By Mariana Barbosa Fialho
The Big Mystery of the Universe Origins:
Have you ever heard about antimatter?
At school, students learn that all matter in the universe is composed by little structures called atoms. These atoms, in turn, are composed by particles, called proton, neutron and electron. Finally, those structures are composed by quarks and gluons. This is what the scientists believed until 1928, but what if I told you that there is something called antimatter, that is basically the same as matter but with opposite charge? This is what the mathematician and physicist Paul Dirac published in his work “Proceedings of the Royal Society”, in 1928. According to this study and others that followed it, with the Big Bang, equal quantities of matter and antimatter were created in the universe. However, currently, there is much more matter than antimatter in the universe. So, where did most of the antimatters present in the primordial universe go? That is the question that scientists still try to answer.
The Asymmetry Between Matter and Antimatter:
Trying to answer the question that persisted for years, scientists started studying the proprieties of antiparticles. With that, they discovered that the physical laws do not apply equally to matter and antimatter. Therefore, physicists in the field of particle physics are focused on exploring these differences, that can reveal a lot about the universe and its origins. An experiment made by APHA scientists at The Large Hadron Collider showed that gravity influences matter and antimatter equally. For that, scientists created an anti-hydrogen atom, gradually ramping down its current in the top and bottom magnets of the trap, in a way that allows the observation of the action of gravity on these anti-atoms.
The results showed that 80% of these anti-hydrogen atoms exited through the bottom of the trap, while only 20% were exited through the top. These results, therefore, allow us to visualize that the behavior of antiparticles when influenced by gravity is similar to the behavior of particles of matter under the same conditions. But does that work for all physical conditions?
The Charge-Parity Violation:
Until 1956, physicists believed that the equations of particle physics we're invariant under mirror inversion. What does it mean? For instance, in a chemical reaction, it is predicted that an inverse reaction occurs at the same rate as the original reaction, the same occurs for particle decay. Nevertheless, that is not true for antiparticles, when considering the weak force influences. After the discovery of what was called “P violation”, it was proposed that the charge conjugation, C, which transforms a particle into its antiparticle, was the solution to the P-violation problem.
The Charge Violation:
The charge conjugation transforms a particle into its corresponding antiparticle. For example,if we apply it to an electron, we will obtain a positron. A study conducted by Chien- Shiung Wu, in 1957, however, showed that the charge and parity violation does not work when talking about weak force. She made an experiment monitoring the decay of cobalt-60 (60Co) by beta decay to the stable isotope Nickel-60 (60Ni), also emitting gama rays that will be analyzed to the experiment. In the decay, one of the neutrons of the nucleus of the Cobalt-60 decay to a proton by emitting an electron and an electron antineutrino.
Therefore, for charge conservation, the experiment made with the correspondent antiparticles of the electron and the antineutrino should provide the same results. However, it is known that the same experiment made with positrons and neutrinos does not provide the same results obtained in the experiment carried out with an electron and an antineutrino. This way, the experiment developed by Chien-Shiung Wu proves that it exists a charge violation.
The Parity Violation:
The parity is the transformation that inverts the space coordinates. So, if there´s a particle moving with velocity v from left to right, the mirror image obtained of that particle is it moving with a velocity –v from right to left.
If considering parity conjugation in the experiment made by Chien- Shiung Wu, for parity conservation, the emission anisotropy should not be significant. Nonetheless, the results obtained showed that the gama ray anisotropy was approximately 0.6, which means that while 60% of the electrons were emitted in one direction, only 40% were emitted in the opposite direction. This asymmetry observed, therefore, led Dr. Wu to conclude that this prove that it exists a violation in parity between matter and antimatter as well.
Despite all the studies that showed that it exists a charge violation and a parity violation between matter and antimatter, it was in 1956 that Reinhard Oehme, in a letter to Chen-Ning Yang, showed that the parity violation ends up by culminating in the charge violation, when talking about weak decays. This way, what is actually observed in the behavior of antiparticles is a parity-charge violation.
Given all the results obtained by the previously mentioned experiments, it is common to question: what are the consequences of this PC violation? Physicists are still investigating it through the conduction of other experiments, but what can be said is that it changes completely the comprehension that one has of the Standard Model, or, in other words, the comprehension of the particles behavior and the fundamental forces influences on it.
Moreover, mathematically, the unitarity triangle, which is an abstract diagram that represents the interactions between different types of quarks, this way describing the matter interactions, get “broken”, in a certain way. With that, this mathematical tool can be used to help in the investigation of the antimatter behavior.
The Dark Matter and the Axion:
In the year of 1977, the particle physicists Roberto Peccei and Helen Quinn proposed a new symmetry capable of solving the CP violation problem.
How did they come up with this model?
In the Standard Model, the CP violation in the strong sector should manifest as a significant electric dipole moment for the neutron. Nevertheless, this parameter Θ (related to the eletric dipole moment of the particle) is of the order of 10–10 , which is much lower than expected. This way, the electric dipole moment for the neutron is negligible and, therefore, the CP violation can be neglected.
From this result, Peccei and Quinn created a new symmetry (PQ symmetry) associated with a hypothetical particle, the axion. The axion is a particle associated to a new scalar field a(x), whose dynamics naturally adjusts Θ to approximately zero, therefore, solving the CP violation problem.
This way, the equation that describes the CP violation:
With the addition of a term associated with the axion, it is adjusted to:
Now, the title of this section is “The dark matter and the axion”; So, what is dark matter after all?
The simplest answer to this question is that we do not know. The fact is that, in our universe, all the planets, satellites, black holes, galaxies and all we know is only approximately 5% of the total mass of our universe. This way, what would make up the other largest part of our universe? This is what we call dark matter.
The dark matter needs to be something that does not interact with mass or light, since we cannot detect it. In fact, that is one of the few things we know about dark matter. Despite that, there are good candidates for being dark matter. Here, we have presented one, which is the axion, and will be presenting another theory related to that, the supersymmetry.
A good candidate to dark matter must have characteristics similar to those that dark matter must have. Therefore, it must interact minimally with other particles/ electromagnetic radiation, must have low mass and also must be stable. In fact, if existing, the axion must have characteristics that fit this pattern, according to studies that investigate its proprieties. The axion, thus, is, currently, the best candidate for dark matter.
The Supersymmetry Theory:
Even though the axion is a good candidate, there is another theory that is also widely accepted to define what dark matter is. The supersymmetry theory (SUSY) proposes that for each Fermion, which is a particle of half- integer spin (e.g. electrons, quarks and neutrinos), there is a partner boson, which is a particle of integer spin (e.g. photons, W and Z bosons, and gluons) and vice versa. This way, the theory says that for each particle of the Standard Model, there is a correspondent supersymmetric particle with a different spin. In this model, the most the lightest particle theorized is neutralino, which has characteristics, such as its weak interaction with matter, that make it a good candidate for dark matter. Therefore, the supersymmetry theory not only presents a good resolution to the dark matter problem, but also suggests a possible unification of the fundamental forces of nature. It is, thus, a very relevant theory that, however, needs to be better developed and explored.
Conclusion:
As a conclusion, this work aims to investigate the asymmetry encountered between matter and antimatter, in a way that can obtain data that will contribute to the better understanding of particles behavior, and, therefore, better comprehend the universe we live in. For doing that, this work will explore theories that can both explain the dark matter mystery and the matter- antimatter asymmetry in our universe, mainly focusing on the study of supersymmetry to understand how antimatter can transform into dark matter. Moreover, it will use of mathematical tools, such as the unitarity triangle, so as to describe the universe behaves mathematically and interpretate it physically.
For more information, contact: mariana.TKS.123@gmail.com
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