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Neutrinoless Double Beta Decay

Neutrinoless Double Beta Decay. Student : Alina Hriscu Supervisors : Olaf Scholten Gerco Onderwater 30 November 2005. Summary. - Beta decay(-review-) -Two neutrinos double beta decay (2 ν 2 β ) -Zero neutrinos double beta decay(0 ν 2 β ) -Majorana particles

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Neutrinoless Double Beta Decay

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  1. Neutrinoless Double Beta Decay Student: Alina Hriscu Supervisors: Olaf Scholten Gerco Onderwater 30 November 2005

  2. Summary • -Beta decay(-review-) • -Two neutrinos double beta decay (2ν2β) • -Zero neutrinos double beta decay(0ν2β) • -Majorana particles • -Calculus of half-life time • -elementary particle problem • -nuclear structure point of view • -Experiments on neutrinoless double beta decay • -Conclusions • -References

  3. Beta decay -Decay of a neutron in a nucleus, into a proton -Theory of β decay -parity is violated -neutrinos exist in nature only as “left-handed ” particles (antineutrinos “right-handed”)and only these can interact

  4. Fermi theory -analogy with electromagnetic interaction -Dirac-Pauli representation with γ matrixes -Matrix elements are calculated

  5. n e- e- W- W+ (A,Z+1) n p (A,Z) EC β-decay n e+ W+ p n b+decay Other possible b decays e+ W+ p n neutrino conversion -In nature-only insupernovae-

  6. β decay sequential β decay (A,Z) (A,Z) Q Q (A,Z+1) (A,Z+1) (A,Z+2) (2ν)ββ decay

  7. 2νββ decay -Simultaneous transmutation of two neutrons in two protons inside a nucleus thanks to β-decay ex: 54Xe136 -> 56Ba136 + 2e- -It is possible whenever beta decay is forbidden by energy conservation or by angular momentum mismatch -Conserves the lepton number -Allowed in SM

  8. 2νββ decay • -Very rare process • -It is possible only for heavy nuclei (nuclei which can ββ decay have complicated nuclear structure) • -It has been observed experimentally • -Half-life time of order of

  9. The ββ emitting isotopes

  10. 0νββ decay • If the two neutrinos are missing… • Violates the lepton conservation rule (left side has lepton nb=0,right side equal to 2)=>beyond SM • Possible only if • neutrinos are Majorana particles • neutrinos have non-zero mass • Existence of right-handed currents in weak interaction • Has not been (yet) observed experimentally

  11. n p W+ e- ν e- W+ n p • Feynman diagram for 0νββ decay ν->e- (in SM)

  12. Calculating the half-life time • Two aspects: • Elementary particle • Nuclear structure • Half-life time (for Majorana neutrinos) • So,if we know the half-life time and the matrix elements=> we can obtain the NEUTRINO MASS • Elem. particle properties: neutrino masses and mixing • Nuclear strucure calculations: the matrix elements

  13. Majorana Particles • Majorana particles –the particle is the same as the antiparticle, opposite as the Dirac particles • Ex: Dirac particles: electron,proton Majorana particles: photon, *all particles with spin ½ known (fermions) are Dirac particles We define left- and right-handed components of Dirac 4-spinor by: Construct left-handed neutrinos - charge conjugate field, which for neutrinos is also neutral

  14. Neutrino-fields-can be linear combination of ,since • We define independent Majorana neutrino fields that are their own charge conjugate (antiparticles) • -Since the helicity flips,Majorana particles have nonzero mass

  15. Neutrinos masses and mixing-the See-saw mechansim • If neutrinos have masses,flavour is mixed, and a leptonic mixing matrix will appear • -Since we can write the neutrino fields as linear combination of • Mass term in Lagrangian can couple these two kinds of fields themselves and to eachother:

  16. If ML and MR are zero, the Majorana ν-left pair with ν-right to form N Dirac neutrinos • The Seesaw mechanism: assuming a hierarchy in the values of elements of • with μ negligible or zero,one (set)of particle(s)become heavy,while another becomes light • In the simple seesaw model, there are as many right- as left-handed neutrinos such that we have three light and three heavy neutrinos, the lightest of which has mass

  17. Neutrino mixing • Mixes of light neutrinos: • For three active neutrinos,the mixing matrix can be written: • The effective neutrino mass:

  18. Calculating the nuclear matrix elements • If the 0νββ decay will be observed,it is important to have accurate values of nuclear matrix elements in order to obtain quantitative results • The hadronic part contributing to the half time must be evaluated between initial and final states in the intermediate nucleus summed over • Many body techniques which lead to such results are: • QRPA (neutron-proton Quasiparticle Random Phase Approximation) -treats a large fraction of nucleons as active and allows these a large single-particle space to move in -suitable for collective motion • SHELL MODEL -Treats a small fraction of the nucleons in a limited single-particle space, but allows nucleons to corrrelate in arbritary ways These methods have been applied to 2νββ decay(which was observed) ->result: RPA model gave the most precise n.m.e –same order of magnitude -it is expected to give better results for 0νββ, too –one order of magnitude difference

  19. Why is that so difficult? • Theorists are making real efforts to reduce the uncertainty in calculated n.m.e • Matrix elements: • <Z+2| Oβ Oβ|Z>= <Z+2| Oβ |Z+1> <Z+1| Oβ|Z> • ββdecay sequential β decay • Graphical representation • |Z>0 • |Z+1>0 sequential β decay • |Z+2>0

  20. Values of the calculated n.m.e for 2νββ decay and experimental ones Predicted n.m.e. and half-times vs. experimental ones for decay ; WS=Wood-Saxon basis for calculated n.m.e(still QRPA); calculated for decay in ground and excited states of daughter nucleus

  21. …and calculated ones for 0νββ • In one of the most recent QRPA model: Rodin(2003) Compared with the SM results;except of Mo similar results

  22. Predicted life-time for some calculated n.m.e (2ν) The outliers predict wrong life-time; the n.m.e of Rodin and SM are quite close

  23. Experimental 0νββ? • If an experiment observes 0νββ it will have profound physics implications • =>extraordinary evidence is required • Difficulties: • Very slow process(one of the most slowest in nature)=>requires a lot of material(500 kg to 1 tone) • Extremely high energy resolution is required • Only very pure material is used (contaminations may give background signals) • The material is difficult to obtain - experimentalists have to enrich nuclei;also,very expensive • The experiment must take place in underground-mines or like others,under a mountain • Even in the best conditions, false peaks may appear(cosmic rays,walls) -Since enormous blocks of material are used,how can they determine the energy of only one decay A lot of experiments are running to date,and others are being prepared One of the most advanced : Heidelberg-Moscow

  24. 2νββ N 0νββ 2νββ N 0νββ E E Energetic resolution • 0νββ decay being a 2 body decay, experimentally,only the energy of the two outgoing electrons needs to be measured • Ideal case-infinite resolution Real case-finite resolution

  25. Importance of energy resolution

  26. Heidelberg-Moscow experiment • German-Russian experiment • In Gran Sasso Underground Laboratory in Italy • Operating with 76Ge(the sample and the detector) • -experiment is possible due to simultaneous use of large source strengths with high resolution detectors • In the underground lab the flux of cosmic muon is reduced by 6 orders of magnitude • They claim to have “seen” 0νββ decay

  27. H-M experimental setup

  28. What do they have? Peak expected here

  29. Their results Their best values with 50% uncertainty in n.m.e

  30. Conclusions • If 0νββ decay will be observed, it will reveal the identity of neutrino,so a fundamental issue will be answered • If a nonzero rate is seen=> ν=Majorana particle • If no signal is seen=> ν=Dirac particle • Experimental proposal are promising • To obtain quantitative results (neutrino masses and hierarchy) from the experiment, good theoretical results are required • uncertainties in calculus of n.m.e must be reduced

  31. References • Neutrinoless double beta decay from a modern perspective, J.D. Vergados (Phys. Rep 361(2002) 1-56) • Weak interaction and nuclear-structure aspects of nuclear double beta decay-Jouni Suhonen,Osvaldo Civitarese (Phys. Rep 300(1998)123-214) • Double beta decay –Topical review, S.R. Elliot,J. Engel(Jour. Of Phys G. 30(2004)183-215) • Renormalized proton-neutron QRPA and double beta decay of 82Se to excited states in 82Kr, J. Suhonen, J. Toivanen, A.S. Barabash, I.A. Vanushin, V.I. Umatov, R. Gurriar´an, F. Hubert, Ph. Hubert(Z. Phys. A 358, 297–301 (1997)) • Evidence for neutrinoless double beta decay, Klapdor-Kleingrothaus, Modern Physics Letters A [Particles and Fields; Gravitation; Cosmology and Nuclear Physics], Vol. 16, No. 37 (2001) 2409-2420

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