Twenty years ago, physicists began to investigate the mysterious asymmetry of the internal structure of the proton. The results of their work, published at the end of February 2021, explain how antimatter helps stabilize the nucleus of each atom.
Very rarely is it mentioned that protons - the positively charged particles at the center of an atom - are partly antimatter.
At school, we were told that a proton is a group of three elementary particles called quarks - two u-quarks (up) and one d-quark (down), whose electric charges +2/3 and -1/3, respectively, add up the proton has a charge of +1. But behind this elementary picture lies a much stranger and still unsolved story.
From a distance, it seems that the proton is made up of three particles called quarks. But if you take a closer look, you can see many particles appearing and disappearing.In
fact, a vortex of a changing number of six types of quarks, their oppositely charged counterparts from antimatter (antiquarks) and gluons, elementary massless particles that bind other particles together, are transformed inside the proton. in them and multiply rapidly. Somehow, this bubbling vortex turns out to be completely stable and seemingly simple, imitating in certain aspects a trio of quarks. “The way this all works, frankly, looks like a miracle,” said Donald Gisaman , a nuclear physicist at Argonne National Laboratory in Illinois.
Thirty years ago, researchers discovered a striking property of this "proton sea". Theorists expected that different types of antimatter would be evenly distributed in it, but it seemed that the number of lower anitquarks significantly exceeded the number of upper antiquarks. Then, ten years later, another group of researchers noticed hints of unexplainable variations in the ratio of the upper and lower antiquarks. But these results were on the verge of the sensitivity of the experiment.
So 20 years ago, Donald Gisaman and his colleague Paul Rymer began working on a new experiment to better understand this issue. The experiment, dubbed SeaQuest, is finally over and the researchers published it results in the journal Nature. They measured the proton's intrinsic antimatter more thoroughly than ever before and found that for each upper antiquark, there are, on average, 1.4 lower antiquarks.
Samuel Velasco / Quanta Magazine
These data directly support two theoretical models of the proton sea. “This is the first real evidence to support these models,” Reimer said.
One is the pion cloud model , a popular approach that has been around for decades that emphasizes the proton's tendency to emit and reabsorb particles called pions, which belong to a group of particles known as mesons. The second, the so-called statistical model , considers the proton as a container filled with gas.
Further planned experiments will help researchers choose one of these two models. But whichever is true, the SeaQuest proton intrinsic antimatter dataset will be of immediate benefit, especially to physicists who collide protons at near-light speeds at the Large Hadron Collider. With accurate information about the composition of colliding objects, they will be able to more efficiently disassemble the products left after the collision, in search of evidence of the existence of new particles or effects. Juan Rojo from the Free University of Amsterdam, which assists in the analysis of the LHC data, believes that the results of the SeaQuest experiment could have a large impact on the search for new physics, which is currently "limited by our knowledge of the structure of the proton, in particular about its antimatter."
The third is not superfluous
For a short period of time, about half a century ago, physicists believed they had dealt with the proton.
In 1964, Murray Gell-Mann and George Zweig independently proposed a model that later became known as quark : the idea was that protons, neutrons and their associated rarer particles are beams of three quarks (as they were calledGell-Mann), and pions and other mesons are composed of one quark and one antiquark. This scheme explained the cacophony of particles flying from high-energy particle accelerators, since the spectrum of their charges could be built from two- and three-part combinations. Then, around 1970, researchers at the Stanford Linear Accelerator (SLAC) seemed to confirm the quark model: by firing high-speed electrons at protons, they saw electrons bounce off objects inside.
But the picture soon became less clear. “As we tried harder to measure the properties of these three quarks, we found something else was going on,” said Chuck Brown, an 80-year-old SeaQuest team member at the National Accelerator Laboratory. Enrico Fermi (Fermilab), who has been working on quark experiments since the 1970s.
The study of the momentum of the three quarks showed that their masses constitute a small part of the total mass of the proton. In addition, when researchers at SLAC were shooting electrons at higher speeds at protons, they saw that the electrons were repelling more particles inside. The faster the electrons, the shorter their wavelength, which made them sensitive to the finer elements of the proton; it is like increasing the resolution of a microscope. More and more internal particles were discovered, which seemed to have no end. “We do not know where the limit is and what the highest resolution can be obtained,” Gisaman said.
The results began to make more sense when physicists developed a true theory that the quark model is only getting closer to: quantum chromodynamics, or QCD. QCD, formulated in 1973, describes the "strong force," the greatest force in nature by which particles called gluons bind beams of quarks.
QCD predicts the same vortex that was revealed in scattering experiments. Difficulties arise due to the fact that gluons feel the very force that they carry. This is how they differ from photons, which carry a simpler electromagnetic force. This "arbitrariness" creates disorder inside the proton, giving gluons complete freedom of action for the emergence, multiplication and splitting into short-term pairs of quarks and antiquarks. Balancing each other, these closely spaced oppositely charged quarks and antiquarks go unnoticed from afar. Only three unbalanced "valence" quarks - two up and down - make up the total charge of the proton. But physicists realized that by firing electrons at higher speeds, they hit smaller targets.
However, the oddities did not end there.
Because of the arbitrariness of gluons, the QCD equations cannot be solved; therefore, physicists have failed and still cannot calculate accurate predictions of the theory. But they had no reason to assume that gluons would split into one type of quark-antiquark pair (namely, the lower one) more often than into another. “We expected an equal number of both pairs to appear,” said Mary Ahlberg , a nuclear theorist at Seattle University, explaining her rationale at the time.
Mary Ahlberg, a nuclear physicist at the University of Seattle, and her co-authors have long argued that the pion plays an important role in the formation of the essence of the proton.
Photo courtesy of Seattle University
This is why researchers at the New Muon Collaboration in Geneva were so shocked by the results of the muon scattering experiment. In 1991. they collided muons (the heavier relatives of electrons) with protons and deuterons, consisting of one proton and one neutron, compared the results and concluded that there are more lower antiquarks in the proton sea than upper antiquarks.
Parts of a proton
Soon, theorists proposed several possible explanations for the asymmetry of the proton.
One of them is associated with a peony. Since the 1940s, physicists have watched protons and neutrons exchange pions inside atomic nuclei, like players on a team throwing basketballs to each other to keep them together. Reflecting on the structure of the proton, the researchers came to the conclusion that it can also toss a basketball to itself, that is, it can briefly emit a positively charged pion, turning into a neutron for this time, and then reabsorb it. “If during an experiment you think that you are looking at a proton, you are not, because for some time this proton will go into the state of a neutron-pion pair,” Ahlberg said.
More precisely, a proton turns into a neutron and a pion, consisting of one up quark and one down antiquark. Because this ghostly peony has a lower antiquark (a peony with an upper antiquark cannot materialize so easily), theorists such as Ahlberg, Gerald Miller, and Tony Thomas have argued that the pion cloud model explains the greater number of lower proton antiquarks detected by measurements.
Samuel Velasco / Quanta Magazine
Other arguments have emerged as well. Claude Burrely and his colleagues from France have developed a statistical model that considers the internal particles of a proton as gas molecules in a room, chaotically moving at different speeds, depending on whether the particle has an integer or half-integer angular momentum. When tuned with data from numerous scattering experiments, the model assumed a predominance of antiquarks.
The predictions of the two aforementioned models were not identical. Most of the total mass of a proton is made up of the energies of individual particles that break into and out of the proton sea, and these particles carry different energies. Models have differently predicted how the ratio of high to low antiquarks should change as they count antiquarks that carry more energy. Physicists measure a related quantity called the antiquark momentum fraction.
When researchers at Fermilab in 1999 under NuSea experiment measuredthe ratio of upper and lower antiquarks as a function of the antiquark momentum, the result of their work simply inspired everyone, recalls Ahlberg. These data indicate that among antiquarks with a large momentum (so large that they were on the verge of the instrument's detection range), there were suddenly more upper antiquarks than lower ones. "Every theorist said, 'Wait a minute,' Ahlberg said." Why did the curve turn when these antiquarks got a lot of momentum? "
While theorists were racking their brains over this question, Gisaman and Reimer, who were working on the NuSea experiment and knew that data on the brink sometimes should not be trusted, decided to build an experiment where it would be possible to investigate a wider range of antiquark pulses under comfortable conditions. They named it SeaQuest.
From what was
With a bunch of questions about the proton, but no money, they began to assemble an experiment from the used parts. “Our motto was: reduce waste, reuse, recycle,” Reimer said.
They purchased several old scintillators from the Hamburg laboratory, the remaining particle detectors at Los Alamos National Laboratory, and the radiation-blocking iron plates that were originally used at Columbia University's cyclotron in the 1950s. They were able to use the room-sized magnet used in the NuSea experiment and carry out their new experiment at the proton accelerator at Fermilab. The resulting "Frankenstein" from these details, however, was not devoid of its charm. According to Brown, who helped find all the parts, the audible indicator signaling that the protons are entering the device was made 50 years ago: "When it beeps, it becomes warm in the soul."
Nuclear physicist Paul Rymer (top) with the SeaQuest experiment device
An experiment at Fermilab, built mostly from used parts
. Finally, they started it. In the experiment, protons hit two targets: a hydrogen bubble, which is essentially a proton, and a deuterium bubble, the nucleus of which consists of one proton and one neutron.
When hitting either of the two targets, one of the valence quarks of the proton sometimes annihilates with one of the antiquarks of the proton or neutron of the target. “Annihilation has a unique signature and produces muon and anti-muon,” Rymer said. These particles, along with other "debris" from the collision, then crash into the old iron plates. “Muons can pass through them, and all other particles are blocked,” he said. By detecting muons on the back of the plates and restoring their original trajectories and velocities, "you can reconstruct the chronology of events to find out what fraction of the momentum is carried by antiquarks."
Since protons and neutrons mirror each other, where one has particles of the upper type, the other has particles of the lower type, and vice versa. By comparing the data from the two bubbles, one can immediately see the ratio of the upper antiquarks to the lower antiquarks in the proton, but this, of course, was preceded by 20 years of work.
In 2019, Ahlberg and Miller calculated the results of the SeaQuest experiment based on the pion cloud model . Their predictions are in line with new SeaQuest data.
New data that show a gradual rise and then a plateau in the ratio between lower and upper antiquarks, rather than a sudden reversal, also coincides with the results of a more flexible statistical model.developed by Burrely and colleagues. Yet Miller calls this competing model “descriptive, not predictive,” because it is tuned to fit the data rather than elicit a physical mechanism that explains the dominance of antiquarks. “And in our calculations, I am proud of the fact that they represent a true forecast,” said Ahlberg. “We didn’t tweak any parameters beforehand.”
In an email, Burrely argued that "the statistical model is more powerful than the Alberg and Miller model" because it takes into account scattering experiments with both polarized and non-polarized particles. Miller strongly disagreed, noting that the pion cloud model explains not only the antimatter composition of the proton, but also the magnetic moments of various particles, the distribution of charges and decay times, as well as "the binding and therefore the existence of all nuclei." He added that the pion mechanism “is important in a broad sense for questions such as“ Why are there nuclei? Why do we exist? "
In the ultimate quest to understand the proton, spin or intrinsic angular momentum may be the decisive factor. A muon scattering experiment in the late 1980s showedthat the spins of the three valence quarks of the proton are no more than 30% of the total spin of the proton. The "proton spin crisis" can be expressed by the following question: "what makes up the remaining 70%?" And as veteran explorer Chuck Brown, a Fermilab veteran, said again, "There must be something else."
Experimenters will investigate the spin of the proton sea at Fermilab and then at the projected electron-ion collider at Brookhaven National Laboratory. Ahlberg and Miller are already working on calculations of the complete "meson cloud" surrounding the protons, which, in addition to pions, includes the rarer "rho mesons". Unlike pions, ro-mesons have spin, so they must somehow influence the total spin of the proton, which Ahlberg and Miller hope to determine.
Fermilab's SpinQuest experiment , which involves many SeaQuest researchers and uses the details of that experiment, is almost ready to go , Brown said . “With luck, we'll get the data this spring; this will depend, at least in part, on progress in developing a vaccine against the virus. It's funny that the solution to such a deep and incomprehensible issue of the internal structure of the nucleus depends on the situation with the COVID virus in the country. Everything in the world is interconnected, isn't it? "