Doubly Cabibbo-suppressed (DCS) nonleptonic weak decays of antitriplet charmed baryons are studied systematically in this work. The factorizable and nonfactorizable contributions can be classified explicitly in the topological-diagram approach and tr
eated separately. In particular, the evaluation of nonfactorizable terms is based on the pole model in conjunction with current algebra. All three types of relevant non-perturbative parameters contributing factorizable and nonfactorizable terms are estimated in the MIT bag model. Branching fractions of all the DCS decays are predicted to be of order $10^{-4}sim 10^{-6}$. In particular, we find that the three modes $Xi_c^+to Sigma^+ K^0, Sigma^0 K^+$ and $Xi_c^0to Sigma^- K^+$ are as large as $(1sim 2)times 10^{-4}$, which are the most promising DCS channels to be measured. We also point out that the two DCS modes $Xi_c^+to Sigma^+ K^0$ and $Xi_c^0to Sigma^0 K^0$ are possible to be distinguished from $Xi_c^+to Sigma^+ K_S$ and $Xi_c^0to Sigma^0 K_S$. The decay asymmetries for all the channels with a kaon in their final states are found to be large in magnitude and negative in sign.
The hadronic two-body weak decays of the doubly charmed baryons $Xi_{cc}^{++}, Xi_{cc}^+$ and $Omega_{cc}^+$ are studied in this work. To estimate the nonfactorizable contributions, we work in the pole model for the $P$-wave amplitudes and current al
gebra for $S$-wave ones. For the $Xi_{cc}^{++}to Xi_c^+pi^+$ mode, we find a large destructive interference between factorizable and nonfactorizable contributions for both $S$- and $P$-wave amplitudes. Our prediction of $sim 0.70%$ for its branching fraction is smaller than the earlier estimates in which nonfactorizable effects were not considered, but agrees nicely with the result based on an entirely different approach, namely, the covariant confined quark model. On the contrary, a large constructive interference was found in the $P$-wave amplitude by Dhir and Sharma, leading to a branching fraction of order $(7-16)%$. Using the current results for the absolute branching fractions of $(Lambda_c^+,Xi_c^+)to p K^-pi^+$ and the LHCb measurement of $Xi_{cc}^{++}toXi_c^+pi^+$ relative to $Xi_{cc}^{++}toLambda_c^+ K^- pi^+pi^+$, we obtain $B(Xi_{cc}^{++}toXi_c^+pi^+)_{rm expt}approx (1.83pm1.01)%$ after employing the latest prediction of $B(Xi_{cc}^{++}toSigma_c^{++}overline{K}^{*0})$. Our prediction of $mathcal{B}(Xi_{cc}^{++}toXi_c^+pi^+)approx 0.7%$ is thus consistent with the experimental value but in the lower end. It is important to pin down the branching fraction of this mode in future study. Factorizable and nonfactorizable $S$-wave amplitudes interfere constructively in $Xi_{cc}^+toXi_c^0pi^+$. Its large branching fraction of order 4% may enable experimentalists to search for the $Xi_{cc}^+$ through this mode. That is, the $Xi_{cc}^+$ is reconstructed through the $Xi_{cc}^+toXi_c^0pi^+$ followed by the decay chain $Xi_c^0to Xi^-pi^+to ppi^-pi^-pi^+$.
The LHCb experiment observed B+ --> pi+ mu+ mu- decay with 1.0 fb^-1 data, which is the first measurement of a flavor changing neutral current b --> d l+ l- decay (l = e, mu). Based on QCD factorization, we give Standard Model predictions for the bra
nching ratios, direct CP asymmetries, and isospin asymmetry for B --> pi l+ l- decays, in the kinematic region where the dilepton invariant mass is small. We find that the contribution from weak annihilation enhances the direct CP asymmetry for low l+ l- pair mass. Anticipating improved measurements, we assess the utility of B+ --> pi+ l+ l- observables, when combined with B0 --> pi- l+ nu and B+ --> K+ l+ l-, for determining CKM parameters in the future.
The very rare Bd0 --> mu+ mu- decay may be the last chance for New Physics in flavor sector at the LHC, before the 13 TeV run in 2015. Partially motivated by the known tension in sin(2beta/phi_1), enhancement beyond (3-4) x 10^-10 would likely imply
the effect of a fourth generation of quarks. If observed at this level, the 126 GeV boson may not be the actual Higgs boson, while the b --> d quadrangle (modulo m_t) would jump out. The 2011-2012 data is likely not sensitive to values below 3 x 10^-10, and the mode should continue to be pursued with the 13 TeV run.
With full 2011 LHC data analyzed, there is no indication for deviation from Standard Model (SM) in CP violating phase for Bs --> J/psi phi, nor in the forward-backward asymmetry for B0 --> K*0 mu+ mu-. SM sensitivity, however, has been reached for Bs
--> mu+ mu- rate, and there may be some hint for a suppression. We illustrate that, if a suppressed B(Bs --> mu+ mu-) bears out with 2012 data, it would imply a lower bound on the fourth generation quark mixing product |V_{ts}^*V_{tb}|.
We show that simultaneous precision measurement of the CP-violating phase in time-dependent Bs --> J/psi phi study and the Bs --> mu+ mu- rate, together with measuring m_t by direct search at the LHC, would determine V_{ts}^*V_{tb} and therefore the
b --> s quadrangle in the four-generation standard model. The forward-backward asymmetry in B --> K* l+ l- provides further discrimination.
