Penguin-Induced Radiative Baryonic B Decays

Weak radiative baryonic B decays B\to\B_1\ov \B_2\gamma mediated by the electromagnetic penguin process $b\to s\gamma$ have appreciable rates larger than their two-body counterparts B\to\B_1\ov \B_2. The branching ratios for $B^-\to\Lambda\bar p\gamma$ and $B^-\to\Xi^0\bar\Sigma^-\gamma$ are sizable, falling into the range of $(1\sim 6)\times 10^{-6}$ with the value preferred to be on the large side, and not far from the bottom baryon radiative decays $\Lambda_b\to\Lambda\gamma$ and $\Xi_b\to\Xi\gamma$ due to the large short-distance enhancement for $b\to s\gamma$ penguin transition and the large strong coupling of the anti-triplet bottom baryons with the B meson and the light baryon. These penguin-induced radiative baryonic B decay modes should be accessible by B factories.Comment: 8 pages, 1 figure. Branching ratios are corrected as previous values are too large by a factor of 2 and a new reference is adde

Penguin-induced Radiative Baryonic B Decays Revisited

Weak radiative baryonic B decays $\bar B\to B_1\bar B_2\gamma$ mediated by the electromagnetic penguin process $b\to s\gamma$ are re-examined within the framework of the pole model. The meson pole contribution that has been neglected before is taken into account in this work. It is found that the intermediate $K^*$ contribution dominates in the $\Sigma\bar p\gamma$ mode and is comparable to the baryon pole effect in $\Lambda\bar p\gamma$ and $\Xi\bar\Lambda\gamma$ modes. The branching ratios for $B^-\to\Lambda\bar p\gamma$ and $B^-\to\Xi^0\bar\Sigma^-\gamma$ are of order $2.6\times 10^{-6}$ and $6\times 10^{-7}$, respectively. The threshold enhancement effect in the dibaryon mass spectrum is responsible by the meson pole diagram. We also study the angular distribution of the baryon in the dibaryon rest frame. The baryon pole diagrams imply that the antibaryon tends to emerge in the direction of the photon in the baryon-pair rest frame. The predicted angular asymmetry agrees with experiment for $B^-\to\Lambda\bar p\gamma$. Measurements of the correlation of the photon with the baryon allow us to discriminate between different models for describing the radiative baryonic B decays. For decays $B\to\Xi\bar\Sigma\gamma$, a large correlation of the photon to the $\bar\Sigma$ and a broad bump in the dibaryon mass spectrum are predicted.Comment: 10 pages, 4 figure

Exclusive Baryonic B Decays Circa 2005

The status of exclusive two-body and three-body baryonic B decays is reviewed. The threshold enhancement effect in the dibaryon invariant mass and the angular distributions in the dibaryon rest frame are stressed and explained. Weak radiative baryonic B decays mediated by the electromagnetic penguin process $b\to s\gamma$ are discussed. Puzzles with the correlation observed in $B^-\to p\bar pK^-$ decay and the unexpectedly large rate observed for $B\to\Lambda_c\bar\Lambda_cK$ are examined. The former may indicate that the $p\bar p$ system is produced through some intermediate states, while the latter implies the failure of naive factorization for $\Lambda\bar\Lambda K$ modes and may hint at the importance of final-state rescattering effects.Comment: 21 pages, 9 figures, talk presented at 3rd International Conference on Flavor Physics, Oct 3-8, 2005, National Central Univ. Chung-li, Taiwa

Weak Radiative Decays of Beauty Baryons

Weak radiative decays of beauty baryons into strange baryons, induced by the electroweak penguin, are estimated by using a quark model approach. Relations between formfactors in the semileptonic and in the weak radiative decays are derived within the heavy quark effective theory. The partial decay widths are found to be of the order of $10^{-15}{\rm MeV}$ for $\Lambda_b\rightarrow\Lambda\gamma$ and $\Xi_b\rightarrow\Xi\gamma$ and of the oder of $10^{-13}{\rm MeV}$ for $\Omega_b\rightarrow\Omega\gamma$. The $\Omega_b$ radiative decay is thus expected at the sizable branching ratio of approximately $10^{-4}$.Comment: 10 pages, latex, no figure

Radiative Baryonic BB Decays

We study the structure-dependent contributions to the radiative baryonic $B$ decays of $B \to {\bf B}{\bf \bar B'}\gamma$ in the standard model. We show that the decay branching ratios of $Br(B \to {\bf B}{\bf \bar B'}\gamma)$ are $O(10^{-7})$, which are larger than the estimated values of $O(10^{-9})$ induced from inner bremsstrahlung effects of the corresponding two-body modes. In particular, we find that $Br(B^- \to \Lambda \bar p \gamma)$ is around $1 \times 10^{-6}$, which is close to the pole model estimation but smaller than the experimental measurement from BELLE.Comment: 11 pages, 2 figures, Revtex4, new fitting is include

Charmless Exclusive Baryonic B Decays

We present a systematical study of two-body and three-body charmless baryonic B decays. Branching ratios for two-body modes are in general very small, typically less than $10^{-6}$, except that \B(B^-\to p \bar\Delta^{--})\sim 1\times 10^{-6}. In general, $\bar B\to N\bar\Delta>\bar B\to N\bar N$ due to the large coupling constant for $\Sigma_b\to B\Delta$. For three-body modes we focus on octet baryon final states. The leading three-dominated modes are $\bar B^0\to p\bar n\pi^-(\rho^-), n\bar p\pi^+(\rho^+)$ with a branching ratio of order $3\times 10^{-6}$ for $\bar B^0\to p\bar n\pi^-$ and $8\times 10^{-6}$ for $\bar B^0\to p\bar n\rho^-$. The penguin-dominated decays with strangeness in the meson, e.g., $B^-\to p\bar p K^{-(*)}$ and $\bar B^0\to p\bar n K^{-(*)}, n\bar n \bar K^{0(*)}$, have appreciable rates and the $N\bar N$ mass spectrum peaks at low mass. The penguin-dominated modes containing a strange baryon, e.g., $\bar B^0\to \Sigma^0\bar p\pi^+, \Sigma^-\bar n\pi^+$, have branching ratios of order $(1\sim 4)\times 10^{-6}$. In contrast, the decay rate of $\bar B^0\to\Lambda\bar p\pi^+$ is smaller. We explain why some of charmless three-body final states in which baryon-antibaryon pair production is accompanied by a meson have a larger rate than their two-body counterparts: either the pole diagrams for the former have an anti-triplet bottom baryon intermediate state, which has a large coupling to the $B$ meson and the nucleon, or they are dominated by the factorizable external $W$-emission process.Comment: 46 pages and 3 figures, to appear in Phys. Rev. D. Major changes are: (i) Calculations of two-body baryonic B decays involving a Delta resonance are modified, and (ii) Penguin-dominated modes B-> Sigma+N(bar)+p are discusse

Effective Lagrangian Approach to Weak Radiative Decays of Heavy Hadrons

Motivated by the observation of the decay $\bar{B}\to \bar{K}^*\gamma$ by CLEO, we have systematically analyzed the two-body weak radiative decays of bottom and charmed hadrons. There exist two types of weak radiative decays: One proceeds through the short-distance $b\to s\gamma$ transition and the other occurs through $W$-exchange accompanied by a photon emission. Effective Lagrangians are derived for the $W$-exchange bremsstrahlung processes at the quark level and then applied to various weak electromagnetic decays of heavy hadrons. Predictions for the branching ratios of $\bar{B}^0\to D^{*0} \gamma,~\Lambda_b^0\to\Sigma_c^0\gamma,~\Xi_b^0\to \Xi_c^0\gamma$ and \Xi_b^0\to\xip_c^0\gamma are given. In particular, we found ${\cal B}(\bar{B}^0 \to D^{*0}\gamma)\approx 0.9\times 10^{-6}$. Order of magnitude estimates for the weak radiative decays of charmed hadrons: $~D^0\to \bar{K}^{*0}\gamma,~\Lambda_c^+\to\Sigma^+\gamma$ and $\Xi_c^0\to\Xi^0\gamma$ are also presented. Within this approach, the decay asymmetry for antitriplet to antitriplet heavy baryon weak radiative transitions is uniquely predicted by heavy quark symmetry. The electromagnetic penguin contribution to $\Lambda_b^0\to\Lambda\gamma$ is estimated by two different methods and its branching ratio is found to be of order $1\times 10^{-5}$. We conclude that weak radiative decays of bottom hadrons are dominated by the short-distance $b\to s\gamma$ mechanism.Comment: 28 pages + 3 figures (not included), CLNS 94/1278, IP-ASTP-04-94. [Main changes in this revised version: (i) Sect 2 and subsection 4.1 are revised, (ii) A MIT bag method for calculating the decay rate of $Lambda_b \to\Lambda+gamma$ is presented, (iii) All predictions are updated using the newly available 1994 Particle Data Group, and (iv) Appendix and subsections 3.3 and 4.4 are deleted.

Anatomy of the pQCD Approach to the Baryonic Decays Λb→pπ,pK\Lambda_b \to p\pi, p K

We calculate the CP-averaged branching ratios and CP-violating asymmetries for the two-body charmless hadronic decays $\Lambda_b \to p \pi, pK$ in the perturbative QCD (pQCD) approach to lowest order in $\alpha_s$. The baryon distribution amplitudes involved in the factorization formulae are considered to the leading twist accuracy and the distribution amplitudes of the proton are expanded to the next-to-leading conformal spin (i.e., "P" -waves), the moments of which are determined from QCD sum rules. Our work shows that the contributions from the factorizable diagrams in $\Lambda_b \to p \pi, pK$ decays are much smaller compared to the non-factorizable diagrams in the conventional pQCD approach. We argue that this reflects the estimates of the $\Lambda_b \to p$ transition form factors in the $k_T$ factorization approach, which are found typically an order of magnitude smaller than those estimated in the light-cone sum rules and in the non-relativistic quark model. As an alternative, we adopt a hybrid pQCD approach, in which we compute the factorizable contributions with the $\Lambda_b \to p$ form factors taken from the light cone QCD sum rules. The non-factorizable diagrams are evaluated utilizing the conventional pQCD formalism which is free from the endpoint singularities. The predictions worked out here are confronted with the recently available data from the CDF collaboration on the branching ratios and the direct CP asymmetries for the decays $\Lambda_b \to p \pi$, and $\Lambda_b \to p K$. The asymmetry parameter $\alpha$ relevant for the anisotropic angular distribution of the emitted proton in the polarized $\Lambda_b$ baryon decays is also calculated for the two decay modes.Comment: 47 pages, 9 figures, 8 tables; typos and references corrected; version corresponds to the one accepted for publication in Phys. Rev.