Narrow Results

We study the CP-violating asymmetry , which arises, in η → π⁺π^-e⁺e^-, from the angular correlation of the e⁺e^- and π⁺ π^- planes due to the interference between the magnetic and electric decay amplitudes. With the phenomenologically determined magnetic amplitude and branching ratio as input, the asymmetry, induced by the electric bremsstrahlung amplitude through the CP-violating decay η → π⁺π^-, and by an unconventional tensor type operator, has been estimated respectively. The upper bound of from the former is about 10^-3, and the asymmetry from the latter might be up to O(10^-2). One can therefore expect that this CP asymmetry would be an interesting CP-violating observable for the future precise measurements in the η factories.

Possible manifestations of new physics in rare (exotic) decays of orthopositronium (o - Ps) are briefly reviewed. It is pointed out that models with infinite additional dimension(s) of Randall–Sundrum type predict disappearance of orthopositronium into additional dimension(s). The experimental signature of this effect is the invisible decay of orthopositronium. We point out that this process may occur at a rate within two or three orders of magnitude of the present experimental upper limit. We also propose a model with a light weakly interacting boson leading to o - Ps → invisible decays at the experimentally interesting rate. We discuss this in details and stress that the existence of invisible decay of orthopositronium in vacuum could explain the o - Ps decay rate puzzle. Thus, our result enhances the existing motivation and justifies efforts for a more sensitive search for o - Ps → invisible decay in a near future experiment.

Using the most general, model-independent effective Hamiltonian, the rare decay with polarized photon is studied. The sensitivity of the branching ratio and photon polarization to the new Wilson coefficients are investigated. It is shown that these physical observables are sensitive to the vector and tensor type interactions, which would be useful in search of new physics.

We investigate the CP violating asymmetry, the forward backward asymmetry and the CP violating asymmetry in the forward–backward asymmetry for the inclusive B → X_dℓ⁺ℓ^- decays for the ℓ = e,μ,τ channels in the standard model. It is observed that these asymmetries are quite sizeable and B → X_dℓ⁺ℓ^- decays seem promising for investigating CP violation.

We review the effective Lagrangian of the Higgs penguin in the Standard Model and its minimal supersymmetric extension (MSSM). As a master application of the Higgs penguin, we discuss in some detail the B-meson decays into a lepton–antilepton pair. Furthermore, we explain how this can probe the Higgs sector of the MSSM provided that some of these decays are seen at Tevatron Run II and B-factories. Finally, we present a complete list of observables where the Higgs penguin could be strongly involved.

We consider the effect of new physics on the branching ratio of B_s → l⁺l^-γ where l = e, μ. If the new physics is of the form scalar/pseudoscalar, then it makes no contribution to B_s → l⁺l^-γ, unlike in the case of B_s → l⁺l^-, where it can potentially make a very large contribution. If the new physics is in the form of vector/axial-vector operators, then the present data on B → (K, K*) l⁺l^- does not allow a large enhancement for B(B_s → l⁺l^- γ). If the new physics is in the form of tensor/pseudotensor operators, then the data on B → (K, K*) l⁺l^- gives no useful constraint but the data on B → K* γ does. Here again, a large enhancement of B(B_s → l⁺l^-γ), much beyond the Standard Model expectation, is not possible. Hence, we conclude that the present data on b → s transitions allow a large boost in B(B_s → l⁺l^-) but not in B(B_s → l⁺l^-γ).

Our previous analysis on the contributions of mirror matter admixtures in ordinary hadrons to K_L→γγ is extended to study the relevance of such contributions to the K_L→μ⁺μ^- rare decay. The mixing angle of the admixtures previously determined to describe the enhancement phenomenon in two-body non-leptonic decays of strange hadrons is used, along with recent results for the description of the strong and electromagnetic interaction parts of the transition amplitudes. We find that these admixtures give a significant contribution with a small SU(2) breaking of only 2.8%, we also find a value of ~ -17.9° for the η–η′ mixing angle consistent with some of its determinations in the literature and a preferred negative value around -16 for the local counterterm contribution χ₁+χ₂ consistent with the existence of a unique counterterm assuming lepton universality. We conclude that those mixings may be relevant in low energy physics and should not be ignored.

The current experimental status of the searches for the very rare decays and is discussed. These channels are highly sensitive to various extensions of the Standard Model, especially in the scalar and pseudoscalar sector. The recent, most sensitive measurements from the CDF, ATLAS, CMS and LHCb collaborations are discussed and the combined upper exclusion limit on the branching fractions determined by the LHC experiments is shown to be 4.2×10^-9 for and 0.8×10^-9 for . The implications of these tight bounds on a selected set of New Physics models is sketched.

We study a new contribution, which is a tree-level transition from long-distance dynamics, to K⁺→π⁺ℓ⁺ℓ^- (ℓ = e, μ) decays. It is found that this tree-diagram can be calculated unambiguously and would generate the additional contribution to the vector, axial-vector, scalar and pseudoscalar form factor of the decay. Our analysis shows that these form factors are strongly suppressed and this long-distance contribution is well under control.

We study rare decays , and in NSIs. We calculate the NSIs branching ratios of these decays. There is a strong dependence of these on new physics parameter. They provide stringent constraints on , and (α, β = e, μ).

The form factors of the rare ${\mathrm{\Lambda }}_b\to n{l}^{+}{l}^{-}$ decays are calculated in the framework of the relativistic quark–diquark picture of baryons with the consistent account of the relativistic effects. Their momentum transfer squared dependence is determined explicitly in the whole accessible kinematical range. The decay branching fractions, forward–backward asymmetries and the fractions of longitudinally polarized dileptons are determined. The branching fraction of the rare ${\mathrm{\Lambda }}_b\to n{\mu }^{+}{\mu }^{-}$ decay are found to be $\mathrm{\text{Br}}({\mathrm{\Lambda }}_b\to n{\mu }^{+}{\mu }^{-})=(3.75\pm 0.38)\times 10^{-8}$ and thus could be measured at the LHC. Prediction for the branching fraction of the rare radiative ${\mathrm{\Lambda }}_b\to n\gamma$ decay is also given.

The contributions of the vector leptoquarks of Pati–Salam type to the branching ratios of $K_L^0,B^0,B_s\to l{l}^{\prime }$ decays are calculated with account of the fermion mixing in the leptoquark currents of the general type. Using the general parametrizations of the mixing matrices the lower vector leptoquark mass limit $m_V$$>$ 86 TeV is found from the current experimental data on these decays. The branching ratios of the decays $B^0,B_s\to l{l}^{\prime }$ predicted at $m_V$ = 86 TeV are calculated. These branching ratios for the decays $B^0,B_s\to {\mu }^{+}{\mu }^{-},e\mu$ are close to the experimental data whereas those for the decays $B^0\to e^{+}{e}^{-},e\tau ,\mu \tau$ and $B_s\to e^{+}{e}^{-}$ are by order of 2–4 less than their current experimental limits. For the decays $B_s\to e\tau ,\mu \tau$ these branching ratios are of orders $10^{-10}$ and $10^{-9}$, respectively. The predicted branching ratios will be useful in the current and future experimental searches for these decays.

We investigate the nonstandard neutrino interactions (NSI) in the rare decays of $B_c^{+}$ mesons involving neutrinos in the final state. It is suggested that the interference between Standard Model and nonstandard interaction can provide sizeable contribution. We calculate the limits on NSI free parameters (${𝜖}_{\tau \tau }^{uL}$, ${𝜖}_{\tau \tau }^{dL}$) and compare them with experimental data.

The contributions of the chiral gauge leptoquarks $V^{L,R}$ induced by the chiral four-color quark–lepton symmetry to the branching ratios of $K_L^0,B^0,B_s\to l_1{l}_2$ decays are calculated and analyzed using the general parametrizations of the fermion mixing matrices in the leptoquark currents. From the current experimental data on these decays under assumption $m_{V^L}\ll m_{V^R}$, the lower mass limit $m_{V^L}cos{\gamma }_L>5.68\mathrm{\text{TeV}}$ is found, which in particular case of equal gauge coupling constants gives $m_{V^L}>8.03\mathrm{\text{TeV}}$. The branching ratios of the decays under consideration predicted by the chiral gauge leptoquarks are calculated and analyzed in dependence on the leptoquark masses and the mixing parameters. It has shown that in consistency with the current experimental data, these branching ratios for $B_s,B^0\to \mu e$ decays can be close to their experimental limits and those for $B_s,B^0\to \tau e,\tau \mu$ decays can be of order of $10^{-7}$. The calculated branching ratios will be useful in the further experimental searches for these decays.

We study the $B\to K^{(\ast )}\nu \bar{\nu }$ decays within the Standard Model (SM) by using the relevant transition form factors obtained from the covariant confined quark model (CCQM) developed by us. The $B\to K$ and $B\to K^{\ast }$ transition form factors are calculated in the full kinematic $q^2$ range. The branching fractions are then calculated. It is shown that our results are in an agreement with those obtained in other theoretical approaches. Currently, the BaBar and Belle collaborations provide us by the upper limits at 90% confidence limit. The obtained bounds are roughly an order of magnitude larger than the SM predictions. This should stimulate experimental collaborations to set up experiments that allow one to obtain more accurate branching values, which is quite achievable on the updated LHCb and Belle machines. If the discrepancies between theory and experiment are confirmed, this will open up opportunities for constructing models with new particles and interactions leading to an extension of the SM.

After a decade of no measurements of pion and muon rare decays, PIBETA, a new experimental program is producing its first results. We report on a new experimental study of the pion beta decay, π⁺→π⁰e⁺ν, the π_e2γ radiative decay, π⁺→e⁺νγ, and muon radiative decay, . The new results represent four- to six-fold improvements in precision over the previous measurements. Excellent agreement with Standard Model predictions is observed in all channels except for one kinematic region of the π_e2γ radiative decay involving energetic photons and lower-energy positrons.

Using data taken on 2002 at the CERN SPS accelerator, the NA48/1 collaboration has made the first observation of the rare decays K_S→π⁰e⁺e^- and K_S→π⁰μ⁺μ^-. Seven events were found in the electron channel with an estimated background of . For the muon channel 6 events were observed with a background of . Using vector a matrix element with unity form factor the resulting branching ratios are: and .

A high statistics study of charged kaon decays, using a novel design for simultaneous K⁺/K^- beams and an upgraded NA48 setup, is in progress. The main goal is to measure the CP-violating asymmetry in the matrix element linear slope of the K^±→π⁺π^-π^± decays with an accuracy of 2.2×10^-4. In addition, the analogous CP-violating asymmetry is measured in K^±→π⁰π⁰π^± decays. More than 10⁶ K_e4 decays allow to measure the scattering length parameter with an accuracy better than 0.01, and some other rare decays of charged kaons are studied as well.

Results of a search for the Flavor-Changing Neutral Current decay using collision data at collected at Fermilab Tevatron collider by the CDF and DØ detectors are presented. CDF reports upper limits on and at the 95% C.L. using 171 pb^-1. The DØ Collaboration used 240 pb^-1 to set an even more stringent limit on the branching ratio for of 5.0 · 10^-7 at the 95% C.L.

We summarize the theoretical virtues of the rare decays and emphasize the unique role of in probing the nature of physics beyond the Standard Model, in particular concerning possible new sources of CP violation and flavor-symmetry breaking. A brief summary of the prospects for the measurement of the rate is also given.