One of the most fundamental mysteries, in physics, is what is known as the baryon asymmetry. It refers to the inexplicable imbalance of matter and oppositely-charged antimatter, in the universe, despite the fact that the Big Bang should have produced equal amounts of both. In a recent research, scientists, from CERN’s Baryon Antibaryon Symmetry Experiment (BASE) collaboration, have experimentally confirmed the previous assumption that matter and antimatter particles are perfect mirror images of each other, with only their charges reversed. The discovery, which could be a step towards solving the age-old mystery, is the result of an incredibly accurate comparison of the charge-to-mass ratio of protons and their antimatter equivalent, antiprotons.
As some of us already know, everyday matter is comprised of protons, neutrons or electrons. In particle physics, antimatter contains antiparticles, namely antiprotons, antineutrons or positrons, which have the same mass, but opposite charge, as their counterparts. When the universe was created some 13.7 billion years ago, equal amounts of both matter and antimatter were produced. Total annihilation occurs when a matter particle and an antimatter particle come in the vicinity of one another. Based on this theory, however, one can argue that the universe is neutral in charge and has no baryon asymmetry, whatsoever. Since that does not seem to be the case, researchers, over the years, have provided several hypotheses to explain the vast difference between the amounts of matter and antimatter, present in the universe.
Some scientists, like James Cronin and Val Fitch, believe that the standard model of particle physics – a branch of science that examines the nature of particles and their various interactions – is incomplete. According to them, the baryon asymmetry could be due to differences in properties between matter and antimatter. Commonly known as the CP (charge-parity) symmetry violation, the contrast could be with regard to any of the fundamental properties. For instance, the antiprotons might be more susceptible to decay than protons, thus resulting in greater number of matter particles than antimatter particles. The CP symmetry violation, however, remains unsupported by any kind of empirical evidence. Stefan Ulmer, a physicist at Japan-based Institute of Physical and Chemical Research (RIKEN) and the study’s chief author, explained:
Any detected CPT violation will have huge impact on our understanding of nature… This is an important issue because it helps us to understand why we live in a universe that has practically no antimatter, despite the fact that the Big Bang must have led to the creation of both. If we had found violations of CPT, it would mean that matter and antimatter might have different properties—for example that antiprotons might decay faster than protons—but we have found within quite strict limits that the charge-to-mass ratios are the same.
In the new research, recently published in the Nature journal, the team undertook the complicated task of comparing the charge-to-mass ratio of protons and antiprotons. One of the chief properties of a particle, the charge-to-mass ratio can be measured by carefully analyzing the oscillations of a particle, in a magnetic field. To that end, the scientists used a device, called Antiproton Decelerator, to generate low-energy antiprotons for the experiment. Using a approach similar to the one designed by CERN’s TRAP collaboration back in the 1990s, they successfully recorded 13,000 measurements over a period of 35 days. It is also important to note that they used negatively-charged hydrogen ions as proxy for protons.
For the experiment, the researchers trapped a single pair of antiproton and negative hydrogen ion in what is known as a magnetic Penning trap, and then slowed them to ultra-low energies. Following that, the cyclical movement of the particles in the magnetic field, known as cyclotron frequency, was observed and measured. The cyclotron frequency, of a particle, is believed to be proportional to its charge-to-mass ratio as well as the strength of the given magnetic field. Based on their observations, the scientists finally developed an in-depth, and incredibly precise, comparison of the charge-to-mass ratio of a proton and an antiproton. Speaking about the project, Ulmer said:
We found that the charge-to-mass ratio is identical to within 69 parts per thousand billion, supporting a fundamental symmetry between matter and antimatter. Ultimately, we plan to achieve measurements that are at least ten or a hundred times more precise than the current standard.
The research, therefore, confirms the law of CPT symmetry, and further limits the possibility of any kind of CP symmetry violations. CPT (or charge-parity-time) invariance says that a system remains unaltered, under simultaneous reversal of charge, parity (basically a 180° rotation in space) and time. It, in turn, states that matter and antimatter particles should be perfect mirror images of each other, except only their charges. Rolf Heuer of CERN said:
Research performed with antimatter particles has made amazing progress in the past few years. I’m really impressed by the level of precision reached by BASE. It’s very promising for the whole field.
Additionally, the study has confirmed Einstein’s weak equivalence principle, which states that all particles, both matter and antimatter, are equally affected by gravity, irrespective of their individual mass and charge. Christian Smorra, a scientist working for BASE, said:
There are many reasons to believe in physics beyond the standard model, including the mystery of dark matter and, of course, the imbalance between matter and antimatter. These high-precision measurements put important new constraints and will help us to determine the direction of future research.