Current problems inneutrino physics

D.OrestanoDip. di Matematica e Fisica

Universita' degli Studi Roma Tre

Summary Lecture 1• Neutrino history

– Where does the idea of the neutrino comefrom?

– The discovery of the neutrino– Two-component theory and Goldhaber

experiment – The two neutrino experiment

decay1898 discovery ofradioactivityProblem with decay:1911 Lise Meitner e OttoHahn - electrons withcontinous spectrum areemitted. If it had been a two bodydecay of a nucleus X,transformed into anucleus Y while emettingan electron, thespectrum would havebeen monochromatic ,with E= (MX

2-MY2+me

2)/2MX

+ problems with angular

momentum conservation!?

Bohr came to propose that in this case EnergyConservation would not holdThe solution was found by Pauli with the hypothesisthat in this decay a third undetectable particle isemittedThe new particle had to be

Neutral (no ionization seen)Could not be a photon (no indication in experimentaldata of neutral electromagnetic energy deposits)Had to be an entirely new particle

W.Pauli's letter to the Tubinga congress participants on Dec. 4th 1930

1931 Fermi proposes the name neutrino for the newparticle1932 Chadwick discovers the neutron1933 the nucleus structure is defined and the way isopen for the interpretation of the decay withouthaving to assume that the emitted particles werepreviously present within the nucleaus1933 Fermi and Perrin propose to study the end-point ofthe spectrum to get information on the neutrino mass

1934 Fermi Theory: Neutrino hypotheis, + Dirac theory ()- decay can be ascribed to the transmutation of a neutron into aproton

n p e- This process can be treated as a 4-fermions pointlike interactionwithin the Dirac formalism

Inverse process

With: - G: Fermi coupling constant;

- p: momentum in the Centre-of-Mass (CM) frame

- vi & vf: relative velocities of the 2 particles;

- |M|=1 for Fermi transitions, 3 for Gamow-Teller ones

With E expressed in MeV:

A (anti-)neutrino of 1 MeV has an interaction lengthin water of few light years!

Hans Bethe: “It seems practically impossible todetect neutrinos in the free state.”

1/(σ n)=1/(12x1023σ )≈1019 cm

Ideas on neutrino detectionSimultaneously and independently

– Bruno Pontecorvo at Chalk River Laboratory,Canada, 1946

– Luis Alvarez at Laurence Berkeley RadiationLaboratory, California, 1949

Figure out a detection technique based upon the inverse decay which will be subsequently adopted by Davis insolar neutrinos detection.But where to find an intense neutrino source?

Neutrino sources: the nuclear reactors

Each fission reaction releases on average200MeV In a 3 GW thermal reactor (3x109)/(2x108x1.6x10-19)~1020 fission reactionstake then place each secondIn the thermal neutrons' induced fission of U-235two fragments with typically A=94 and A=140 areproduced. To reach the stability curve (Zr-94 and Ce-140)these nuclei need to go from 92 to 98 protons,thus 6 decays take place, leading to 6 anti-neutrinos per fission, i.e. 6x1020 per second

Convoluted with theneutrino cross-section(red), the spectrum givesthe observed distribution(blue)

Neutrinos typically carry away 1.6 MeV each, i.e.6% of the energy Besides the neutrinos produced in the fissionchain of U-235 there are others from the U-238,the Pu-239 and the Pu-241 chainsTherefore the spectrum has different components

Max around 4 MeV

The Savannah River SiteAlvarez convinced the Atomic Energy Commission toforesee an experimental hall near by the reactor (toproduce tritium for the H bomb) being constructed at aSavannah RiverHe then realized that the cosmic ray flux at the sitewould have been too high to allow measurements ofrare processesNB: He was not yet aware that the radiochemicalreaction he was foreseeing to use would have beensensitive only to neutrinos and not to reactor anti-neutrinos

Reines and CowanReines and Cowan (Los AlamosLaboratory) decided instead touse the inverse decay reactionto detect anti-neutrinosHaving abandoned the initial planof using a controlled nuclearexplosion they first installed theirexperiment by the Hanfordreactor (1953), and then moved toSavannah River (1956)Most hydrogen atoms bound inwater molecules have a singleproton for a nucleus. Thoseprotons serve as a target for theantineutrinos from a reactor

• The anti-neutrino is detected through the inverse decay reaction• The occurrence of such reaction is signaled by the production of a

positron and a neutron• The positron is the lightest particle and carries away most of the

available energy Te=Enu+mp-mn-me=Enu-1.8MeV

(there a 1.8 MeV Threshold for this process to occurr)• In the target the positron slows down and annihilates. Two

photons are emitted in opposite directions (1), each of energy me

• In the target the neutron slows down and thermalizes (timescale ~microseconds). If a neutron absorber is present the neutron can becaptured and one (of more) photon(s) (2) will be emitted

Anti-neutrino detection

The key trick allowing background reduction is the delayed coincidence of (1) and (2)

Los Alamos Science number 25, 1997 http://library.lanl.gov/cgi-bin/getfile?00326606.pdf

Los Alamos Science number 25, 1997 http://library.lanl.gov/cgi-bin/getfile?00326606.pdf

Nobel prize 1995http://www.nobelprize.org/nobel_prizes/physics/laureates/1995/reines-facts.html

3 events/h

P in weak interactions• 1957: discovery of parity violation in weak interactions• The phenomenological Hamiltonian describing weak

interactions needs to be modified to include parityviolating terms accounting for the maximal violationobserved

• Terms proportional to (1-5) and (1+5) appear in frontof the spinors describing the fermions. These arehelicity projectors which select the negative andpositive helicities normally coupled together in the 4components Dirac spinors

• For null masses the left-handed and right-handed

solutions decouple (Weyl equations)

Neutrino Helicity • Goldhaber: an ingenious experiment in which the

neutrino helicity can be correlated with that of adetectable particle, the photon

• The metastable nucleus of Europium 152 undergoes anelectron capture from shell K, emitting an excitedSamarium nucleus and a neutrino

• Eu is initially at rest, so theneutrino and the Sm* takeopposite directions

• The orbital angular momentaare all null. The angularmomentum conservationimplies two alternativeconfigurations: in both ofthem the helicities of theneutrino and that of the Sm*coincide

*Smp

p

*Sms

s

R

L

*

•Sm* decays to the ground state(with spin 0) emitting a 960 KeV ray da 960 KeV.

•The emitted has the spin directedalong the Sm*'s one.

•If the is emitted in the Sm* flightdirection it will have the samepolarization as the Sm* which thencoincides with that of the neutrino

•The problem then becomes measuring the polarization, but only for theones emitted in the direction of the Sm* recoil

•The “forward” emitted have higher energy wrt ones emitted in the Sm*rest frame (Doppler effect) they have the extra energy needed to bereabsorbed by a Sm nucleus (that would recoil), the others don't

•The selection of forward emitted isbased upon their capability ofinducing a resonant scattering on Sm

Evidence for the resonant scattering photons emitted along the Sm*direction can be selected

•To measure their polarization thephotons are diffused through magnetizediron comparing yields for the two oppositemagnetic field orientations

•The scattering cross section depends onthe photon polarization with respect to themagnetic field direction

•A value of PL=0.66 is measuredneutrinos are left-handed

1957: first ideas on neutrinooscillations

• Pontecorvo formulated the hypothesis that, similarly toneutral Kaons, neutrinos could oscillate.

• The oscillations would involve left-handed neutrinosturing into right-handed ones and vice-versa

• We will resume this hypothesis later on...

decay• 3 body decay to an electron an 2 undetected particles

• If so one would expect to have

• with

• Experimentally < 5x10-13 :

?

?

Search for the second neutrino

Experiment proposed by B.Pontecorvo in 1959Executed in 1962 at Brookhaven by Lederman,Steinberger and SchwarzProduction of a “muonic” neutrino beamDetection of neutrinos with

A large mass detectorAble to distinuish muons from electrons

ee Electron familyMuon family

μ

μ-

e

e-

Neutrino beams• First particel accelerators in the ’50s• Proton beams up to few GeV• In their interactions copius production of s, both

charged and neutrals • Charged s decay to muon plus an undetected particle

assumed to be a neutrino

this is how neutrino beams can be produced• If electron and muon lepton numbers are conserved

separately these would be beams of muon neutrinos• BNL set-up:

– Proton beam of 15 GeV on Be target– 21m of decay path– 13.5m of iron

15 GeV > K productionthreshold

The relative productionyields of π and K wasalready well known

Not only pions…

100%

64%

Caution! K3e decay at 5% small electron neutrino contamination

The detector• 10 modules of 1t each• A sandwitch of gas gaps alternate and Aluminium

plates with an applied voltage difference lastre di • Ionization from charge particles generates sparks

along the particle track• Muon tracks are straight• While electrons produce showers

Results:

Observation of the muonic neutrino!

Bibliography • M. Goldhaber, L. Grodzins, and A. W. Sunyar,

“Helicity of Neutrinos.” Phys. Rev.,109, 1015(1958).

• C. L. Cowan, Jr., F. Reines, F. B. Harrison, H.W. Kruse, and A. D. McGuire “Detection of theFree Neutrino: a Confirmation” Science 20July 1956: 103-104.

• G. Danby et al., “Observation of High EnergyNeutrino Reactions and the Existence of TwoKinds of Neutrinos.” Phys. Rev. Lett., 9, 36(1962)

• Nobel lectures 1988• Los Alamos Science: articles on neutrino at

http://la-science.lanl.gov/lascience25.shtml