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Research Article | Open Access
Madan Singh, "The Texture One-Zero Neutrino Mass Matrix with Vanishing Trace", Advances in High Energy Physics, vol. 2018, Article ID 2863184, 13 pages, 2018. https://doi.org/10.1155/2018/2863184
The Texture One-Zero Neutrino Mass Matrix with Vanishing Trace
In the light of latest neutrino oscillation data, we have investigated the one-zero Majorana neutrino mass matrix with zero sum condition of mass eigenvalues in the flavor basis, where charged lepton mass matrix is diagonal. Among the six possible one-zero cases, it is found that only five can survive the current experimental data, while case with (1, 1) vanishing element of is ruled out, if zero trace condition is imposed at confidence level (CL). Numerical and some approximate analytical results are presented.
The Double Chooz, Daya Bay, and RENO Collaborations [1–3] have finally established the nonzero and relatively large value of the reactor mixing angle ; hence the number of known available neutrino oscillation parameters approaches five, namely, two mass-squared differences and three neutrino mixing angles . However, any general neutrino mass matrix contains more parameters than can be measured in realistic experiments. In fact, assuming the Majorana-type nature of neutrinos, the neutrino mass matrix contains nine real free parameters: three neutrino masses , three flavor mixing angles , and three CP violating phases .
In order to reduce the number of free parameters, several phenomenological ideas, in particular texture zeros [4–26], have been widely adopted in the literature. The imposition of texture zeros in neutrino mass matrix leads to some important phenomenological relations between flavor mixing angles and fermion mass ratios [24–27]. In the flavor basis, where charged lepton mass matrix is diagonal, at most two zeros are allowed in neutrino mass matrix, which are consistent with neutrino oscillation data [25, 26]. The analysis of two texture zero neutrino mass matrices limits the number of experimentally viable cases to seven. The phenomenological implications of one texture zero neutrino mass matrix have also been studied in the literature [20–23] and it has been observed that all the six cases are viable with experimental data. However, the imposition of single texture zero condition in neutrino mass matrix makes larger parametric space for viability with the data available compared with two-zero texture. In order to impart predictability to one-zero texture, additional constraints in the form of vanishing determinant  or trace can be incorporated. The phenomenological implication of determinantless condition on one-zero texture have been rigorously studied in [20, 28–32]. The implication of traceless condition was first put forward in  wherein the anomalies of solar and atmospheric neutrino oscillation experiments as well as the LSND experiment were simultaneously explained in the framework of three neutrinos. In , Zee has particularly investigated the case of CP conserving traceless neutrino mass matrix for explaining the solar and atmospheric neutrino deficits. Further motivation of traceless mass matrices can be provided by models wherein neutrino mass matrix can be constructed through a commutator of two matrices, as what happens in models of radiative mass generation . In , Alhendi et al. have studied the case of two trackless submatrices of Majorana mass matrix in the flavor basis and carried out a detailed numerical analysis at confidence level. The phenomenological implications of traceless neutrino mass matrix on neutrino masses, CP violating phases, and effective neutrino mass term are also studied in , for both normal and inverted mass ordering and in case of CP conservation and violation, respectively. In the present work we impose the traceless condition on texture one-zero Majorana mass matrix and investigate the outcomes of such condition on the parametric space of neutrino masses and CP violating phases .
Assuming the Majorana nature of neutrinos, neutrino mass matrix is complex symmetric. In the flavor basis, if one of the elements is considered to be zero, the number of possible cases turns out to be six, which are given below:where “” stands for nonzero element and complex matrix element.
Among these possible cases, there exists a permutation symmetry between certain pair of cases, namely, and , while cases and transform onto themselves independently. The origin of permutation symmetry is explained from the fact that these pairs are related by exchange of 2-3 rows and 2-3 columns of neutrino mass matrix. The corresponding permutation matrix is given bywhich leads to the following relations among the neutrino oscillation parameters:where and superscripts denote the cases related by 2-3 permutation symmetry.
The rest of the work is planned as follows: In Section 2, we discuss the methodology used to reconstruct the Majorana neutrino mass matrix and subsequently obtain some useful phenomenological relations of neutrino mass ratios and Majorana phases by incorporating texture one-zero and zero trace conditions simultaneously. In Section 3, we present the numerical analysis using some approximate analytical relations. In Section 4, we summarize our work.
In the flavor basis, the Majorana neutrino mass matrix , depending on three neutrino masses and the flavor mixing matrix can be expressed asThe mixing matrix can be written as , where denotes the neutrino mixing matrix consisting of three flavor mixing angles and one Dirac-like CP violating phase, whereas the matrix is a diagonal phase matrix; that is, = diag with and being the two Majorana CP violating phases. The neutrino mass matrix can then be rewritten aswhere , , and
For the purpose of calculations, we have adopted the parameterization of the mixing matrix considered by ; for example,where , for , and is the CP violating phase.
If one of the elements of is considered zero, that is, , it leads to the following constraint equation:where , run over , , and .
The traceless condition implies that sum of the mass eigenvalues in neutrino mass matrix is zero; that is,Using (7) and (8), we obtainThe magnitudes of neutrino mass ratios are given byUsing (9), we find the following analytical relations for Majorana phases :Thus neutrino mass ratios and two Majorana-type CP violating phases can fully be determined in terms of three mixing angles and the Dirac-type CP violating phase . The ratio of two neutrino mass-squared differences in terms of neutrino mass ratios and is given bywhere and  corresponds to solar and atmospheric neutrino squared differences, respectively. The sign of is still not known experimentally; that is, or corresponds to the normal or inverted mass ordering of neutrinos.
The expressions for three neutrino masses can be given asThus the neutrino mass spectrum can be fully determined.
The expression for Jarlskog rephasing parameter , which is a measure of CP violation, is given by
3. Numerical Results and Discussion
The experimental constraints on neutrino parameters at , , and confidence level (CL) are given in Table 1.
The effective Majorana mass term relevant for neutrinoless double beta decay is given byThe future observation of decay would imply lepton number violation and Majorana character of neutrinos. For recent reviews see [40–43]. There are a large number of projects such as CUORICINO , CUORE , GERDA , MAJORANA , SuperNEMO , EXO , and GENIUS  which target achieving a sensitivity up to 0.01 eV for . For the present analysis, we assume the upper limit on to be less than 0.5 eV at 3 CL . The data collected from the Planck satellite  combined with other cosmological data put a limit on the sum of neutrino masses asHere, we take rather more conservative limit on sum of neutrino masses (Σ) (i.e., eV) at CL. We span the parameter space of input neutrino oscillation parameters by choosing the randomly generated points of the order of . Using Eq. (12), the parameter space of CP violating phases , effective mass term , neutrino masses can be subsequently constrained. In order to interpret the phenomenological results, some approximate analytical relations (up to certain leading order term of ) have been used in the following discussion. The exact analytical relations of neutrino mass ratios have been provided in Table 2.
Using (6) and (9), in the leading order term of , we obtain the following analytical relations:For NO, using (12), we obtain . Using experimental range of oscillation parameters, we find , which excludes the experimental range of and for IO we havewhich is again inconsistent with current experimental data as . Therefore, Case is ruled out with the latest neutrino oscillation data at CL.
Using (6) and (9), we obtain the following analytical relations in the leading order approximation of :From (19), one can obtain the neutrino mass ratiosand the Majorana CP violating phasesThe correlation plots for case have been compiled in Figures 1(a), 1(b), 1(c), and 1(d) and Figures 2(a), 2(b), 2(c), and 2(d), respectively. It is found from the analysis that case favors both normal (NO) and inverted mass ordering (IO) at CL. The parameter space of CP violating phases , , is found to be constrained to very small ranges for NO (Figures 1(a) and 1(b)). However, for IO, comparatively significant allowed parameter space is available for , , (Figures 2(a) and 2(b)).
In the leading order approximation of , the effective mass term in decay turns out to bewhich lies well within the sensitivity limit of neutrinoless double beta decay experiments. Figures 1(c) and 2(c) show the correlation plot between and for NO and IO, respectively. The Jarlskog rephrasing parameter is found to be nonvanishing for NO (Figure 1(d)); however, cannot be excluded for IO (Figure 2(d)).
Cases and are related via permutation symmetry; therefore the phenomenological results for case can be obtained from case by using (3). The correlation plots for case have been complied in Figures 3(a), 3(b), 3(c), and 3(d) (NO) and Figures 4(a), 4(b), 4(c), and 4(d) (IO), indicating the parameter space of , , , , .
Using (6) and (9), we deduce the following analytical expressions in the leading order of term:Since in the leading order approximation of , we have to work next to leading order, and we getUsing (27), the neutrino mass ratios can be given asand the Majorana CP violating phases are asUsing the best fit values from latest global fits on neutrino oscillation data (Table 1), the neutrino mass spectrum can be given as follows:implying that only NO is allowed. Figures 5(a) and 5(b) show the correlation plot between Majorana phases and Dirac CP violating phase . The parameter space for is found to be restricted near and . The prediction is significant considering the latest hint on near in the recent global fits on neutrino oscillation data (Table 1). The Majorana phases are found to be constrained near and . In Figure 5(d), it is explicitly shown that is nonzero implying that case is necessarily CP violating.
In the leading order of , the effective mass term in decay can be approximated aswhich is well within the accessible limit of next generation neutrinoless double decay experiments. The correlation plot between and has been provided for case in Figure 5(c).
With the help of (6) and (9), we obtain the following analytical expressions in the leading order of term:Since in the leading order approximation of , we have to work next to leading order, and we obtainFrom (33), the neutrino mass ratios can be given as follows:and the Majorana CP violating phases are given as