Why do we exist? This is perhaps the deepest question that may seem completely outside the scope of particle physics. But our new experiment at the Large Hadron Collider at CERN has brought us closer to the answer. To understand why we exist, you first need to go 13.8 billion years ago, in the time of the Big Bang. This event produced an equal amount of the substance we are made of and antimatter.
It is believed that each particle has an antimatter partner, which is almost identical to it, but has the opposite charge. When a particle and its antiparticle meet, they annihilate - disappear in a flash of light.
Where is all the antimatter?
Why the universe we see is composed entirely of matter is one of the greatest mysteries of modern physics. If there was once an equal amount of antimatter, everything in the universe would annihilate. And so, a recently published study seems to have found a new source of asymmetry between matter and antimatter.
Arthur Schuster was the first to speak about antimatter in 1896, then in 1928 Paul Dirac gave it a theoretical basis, and in 1932 Karl Anderson discovered it in the form of anti-electrons, which are called positrons. Positrons are born in natural radioactive processes, such as the decay of potassium-40. This means that a regular banana (containing potassium) emits a positron every 75 minutes. It then annihilates with electrons in matter, producing light. Medical applications like PET scanners also produce antimatter in a similar process.
The main building blocks of the substance of which atoms are composed are elementary particles - quarks and leptons. There are six kinds of quarks: up, down, strange, charmed, true, and beautiful. Likewise, there are six leptons: electron, muon, tau, and three types of neutrinos. There are also antimaterial copies of these twelve particles, which differ only in their charge.
Antimatter particles, in principle, should be the perfect mirror image of their normal satellites. But experiments show that this is not always the case. Take, for example, particles known as mesons, which are made up of one quark and one antiquark. Neutral mesons have an amazing feature: they can spontaneously turn into their anti-meson and vice versa. In this process, a quark turns into an antiquark or an antiquark turns into a quark. However, experiments have shown that this can happen more often in one direction than in another - as a result of which there is more matter over time than antimatter.
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The third time is magical
Among particles containing quarks, such asymmetries were found only in strange and beautiful quarks - and these discoveries became extremely important. The very first observation of asymmetry involving strange particles in 1964 allowed theorists to predict the existence of six quarks - at a time when only three were known to exist. The discovery of asymmetry in beautiful particles in 2001 was the final confirmation of the mechanism that led to the six-quark picture. Both discoveries earned Nobel Prizes.
Both strange and beautiful quarks carry negative electrical charges. The only positively charged quark that, in theory, should be able to form particles that can exhibit the asymmetry of matter and antimatter is the charmed one. The theory suggests that he does this, his effect should be insignificant and difficult to find.
But the LHCb experiment at the Large Hadron Collider was able to observe such asymmetry in particles called D-mesons, which are composed of charmed quarks - for the first time. This is made possible by the unprecedented amount of charmed particles produced directly in collisions at the LHC. The result shows that the probability that this is a statistical fluctuation is 50 per billion.
If this asymmetry is not born from the same mechanism that leads to the asymmetries of strange and beautiful quarks, there is room for new sources of asymmetry of matter-antimatter, which can add to the general asymmetry of those in the Universe. And this is important, since several known cases of asymmetry cannot explain why there is so much matter in the universe. The charm quark discovery alone won't be enough to fill this problem, but it is an important piece of the puzzle in understanding fundamental particle interactions.
Next steps
This discovery will be followed by an increase in the number of theoretical works that help in the interpretation of the result. But more importantly, she will outline further tests to deepen our understanding of our discovery - and some of those tests are already in progress.
In the coming decade, the upgraded LHCb experiment will increase the sensitivity of such measurements. It will be complemented by the Belle II experiment in Japan, which is just getting started.
Antimatter is also at the heart of a number of other experiments. Whole antiatoms are produced at CERN's Antiproton Moderator, and they provide a range of highly accurate measurement experiments. Experiment AMS-2 aboard the International Space Station is in search of space-derived antimatter. A number of current and future experiments will be devoted to the question of whether there is a matter-antimatter asymmetry among neutrinos.
Although we still cannot fully unravel the mystery of the asymmetry of matter and antimatter, our latest discovery opened the door to an era of precise measurements that can reveal yet unknown phenomena. There is every reason to believe that one day physicists will be able to explain why we are here at all.
Ilya Khel