1 / 26

Massive neutrinos Dirac vs. Majorana

Massive neutrinos Dirac vs. Majorana. Niels Martens Supervisor: Dr. J.G. Messchendorp. Outline. Introduction Helicity Chirality Parity violation in weak interactions Theory SM: massless lefthanded neutrinos Massive neutrinos Dirac mass Majorana mass Dirac-Majorana mass terms

merv
Download Presentation

Massive neutrinos Dirac vs. Majorana

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Massive neutrinosDirac vs. Majorana Niels Martens Supervisor: Dr. J.G. Messchendorp

  2. Outline • Introduction • Helicity • Chirality • Parity violation in weak interactions • Theory • SM: massless lefthanded neutrinos • Massive neutrinos • Dirac mass • Majorana mass • Dirac-Majorana mass terms • Possible scenarios • Experiments • Neutrinoless double beta decay • Results Heidelberg-Moscow cooperation

  3. Helicity & Chirality • Helicity: projection of spin in the direction of momentum • Ill-defined when m≠0 (Lorentz transformation)  Chirality states (eigenstates of weak interaction): superposition of helicity states

  4. Parity violation in weak interactions • Parity operation: x  -x • V  -V • A  A • Goldhaber experiment (1957): measuring neutrino helicity • Electron capture in 152Eu • Two co-linear events of opposite parity expected:

  5. Parity violation in weak interactions • Only lefthanded photons observed  only lefthanded neutrinos • Later experiments: only righthanded anti-neutrinos P

  6. Neutrinos in the Standard Model • Fermion; spin-½ • Massless • only lefthanded neutrinos, righthanded anti-neutrinos

  7. Neutrinos in the standard model • Massless spin-½ particles are described by the Dirac eqation for massless particles:

  8. Massive neutrinos – Dirac neutrino • Flavour oscillations  (small) neutrino mass!! • How to incorporate this in SM/ extend SM? • Dirac mass • Boost can change handedness • coupling between two helicity states • A single four-component spinor

  9. Massive neutrinos – Dirac neutrino • Dirac mass term in Lagrangian • What other mass terms are possible?

  10. Massive neutrinos – Majorana neutrino (2) Majorana mass • Neutrino is chargeless, so it can be its own antiparticle  mM couples particle and antiparticle

  11. General case: Dirac-Majorana-mass (3) Dirac-Majorana mass term • Diagonalizing M gives two mass eigenvalues:

  12. Different scenarios (a) : pure Dirac case  (Dirac field) • : pure Majorana case 

  13. Different scenarios (c) Seesaw model • Explains: • light mass of neutrinos • the experimental fact that only lefthanded neutrinos couple to the weak interaction.

  14. Related experiments • Tritium β-decay • Flavor oscillations • Neutrinoless double β-decay

  15. Neutrinoless double β-decay • β—decay: • Double β--decay: • Could any nucleus be used? No: * * Single β-decay must be forbidden 

  16. Neutrinoless double β-decay • Semi-empirical mass/Weizsäcker formula:

  17. Neutrinoless double β-decay • 35 naturally occurring isotopes which decay via 2β-, all even-even

  18. Neutrinoless double β-decay • So how can 2β- show that the neutrino is a majorana particle? Neutrinoless double beta decay X

  19. Neutrinoless double β-decay • 2 necessary conditions: • Particle-antiparticle matching  • Helicity matching  • If neutrinoless double β-decay occurs, the neutrino is a massive majorana particle. Virtual neutrino line

  20. Neutrinoless double β-decay • Experimental signatures: • Two e- from same place at same time • Daughter nucleus (Z+2,A) • Neutrinoless case: sharp defined kinetic energy of electrons, instead of continuous spectrum

  21. Neutrinoless double β-decay • Theoretical uncertainty (76Ge): 1.5 < |M| < 4.6 • Half-lives • β : from seconds to 105 y • 2νββ: ~1020 y • 0 νββ: > 1025 y • mν ~ 50 meV  100 kg needed for 1 event/y

  22. Neutrinoless double β-decay • Experimental difficulties: • Count rate: How to measure T1/2 beyond 1025 y!? • Source strength: expensive! • Background: Cosmic rays, 2νββ, natural radioactive decay • Energy resolution

  23. Heidelberg-Moscow Experiment Source strength  11,0 kg enriched 76Ge: Source = detector Background  find a mountain and dig a hole Enormous half-lives  experiment run from 1990 till 2003 (but, stability then becomes a problem)

  24. Heidelberg-Moscow experiment

  25. Conclusions • None… yet • Since neutrinos do have mass, the SM has to be extended. • Theoretically, massive neutrinos can have a Dirac and/or Majorana nature. • Reliable 0νββ observations would prove that the neutrino is a Majorana particle and give the neutrino mass, but at the moment 0νββ-experiments face many difficulties.

  26. Bibliography • C. Giunti & C.W. Kim, Fundamentals of neutrino physics and astrophycis, Oxford University Press, 2007 • K. Zuber, Neutrino Physics, IOP Publishing, 2004 • H.V. Klapdor-Kleingrothaus et al. / Physics Letters B 586 (2004) 198–212

More Related