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Hic sunt neutrini 04/03/2013 20:46 #60

Hic sunt neutrini
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Why the neutrino passion? Its fascination comes from combining essential simplicity with scientific challenges of major mysteries still open. In Nature - and thus also for us - it has also a "social role", since the Big Bang: one morning I will perhaps wake up with the idea to tell you about it.

The usual quarks

Think for a moment of the quarks. They are normally bound in complex systems such as protons and neutrons, in turn bound in atomic nuclei. Their study cannot disregard side effects, sometimes complex, due to the fact that they are not free. To investigate the essence of the phenomena, observations have to be purged from side effects. This not done conceptually new: Galileo taught us that in order to study the falling bodies we must disregard what derives from friction with the air. It 's just in high-energy experiments that quarks come to the so-called "asymptotic freedom". In crude words but basically correct, the very high energy particles are able to penetrate into nuclei, protons and neutrons and "see" quarks as if they were isolated, free. We free ourselves from side effects also in searching for or studying high mass particles such as the Higgs boson or the W and Z particles mediating the Weak Interactions: the energy E = mc2 made available in their spontaneous decay shoots out particles that "pierce the screen" and stand out from the rest.

Neutrinos: truly special

Neutrinos are free from birth, they are not tied to anything. They can be treated as "mathematical points", but very special ones. It is practical reality what in classical mechanics is an abstraction conceived in order not to be bothered by effects related to the size of the bodies. Their interactions are simple and clear. They are not contaminated by side interactions of particles bound to them. Look in the figure at the interaction of a neutrino which came from CERN and was observed in the OPERA experiment at Gran Sasso. Then compare it with one of the interactions in the high-energy collider LHC at CERN, that you have in your memory or can find on the web. What a beautiful simplicity. Yet this interaction plays an important role in clarifying what until a few years ago it was the mystery of the neutrino mass. It demonstrates the rare and magical transformation - suggested by Bruno Pontecorvo in 1957, as a result to be expected in the case of non-zero neutrino mass - of a neutrino said of muon type in a neutrino currently considered as different, the tau neutrino. In the specific language of physics, this process is called "neutrino oscillation". You know, in physics is like on a boat: in order to be fast without being confusing, the words used on the sea are different from those used on land. One says "luff" and not "go more upwind."

When complexity is beautiful

Let's open a brief parenthesis. The complexity fascinates, however, when one deals with "complex systems", to be treated just like systems. Don't you look with awe and wonder the figures drawn in the sky by a flock of birds? Changeable and unpredictable but with all birds in coherent motion: it makes you understand that there must be scientific methodologies suitable to study complex systems. Read the article by Massimo Pica Ciamarra on this Forum.

Area of mystery

Let's return to the neutrino. It is assumed that the Latin phrase "hic sunt leones" (here there are lions) denoted unexplored areas of Africa in geographical maps of ancient Rome. Let's take the freedom to paraphrase it in "hic sunt neutrini". Neutrinos are still a largely unexplored area of Science: an experiment in neutrino physics is still an adventure for the experimental challenge it poses. Experimental difficulties and simplicity of the interactions come from the same underlying reason, they are the two sides of the same coin: the neutrino interactions are purely "weak". Their probability of interaction with matter is extremely low, so they go through an experimental apparatus without being "seen", almost ever. This implies great experimental difficulties. But, when they occur, their "weak" interactions are pure and unaffected by "electro-magnetic" interactions (own of electrically charged particles) or by "strong" interactions that bind quarks within protons and neutrons.

Neutrinos are free particles travelling at the speed of light. As a matter of fact, it was recently discovered that neutrinos have non-zero mass, instead of being zero as previously hypothesized. However extremely small. The peculiarity (that we do not describe) of the interaction shown in the figure is that it gives conclusive evidence. The speed of the neutrino is therefore extremely close but not exactly equal to that of light.

The discovery that the neutrino has non-zero mass is the first step towards new ways beyond the "Standard Theory" of elementary particles as it is currently formulated. But it does not exhaust the mysteries of the neutrino. We will illustrate one of them, pointed out by Ettore Majorana.

Neutrino today

Wolfgang Pauli postulated the existence of the neutrino in 1930, noting that the spontaneous disintegration of nuclei with the emission of electrons or anti-electrons (positrons) - called "beta decay" - must also produce an electrically neutral, hence invisible, particle with practically zero mass. He qualified this bold hypothesis as a "desperate remedy" to respect the principles of conservation of energy and angular momentum: the neutrino is in charge of fixing things. Those produced in association with positrons were considered as "neutrinos" and those, distinct, emitted together with electrons as "anti-neutrinos".

The neutrino has a "spin angular momentum", that is it behaves like an invisibly spinning top. It 's a mystery how it can show an effect usually of rotating bodies, without any evidence of non-zero size. But also a tennis player is not able to see the rotation of a spinning ball. Yet, he well realizes it when the ball bounces on one side or the other, confusing him. Also neutrinos are "spinning", transferring in colloquial language the fact that they have a "spin angular momentum". We become aware of it from how they interact with matter. As for the tennis balls we become aware from the rebound.

The spin angular momentum of neutrinos has well-defined absolute value (equal to that of electrons and quarks). According to the direction of their invisible rotation, neutrinos are called "left-handed" or "right-handed". Even a tennis player may qualify so the spinning balls, depending on whether the bounce is deflected to one side or the other.

Does the neutrino have its "anti"?

Ettore Majorana drew a sharp consequence from the "experimental fact" that neutrinos are always and only left-handed and anti-neutrinos right-handed. Why this waste of words? Wouldn't be enough to differentiate them by simply speaking of "left-handed neutrinos" and "right-handed neutrinos", that is only of neutrinos? Why to doubly differentiate them as left-handed/right-handed and as neutrino/anti-neutrino? In fact, neutrinos have no electrical charge and therefore there is no need to differentiate them as neutrinos or anti-neutrinos in relation to it, as it is for example for electrons and positrons. Majorana remarked that it is simple and rational to say that there are only "neutrinos" and that they can be left-handed or right-handed depending on the sign taken by the spin angular momentum. The same top can spin clockwise or anti-clockwise: it just depends on how it is launched, there is no anti-top.

Simple and practical, but let's see it emotionally putting ourselves in the "shoes" of ... a neutrino. Imagine its feelings when it finds out that the strange being called anti-neutrino is nothing other than it's mirror image, in which left and right are symmetrically exchanged. Isn't it an emotion for children, when they make a similar "discovery"? The relationship with a mirror remains complex even for adults. It is an intriguing relationship also for animals, that do not always learn to recognize their image or do not learn it immediately: Have you ever seen a kitten playing with its image?

Let's go back to the brilliant simplicity of Majorana. All nice and logical, but we have to prove it experimentally by observing a process that occurs only if his view is correct. Such a very rare (if it exists) process is the so-called "double-beta decay without neutrinos' emission." Even without going into its description, you already understand the motivation for the difficult experimental challenge to observe it. One of the challenges, one of the mysteries. The neutrino is more than eighty years old and still it is young. "Hic sunt neutrini": the neutrino and many other fundamental questions that Science arises provide new space for your adventures.

You might ask, what is the practical use? Science, or more appropriately Sciences, expand the frontiers of knowledge. It is an intellectual requirement of the "human species" that since prehistoric times, and very impressively in recent centuries, has led to such a strong differentiation and to live better. There are studies that lead to immediate applications, others do not. Sciences have to be taken as a whole.
Paolo Strolin

Professore Emerito di Fisica Sperimentale
Università di Napoli "Federico II"
Complesso Univ. Monte S. Angelo
Via Cintia - 80126 Napoli - Italy

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