Weak Radiative Decay  b ->    and Quark-Confined Effects in the Covariant Oscillator Quark Model

Abstract

Motivated by the observation of the decay B → K * γ by the CLEO collaboration, we have systematically analyzed the weak radiative decay Λ b → Λ γ, evaluating the confined effects of quarks in the covariant oscillator quark model. This decay process receives both short distance (electromagnetic penguins at the one loop level) and long distance contributions in the quark level. The long distance contributions are analyzed using the vector meson dominance (VMD) method. The estimated branching ratio is found to be 0.23 × 10 −5 . §1. IntroductionWeak radiative (flavor changing neutral current) decays of hyperons have attracted the interest of physicists during the last three decades. In the Standard Model (SM) these processes are forbidden at the tree level and are strongly suppressed by the GIM mechanism. Hence they offer a unique possibility to test the CKM sector of the SM and possibly open a door to physics beyond it. Experimental data are now available for the light baryon sector, i.e., Σ + → p γ, Λ → n γ, Ξ − → Σ − γ, Ξ 0 → Σ 0 γ and Ξ 0 → Λ γ, 1) which involve transitions of the type s → dγ. Recently the rare decay B → K * γ has been observed by the CLEO collaboration, 2) which is dominated by the quark level process b → sγ. Therefore, one may expect that there is also some possibility for the rare decay of heavy baryons. For the heavy hadron decay processes of this type, it is considered that the confined effects of quarks play generally an important role, since there is a large difference between the initial and final hadron masses. In a preceding paper 3) we investigated the rare B → K * γ process, evaluating the confined effects in the framework of the covariant oscillator quark model (COQM). 4) In this paper we study the weak radiative decay Λ b → Λ γ, which results from the interplay of electroweak and gluonic interactions. At the quark level, there are two essential mechanisms responsible for the weak radiative decays (b → sγ): the short distance (SD) electroweak penguin and the long distance (LD) contributions. Recently, an investigation 5) of long distance contributions to these decays was made using vector meson dominance (VMD) at the quark level as b → s[ψ] followed by the conversion [ψ] → γ. We estimate the contributions to the decay Λ b → Λ γ arising

Analysis of the radiativeΛb→Λγtransition in the standard model and scenarios with one or two universal extra dimensions

“…From this table we see that, within the errors, our result is consistent with those of QCD sum rules [56,57] and a special current [59] and exactly the same with pole model's prediction [60]. However, our prediction differs considerably from these of light-cone QCD sum rules [55], covariant oscillator quark model (COQM) [58] and Ioffe current [59]. The difference between our SM prediction on the branching ratio with that of Ref.…”

“…1, we see that there are distinctive differences between the SM predictions and those of the UED models, especially the UED6 for N KK ¼ 15, at small values of the [55] ð0:63 À 0:73Þ Â 10 À5 Three-point QCD sum rule [56] ð3:1 AE 0:6Þ Â 10 À5 QCD sum rule [57] ð3:7 AE 0:5Þ Â 10 À5 COQM [58] 0:23 Â 10 À5 Special current [59] ð1:99 þ0:34 À0:31 Þ Â 10 À5 Ioffe current [59] ð0:61 þ0:14 À0:13 Þ Â 10 À6 Pole model [60] ð1:0 À 4:5Þ Â 10 À5 PDG [54] <1:3 Â 10 À3 (CL ¼ 90%Þ compactification factor 1=R. These differences exist in the lower limits obtained by different FCNC transitions in the UED5 and UED6, cosmological constraints, electroweak precision tests [25,29,31,32], and the latest results of the Higgs search at the LHC and of the electroweak precision data for the S and T parameters [34]; however, they become small when 1=R approaches 1 TeV.…”

Rare decays of Λ b →Λ γ and Λ b →Λ l + l − in universal extra-dimension model

“…Unlike mesonic decays, the investigations of FCNC b → s transition for bottom baryonic decays Λ b → Λγ and Λ b → Λl + l − [2,3,4,5,6,7,8,9] are much behind because more degrees of freedom are involved in the bound state of baryon system at the quark level. It should be pointed out that such baryonic decays can offer the unique ground to extract the helicity structure of effective…”

Perturbative QCD calculation forΛb→Λγin the standard model

“…In this work we study Λ b → Λγ. In this decay more experimental information about the heavy b quark inside the hadron which is not available in inclusive and mesonic b-hadron decays, such as spin polarization during hadronization, and the handedness of the couplings at the quark level, can be extracted [9,10,11,12,13]. Therefore the baryonic b-hadron radiative decay can provide a new test for theoretical methods for b-quark hadronization.…”