arXiv subjects
Publications and source records attributed to Marina Nielsen.
We consider the nonleptonic and semileptonic decays of $D_s$-mesons into $ϕ$ and $f_0(980)$ mesons. QCD sum rules are used to calculate the form factors associated with these decays, and the correspondig decay rates. On the basis of data on $D_s^+\toπ^+π^+π^-$, which goes dominantly via the transition $D_s^+\to π^+f_0(980)$, we conclude that there is space for a sizeable light quark component on $f_0(980)$.
Improving previous calculations, we compute the $J/ψπ\to {charmed mesons}$ cross section using QCD sum rules. Our sum rules for the $J/ψπ\to \bar{D} D^*$, $D \bar{D}^*$, ${\bar D}^* D^*$ and ${\bar D} D$ hadronic matrix elements are constructed by using vaccum-pion correlation functions, and we work up to twist-4 in the soft-pion limit. Our results suggest that, using meson exchange models is perfectly acceptable, provided that they include form factors and that they respect chiral symmetry. After doing a thermal average we get $<σ^{πJ/ψ} v>\sim 0.3$ mb at $T=150\MeV$.
A crucial input for recent meson hyperon cloud model estimates of the nucleon matrix element of the strangeness current are the nucleon-hyperon-K* (NYK*) form factors which regularize some of the arising loops. Prompted by new and forthcoming information on these form factors from hyperon-nucleon potential models, we analyze the dependence of the loop model results for the strange-quark observables on the NYK* form factors and couplings. We find, in particular, that the now generally favored soft N-Lambda-K* form factors can reduce the magnitude of the K* contributions in such models by more than an order of magnitude, compared to previous results with hard form factors. We also discuss some general implications of our results for hadronic loop models.
We study isospin breaking instanton corrections to the operator product expansion of the nucleon correlation functions. After a comparison with quark model calculations based on the 't Hooft interaction, we examine the role of instantons in the corresponding QCD sum rules. Instanton contributions are found to be absent in the chirally even sum rule, but significant in the chirally-odd one. They improve the consistency of both sum rules and favor a value of the isovector quark condensate close to the chiral estimate.
We calculate the electromagnetic pion form factor at intermediate space-like momentum transfer from the QCD sum rule for the correlation function of two {\it pseudoscalar } interpolating fields and the electromagnetic current. This correlator receives essential contributions from direct ({\it i.e.} small-scale) instantons, which we evaluate under the assumption of an instanton size distribution consistent with instanton liquid and lattice simulations. The resulting form factor is in good agreement both with the sum rule based on the axial-current correlator and with experiment.
A model combining vector meson dominance, $ω- ϕ$ mixing and a kaon cloud contribution is used to estimate the strangeness vector form factors of the nucleon in the momentum range of the planned measurements at MIT-Bates and CEBAF. We compare our results with some other theoretical estimates and discuss the nucleon strangeness radius in models based on dynamically dressed, extended constituent quarks. For a quantitative estimate in the latter framework we use the Nambu-Jona-Lasinio model.
Within finite-density QCD sum-rule approach we investigate the self-energies of $Σ$ hyperons propagating in nuclear matter from a correlator of $Σ$ interpolating fields evaluated in the nuclear matter ground state. We find that the Lorentz vector self-energy of the $Σ$ is similar to the nucleon vector self-energy. The magnitude of Lorentz scalar self-energy of the $Σ$ is also close to the corresponding value for nucleon; however, this prediction is sensitive to the strangeness content of the nucleon and to the assumed density dependence of certain four-quark condensate. The scalar and vector self-energies tend to cancel, but not completely. The implications for the couplings of $Σ$ to the scalar and vector mesons in nuclear matter and for the $Σ$ spin-orbit force in a finite nucleus are discussed.
Classical considerations suggest that the probability distribution $P(R)$, where $R$ is the ratio of neutral pions to total pions emitted from a disoriented chiral condensate (which has been hypothesized to form in heavy ion reactions) is $R^{-1/2}/2$. Quantum mechanical isospin correlations between the condensate and the remainder of the system can alter this. Moments of the P(R) distribution can be expressed in terms of expectation values of $(I^2/N^2)^m$ where $I$ is the isospin carried by the condensate, $N$ is the number of pions emitted and m is an arbitrary integer. We find that the probability distribution is very similar to the classical distribution for $0.1 < R < 0.9$ unless the isospin carried by the condensate is very large.
We consider a simple model for the strangeness radius of the nucleon. The model is based on vector meson dominance (VMD) and $ω- ϕ$ mixing in addition to a kaon cloud contribution. We find that the VMD contribution is similar in magnitude and of the same sign as the kaon contribution to the Sachs strangeness radius and is significantly larger than the kaon contribution to the Dirac radius. Two figures (not included) can be obtained through the authors (marina@quark.umd.edu).