arXiv subjects

G. C. Branco

Publications and source records attributed to G. C. Branco.

At least 91 records · Page 5Linked to original sources

arXiv2000

Unitarity triangles and geometrical description of CP violation with Majorana neutrinos

We generalize the geometrical description of CP violation in the Standard Model in terms of a unitarity triangle. For three left-handed Majorana neutrinos CP violation in the lepton sector is determined by three unitarity triangles. With three additional right-handed neutrinos 15 quadrangles are required to characterize CP violation. We show the relation of the unitarity polygons with physical observables.

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arXiv2000

Reply to ``Is $U_{e3}$ really related to the solar neutrino solutions?''

In a recent paper [1] we showed that, assuming no fine tuning between certain elements of the neutrino mass matrix, one can link the element U_{e3} of the lepton mixing matrix to solar and atmospheric neutrino oscillation parameters. This result has been recently criticized by Haba and Suzuki [2]. In the present note we show that their criticism is not valid and just reflects their failure to understand the content of our paper.

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arXiv2000

Yukawa Structure with Maximal Predictability

A simple Ansatz for the quark mass matrices is considered, based on the assumption of a power structure for the matrix elements and the requirement of maximal predictability. A good fit to the present experimental data is obtained and the position of the vertex of the unitarity triangle, i.e. (\barρ,\barη), is predicted.

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arXiv2000

Mixing and CP violation with Quasidegenerate Majorana Neutrinos

We study the exactly degenerate case for three lefthanded Majorana neutrinos building a general parametrization for the leptonic mixing matrix characterized by two angles and one CP violating phase and identify a weak-basis invariant relevant, in this case, for CP violation. After lifting the degeneracy, this parametrization accommodates the present data on atmospheric and solar neutrinos, as well as neutrinoless double beta decay. Some of the leptonic mixing ansätze suggested in the literature correspond to special cases of this parametrization.

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arXiv2000

Large Neutrino Mixing with Universal Strength of Yukawa Couplings

We analyse, within the framework of universal strength for Yukawa couplings (USY), various structures for the Dirac and Majorana neutrino mass matrices giving rise, through the see-saw mechanism, to a degenerate mass spectrum. A specific USY ansatz is presented for the charged lepton and neutrino effective mass matrix, leading to quasi-degenerate neutrinos and a leptonic mixing matrix which provides a large angle solution for both the atmospheric and solar neutrino problems.

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arXiv1999

Linking solar and long baseline terrestrial neutrino experiments

We show that in the framework of three light neutrino species with hierarchical masses and assuming no fine tuning between the entries of the neutrino mass matrix, one can use the solar neutrino data to obtain information on the element $U_{e3}$ of the lepton mixing matrix. Conversely, a measurement of $U_{e3}$ in atmospheric or long baseline accelerator or reactor neutrino experiments would help discriminate between possible oscillation solutions of the solar neutrino problem.

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arXiv1999

Texture Zeros and Weak Basis Transformations

We investigate the physical meaning of some of the "texture zeros" which appear in most of the Ansatze on quark masses and mixings. It is shown that starting from arbitrary quark mass matrices and making a suitable weak basis transformation one can obtain some of these sets of zeros which therefore have no physical content. We then analyse the physical implications of a four-texture zero Ansatz which is in agreement with all present experimental data.

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arXiv1999

Seesaw mechanism and structure of neutrino mass matrix

We consider the seesaw mechanism of neutrino mass generation in the light of our present knowledge of the neutrino masses and mixing. We analyse the seesaw mechanism constrained by the following assumptions: (1) minimal seesaw with no Higgs triplets, (2) hierarchical Dirac masses of neutrinos, (3) large lepton mixing primarily or solely due to the mixing in the right-handed neutrino sector, and (4) unrelated Dirac and Majorana sectors of neutrino masses. We show that large mixing governing the dominant channel of the atmospheric neutrino oscillations can be naturally obtained and point out that this constrained seesaw mechanism favours the normal mass hierarchy for the light neutrinos leading to a small $V_{e3}$ entry of the lepton mixing matrix and a mass scale of the lightest right handed neutrino $M\simeq 10^{10} - 10^{11}$ GeV. Any of the three main neutrino oscillation solutions to the solar neutrino problem can be accommodated. The inverted mass hierarchy and quasi-degeneracy of neutrinos are disfavoured in our scheme.

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arXiv1999

A Real CKM Matrix and Physics Beyond the Standard Model

We study the possible existence of a real Cabibbo-Kobayashi-Maskawa (CKM) matrix, with CP violation originating from physics beyond the standard model (SM). We show that present experimental data allow for a real CKM matrix provided that new physics also contributes to Delta m_{B_d} by at least 20% of the SM contribution (for rho > 0), besides generating CP violation in the kaon sector. The naturalness of a real CKM matrix is studied within the framework of general multi-Higgs-doublet models with spontaneous CP violation. As an example, we discuss a specific two-Higgs-doublet model and its implications for CP asymmetries in non-leptonic neutral B-meson decays.

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arXiv1999

CP violation from new quarks in the chiral limit

We characterize CP violation in the SU(2)xU(1) model due to an extra vector-like quark or sequential family, giving special emphasis to the chiral limit m_{u,d,s}=0. In this limit, CP is conserved in the three generation Standard Model (SM), thus implying that all CP violation is due to the two new CP violating phases whose effects may manifest either at high energy in processes involving the new quark or as deviations from SM unitarity equalities among imaginary parts of invariant quartets (or, equivalently, areas of unitarity triangles). In our analysis we use an invariant formulation, independent of the choice of weak quark basis or the phase convention in the generalized Cabibbo-Kobayashi-Maskawa matrix. We identify the three weak-basis invariants, as well as the three imaginary parts of quartets B_{1-3} which, in the chiral limit, give the strength of CP violation beyond the SM. We find that for an extra vector-like quark |B_i| <= 10^{-4}, whereas for an extra sequential family |B_i| <= 10^{-2}.

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arXiv1998

Degenerate and Quasi Degenerate Majorana Neutrinos

We study mixing and CP violation of three left-handed Majorana neutrinos in the limit of exactly degenerate masses, identify the weak-basis invariant relevant for CP violation and show that the leptonic mixing matrix is parametrized only by two angles and one phase. After the lifting of the degeneracy, this parametrization accommodates the present data on atmospheric and solar neutrinos, as well as double beta decay. Some of the leptonic mixing ansatze suggested in the literature correspond to special cases of this parametrization.

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arXiv1998

Electroweak Baryogenesis in the Presence of an Isosinglet Quark

We consider the possibility of electroweak baryogenesis in a simple extension of the standard model with an extra singlet complex scalar and a vector-like down quark. We show that in the present model the first-order electroweak phase transition can be strong enough to avoid the baryon asymmetry washout by sphalerons and that the CP-violating effects can be sufficient to explain the observed baryon-to-entropy ratio nB/s ~ 10^(-10). Other appealing features of the model include the generation of a CKM phase from spontaneous CP breaking at a high energy scale and a possible solution of the strong CP problem through the natural suppression of the parameter theta.

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arXiv1998

Quasi degenerate Neutrino Masses with Universal Strength Yukawa Couplings

A simple ansatz is proposed for neutrino and charged lepton mass matrices, within the framework of universal strength for Yukawa couplings. In this framework all Yukawa couplings have equal moduli and the flavour dependence is only in their phases. We take into account the solar neutrino deficit and the atmospheric neutrino anomaly, assuming three neutrino families only. The ansatz leads in a natural way to small mixing involving neutrinos of quasi degenerate masses, as required to explain the solar neutrino deficit in the nonadiabatic MSW solution, while having the large mixing necessary to explain the atmospheric neutrino anomaly.

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arXiv1997

How sensitive to FCNC can $B^0$ CP asymmetries be?

We show that the study of CP asymmetries in neutral B-meson decays provides a very sensitive probe of flavour-changing neutral currents (FCNC). We introduce two new angles, $α_{SM}$ and $β_{SM}$, whose main feature is that they can be readily obtained from the measurement of the CP asymmetries $a_{J/ψK_s}$, $a_{π^+ π^-}$ and the ratio $R_u \equiv|V_{ud}V_{ub}^*|/|V_{cd}V_{cb}^*|$, providing a quantitative test of the presence of new physics in a model-independent way. Assuming that new physics is due to the presence of an isosinglet down-type quark, we indicate how to reconstruct the unitarity quadrangles and point out that the measurements of the above asymmetries, within the expected experimental errors, may detect FCNC effects, even for values of $|\sum_{i=1}^3 V_{id} V_{ib}^* / (V_{td} V_{tb}^*)| $ at the level of a few times $10^{-2}$.

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arXiv1996

A New Approach to the Yukawa Puzzle

We do a systematic analysis of the question of calculability of CKM matrix elements in terms of quark mass ratios, within the framework of the hypothesis of universality of strength for Yukawa couplings (USY), where all Yukawa couplings have equal moduli, and the flavor dependence is only in their phases. We use the fact that the limit $m_u=m_d=0$ is specially simple in USY, to construct the various ansätze. It is shown that the experimentally observed CKM matrix can be obtained within USY ansätze corresponding to simple relations among phases of Yukawa couplings. Within USY, one finds a natural explanation why Cabibbo mixing is significantly larger than the other CKM mixings. In the most successful of the USY ansätze, one obtains in leading order: $|V_{us}|=\sqrt{m_d/{m_s}}$ ; $ |V_{cb}|=\sqrt{2}(m_s/m_b)$ $ |V_{ub}|= (1/\sqrt{2})\sqrt{m_dm_s/m_b^2}$ ; $|V_{td}|= 3|V_{ub}|$. We study the behavior of this USY ansatz under the renormalization group.

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arXiv1996

Relating the scalar flavour-changing neutral couplings to the CKM matrix

We build a class of two-Higgs-doublet models in which the flavour-changing couplings of the neutral scalars are related in an exact way to elements of the quark mixing matrix. In this framework, we explore the different possibilities for CP violation and find some interesting scenarios, like a realization of the superweak idea without CP violation in the $B$-meson system. In another scenario the neutral scalars can be relatively light, and their contributions to the $B^0$--$\bar{B}^0$ transitions can alter the standard-model predictions for CP violation in that system.

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arXiv1995

CP-Violating Observables in Tagged $D^0$--$\bar{D}^0$ Decays

We perform a systematic study of CP-violating observables in \ddbar\ decays. We show that given a final state, which may be a CP eigenstate or not, all the observables can in principle be measured if decay-time information is available and if, for CP non-eigenstates, both decay rates into $f$ and $\bar f$ are considered. As an illustration of our analysis we discuss an effective superweak scenario.

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arXiv1995

Predicting Vckm with Universal Strength of Yukawa Couplings

We study the question of calculability of the Cabibbo-Kobayashi-Maskawa (CKM) matrix elements within the framework of universal strength for Yukawa couplings (USY). We first classify all solutions leading to $m_u=m_d=0$ within USY and then suggest a highly predictive ansatz where all the moduli of the CKM matrix elements are correctly predicted in terms of quark mass ratios, with no free parameters.

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