Neutral Current Sector Research Articles

New physics effects in purely leptonic B^*_s decays

Recently several measurements in the neutral current sector brightarrow s l^+l^- (l=e or mu ) as well as in the charged current sector b rightarrow c tau {bar{nu }} show significant deviations from their Standard Model predictions. It has been shown that two different new physics solutions can explain all the anomalies in brightarrow s l^+l^- sector. Both these solutions are in the form of linear combinations of the two operators ({bar{s}}gamma ^{alpha }P_Lb)({bar{mu }}gamma _{alpha }mu ) and ({bar{s}}gamma ^{alpha }P_Lb)({bar{mu }}gamma _{alpha }gamma _5mu ). We show that the longitudinal polarization asymmetry of the muons in B^*_srightarrow mu ^+,mu ^- decay is a good discriminant between the two solutions if it can be measured to a precision of 10%, provided the new physics Wilson coefficients are real. If they are complex, the theoretical uncertainties in this asymmetry are too large to provide effective discrimination. We also investigate the potential impact of b rightarrow c tau {bar{nu }} anomalies on b rightarrow s tau ^+ tau ^- transitions. We consider a model where the new phyics contributions to these two transitions are strongly correlated. We find that the branching ratio of B^*_srightarrow tau ^+,tau ^- can be enhanced by three orders of magnitude.

Flavour physics and extra-dimensions

Randall-Sundrum (RS) model of warped extra-dimensions were originally proposed to explain the Planck-weak scale hierarchy. It was soon realised that modifications of the original setup, by introducing the fields in the bulk, has several interesting features. In particular it imbues a rich flavour structure to the fermionic sector thereby offering an understanding of the Yukawa hierarchy problem. This construction is also useful in explaining the recently observed deviations in the decay of the B mesons. We consider two scenarios to this effect : A) Right handed muon fields coupled more to NP that the corresponding muon doublets (unorthodox case). Non-universality exists in the right handed sector. B) Standard scenario with anomalies explained primarily by non-universal couplings to the lepton doublets. Further, we establish correlation with the parameter space consistent with the flavour anomalies in the neutral current sector and obtain predictions for rare K- decay which are likely to be another candle for NP with increased precision. The prediction for rare K- decays are different according to the scenario, thereby serving as a useful discriminatory tool. We also discussthe large flavour violation in the lepton sector and present an example with the implementation of bulk leptonic MFV which is essential to realize the model with low KK scales. Further we consider a radical solution, called GUT RS models, where the RS geometry can work as theory of flavour in the absence of flavour symmetries. In this case the low energy brane corresponds to the GUT scale as a result of which RS is no longer solution to the gauge hierarchy problem. The Kaluza Klein (KK) modes in this setup are naturally heavy due to which the low energy constraints can be easily avoided. We use this framework to discuss the supersymmetric version of the RS model and provide means to test this scenario by considering rare lepton decays like τ → μγ.

Flavour violation in a supersymmetric T′ model

We describe the phenomenology of the flavour changing neutral current sector of a supersymmetric model, invariant under the T' discrete flavour group. This model has been proposed in Ref.[1] and describes realistic leptonic and hadronic masses and mixings and predicts the amount of flavour changing in terms of the small flavour breaking parameter u in [0.007, 0.05]. For small values of u, the model almost reduces to the MSUGRA framework, while sizable deviations from MSUGRA can be, instead, observed when larger values of u and tan(beta) are considered. We analyse in detail the T' BR(mu --> e gamma) prediction, concerning the leptonic sector, while for the hadronic sector we concentrate on b --> s gamma and neutral B meson mass differences. Moreover, for the first time a comparative study on leptonic and hadronic observables for the T' model is performed. The experimental data on FCNC observables severely constrain the model in the small m_0 region. Conversely for larger m_0, the T' model satisfies all the bounds.

Windows over a new low energy axion

We outline some general features of possible extensions of the Standard Model that include anomalous U(1) gauge symmetries, a certain number of axions and their mixings with the CP-odd Higgs sector. As previously shown, after the mixing one of the axions becomes a physical pseudoscalar (the axi-Higgs) that can take the role of a modified QCD axion. It can be driven to be very light by the same non-perturbative effects that are held responsible for the solution of the strong CP-problem. At the same time the axi-Higgs has a sizeable gauge interaction, which is not allowed to the Peccei–Quinn axion, possibly explaining the PVLAS results. We point out that the Wess–Zumino term, typical of these models, can be both interpreted as an anomaly inflow from higher dimensional theories (second window) but also as a result of partial decoupling of an extra Higgs sector (and of a fermion) that leaves behind an effective anomalous Abelian theory (first window) in a broken Stückelberg phase. The possibility that the axi-Higgs can be heavy, of the order of the Higgs mass or larger, however, cannot be excluded. The potentialities for the discovery of this particle and of anomaly effects in the neutral current sector at the LHC are briefly discussed in the context of a superstring inspired model (second window), but with results that remain valid also if any of the two possibilities is realized in Nature.

Updated parameter limits of the left-right symmetric model

Bounds of the neutral current sector parameters of the left-right symmetric model are investigated taking into account the low-energy data, LEP-data and CDF-result for the top mass m t = 174 ± 10 −12 +13. It is found that in the case of the minimal scalar sector with a left- and a right-handed triplet and a bidoublet Higgses the mass of the heavy neutral gauge boson M Z′ should be larger than 1.2 TeV, assuming equal left- and right-handed gauge couplings and a negligible VEV of the left-handed triplet. For larger values of the ratio g L g R smaller values of M Z′ are allowed.

The τ polarization measurement at LEP and the nature of the τ-W-ντ coupling

The recent measurement of the τ polarization at LEP is explicitly analyzed in terms of the τ -W- ν τ . When the validity of the neutral current prediction for P τ and lepton universality in the neutral current sector is assumed, the analysis of the leptonic τ decays excludes the possibility of a (V+A)-(V+A) interaction, which is not constrained by the existing ϱ measurement, 3σ level. From the semileptonic τ decays, a value of the chirality parameter is obtained, h ν τ = −2g V g A (g V 2 + g A 2 ) = 1.22±0.33 , which is consistent with h ν τ = 1 expected for pure V-A coupling and favours a negative helicity of the v τ .

The gluonic contribution to polarized deep-inelastic scattering in the standard model

We study the gluonic contribution to the spin-dependent deep-inelastic proton structure functions g 1 p , g 3 p and g 4 p that appear when full neutral current sector (i.e. γ-, Z 0-exchange and γZ 0-interference) and the charged current sector (W ±-exchange) are taken into account, which is indispensable if polarized lepton-proton experiments at large Q 2 should be carried out in the future. It is shown that the gluonic contribution enters in such a way that experimental results for the first moment of the structure functions will not help us to disentangle the recently proposed scenarios for the proton spin, but that one might obtain information about the polarized valence quark content of the proton from a measurement of the parity violating structure functions g 3 p or g 4 p .

Neutral current effects in a model with a naturally large neutrino magnetic moment

A phenomenological analysis is performed on the neutral current sector of a model recently proposed by Barbieri and Mohapatra, which predicts in a natural way a large electron neutrino magnetic moment. Limits on the mass and the mixing of an extra Z′ boson predicted by the model are extracted from the present measurements of the standard model ϱ parameter and from the present neutral current data. Prospects at LEP and future hadron colliders are also given.

Calculability of the Flavor-Mixing Parameters

Despite the impressive success of the standard model, the repetition of the family-generation of leptons and quarks persists as one of the most challenging unresolved problems in particle physics. The fermion family problem is to understand the fermion mass hierarchy of the leptons and quarks as observed in family-generations. Any consistent solution to this problem should also describe the flavor mixings in the charged-current sector including the CP-invariance violation as well as the empirical flavor-conservation up to the αGf order in the neutral weak interaction sector. In the standard model the fermion masses are generated by spontaneous breaking of the electroweak symmetry via the Higgs mechanism, but they are arbitrary free parameters. In other words, the standard model can be adjusted to accommodate the fermion family-generations but lacks calculability for the fermion mass spectrum. Because there are several fermion generations, the weak interaction eigenstates are not necessarily the mass eigenstates and therefore there will inevitably be mixings between different flavor states in the charged current sector thus introducing further arbitrary parameters to the standard model. However, the neutral-current sector is flavor diagonal up to αGf order in the standard model because the fermions in each generation are assigned in the same dimensional representations.

A neutrino with a large magnetic moment and a naturally small mass

We examine the question of having large magnetic moment for the electron neutrino (needed in one proposed solution to the solar neutrino puzzle) while keeping its mass naturally small in unified gauge theories. We present a phenomenologically viable model realizing this idea in an SU(3) L×U(1) extension of the standard model, where one embeds an SU(2) gauge symmetry transforming a neutrino to an antineutrino. The model predicts new physics in the neutral and charged current sector, which are testable in the near future.

Neutral current phenomenology of supersymmetric SU(2) × U(1) × ≈U(1) models

The neutral current sector of a class of supersymmetric SU(2) × U(1) × ≈ U(1) models is parametrized. Bounds on the neutral boson masses are obtained from the low energy data, and the implications of future experimental findings for these models are discussed.

The link between the emergence of the GIM mechanism and the nature of the neutral-current parity violation

We show that minimization of the Higgs potential within the unifying symmetry [SU(4)] 4 together with the requirement that the GIM mechanism should emerge as a consequence of spontaneous breakdown of the symmetry on the flavor side leaves us with onechoice regarding the nature of parity violation: charged current as well as neutral current parity violations must have one and the same origin; if the former is à la SU(2) L×U(1), so must be the latter. Furthermore, there appear to be only two possible alternative forms for the low-energy electroweak symmetry: (i) the familiar SU(2) L×U(1), and (ii) and extended symmetry SU(2) L×U(1) L×Ũ(1) R differing from the former only in the parity-conserving neutral current sector.

Neutrino pairing as the origin of parity violation in a chiral flavor theory of weak interactions

In this paper we study the space-time and internal-symmetry aspects of weak interactions together with their interplay. We propose that the weak interaction be obtained by gauging the strong-interaction chiral flavor group and consider explicitly the group $\mathrm{SU}{(4)}_{L}\ifmmode\times\else\texttimes\fi{}\mathrm{SU}{(4)}_{R}\ifmmode\times\else\texttimes\fi{}\mathrm{U}(1)$ as a candidate unified gauge theory of weak and electromagnetic interactions. In the symmetry limit the theory is both parity conserving and flavor conserving. The neutrinos in the theory are four-component spinors. We introduce a new dynamical mechanism for obtaining parity violation in weak interactions, namely neutrino pairing, in which we allow pairs of right-handed neutrinos to condense into the vacuum. The group theory associated with this pairing produces maximal parity violation in both the lepton and quark charged-current sectors of the weak interaction and also the conventional Weinberg mixing pattern in the neutral-current sector, while keeping the observed left-handed electron and muon neutrinos massless. In the right-handed sector of the theory, however, the pairing causes the right-handed electron and muon neutrinos to combine into one massive fourcomponent spinor. The unusual phenomenology associated with this new observable massive neutral lepton is studied in detail. Using other representations of the gauge group we also break flavor spontaneously to provide a group-theoretical origin for the Cabibbo angle. Through our approach we can incorporate current algebra and the Gell-Mann-Oakes-Renner Hamiltonian phenomenology into weak-interaction theory. We discuss briefly the implications of our work for constructing grand-unified theories of the strong, electromagnetic, and weak interactions.

SU(2)L ⊗ SU(2)R ⊗ U(1) gauge models confronting neutral current data

We study the neutral current (NC) sector of the general SU(2) L ⊗ SU(2) R ⊗ U(1) gauge model of weak and electromagnetic interactions with L-R symmetric couplings g L = g R , and with conventional fermion multiplet assignments. We make a least-squares fit of fourteen experimental quantities in the full parameter space, as well as in several sub-spaces defining specific models. We discuss all previously studied SU(2) L ⊗ SU(2) R ⊗ U(1) models, the Weinberg-Salam model, an SU(2) ⊗ U(1) vector model, with and without parity violation in atoms. We give 90% confidence ranges of all the parameters, NC coupling constants, and the muon forward-backward asymmetry in unpolarized colliding e + e − beams. Among the coupling constants the same two linear relations must hold as given in SU(2) ⊗ U(1), and in addition the new relations β = −2g A , δ = 0, α − β + 3( γ − δ ) = 0, α − 3 γ − 4(h AA + g A ) = 0, β = −4h VA , δ = 0 .

The price of natural parity conservation in the neutral current sector

We study the requirement that the neutral current sector in SU(2) L ⊗ SU(2) R ⊗ U(1) gauge models be naturally parity conserving in the tree approximation, and therefore calculable. A class of models which reproduces automatically the Weinberg-Salam neutrino neutral current phenomenology cannot be made natural in this sense and at the same time compatible with charged current phenomenology. At best one has small but uncalculable violations of both Weinberg-Salam neutrino phenomenology and parity conservation in neutral currents, as well as proliferation of fermion and scalar fields and possible difficulties with neutral strangeness changing transitions. A second class of models has calculable p parity violation in neutral currents and allows a more restricted fermion and scalar sector, but the Weinberg-Salam neutrino phenomenology does not emerge naturally. For the latter class we evaluate the parity violation induced by radiative corrections, relevant to atomic physics experiments. While technically calculable, it is not practically so, because of their dependence on unknown parameters of the Higgs sector. We also discuss radiative corrections to Cabibbo universality in these models.

SU(3) L × SU(3) R as gauge groups of weak and associated charges

Consequences of an SU(3) L × SU(3) R gauge group underlying weak-electromagnetic and associated interactions are studied. A specific pattern of symmetry breaking relates the lepton mass spectrum and mixing pattern, the effective (anti) neutrino-induced neutral current and (approximate) parity conservation in the neutral-current sector.