This article is taken from the monthly n ° 883 of Sciences et Avenir-La Recherche, dated September 2020.
1. Where has the antimatter gone?
Would there exist somewhere in the Universe, well hidden beyond the region within reach of telescopes, antigalaxies containing anti-stars? This serious hypothesis raises a host of questions, the first of which is: why have astronomers never detected the flow of gamma rays resulting from the annihilation that should have been produced by the encounter between matter and antimatter that existed in equal parts at the time of the Big Bang? In the meantime, antimatter only appears in a secondary and furtive manner in the Universe which is within our reach: in laboratories or in nature, in the heart of lightning or during the impact of cosmic rays. This observation pushes physicists to seek the process which, during the history of the cosmos, broke this original symmetry which existed between matter and antimatter. A “symmetry breaking” which could explain the victory of matter over its double. Between the two, nature indeed imposes subtle differences: the negative or positive electric charges carried by the particles to which are added some quantum properties such as parity, that is to say their orientation in space, like a inverted image in a mirror.
So, logically, if the antimatter has disappeared, it is because an event occurring in the childhood of the Universe acted differently according to the charge and the parity of the particles. An explanation was provided in 1967 by Russian physicist Andrei Sakharov, winner of the Nobel Peace Prize (1975). Its explanation concerns baryogenesis, that is to say the formation of the particles that today make up the atomic nucleus, neutrons and protons (nucleons). They are each made up of three quarks of different “flavors”, those elementary particles that came together when the Universe was a few fractions of a second old. However, immediately after the Big Bang, matter-antimatter asymmetry probably arose, long before these nucleons managed to form the nuclei of the atoms that constitute matter. Andrei Sakharov’s hypothesis is that the conditions were such that each time a billion antinucleons were formed, a billion nucleons were also formed… plus one! And it is to this extra nucleon that we owe our existence. Indeed, still according to him, the antinucleons have all been annihilated by their double matter… except the tiny surplus which formed our Universe. However, this commonly accepted explanation is not enough to account for the total disappearance of antimatter as we see it today. The mystery remains.
2. How to make it?
The simplest and most abundant chemical element in the Universe is hydrogen, formed by an electron around a proton. The recipe for antihydrogen follows: an anti-electron (positron) around an antiproton. But to make it – and keep it – is another matter! “The antiproton is always made with its double. We must provide at least the mass energy of the two particles to materialize them, explains Pauline Comini, from CEA (Saclay, Essonne). Then, you have to manage to recover the antiparticle and keep it so that it does not come into contact with matter, thanks to a magnetic field and a very high vacuum. “ Because of these difficulties, it was not until 1995 that nine antihydrogens could finally be manufactured.
Since then, physicists have made a lot of progress: the important step was the development of the antiproton decelerator (AD) at CERN, near Geneva (Switzerland), a unique machine in its kind since it slows down particles . It is the essential tool for making anti-atoms. In fact, the proton synchrotron (PS), which also powers the ring of the Large Hadron Collider (LHC) at CERN, sends its protons to a metal target. Many antiprotons are born from this collision. But their speed is such that it is not easy to control them to associate them with a positron in order to manufacture an anti-hydrogen. They must therefore be slowed down… down to 1/10 the speed of light! Thus, AD produces 20 to 30 million antiprotons every two minutes.
In 2021, a new deceleration ring will come to lend a hand: ELENA (Extra Low Energy Antiproton) will further reduce the energy of the antiprotons from 5.3 megaelectronvolts (MeV) to 0.1 MeV, or approximately 1/100 of the speed of light. The goal: to facilitate the capture of antiprotons for the different experiments.
“Today, the G-bar experiment is about to break the record for the accumulation of positrons, i.e. four billion, continues Pauline Comini. And the Alpha experiment is capable of simultaneously trapping 500 antihydrogens. “ However, the prize for storage goes to the Base experiment, still at CERN, which studies the magnetic properties of the proton and the antiproton: “The researchers kept 1,000 antiprotons for over a year in their sealed flask, where the vacuum approaches that of outer space. Their lifespan is estimated at more than eleven years, says the physicist. But during the manipulations, some are lost. After a year, there were only a few dozen left! “
When Dirac predicted antimatter
In 1928, the British physicist Paul Dirac worked on an equation – which will bear his name – and which attempts to combine Einstein’s special relativity and the very young quantum theory. This admits two solutions: one is the electron, a well-known particle of matter, with positive energy. The other would be a particle identical to the electron in every way, but of negative energy… a configuration that the researchers then thought impossible. Dirac thus predicts that each particle of matter is associated with an antimatter particle, quite similar but of opposite charge. Thus, the electron would form a pair with the anti-electron (called a positron). In 1932, the American Carl Anderson was the first to experimentally identify the trace of this antiparticle! The following year, Paul Dirac shared the Nobel Prize in physics with the Austrian Erwin Schrödinger, one of the fathers of quantum mechanics.
3. Does antimatter “fall” in the air?
At CERN, three experiments (Aegis, Alpha-g and Gbar) attempt to resolve this surprising question: unlike matter that falls towards the ground, does antimatter rise towards the sky? For these experiments, it would be much more practical for researchers to have a charged particle that sets in motion in an electromagnetic field. “However, the gravitational force is so weak that the effects of the electromagnetic force – due to the charge of the particle – would mask it “, explains Yves Sacquin, from CEA. We must therefore carry out these experiments with anti-hydrogen, which is neutral. Making this anti-atom is therefore the first difficulty. To do this, the three experiments use different approaches. For Alpha-g, the anti-atom is formed within the vertical experiment chamber, one meter high, by injecting positrons and antiprotons into it. “Then, the antihydrogen oscillates between the top and the bottom of the chamber, and if everything goes for it as for its double of matter, once arrived at the bottom, it gains in energy and its temperature increases by 1.2 millikelvin “, continues the physicist. The energy level of the trap is then lowered so that the anti-hydrogens that reach this energy can exit. And then it suffices to count them to estimate the effect of gravity on antihydrogen.
The other two experiments (Aegis and Gbar) manufacture their antihydrogens from a gas of positronium, an “exotic atom” made up of a positron and an electron that lives only 142 nanoseconds. “Sufficient time to make it interact with antiprotons and thus produce an antihydrogen atom “, specifies Yves Sacquin. After this first step, the two experiences differ: in Aegis, the antihydrogen acquires an initial speed and, like a thrown balloon, its trajectory is affected by its speed and the gravity it undergoes. Its impact on the detector plate is offset by a few tens of micrometers and this precise measurement makes it possible to advance an estimate of the gravity.
In the Gbar experiment, it is the free fall of an antihydrogen in the gravity field that is observed. But to follow it, it must fall a thousand times slower than in Aegis. “To do this, we add an electric charge to it to obtain ionized antihydrogen, which we know how to slow down in a special trap “, adds the researcher. By falling – upwards or downwards – it impacts the walls of the enclosure made up, of course, of material. This results in an annihilation reaction which emits secondary particles, pions (Pi). “The latter are analyzed by detectors which thus reconstitute the drop in antihydrogen. “ The slightest difference with the drop in hydrogen will allow physicists to speculate on the nature of antimatter. They will thus know if this famous negative gravity, which many researchers doubt, is a valid hypothesis. See you mid-2021.
The GBar experiment, at Cern in Geneva, aims to observe the free fall of an antihydrogen atom obtained from a positronium gas. Credits: JULIEN ORDAN / CER