Weakly interacting K --> pi X^0 emission with m_{X^0} simeq m_{pi^0} is out of sight of the current K^+ --> pi^+ nu nubar study, but it can be sensed by the K_L --> pi^0 nu nubar search. This evades the usual Grossman-Nir bound of B(K_L --> pi^0 nu n
ubar) < 1.4 x 10^-9, thus the KOTO experiment is already starting to probe New Physics. An intriguing possibility is the Z gauge boson of a weak leptonic force that couples to L_mu - L_tau (the difference between the muon and tauon numbers), which may explain the long-standing muon g-2 anomaly, but is constrained by nu_mu N --> nu_mu N mu^+ mu^- scattering to m_{Z} lesssim 400 MeV. An explicit model for K --> pi Z is given, which illustrates the link between rare kaon and B --> K mu^+ mu^-, K^{(*)} nu nubar decays. Complementary to these searches and future lepton experiments, the LHC might discover the scalar boson phi responsible for light m_{Z} generation via phi --> Z Z --> 2(mu^+ mu^-).
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.
We investigate a model in which tiny neutrino masses are generated at the two-loop level by using scalar leptoquark and diquark multiplets. The diquark can be singly produced at the LHC, and it can decay into a pair of leptoquarks through the lepton
number violating interaction. Subsequent decays of the two leptoquarks can provide a clear signature of the lepton number violation, namely two QCD jets and a pair of same-signed charged leptons without missing energy. We show that the signal process is not suppressed while neutrino masses are appropriately suppressed.
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.
