Quark-lepton Research Articles

Reactor neutrino experiments with a large liquid scintillator detector

We discuss several new ideas for reactor neutrino oscillation experiments with a Large Liquid Scintillator Detector. We consider two different scenarios for a measurement of the small mixing angle $\theta_{13}$ with a mobile $\bar{\nu}_e$ source: a nuclear-powered ship, such as a submarine or an icebreaker, and a land-based scenario with a mobile reactor. The former setup can achieve a sensitivity to $\sin^2 2\theta_{13} \lesssim 0.003$ at the 90% confidence level, while the latter performs only slightly better than Double Chooz. Furthermore, we study the precision that can be achieved for the solar parameters, $\sin^2 2\theta_{12}$ and $\Delta m_{21}^2$, with a mobile reactor and with a conventional power station. With the mobile reactor, a precision slightly better than from current global fit data is possible, while with a power reactor, the accuracy can be reduced to less than 1%. Such a precision is crucial for testing theoretical models, e.g. quark-lepton complementarity.

Predictions forμ→eγin Supersymmetry from Nontrivial Quark-Lepton Complementarity and Flavor Symmetry

We compute the effect of nondiagonal neutrino mass inli→ljγin Supersymmetry (SUSY) theories with nontrivial quark-lepton complementarity and a flavor symmetry. The correlation matrixVM=UCKMUPMNSis such that its (1,3) entry, as preferred by the present experimental data, is zero. We do not assume thatVMis bimaximal. Quark-lepton complementarity and the flavor symmetry strongly constrain the theory and we obtain a clear prediction for the contribution toμ→eγand theτdecaysτ→eγandτ→μγ. If the Dirac neutrino Yukawa couplings are degenerate but the low-energy neutrino masses are not degenerate, then the lepton decays are related among them by theVMentries. On the other hand, if the Dirac neutrino Yukawa couplings are hierarchical or the low-energy neutrino masses are degenerate, then the prediction for the lepton decays comes from theUCKMhierarchy.

Quark lepton complementarity and renormalization group effects

We consider a scenario for the quark-lepton complementarity relations between mixing angles in which the bimaximal mixing follows from the neutrino mass matrix. According to this scenario in the lowest order the angle ${\ensuremath{\theta}}_{12}$ is $\ensuremath{\sim}1\ensuremath{\sigma}$ (1.5\ifmmode^\circ\else\textdegree\fi{}--2\ifmmode^\circ\else\textdegree\fi{}) above the best fit point coinciding practically with the tribimaximal mixing prediction. Realization of this scenario in the context of the seesaw type-I mechanism with leptonic Dirac mass matrices approximately equal to the quark mass matrices is studied. We calculate the renormalization group corrections to ${\ensuremath{\theta}}_{12}$ as well as to ${\ensuremath{\theta}}_{13}$ in the standard model (SM) and minimal supersymmetric standard model (MSSM). We find that in a large part of the parameter space corrections $\ensuremath{\Delta}{\ensuremath{\theta}}_{12}$ are small or negligible. In the MSSM version of the scenario, the correction $\ensuremath{\Delta}{\ensuremath{\theta}}_{12}$ is in general positive. Small negative corrections appear in the case of an inverted mass hierarchy and opposite $CP$ parities of ${\ensuremath{\nu}}_{1}$ and ${\ensuremath{\nu}}_{2}$ when leading contributions to ${\ensuremath{\theta}}_{12}$ running are strongly suppressed. The corrections are negative in the SM version in a large part of the parameter space for values of the relative $CP$ phase of ${\ensuremath{\nu}}_{1}$ and ${\ensuremath{\nu}}_{2}$: $\ensuremath{\varphi}g\ensuremath{\pi}/2$.

Neutrino mass and New physics

Reconstruction of the neutrino mass and flavor spectrum is described. Essentially two processes are relevant for interpretation of the neutrino results which were used in determination of neutrino parameters: oscillations (averaged and non-averaged) in vacuum and matter and the adiabatic flavor conversion in matter (the MSW-effect). Detailed physics picture of these processes is elaborated and their realizations in solar and atmospheric neutrinos as well as in K2K, KamLAND and MINOS experiments are described. Important bounds have been obtained from neutrinoless double beta decay and cosmology. Implications of the obtained results to fundamental physics are discussed. Among various mechanisms for small neutrino masses we consider the seesaw (which has the highest priority) and overlap suppression in extra dimensions. The observed pattern on neutrino mixing may testify for existence of new symmetries of nature. One of the key issues on the way to underlying physics is comparison of the quarks and lepton masses and mixing. In this connections concepts of quark-lepton symmetry and unification, quark-lepton universality and quark-lepton complementarity are described.

Up quark masses from down quark masses

The quark and charged lepton masses and the angles and phase of the CKM mixing matrix are nicely reproduced in a model which assumes SU ( 3 ) ⊗ SU ( 3 ) flavor symmetry broken by the v.e.v.'s of fields in its bi-fundamental representation. The relations among the quark mass eigenvalues, m u / m c ≈ m c / m t ≈ m d 2 / m s 2 ≈ m s 2 / m b 2 ≈ Λ GUT 2 / M Pl 2 , follow from the broken flavor symmetry. Large tan β is required which also provides the best fits to data for the obtained textures. Lepton–quark grandunification with a field that breaks both SU ( 5 ) and the flavor group correctly extends the predictions to the charged lepton masses. The seesaw extension of the model to the neutrino sector predicts a Majorana mass matrix quadratically hierarchical as compared to the neutrino Dirac mass matrix, naturally yielding large mixings and low mass hierarchy for neutrinos.

Quark–lepton unification and eight-fold ambiguity in the left–right symmetric seesaw mechanism

In many extensions of the Standard Model, including a broad class of left–right symmetric and grand unified theories, the light neutrino mass matrix is given by the left–right symmetric seesaw formula M ν = f v L − v 2 v R Y ν f −1 Y ν , in which the right-handed neutrino mass matrix and the SU ( 2 ) L triplet couplings are proportional to the same matrix f. We propose a systematic procedure for reconstructing the 2 n solutions (in the n-family case) for the matrix f as a function of the Dirac neutrino couplings ( Y ν ) i j and of the light neutrino mass parameters, which can be used in both analytical and numerical studies. We apply this procedure to a particular class of supersymmetric SO ( 10 ) models with two 10-dimensional and a pair of 126 ⊕ 126 ¯ representations in the Higgs sector, and study the properties of the corresponding 8 right-handed neutrino spectra. Then, using the reconstructed right-handed neutrino and triplet parameters, we study leptogenesis and lepton flavour violation in these models, and comment on flavour effects in leptogenesis in the type I limit. We find that the mixed solutions where both the type I and the type II seesaw mechanisms give a significant contribution to neutrino masses provide new opportunities for successful leptogenesis in SO ( 10 ) GUTs.

QUARK–LEPTON SYMMETRY AND COMPLEMENTARITY

We argue that the difference between the observed approximate quark–lepton complementarity and the theoretical prediction based on realistic quark–lepton symmetry within the seesaw mechanism may be adjusted by means of a triplet contribution in the seesaw formula.

Phenomenology of minimal lepton flavor violation

We extend the effective theory of minimal lepton flavor violation (MLFV) by including four-lepton operators. We compute the rates for μ → 3 e and τ → 3 ℓ decays and point out several new ways to test the hypothesis of MLFV. We also investigate to what extent it will be possible from (future) experimental information to pin down the contributions of different effective operators. In particular we look for experimental handles on quark–lepton operators of the type ℓ ¯ i Γ ℓ j × q ¯ Γ q by working out their contribution to hadronic processes such as τ → μ π 0 , π 0 → μ e ¯ , ϒ → τ μ ¯ , as well as to purely leptonic decays such as μ → 3 e through loop effects.

Quark–lepton universality and large leptonic mixing

A unified description of fermionic mixing is proposed which assumes that in certain basis (i) a single complex unitary matrix V diagonalizes mass matrices of all fermions to the leading order, (ii) the SU ( 5 ) relation M d = M l T exists between the mass matrices of the down quarks and the charged leptons, and (iii) M d † = M d . These assumptions automatically lead to different mixing patterns for quarks and leptons: Quarks remain unmixed to leading order (i.e. V CKM = 1 ) while leptons have non-trivial mixing given by a symmetric unitary matrix V PMNS 0 = V T V . V depends on two physical mixing angles and for values of these angles ∼ 20 ° – 25 ° it reproduces the observed mixing patterns rather well. We identify conditions under which the universal mixing V follows from the universal mass matrices of fermions. Relatively small perturbations to the leading order structure lead to the CKM mixing and corrections to V PMNS 0 . We find that if the correction matrix equals the CKM matrix, the resulting lepton mixing agrees well with data and predicts ( V PMNS ) e 3 > 0.08 .

Rare t-quark decays t → cl +j l −k and t → cṽ j v k induced by doublets of scalar leptoquarks within the minimal four-color-symmetry model

The rare t-quark decays t → clj+lk− and t → cṽjkk induced by scalar-leptoquark doublets are considered within the minimal model involving four-color quark-lepton symmetry and the Higgs mechanism of quark and lepton mass splitting. The partial widths with respect to the decays being considered and the total widths Γ(t → cl+′l−) = Σj,k Γ(t → clj+lk−) and Γ(t → cl+′l−) = Σj,kΓ(t → cṽjvk) with respect to, respectively, the charged leptonic and neutrino modes are calculated. It is shown that, at scalar-leptoquark masses higher than the t-quark mass (mS > mt), the branching ratios for these modes are Br(t → cl+′l−) ≈ (3.5−0.4) × 10−5 and Br(t → cṽ′v) ≈ (7.1−0.8) × 10−5 at ms = 180–250 GeV and an appropriate value of the leptoquark-mixing angle (sin β ≈ 0.2) and can increase for mS < mt to Br(t → cl+′l−) ≈ 0.03−0.002 and Br(t → cl+′l−) ≈ 0.46−0.05 for the charged mode at mS = 150–170 GeV for sin β ≈ 1 and sin β ≈ 0.2, respectively. In the cases being considered, t-quark decays to pairs of charged leptons can be accessible to detection at LHC. In the last case, these decays could manifest themselves (for example, in dilepton events) at the Tevatron as well.

Neutrino masses and mixing

Status of determination of the neutrino masses and mixing is formulated and possible uncertainties especially due to presence of the sterile neutrinos are discussed. The data hint an existence of special ‘‘neutrino’’ symmetries. If not accidental these symmetries have profound implications and can substantially change the unification program. The key issue on the way to the underlying physics is relations between quarks and leptons. The approximate quark-lepton symmetry or universality can be reconciled with strongly different patterns of masses and mixings due to nearly singular character of the mass matrices or screening of the Dirac structures in the double see-saw mechanism.

Quark-lepton complementarity with quasidegenerate Majorana neutrinos

A basis independent formulation of quark-lepton complementarity is implemented at a high scale for quasidegenerate Majorana neutrinos. It is shown that even with the renormalization group evolution in the minimal supersymmetric standard model, the scenario can be consistent with the data provided a nontrivial role is played by the Majorana phases. Correlated constraints are found on these phases and the neutrino mass scale using the current data. We also indicate how future accurate measurements of the mixing angles can serve as tests of this scenario and restrict the values of the Majorana phases.

Neutrino masses, mixing and leptogenesis in a two Higgs doublet model “for the third generation”

We examine neutrino oscillations in a two Higgs doublet model (2HDM) in which the second doublet couples only to the third generation right-handed up-fermions, i.e., to tR and to N3 which is the heaviest right-handed Majorana neutrino. The inherently large tanβ of this model can naturally account for the large top mass and, based on a quark–lepton similarity ansatz, when embedded into a see-saw mechanism it can also account for the observed neutrino masses and mixing angles giving a very small θ13: −0.017≲θ13≲0.021 at 99% CL, and a very restrictive prediction for the atmospheric mixing angle: 42.9°≲θatm≲45.2° at 99% CL. The large value of tanβ also sets the mass scale of the heaviest right-handed Majorana neutrino N3 and triggers successful leptogenesis through a CP-asymmetry in the decays of the N1 (lightest right-handed Majorana) which is tan2β enhanced compared to the CP-asymmetry obtained in models for leptogenesis with one Higgs doublet or in the MSSM. This enhancement allows us to relax the lower bound on MN1 and consequently also the lower bound on the reheating temperature of the early universe.

Energy scale independence of Koide’s relation for quark and lepton masses

Koide's mass relation of charged leptons has been extended to quarks and neutrinos, and we prove here that this relation is independent of energy scale in a huge energy range from $1 {GeV}$ to $2\times10^{16} {GeV}$. By using the parameters $k_u$, $k_d$ and $k_{\nu}$ to describe the deviations of quarks and neutrinos from the exact Koide's relation, we also check the quark-lepton complementarity of masses such as $k_{l}+k_{d} \approx k_{\nu}+k_{u} \approx 2$, and show that it is also independent (or insensitive) of energy scale.

FERMIONIC DECAYS OF SCALAR LEPTOQUARKS AND SCALAR GLUONS IN THE MINIMAL FOUR-COLOR SYMMETRY MODEL

Fermionic decays of the scalar leptoquarks [Formula: see text], [Formula: see text], Sm and of the scalar gluons F=F1, F2 predicted by the four-color symmetry model with the Higgs mechanism of the quark–lepton mass splitting are investigated. Widths and branching ratios of these decays are calculated and analyzed in dependence on coupling constants and on masses of the decaying particles. It is shown that the decays [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text] are dominant with the widths of order of a few GeV for mS, mF&lt; 1 TeV and with the total branching ratios close to 1. In the case of mS&lt;mt the dominant scalar leptoquark decays are [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text] with the total branching ratios [Formula: see text], [Formula: see text] and [Formula: see text]. A search for such decays at the LHC and Tevatron may be of interest.

A D-brane inspired [formula omitted] model

Motivated by D-brane scenarios, we consider a non-supersymmetric model based on the gauge symmetry U(3)C×U(3)L×U(3)R which is equivalent to the SU(3)3 “trinification” model supplemented by three U(1)'s. Two U(1) combinations are anomalous while the third U(1)Z′ is anomaly free and contributes to the hypercharge generator. This hypercharge embedding corresponds to sin2θW=619 in the case of full gauge coupling unification. The U(3)3 symmetry is broken down to the Standard Model by vevs of two (1,3,3¯)-scalar multiplets supplemented by two Higgs fields in (1,3,1) and (1,1,3) representations. The latter break U(1)Z′ and provide heavy masses to the extra lepton doublets. Fermions belong to (3,3¯,1)+(3¯,1,3)+(1,3,3¯) representations as in the trinification model. The model predicts a natural quark–lepton hierarchy, since quark masses are obtained from tree-level couplings, while charged leptons receive masses from fourth order Yukawa terms, as a consequence of the extra Abelian symmetries. Light Majorana neutrino masses are obtained through a see-saw type mechanism operative at the SU(3)R breaking scale of the order MR⩾109 GeV.

Charged lepton corrections to neutrino mixing angles and CP phases revisited

We re-analyze charged lepton corrections to neutrino mixing angles and CP phases, carefully including CP phases from the charged lepton sector. We present simple analytical formulae for including the charged lepton corrections and derive compact new results for small neutrino and charged lepton mixings θ 13 ν and θ 13 e . We find a generic relation θ 12 + 1 2 θ 12 e cos ( δ − π ) ≈ θ 12 ν , which relates the prediction from the neutrino sector θ 12 ν to the charged lepton mixing θ 12 e and to the MNS neutrino oscillation phase δ. We apply our formula to the examples of bimaximal or tri-bimaximal neutrino mixing. One implication is that the so-called quark–lepton complementarity relation θ 12 + θ C = 45 ° can only hold for δ = π and it gets modified in the presence of leptonic CP violation. On the other hand, the lepton mixing θ 13 generated from the charged lepton correction θ 12 e is independent of CP phases and given by θ 13 = 1 2 θ 12 e . Combining these results leads to a model-independent sum rule: θ 12 + θ 13 cos ( δ − π ) ≈ θ 12 ν where θ 12 ν = ( 35.26 ° ) 45 ° in the case of (tri-)bimaximal neutrino mixing, for example.

Running of low-energy neutrino masses, mixing angles and CP violation

We calculate the running of low-energy neutrino parameters from the bottom–up, parameterizing the unknown seesaw parameters in terms of the dominance matrix R. We find significant running only if the R matrix is non-trivial and the light-neutrino masses are moderately degenerate. If the light-neutrino masses are very hierarchical, the quark–lepton complementarity relation θ c + θ 12 = π / 4 is quite stable, but θ 13 , 23 may run beyond their likely future experimental errors. The running of the oscillation phase δ is enhanced by the smallness of θ 13 , and jumps in the mixing angles occur in cases where the light-neutrino mass eigenstates cross.

Deviations from tribimaximal neutrino mixing

Current neutrino data are consistent with the so-called tribimaximal mixing scenario, which predicts sin2θ12=1/3, zero Ue3 and maximal θ23. This implies a special form of the neutrino mass matrix. Introducing small breaking terms in this mass matrix generates deviations from the tribimaximal scheme and leads to testable correlations between the parameters. They depend on where the perturbation is located in the mass matrix. A special case of such perturbations are radiative corrections. Alternative deviations from tribimaximal mixing may stem from contributions of the charged lepton sector. If there is quark–lepton unification and it is the CKM matrix which corrects the tribimaximal mixing scheme, then almost maximal CP violation and sizable deviation from zero Ue3 are implied.

Ambiguities pertaining to quark–lepton complementarity

Recently the possible origin of the so-called quark–lepton complementarity relations has received a considerable amount of attention. We point out some of the inherent ambiguities in such analyses.