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Register a new userAnalysis of Light Neutrino Exchange and Short-Range Mechanisms in $0 ubetabeta$ Decay
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Neutrinoless double beta decay ($0 ubetabeta$) is a crucial test for lepton number violation. Observation of this process would have fundamental implications for neutrino physics, theories beyond the Standard Model and cosmology. Focussing on so called short-range operators of $0 ubetabeta$ and their potential interplay with the standard light Majorana neutrino exchange, we present the first complete calculation of the relevant nuclear matrix elements, performed within the interacting boson model (IBM-2). Furthermore, we calculate the relevant phase space factors using exact Dirac electron wavefunctions, taking into account the finite nuclear size and screening by the electron cloud. The obtained numerical results are presented together with up-to-date limits on the standard mass mechanism and effective $0 ubetabeta$ short-range operators in the IBM-2 framework. Finally, we interpret the limits in the particle physics scenarios incorporating heavy sterile neutrinos, Left-Right symmetry and R-parity violating supersymmetry.
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We report a study of the CUORE sensitivity to neutrinoless double beta ($0 ubetabeta$) decay. We used a Bayesian analysis based on a toy Monte Carlo (MC) approach to extract the exclusion sensitivity to the $0 ubetabeta$ decay half-life ($T_{1/2}^{0 u}$) at $90%$ credibility interval (CI) -- i.e. the interval containing the true value of $T_{1/2}^{0 u}$ with $90%$ probability -- and the $3 sigma$ discovery sensitivity. We consider various background levels and energy resolutions, and describe the influence of the data division in subsets with different background levels. If the background level and the energy resolution meet the expectation, CUORE will reach a $90%$ CI exclusion sensitivity of $2cdot10^{25}$ yr with $3$ months, and $9cdot10^{25}$ yr with $5$ years of live time. Under the same conditions, the discovery sensitivity after $3$ months and $5$ years will be $7cdot10^{24}$ yr and $4cdot10^{25}$ yr, respectively.
Radiation background studies pertaining to $0 ubetabeta$ decay in $^{124}$Sn have been carried out. A TiLES setup has been installed at TIFR for this purpose. Neutron-induced background is studied in the TIN.TIN detector materials using fast neutron activation technique. The neutron flux ($E_nleq15$ MeV) resulting from SF and ($alpha, n$) interactions for the rock in the INO cavern is estimated using MC simulations. A two layer composite shield of borated paraffin (20 cm) + Pb (5 cm) is proposed for the reduction of neutron flux.
We present results from a search for neutrinoless double-$beta$ ($0 ubetabeta$) decay using 36.6 g of the isotope $^{150}$Nd with data corresponding to a live time of 5.25 y recorded with the NEMO-3 detector. We construct a complete background model for this isotope, including a measurement of the two-neutrino double-$beta$ decay half-life of $T^{2 u}_{1/2}=$[9.34 $pm$ 0.22 (stat.) $^{+0.62}_{-0.60}$ (syst.)]$times 10^{18}$ y for the ground state transition, which represents the most precise result to date for this isotope. We perform a multivariate analysis to search for zeronu decays in order to improve the sensitivity and, in the case of observation, disentangle the possible underlying decay mechanisms. As no evidence for zeronu decay is observed, we derive lower limits on half-lives for several mechanisms involving physics beyond the Standard Model. The observed lower limit, assuming light Majorana neutrino exchange mediates the decay, is $T^{0 u}_{1/2} >$ 2.0 $times 10^{22}$ y at the 90% C.L., corresponding to an upper limit on the effective neutrino mass of $langle m_{ u} rangle$ $<$ 1.6 - 5.3 eV..
Study of the neutrinoless double beta decay and searches for the manifestation of the neutrino mass in ordinary beta decay are the main sources of information about the absolute neutrino mass scale, and the only practical source of information about the charge conjugation properties of the neutrinos. Thus, these studies have a unique role in the plans for better understanding of the whole fast expanding field of neutrino physics.
The interference of charge-changing interactions, weaker than the V-A Standard Model (SM) interaction and having a different Lorentz structure, with that SM interaction, can, in principle, produce effects near the end point of the Tritium beta decay spectrum which are of a different character from those produced by the purely kinematic effect of neutrino mass expected in the simplest extension of the SM. We show that the existence of more than one mass eigenstate can lead to interference effects at the end point that are stronger than those occurring over the entire spectrum. We discuss these effects both for the special case of Dirac neutrinos and the more general case of Majorana neutrinos and show that, for the present precision of the experiments, one formula should suffice to express the interference effects in all cases. Implications for sterile neutrinos are noted.
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