Quarks and anti-quarks form via matter-antimatter pair production. Because of their nature, these particles instantly annihilate each other. However, during the Big Bang, a slight asymmetry in this pair production resulted in approximately 1 extra particle of matter for every 10 billion produced.
It turns out that this 1 in 10 billion ratio of “leftover particles” happens to be the exact amount of mass necessary for the formation of stars, galaxies, and planets. As much as 2 in 10 billion, and the universe would have just been filled with black holes. As little as 0.5 in 10 billion, and there wouldn’t have been enough density for galaxies to form.
The matter-antimatter asymmetry problem 3
Researchers have observed spontaneous transformations between particles and their antiparticles, occurring millions of times per second before they decay. Some unknown entity intervening in this process in the early universe could have caused these "oscillating" particles to decay as matter more often than they decayed as antimatter.
Matter-antimatter mystery remains unsolved 2
There is little wiggle room for disparities between matter and antimatter protons, according to a new study published by the BASE experiment at CERN.
Charged matter particles, such as protons and electrons, all have an antimatter counterpart. These antiparticles appear identical in every respect to their matter siblings, but they have an opposite charge and an opposite magnetic property. This recalcitrant parity is a head-scratcher for cosmologists who want to know why matter triumphed over antimatter in the early universe.
“We’re looking for hints,” says Stefan Ulmer, spokesperson of the BASE collaboration. “If we find a slight difference between matter and antimatter particles, it won’t tell us why the universe is made of matter and not antimatter, but it would be an important clue.”
Ulmer and his colleagues working on the BASE experiment at CERN closely scrutinize the properties of antiprotons to look for any miniscule divergences from protons. In a paper published today in the journal Nature Communications, the BASE collaboration at CERN reports the most precise measurement ever made of the magnetic moment of the antiproton.
“Each spin-carrying charged particle is like a small magnet,” Ulmer says. “The magnetic moment is a fundamental property which tells us the strength of that magnet.”
The BASE measurement shows that the magnetic moments of the proton and antiproton are identical, apart from their opposite signs, within the experimental uncertainty of 0.8 parts per million. The result improves the precision of the previous best measurement by the ATRAP collaboration in 2013, also at CERN, by a factor of six. This new measurement shows an almost perfect symmetry between matter and antimatter particles, thus further constricting leeway for incongruencies which might have explained the cosmic asymmetry between matter and antimatter.
The measurement was made at the Antimatter Factory at CERN, which generates antiprotons by first crashing normal protons into a target and then focusing and slowing the resulting antimatter particles using the Antiproton Decelerator. Because matter and antimatter annihilate upon contact, the BASE experiment first traps antiprotons in a vacuum using sophisticated electromagnetics and then cools them to about 1 degree Celsius above absolute zero. These electromagnetic reservoirs can store antiparticles for long periods of time; in some cases, over a year. Once in the reservoir, the antiprotons are fed one-by-one into a trap with a superimposed magnetic bottle, in which the antiprotons oscillate along the magnetic field lines. Depending on their North-South alignment in the magnetic bottle, the antiprotons will vibrate at two slightly different rates. From these oscillations (combined with nuclear magnetic resonance methods), physicists can determine the magnetic moment.
The challenge with this new measurement was developing a technique sensitive to the miniscule differences between antiprotons aligned with the magnetic field versus those anti-aligned.
“It’s the equivalent of determining if a particle has vibrated 5 million times or 5 million-plus-one times over the course of a second,” Ulmer says. “Because this measurement is so sensitive, we stored antiprotons in the reservoir and performed the measurement when the antiproton decelerator was off and the lab was quiet.”
BASE now plans to measure the antiproton magnetic moment using a new trapping technique that should enable a precision at the level of a few parts per billion—that is, a factor of 200 to 800 improvement.
Members of the BASE experiment hope that a higher level of precision might provide clues as to why matter flourishes while cosmic antimatter lingers on the brink of extinction.
“Every new precision measurement helps us complete the framework and further refine our understanding of antimatter’s relationship with matter,” Ulmer says.
Davies, Goldilocks enigma :
What Happened to All the Antimatter?
The possibility that protons might not be absolutely stable is also germane to the origin of the universe. If matter can disappear, then it can also appear (by the reverse process). This yields a clue to one of the deepest puzzles of cosmology: the origin of matter. Somehow it was made, in a flash, from the heat energy of the big bang. But cosmologists want to know exactly how it happened and why that particular amount (10^50 tons in the observable universe) got made. When matter is made in the lab by high-energy collisions, the same quantity of antimatter appears too. If the universe contained equal amounts of matter and antimatter, we’d be in for trouble. Whenever antimatter and matter mingle, they quickly annihilate in a burst of gamma rays. Even in outer space, lots of mingling happens, when gas clouds collide, for example. Unless matter and antimatter are quarantined on a very large scale (much larger than the size of a galaxy), the universe would be flooded with distinctive gamma radiation. Astronomers have looked for it, without success, and so have concluded that less than one millionth of our galaxy is in the form of antimatter. Most cosmologists assume that the entire observable universe is made overwhelmingly of matter. So that presents a puzzle: how did the big bang make 10^50 tons of matter without also making 10^50 tons of antimatter? Evidently the symmetry between matter and antimatter cannot be exact. Something must have broken it, slightly favoring matter over antimatter. Grand unified theories naturally encompass the necessary symmetry-breaking. If a proton can turn into a positron, then (by going the other way) an electron-positron pair can turn into an electron-proton pair—in effect, a hydrogen atom with no antihydrogen atom accompanying it. But however it is done, the story of the origin of matter would go something like this.
The heat radiation released after the big bang created copious quantities of both matter and antimatter, all mixed together, but containing a slight excess of matter. As the universe cooled, the antimatter would be totally destroyed by virtue of its being in intimate contact with matter, leaving unscathed the small residue of excess matter—about one part in a billion. The wholesale annihilation of the antimatter, and most of the matter, flooded the universe with gamma-ray photons. Where are they now? The answer is that they lost most of their energy as the universe expanded and cooled, eventually be coming microwave photons. They constitute the cosmic microwave background radiation. So this radiation is a fading remnant of the primordial extermination visited on antimatter at the dawn of time. Viewed this way, matter is almost a cosmic afterthought. But what an important afterthought it is! Without matter there could be no life. Therefore our very existence, not to mention the existence of the visible universe, hinges on the minute degree of symmetry-breaking between matter and antimatter, which in turn depends on how quarks, leptons, and the forces that act between them are amalgamated together in some as yet undetermined grand unification.
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