Project title: Neutrino properties derived from the study of rare decay processes at low and high Energies.

Project code: PN-III-P4-ID-PCE-2016-0078

Contractor: CIFRA-INCDFM

Project director: Stoica Sabin

Project Summary:

Neutrinos are tiny, neutral particles that play a key role in many physical processes from different nuclear physics, astrophysics, particle physics, cosmology, etc. In the same time, they are the only elementary particles whose fundamental properties, like their absolute masses and the mechanism generating them, mass hierarchy, their character (Dirac or Majorana ?), the possible existence of sterile neutrinos, etc, are still unknown. The knowledge of these properties would lead to a significant progress in understanding of the Standard Model (SM), and the structure and evolution of the universe. That is why the neutrino physics, represents nowadays a priority domain of research in physics, from which one expects significant discoveries in the next future.

A very appealing process for the study of neutrinos is the double beta decay (DBD) process, the rarest spontaneous nuclear decay measured until now. Particularly, the neutrinoless double beta (0νββ) decay, a beyond Standard Model (SM) process that may occur without emission of neutrinos and hence with lepton number violation (LNV), was considered until recently the most sensitive process that can provide us with information about the above mentioned neutrino properties.

On the other hand, similar information on the neutrino properties as that obtained from the DBD study, can be also provided by analyzing of rare decays of mesons and baryons at high energy through same sign dilepton channels, which are also triggered by LNV processes. This is because the present integrated luminosity reached by the LHC experiments at CERN, makes the analysis of such processes competitive with that of the actual 0νββ decay experiments. Study of these processes is included in the European strategy of particle physics and is approached more and more within the ATLAS, CMS and LHCb collaborations. The opportunity to explore the complementarity existing between LNV processes at low and high energy represents now a very new and promissing line of research.

This project proposes itself a theoretical study of the neutrino properties by combining the results obtained from the investigation of the 0νββ decay (low-energy process) with results provided by the analysis of several LNV decay channels (high energy processes) within the LHCb experiment. First, we aim to develop a novel, approach to calculate accurately the phase space factors (F0ν) and nuclear matrix elements (M0ν), two key quantities entering the 0νββ lifetime formulas, that were computed separately until now in literature. Their computation will be performed in a unitary manner, by deriving a unique formula for their product (which matter as a whole for accurate theoretical predictions), and then building a new numerical code where the same nuclear approximations and input parameters be used consistently, for both quantities. In order to eliminate errors associated with large uncertainties of some quantities involved in calculations (the most uncertain being the value of axial-vector constant), we will combine our theoretical calculations with experimental results taken from different 0νββ decay experiments. In this way we aim to reduce significantly the uncertainties in the F0ν and M0ν calculation, and thus to get an important achievement in the domain. Then, having calculated these products, we will derive reliable neutrino parameters and constrain different BSM decay mechanisms, using the actual bounds for the experimental 0νββ decay lifetimes. Such results are also much expected in literature.

Further, we will use the information on neutrinos derived from the 0νββ decay study and from other low-energy processes (as single beta decay, for example), to investigate several rare decays channels of Σ, Ξ and Λc hyperons in order to predict Br bounds and constrain other BSM parameters, in scenarios with sterile neutrinos that manifest at high energy. Such a complementary analysis it is not yet done, on our best knowledge, and represents another important novel ingredient of the project.

The specific objectives of the project include: i) computing in a unitary manner the product F0ν x |M0ν|2 for several possible 0νββ decay mechanisms (exchange of light LH and heavy RH neutrinos, exchange of SUSY particles, exchange of light neutrinos with Majoron emission, etc.); ii) getting new, more reliable, predictions for 0νββ decay lifetimes; iii) deriving neutrino mass parameters for active and sterile flavors, in different BSM scenarios; iv) investigating of several rare hyperon decay channels triggered by LNV processes, and their analysis with data collected at LHCb; v) dissemination of the results through publications in ISI journals and presentations at international conferences; vi) continuation and strengthening of our international collaborations with top research groups in domain; vii) guidance of the young researchers from the project team to accomplish a master or PhD program and reach an international competitive level in the domain.

We expect the project results to significantly contribute to the progress in the following issues: reliable computation of the 0νββ decay rates and lifetime predictions, improving the constraints on BSM parameters associated with different neutrino flavors and BSM scenarios, constraining Br for LNV processes at high energy and opening of new LHCb analizes, etc.

Besides the direct impact in neutrino physics, DBD, nuclear and particle physics, the project results may have a broader impact, since a better understanding of the neutrino properties, means progress in deciphering of important issues as baryon asymmetry of the universe, DM composition (massive neutrinos are suitable candidates), stellar evolution, nucleosynthesis, etc., which may lead to technological developments with an important economic and social impact.

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