The Low-Scale Approach to Neutrino Masses

Hindawi Publishing Corporation Advances in High Energy Physics Volume 2014, Article ID 831598, 15 pages http://dx.doi.org/10.1155/2014/831598 Review Article The Low-Scale Approach to Neutrino Masses Sofiane M. Boucenna,1 Stefano Morisi,2 and José W. F. Valle1 1 AHEP Group, Instituto de F´ısica Corpuscular-Parque Cient´ıfico (IFIC), Universitat de Valencia,` C/Catedratico´ JoseBeltr´ an´ 2, 46980 Paterna, Valencia,` Spain 2 DESY, Platanenallee 6, 15735 Zeuthen, Germany Correspondence should be addressed to Stefano Morisi; [email protected] Received 14 March 2014; Accepted 14 July 2014; Published 14 August 2014 Academic Editor: Abhijit Samanta Copyright © 2014 Sofiane M. Boucenna et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The publication of this article was funded by SCOAP3. In this short review we revisit the broad landscape of low-scale (3) ⊗(2) ⊗(1) models of neutrino mass generation, with view on their phenomenological potential. This includes signatures associated to direct neutrino mass messenger production at the LHC, as well as messenger-induced lepton flavor violation processes. We also briefly comment on the presence of WIMP cold dark matter candidates. 1. Introduction (ii) its flavor structure which should account for the observed oscillation pattern, The flavor problem, namely, why we have three families of fermions with the same standard model quantum numbers, (iii) its dimensionality, but with very hierarchical masses and a puzzling pattern of (iv) its characteristic scale, and mixing parameters, constitutes one of the most challenging (v) its underlying mechanism. open problems in particle physics. In this regard neutrinos are probably the most mysterious particles. Indeed, while This leads to considerable theoretical freedom which makes the discovery of the Higgs boson by the ATLAS and CMS modelbuildinganespeciallyhardtask,adifficultywhichto experiments at the Large Hadron Collider (LHC) at CERN a large extent persists despite the tremendous experimental [1–3] has clarified to some extent the nature of electroweak progress of the last fifteen years [10, 11]. symmetry breaking, the origin of neutrino masses remains Indeed the origin of neutrino mass remains so far a elusive. With standard model fields one can induce Majorana mystery. From oscillation studies we can not know the neutrino masses through the nonrenormalizable dimension- absolute neutrino mass scale. Still we know for certain that 5operator neutrinos are the lightest known fermions. Their mass must be below the few eV scale from tritium beta decay studies at Odim=5 = (1) the Katrin experiment [12], with somewhat stronger, though Λ more model dependent limits coming from cosmology [13] † and from negative neutrinoless double beta decay searches or higher order ones, for example, ( ) [4–9], [14]. Unfortunately this vast body of information is far where is a dimensionless coupling and Λ denotes some from sufficient to underpin the nature of the neutrino mass unknowneffectivescale.However,strictlyspeaking,westill generation mechanism. do not know whether neutrinos are Dirac or Majorana Mechanisms inducing neutrino mass may be broadly fermions, and many issues remain open regarding the nature divided on the basis of whether the associated messengers of the associated mass-giving operator, for example, lie at the high energy scale, related say, to some unification (i) its underlying symmetries, such as total lepton num- scheme or, in contrast, they involve new physics at the TeV ber, scale, potentially accessible at the LHC. 2 Advances in High Energy Physics For simplicity here we tacitly assume neutrino masses to are viable models where -parity is an exact symmetry of come from Weinberg’s operator in (1). This operator can arise the Lagrangian but breaks spontaneously through the Higgs in a rich variety of different pathways15 [ ]. For instance in mechanism [35, 36]byan=1vacuum expectation thecaseofthestandardtype-Iseesawmechanism[16–21]the value. As we will explain in the next section this scheme is right-handed neutrino messengers have a Majorana mass at hybrid in the sense that it combines seesaw and radiative somelargescale,fittingnaturallyinGrandUnifiedTheories contributions. In all of the above one can assume that the (GUTs).Thereare,however,manyalternativerealizationsof neutrino mass messengers lie at the TeV mass scale and hence the dimension-5 operator, such as the type-II [19, 22–25]and have potentially detectable consequences. type-III seesaw [26] constructions, in which the messengers In this review we consider the low-scale approach to have nontrivial gauge quantum numbers. Such schemes are neutrinomasses.Wechoosetomapoutthepossibleschemes bona fide high-scale seesaw in the sense that, to account taking their potential phenomenological implications as for the observed neutrino masses with reasonable strength guiding criteria, focusing on possible signatures at the LHC for the relevant neutrino Yukawa couplings, one needs very andleptonflavorviolation(LFV)processes(Figure 1). The large scales for the messenger mass, hence inaccessible to paper is organized as follows: in Section 2 we review low collider experiments. Of course within such scenarios one energy seesaw schemes; in Section 3 we discuss one-, two-, may artificially take TeV scales for the messenger mass by and three-loop radiative models. In Section 4 we discuss the assuming tiny Yukawas, so as to account for the smallness supersymmetric mechanism and we sum up in Section 5. of neutrino mass (One can avoid this in schemes where ad hoc cancellations [27]orsymmetries[28, 29]preventseesaw- 2. Seesaw Mechanism produced masses. We do not consider such a special case in this review. Similarly we will not assume any family symmetry 2.1. High-Scale Seesaw. Within minimal unified models such restricting the flavor structure of models.). However by as SO(10), without gauge singlets, one automatically encoun- doing so one erases a number of potential phenomenological ters the presence of new scalar or fermion states that can act implications. Hence we call such standard seesaw varieties as neutrino mass mediators inducing Weinberg’s operator in as high-scale seesaw.Ithaslongagobeenrealized[19]that, (1). This leads to different variants of the so-called seesaw carryingnoanomalies,singletscanbeaddedinanarbitrary mechanism. One possibility is to employ the right-handed number to any gauge theory. Within the framework of the neutrinos present in the 16 of SO(10) and broadly called (3) ⊗(2) ⊗ (1) standard model gauge structure, the type-I seesaw schemes [16–21](seeFigure 2). Similar unified models can be labeled by an integer, ,thenumberofsinglets. constructions can also be made substituting the right-handed For example, to account for current neutrino oscillation data, neutrino exchange by that of an exotic hypercharge-neutral a type-I seesaw model with two right-handed neutrinos is isotriplet lepton [26] sufficient (=2). Likewise for models with =1in + 0 − which another mechanism such as radiative corrections (see Σ=(Σ ,Σ ,Σ ), (2) below) generates the remaining scale. Models with >3 are especially interesting, where one can exploit the extra which is called type-III seesaw [26]. An alternative mediator freedom to realize symmetries, such as lepton number ,soas is provided by a hypercharge-carrying isotriplet coming from to avoid seesaw-induced neutrino masses, naturally allowing the 126 of SO(10) and goes by the name type-II seesaw for TeV-scale messengers. This is the idea behind the inverse mechanism [19, 22, 23, 25](seeFigure 2). [30] and linear seesaw schemes [31–33] described in the next The three options all involve new physics at high scale, section. We call such schemes as genuine low-scale seesaw typically close to the unification scale. While being model constructions. A phenomenologically attractive alternative dependent, the expected magnitude of the mass of such to low-scale seesaw are models where neutrino masses arise messengers is typically expected to be high, say, associated radiatively [34]. to the breaking of extra gauge symmetries, such as the - In principle one can assume the presence of supersym- generator. metry in any such scheme, though in most cases it does Within standard type-I or type-III seesaw mechanism not play an essential role for neutrino mass generation, per with three right-handed neutrinos the isodoublet neutrinos se. However we give an example where it could, namely, get mixed with the new messenger fermions by a 6×6 when the origin of neutrino mass is strictly supersymmetric seesaw block diagonalization matrix that can be determined because -parity breaks. Indeed, neither gauge invariance perturbatively using the general method in [21]. For example nor supersymmetry requires -parity conservation. There in the conventional type-I seesaw case the 6×6matrix that diagonalizes the neutrino mass is unitary and is given by 1 ( − ∗ (∗)−1−1 ) ∗ (∗)−1 2 1 2 =( )+(3), 1 (3) −−1 ( − −1 ∗ (∗)−1) 1 2 2 Advances in High Energy Physics 3 LHC Low-scale models LFV Neutrinos Figure 1: Low-scale neutrino mass models at the crossroad of high and low energy experiments. Δ NR NR (a) (b) Figure 2: Neutrino mass generation in the type-I seesaw (a) and type-II seesaw (b). The black disks show where lepton number violation takes place. where 1 and 2 are the unitary matrices

Load more