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Did you know?

The ceaseless transformation of the three neutrinos

  • Did you know that every second about 100 billion neutrinos pass trhough each square centimeter of your body?

    A huge number of neutrinos is produced in nuclear reactions inside the Sun, and quickly escapes from our star in all directions. They are so many that every squared meter of the Earth's surface facing the Sun is crossed by about 3 million billion (yes, 3 1015) neutrinos every second! Which means that during our lifetime, our body receives as a whole something like 1021 (a thousand billion billion) neutrinos. This is an unimaginable quantity, but thanks to the weakness of neutrino interactions with matter, we don't notice this at all! Indeed, most of the neutrinos just pass through our planet completely undisturbed, and it is estimated that only 1 or 2 of those worrying 1021 happen to meet one of the particles we're made of.

    Did you know that if the Sun switched suddenly off, neutrinos would tell us in 8 minutes, while we would have sunlight for a million years more?

    This is another consequence of neutrinos' elusivity: both photons and neutrinos are produced by the nuclear reactions taking place in the core of the Sun. Afterwards, they need to reach the surface of the star before being able to travel in the empty space towards us., which, as you probably know, takes about 8 minutes to light, and almost the same to neutrinos. It is exactly the journey from the nucleus to the surface that makes all the difference: for neutrinos, the Sun is transparent, so it is very fast for them. Photons, instead, participate to the electromagnetic interaction, and therefore they have a much higher probability to interact with the electrons and nuclei that compose the solar plasma. This means that they have to walk a very very tortuous path through continuous absorptions, scatterings and re-emissions, until they slowly cross, one after the other, all the outer layers of the Sun. This takes on average millions years! So, what would happen if the Sun switched off in this instant? The huge amount of photons that are already on their way to the Sun's surface will just go on in their journey, and sunlight would be granted for some other million years. However, the absence of neutrino flux would be noticed by all the neutrinos detectors we have on Earth in less than 10 minutes. 

The Dark Matter Mystery

  • Did you know that billions of dark matter particles can be passing through the human body every second, but the average number of interactions is at most 1 per minute?

    One of the leading candidates for dark matter is a type of particle that can have weak and gravitational interactions, but does not have electromagnetic or strong interactions. Such a particle is called Weakly Interacting Massive Particle (WIMP). On average at most one WIMP interacts with the human body per minute, although billions of them can pass through the body every second.

    Did you know that 85% of the matter content of the universe is made of dark matter and we don’t know its nature yet?

    Many observational evidences suggest that galaxies contain an invisible form of matter which does not interact electromagnetically. The total mass of a galaxy can be measured from the velocities of the stars that orbit the center of the galaxy. We can measure the mass of the luminous part of galaxies, and thus can estimate the remaining mass which is invisible and is called dark matter. Dark matter is estimated to be more than 5 times the ordinary matter in the universe.

Results on neutrinoless double beta decay of Ge-76 from GERDA Phase I

  • Did you know the majority of particles also have a corresponding antiparticle?

    Many concepts in the world come with their opposite: good and bad, black and white, up and down, etc. This is true for a lot of things but not for the totality. For example, the majority of particles also have a corresponding antiparticle, but for some of them, the antiparticle is the particle itself. These are called intrinsically neutral particles.


    The fermionic (i.e. spin 1/2) sector is the only one in the Standard Model for which we don’t yet know if there exist intrinsically neutral fundamental particles. In general, an intrinsically neutral fermion is called a Majorana particle. Otherwise, it is called a Dirac particle. We know, for example, that Majorana fermions can emerge in superconducting materials as quasi-particle bound states, but we don’t know if fundamental Majorana fermions exist. Currently, the only known candidates for this role are the neutrinos.


    The situation is more clear for the other sectors in the Standard Model. For example, if the analysis at LHC would confirm that the observed Higgs particle is indeed the Standard Model Higgs boson, it would be the first example of a scalar (i.e. spin 0) intrinsically neutral fundamental particle. Before the LHC discovery we only knew examples of composite intrinsically neutral scalar particles, as for instance the neutral pion, which is a meson formed by a bounded quark-antiquark pair.


    In the vector (i.e. spin 1) sector there are two examples of intrinsically neutral fundamental particles: the photon, that is the carrier of electromagnetic interactions, and the Z boson, that mediates neutral weak interactions.


    Also the graviton, the unique spin 2 fundamental particle which is at the moment only hypothetical, should be an intrinsically neutral particle.

    If neutrinos are Majorana particles, they could generate a peculiar process that consists in a nuclear decay without any neutrino emission. This process is called neutrinoless double beta decay and is forbidden if neutrinos are Dirac particles.


    A possible future observation of a neutrinoless double beta decay would enable us to conclude that neutrinos are Majorana particles, fitting an intrinsically neutral fundamental particle also in the fermionic sector.


    Text by Michele Lucente.

Higgs boson, the missing piece in the Standard Model of Particle Physics

  • Why is it called the Higgs particle?

    Peter Higgs wrote two papers on the existence of such a particle, becoming the first to mention explicitly that the theory demanded a new particle in nature, which was given the name Higgs boson in 1972.

    In case they decide to award the Nobel price for the discovery, who should be honored?

    (taken from "The Guardian")

    Traditionally, the science Nobel prizes are given to a maximum of three people, whose contributions are judged to be the most important. Two teams of scientists at Cern, amounting to thousands of people, carried out the painstaking work of spotting traces of the particle amid the subatomic debris of more than a thousand trillion collisions inside the Large Hadron Collider. All deserve credit for that effort. But this is the least of the Nobel committee's problems. The prize is more likely to go to theoretical physicists who worked on the theory of particle masses almost 50 years ago. Here the parentage becomes more muddled.

    Six physicists published the theory within four months of each other in 1964. They built on the work of others.

    The first to publish, that August, were Robert Brout and François Englert at the Free University of Brussels. Brout died in 2011, and the award cannot be given posthumously.

    Second to publish was Peter Higgs, with two papers on the theory in September and October 1964. In his second paper, he became the first to mention explicitly that the theory demanded a new particle in nature, which was given the name Higgs boson in 1972. Drawing attention to the particle was crucial, because it gave scientists something concrete to hunt.

    Third to publish was a group of three theorists, including two US researchers, Dick Hagen and Gerry Guralnik, and a British physicist, Tom Kibble. Their work was published in November. All three teams worked independently.

    So there are at least five living physicists who can lay claim to the Nobel prize. If the particle discovered at Cern is confirmed to be the Higgs boson, then Higgs is certain to be honoured. That leaves four physicists competing for two places. Englert published first, and would be hard to dismiss. That leaves one place left.

    The quandary raises a familiar issue for the Nobel committee. Restricting those honored with a Nobel helps maintain their prestige. But in modern science, few discoveries are born in final form from so few parents.

    How did the Higgs boson get the nickname ?the God particle?

    Nobel laureate Leon Lederman, a Fermilab physicist, wrote a book in the early 1990s about particle physics and the search for the Higgs boson. The publisher chose this nickname for improving the book sales. Many scientists do not like that nickname.

    What would the world look like without the Higgs boson or a similar particle?

    The world would not be as we know it today. Without the Higgs boson or something like it giving mass to the basic building blocks of matter, electrons would not form unions with protons to make atoms. No atoms=no molecules=no ordinary matter =no life!