IL NUCLEARE

Direttore

Ettore GUniversità degli Studi di MilanoPiero Caldirola International Centre for the Promotion of Science

Comitato scientifico

Ignazio LInstitute for Scientific Methodology

Elio SUniversità degli Studi di Milano–Bicocca

Comitato redazionale

Francesca BUniversità degli Studi di Pavia

Francesco CEuropean Organization for Nuclear Research CERN

Comitato editoriale

Giuseppe BIstituto Nazionale di Fisica Nucleare

Laszlo S BUniversidad Simón Bolívar

Elio SUniversità degli Studi di Milano–Bicocca

Ignazio LInstitute for Scientific Methodology

IL NUCLEARE

La Fisica Nucleare ha portato a scoperte fondamentali ed è tuttora uncampo di indagine alle frontiere della ricerca che permette in modopeculiare ed esclusivo lo studio della materia elementare in condizioniestreme.

Non meno importante è il suo utilizzo in ricerche e applicazionitecnologiche di immediato interesse per la Società, tra cui oggi sonodi particolare importanza la produzione controllata e sicura di energiae le applicazioni mediche per la diagnosi e la terapia di tumori.

Conclusioni analoghe si raggiungono se si considerano le ricerchesulla radioattività: accanto a studi di carattere fondamentale, leapplicazioni di tipo medico ed industriale, per il controllo ambientale,la sicurezza, la datazione di reperti sono innumerevoli.

Questa collana si propone la pubblicazione di testi volti a descriverequesta variegata moltitudine di argomenti e a rappresentare una fontedi informazioni obiettive e documentate.

Neutrino: the mutant particle

Edited by

Elena CanoviGiampaolo Cò

Daniele MontaninoFrancesco Vissani

Contributions ofMaria Benedetta Barbaro, Omar Benhar

Paolo Bernardini, Giampaolo Cò, Vincenzo Flaminio,Carlotta Giusti, Eligio Lisi, Gianpiero Mangano

Camillo Mariani, Alessandro Mirizzi, Daniele Montanino,Francesco Ronga Francesco Terranova, Francesco Vissani

Copyright © MMXVIARACNE editrice int.le S.r.l.

www.aracneeditrice.itinfo@aracneeditrice.it

via Quarto Negroni, Ariccia()

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No part of this book may be reproducedby print, photoprint, microfilm, microfiche, or any other means,

without publisher’s authorization.

I edition: August

Contents

IntroductionElena Canovi, Giampaolo Cò, Daniele Montanino, Francesco Vissani

Neutrinos and weak interactionGiampaolo Cò

Neutrino oscillationDaniele Montanino

A brief history of neutrino oscillationsFrancesco Ronga

Majorana’s conundrumFrancesco Vissani

Neutrino interaction with matterMaria Benedetta Barbaro, Omar Benhar, Carlotta Giusti

The future of neutrino research in EuropeFrancesco Terranova

Neutrino experiments in the USACamillo Mariani

Contents

Neutrino astrophysicsVincenzo Flaminio

Neutrinos and SupernovaeAlessandro Mirizzi

Neutrinos and cosmologyGianpiero Mangano

The frontier of the sterile neutrinosPaolo Bernardini

Neutrinos: messengers of new physicsEligio Lisi

The authors

Neutrino: the mutant particleISBN 978-88-548-9580-5DOI 10.4399/97888548958051pag. 9–10 (august 2016)

Introduction

Elena Canovi∗, Giampaolo Cò ∗∗Daniele Montanino∗∗, Francesco Vissani∗∗∗

On the first week of October the Royal Swedish Academy ofSciences announced that the Nobel Prize in Physics was assignedto Takaaki Kajita and Arthur B. McDonald for the discovery ofneutrino oscillations, which shows that neutrinos have mass. This isthe academic acknowledgement of a result which has deeply modifiedour understanding of fundamental physics. In about twenty years ofexciting discoveries (and eighty from Pauli and Fermi’s ideas) neutrinophysics has changed from a pioneering discovery activity into a matureprecision science.

In this volume we collect a set of articles presenting modernissues of neutrino physics. These articles, with the exception ofthat of G. Mangano, were already published in Italian on Ithaca, anon–line journal devoted to present scientific topics to a public ofnon experts, high–school teachers, Physics students. The traditionaldescription of neutrino physics and of weak interaction, before thediscovery of neutrino oscillations, is presented in the article writtenby Giampaolo Cò. The theoretical framework and the empiricalevidences of neutrino oscillations are described in the article byDaniele Montanino.

The following contribution, written by Francesco Ronga, isthe chronicle of the announcement of the discovery of neutrinooscillations in the summer of written by one of the protagonistsof the discovery. Besides its historical interest, this article offers aninsider view of the dynamics of scientific research and of the large

∗ Gran Sasso Science Institute, L’Aquila.∗∗ Department of Mathematics and Physics “Ennio De Giorgi” – University of Salento

and INFN, Lecce.∗∗∗ Gran Sasso Science Institute and Gran Sasso National Laboratories – INFN, L’Aquila.

Introduction

international physics collaborations and experiments. Even thoughneutrino oscillation is now a widely accepted fact, the mysteriesof the structure of neutrinos are not fully clarified. One of mainpending question was raised by Majorana: whether the neutrinois identical to its antiparticle, as it happens, for example, to thephoton. This is discussed in the contribution by Francesco Vissani.Evidently, our knowledge about neutrinos is tightly related to thepossibility of observing their interactions. Thus, the knowledge oftheir interaction with matter, discussed in article of Maria Barbaro,Omar Benhar and Carlotta Giusti, is essential. An overview ofthe situation of neutrino experiments planned in Europe and inthe United States is given in the articles by Francesco Terranovaand Camillo Mariani. Vincenzo Flaminio’s contribution describespresent and future activities concerning neutrinos in astrophysics,while the article by Gianpiero Mangano deals with neutrinos andcosmology. Alessandro Mirizzi describes the role of neutrinos insupernovae explosions. Paolo Bernardini discusses the results ofobservations which could indicate the presence of new types ofneutrinos, insensitive to ordinary weak interactions. Finally, Eligio Lisiindicates how the study of neutrinos helped and will help us widenour scientific horizons, emphasizing the possibilities of investigatingphenomena which go beyond our present understanding.

We wish to stimulate the interest of the reader for neutrino physics,a lively branch of current scientific research, full of perspectives andexpectations, and likely to surprise us again.

These essays offer many entrance points to neutrino physics andthey express a great variety of points of view. We have collectedthem in this volume and we offer them to the reader, in the hope tostimulate further interest in neutrino physics, a lively branch of thescientific research, full of perspectives and expectations and that islikely to surprise us again in the future.

Neutrino: the mutant particleISBN 978-88-548-9580-5DOI 10.4399/97888548958052pag. 11–30 (august 2016)

Neutrinos and weak interaction

Giampaolo Cò∗

: This introductory article is addressed to those readers who do nothave great familiarity with the neutrino physics and the weak interaction.I shall provide a short presentation of some well–established facts that willbe considered and discussed in the other articles of the present volume.

. A little bit of history

It is common wisdom to choose the as the year of the discoveryof the radioactivity. In this year, Henry Becquerel observed thatsome photosensitive slides, conserved in a drawer which was wellsealed from the external light, had been impressed. The origin of thisphenomenon was related to the presence of some material which,consequently, resulted to emit radiation.

In the first years of it was already evident that the radiationemitted from radioactive materials could be catalogued in onlythree different types, which were called α, β and γ following theirpenetration power in the matter. It was found later that the α rays, theleast penetrating, are nuclei of He, the γ rays the most penetratingones, are high–energy photons, and the β rays are electrons.

The α and γ decays presented discrete spectra. This means that, fora specific radioactive material, the energies of the α or the γ rays wereconstant. This fact was immediately understood in terms of the energyconservation. In fact, the value of the energies which were measuredfor the α or the γ rays coincided with the difference between the massof the parent nucleus, the nucleus which undergoes the radioactivedecay, and that of the sum of the masses of the decay products.

∗ Department of Mathematics and Physics “Ennio De Giorgi” – University of Salentoand INFN, Lecce.

Giampaolo Cò

On the contrary, the energy spectrum of the electrons measuredin the β decay, see Figure , was continuous for each material.Furthermore, calorimetric measures [, ] showed that, on average, theelectrons transported less than half of the available energy, obtainedby the comparison between the masses of the parent and daughternuclei. The hypotheses proposed to explain these observations raisedmany problems. It was even questioned wether in β decay processesthe energy had to be conserved.

In the , Wolfgang Pauli, in a nowadays very famous letter,proposed the idea of the existence of a particle without electriccharge, therefore very difficult to detect, which would have beenemitted together with the electron, in such a way that the sum of theenergies of the two particles would be constant. Pauli named neutronthis particle, which must be a fermion to satisfy the conservation ofthe statistics.

Figure . Energy spectrum of the electron emitted in the β decay of the Binucleus. The maximum energy Emax, of . MeV, is given by the differencebetween the mass of the parent nucleus, Bi, and that of the daughter nucleus,Po. The vertical line indicates as, on average, the available energy is dividedbetween electron and antineutrino. The average energy of the electron is measuredin calorimetric experiments

Neutrinos and weak interaction

Pauli’s letter

Physics Institute of Politechnical School, ZürichZürich, Dec. th , Gloriastrasse

Dear Radioactive Ladies and Gentlemen,as the bearer of these lines, to whom I graciously ask you to listen,

will explain to you in more detail, because of the “wrong” statistics of theN– and Li– nuclei and the continuous beta spectrum, I have hit upona desperate remedy to save the “exchange theorem” of statistics and theenergy conservation law. It is the possibility that in the nuclei there couldexist electrically neutral particles, which I will call neutrons, that have spin/ and obey the exclusion principle and that further differ from light quantain that they do not travel with the velocity of light. The mass of the neutronsshould be of the same order of magnitude as the electron mass and in anyevent not larger than . proton mass. The continuous beta spectrumwould then make sense with the assumption that in beta decay, in additionto the electron, a neutron is emitted such that the sum of the energies ofneutron and electron is constant. Now it is also a question of which forcesact upon neutrons. For me, the most likely model for the neutron seems tobe, for wave–mechanical reasons (the bearer of these lines knows more),that the neutron at rest is a magnetic dipole with a certain moment µ. Theexperiments seem to require that the ionizing effect of such a neutron cannot be bigger than the one of a gamma–ray, and then µ is probably notallowed to be larger than e× − cm. But, so far, I do not dare to publishanything about this idea, and trustfully turn first to you, dear Radioactivepeople, with the question of how likely it is to find experimental evidencefor such a neutron if it would have the same or perhaps a times largerability to get through matter than a gamma–ray.

I admit that my remedy may seem almost improbable because oneprobably would have seen those neutrons, if they exist, for a long time.But nothing ventured, nothing gained, and the seriousness of the situation,due to the continuous structure of the beta spectrum, is illuminated by aremark of my honorable predecessor, Mr. Debye, who told me recentlyin Bruxelles: “Oh, It’s better not to think about this at all, like new taxes”.Therefore one should seriously discuss every way of rescue. Thus, dearradioactive people, scrutinize and judge. Unfortunately, I cannot personallyappear in Tübingen since I am indispensable here in Zürich because of aball on the night from December th to th. With my best regards to you,and also to Mr. Back.

Your humble servantW. Pauli

. Pauli refers to the fact that the emission of a single fermion, the electron, violatesthe conservation of the spin statistics since it would transform a spin integer system in astate that, globally, would have semi–integer spin, or viceversa. (N.o.A.).