Physicists study the presence of matter and antimatter in the universe – and how this phenomenon can tell us more about where we came from.
This is undoubtedly the most profound issue that exists and may seem entirely outside the scope of particle physics. But our new experiment in the Great Hadron Collider of the European Organization for Nuclear Research (CERN) has taken us one step closer to discovering this.
To understand why, let’s go back to 13.8 billion years ago, until the Big Bang.
This event produced a vast amount of the matter from which you are made and also something called antimatter. Each particle is believed to have an antimatter mate virtually identical to itself, but with the opposite charge.
When a particle and its antiparticle meet, they vanish – disappearing in an explosion of light.
One of the greatest mysteries of modern physics is the reason that the universe we see today is made entirely of matter. If there were an equal amount of antimatter, everything in the universe would have been annihilated.
Our research has revealed a new source of this asymmetry between matter and antimatter.
Antimatter was first thought of by Arthur Schuster in 1896, then a theoretical basis for Paul Dirac in 1928, and discovered in the form of anti-electrons, dubbed positrons, by Carl Anderson in 1932.
The positrons occur in natural radioactive processes, as in the decay of potassium-40. This means that the banana (which contains potassium) emits a positron every 75 minutes. Then annihilate with electrons of matter to produce light. Medical applications, such as PET scans, produce antimatter in the same process.
The fundamental building blocks of matter that make up atoms are the elementary particles called quarks and leptons.
There are six types of quarks: up, down, strange, charm, bottom and top. In the same way, there are six leptons: the electron, the muon, the tau, and three neutrinos.
There are also antimatter copies of these twelve particles that differ only in their charge.
The antimatter particles must, in principle, be perfect mirror images of their regular companions. But experiments show that this is not always the case.
Take as example particles known as mesons, which are made of a quark and an anti-quark. Neutral mesons have a fascinating feature: they can spontaneously transform into your anti-meson and vice versa.
In this process, the quark turns into an anti-quark, or the anti-quark turns into a quark. However, experiments have shown that this can happen more in one direction than in the other – creating more matter than antimatter over time.
Among quark-containing particles, only those that include the odd and the lower quarks exhibit such asymmetries-and these were fundamental discoveries.
The first observation of asymmetry involving foreign particles occurred in 1964 and allowed theorists to predict the existence of six quarks – at a time when only three were known.
The discovery in 2001 of asymmetry in background particles was the final confirmation of the six-quark mechanism. Both developments led to the Nobel Prize.
Both the odd quark and the background quark carry a negative electric charge. The only positively charged quark that theoretically should be able to form particles that may exhibit asymmetry of matter and antimatter is the charm.
The theory suggests that if this happens, the effect should be tiny and difficult to detect.
However, now the LHCb experiment has been able to observe this asymmetry of particles called D-meson – which are composed of charm quarks – for the first time.
This is possible thanks to the unprecedented amount of charm particles produced directly in the collisions of the LHC, of which I was pioneer a decade ago. The result indicates that the chance of this being a statistical fluctuation is about 50 in a billion.
If this asymmetry does not come from the same mechanism that causes the asymmetries of the strange quarks and the background, this opens space for new sources of asymmetry between matter and antimatter that can be added to the total in the primitive universe.
And this is important because the few known cases of asymmetry cannot explain why the universe contains much matter. The discovery of the charm particle by itself will not be enough to fill this gap, but it is a key piece in understanding the interactions of fundamental particles.
The discovery will be followed by an increase in the number of theoretical papers, which help interpret the result. But more importantly, after our discovery this will outline further testing to deepen understanding – several of them are already underway.
Over the next decade, the updated LHCb experiment will increase sensitivity for these types of measurements. This will be complemented by the Belle II experiment in Japan, which is just beginning to operate. These are exciting prospects for the research of antimatter-matter asymmetry.
Antimatter is also at the heart of several other experiments. Whole anti-atoms are being produced in CERN’s Antiproton Decelerator, which feeds other tests that perform high-precision measurements.
The AMS-2 experiment aboard the International Space Station is one that is looking for antimatter of cosmic origin. And a series of current and future operations will address the issue of the existence of asymmetry of matter and antimatter among neutrinos.
While we have yet to completely solve the mystery of the asymmetry between antimatter-matter in the universe, our latest discovery has opened the door to an era of precision measurements that have the potential to discover yet unknown phenomena.
There is every reason to be optimistic that physics will one day be able to explain why we are here.