Leptoquark mechanism of neutrino masses within the grand unification framework

We demonstrate viability of the one-loop neutrino mass mechanism within the framework of grand unification when the loop particles comprise scalar leptoquarks (LQs) and quarks of the matching electric charge. This mechanism can be implemented in both supersymmetric and non-supersymmetric models and requires the presence of at least one LQ pair. The appropriate pairs for the neutrino mass generation via the up-type and down-type quark loops are $S_3$-$R_2$ and $S_{1,\,3}$-$\tilde{R}_2$, respectively. We consider two phenomenologically distinct regimes for the LQ masses in our analysis. First regime calls for very heavy LQs in the loop. It can be naturally realised with the $S_{1,\,3}$-$\tilde{R}_2$ scenarios when the LQ masses are roughly between $10^{12}$ GeV and $5 \times 10^{13}$ GeV. These lower and upper bounds originate from experimental limits on partial proton decay lifetimes and perturbativity constraints, respectively. Second regime corresponds to the collider accessible LQs in the neutrino mass loop. That option is viable for the $S_3$-$\tilde{R}_2$ scenario in the models of unification that we discuss. If one furthermore assumes the presence of the type II see-saw mechanism there is an additional contribution from the $S_3$-$R_2$ scenario that needs to be taken into account beside the type II see-saw contribution itself. We provide a complete list of renormalizable operators that yield necessary mixing of all aforementioned LQ pairs using the language of $SU(5)$. We furthermore discuss several possible embeddings of this mechanism in $SU(5)$ and $SO(10)$ gauge groups.

The one-loop contributions towards neutrino masses that we study have been considered extensively in the literature [1,2,[18][19][20][21][22]. Our intention, in contrast to the existing studies, is to analyse possibilities to have a more fundamental origin of this mechanism and to provide several realistic examples.
To start, we present an overview of the most salient features of this mechanism. Only then do we proceed to discuss two distinct implementations of this approach to address the issue of neutrino mass within the grand unification framework. We list the transformation properties of scalar LQs under the SM gauge group in Table I. We adopt symbolic notation to represent LQ multiplets [14]. We also denote a given representation with the associated dimensionality whenever possible. To single out a particular electric charge eigenstate from a given LQ multiplet we use superscripts [10]. For example, S 3 comprises three electric charge eigenstates that we label S . This fixes the hypercharge normalisation we use throughout the manuscript.
The mechanism we want to study, in its minimal form, requires the presence of one scalar multiplet that transforms asR 2 and another one that has the transformation properties of either S 1 or S 3 in addition to the SM particle content. The following two features are crucial if one is to generate neutrino mass(es) at the one-loop level. Firstly,R −1/3 2 (S 1 and S 1/3 3 ) can couple neutrinos to the right-chiral (left-chiral) down-type quarks. The relevant parts of the Yukawa interactions are whereỹ RL 2 , y LL 1 , y LL 3 , and y D are 3×3 matrices in flavor space. 1 H(≡ (1, 2, 1/2)) is the Higgs boson of the SM, τ k , k = 1, 2, 3, are Pauli matrices, and a, b, c = 1, 2 are the SU (2) group space indices. . ( The mechanism is very economical since the same scalar field H, upon the electroweak symmetry breaking, provides masses for the SM charged fermions and introduces a mixing term for the LQs. The particles that propagate in the loop that generates neutrino Majorana mass(es) are the down-type quarks and scalar LQs of the matching electric charge. The associated one-loop Feynman diagrams are presented in the left panel of Fig. 1. The effective neutrino mass matrix in the basis of the physical down-type quarks and LQs reads [18] (m N ) αβ = 3 sin 2θ 1, 3 32π 2 where 33 ) are the down-type quark masses, α, β, δ = 1, 2, 3 are flavor indices, and x iδ = m 2 δ /m 2 LQ i . Before we proceed we have one specific comment with regard to the previous discussion. It concerns a possibility that the fermions that propagate in the neutrino mass loop are the up-type quarks instead of the down-type quarks. This seems to be a viable possibility if one starts with the R 2 -S 3 combination. The most essential Yukawa interactions for this scenario are where y RL 2 and y U are 3×3 matrices in flavor space. The couplings of R ing matrix. These couplings, though needed, are not enough to complete the neutrino mass loop since R 2 and S 3 cannot couple directly through H at renormalizable level. One possible remedy is to have an operator of dimension five of the form R † 2 S † 3 HHH that is suppressed by an appropriate scale. Another possibility is to mix R but only if all three multiplets, i.e., R 2 ,R 2 , and S 3 , are present in the set-up [18].
Third option is to have one additional scalar S(≡ (1, 3, 1)) that acquires a VEV. The tree-level mixing of is then possible and the off-diagonal entry of the relevant 2 × 2 squared-mass matrix is proportional to a product of the VEVs of neutral fields in S and H. The scalar interactions that are needed to implement the second and third option are where λ 2 is a dimensionful parameter, whereas κ 1 and κ 2 are both dimensionless parameters.
pair is shown in the right panel of Fig. 1. We will make further comments on this potentially important contribution towards neutrino masses later on.
Our aim is to implement the one-loop neutrino mass mechanism in the framework of grand unification. We accordingly investigate viability of two distinct regimes in Section II using mainly the language of SU (5) gauge group. First regime corresponds to a scenario where the LQs behind the neutrino mass generation reside at a very high energy scale. This possibility is discussed in Section II A. Second regime corresponds to a scenario where the neutrino masses are generated with the Large Hadron Collider (LHC) accessible scalar LQs. We demonstrate viability of that scenario in Section II B. The summary of our findings is presented in Section III.

II. GRAND UNIFICATION VS. ONE-LOOP NEUTRINO MASS
Let us proceed with a realistic implementation of the one-loop neutrino mass mechanism with scalar LQs in the grand unification framework. We primarily use the language of the SU (5) gauge group in what follows. The SM fermions reside in 10 α and 5 α of SU (5), where α(= 1, 2, 3) is a flavor index [6]. The exact decompositions of 10 α and 5 α under , respectively. Possible embeddings of scalar LQs in the SU (5) representations are presented in Table I. We clearly need to have either one 10or one 15-dimensional scalar representation in order to introduce oneR 2 multiplet in any SU (5) model. Relevant contraction that yieldsỹ RL 2d R ν LR −1/3 2 term whenR 2 is part of 10-dimensional (15-dimensional) representation is y αβ 5 α 5 β 10 (y αβ 5 α 5 β 15). We identify (ỹ RL 2 ) αβ to be −y αβ / √ 2, where y αβ are elements of an antisymmetric (symmetric) complex matrix in the case whenR 2 originates from 10-dimensional (15-dimensional) representation.
The mass mechanism that we discuss can also be implemented in the SO(10) framework. See Table I  where we assume that one 16-dimensional SO(10) representation comprises one generation of the SM fermions and one right-chiral neutrino.
Finally, one needs to provide the mixing term for at least one of the relevant LQ pairs in order to complete the neutrino mass loop. There are two very different regimes for the scalar LQ masses that we can envisage with this in mind. First option is that the LQs behind the neutrino mass generation reside at a very high energy scale. This could provide compliance of the set-up with the experimental bounds on proton decay. The main issue with this regime could be associated with the size of the relevant lepton-quark-LQ couplings. Namely, these couplings might need to be unrealistically large in order to (re)produce neutrino mass scales that are compatible with experimental observations. It turns out that this is not the case and we accordingly demonstrate in Section II A why and how this particular scenario can be realised within the grand unification frameworks.
Second option is that the scalar LQs are very light. That scenario is especially appealing since the LHC accessible LQs could also affect flavor physics observables. The main difficulty with this particular set-up is to explain observed levels of matter stability. 2 Namely, S 1 and S 3 can both have "diquark" couplings that, in combination with the lepton-quark-LQ couplings that are needed to generate neutrino masses, destabilise protons and bound neutrons. 3 To avoid conflict with stringent limits on proton lifetime one would need to either forbid or substantially suppress these "diquark" operators. This might be very difficult from the model building point of view since unification of matter multiplets dictates common origin of both types of couplings. One would also need to prevent mixing between these LQs and any other LQ in the theory that has "diquark" couplings to insure stability of matter. This might also represent a challenge since one needs to mix specific LQ multiplets in order to generate neutrino masses in the first place. We show that both of these issues can be successfully addressed for the S 3 -R 2 and S 3 -R 2 scenarios in Section II B. The S 1 -R 2 option, on the other hand, is problematic due to difficulty with suppression of the S 1 "diquark" couplings in the simplest of models and we opt not to discuss it in the light LQ regime.

A. Heavy leptoquark regime
Let us turn our attention to a scenario where the LQs are heavy. We assume in what follows that all the LQ masses need to be at or exceed 10 12 GeV to insure proton stability. This is a very conservative estimate since it is certainly above a lower bound that can be extracted from the latest data on proton stability within the SU (5) framework [38]. We show that the one-loop neutrino masses can be realised in this part of phenomenologically available parameter space if the fermions in the neutrino mass loop are exclusively the down-type quarks.
The mixing angle between either S 1 andR The necessary mixing between S 1 (∈ 5) andR 2 (∈ 15) can be generated through the contractions of the form 5 i 5 j 15 ij and 5 i 5 j 15 jk 24 i k . These, again, yield an angle θ 1 that is comparable in strength to our estimate for θ 3 . We can furthermore safely assume that m b (≈ 1 GeV) contribution dominates the sum in Eq. (4). Putting all this together implies that where we suppress flavor indices and assume that the mass splitting between LQs is not substantial, i.e., we take that ln(m 2 LQ 2 /m 2 LQ 1 ) ≈ 1. The approximation of Eq. (10) shows that the entries in the product (ỹ RL 2 y LL 1, 3 ) do not have to be very large to correctly describe the neutrino mass scale. For example, in the non-degenerate normal hierarchy case for the neutrino masses the largest entry on the left side of Eq. (10) needs to be at the level of 5 × 10 −2 eV which would imply that (ỹ RL 2 y LL 1, 3 ) ∼ 5 × 10 −3 . 4 The back-ofthe-envelope estimate we present clearly demonstrates viability of this option. Note that there is an upper bound on the heavier of the two LQs in this set-up if one demands perturbativity of Yukawa coupling entries inỹ RL 2 and y LL 1, 3 matrices. We find it to be roughly at 5 × 10 13 GeV. This implies that the two LQs must reside in relatively narrow mass window from 10 12 GeV to 5 × 10 13 GeV in order to accommodate all the relevant constraints. One can then infer that ln(m 2 LQ 2 /m 2 LQ 1 ) < ∼ 5 which is in agreement with our initial assumption.  This particular possibility to generate neutrino masses, in our view, has been overlooked in the literature on grand unification. For example, there are two non-supersymmetric models that already have all the necessary ingredients to incorporate this particular scenario. The first model [40] introduces one 10dimensional scalar representation on top of 5, 24, and 45 in order to generate neutrino masses through the Zee mechanism [41]. The second model [37] resorts to one 15-dimensional scalar representation in addition to 5, 24, and 45 in order to generate neutrino masses through the type II see-saw mechanism [30,31].
Again, both of these models can accommodate the one-loop mechanism we discuss.
The heavy LQ regime is also tailor-made for the SO(10) framework. This could especially be beneficial in the scenarios that fail to accommodate neutrino masses in satisfactory manner. Clearly, it is sufficient to have either 120or 126-dimensional representation to introduce LQs that transform as S 1 , S 3 , andR 2 . that exists regardless whether the theory is supersymmetric or not that yields a mixing between S 1 (∈ 10) andR 2 (∈ 126), Here, 10 and 126 are scalar representation that generate masses of the SM charged fermions.

B. Light leptoquark regime
To demonstrate that the collider accessible LQ scenario is a viable option to generate neutrino masses one needs to address the issue of the LQ mixing. Namely, if the genuine LQ states mix with the states that have "diquark" couplings it is hard to imagine that matter stability holds at the experimentally observed levels. We focus exclusively on a scenario whenR 2 originates from 15-dimensional representation. The analysis for the 10-dimensional representation case is completely analogous as we show in Appendix A.
We note that R 2 ,R 2 , and S 3 do not have "diquark" couplings [25] at renormalizable level if the charged fermion mass relations are given with Eqs. the up-type quarks is symmetric in accordance with Eq. (9) which has implications for the gauge-mediated proton decay [29]. We plan to pursue the phenomenology of this set-up in the future works. In this respect, the state S 3 with mass close to the LHC reach has been proven to play a beneficial role in addressing hints of lepton flavor universality violation in b → s and b → c ν processes [27,28].
We have, in our analysis, neglected possible VEVs of electrically neutral fields in 15and 24-dimensional representations. The former (latter) field resides in the (1, 3, 1) ((1, 3, 0)) component of 15 (24). We normalize these additional VEVs of 15 ≡ 15 ij and 24 ≡ 24 i j to be 15 55 = v 15 and 24 4 4 = − 24 5 5 = v S , respectively. The presence of these VEVs introduces seven additional SU (5) operators that one needs to include in the analysis of the LQ mixing. We list these operators in Appendix A.
The one-loop mechanism we discuss is not the only possible contribution towards neutrino masses in the light LQ regime. Note that the VEV of the 15-dimensional representation can generate neutrino mass(es) of Majorana nature through the type II see-saw mechanism [30,31]. 5 More importantly, the up-type quarks can also contribute towards neutrino mass generation since the scalars R 2/3 2 ,R 2/3 2 , and S −2/3 * 3 mix with or without the VEV of the 15-dimensional representation [18]. In the latter case we find that the up-type quark contribution is completely negligible. In the former case the mixing angle θ 2 between R For the latest direct bounds on LQ masses from the LHC data see, for example, Refs. [34,35]. Note that v 15 is bounded from above due to the existing electroweak precision measurements of the so-called ρ parameter [36]. This bound can be avoided if one judiciously adjusts v 15 and v S to be approximately equal [37]. This can increase the maximum allowed value of v 15 but only by a factor of ten. The leading neutrino mass contributions due to propagation of the up-type quarks and the down-type quarks are thus proportional to O(10 −3 )m t and O(1)m b , respectively, and can be comparable in strength in some parts of the available parameter space. A self-consistent study of neutrino mass(es) should take into account all these contributions ifR 2 originates from 15-dimensional representation and the VEV proportional to v 15 is turned on. IfR 2 originates from 10-dimensional representation the only relevant contribution in this regime is due to the down-type quark loop.

III. CONCLUSIONS
The one-loop neutrino mass mechanism with scalar LQs in the loop can be embedded within the framework of grand unification regardless of whether the scenario is supersymmetric or not. There exist two distinct regimes for the LQ masses.
One option is to have heavy LQs in the loops that generate neutrino masses. This option can be naturally realised with the S 1, 3 -R 2 combinations of LQs. The type II see-saw mechanism contribution could also be present and important in some parts of the accessible parameter space. The nice feature of the heavy LQ limit is that the masses of the LQs in the loop can only be between 10 12 GeV and 5 × 10 13 GeV in order to simultaneously avoid experimental limits on partial proton decay lifetimes and still satisfy perturbativity constraints on the lepton-quark-LQ couplings.
The other option is to have collider accessible LQs in the loop. That particular limit can be realised via the loops that contain the down-type quarks and scalars of the matching electric charge that are the mixture of S 3 andR 2 multiplets. The S 1 -R 2 combination is not a viable option in this limit due to existence of "diquark" couplings of S 1 in the minimal set-up. If the theory also contains an SU (2) triplet scalar (1, 3, 1) that gets the VEV one needs to take into account two additional neutrino mass contributions. One is the type II see-saw contribution and the other one is the one-loop contribution due to propagation of the uptype quarks and the scalar states of the same electric charge that originate from the mixture of S 3 and R 2 multiplets. These three mechanisms can coexist and be of equal importance in some parts of available parameter space.
We discuss possible origins of scalar LQs that are needed to complete the neutrino mass generating loops using the language of SU (5). We also provide a list of all SU (5) contractions that generate the LQ mixing terms. We furthermore argue that all the necessary ingredients to implement the one-loop neutrino mass mechanism are present in any SO(10) theory with the standard embedding of the matter fields that generates charged fermion masses through renormalizable contractions. ture of 10 ij (= −10 ji ) in the SU (5) group space. These contractions are 5 i 10 ij 5 j , 45 ij k 10 jl 45 lk i , and ijlmn 5 i 10 ko 45 jl k 45 mn o . Also, one needs to add two more operators -5 i 10 jk 45 jk i and 5 i 10 lm 45 lm j 24 j i -that are specific for the 10-dimensional representation case.