of three quarks, mesons are made of one quark and one antiquark. Quarks come in six different “flavors,” and the phi meson is made of a quark-antiquark pair with the same flavor, called strange.
The scientists wanted to know if, in the instant after collision, the swirling momentum of the quark-gluon soup could cause phi mesons to spin along with it, like a beach ball in a whirlpool. This effect, called spin polarization, wasn’t a given but had been seen in other exotic particles. Measuring if and how the particles coupled with the churning quarks and gluons, the researchers hoped, would provide an unparalleled glimpse of how the strong force assembles the visible matter all around us.
This task is no easy feat, requiring automated software (and sharp-eyed scientists) to pinpoint phi mesons from thousands of new particles that are produced by each collision. “It’s like two dumpster trucks running into each other, and then all the garbage flies out, and you want to watch an apple and a banana flying in two different directions,” says Karl Slifer, a nuclear physicist at the University of New Hampshire, who was not involved in the new experiment. “It’s just a huge amount of information just to sift through.”
After finding their elusive quarry, the scientists saw something wholly unexpected. The phi mesons were indeed spinning along with the quark-gluon soup but in ways very far from theoretical predictions.
Here’s how those predictions work: Phi mesons can spin in one of three directions. If their spin was unaffected by that swirling pool of particles, then each of those spin directions would be equally common—each manifesting around a third of the time. Even a small deviation from those one-third odds would indicate that the phi meson’s spin had been influenced by the circling momentum around it.
What the researchers saw instead was a massive deviation from the one-third probability, 1,000 times larger than conventional models could explain. The usual variables, such as the spin-altering interference from electromagnetic fields, just couldn’t account for such a large difference. The team announced its preliminary results in 2017, to the bafflement of theorists.
“We were really scratching our heads and saying, ‘What is going on?’” says Xin-Nian Wang, a theoretical physicist at Lawrence Berkeley National Laboratory in California, who was not involved in the initial Brookhaven experiments but has helped peer-review papers reporting the results. “I didn’t believe them at the time: ‘No, this is unbelievable; this is too big,’” he recalls.
But the equally skeptical Brookhaven team ran the analysis again and again, arriving each time at the same seemingly impossible result. “And then we realized it could be something that we, as theorists, didn’t understand,” Wang says.
The theorists had missed something big: Electromagnetic fields may not be strong enough to affect the phi mesons’ spin, but what about fields generated by the strong force? Fields are created by charged particles in motion. Just as electromagnetic fields arise from moving electrons, strong fields arise from moving quarks and gluons. Preexisting models essentially ignored the possibility of strong fields because their effects would usually be irrelevant. Even across typical subatomic distances, the random movements of quarks and gluons that would create such fields would cancel out, resulting in no effect on the system overall.
But on super tiny scales—think of distances smaller than a proton’s infinitesimal span—the cumulative details of all those random movements may actually matter. This is what Wang and his colleagues have proposed in a recent preprint study: the movement of the phi mesons themselves creates a strong force field whose minuscule fluctuations are affecting mesons’ spin polarization.
“There exists no other theory which 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 experiments represent the first time physicists have observed such fluctuations in the strong force field. “This is completely new. I think that the consequences are probably far-reaching,” Wang says.
For starters, “it tells you much, much more information” about what is happening with strong interactions in the quark-gluon fireball, says Qun Wang, a theoretical physicist at the University of Science and Technology China and one of Xin-Nian Wang’s co-authors in the recent preprint paper. Understanding those interactions is, in most respects, the most urgent goal of today’s particle accelerator experiments.
To test the new hypothesis, the Brookhaven scientists plan to re-create their experiment with a different meson, called the J/psi meson, which is made of a quark-antiquark pair of a different favor. If phi mesons can add to the strong force field, their cousins should, too—and J/psi mesons’ spin polarizations should be similarly impacted by the resulting fluctuations.
Despite dealing with subatomic physics, such work would be no small thing. Tracing the tiniest motions of ephemeral particles in a vortex of trillion-degree-Celsius plasma is akin to reconstructing the briefest, brightest candle from ashes alone. “You must appreciate the challenge,” Mohanty says. That anyone would attempt it at all attests to the fundamental truths it may bring within reach: a better view of the force that literally binds us all together.
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