“All of our observations find a complete symmetry between matter and antimatter, which is why the universe should not actually exist,” explained Christian Smorra, member of the BASE collaboration at the CERN research center. “An asymmetry must exist here somewhere but we simply do not understand where the difference is. What is the source of the symmetry break?”
“We know that matter and antimatter are created by the strong force in equal amounts,” wrote Wendy Taylor, an Experimental Particle Physicist at York University, former Canada Research Chair, and the lead for York University’s ATLAS experiment group at CERN, in an email to The Daily Galaxy. “After the Big Bang,” she explains,”when the temperature of the universe cooled enough for particles to form, the universe had equal amounts of matter and antimatter. But now we live in a universe that is predominantly matter.”
What Happened to All the Antimatter?
“When matter and antimatter of the same particle type interact they annihilate each other. For example, when an electron and positron collide, they annihilate and produce two photons that conserve the energy and momentum of the original electron-positron pair. Interactions between protons and antiprotons, both of which are composed of quarks and gluons, leave behind several unstable mesons (other elementary particles), which quickly decay into photons, electrion, positrons and neutrinos. If the Universe started with an equal amount of matter and antimatter and then the matter and antimatter annihilate each other, how are we left with a Universe composed mostly of matter?
“What happened to all the antimatter? Certain particles are unstable (think of radioactive uranium),” Taylor explains.“Unstable particles decay by the weak force into lower mass stable particles. We know that there is an asymmetry in these weak decays. That is, unstable antimatter particles decay preferentially into matter particles whereas the reverse is not true for unstable matter particles. The weak force has given us a universe that is predominantly matter (by the effect known as CP violation, where C stands or Charge-Conjugation and P stands for Parity).”
The Search Goes On
“However, that is not the end of the story,” Taylor concludes, “Although the weak force gives us more matter than antimatter, it is not enough to explain the huge disparity between matter and antimatter that we see in the universe today! This is one of the outstanding questions in physics and astronomy, and both experimental and theoretical physicists are using their creativity and talent to learn the answer.”
The Antimatter Factory
The search goes on. No difference in protons and antiprotons have yet been found which would help to potentially explain the existence of matter in our universe. However, physicists in the BASE collaboration at the CERN research center have been able to measure the magnetic force of antiprotons with almost unbelievable precision. Nevertheless, the data do not provide any information about how matter formed in the early universe as particles and antiparticles would have had to completely destroy one another.
The Antimatter Factory at CERN is the world’s only facility that produces low-energy antiprotons—the antimatter equivalents of protons.
The 2017 BASE measurements revealed a large overlap between protons and antiprotons, thus confirming the Standard Model of particle physics. Around the world, scientists are using a variety of methods to find some difference, regardless of how small. The matter-antimatter imbalance in the universe is one of the hot topics of modern physics.
The Magnetic Moment
The multinational BASE collaboration at the European research center CERN brings together scientists from around the world to compare the magnetic properties of protons and antiprotons with great precision. The magnetic moment is an essential component of particles and can be depicted as roughly equivalent to that of a miniature bar magnet. The so-called g-factor measures the strength of the magnetic field.
“At its core, the question is whether the antiproton has the same magnetism as a proton,” explained Stefan Ulmer, spokesperson of the BASE group. “This is the riddle we need to solve.”
The BASE collaboration published high-precision measurements of the antiproton g-factor back in January 2017 but the current ones are far more precise. The current high-precision measurement determined the g-factor down to nine significant digits. This is the equivalent of measuring the circumference of the earth to a precision of four centimeters. The value of 2.7928473441(42) is 350 times more precise than the results published in January.
“This tremendous increase in such a short period of time was only possible thanks to completely new methods,” said Ulmer. The process involved scientists using two antiprotons for the first time and analyzing them with two Penning traps.
Antiprotons are artificially generated at CERN and researchers store them in a reservoir trap for experiments. The antiprotons for the current experiment were isolated in 2015 and measured between August and December 2016, which is a small sensation as this was the longest storage period for antimatter ever documented. Antiprotons are usually quickly annihilated when they come into contact with matter, such as in air. Storage was demonstrated for 405 days in a vacuum, which contains ten times fewer particles than interstellar space. A total of 16 antiprotons were used and some of them were cooled to approximately absolute zero or minus 273 degrees Celsius.
The new principle uses the interaction of two Penning traps. The traps use electrical and magnetic fields to capture the antiprotons. Previous measurements were severely limited by an ultra-strong magnetic inhomogeneity in the Penning trap. In order to overcome this barrier, the scientists added a second trap with a highly homogeneous magnetic field.
“We thus used a method developed at Mainz University that created higher precision in the measurements,” explained Ulmer. “The measurement of antiprotons was extremely difficult and we had been working on it for ten years. The final breakthrough came with the revolutionary idea of performing the measurement with two particles.” The larmor frequency and the cyclotron frequency were measured; taken together they form the g-factor.
The g-factor ascertained for the antiproton was then compared to the g-factor for the proton, which BASE researchers had measured with the greatest prior precision already in 2014. In the end, however, they could not find any difference between the two. This consistency is a confirmation of the CPT symmetry, which states that the universe is composed of a fundamental symmetry between particles and antiparticles.
The BASE scientists now want to use even higher precision measurements of the proton and antiproton properties to find an answer to this question. The BASE collaboration plans to develop further innovative methods over the next few years and improve on the current results.
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