brief history of neutrino

brief history of neutrino

Brief History of the Neutrino

1) 1896: Henri Becquerel discovers natural radioactivity while studying phosphorescent properties of uranium salts.

É a rays: easy to absorb, hard to bend, positive charged, mono-energetic;

É b ray: harder to absorb, easy to bend, negative charge, spectrum?;

É g rays: no charge, very hard to absorb. 2) 1897: J.J Thompson discovers

the electron

3)1911: Lise Meitner and Otto Hahn first shown that the energies of electrons mitted by beta decay had

a continuous rather than discrete spectrum

4) 1914: Chadwick presented definitive

evidence for a continuous beta-ray

spectrum

F A Scott, Phys Rev 48, 391 (1935)

 spectra were

continuous

Instead of discrete

Origin continuous b-ray spectrum was unknown

Different options include several different energy loss mechanisms

It took 15 years more to decide that the “real” beta- ray spectrum was really continuous Reason for

continuous spectrum was a total mystery:

+ QM: Spectra are discrete

+ Energy-momentum conservation: N → N’ + e

-electron energy and momentum well defined

Nuclear physics before 1930: nucleus = nP P + ne

e-É Example: 4 He = 4P +2e - work well.

É However: 14 N = 14P +7e - is expected to be a fermion, but it was experimentally known that is a boson!

É There was also a problem with the magnetic moment of

nuclei: mN , mP á me (m=eh/4mc) How can the nuclear

magnetic moment be so much smaller than the electron one

if the nucleus contains electrons?

SOLUTION: bound, nuclear electrons are very weird!

+ This can also be used to solve the continuous

b-ray spectrum: energy need not to be

conserved in nuclear processes! (N Bohr)

“…This would mean that the idea of energy and its conservation fails in dealing with processes

involving the emission and capture of nuclear

electrons This does not sound improbable if we

remember all that it has been said about

peculiar properties of electrons in the nucleus.”

(G Gamow, 1931)

Enter the neutrino…

Weakly interacting massless neutral fermion

particle with small mass and no electric charge in order that:

+ resolve the problem of continuous beta-ray spectra,

+ reconcile nuclear model with spin-statistics theorem.

Chadwick’s neutron if different from Pauli’s neutron = neutrino (Fermi).

Adapted summary

of an English

translation to

Pauli’s letter dated December 4, 1930

observing the unobservable…

1) 1956: “Discovery” of the electron neutrino (Reines and Cowan) in the Savannah River Nuclear Reactor site

2) 1962, the second neutrino: nm ∫ ne

(Lederman, Steinberger, Schwarts at

Brookhaven National Laboratory-BNL

How to see them ?

observing atmospheric neutrinos

300 events/yr

Need huge detector

3) 2001: nt directly

observed (DONUT

experiment at

FERMILAB: