Neutrino Mass Models in Extra Dimensions

Neutrinos play a crucial role in many areas of physics from very short distances to astrophysics and cosmology. It is a long held believe that they are good probes of physics at the GUT scale. Recent developments have made it clear that they can also be of fundamental importance for the physics of extra dimensions if these exist. Here we pedagogically review the construction of neutrino mass models in extra dimensions within the brane world scenarios. These models are usually nontrivial generalizations of their four dimensional counterparts. We describe the theoretical tools that have been forged and the new perpectives gained in this rapidly developing area. In particular we discuss the issues involve in building models without the use of right-handed singlets. It is very difficult to directly test the origin of neutrino masses in different models be it in four or more dimensions. We point out that different models give very different indirect signatures in the TeV region and in precision measurements.Comment: Invited Talk at International Conference on Flavor Phyiscs 2003, KIAS, S.Kore

Top Quark Forward-Backward Asymmetry and Anomalous Right-Handed Charge Currents

Recent measurements of the top quark forward-backward asymmetry at the Tevatron could hint at new physics with an unexpected flavor structure. The significance of such an abnormal flavor structure in alleviating the tension between the various determinations of $|V_{ub}|$ via right-handed charge currents is studied. In particular, we elaborate on how the associated new flavor changing couplings naturally allow for the generation of anomalous loop-induced right-handed charge currents which can simultaneously remove the tension in the determinations of $|V_{ub}|$ and escape the tight indirect bounds from B \ra X_s \gamma.Comment: 20 pages, 8 figures, references added, minor changes to text, new collider signatures added, matches version accepted for publication in JHE

A Minimal Model of Majoronic Dark Radiation and Dark Matter

We extend the singlet Majoron model of dark radiation by adding another singlet scalar of unit lepton charge. The spontaneous breaking of global $U(1)_L$ connects dark radiation with neutrino mass generation via the type-I seesaw mechanism. The model naturally has a stable scalar dark matter field. It also predicts the existence of a light scalar of mass less than 1 GeV that mixes with the Standard Model Higgs boson. We perform a numerical analysis of the parameters of the model by imposing constraints from giving correct relic abundance and satisfying bounds from direct dark matter detection, rare decays of B-meson, and invisible width of the Higgs boson. The viability of the model in accommodating the gamma rays from the Galactic center is discussed as well. The model gives rise to new rare Higgs boson decays such as four-muon final states with displaced vertices. Another unique signal is two muons and missing energy recoil against the muon pair. Our result also shows that such a bridge between dark radiation and the seesaw mechanism will put the seesaw scale in the range of 1-100 TeV.Comment: 27 pages,17 figures. To match the published versio

An Effective Operators Analysis of CP Violation : The Semileptonic Case

Aiming at a model-independent analysis of possible new physics effects in semileptonic processes at various energy scales, we list and study a complete set of $SU(3)_c\times SU(2)_L\times U(1)_Y$ invariant 4-Fermi operators which consist of a pair of quarks and a pair of leptons above the electroweak symmetry breaking. We give a full 1-loop renormalization group treatment of the evolution of the Wilson coefficients associated with these 4-Fermi operators between low energy ($\sim$ meson masses) and the cutoff scale $\Lambda$, $\sim (1-10)$ TeV, where we assume new degree of freedom beyond standard model will begin to appear and an ultra-violet completion of our effective theory will take place. Motivated by the existing phenomenological bounds, we argue that the new CP violation can only stem from the scalar and tensor types of 4-Fermi interaction. Some interesting constraints are obtained by studying the universality of kaon and pion leptonic decays, CP violating polarization of $K^+_{\mu 3}$, charged lepton anomalous magnetic moments, and (\mu\ra e \gamma) like rare decays. In particular, we can use the limit of electron dipole moment to constrain the size of the CP violating triplet correlation in the e^+ e^- \ra t \bar{t} process.Comment: 25 pages, 6 figure

Neutron-Antineutron Oscillations in a Warped Extra Dimension

We investigate neutron-antineutron oscillations in the Randall-Sundrum warped extra dimensional scenario. The four dimensional effective strengths of the relevant operators that induce the oscillations are calculated up to an arbitrary coupling along with their corresponding enhancements due to QCD 1-loop running effects. We find that the $\Delta B = 2$ operators can be geometrically suppressed without fine tuning to within current experimental limits with a warped down four dimensional mass scale which can be as low as a fraction of a TeV.Comment: 7 pages, 2 figures, accepted for publication in Phys. Rev. D, small change

Phenomenology of a 5D Orbifold SU(3)WSU(3)_W Unification model

We study the phenomenology of a 5D $SU(3)_W$ model on a $S_1/(Z_2\times Z'_2)$ orbifold in which the minimal scalar sector plays an essential role of radiatively generating neutrino Majorana masses without the benefits of right-handed singlets. We carefully examine how do the exotic scalars affect the renormalization group (RG) equations for the gauge couplings and the 5D $SU(3)_W$ unification. We found that the compactification scale of extra dimension is in the range of $1/R\sim 1.5-5$ TeV. The possibility of the existence of relatively low mass Kaluza-Klein excitations makes the phenomenology of near term interest. Some possible bilepton signatures can be searched for in future colliders and in neutrino scattering experiments with intense neutrino beams. The low energy constraints from muon physics and lepton number violating decay process induced by bilepton are also discussed. These constraints can provide new information on the structure of Yukawa couplings which might be useful for future model building.Comment: 17 pages, 14 EPS figures, REVTe

Neutrino Masses in 5D Orbifold SU(5) Unification Models without Right-handed Singlets

We explore a mechanism for radiatively generating neutrino Majorana masses in a 5 dimensional orbifold SU(5) unification model without introducing right-handed singlets. The model is non-supersymmetric and the extra dimension is compactified via a $s_1/(Z_2\times Z'_2)$ orbifold geometry. The necessary lepton number violating interaction arises from the Yukawa interactions either between a 10-plet or a 15-plet bulk scalar field and the fermion quintuplets which are residents on the SU(5) symmetrical brane located at one of the orbifold fixed points. The model is engineered to give realistic charged fermion masses and mixing and in the same time avoiding the rapid proton and neutron decays by geometric construction. The gauge unification can be maintained by adding extra fermion or scalar fields. The unification scale is found to be larger then $10^{15}$ GeV by adding a bulk vector decuplet pair whose zero mode has masses around $10\sim 100$ TeV range. We found that neutrino mass matrix of the normal hierarchy type is favored by using 15-plet scalar. We give a solution of this type which has detectable \mu\ra 3e transition. On the other hand, by introducing 10-plet scalar, the leading neutrino mass matrix can only be inverted hierarchical and gives at most bi-maximal mixing.Comment: 25 pages, 8 figure

Renormalization Group Study of the Minimal Majoronic Dark Radiation and Dark Matter Model

We study the 1-loop renormalization group equation running in the simplest singlet Majoron model constructed by us earlier to accommodate the dark radiation and dark matter content in the universe. A comprehensive numerical study was performed to explore the whole model parameter space. A smaller effective number of neutrinos $\triangle N_{eff}\sim 0.05$, or a Majoron decoupling temperature higher than the charm quark mass, is preferred. We found that a heavy scalar dark matter, $\rho$, of mass $1.5-4$ TeV is required by the stability of the scalar potential and an operational type-I see-saw mechanism for neutrino masses. A neutral scalar, $S$, of mass in the $10-100$ GeV range and its mixing with the standard model Higgs as large as $0.1$ is also predicted. The dominant decay modes are $S$ into $b\bar{b}$ and/or $\omega\omega$. A sensitive search will come from rare $Z$ decays via the chain $Z\rightarrow S+ f\bar{f}$, where $f$ is a Standard Model fermion, followed by $S$ into a pair of Majoron and/or b-quarks. The interesting consequences of dark matter bound state due to the sizable $S\rho \rho$-coupling are discussed as well. In particular, shower-like events with an apparent neutrino energy at $M_\rho$ could contribute to the observed effective neutrino flux in underground neutrino detectors such as IceCube.Comment: 33 pages,11 figures, published versio

Alternative Perspective on Gauged Lepton Number and Implications for Collider Physics

A new anomaly-free gauged $U(1)_\ell$ lepton-number model is studied. Two standard model lepton generations acquire the same but oppositive sign $U(1)_\ell$ charges, while four exotic chiral leptons cancel the anomalies of the remaining lepton family. We discuss a simplified case which has the universal Yukawa couplings. It agrees with all the experimental constraints and predicts $m_e, m_\mu \ll m_\tau$, and the latter is of the electroweak scale. Due to the interference between the SM and $U(1)_\ell$ gauge interactions, this model robustly predicts that $e,\mu,\tau$ have distinctive forward-backward asymmetries at the $e^+e^-$ colliders. It can be searched for at the $e^+e^-$ machine with $\sim$ TeV center-of-mass energy and an integrated luminosity $\sim ab^{-1}$.Comment: 13 pages, 1 figure. Matches published versio

An Effective Operators Analysis of Leptonic CP Violation : Bridging High and Low Energy Processes

We study the leptonic CP violation by employing the complete set of dimension-six pure leptonic effective operators. Connection among the observable at different energy scales can be made by the running of the renormalization group equations. Explicitly, we study the charged lepton electric dipole moment, muon Michel decay, and the triple spin-momentum correlations at the Linear Collider. We found the electron electric dipole moment, which starts at 2-loop level, severely constrains the possibilities to detect the CP violating signatures in muon decay and at the linear colliders.Comment: 16 pages, 9 figures, discussion on dipole operator and reference adde