Branko Guberina

Rare decays of the Z0

We study radiative and Higgs decay modes of the Z0 into hypothetical heavy quark bound states (onia). In general, we find that the rates for these decays are extremely small. However, in some cases, there is some enhancement if the onia are rather near in mass to the Z0. We examine the experimental prospects for detecting, by these means, spin-parity onium states which are otherwise difficult to reach or discovering Higgs mesons. We conclude, unfortunately, that these prospects are not very bright.

Quantum chromodynamic effects and CP violation in the Kobayashi-Maskawa model

We study, in the context of the Kobayashi-Maskawa model, the effects that quantum chromodynamics has on the predictions for CP violation in kaon decays. We derive an effective hamiltonian for ΔS = 1 transitions in which all-order corrections in the strong coupling constant αs are incorporated, including those arising from penguin diagrams. We examine, in particular, the predictions of this hamiltonian for the ratio of the CP-violating parameters |ϵ′/ϵ|. Our results for this ratio are well within the present experimental bound, but are considerably smaller than the recent calculation of Gilman and Wise. The discrepancy between our results and those of Gilman and Wise arises because of the different way we have estimated the amplitude for K0 decay. A discussion of the reliability of both our estimate and that of Gilman and Wise is included.

D-meson lifetimes and decays

Abstract We discuss the theoretical implications of having the D + lifetime be much greater than that of the D 0 . A consistent phenomenology of D-decay obtains if one assumes sextet dominance and strong color clustering.

Dynamical dark energy with a constant vacuum energy density

Abstract We present a holographic dark-energy model in which the Newton constant G N scales in such a way as to render the vacuum energy density a true constant. Nevertheless, the model acts as a dynamical dark-energy model since the scaling of G N goes at the expense of deviation of concentration of dark-matter particles from its canonical form and/or of promotion of their mass to a time-dependent quantity, thereby making the effective equation of state (EOS) variable and different from −1 at the present epoch. Thus the model has a potential to naturally underpin Diracs suggestion for explaining the large-number hypothesis, which demands a dynamical G N along with the creation of matter in the universe. We show that with the aid of observational bounds on the variation of the gravitational coupling, the effective-field theory IR cutoff can be strongly restricted, being always closer to the future event horizon than to the Hubble distance. As for the observational side, the effective EOS restricted by observation can be made arbitrary close to −1, and therefore the present model can be considered as a “minimal” dynamical dark-energy scenario. In addition, for nonzero but small curvature ( | Ω k 0 | ≲ 0.003 ) , the model easily accommodates a transition across the phantom line for redshifts z ≲ 0.2 , as mildly favored by the data. A thermodynamic aspect of the scenario is also discussed.

Non-saturated holographic dark energy

It has been well established that holographic dark energy (HDE) is a serious candidate for being the dark energy of the universe. Here we deal with models where the holographic bound for dark energy is not saturated for a large portion of the history of the universe. This is particularly compelling when the IR cut-off is set by the Hubble scale, since otherwise a transition from a decelerated to an accelerated era cannot be obtained for a spatially flat universe. We demonstrate using three generic but disparate dynamical models, two of them containing a variable Newton constant, that a transition between the two eras is always obtained for the IR cut-off in the form of the Hubble scale and the non-saturated HDE. We also give arguments for why such a choice for the dark energy is more consistent and favoured over the widely accepted saturated form.

Quarkonium sum rules: A critical reappraisal

Abstract We reanalyze the ITEP charmonium sum rules incorporating experimental errors and allowing for variations in input parameters. The necessity of a gluon condensate term is apparent for the ratio sum rules, but the presence or absence of this term cannot be established using the moment sum rules. Plausibility arguments are given for trusting the results of the ratio sum rules and the advantages of mass-independent double ratio sum rules are stressed. Sum rules for the bottomonium system are also analyzed and the results are shown to be consistent with what is obtained in the charmonium sector.

Hint for quintessence-like scalars from holographic dark energy

We use the generalized holographic dark energy model, in which both the cosmological constant (CC) and Newtons constant GN are scale dependent, to set constraints on the renormalization-group (RG) evolution of both quantities phrased within quantum field theory (QFT) in a curved background. Considering the case in which the energy-momentum tensor of ordinary matter stays individually conserved, we show from the holographic dark energy requirement that the RG laws for the CC and GN are completely determined in terms of the lowest part of the particle spectrum of an underlying QFT. From simple arguments one can then infer that the lowest-mass fields should have a Compton wavelength comparable with the size of the current Hubble horizon. Hence, although the models with variable CC (or with both the CC and the GN varying) are known to lead to successful cosmologies without introducing a new light degree of freedom, we nonetheless find that holography actually brings us back to the quintessence proposal. An advantage of having two different components of the vacuum energy in the cosmological setting is also briefly mentioned.

Effects of QCD on the Cabibbo patterns in B-meson decays

Abstract The Cabibbo patterns for B-meson decays in the Kobayashi-Maskawa model are altered once QCD corrections are included. We estimate these QCD effects and discuss possible experimental signatures.

The decay K+→φ+φ0 in the standard model

Abstract We present a direct calculation of the ΔI = 3 2 amplitude which governs the K + →φ + φ 0 . The calculational procedure is similar to the one which has been recently used by two of us to evaluate the B -parameter of K 0 K 0 mixing. The succesful result we find reinforces our belief in the value of the B -parameter obtained previously by two of us.

Problem with the |ΔI| = 12 rule in the standard model

34 paginas, 9 figuras, 5 tablas.-- Trabajo presentado al 23o International Conference On High-Energy Physics celebrado del 16 al 23 de Julio de 1986 en Berkeley (USA).