Bruno Pontecorvo-pioneer of neutrino oscillations

S. BilenkyJINR(Dubna)

September 7, 2013

I Bruno Pontecorvo was born on August 22 1913 in Pisa(Marina di Pisa)

I His father was owner of a textile factory. The factory wasfounded by Pellegrino Pontecorvo, Bruno grandfather.

I After the war during many years the factory was closed andthe building was not used. Now the Pisa department of INFNis in the building of the Pontecorvo‘s factory. The squarebefore the building is called Largo Bruno Pontecorvo

I There were 8 children in the family: 5 brothers and 3 sistersAll of them were very successful

I Three brothers became famous:I Biologist Guido (the eldest brother)I Physicist BrunoI Movie director Gillo

I Bruno entered engineer faculty of Pisa University. However,after two years he decided to switch to physics

I From his autobiography. My brother Guido declaredauthoritatively “Physics! I would like to say that you must goto Rome. In Rome there are Fermi and Rasetti”.

I Bruno passed through exam that was taken by Fermi andRasetti and was accepted at the third year of the Faculty ofPhysics and Mathematics of the Rome University withspecialization in experimental physics

I First as a student and later as researcher from 1931 till 1936Bruno worked in the Fermi group

I The experiment performed by Amaldi and Pontecorvo lead tothe discovery of the effect of slow neutrons, the mostimportant discovery made by the Fermi‘s groupAt that time Bruno was 21

I For the discovery of the effect of slow neutrons Fermi wasawarded the Nobel Prize.This effect opened the road to all applications of neutrons(reactors, isotopes for medicine,... and atomic bombs)

I In 1936 Bruno received a premium of the Italian Ministry ofEducation and went to Paris to work with F. Joliot-Curie

I In Paris he studied nuclear isomers, metastable nuclear stateswith large spins. He made first experiments on theobservation of electrons of the conversion in decays ofisomers, on production of nuclear isomers in process of theirradiation of nuclei by high-energy -quanta etc

I For the study of the nuclear isomerism Bruno gotCurie-Carnegie premium. Fermi congratulated Bruno onexcellent results. Bruno was very happy and proud by Fermi‘scongratulation (as he wrote in his autobiography, he thoughtthat Fermi, who usually called him great champion, hadrespect to him only as an expert in tennis)

I In 1940 before Germans occupied Paris Bruno with wife andson escaped to US.

I From 1940 till 1942 he worked in a private oil company inOklahoma. He developed and realized a method of neutronwell logging for oil (and water) prospering

I This was the first practical application of the effect of slowneutrons. The Pontecorvo‘s method of neutron well logging iswidely used nowadays

I In 1943 B. Pontecorvo was invited to take part in theAnglo-Canadian Uranium Project in Canada. Bruno (30) wasscientific leader of the project of the research reactor whichwas built in 1945 and was the first nuclear reactor outside ofUSA

I In Canada Pontecorvo started research in elementary particlephysics

I After famous Conversi, Pancini and Piccioni experiment(1947) from which it followed that muon weakly interact withnuclei Bruno Pontecorvo and E. Hincks performed a series ofpioneer experiments on the investigation of fundamentalproperties of muon

I They proved thatI The charged particle emitted in µ-decay is electron.I Muon decays into three particles.I Muon does not decay into electron and γ-quantum.

I Pontecorvo suggested that the muon is a particle with spin1/2 and in the process of muon capture by a nucleus neutrinois emitted.

I He compared the probabilities of the processesI µ− + (A,Z )→ ν + (A,Z − 1)I e− + (A,Z )→ ν + (A,Z − 1)

and found that the constants which characterize these twoprocesses were of the same order of magnitude

I He came to the conclusion that exist “ fundamental analogybetween β-processes and processes of absorption of muons“.Thus, in 1947 Pontecorvo came to the idea of the existence ofan universal weak interaction which include eν and µν pairs.Later the idea of µ− e universality was put forward by Puppi,Klein, Tiomno and Wheeler.

I Soon after famous Fermi paper on the theory of the β-decay(1934) Bethe and Pierls estimated cross section of theinteraction of postulated by Pauli neutrino with a nucleus

I The estimated cross section was extremely small. At ∼ MeVenergies σ < 10−44cm2

I At that time did not existed methods of detection of processeswith such small cross section. During many years neutrinowas considered as an “undetectable particle“

I The first physicist who challenged this opinion and proposed amethod of neutrino detection was Pontecorvo (Canada, 1946)

I Pontecorvo‘s method was based on the observation of decayof daughter nucleus produced in the reactionν + (A,Z )→ e− + (A,Z + 1)

I He considered as the most promising the reactionν +37 Cl → e− +37 Ar

I many reasons: C2Cl4 is a cheap, non-flammable liquid, 37Ar isunstable nucleus with a convenient half-life (34.8 days), a fewatoms of 37Ar (rare gas), produced during an exposition time,can be extracted from a large detector, etc

I The Pontecorvo Cl − Ar radiochemical method was realizedby R. Davis in his first experiment on the detection of thesolar neutrinos

I In 2002 R. Davis was awarded the Nobel Prize for thediscovery of the solar neutrinos

I Radiochemical method of neutrino detection based on theobservation of the reaction ν +71 Ga→ e− +71 Ge, proposedby V. Kuzmin, was used in the GALLEX-GNO and SAGE solarneutrino experiments in which main p − p neutrinos weredetected

I In the seminal Chalk River paper (1946) Pontecorvo paidattention to the following intensive sources of neutrinos

I The sunI ReactorsI Radioactive materials produced in reactors

I In 1948 Pontecorvo invented low-background proportionalcounter with high amplification. This counter was crucial fordetection of solar neutrinos in Homestake , GALLEX andSAGE experiments

I In 1950 Bruno with family (wife and three sons) moved fromEngland to USSR

I He started to work in Dubna where at that time was thelargest accelerator in the world (460 MeV later 680 MeV)

I Pontecorvo and his group performed experiments on theproduction of π0 in neutron-proton and neutron-nucleicollisions, on pion-nucleon scattering and others

I Bruno always thought about neutrino

I In the end of the fifties in Dubna a project of a meson factorywas developed (unfortunately the project was not realized). Inconnection with this project Pontecorvo thought about afeasibility of neutrino experiments with neutrinos from decaysof pions and kaons produced at high intensity accelerators

I Bruno came to the conclusion that experiments withaccelerator neutrinos are possible (independently M.A. Markovand Schwartz came to the same conclusion)

I Bruno understood that experiments with high energyaccelerator neutrinos give perfect possibility to answer thefundamental question

I Are νe and νµ the same or different particles?

I In 1959 he published a paper on accelerator neutrinos.

I Pontecorvo‘s proposal was realized in the famous Brookhavenneutrino experiment (1962). It was proved that νe 6= νµ

I In 1988 Lederman, Schwartz and Steinberger were awardedthe Nobel Prize for the discovery of the muon neutrino leadingto classification of particles in families

I NEUTRINO MASSES, MIXING AND OSCILLATIONSThis bright and courageous idea of Bruno Pontecorvo createda new era in neutrino physics

I Bruno came to idea of neutrino oscillations in 1957-58 soonafter the two-component neutrino theory appeared. He wasvery much impressed by a possibility of K 0 � K̄ 0 oscillationssuggested by Gell-Mann and Pais

I The K 0 � K̄ 0 oscillations are based on the following factsI K 0 and K̄ 0 are different particles with strangeness -1 and +1,

respectively. The strangeness is conserved in the stronginteraction

I Weak interaction does not conserve the strangeness. As aresult states of K 0 and K̄ 0 are “mixed“ states, superpositionsof states of K 0

1 and K 02 , particles with definite masses and

transitions between them in vacuum are possible

I In 1957 Pontecorvo put the following question: “...whetherthere exist other mixed neutral particles (not necessarilyelementary ones) which are not identical to correspondingantiparticles and for which particle- antiparticle transitions arenot strictly forbidden“

I He came to a conclusion such a system could be muonium(µ+ − e−) and antimuonium (µ− − e+) and wrote a paper onthat (1957).

I In this paper he mentioned neutrino. The problem wastwo-component neutrino theory established at that time.According to this theory only νL and ν̄R exist (one neutrinotype was known at that time). Transitions between them areforbidden

I Some rumor helped Pontecorvo to realize idea of neutrinooscillations in the case of one neutrino

I In 1957 R.Davis searched for 37Ar production in the processof interaction of reactor antineutrino with 37Cl . A rumorreached Pontecorvo that Davis observed such events

I He suggested that these “events“ could be due to transitionsof reactor antineutrinos into right-handed neutrinos on theway from the reactor to the detector and published a firstpaper dedicated to neutrino oscillations (1958)

I Pontecorvo considered transitions ν̄R → νR and νL → ν̄LI Thus, he had to assume that not only the lepton number is

not conserved but also in addition to ν̄R and νL, quanta ofνL(x), existed also νR and ν̄L, quanta of νR(x)

I In order to explain Davis “events“ Pontecorvo had to assumethat “a definite fraction of particles (νR ) can induce theCl − Ar reaction“

I In the 1958 paper (and this is the most important)Pontecorvo pointed out that in reactor experiments a deficitof antineutrino events will be observed “ due to the fact thatthe neutral lepton beam which at the source is capable ofinducing the reaction ν̄ + p → e+ + n changes its compositionon the way from the reactor to the detector“

I Later the anomalous events disappeared and B. Pontecorvounderstood that νR and ν̄R must be noninteracting, sterileparticles. The terminology sterile neutrino, which is standardnowadays, was introduced by B. Pontecorvo

I Starting from this first paper all his life Bruno believed inexistence of neutrino oscillations. He wrote “Effects oftransformation of neutrino into antineutrino and vice versamay be unobservable in the laboratory but will certainly occur,at least, on an astronomical scale.“

I In the next paper on neutrino oscillations, written in 1967after the second type of neutrino was discovered, Pontecorvoconsidered oscillations between active neutrinos νµ → νe andalso between active and sterile νµ → ν̄eL etc.

I He discussed in this paper the effect of neutrino oscillationsfor the solar neutrinos and came to the conclusion that “theflux of observable sun neutrinos must be two times smallerthan the expected neutrino flux.“

I In 1970 R. Davis obtained the first result of the detection ofthe solar neutrinos.

I It occurred that the detected flux of the solar neutrinos was2-3 times smaller than the predicted flux

I This created so called solar neutrino problemI Bruno Pontecorvo anticipated the solar neutrino problem

I In Pontecorvo and Gribov paper (1969) neutrino oscillationsbetween νe and νµ (no sterile neutrinos) were considered

I For the mixing they found

νeL = cos θν1L + sin θν2L, νµL = − sin θν1L + cos θν2L

I ν1 and ν2 are fields of truly neutral Majorana neutrinos withmasses m1 and m2

I νe survival probability in vacuum in such a scheme

P(νe → νe) = 1− 1

2sin2 2θ(1− cos

∆m2L

2E)

∆m2 = m22 −m2

1

I Oscillations of the solar neutrinos were considered in somedetails

I In 1975 Pontecorvo and me started a long term collaboration(about 15 years) on the study and development of the idea ofneutrino masses, mixing and oscillations

I Our first paper was based on the idea of quark-lepton analogy

I In this case for two neutrinos we have the same mixing as inthe Majorana case but ν1 and ν2 are fields of Dirac neutrinosand antineutrinos with masses m1 and m2

I There is no conserved total lepton number L in the Majoranacase. In the Dirac case L is conserved

I We published many papers

Summarizing

I There are three types of neutrino mixing

I Majorana neutrino mixing

νlL =3∑

i=1

UliνiL l = e, µ, τ, νci = νi

νi is the field of the Majorana neutrino with mass mi , U is aunitary 3× 3 mixing matrix

Transitions only between flavor neutrinos νl � νl′I Dirac neutrino mixing

νlL =3∑

i=1

UliνiL l = e, µ, τ, νci = νi

νi is the field of the Dirac neutrino with mass mi , U is a 3× 3unitary mixing matrix

Transitions only between flavor neutrinos νl � νl′

I Majorana and Dirac mixing

ναL =3+n∑i=1

UαiνiL α = e, µ, τ, s1, ...sn νci = νi

νi is the field of the Majorana neutrino with mass mi , U is aunitary (3 + n)× (3 + n) mixing matrix

Transitions between flavor neutrinos νl � νl ′ and flavor andsterile neutrinos νl � νsi

I We considered non stationary picture of neutrino oscillations

I In CC weak processes together with lepton e+, µ+, τ+ flavorneutrinos νe , νµ, ντ are produced

I Mixed states of flavor neutrinos|νl〉 =

∑i U

∗li |νi 〉

|νi 〉 is the state of neutrinos with mass mi , momentum ~p and

energy Ei =√p2 + m2

i

I If at t = 0 flavor neutrinos νl is produced

|νl〉t =∑i

U∗li e

−Ei t |νi 〉

I Flavor neutrino transition probability

P(νl → νl ′) = |∑i

Ul ′ie−∆m2

2i L

2E U∗li |2

L ' t (OPERA and other experiments)L is the distance between neutrino production and detection

pointsIf there are no sterile neutrinos, six parameters : ∆m2

23 and∆m2

12, θ12, θ23, θ13, δ

I In 1998 after many years of heroic efforts oscillations ofatmospheric neutrinos were discovered in the

Super-Kamiokande experiment

I Golden years of neutrino oscillations started

I In 2001 oscillations of solar neutrinos were discovered in theSNO experiment

I In 2002 oscillations of reactor neutrinos were discovered in theKamLAND experiment

I Many recent neutrino oscillation experiments confirm thisdiscovery

I Discovery of neutrino oscillations was a great triumph of ideasof Pontecorvo who came to idea of neutrino oscillations at a

time when common opinion favored massless neutrinos and noneutrino oscillations

From my point of view the history of the neutrino oscillationsis an illustration of an importance of analogy in physics. It isalso an illustration of the importance of new courageous ideas

which are not always in agreement with general opinion

I The situation with neutrino oscillations today

I Neutrino oscillation data are perfectly described by the threeneutrino mixing

∆m223 = (2.41±0.10)·10−3 eV2, ∆m2

12 = (7.53±0.18)·10−5 eV2

0.407 < sin2 θ23 < 0.585, tan2 θ12 = 0.436+0.029−0.023

sin2 2θ13 = 0.090± 0.009

I The CP phase δ is unknownFor absolute values of neutrino mass upper bound

mβ < 2.3 eV

I Small neutrino masses and neutrino mixing is the onlysignature in the particle physics of a NEW BEYOND THE

STANDARD MODEL PHYSICSI Future very challenging problems

I Are νi Majorana or Dirac particles? (0νββ-decay)I Character of neutrino mass spectrum (Normal or inverted?)I What is the value of the CP phase δ?I Are there transitions into sterile states? (at the moment exist

some indications)I ...

I The years of work and friendship with Bruno Pontecorvo werethe happiest and unforgettable years in my life. His wide and

profound knowledge of physics, his love of physics, hisingenious intuition and his ability to understand complicated

problems in a clear and simple way were gifts of God.Bruno Pontecorvo was a true scientist in the best, classical

sense of the word. When he thought about some problem hethought about it continuously from early morning till late

evening. He devoted all his resources and great intellect toscience, and though he was not indifferent to the recognition

of his contribution to physics, his main stimulus was search forthe truth

I More than ten last years were for Bruno Pontecorvo years ofcourageous struggle against Parkinson illness. His love to

physics and to neutrino and help of family and friends helpedhim to overcome difficult problems of the illness. He neverstopped to work, to think about neutrinos and to continueactive life. Two days before his death Bruno came to his

office at the second floor of the Laboratory of NuclearProblem in Dubna, where he worked 43 years When he wasleaving the Laboratory he looked into window at golden and

yellow birches and said to his secretary Irina Pokrovskaya:Look how beautiful are these colors... It was nice Russian

Golden Autumn, September 22d 1993.

I Bruno Pontecorvo was one of the first who understoodimportance of neutrinos for elementary particle physics and

astrophysics.He felt and understood neutrinos probably betterthan anybody else in the world. Starting from his Canadiantime he thought about neutrino the whole his life. He wasnever confined by narrow theoretical frameworks. He was

completely open-minded, without any prejudices, verycourageous and with very good intuition and scientific taste.

Bruno Pontecorvo was very bright, wise, exceptionallyinteresting and very friendly personality. People liked him and

he had many friends in Italy, Russia, France, Canada andmany other countries. The name of Bruno Pontecorvo will beforever connected with neutrino as the name of the founder

and father of modern neutrino physicsHe will remain with us in our memory and our hearts as a

great outstanding physicist, as a man of of great impact andhumanity