Revelations of the neutrino: Weak interaction (beta decay, double beta decay) Sebastian Liebschner 15.11.2012

Outline Beta decay – experimental results Neutrino hypothesis Detection Properties of neutrinos Weak interaction 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: ) 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 2/23

1. Beta decay – experimental results - measurement of electron/positron energy provided next unexpected result  - instead of discrete  continuous spectrum 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) 6/23

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

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) Hans Bethe (1906-2005) 9/23

3.Detection Project Poltergeist 10/23

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

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

3.Detection 200l reservoir The neutrino detector „Herr Auge“: 90 photomultiplier 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: 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 final state initial state 17/23

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