Flavor Space Research Articles

Light colored scalar as messenger of up-quark flavor dynamics in grand unified theories

The measured forward-backward asymmetry in the t tbar production at the Tevatron might be explained by the additional exchange of a colored weak singlet scalar. Such state appears in some of the grand unified theories and its interactions with the up-quarks are purely antisymmetric in flavor space. We systematically investigate the resulting impact on charm and top quark physics. The constraints on the relevant Yukawa couplings come from the experimentally measured observables related to D0--D0bar oscillations, as well as di-jet and single top production measurements at the Tevatron. After fully constraining the relevant Yukawa couplings, we predict possible signatures of this model in rare top quark decays. In a class of grand unified models we demonstrate how the obtained information enables to constrain the Yukawa couplings of the up-quarks at very high energy scale.

Flavour constraints on the Aligned Two-Higgs-Doublet Model and CP violation

The Aligned Two-Higgs-Doublet Model (A2HDM) [1] describes a particular way of enlarging the scalar sector of the Standard Model (SM), with a second Higgs doublet which is aligned to first the one in flavour space. This implies the absence of flavour-changing neutral currents (FCNCs) at tree level and the presence of three complex parameters. Within this general approach, we analyze the charged Higgs phenomenology, including CP asymmetries in the K and B systems [2].

Charged-Higgs phenomenology in the aligned two-Higgs-doublet model

The alignment in flavour space of the Yukawa matrices of a general two-Higgs-doublet model results in the absence of tree-level flavour-changing neutral currents. In addition to the usual fermion masses and mixings, the aligned Yukawa structure only contains three complex parameters, which are potential new sources of CP violation. For particular values of these three parameters all known specific implementations of the model based on discrete Z_2 symmetries are recovered. One of the most distinctive features of the two-Higgs-doublet model is the presence of a charged scalar. In this work, we discuss its main phenomenological consequences in flavour-changing processes at low energies and derive the corresponding constraints on the parameters of the aligned two-Higgs-doublet model.

THE SENSORY PROFILES OF KIWIFRUIT HYBRIDS INVOLVING ACTINIDIA ERIANTHA

In an attempt to broaden the consumer appeal of kiwifruit, breeders are developing a range of novel fruit with enhanced convenience characteristics, particularly ease of eating. Kiwifruit skin is normally unpalatable, so consumers usually scoop out the flesh with a spoon. The ability to remove flesh cleanly without the need for utensils may therefore improve perceptions of convenience. Selections of Actinidia chinensis and A. deliciosa (particularly the green kiwifruit A. deliciosa var. deliciosa 'Hayward') are poor peeling with good flavour, while selections of a related species Actinidia eriantha are generally peelable but with typically poor flavour. Hybrid populations between Actinidia deliciosa (A. Chev.) C.F. Liang et A.R. Ferguson and A. eriantha Benth. include selections that are highly peelable, but with unknown flavour profiles. The aim of the current study was to understand the range of sensory attributes (including the existence of novel flavours/odours/textures) present in A. deliciosa and A. eriantha hybrids in relation to 'Hayward' and another commercially available cultivar, 'Hort16A' (A. chinensis) to determine whether hybridisation may be a viable approach for breeding peelable cultivars with appealing flavours. The current study confirmed that introduction of A. deliciosa parentage to A. eriantha selections produces fruit that occupy a much broader flavour space than those of A. eriantha on their own, and indicates that hybridisation is a feasible means of creating good flavour cultivars with added convenience attributes.

Constraints on the Aligned Two-Higgs-Doublet Model and CP violation

The Aligned Two-Higgs-Doublet Model (A2HDM) [Pich A. and Tuzon P. Yukawa Alignment in the Two-Higgs-Doublet Model. Phys. Rev. D80 , (2009), 091702. 0908.1554 ] describes a particular way of enlarging the scalar sector of the Standard Model (SM), with a second Higgs doublet which is aligned to first the one in flavour space. This implies the absence of flavour-changing neutral currents (FCNCs) at tree level and the presence of three complex parameters. Within this general approach, we analyze the charged Higgs phenomenology, including CP asymmetries in the K and B systems [Jung M., Pich A. and Tuzon P. Charged-Higgs phenomenology in the Aligned two-Higgs-doublet model 1006.0470 ].

Flavour geometry and effective Yukawa couplings in the MSSM

We present a new geometric approach to the flavour decomposition of an arbitrary soft supersymmetry-breaking sector in the MSSM. Our approach is based on the geometry that results from the quark and lepton Yukawa couplings, and enables us to derive the necessary and sufficient conditions for a linearly-independent basis of matrices related to the completeness of the internal [SU(3)⊗U(1)]5 flavour space. In a second step, we calculate the effective Yukawa couplings that are enhanced at large values of tan β for general soft supersymmetry-breaking mass parameters. We highlight the contributions due to non-universal terms in the flavour decompositions of the sfermion mass matrices. We present numerical examples illustrating how such terms are induced by renormalization-group evolution starting from universal input boundary conditions, and demonstrate their importance for the flavour-violating effective Yukawa couplings of quarks.

Yukawa alignment in the two-Higgs-doublet model

In multi-Higgs-doublet models the alignment in flavour space of the relevant Yukawa matrices guarantees the absence of tree-level flavour-changing couplings of the neutral scalar fields. We analyze the consequences of this condition within the two-Higgs-doublet model and show that it leads to a generic Yukawa structure which contains as particular cases all known specific implementations of the model based on Z_2 symmetries. All possible freedom in the Yukawa sector gets parametrized in terms of three complex couplings. In spite of having flavour conservation in the neutral scalar couplings, the phases of these three parameters represent potential new sources of CP violation.

Composite Higgs-mediated flavor-changing neutral current

We discuss how, in the presence of higher-dimensional operators, the standard model fermion masses can be misaligned in flavor space with the Yukawa couplings to the Higgs boson, even with only one Higgs doublet. Such misalignment results in flavor-violating couplings to the Higgs and hence flavor-changing neutral current processes from tree-level Higgs exchange. We perform a model-independent analysis of such an effect. Specializing to the framework of a composite Higgs with partially composite standard model gauge and fermion fields, we show that the constraints on the compositeness scale implied by ${ϵ}_{K}$ can be generically as strong as those from the exchange of heavy spin-1 resonances if the Higgs is light and strongly coupled to the new states. In the special and well-motivated case of a composite pseudo-Goldstone Higgs, we find that the shift symmetry acting on the Higgs forces an alignment of the fermion mass terms with their Yukawa couplings at leading order in the fermions' degree of compositeness, thus implying much milder bounds. As a consequence of the flavor-violating Higgs couplings, we estimate $\mathrm{BR}(t\ensuremath{\rightarrow}ch)\ensuremath{\sim}{10}^{\ensuremath{-}4}$ and $\mathrm{BR}(h\ensuremath{\rightarrow}tc)\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ both for a pseudo-Goldstone (if ${t}_{R}$ is fully composite) and for a generic composite Higgs. By virtue of the AdS/CFT correspondence, our results directly apply to 5-dimensional Randall-Sundrum compactifications.

GenEvA (II): a phase space generator from a reweighted parton shower

We introduce a new efficient algorithm for phase space generation. A parton shower is used to distribute events across all of multiplicity, flavor, and phase space, and these events can then be reweighted to any desired analytic distribution. To verify this method, we reproduce the e+e- -> n jets tree-level result of traditional matrix element tools. We also show how to improve tree-level matrix elements automatically with leading-logarithmic resummation. This algorithm is particularly useful in the context of a new framework for event generation called GenEvA. In a companion paper [arXiv:0801.4026], we show how the GenEvA framework can address contemporary issues in event generation.

Multi-angle effects in collective supernova neutrino oscillations

We study two-flavor collective neutrino oscillations in the dense-neutrino region above the neutrino sphere in a supernova (SN). The angular dependence of the neutrino-neutrino interaction potential causes ‘multi-angle’ effects that can lead either to complete kinematical decoherence in flavor space or only to small differences between different trajectories. This nonlinear system switches abruptly between ‘self-maintained coherence’ and ‘self-induced decoherence’ among the angular modes, depending on the strength of the deleptonization flux. For a realistic SN the quasi single-angle behavior is probably typical, simplifying the numerical treatment and probably allowing for the survival of observational features of flavor oscillations.

F-term induced flavor mass spectrum

New mechanism of generating flavor mass spectrum is proposed by using an O'Raifeartaigh-type supersymmetry breaking model. A desired bilinear form of fermion mass spectrum is naturally realized through F-components of gauge-singlet (nonet of SU(3) flavor symmetry) superfields, and the suitable charged-lepton mass relation is reproduced. The charged-slepton mass spectrum is non-degenerate in general, and can be even hierarchical (proportional to the charged-lepton masses in the specific case). Flavor changing neutral processes are suppressed since the charged-lepton and slepton (except for right-handed sneutrino) mass matrices are diagonalized simultaneously in the flavor space. The right-handed sneutrinos are light with the similar ratio to the lepton sector (νR/e ~ mν/me).

Simple picture for neutrino flavor transformation in supernovae

We can understand many recently discovered features of flavor evolution in dense, self-coupled supernova neutrino and antineutrino systems with a simple, physical scheme consisting of two quasistatic solutions. One solution closely resembles the conventional, adiabatic single-neutrino Mikheyev-Smirnov-Wolfenstein (MSW) mechanism, in that neutrinos and antineutrinos remain in mass eigenstates as they evolve in flavor space. The other solution is analogous to the regular precession of a gyroscopic pendulum in flavor space, and has been discussed extensively in recent works. Results of recent numerical studies are best explained with combinations of these solutions in the following general scenario: (1) Near the neutrino sphere, the MSW-like many-body solution obtains. (2) Depending on neutrino vacuum mixing parameters, luminosities, energy spectra, and the matter density profile, collective flavor transformation in the nutation mode develops and drives neutrinos away from the MSW-like evolution and toward regular precession. (3) Neutrino and antineutrino flavors roughly evolve according to the regular precession solution until neutrino densities are low. In the late stage of the precession solution, a stepwise swapping develops in the energy spectra of ${\ensuremath{\nu}}_{e}$ and ${\ensuremath{\nu}}_{\ensuremath{\mu}}/{\ensuremath{\nu}}_{\ensuremath{\tau}}$. We also discuss some subtle points regarding adiabaticity in flavor transformation in dense-neutrino systems.

Importance of the Heavier Singlet Neutrinos in Leptogenesis

We argue that fast interactions of the lightest singlet neutrino N1 would project part of a preexisting lepton asymmetry L{p} onto a direction that is protected from N1 washout effects, thus preventing it from being erased. In particular, we consider an asymmetry generated in N2 decays, assuming that N1 interactions are fast enough to bring N1 into full thermal equilibrium. If N1 decays occur at T > or = 10{9} GeV, that is, before the muon Yukawa interactions enter into thermal equilibrium, then generically part of L{p} survives. In this case some of the constraints implied by the standard N1 leptogenesis scenario hold only if L{p} approximately 0. For T < or = 10{9} GeV, L{p} is generally erased, unless special alignment or orthogonality conditions in flavor space are realized.

Erratum: Self-induced conversion in dense neutrino gases: Pendulum in flavor space [Phys. Rev. D74, 105010 (2006)

Neutrino-neutrino interactions can lead to collective flavour conversion effects in supernovae and in the early universe. We demonstrate that the case of "bipolar" oscillations, where a dense gas of neutrinos and antineutrinos in equal numbers completely converts from one flavour to another even if the mixing angle is small, is equivalent to a pendulum in flavour space. Bipolar flavour conversion corresponds to the swinging of the pendulum, which begins in an unstable upright position (the initial flavour), and passes through momentarily the vertically downward position (the other flavour) in the course of its motion. The time scale to complete one cycle of oscillation depends logarithmically on the vacuum mixing angle. Likewise, the presence of an ordinary medium can be shown analytically to contribute to a logarithmic increase in the bipolar conversion period. We further find that a more complex (and realistic) system of unequal numbers of neutrinos and antineutrinos is analogous to a spinning top subject to a torque. This analogy easily explains how such a system can oscillate in both the bipolar and the synchronised mode, depending on the neutrino density and the size of the neutrino-antineutrino asymmetry. Our simple model applies strictly only to isotropic neutrino gasses. In more general cases, and especially for neutrinos streaming from a supernova core, different modes couple to each other with unequal strength, an effect that can lead to kinematical decoherence in flavour space rather than collective oscillations. The exact circumstances under which collective oscillations occur in non-isotropic media remain to be understood.

GENXICC: A generator for hadronic production of the double heavy baryons [formula omitted], [formula omitted] and [formula omitted

We write down a generator program for the hadronic production of the double-heavy baryons Xi(cc), Xi(bc), and Xi(bb) according to relevant publications. We name it as GENXICC and we test it by comparing its numerical results with those in references. It is written in a PYTHIA-compatible format and it can be easily implemented into PYTHIA. GENXICC is also written in modularization manner, with make, a GNU C compiler, one may apply the generator to various situations or experimental environments very conveniently.

Nonlocality effects on spin-one pairing patterns in two-flavor color superconducting quark matter and compact star applications

We study the influence of nonlocality in the interaction on two spin-one pairing patterns of two-flavor quark matter: the anisotropic blue-color pairing besides the usual two-color superconducting matter (2SCb), in which red and green colors are paired, and the color-spin locking phase (CSL). The effect of nonlocality on the gaps is rather large and the pairings exhibit a strong dependence on the form factor of the interaction, especially in the low-density region. The application of these small spin-one condensates for compact stars is analyzed: the early onset of quark matter in the nonlocal models may help to stabilize hybrid star configurations. While the anisotropic blue-quark pairing does not survive a big asymmetry in flavor space as imposed by the charge neutrality condition, the CSL phase as a flavor independent pairing can be realized as neutral matter in compact star cores. However, smooth form factors and the mismatch between the flavor chemical potential in neutral matter make the effective gaps of the order of magnitude ≃10 keV, and a more systematic analysis is needed to decide whether such small gaps could be consistent with the cooling phenomenology.

Tri-bimaximal neutrino mixing and quark masses from a discrete flavour symmetry

We build a supersymmetric model of quark and lepton masses based on the discrete flavour symmetry group T ′ , the double covering of A 4 . In the lepton sector our model is practically indistinguishable from recent models based on A 4 and, in particular, it predicts a nearly tri-bimaximal mixing, in good agreement with present data. In the quark sector a realistic pattern of masses and mixing angles is obtained by exploiting the doublet representations of T ′ , not available in A 4 . To this purpose, the flavour symmetry T ′ should be broken spontaneously along appropriate directions in flavour space. In this paper we fully discuss the related vacuum alignment problem, both at the leading order and by accounting for small effects coming from higher-order corrections. As a result we get the relations: m d / m s ≈ | V u s | and m d / m s ≈ | V t d / V t s | .

On the impact of flavour oscillations in leptogenesis

When lepton flavour effects in thermal leptogenesis are active, they introduceimportant differences with respect to the case in which they are neglected,the so-called one-flavour approximation. We investigate analytically andnumerically the transition from the one-flavour to the two-flavour case when theτ-lepton flavour becomes distinguishable from the other two flavours. We study the impactof the oscillations of the asymmetries in lepton flavour space on the final leptonasymmetries, for the hierarchical right-handed neutrino mass spectrum. Flavouroscillations project the lepton state on the flavour basis very efficiently. We concludethat flavour effects are relevant typically for M1 1012 GeV, where M1 is the mass of the lightest right-handed neutrino.

Self-induced conversion in dense neutrino gases: Pendulum in flavor space

Neutrino-neutrino interactions can lead to collective flavor conversion effects in supernovae and in the early universe. We demonstrate that the case of bipolar oscillations, where a dense gas of neutrinos and antineutrinos in equal numbers completely converts from one flavor to another even if the mixing angle is small, is equivalent to a pendulum in flavor space. Bipolar flavor conversion corresponds to the swinging of the pendulum, which begins in an unstable upright position (the initial flavor), and passes through momentarily the vertically downward position (the other flavor) in the course of its motion. The time scale to complete one cycle of oscillation depends logarithmically on the vacuum mixing angle. Likewise, the presence of an ordinary medium can be shown analytically to contribute to a logarithmic increase in the bipolar conversion period. We further find that a more complex (and realistic) system of unequal numbers of neutrinos and antineutrinos is analogous to a spinning top subject to a torque. This analogy easily explains how such a system can oscillate in both the bipolar and the synchronized mode, depending on the neutrino density and the size of the neutrino-antineutrino asymmetry. Our simple model applies strictly only to isotropic neutrino gasses. In more general cases, and especially for neutrinos streaming from a supernova core, different modes couple to each other with unequal strength, an effect that can lead to kinematical decoherence in flavor space rather than collective oscillations. The exact circumstances under which collective oscillations occur in nonisotropic media remain to be understood.

Flavour issues in leptogenesis

We study the impact of flavour in thermal leptogenesis, including the quantum oscillationsof the asymmetries in lepton flavour space. In the Boltzmann equations we find differentnumerical factors and additional terms which can affect the results significantly. Fordegenerate light neutrinos, the upper bound on the CP asymmetry in a specific flavour isweaker than the bound on the sum. For hierarchical right-handed neutrinos—when flavourdynamics is included—there is no model-independent limit on the light neutrino massscale, and the lower bound on the reheat temperature is relaxed by a factor of∼(3–10).