Bruno Pontecorvo and neutrino physics
aa r X i v : . [ phy s i c s . h i s t - ph ] O c t Bruno Pontecorvo and neutrino physics
Ubaldo DoreDipartimento di Fisica, Universit`a di Roma “La Sapienza”,and I.N.F.N., Sezione di Roma, P. A. Moro 2, Roma, ItalyLucia ZanelloDipartimento di Fisica, Universit`a di Roma “La Sapienza”,and I.N.F.N., Sezione di Roma, P. A. Moro 2, Roma, Italy bstract In this paper the contribution of Bruno Pontecorvo inthe field of neutrino physics will be illustrated. Specialemphasis will be given to the physics of oscillations thathe was the first to propose . ontents .6 References . . . . . . . . . . . . . . . . . 36 .1 Introduction This paper is dedicated to illustrate the Bruno Pontecorvocontribution to neutrino Physics.BP contribution to neutrino physics make him one themajor contributors to the development of particle physicsin the XX century. His ideas and proposals were then thesubject of many successful experiments that gave to theirauthors Nobel Prize award. We will describe his ideas inthis paper. Many ideas about neutrino physics were putforward by BP much in advance of the times. The mostimportant were: • As soon as nuclear reactors were built he realized thefact that the produced intense flux of neutrinos wouldhave made the detection of these particles possibleat a time in which many physicists thought that thisobservation was impossible. • He suggested the use of the neutrino chlorine reactionthat was then used in the Homestake experiment thatstarted the experimental oscillation physics. • The results of the Conversi-Pancini-Piccioni experi-ment brought him to a first introduction of the con-cept of Universality in weak interactions. • Neutrino experiments at accelerators and the exis- ence of two types of neutrino was anticipated byBP. • The hypothesis of neutrino mixing and so of oscilla-tions was put forward by BP. The existence of oscil-lations and so the fact that neutrinos have mass hasbeen one of the most important results of particlephysics in the last years.The paper is organized in the following way • Brief introduction to neutrino physics • Biography of Bruno Pontecorvo • The scientific production • The big intuition ”OSCILLATIONS”.
In 1930 Pauli [1] suggested, to save energy conservationin beta decay, that a neutral light particle was emitted inthe process, he named it neutron in June 1931.In 1932 the neutron was discovered by Chadwick [2] sothe new Pauli particle was called neutrino by E. Fermi.
The smallness of cross section made the neutrinos still ahypothesis; many physicists thought that neutrino detec-tion was almost impossible.Pontecorvo showed [23] (see paragraph 0.4.1) that in factthe detection could be possible.The detection of free neutrinos was accomplished by Cowanand Reines [26] who observed the reaction ν e + p → n + e + This experiment was the first to detect free neutrinos and eines was awarded the Nobel prize in 1995. For a complete review of current status of knowledge onneutrino physics see [3]We summarize briefly our present knowledge of neutrinoproperties: • Neutrinos are chargeless fermions interacting onlythrough weak interactions. • Neutrinos interact through the exchange of W (chargedcurrents) or Z0 (neutral currents). • the V-A theory requires, in the limit of massless neu-trinos, that only left handed neutrinos be active. Theopposite is true for antineutrinos. • In the Minimum Standard Model(MSM) there are3 type of massless neutrinos (we shall see that themassless condition must be relaxed) and a corre-sponding number of antineutrinos. • Weak interactions have the same strength for thethree species, (Universality). • Neutrinos are coupled to the corresponding chargedleptons so we have 3 leptonic doublets e − ν e (cid:19) , (cid:18) µ − ν µ (cid:19) , (cid:18) τ − ν τ (cid:19) . • Leptons in each doublet carry an additive leptonicnumber L e , L µ , L τ , which has opposite sign for an-tiparticles. • Leptonic numbers are separately conserved.There are still questions that must be answered.Are neutrinos Dirac or Maiorana particles?Neutrinos have a mass but the absolute scale of this massis still unknown
Bruno Pontecorvo was born in Pisa in 1913 from a knownjewish family. He had three brothers and two sisters , onebrother was Guido(1907-1999) professor of genetics, theother was Gillo(1919-2008) the well-known film director.In 1929 he left Pisa and enrolled in the third year of thedegree in Physics at the University of Rome. After hisgraduation he became the youngest member of the FermiGroup. In 1934 he collaborated to the famous experimenton slow neutrons [11],[12].In 1936 he moved to Paris in the Joliot Curie laboratory.He was of a jewish family, so after the German invasion e had to flee to Spain and then to the United Stateswith his family: his wife Marianne Nordblon and his firstson Gil. He found a job in an oil search company. In1943 he was called to participate to the construction ofa nuclear reactor in Canada. He stayed there until 1948.In that period his sons Tito and Bruno were born.In 1948he became a British citizen and moved to England calledby J. Cockroft. In 1950 he traveled to Italy, officially onholiday. He went with his family to Stockolm and thento the Soviet Union in the Dubna laboratories.In Soviet union he was called Bruno Maximovic.From Dubna he made several trips to Italy starting in1978. Figure 2: Bruno Pontecorvo,Emilio Segre and Edoardo Amaldi in Rome 1978
The last ten years of BP life were a courageus struggle gainst Parkinson disease. He never stopped to work onphysics and oscillations Figure 3: Dubna 1988 Neutrino oscillations
He remained in the Soviet Union until his death in 1993.Now his tombstone is in the Cimitero Acattolico in Rome.the tomstone is shown in figure 4For the reasons that made him decide to go to the SovietUnion one can quote the words of V.P. Dzhelepov [22], adistinguished russian physicist:
Bruno was an Italian communist, at the time of ar-rival in the Soviet Union he was a communist-idealistsincerely believing in the strength and rightness ofthe type of development chosen by Russia.
It must be noted that there was a malevolent interpreta-tion of his arrival in USSR :he was a soviet spy and hefled before being unmasked.the Impact of BP on neutrino physics is well recognizedin the Scientific world.we will give three examplesValentine Telegdi quoted by L.Okun [ ? ] Almost all important idea in neutrino physics are dueto Pontecorvo
Jack Steinberger [ ? ] ew of us in particle physics can boast of a singleoriginal and important idea. Bruno wealth of seminalsuggestions establish him as a truly unique contribu-tor to the the advance of particle physics in the pasthalf century Nicola Cabibbo [ ? ] Con Pontecorvo scompare uno dei grandi scienziatidel XX secolo uno dei pionieri della fisica delle alteenergie.
BP started his scientific activity in Rome in the groupof E. Fermi, participating to the famous experiments onthe slowing down of neutrons in hydrogenous materials[11],[12]From 1936 to 1938 he worked in Paris with F. Joliot Curieon nuclear isomerism [13]From 1938 to 1940 he worked in the ’Well survey inc’,Tulsa USA where he developed a system [14] for findingoil or water underground. The system used a neutronsource and was the first application of the Rome results onthe slowing down of neutrons. In the following, activitiesin Canada and Dubna will be presented. .4.1 Activities in Canada BP lived in Canada from 1943 to 1950. At the begin-ning he stayed in Montreal where he worked on the NRXproject, a heavy water, natural uranium reactor. Heworked on the design of the reactor that started in 1947yielding a flux of 6.10 ν e /cm secThe reactor was then built near the Chalk river , and helived in the nearby Deep River town. We want to recallhis contributions on the following arguments • muon decay • universality • neutrino detectors • proportionals countersmuon decayIn 1950 he published, together with E.P.Hincks [15], theresult of an experiment on the decay of the penetratingcosmic ray particle, the muon, that was known to havea 2.2 µ s lifetime, while decay products were not known.The only thing known was that one charged particle wasemitted but there was no information on its nature andon the nature of the emitted neutral particle(s). In the ex-periment, muons were stopped in an absorber and were etected by an arrangement of Geiger-Muller counters.The decay products were detected with a delayed coinci-dence technique. The results of the experiment were • the charged particle emitted is an electron • the average energy of the electron is greater than 25Mev • there is no evidence of emission of electrons with en-ergy larger than 50 MeV. • the shape of the energy spectrum of the electron ex-cludes the possibility of being a single lineThe conclusion was The average energy and the form of the energy spec-trum of decay electrons are, within the accuracy oftheory and experiments, in agreement with theoreti-cal expectations of a process µ → e + ν + ν UniversalityThe similarity of e-N and µ -N interactions later knownas Universality was for the first time pointed out by BP.In 1946 a paper of Conversi, Pancini and Piccioni [30]was published on the results of the behaviour of the neg-ative and positive particles of the hard component of cos-mic rays. This experiment showed that negative particles here not captured when stopped in light elements as wasexpected if the negative particle had been the Yukava par-ticle. Lattes, Occhialini and Powell [31] in 1947 in factshowed that cosmic ray muons were the decay productsof a particle now known as π . Thinking about the Con-versi experiment, BP made some relevant considerationsas shown in the letter he wrote to Giancarlo Wick in 1947.letter to GC Wick [16] Deep River 8 maggio 1947,Caro Giancarlo ... se ne deduce una similarita’ traprocessi beta e processi di assorbimento ed emissionedi mesoni, che, assumendo non si tratti di una coin-cidenza, sembra di carattere fondamentale.
English translation
It can be deduced a similarity between beta processesand processes of absorpion or emission of mesons,that, assuming that it is not coincidence, seems tobe of fundamental character
He then published his considerations [17]
We notice that the probability (10 sec − ) of cap-ture of a bound negative meson is of the order ofthe probability of ordinary K-capture processes, whenallowance is made for the difference in the disinte-gration energy and the difference in the volumes ofthe K-shell and of the meson orbit. We assume that his is significant and wish to discuss the possibil-ity of a fundamental analogy between beta-processesand processes of emission and absorption of chargedmesons This paper introducing the concept of Universality hadnot a large echo. In fact after 2 years there was a paper ofG.Puppi that introduced the concept of a new fundamen-tal force, the ’Universal weak interaction’, [18] in whichthere was no reference to the Pontecorvo ideas. The samehappened in the papers of other authors all concerningthe universality of the weak interactions [19], [20],[21]Neutrino detectorsIn 1946 in his paper ’inverse beta processes ’ [23] BP dis-cussed the possibility of detection of neutrinos. Althoughthe detection of the inverse beta decay ν + Z → ( Z −
1) + e + was at that time considered not possible. due the calcu-lated cross section 10 − cm given by Bethe and Peierls[25], the use of powerful neutrino sources as the nuclearreactors could make the detection of the above processpossible. He wrote It is true that the actual β transition is certainly notdetectable in practice. However the nucleus of charge(Z ±
1) produced in the reaction may be radioactive ith a decay period well known. The essential pointin this method is that the radioactive atoms producedmust have different chemical properties of the irra-diated atoms. Consequently it may be possible toconcentrate the radioactive atoms from a very largevolume The principal requirements for the irradiated materialhad to be • the irradiated material must be not too expensive • the produced nucleus must be radioactive with a pe-riod of at least one day, because of the long timeinvolved in the separation • The separation of radioactive atoms must be rela-tively simple • the background must be as small as possibleBP suggested the reaction ν + C l → Ar + e As a source of neutrinos he suggesteda) neutrino flux from the sunb) neutrinos from recently developed nuclear reactorsIt must be noted that the sun produces neutrinos, while inreactors antineutrinos are produced, at that the differencebetween neutrinos and antineutrinos was not yet clear. he above process was used in the solar neutrino Homes-take Davis experiment that started in 1962. The neutrinodeficit compared with the prediction of the standard solarmodel gave origin to the ”neutrino puzzle’. The final re-sult of the experiment, that lasted several decades, werepublished in 1998 [24]. In this paper the Pontecorvo sug-gestion was recognized.In 1946 the detection of the inverse beta decay process ν + p → n + e with neutrinos from nuclear plants was not feasible.The development of the liquid scintillator technique tenyears later allowed the detection of the above process.[26].The Reines experiment was the first experimental proofof the existence of the neutrino.proportional countersBP together with Hanna and Kirkwood developed a newtechnique of proportional counters, based on very largeamplification in the gas. Sensitivity to a few ion cou-ples was reached [27]. This development was essential inthe solar neutrino Davis experiment and later in the He proportional counters of the SNO experiment.The new technique was used in experiments of low energyspectrometry and used in the measurement of the tritium β spectrum. n the experiment, published in 1949, a first measure-ment of the neutrino mass was obtained [28].The obtainedvalue was m ν ≤ eV In 1950, BP started his activity in the Dubna laborato-ries (JINR). His notes and reports were in Russian. TheEnglish translation of part of this material can be foundin Selected Scientific Works recollection on Bruno Pon-tecorvo [29].He participated in experiments at the Dubna and Ser-pukov accelerators.Out of the experiments at the Dubna Synchrocyclotronwe can recall • muon capture [32] • measurement of the pion interactions[33] • search for the production of Λ in proton interactionsat 700 MeV [35]. • search for anomalous scattering of muon neutrinosby nucleons [34]Results of experiments at the Serpukhov accelerator canbe found in [36],[37]. here are several arguments that he considered. We willgive a brief resume. (One of the more important contri-bution of Pontecorvo, Oscillations, will be considered innext section 0.5)Neutrino beamsIn 1959 BP started to think to the possibility of per-forming experiments with neutrino produced at accelera-tors. The first problem to be solved was the possibility ofthe existence of two types of neutrinos, namely neutrinosfrom beta decay and neutrinos from pion and muon de-cay.He discussed the problem in [38]. One year later M.Schwarz discussed the same problem [39]. Schwarz thenpartecipated in an experiment at the AGS in BrookhavenUSA together with L. Lederman and J. Steinberger. Theyproved that there are two type of neutrinos [40] named ν e and ν µ .For this result the three authors were awarded theNobel prize in 1988. BP also proposed a non conventionaltechnique to produce neutrino beams.Conventional high energy neutrino beams are producedby pion and K decays produced in proton interactions .Their flavor content is: neutrinos ,(antineutrinos) com-ing from the decay of ( π, K ) + and ( π, K ) − . These beamshave a small ν e contribution coming from the three bodydecay of K mesons. Bruno’s innovative idea was the”beam dump” [42], a technique proposed to search for hort lived particles that decay before interacting. Pro-tons are made to interact in heavy materials. Pion andKaons interact before decaying, so the only producedneutrinos come from the decay of short lifetime parti-cles (charms). In this case the contribution of ν e and ν µ are comparable.Neutrino and astrophysicsPontecorvo published several papers on this argument.Many of these are given in the following table n year title refer1 1959 Universal Fermi interactions and astrophysics [44]2 1961 Neutrino and density of matter in the Universe [45]3 1963 Neutrino and its role in Astrophysics [46]4 1969 Neutrino Astronomy and lepton charge [47] All these papers can be found translated in English in ref[29]
The contribution of Bruno Pontecorvo to the field of neu-trino oscillation was fundamental and he defended theconcept of oscillations in years in which the neutrino wereconsidered massless and so oscillation impossible. In thissection we will present Basics of neutrino oscillation theory • Experimental results • The Bruno Pontecorvo contribution
As in the quark sector the weak interactions states are alinear superposition of the mass eigenstates. These statesare connected by an unitary matrix U ν α = X j U αj · ν j with index α running over the three flavor eigenstatesand index j running over the three mass eigenstates.The matrix U is called the Pontecorvo–Maki–Nakagawa–Sakata. In the general case, a 3 × θ = θ , θ = θ , θ = θ and a CP violating phase δ . A frequently usedparametrization of the U matrix is the following U = c s − s c c s e − iδ − s e + iδ c c s − s c
00 0 1 where c jk = cos( θ jk ) and s jk = sin( θ jk ). iven three neutrino masses, we can define two indepen-dent square mass differences ∆ M and ∆ M . ∆ M = M − M , ∆ M = M − M It has experimentally been shown that | ∆ M | ≪ | ∆ M | and so ∆ M ≃ ∆ M .The mass spectrum is formed by a closely spaced doublet ν and ν , and by a third state ν relatively distant.In many cases oscillations can be studied considering thesimplified approximation of two family mixing. With twomass states M and M , the mixing matrix is reducedto 2 × θ (omitting irrelevant phase factors): (cid:18) cos θ sin θ − sin θ cos θ (cid:19) Consider for example a ν e beam and ν e → ν µ oscillationsAt a distance L from from the source the probability ofdetecting a ν e as ν µ , is P ( ν e → ν µ ) = sin (2 θ ) sin (∆ M L/ E )where ∆ M = M − M . Choosing to express ∆ M in eV , L in m(Km) E in MeV(Gev) we have P ( ν e → ν µ ) = sin (2 θ ) sin (1 . M L/E ) e emphasize that oscillations are sensitive only to thedifference of the square masses. Radiochemical experimentsThe experimental story of the oscillations started with thesolar neutrino ’puzzle’. The flux of neutrinos coming fromthe sun, as detected in the Homestake neutrino detector,was below the expected one. Nuclear reactions in the Sungiving rise to neutrinos start with the PP reaction p + p → H + e + + ν e neutrinos produced in this reaction constitute 99% of allneutrinos emitted by the sun . The energy spectrumof neutrinos has an end point at 0.42 MeV.Additionalneutrinos are emitted in the chain process initiated bythr PP reaction H + p → H e + γH e + H e → Be + γBe + e − → Li + ν e Be + p → B + γ ( Be ) ∗ → H e B → ( Be ) ∗ + e + + ν e The net result of the chain is4 p + 2 e − → H e + 2 ν e + γ. he Q of the reaction is 26 MeV and the neutrinos takeaway on the average 0.5 MeV.In the Davis experiment solar netrinos coming from thesun were detected via the reaction ν e + C l → Ar + e as suggested by BP.The Davis detector was a large tank containing 100000gallons of tetrachloroetilene. It was located in the Home-stake mine at a depth of 4800 meters.Using physical and chemical methods the amount of pro-duced Ar was extracted from the target material. The Ar is unstable, so the extraction had to be performedperiodically Auger electrons or photons emitted in thedecay were detected in proportional counters.The experiment ran from 1970 to 1995 and the resultswere compared with the model of neutrino emission fromthe sun the Standard Solar Model (SSM) The main con-tributor to the computation of SSM model and of hisresults was J.Bahcall [55]. The final result of the experi-ment was Φ( Davis ) / Φ( SSM ) = 0 . ± . he disagreement between SSM and results, the ’solarneutrino puzzle’ was largely discussed in the physics com-munity and many explanations were given, but only BPindicated oscillations as the origin of the discrepancy.Following the Davis results other experiments were done.The Gran Sasso Laboratories (Italy)experiment Gallex[59] and then GNO [60]and Baksan(Russia)[58] radio-chemical experiments studied the process ν e + Ga → Ge + e − The threshold of this reaction is 0.223 Mev so it is sen-sitive to the PP reaction that has an energy endpoint at0.42 Mev while the Chlorine experiment had a thresholdof 0.813 Mev and therefore was not sensitive to the PPreaction that is the origin of the the large majority ofsolar neutrinos.The weighted average of all Gallium results is [57]
C apture Rate = 67 . ± . SN U to be compared with the prediction of 128 of the SSM (1SNU(standard neutrino unit)=10 − neutrino captures/(atomsec)).Real time experimentsIn the Kamioka mine in Japan the experiment Kamiokanderan from 1983 until 1988 followed by the Superkamiokandeone, that started in 1996, and is still running. oth experiments were large water Cherenkov counters(Kamiokande 3 ktons, Superkamiokande 50 ktons)in which the Cherenkov light emitted by fast particles iscollected by photomultipliers.Solar neutrinos were detected via the reaction ν x + e → ν x + e all neutrinos contribute to the reaction but the main con-tribution is given by ν e because in this case we can haveboth charged current (CC) interaction and neutral cur-rent(NC) ones, while in the case of ν µ and ν τ only neutralcurrent reactions are allowed. The ratio is CC/NC= ∼ . ± . stat. ) ± . sys ) . cm − sec − ratio Φ( ν x ) / Φ( SSM ) = 0 . ± . ± . . ± . stat. ) ± . sys ) . cm − sec − ratio Φ( ν x ) / Φ( SSM ) = 0 . ± . ± . onfirmation of the oscillation hypothesisThe results on the ratio given above rely on the correct-ness of the SSM and so the interpretation of the value ofthe ratio as due to oscillations needs confirmation.The confirmation came in the year 2003 ’annus mirabilis’from two experiments, SNO and Kamland.The SNO experiment [64], Sudbury Neutrino observa-tory, was a 1000 tons heavy water Cherenkov detector.In Deuterium the following three reactions were observed:1) ν e + d → p + p + e − charged current interaction ac-cessible only to ν e ν x + d → p + n + ν x neutral current interaction accessibleto all neutrinos. This is an unique feature of the SNOexperiment that allows a direct verification of the SSM.3) ν x + e → ν x + e accessible to ν e and, with smallercross section, to ν µ and ν τ .Reactions 1 and 3 were observed via the detection of theCherenkov light emitted by electrons, while reaction 2was detected via the observation of the neutron in the fi-nal state.Various neutron detection techniques were usedin successive parts of the experiment. The results weresummarized in the following way R ee = Φ( C C ) / Φ( N C ) = Φ( ν e / Φ( total ) = 0 . ± . +0 . − . The ν e flux is depressed, in agreement with the results of ll solar experimentsΦ( ν x ) / Φ( SSM ) = 1 ± . ν e are depressed this means that neutrinos intheir path from Sun to Earth have changed their flavor.The Kamland experimentThe detector is located in the Kamioka mine in Japan.It consists of 1 kton scintillator contained in a balloonviewed by photomultipliers. 53 nuclear reactors surroundKamland at an average distance of 150 km. Emitted antineutrinos interact in the hydrogen of the scintillator ν µ + p → n + e + The reaction products are detected as a pulse delayedpair, the first pulse being due to the annihilation of thepositron, the second to delayed gammas emitted in thecapture of the moderated neutron. This is the same tech-nique used in the first detection of free neutrinos andin all ν e reactor neutrinos detection experiments. Thesurvival probability, ie the probability that antineutrinosoriginated in the reactors reach the detectors has beencomputed to be0 . ± . stat ± . syst nterpreting this result in terms of oscillations one obtainsthe same parameters that have been found in the analysisof solar neutrinos.A global two flavor analysis of Kamland data and solardata [65] gives∆ M = (7 . +0 . − . ) × − eV tan θ = 0 . +0 . − . . This is another proof of the interpretation of solar neu-trino deficit in terms of oscillation.After the results of these two experiments, neutrino os-cillations from a possible theory become a well definedphysical phenomenon.
While the observation of solar neutrinos concerns the dis-appearance of ν e and so the (1,2) mixing parameters, in-formation on the (2,3) mixing comes from the study ofatmospheric ν µ disappearance experiments. Neutrinosare generated by the decay of hadrons produced by pri-mary cosmic rays in the upper part of the atmosphere. ν µ are produced by the decay of pions and kaons while the ν e are produced together with ν µ by the decay of muons.Many underground experiments have been made, the one hat has given a definite proof of disappearance of ν µ is the Superkamiokande experiment that used the samedetector used for solar neutrinos. While the ν e behaveaccording to Montecarlo computations a clear deficit ofupward ν µ ie from ν µ that have traversed the Earth wasobserved . The experiment studied the double ratio R=( µ/e ) data / ( µ/e ) M C that should be 1 in the absence ofoscillations and that turned out to be [66] R = 0 . ± . ± . ν µ deficit have been obtainedalso in other underground detectors [67] and [68]A direct proof of disappearance of muon neutrinos hasbeen obtained in two long baseline experiments K2K [69]and Minos [70]These experiments utilize a two detector scheme. Thedistance between the two detectors and the energy ofthe beams are choosen to access the ∆ M region givenby the SK result. The results of the three experiments,either with atmospheric or with accelerators, given in thefollowing table, are in good agreement and this fact isagain a proof of the interpretation of results in termsof oscillations. All these experiments are disappearanceexperiments , the possibility of ν µ → ν e is excluded bythe SuperK result in which ν e are in good agreement xperiment ∆ M · − eV sin θ ATMO SK [66] 1.5-3.4 ≥ ≥ ± ≥ with the expectations and by the result of the Choozreactor experiment so that the only possibility left is the ν µ → ν τ but no direct evidence of this reaction has beenuntil now given. The Opera experiment [71] from CERNto LGNS will look for the appearence of τ produced by ν τ interactions. The difficulty, given the short lifetime of τ ,of detecting ν τ will be addressed using the high granulariyof nuclear emulsions. As it has been shown in the section 0.4.1 Pontecorvo sug-gested the possibility of measuring the reaction ν + C l → Ar + e and as source of neutrinos he suggested solar neutrinos.The solar neutrino Davis experiment started the ’neutrinopuzzle’ and the originality of the BP contribution waslargely acknowledged.In 1957 R.Davis was doing the same experiment at theSavannah river reactor using ν e . a rumor reached B.P in ubna, that the process had been observed. The rumourwas false and Davis obtained a null result thus showinga difference between neutrinos and antineutrinos.The process ν e + C l → Ar + e does not conserve leptonic number, so he started to thinkto processes that violate leptonic number and that thereason that the reaction did happen was the transition ν e → ν e in vacuum. He published two fundamental pa-pers in 1957:1)’Mesonium and anti-mesonium’, The conclusion of thepaper was [48] .. if the conservation of neutrino chargetook no place the neutrino antineutrino transitionwould be in principle possible .2)’Inverse beta processes and non conservation of leptoncharge’ [49] In the hypothesis of non conservation of neutrino chargea beam of neutral leptons from a reactor which atfirst consists mainly of antineutrino will change itscomposition at a certain distance from the reactor
We note that this effect has been observed in the Kam-land experiment.At that time only one type of neutrinos was known. Afterthe discovery of two types of neutrinos BP extended thisconcept to flavour oscillations [50]. n 1967 BP published a paper ’neutrino experiments andthe question of lepton charge conservation’ [41] in whichhe, given the evidence for ν and ν µ diffference, gave ex-amples of processes that could test the leptonic chargeconservation.BP was very interested on the oscillation problem, manytimes in collaboration with S. Bilenky. He published sev-eral papers on solar neutrinos [52],[51] [53],[54]. In thelast paper he explained the result of the Davis experi-ment in terms of oscillations.He wrote: It appears that the explanation in terms of neutrinomixing is much more attractive and natural than otherexplanations
As shown in the chapter on the neutrino oscillation the-ory the mixing matrix U contains 3 angles and possibly aCP violating term δ .The angles θ and θ are reason-ably well known while for θ only upper limits are given,the more stringent has been given by the Chooz Exper-iment [72]; the phase δ is unknown. The determinationof θ is very important because a not too small value ofthis quantity will open the possibility of the phase δ ofCP violation in the neutrino field to be measured .Exper- ments have been proposed and are in preparation , T2K[74] will look in a neutrino beam from Jaeri Japan to Su-perkamiokande through the detection of the subdominat ν µ → ν e reaction. The reactor experiment DayaBay [75]will try to improve the limit on ν e → ν x of Chooz by afactor 10 ppendix Basics of neutrino oscillation theoryAs in the quark sector the weak interactions states are a linearsuperposition of the mass eigenstates. These states are con-nected by an unitary matrix U ν α = X j U αj · ν j with index α running over the three flavor eigenstates and indexj running over the three mass eigenstates.The matrix U is called the Pontecorvo–Maki–Nakagawa–Sakata.In the general case, a 3 × θ = θ , θ = θ , θ = θ and a CP violatingphase δ . A frequently used parametrization of the U matrix isthe following U = c s − s c c s e − iδ − s e + iδ c c s − s c
00 0 1 where c jk = cos( θ jk ) and s jk = sin( θ jk ).Given three neutrino masses, we can define two independentsquare mass differences ∆ M and ∆ M . ∆ M = M − M ,∆ M = M − M It has experimentally been shown that | ∆ M | ≪ | ∆ M | andso ∆ M ≃ ∆ M .The mass spectrum is formed by a closely spaced doublet ν and ν , and by a third state ν relatively distant.In many cases oscillations can be studied considering the simpli-fied approximation of two family mixing. With two mass states M and M , the mixing matrix is reduced to 2 × cterized by a single parameter,the mixing angle θ (omittingirrelevant phase factors): (cid:18) cos θ sin θ − sin θ cos θ (cid:19) Consider for example a ν e beam and ν e → ν µ oscillationsAt a distance L from from the source the probability of detectinga ν e as ν µ , is P ( ν e → ν µ ) = sin (2 θ ) sin (∆ M L/ E )where ∆ M = M − M . Choosing to express ∆ M in eV , Lin m(Km) E in MeV(Gev) we have P ( ν e → ν µ ) = sin (2 θ ) sin (1 . M L/E )we emphasize that oscillations are sensitive only to the differenceof the square masses. ibliography [1] W.Pauli, On the earlier and more recent story of neutrino.The English translation of the original paper can be foundin Neutrino Physics edited by K. Winter pg 1[2] J. Chadwick, Possible existence of the neutron. Nature
Physics Letters β decay and Majorana neutrinos. TwelfthInternational workshop on neutrino telescopes
Mod. Phys. Lett.
A16,2409,2001.[6] G.W Rodeback and J.S.Allen, Neutrino recoil following thecapture of orbital electrons in A . Phys. Rev.
Phys. Rev.
Phys. Rev.
9] F. Reines and C.L.Cowan, Detection of free antineutrino
Phys. Rev.
Europhys J. C
Ricerca scientifica
Ricerca scientifica
Nature
Oil and gas jour-nal µ sec meson, Phys. Rev.
Phys. Rev
Nuovo Ci-mento
Nature
Phys. Rev
21] J.Tiomno and J.A. Wheeler, Energy spectrum of electronsfrom muon decay.
Rev. Mod. Phys
Chalk River Report ,Pd-205,1946[24] B.T. Cleveland et al, Measurement of the solar neutrinoflux with the Homestake neutrino detector
Astrophysicajournal
Nature
Phys Rev
Phys Rev Phys Rev
Societa Italiana difisica
Bologna 1997[30] M.Conversi,E.Pancini and O.Piccioni, On the disintegrationof negative mesons,
Phys Rev.
Na-ture
32] O.A. Zaimidoroga et al, Measurement of the total muoncapture rate in He-3
Phys.Lett
Nuovo Cim.A
NuovoCim.A
Phys.Lett
Z theor expphysics
Yad.Fiz
Yad.Fiz
Z theor expphysics
Phys Rev lett
Phys Rev lett
41] B. Pontecorvo, Neutrino experiments and the questionof leptonic charge conservation
Z theor exp physics
Z theor exp physics
Lettere Nuovo Cimento
Z theor exp physics
Z theor exp physics
USp.Fiz,Nauk phys lett.
Sov. Phys.JETP , 6:429,(Zh.Eksp.Teor,33,549,1957), 1957.[49] B. Pontecorvo, Inverse beta processes and nonconser-vation of lepton charge.
Sov. Phys. JETP , 7:172–02,(Zh.Eksp.Teor.fiz,34,247,1957), 1958.[50] S. Bilenky, B. Pontecorvo, Lepton mixingand neutrino oscillations phys rep
51] S.M. Bilenky, B. Pontecorvo, Lepton Mixing and the solarneutrino puzzle
Comments Nucl part Phys
Lettere Nuovo Ci-mento
Sov.J.Nucl.Phys.
Dubna Report E
Nuclearphysics B (proc suppl)
Science electronic preprint nucl-ex/0703012 , page 8, 2007.[58] W.J.N. Abdurashitov et al, Measurement of neutrino cap-ture rate in metallic gallium.
Phys. Rev. C , 60:0055801–3[59] W. Hampel et al, Gallex solar neutrino observation,
Phys.Lett. B , 447,127, 1999.[60] W. Altmann et al, Complete results of five years of GNOsolar neutrino observations.
Phys. Lett. B , 616,174,2005.[61] S. Fukuda et al, Solar neutrino data covering solar cycle 22
Phys.Rev Lett
77 ,1683,1996
62] J. Hosaka et al, Solar neutrino measurements insuperkamiokande-I
Phys.Rev.D
Phys.Rev. D
Phys. Rev. C
Phys. Rev. Lett.
Phys. Rev. Lett
Euro Phys. Journal , 36:323, 2004.[68] M. Sanchez et al, Measurement of l/e distribution insoudan2 and their interpretation as neutrino oscillations.
Phys. Rev. D
Phys. Rev D. , 74,072003, 2006.[70] A. Blake et al, neutrino oscillation results from Minos.
J.Phys.Conf.Ser .
CERN-SPSC-2000-028 , 2000.
72] M. Apollonio et al, Search for neutrino oscillations on a longbase-line at the Chooz nuclear power station.
Eur. Phys. J. , C27,331,2003[73] F.Ardellier, Double Chooz, a search for neutrino mixingangle theta-13. electronic preprint hep-ex/0606025 , page173, 2006.[74] K. Nishikawa, Status of j-park facility and the T2K ex-periment.
Twelfth International workshop on neutrino tele-scopes , pages 197–4, 2007.[75] Xineng Guo et al, A precise measurement of the neutrinomixing angle theta(13) using reactor antineutrinos at dayabay. electronic preprint hep-ex/0701029 , page 162, 2007., page 162, 2007.