Revelations of the neutrino:

Weak interaction (beta decay, double beta decay)Sebastian Liebschner

15.11.2012

Outline1.Beta decay – experimental

results2.Neutrino hypothesis3.Detection4.Properties of neutrinos5.Weak interaction6.Double beta decay

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: )

epn 01

11

10

enp 01

10

11

1/23

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

0163

62 LiHe

eLi pp

2/23

1. Beta decay – experimental results- measurement of electron/positron energy

provided next unexpected result

- instead of discrete  continuous spectrum

m

pEkin

2

2

02

2121 )( EcmmMEE kinkin

021

21 E

mm

mEkin

3/23

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

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 (1900-1958)

5/23

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)

eYX AZ

AZ

011

eYX AZ

AZ

011

6/23

2. Neutrino hypothesis- with quark model:

cut down to quark reaction: β- decay: β+ decay:

eud

edu

Charge of quarks:

eQu 3

2

eQd 3

1

7/23

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

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 (1901-1954)

24310 cm

Hans Bethe (1906-2005)

barncm 110 228

9/23

3.Detection

Project Poltergeist 10/2

3

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

epn 01

11

10 enp 0

110

11

epn 01

11

10 enp 0

110

11

n

eep

p n

e e

e

e

11/23

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

01

10

11

ee

CdCdnCd 114*114111312/23

3.Detection200l reservoir

90 photomultiplier

-The neutrino detector „Herr Auge“:

13/23

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

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

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:

3862 10:10:10:1::: gravweakemstrong

16/23

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

ee ee

initial state

final state

17/23

5. Weak interaction - beta-decay: ○ first reaction:

○ second reaction:

Wud

eeW

18/23

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

0Zee

19/23

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

As7633

Ge7632

Se7634

20/23

6. Double β-decay

21/23

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)

eepn

22/23

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

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