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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