I'm very happy while I'm writing this blog because few days ago the announcement about the discovery of a subatomic particle, theoryzed in 1937 by the italian physicist Ettore Majorana, has made by a team of Dutch scientists of the Delft University of Technology in collaboration with a team of dutch collegues (QuTech, Microsoft, Eindhoven University of Technology) and american collegues (JQI Maryland, UC Santa Barbara). What is this particle? How was it detected? Which implications its discovery will have in the everyday reality? Those are the topics I will try to clarify in this blog.
Portrait of Ettore Majorana. Credits: Wikipedia
▀ The Majorana fermion
The Majorana particle I will write about is a fermion.
According to the Standard Model, i.e. the physical model which summarize and catalogues all the fundamental physical particles that have been discovered until now, the Universe is made up of two big families of particles: fermions and bosons. Fermions always have mass, seminteger spin (a parameter that measures in which way the particle rotates around its axis), and they are the elementary constituents of matter; bosons, instead, are particles which can have or not mass, they have an integer spin and they are also at the base of the fourth forces of Nature (gravity, electromagnetism, weak and strong nuclear forces).
The particles that belongs to the family of fermions divide into two groups: quarks and leptons, each of which has six elements, divided into three generations or families. To the group of leptons belong electrons, muons, tauons and neutrinos, which can be of three types: electron, muon or tau. The experiment which has been executed few days ago demonstrated that just neutrinos are the Majorana particles.
What is a neutrino?
A neutrino is an elementary particle which has an extra small mass (from 100,000 to 1 million times less than the one of the electron) and no electrical charge.
The table of elementary particles of the standard model of physics. Credits: physics.stackexchange.com
It is an elusive and "undecided" particle, since each neutrino can freely change its type, passing from one type to another or, as they say in jargon, it can "change flavour". For example, an electron neutrino can become a muon one and vice versa. Neutrinos are not a very social particles: they cross the matter without interacting with it; the only entities with which they interact are the gravitational force and the weak nuclear one, responsible of the radioative decay process. They are not easy to find in Nature, since they correspond to instable states of matter; however big quantities of this particles can be observed in the zones into contact with the atmosphere and near the nuclear centrals, since either the radioactive decay process inside the nuclear centrals or the cosmic radiations are excellent sources of neutrinos.
The three types of neutrino: electron neutrino (red), muon neutrino (green) and electron (blue). The directional arrows indicate how each can morph into different types. Credit: blogs.voanews.com
According to the observations of the british physicist Paul Dirac, fermions are descripted from the Fermi-Dirac statistic and they enjoy the symmetry property, on the basis of which each particle corresponds to an antiparticle, characterized by equal mass but opposite electrical, baryonic or leptonic charge. This theory was experimentally proved few years later than its pubblication, when the antiparticle of the electron, called positron, was detected.
Neutrinos, however, represent a singularity: even if they are fermions, they don't have an antiparticle, because they themselves are their own antiparticle at the same time. This property was teoryzed by the italian physicist Ettore Majorana, who in 1937 solved the Dirac's equation with the aim of removing the solutions with negative energies. The new and original way in which he solved those equations, made him obtain a new result which prospected the possibility, just for the chargeless particles, (neutrons or neutrinos) of being without an antiparticle.
Tracks of neutrinos. Credits: discovermagazine
He published his observations in the essay titled "Symmetric theory of electron and positron".
The antiparticle of neutron was found; the neutrino antiparticles, instead, were never observed. Until few days ago.
▀ In which way the particle was detected?
Before describing the expariment which allowed scientists to detect the presence of the Majorana fermion, it's appropriate to make a clarification: the Majorana particle wasn't observed directly; what scientists observed is a quasiparticle.
A quasiparticle is a particular pattern of electrons and atoms, which behave as if they were particle themselves.
The behavior of the electronic system in question, in the presence of particular applied electric and magnetic fields, reacted as if it was in presence of the Majorana particle.
Scientists used a nanowire made of a conductor material, which put in contact a classic golden electrode with a superconductor one; they observed in the presence of particular electric and magnetic fields a polarization phenomenon with a peak of zero polarization and this peak didn't change not even when the values of the applied fields were changed; this behavior can be justified only with the presence of the Majorana particle.
The experimental circuit. Credits: qutech.nl
Detailed experimental circuit and the theoretical and experimental signals. Credits: qutech.nl
The first experimental results were obteined in 2012 by the same team of scientists and published on Science but the signal was too much noisy for saying with absolute certainty to have detected the track of the Majorana fermion.
Scientists retried after having upgraded and enpowered the measurement tools and the experimental apparatus and, finally, they succeeded. The results were published on Nature.
Is it possible to detect the particle directly?
For years a lot of ongoing experiments in all the world are trying to detect directly the Majorana fermion: some of them are CUORE (Cryogenic Underground Observatory for Rare Events), Lucifer and GERDA in Italy, under Gran Sasso mountain, and NEMO-3, under the tunnel of Frejus, between Italy and France, Kamland-Zen in Japan and Exo in the USA.
The goal of the experiments is to prove definitely the nature of the particle during a particular and rare process of decay of a neutron: this process
takes the name of double beta decay and it is the cascade of two beta dacay, which are more common.
A beta decay is a decay phenomenon in which a neutron becomes a proton and emits an electron and an antineutrino. At the same way, it could happen that a neutron absorbs a neutrino and becomes a proton with the emission of an electron.
If neutrino and antineutrino are the same particle, according to the Majorana's hypothesis, it could happen that in an atomic nucleus, when a neutron becomes a proton and emits an electron and an antineutrino, if the latter is absorbed by a second neutron, then it becomes a proton with the emission of an electron. The final results is a neutrinoless double beta decay.
In the first image there is the classic beta decay; in the second one, instead, there is the reverse beta decay. In the last one, the neutrinoless double beta decay.
This experiment could detect the mass of the particles, since the probability that it verifies is proportional to the probability of having two simultaneous beta decays and proportional to the squared neutrino mass. However, it is an extremely rare event: scientists calculed that a single Tellurium atom can undergo a double beta decay once every ten millions of billions of billions years (10,000,000,000,000,000,000,000,000), an age much higher than the one of the Universe, which is "only" 13 billions years old. The phenomenon happens when two requirements are satisfied:
- the atomic particles (neutrons and protons) are very close each other, as inside the nucleus;
- the atoms have an equal number of protons and neutrons.
For this reason the chemical elements used for the experiments are isothopes of Tellurium, Germanium, Xenon and few other elements. Tellurium is the most frequently used because the one which can be found in Nature contains more than 30% of the isothope 130, the perfect one for the purpose. In order to reduce significantly the observation period within five or ten years, scientists decided to use big amounts of material, around 100 kilos, to observe at the same time billions and billiond of atoms. According to the estimate, by working this way, it will be possible to see almost ten events.
However, there is a big problem: the background noise.
What is that?
It is a noisy signal, caused by several and different phenomena not linked to the one in question, which doesn't allow scientist to distinguish the signal produced by electrons emitted after the double beta dacay.
Few examples of those unwanted signals are: radioactive dacays of materials next to the revelators or the interactions between particles, coming from the envirorment in which is placed the revelator.
▀ Which implications the discovery of the Majorana fermion will have?
The discovery of the particle which violate the Standard Model will have several consequences: first of all, it will totally modify the current view of Physics; it will allow physicians to investigate the mistery of asimmetry between matter and antimatter and it will open the doors to the building of the quantum computer. But let's proceed with order.
The Standard Model says that there is no possibility for neutrinos not to have an antiparticle; moreover, according to it, they have no mass. If this was true, it would be impossible to witness the phenomenon of the "change of flavor" by neutrinos, i.e. the phenomenon on the basis of which a neutrino travelling throught the space changes type, passing from electronic to muonic, for istance. A lot of experiments, like the american Homestake and the japanese Kamiokande and Super-Kamiokande, proved this phenomenon since the Seventies. Other experiments that have to be mentioned are the canadian Sno and Macro, Gallex/Gno, Borexino and a lot more. The first consequence will be the creation of a new model beyond the Standard Model, introducing the not zero mass of the neutrino. This will push scientists to search for new particles (if they exist), new interaction mechanisms and new physical phenomena.
Among the different theoretical implications there is the unsolved mystery of the primacy of the matter in our Universe, despite the fact that at the istant of the Big Bang, an equal amount of matter and antimatter was created. Until now, they always believed that matter was eternal; we can change its shape from a certain form to another but the particles of which it is made continue existing. Well, this axiom could decay because the phenomenon of the neutrinos oscillations (the change of their flavor) suggests that matter can be created or destroyed. Several experiments are ongoing in order to demonstrate few theories that can update the Standard Model and justify why matter predominates in our Universe. Some experiments try to observe creation and destruction of matter; in particular the already mentioned neutrinoless double decay, the proton decay and the electron decay.
Matter and antimatter illustration. Credits: mx.blastingnews.com
The last consequence, but not in order of importance, is the building of the famous quantum computer.
What is a quantum computer?
It is a computer in which the informations are not stored anymore in classical bits but in qubits, i.e. quantum bits. A classic bit can have just two values: 0 and 1, realized throughout electrical signals inside microprocessors and processed by transistor logic gates; a qubit, instead, can have three values: 0, 1 and undetermined, because of the extremely characteristic nature of the subatomic particles; the undetermined value is an overlap of values, i.e. the value of the bit is 0 and 1 at the same time.
It seems impossible but it is what happens in the subatomic reality, in which chaos and undetermination rule; this will exponentially increase the computing power and the speed of computers. A computer of this kind can process much more informations than classical computers do. It could be the beginning of the Artificial Intelligence Era.
Currently, photons are used to realize qubits but they have a big problem: they are very "social" and this causes a foton to interact with other particles during its propagation through the circuit losing the information it transported and causing errors and system slowdowns; the Majorana particle, instead, can be extremely useful in this situation because of its properties. A neutrino is chargeless and it doesn't interact with anything but with the gravitational force and the weak nuclear force; for this reason the neutrino is an extremely stable particle and the probability that the information stored inside it will be lost is extremely lower.
Ultimately, in 1937 Ettore Majorana theorized the existence of a particle, known as Majorana particle after his death, which violated the Dirac's theory, according to which each fermion always has an antiparticle. Between the end of March and the beginning of April 2018, a team of dutch scientists, in collaboration with american collegues found the Majorana quasiparticle, i.e. a pattern of atoms and particles which behave as if they were in presence of the Majorana particle. The experiment demonstrated its existence and the implications will be huge: first of all, they will have to design a new Standard Model, i.e. the model that summarises all the elementary particles which have been found until now; the new model, will be able to justify the asimmetry between matter and antimatter and it will pave the way to the building of quantum computers, whose computing power will be incredibily superior to the current computer on the market. Are you ready for the future? Stay tuned.
Read you later.
Blog written by Pixor for bitlanders. If you have any suggestions and/or questions, please comment without fear. I will be happy to answer you all.
Pixor, student in electronic engineering at the Polytechnic University of Bari, Italy. Curious and passionate about science, technology, animals, travels, history and music.
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