1. Revelations of the neutrino:
Weak interaction (beta decay,
double beta
decay)
Sebastian Liebschner

2. Outline Beta decay – experimental results Neutrino hypothesis
Detection
Properties of neutrinos
Weak interaction
Double beta decay

3. 1. Beta decay – experimental results
- radioactive decay - nucleus emits electron or positron (β- or β+ particle) - mass of nucleus nearly constant  nucleon reaction: - β- decay: - (β+ decay: )
1/23

4. 1. Beta decay – experimental results
- beta decay was observed closer - cloud chambers showed curious results - - anticipated: - but both objects in the same half room disagreement with conservation of momentum
2/23

5. 1. Beta decay – experimental results
- measurement of electron/positron energy provided next unexpected result  - instead of discrete  continuous spectrum
3/23

6. 1. Beta decay – experimental results
- investigation of spin  example:
nucleushalf-integer spin  nucleushalf-integer spin + electronhalf-integer spin
 disagreement with conservation of angular momentum
4/23

7. 2. Neutrino hypothesis
- instead of giving up the conservations of energy, momentum and angular momentum, Wolfgang Pauli theorized a new particle, called neutrino (1930)
Wolfgang Pauli ( )
5/23

8. 2. Neutrino hypothesis
- according to the exp. results, the neutrino has:
○ half-integer spin
○ no electric charge
○ very small mass( next week)
- new equation: β- decay:
β+ decay:
- determination of and necessary, because of conservation of lepton number (lepton L=1, antilepton L=-1)
6/23

9. 2. Neutrino hypothesis with quark model:  cut down to quark reaction:
Charge of quarks:
 cut down to quark reaction:
β- decay:
β+ decay:
7/23

10. 3. Detection - Detection of particles:
○ proton, electron: electromagnetic interaction
○ neutron: collison with protons
○ photon: photoeffect, comptoneffect
- neutrinos don‘t interact
with strong or electro-
magnetic force
nearly go through everything (like a bullet through fog) Pb
8/23

11. 3. Detection
-Fermi calculated cross section from neutrinos with matter: (neutrinos with 10 MeV) (for neutrons with same energie: ) -Bethe: „Nobody can ever detect this particle.“
Enrico Fermi ( )
Hans Bethe ( )
9/23

12. 3.Detection
Project Poltergeist
10/23

13. 3.Detection β- decay: β+ decay: inverse β- decay: inverse β+ decay:
- used in the inverse
β+ decay to create neutron
and positron
 "trigger“ for reaction
11/23

14. 3.Detection -measurement: ○ first E(e+e)=1,02MeV, ○ later E(n)=9,1MeV
12/23

15. 3.Detection 200l reservoir The neutrino detector „Herr Auge“: 90
photomultiplier
13/23

16. 4.Properties
- 3 families/flavours: ○ electron neutrinos νe ○ muon neutrino νμ
○ tau neutrino ντ
- the lepton family number is conserved in
reactions
- Neutrino oscillation is the theorized transformation
of neutrinos in another flavour
conflict with conservation
of lepton family number
14/23

17. 4.Properties - transformation is periodic  oscillation
theory: if oscillation  neutrinos nonzero mass
- neutrino oscillations
observed from
many sources with
different detector
technologies
(e.g. Kamioka, Japan)
 nonzero mass
15/23

18. 5. Weak interaction
- radioactivity at all is effected by a „new force“: the weak interaction - one of the four fundamental forces of nature - originally formulated, in the 1930s, by Fermi - weak force is described with gauge bosons: W+,W- (charged) and Z0 (uncharged) - ratio of the power of all four forces:
16/23

19. 5. Weak interaction
- three types of weak interaction: ○ elastic scattering: only energy and momentum exchange, e. g. ○ charged current: particles couple via W+,W- particle- transformation, e. g. pion decay
final state
initial state
17/23

20. 5. Weak interaction - beta-decay: ○ first reaction: ○ second reaction:
18/23

21. 5. Weak interaction ○ neutral current: particles couple via Z0 and
there is a particle-transformation, e. g.
- process also possible with γ-quant
- in nature there is overlap
of weak and electromagnetic
force
19/23

22. 6. Double β-decay
- double-beta decay (ββ-decay) allowed, if the final state of a nucleus has a larger binding energy than before, e. g. - Germanium-76: ○ has smaller binding enery than , preventing ββ-decay ○ has a larger binding energy  ββ-decay allowed - in general are nuclei with even proton-number and even neutron-number able for ββ-decay
20/23

23. 6. Double β-decay
21/23

24. 6. Double β-decay
- ββ-decay is very rare - two neutrino double-beta decay (2νββ-decay) ○ two β-decays at the same time ○ process is allowed within the standard model (double β+ decay is also possible)
22/23

25. 6. Double β-decay - neutrinoless double-beta decay (0νββ-decay)
○ neutrinos annihilate each other
○ neutrions are there own
anti-particles
(Majorana-fermion)
○ according to theory: at least one
neutrino has to have a nonzero mass
 physics beyond the standard model
if this decay can be detected
23/23

26. 7. References and acknowledgements
- Demtröder: Experimentalphysik 4 - Prof. Dr. K. Zuber - Povh, Rith, Scholz, Zetsche: Teilchen und Kerne - KEK News: Neutrino oscillation experiment - Carsten Hof: Neutrino-Seminar, RWTH Aachen