arXiv subjects
Publications and source records attributed to Ernest Ma.
Neutrino tribimaximal mixing is obtained from the breaking of A_4 to Z_3 in the charged-lepton sector and to Z_2 in the neutrino sector. To enforce this conflicting pattern, extra particles and symmetries are usually invoked, often accompanied by nonrenormalizable interactions and even extra dimensions. It is shown here in a specific renormalizable model how A_4 alone will accomplish this, with only the help of lepton number.
Selected topics in neutrino theory are discussed, including tribimaximal mixing and its A_4 realization, variations of the seesaw mechanism allowing for observable deviations from unitarity of the neutrino mixing matrix, and the radiative generation of neutrino mass from dark matter.
The seesaw mechanism for three neutrinos is discussed, clarifying the situation where the seesaw texture results in three approximately zero mass eigenvalues. The true underlying mechanism is shown to be just the $inverse$ (or $linear$) seesaw, which explains why there could be large mixing. However, these zeroes cannot occur naturally, unless there is a conserved symmetry, i.e. lepton number $L$, either global or gauged, which is softly or spontaneously broken at the TeV scale. We discuss in particular the case where the three heavy singlet neutrinos have $L=3,-2,-1$.
If neutrino masses are obtained via the canonical seesaw mechanism, based on an underlying 2X2 mass matrix, unitarity violation of the neutrino mixing matrix is unavoidable, but its effect is extremely small. On the other hand, in the inverse (and linear) seesaw mechanisms, based on an underlying 3X3 mass matrix, it can be significant and possibly observable. This 3X3 matrix is examined in more detail, and a NEW variation (the lopsided seesaw) is proposed which has features of both mechanisms. A concrete example based on U(1)_N is discussed.
The inverse seesaw mechanism of neutrino mass, i.e. m_nu = (m_D^2/m_N^2)epsilon_L where epsilon_L is small, is discussed in the context of the U(1)_Sigma model. This is a gauge extension of the Standard Model of particle interactions with lepton triplets (Sigma^+,Sigma^),Sigma^-) as (Type III) seesaw anchors for obtaining small Majorana neutrino masses.
In these lectures (at the 2007 Summer School in Akyaka, Mugla, Turkey), I discuss the various mechanisms for obtaining small Majorana neutrino masses, as well as specific models of varying complexity, in the context of the standard model and beyond.
If the canonical seesaw mechanism alone is responsible for neutrino mass, i.e. m_nu = -m_D^2/m_N, it can neither be proved nor disproved at the TeV energy scale. A new verifiable mechanism of neutrino mass is proposed, using the "inverse" seesaw, with new physics at the TeV scale, such that m_nu = m_D^2 epsilon_L/m_N^2, where epsilon_L is a two-loop effect. Dark-matter candidates also appear naturally.
In the recently proposed dark left-right gauge model (DLRM) of particle interactions, the usual left-handed lepton doublet (nu,e)_L transforming under SU(2)_L is accompanied by the UNUSUAL right-handed fermion doublet (n,e)_R transforming under SU(2)_R, where n_R is NOT the Dirac mass partner of nu_L. In this scenario, whereas nu_L is certainly a neutrino, n_R should be considered a SCOTINO, i.e. a dark-matter fermion. Variants of this basic idea are discussed, including its minimal SCOTOGENIC realization.
The three electroweak doublet neutrinos $ν_{e,μ,τ}$ of the Standard Model may acquire small seesaw masses, using either three Majorana fermion singlets $N$ or three Majorana fermion triplets $(Σ^+,Σ^0,Σ^-)$. It is well-known that the former accommodates the U(1) gauge symmetry $B-L$. It has also been shown some years ago that the latter supports a new $U(1)_X$ gauge symmetry. Here we study two variations of this $U(1)_X$, one for two $N$ and one $Σ$, the other for one $N$ and two $Σ$. Phenomenological consequences are discussed.
Recent observatons of high-energy positrons and electrons by the PAMELA and ATIC experiments may be an indication of the annihilation of dark matter into leptons and not quarks. This leptonic connection was foreseen already some years ago in two different models of radiative neutrino mass. We discuss here the generic interactions (nu eta^0 - l eta^+) chi and l^c zeta^- chi^c which allow this to happen, where chi and/or chi^c are fermionic dark-matter candidates. We point out in particular the importance of chi chi to l^+ l^- gamma to both positron and gamma-ray signals within this framework.
In a left-right gauge model of particle interactions, the left-handed fermion doublet (nu,e)_L is connected to its right-handed counterpart (n,e)_R through a scalar bidoublet so that e_L pairs with e_R, and nu_L with n_R to form mass terms. Suppose the latter link is severed without affecting the former, then n_R is not the mass partner of nu_L, and as we show in this paper, becomes a candidate for dark matter which is relevant for the recent PAMELA and ATIC observations. We accomplish this in a specific nonsupersymmetric model, where a generalized lepton number can be defined, so that n_R and W_R are odd under R = (-1)^{3B+L+2j}. Fermionic leptoquarks are also predicted.
The origin of neutrino mass is usually attributed to a seesaw mechanism, either through a heavy Majorana fermion singlet (version 1) or a heavy scalar triplet (version 2). Recently, the idea of using a heavy Majorana fermion triplet (version 3) has gained some attention. This is a review of the basic idea involved, its U(1) gauge extension, and some recent developments.
The neutral member of a Majorana fermion triplet (Sigma^+,Sigma^0,Sigma^-) is proposed as a candidate for the dark matter of the Universe. It may also serve as the seesaw anchor for obtaining a radiative neutrino mass.
In the context of A_4 symmetry, neutrino tribimaximal mixing is achieved through the breaking of A_4 to Z_3 (Z_2) in the charged-lepton (neutrino) sector respectively. The implied vacuum misalignment of the (1,1,1) and (1,0,0) directions in A_4 space is a difficult technical problem, and cannot be treated without many auxiliary fields and symmetries (and perhaps extra dimensions). It is pointed out here that an alternative scenario exists with A_4 alone and no redundant fields, if neutrino masses are "scotogenic", i.e. radiatively induced by dark scalar doublets as recently proposed.
The Supersymmetric Standard Model is a benchmark theoretical framework for particle physics, yet it suffers from a number of deficiencies, chief among which is the strong CP problem. Solving this with an axion in the context of selected new particles, it is shown in three examples that other problems go away automatically as well, resulting in (-)^L and (-)^{3B} conservation, viable combination of two dark-matter candidates, successful baryogenesis, seesaw neutrino masses, and verifiable experimental consequences at the TeV energy scale.
The multiplicative conservation of both lepton and baryon numbers, i.e. (-)^L and (-)^{3B}, is connected to an axionic solution of the strong CP problem in a supersymmetric, unifiable model of quark and lepton interactions. New particles are predicted at the TeV scale, with verifiable consequences at the Large Hadron Collider.
To enforce the conservation of baryon number B and not lepton number L in supersymmetry, a new U(1)_X gauge symmetry is recommended. An example is offered with new particles interacting under U(1)_X which are good candidates for the dark matter of the Universe.
This Brief Review deals with the recent resurgence of interest in adding a second scalar doublet (eta^+,eta^0) to the Standard Model of particle interactions. In most studies, it is taken for granted that eta^0 should have a nonzero vacuum expectation value, even if it may be very small. What if there is an exactly conserved symmetry which ensures =0? The phenomenological ramifications of this idea include dark matter, radiative neutrino mass, leptogenesis, and grand unification.