# Neutrinoless Double Beta Decay

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### Transcript of Neutrinoless Double Beta Decay

Neutrinoless Double Beta Decay Student: Alina HriscuSupervisors: Olaf Scholten Gerco Onderwater

30 November 2005

Summary

-Beta decay(-review-)-Two neutrinos double beta decay (22)-Zero neutrinos double beta decay(02)-Majorana particles-Calculus of half-life time-elementary particle problem-nuclear structure point of view-Experiments on neutrinoless double beta decay-Conclusions-References

Beta decay-Decay of a neutron in a nucleus, into a proton

-Theory of decay -parity is violated -neutrinos exist in nature only as left-handed particles (antineutrinos right-handed)and only these can interact

Fermi theory -analogy with electromagnetic interaction -Dirac-Pauli representation with matrixes

-Matrix elements are calculated

Other possible b decayspne+W+neutrino conversion-In nature-only in supernovae-

(2) decay

2 decay-Simultaneous transmutation of two neutrons in two protons inside a nucleus thanks to -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

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

The emitting isotopes

Isotope

Half-time

T1/2,2 (y) exp.

48Ca

~ 4.0 1019

76Ge

~ 1.4 1021

82Se

~ 0.9 1020

96Zr

~ 2.1 1019

100Mo

~ 8.0 1018

116Cd

~ 3.3 1019

128Te

~ 2.5 1024

136Xe

not observed yet

150Nd

~ 7.0 1018

0 decayIf the two neutrinos are missing

Violates the lepton conservation rule (left side has lepton nb=0,right side equal to 2)=>beyond SMPossible only ifneutrinos are Majorana particlesneutrinos have non-zero massExistence of right-handed currents in weak interactionHas not been (yet) observed experimentally

Feynman diagram for 0 decay

->e-(in SM)

Calculating the half-life timeTwo aspects:Elementary particle Nuclear structureHalf-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

Majorana ParticlesMajorana particles the particle is the same as the antiparticle, opposite as the Dirac particlesEx: Dirac particles: electron,proton Majorana particles: photon,*all particles with spin 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

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

Neutrinos masses and mixing-the See-saw mechansimIf 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:

If ML and MR are zero, the Majorana -left pair with -right to form N Dirac neutrinosThe Seesaw mechanism: assuming a hierarchy in the values of elements of with 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

Neutrino mixingMixes of light neutrinos: For three active neutrinos,the mixing matrix can be written:

The effective neutrino mass:

Calculating the nuclear matrix elementsIf the 0 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 motionSHELL 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 2 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 0, too one order of magnitude difference

Why is that so difficult?

Theorists are making real efforts to reduce the uncertainty in calculated n.m.eMatrix elements: =

decay sequential decay Graphical representation

|Z>0 |Z+1>0 sequential decay |Z+2>0

Values of the calculated n.m.e for 2 decay and experimental onesPredicted 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

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

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

Experimental 0? If an experiment observes 0 it will have profound physics implications=>extraordinary evidence is requiredDifficulties: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 expensiveThe experiment must take place in underground-mines or like others,under a mountainEven 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 decayA lot of experiments are running to date,and others are being prepared

One of the most advanced : Heidelberg-Moscow

Energetic resolution 0 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

Importance of energy resolution

Heidelberg-Moscow experiment

German-Russian experimentIn Gran Sasso Underground Laboratory in ItalyOperating with 76Ge(the sample and the detector)-experiment is possible due to simultaneous use of large source strengths with high resolution detectorsIn the underground lab the flux of cosmic muon is reduced by 6 orders of magnitude

They claim to have seen 0 decay

H-M experimental setup

What do they have?Peak expected here

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

ConclusionsIf 0 decay will be observed, it will reveal the identity of neutrino,so a fundamental issue will be answeredIf a nonzero rate is seen=> =Majorana particleIf no signal is seen=> =Dirac particleExperimental proposal are promisingTo 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

ReferencesNeutrinoless 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. Gurriaran, F. Hubert, Ph. Hubert(Z. Phys. A 358, 297301 (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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