Bounds on heavy Majorana neutrinos in type-I seesaw and implications for collider searches

The neutrino masses and flavor mixings, which are missing in the Standard Model (SM), can be naturally incorporated in the type-I seesaw extension of the SM with heavy Majorana neutrinos being singlet under the SM gauge group. If the heavy Majorana neutrinos are around the electroweak scale and their mixings with the SM neutrinos are sizable, they can be produced at high energy colliders, leaving characteristic signatures with lepton-number violations. Employing the general parametrization for the neutrino Dirac mass matrix in the minimal seesaw scenario, we perform a parameter scan and identify allowed regions to satisfy a variety of experimental constraints from the neutrino oscillation data, the electroweak precision measurements and the lepton-flavor violating processes. We find that the resultant mixing parameters between the heavy neutrinos and the SM neutrinos are more severely constrained than those obtained from the current search for heavy Majorana neutrinos at the LHC. Such parameter regions can be explored at the High-Luminosity LHC and a 100 TeV pp-collider in the future.

With the measurements of nonzero reactor angle θ 13 [1][2][3][4][5], all neutrino oscillation data expect the Dirac CP -phase have been determined [6], which indicate physics beyond the Standard Model (SM). The type-I seesaw extension [7][8][9][10][11][12][13] of the SM is arguably the simplest idea to naturally incorporate the tiny neutrino masses and the flavor mixings into the SM, where heavy Majorana neutrinos which are singlet under the SM gauge group are introduced.
The heavy neutrinos are integrated out at low energies, leading to a dimension five operator [14] among the SM lepton and the Higgs doublets at low energies. After the electroweak symmetry breaking, light Majorana masses for the SM neutrinos are generated thought the type-I seesaw mechanism.
Although the heavy Majorana neutrinos are singlet under the SM gauge group, the heavy mass eigenstates after the seesaw mechanism couple with the weak bosons and the Higgs boson through the mixing with the SM neutrinos. If the heavy neutrinos are around or below the electroweak scale and the mixing with the SM neutrinos is not extremely small, the heavy Majorana neutrinos can be produced at high energy colliders. The smoking gun collider signature of heavy neutrino production at the collider experiments is the same-sign dilepton in the final state which reflects the lepton-number violation due to their Majorana masses. The heavy neutrino signature, once observed at collider experiments, can provide us with a clue to explore the origin of the neutrino masses and flavor mixings.
The mixing of the heavy neutrinos with the SM neutrinos affects not only the production cross section at high energy colliders but also a variety of phenomenologies such as the neutrino oscillation data [15,16], the precision measurement of weak boson decays, and the lepton-flavor-violating decays of charged leptons [17][18][19][20][21] [22][23][24][25][26][27] [28][29][30][31][32][33], which severely constrain the mixing parameters. Therefore, in order to discuss the possibility of the heavy neutrino production at high energy colliders, it is essential to identify allowed regions for the mixing parameters from the current phenomenological constraints. In this letter, for simplicity, we consider the minimal seesaw scenario [34,35] and introduce two right-handed neutrinos to the SM, which is the minimal setup to reproduce the observed neutrino oscillation data with a prediction of one massless neutrino. Employing the general parametrization for the neutrino Dirac mass matrix in the seesaw model, we perform a parameter scan to identify the allowed regions for the mixing parameters.
Let us begin with a brief review of the minimal seesaw. We introduce two flavors of right-handed neutrinos N j R (j = 1, 2). The relevant part of the Lagrangian is written as where i L (i = 1, 2, 3) and H are the SM lepton doublet of the i-th generation and the SM Higgs doublet, respectively, and the Majorana mass matrix of the right-handed neutrinos is taken to be diagonal without loss of generality. After the electroweak symmetry breaking, we obtain the Dirac mass matrix as m D = Y D √ 2 v, where v = 246 GeV is the Higgs vacuum expectation value. Using the Dirac and Majorana mass matrices, the neutrino mass matrix is expressed as Assuming the hierarchy of |m ij D /m k N | 1, we diagonalize the mass matrix and obtain the seesaw formula for the light Majorana neutrinos as We In the presence of , the mixing matrix N is not unitary, namely N † N = 1.
In terms of the neutrino mass eigenstates, the charged current interaction can be written as where α (α = e, µ, τ ) denotes the three generations of the charged leptons, and P L = (1 − γ 5 )/2. Similarly, the neutral current interaction is given by where θ W is the weak mixing angle. Through the mixing R αi , the heavy neutrinos can be produced at high energy colliders, which have been extensively studied . For example, the production cross section of the i-th generation heavy neutrino at the Large where σ LHC is the production cross section of the SM neutrino when its mass is set to be m i N . Similarly, the production cross section at an e + e − collider such as the Large Electron-Positron Collider (LEP) and the International Linear Collider (ILC) is given by where σ LC is the production cross section of the SM neutrino at an e + e − collider when its mass is set to be m i N , and we have used the approximation as we will find in the following.
The minimal seesaw scenario predicts one massless eigenstate. For the light neutrino mass spectrum, we consider both the normal hierarchy (NH) and the inverted hierarchy (IH). In the NH case, the diagonal mass matrix is given by while in the IH case In order to make our discussion simple, we assume the degeneracy of the heavy neutrinos in mass such as M N = m 1 N = m 2 N , so that the light neutrino mass matrix is simplified as for the NH/IH cases. From this formula, we can parameterize the neutrino Dirac mass matrix as [71] 1 where the matrices denoted as D NH/IH are defined as and O is a general 2 × 2 orthogonal matrix given by where X and Y are real parameters.
Due to its non-unitarity, the elements of the mixing matrix N are severely constrained by the combined data from the neutrino oscillation experiments, the precision measurements of weak boson decays, and the lepton-flavor-violating decays of charged leptons [17][18][19][20][21]. We update the results by using more recent data on the lepton-favor-violating decays [73][74][75]: where the diagonal elements are from the precision measurements of weak boson decays (the SM prediction is 1) while the off-diagonal elements are the upper bounds from the leptonfavor-violating decays, namely, the (1,2) and (2,1) elements from the µ → eγ process, the (2,3) and (3,2) elements from the τ → µγ process, and the (1,3) and (3,1) elements from the τ → eγ process. Since N N † 1 − , we have the constraints on such that The most stringent bound is given by the (1, 2)-element which is from the constraint on the lepton-flavor-violating muon decay µ → eγ. Using the general parametrization of the Dirac mass matrix in Eq. (13), we have Here, note that (δ, ρ, Y ) is independent of X since Now we perform a scan for the parameter set {δ, ρ, Y } and identify an allowed region for which (δ, ρ, Y ) satisfies the experimental constraints in Eq. (17). 2 In our analysis, we set M N = 100 GeV and vary the three parameters in the range of −π ≤ δ, ρ ≤ π with the interval of π 20 and 0 ≤ y ≤ 14 with the interval of 0.01875. For the NH case, we show in Fig. 1 our results on the mixing matrix element |R αi | 2 with respect to −π < δ < π. In each panel, the shaded region satisfies the experimental constraints in Eq. (17). We have found |R αi | 2 < 2.94 × 10 −4 . Note that as in Eqs. (7) and (8), the heavy neutrino production cross section is proportional to |R αi | 2 and hence the constraints in Eq. (17) provide us with the upper bound on the cross section. The same results but with respect to Y are shown in Fig. 2. For the IH case, the corresponding results are shown in Fig. 3 and Fig. 4, respectively. Similarly to the NH case, we have found |R αi | 2 < 3.52×10 −4 .
We also show in Fig. 5 and Fig. 6 our results for a combination of the mixing parameters, , in the NH and IH cases, respectively. For comparison, we list in Table I the upper bounds on the mixing parameters from the collider experiments, for M N = 100 GeV. We can see that the upper bounds on the mixing we have obtained are more severe than those listed in Table I. In summary, we have studied the minimal type-I seesaw scenario and the current experimental bounds on the mixing between the heavy Majorana neutrinos and the SM neutrinos.
We have employed the general parameterization for the neutrino Dirac mass matrix so as to reproduce all neutrino oscillation data. In this way, the model is controlled by only three parameters, the Dirac CP -phase, one Majorana phase, and the (complex) angle of the  we have found can be tested at the High-Luminosity LHC or at a 100 TeV pp-collider in the future. We have also performed parameter scan for the effective neutrino mass relevant to the neutrinoless double beta decay and found the range of 0.00154 ≤ |m ν ee |(eV) ≤ 0.00389  The allowed parameter region for a combination of the mixing parameters, , in the NH case.    Although we have shown the results only for the case with the degenerate heavy neutrinos, we have also performed parameter scans for the non-generate case with a few sample values of m 2 N > m 1 N = 100 GeV and found that the upper bound on the mixing parameters reduces from the case with m 2 N = m 1 N = 100 GeV. This observation suggests that the degenerate mass spectrum is preferable in terms of the testability of the type-I seesaw scenario at the future collider experiments. Our parameter scan analysis in this letter is similar to that in Ref. [36], where the inverse-seesaw scenario was considered. A crucial difference of the inverse-seesaw scenario is that we can choose a flavor-blind Dirac mass matrix by encoding all the flavor structures into the small lepton-number violating parameter µ ij and easily avoid the experimental constraints in Eq. (17). However, there is no such freedom in the type-I seesaw scenario, and the neutrino Dirac mass matrix must satisfy all the experimental data such as the neutrino oscillation data, the electroweak precision measurements, and the lepton-flavor violations. As a result, the heavy neutrino production cross section at the high energy colliders are constrained very severely.