Heavy quark spin symmetry breaking in near-threshold JPC=1−−J^{PC}=1^{--} quarkonium-like resonances

A mixing of near-threshold quarkonium-like resonances with heavy meson-antimeson pairs results in an enhancement of heavy quark spin symmetry breaking, since the meson pairs are not eigenstates of the heavy quark spin. The decomposition of $P$-wave states of meson pairs in terms of the heavy-quark-pair spin states is considered in the channel with $J^{PC}=1^{--}$, which is directly produced in $e^+e^-$ annihilation. Specific processes are suggested for experimental study of the effects of the mixing with heavy meson pairs and of the internal spin structure of bottomonium and charmonium resonances.Comment: 15 page

The cˉc{\bar c} c purity of ψ(3770)\psi(3770) and ψ′\psi' challenged

It is suggested that the resonance $\psi(3770)$ may contain a sizeable ($O(10$ in terms of the probability weight factor) four-quark component with the up- and down- quarks and antiquarks in addition to the $c {\bar c}$ pair, which component in itself has a substantial part with the isospin I=1. Furthermore such four-quark part of the wave function should also affect the properties of the $\psi'$ charmonium resonance through the $\psi(3770) - \psi'$ mixing previously considered in the literature. It is argued that an admixture of extra light quark pairs can explain a possible discrepancy between the theoretical expectations and the recent data on the non-$D {\bar D}$ decay width of the $\psi(3770)$ and the ratio of the yield of charged and neutral $D$ meson pairs in its decays, as well as on the extra rate of the $\psi'$ direct decay into light hadrons and the rate of the decay $\psi' \to \pi^0 J/\psi$. It is further argued that the suggested four-quark component of the wave function of the $\psi(3770)$ should give rise to a measurable rate of the decays $\psi(3770) \to \eta J/\psi$ and $\psi(3770) \to \pi^0 J/\psi$.Comment: 13 page

Semiclassical Calculation of Photon-Stimulated Schwinger Pair Creation

We consider the electron-positron pair creation by a photon in an external constant electric field. The presented treatment is based on a purely quasiclassical calculation of the imaginary part of the on-shell photon polarization operator. By using this approach we find the pair production rate for photons with polarization parallel as well as orthogonal to the external electric field in the leading order in the parameter $eE / m ^ 2$, which has been recently found by other methods. For the orthogonal polarization we also find a new contribution to the rate, which is leading in the ratio of the photon energy to the electron mass $\omega/m$. We also reproduce by a purely geometrical calculation the exponential factor in the probability of the stimulated pair creation at arbitrary energy of the photon.Comment: 16 pages, 4 figure

Comment on "Summing One-Loop Graphs at Multi-Particle Threshold"

The propagator of a virtual $\phi$-field with emission of $n$ on-mass-shell particles all being exactly at rest is calculated at the tree-level in $\lambda \phi^4$ theory by directly solving recursion equations for the sum of Feynman graphs. It is shown that the generating function for these propagators is equivalent to a Fourier transform of the recently found Green's function within the background-field technique for summing graphs at threshold suggested by Lowell Brown. Also the derivation of the result that the tree-level on-mass-shell scattering amplitudes of the processes $2 \to n$ are exactly vanishing at threshold for $n > 4$ is thus given in the more conventional Feynman diagram technique.Comment: 7 pages, LaTeX, TPI-MINN-92/46-

Bottomonium dipion transitions

Dipion transitions of the subthreshold bottomonium levels $\Upsilon (nS)\to \Upsilon (n'S) \pi\pi$ with $n>n', n=2,3,4, n'=1,2$ are studied in the framework of the chiral decay Lagrangian, derived earlier. The channels $B\bar B, B\bar B^*+ c.c, B^* \bar B^*$ are considered in the intermediate state and realistic wave functions of $\Upsilon (n S),B$ and $B^*$ are used in the overlap matrix elements. Imposing the Adler zero requirement on the transition matrix element, one obtains 2d and 1d dipion spectra in reasonable agreement with experiment.Comment: 34 pages, 18 figure

Reducing model dependence of spectator effects in inclusive decays of heavy baryons

The dependence of inclusive weak decay rates of heavy hadrons on light spectator quarks is considered. The analysis of a previous work on relating the effects in b baryons to those in charmed baryons is extended to explicit evaluation of the matrix elements of certain four-quark operators over heavy baryons. It is shown that the usually postulated color antisymmetry of these matrix elements is significantly broken. The flavor-singlet shift of inclusive decay rates of b baryons due to the spectator effects is shown to be strongly suppressed in the leading-log approximation. Combined with the results for the flavor non-singlet splittings, this observation allows to argue, in a less model-dependent way, than before, that within the present understanding of the spectator effects it is highly unlikely that the lifetimes of the Lambda_b and the B_d can be split by more than 10%.Comment: 13 pages, 1 figur

Modification by pion exchange of near threshold resonance line shape in open heavy flavor channel

The effect of the pion exchange on the line shape of near threshold bottomoniumlike resonances decaying into an $S$ wave pair of $B^{(*)}$ mesons is considered. It is pointed out that this effect, parametrically enhanced by the heavy meson mass, can be of a practical significance in determining the parameters of the bottomoniumlike resonances such as the known $Z_b(10610)$ and $Z_b(10650)$ states.Comment: 8 pages, 1 figur