arXiv subjects

C. Greub

Publications and source records attributed to C. Greub.

48 records · Page 3Linked to original sources

arXiv1998

Inclusive Decay Rate for $B \to X_d + γ$ in Next-to-Leading Logarithmic Order and CP Asymmetry in the Standard Model

We compute the decay rate for the CKM-suppressed electromagnetic penguin decay $B \to X_d + γ$ (and its charge conjugate) in NLO QCD, including leading power corrections in $1/m_b^2$ and $1/m_c^2$ in the standard model. The average branching ratio of the decay $B \to X_dγ$ and its charge conjugate is estimated to be in the range $6.0 \times 10^{-6} \leq \leq 2.6 \times 10^{-5}$, obtained by varying the CKM-Wolfenstein parameters $ρ$ and $η$ in the range $-0.1 \leq ρ\leq 0.4$ and $0.2 \leq η\leq 0.46$ and taking into account other parametric dependence. In the stated range of the CKM parameters, we find the ratio $R(dγ/sγ) = / $ to lie in the range between 0.017 and 0.074. Theoretical uncertainties in this ratio are found to be small. Hence, this ratio is well suited to provide independent constraints on the CKM parameters. The CP-asymmetry in the $B \to X_d γ$ decay rates is found to be in the range $(7 - 35)%$. Both the decay rates and CP asymmetry are measurable in forthcoming experiments at $B$ factories and possibly at HERA-B.

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arXiv1998

Contribution of $b \to sgg$ through the QCD anomaly in exclusive decays $B^{\pm}\to (η^{\prime},η)(K^{\pm}, K^{\pm})$ and $B^{0}\to (η^{\prime},η)(K^{0},K^{0})$

We compute the decay rates for the exclusive decays $B^{\pm} \to (η^{\prime},η) (K^{\pm}, K^{*\pm})$ and $B^{0}\to (η^{\prime},η) (K^{0}, K^{*0})$ in a QCD-improved factorization framework by including the contribution from the process $b\to sgg \to s (η^{\prime}, η)$ through the QCD anomaly. This method provides an alternative estimate of the contribution $b \to s c\bar{c} \to s(η,η^\prime)$ to these decays as compared to the one using the intrinsic charm content of the $η^{\prime}$ and $η$ mesons determined through the decays $J/ψ\to (η,η^\prime ,η_c) γ$. The resulting branching ratios are compared with the CLEO data on $B^{\pm} \to η^{\prime} K^{\pm}$ and $B^{0} \to η^{\prime} K^{0}$ and predictions are made for the rest.

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arXiv1998

2HDMs predictions for $B \to X_s γ$ in NLO QCD

The decay $B \to X_s γ$ is studied at the Next to Leading Order in QCD in a class of models containing at least two Higgs doublets and with only one charged Higgs boson non-decoupled at low-energy. The two-loop matching condition is calculated and it is found to agree with existing results. The complete dependence of the Wilson coefficients on the matching scale is given. The size of the Next to Leading Order corrections is extensively discussed. Results for branching ratios, possible CP asymmetries and lower bounds on the charged Higgs mass are presented when the convergence of the perturbative series appears fast enough to yield reliable predictions. Regions in the parameter space of these models where the Next to Leading Order calculation is still not a good approximation of the final result for these observables are singled out.

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arXiv1997

An analysis of two-body non-leptonic B decays involving light mesons in the standard model

We report a theoretical analysis of the exclusive non-leptonic decays of B mesons into two light mesons,some of which have been measured recently by the CLEO collaboration. Our analysis is carried out in the context of an effective Hamiltonian based on the Standard Model using next-to-leading order perturbative QCD calculations. Using a factorization ansatz for the hadronic matrix elements, we show that existing data are accounted for in this approach. Thus, theoretical scenarios with a substantially enhanced Wilson coefficient of the chromomagnetic dipole operator (as compared to the SM) and/or those with a substantial color-singlet $c\bar{c}$ component in the wave function of $η^\prime$ are not required by these data. Implications of some of these measurements for the parameters of the CKM matrix are presented.

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arXiv1997

$B \to X_s gamma$ in the Standard Model

We give a summary about the various contributions which have to be calculated in order to obtain the next-to-leading logarithmic result for the branching ratio $BR(B \to X_s γ)$. Combining all these ingredients, which were obtained by different groups, a complete next-to-leading-logarithmic prediction of the inclusive decay rate was recently presented in the literature. The theoretical uncertainty in the partonic decay rate is now at the 10% level, i.e., less than half of the error in the previous leading-logarithmic result. We also mention the impact of non-perturbative corrections which scale like $1/m_b^2$ and discuss in some more detail the recently discovered corrections which scale like $1/m_c^2$. It turns out that the $1/m_b^2$- and the $1/m_c^2$ terms lead to corrections to the branching ratio $BR(B \to X_s γ)$ well below the 10% level.

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arXiv1996

Heavy Majorana neutrinos in e^-e^- collisions

We discuss the process $e^-e^- \to W^- W^-$ mediated by heavy Majorana neutrino exchange in the t- and u channel. In our model the cross section for this reaction is a function of the masses (m_N) of the heavy Majorana neutrinos and mixing parameters (U_{eN}) originating from mixing between the ordinary left-handed standard model neutrinos and additional singlet right-handed neutrino fields. Taking into account the standard model background and contraints from low energy measurements, we present discovery limits in the (m_N,U_{eN}^2) plane. We also discuss how to measure in principle the CP violating phases, i.e., the relative phases between the mixing parameters.

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arXiv1996

Virtual corrections to the decay $b \to s γ$

We calculate the $O(\a_s)$ virtual corrections to the matrix element for $b \to s γ$, taking into account the contributions of the four-Fermi operator $O_2$ and the electromagnetic and color dipole-type operators. The results are combined with existing $O(\a_s)$ Bremsstrahlung corrections in order to obtain the relevant inclusive rate. The new result drastically reduces the large scale dependence of the leading logarithmic approximation. It implies that a very accurate prediction for the branching ratio for $B \to X_s \g $ will become possible once also the corrections to the Wilson coefficients are available.

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arXiv1995

Photon Energy Spectrum in $B \to X_s γ$ and Comparison with Data

A comparison of the inclusive photon energy spectrum in the radiative decay $\BGAMAXS$, measured recently by the CLEO collaboration, with the standard model is presented, using a $B$-meson wave function model and improving earlier perturbative QCD-based computations of the same. The dependence of the photon energy spectrum on the non-perturbative model parameters, $p_F$, the $b$-quark Fermi momentum in the $B$ hadron, and $m_q$, the spectator quark mass, is explicitly shown, allowing a comparison of these parameters with the ones obtained from the analysis of the lepton energy spectrum in semileptonic $B$ decays. Taking into account present uncertainties, we estimate $\BBGAMAXS = (2.55 \pm 1.28) \times 10^{-4}$ in the standard model, assuming $\absvts/\absvcb= 1.0$. Comparing this with the CLEO measurement $\BBGAMAXS = (2.32 \pm 0.67) \times 10^{-4}$ implies $\absvts/\absvcb= 1.1 \pm 0.43$, in agreement with the CKM unitarity.

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arXiv1994

Effects of new physics in the rare decays $B \to K \ell^+ \ell^-$ and $B \to K^\ast \ell^+ \ell^-$

We parametrize phenomenologically possible new physics effects and calculate their influence on the invariant dilepton mass spectrum and the Dalitz plot for the decays $B \to K \ell^+ \ell^-$ and $B \to K^\ast \ell^+ \ell^-$. Especially the decay into $K^*$ yields a wealth of new information on the form of the new interactions since the Dalitz plot is sensitive to subtle interference effects. We also show how transversely polarized $K^*$-mesons give additional information.

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arXiv1994

Branching ratio and direct CP-violating rate asymmetry of the rare decays $B \to K^\ast γ$ and $B \to ργ$

We calculate CP-violating rate asymmetries in the rare radiative decays $B^\pm\rightarrow K^{\ast\pm} γ$ and $B^\pm\rightarrow ρ^\pm γ$. They arise because of the interference between leading-order penguin amplitudes and one-gluon corrections with absorptive phases, and provide unambiguous evidence for direct CP violation. Complementing earlier studies, we also investigate gluon exchange with the `spectator' quark. The bound state effects in the exclusive matrix elements are taken into account by a covariant model, which yields a branching ratio $BR(B\rightarrow K^\ast γ) = (4-5)\times 10^{-5}$ in good agreement with the observed value. The bound state effects increase the CP asymmetry, which is of order $1 \%$ in the channel $B\rightarrow K^\ast γ$ and $15 \%$ for $B \to ργ$.

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arXiv1994

Atomic Alchemy

We consider the transitions between electromagnetic bound states, such as the exclusive weak decay of a muonic atom into an electronic atom: $(μ^- Z) \to (e^- Z) {\bar ν_e }ν_μ. $ We show that relativistic effects in the atomic wavefunctions are crucial for determining the rate. In the case of heavy atoms, the exclusive channel branching ratios exceed $10^{-6},$ possibly bringing the study of these rare decays within experimental reach. Such processes thus provide a detailed laboratory for studying the high momentum tail of wavefunctions in atomic physics; in addition, they provide a simple toy model for investigating analogous exclusive heavy hadronic decays in quantum chromodynamics such as $B \to πe ν.$

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arXiv1994

Effects of non-standard couplings, radiative corrections and neutrino masses on the lepton spectrum in $μ$ and $τ$ decays

We investigate the combined effect of neutrino masses and non-standard couplings on the various lepton spectra in $μ$ and $τ$ decays. We emphasize the energy spectra of the neutrinos, which can be measured in the new KARMEN experiment and hence will yield novel information on deviations from the $V-A$ structure. To constrain these couplings, QED radiative corrections have to be taken into account. We evaluated them and found small corrections to the neutrino spectra.

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