Rare radiative B decays in perturbative QCD

We report on recent progress on perturbative QCD calculations of certain exclusive rare weak $B$ meson decays involving hard photons. In the limit of a photon energy $E_\gamma$ much larger than $\Lambda_{QCD}$, the amplitudes for such processes can be analyzed in a twist expansion in powers of $\Lambda/E_\gamma$. The leading twist amplitude is given by the convolution of a hard scattering amplitude with the $B$ meson light-cone wavefunction. This approach is applied to a calculation of the leptonic radiative $B\to \gamma \ell \nu_\ell$ formfactors and to an estimate of the weak annihilation contribution to the penguin decays $B\to \rho(\omega)\gamma$. As an application we discuss a few methods for constraining the unitarity triangle with exclusive radiative B decays.Comment: 11 pages, 3 included figures. Invited talk given at the 5th International Workshop on Particle Physics Phenomenology, Chi-Pen, Taitung, Taiwan, 8-11 Nov 200

Testing for three-body quark forces in L=1 excited baryons

We discuss the matching of the quark model to the effective mass operator of the 1/Nc expansion using the permutation group S_N. As an illustration of the general procedure we perform the matching of the Isgur-Karl model for the spectrum of the negative parity L=1 excited baryons. Assuming the most general two-body quark Hamiltonian, we derive two correlations among the masses and mixing angles of these states which should hold in any quark model. These correlations constrain the mixing angles and can be used to test for the presence of three-body quark forces.Comment: 6 pages, 2 figures. Invited talk given at the XI Hadron Physics Conference, March 21-26, 2010, Sao Paulo, Brazi

1/Nc expansion and the spin-flavor structure of the quark interaction in the constituent quark model

We study the hierarchy of the coefficients in the 1/Nc expansion for the negative parity L=1 excited baryons from the perspective of the constituent quark model. This is related to the problem of determining the spin-flavor structure of the quark interaction. The most general two-body scalar interaction between quarks contains the spin-flavor structures $t_1^a t_2^a, \vec s_1\cdot \vec s_2$ and $\vec s_1\cdot \vec s_2 t_1^a t_2^a$. We show that in the limit of a zero range interaction all these structures are matched onto the same hadronic mass operator $S_c^2$, which gives a possible explanation for the dominance of this operator in the 1/Nc expansion for the L=1 states and implies that in this limit it is impossible to distinguish between these different spin-flavor structures. Modeling a finite range interaction through the exchange of a vector and pseudoscalar meson, we propose a test for the spin-flavor dependence of the quark forces. For the scalar part of the quark interaction we find that both pion exchange and gluon exchange are compatible with data.Comment: 20 pages, 4 figure

Matching the quark model to the 1/Nc expansion

We compute the coefficients of the effective mass operator of the 1/Nc expansion for negative parity L=1 excited baryons using the Isgur-Karl model in order to compare the general approach, where the coefficients are obtained by fitting to data, with a specific constituent quark model calculation. We discuss the physics behind the fitted coefficients for the scalar part of the most general two-body quark-quark interaction. We find that both pion exchange and gluon exchange lead to the dominance of the same operator at the level of the effective mass operator, which is also observed from data.Comment: 4 pages, 2 figures, contribution to the proceedings of the MENU 2010 conference, May 31- June 4, 2010, Williamsburg VA, US