arXiv subjects
Publications and source records attributed to M. Nielsen.
We derive analytic expressions for infinite products of random 2x2 matrices. The determinant of the target matrix is log-normally distributed, whereas the remainder is a surprisingly complicated function of a parameter characterizing the norm of the matrix and a parameter characterizing its skewness. The distribution may have importance as an uncommitted prior in statistical image analysis.
The $J/ψπ\to \bar{D} D^*, \bar{D} D$, ${\bar D}^* D^*$ and ${\bar D} D^*$ cross sections as a function of $\sqrt{s}$ are evaluated in a QCD sum rule calculation. We study the Borel sum rule for the four point function involving pseudoscalar and vector meson currents, up to dimension four in the operator product expansion. We find that our results are smaller than the $J/ψπ\to {charmed mesons}$ cross sections obtained with models based on meson exchange, but are close to those obtained with quark exchange models.
The DDrho form factor is evaluated in a QCD sum rule calculation for both D and rho off-shell mesons. We study the double Borel sum rule for the three point function of two pseudoscalar and one vector meson currents. We find that the momentum dependence of the form factors is very different if the D or the rho meson is off-shell, but they lead to the same coupling constant in the DDrho vertex. We discuss two different approaches to extract the DDrho coupling constant.
The $D^*Dπ$ form factor is evaluated in a QCD sum rule calculation for both $D$ and $π$ off-shell mesons. We study the Borel sum rule for the three-point function of one pseudoscalar, one axial and one vector meson currents. We find that the momentum dependence of the form factors is very different if the $D$ or the $π$ meson is off-shell, but they lead to the same coupling constant in the $D^*Dπ$ vertex.
The Ge(001)-(2 1 0 3)-Pb surface reconstruction with a lead coverage of 5/3 monolayer is on the borderline between the low-coverage covalently-bonded and high-coverage metallic lead overlayers. This gives rise to an unusual low-temperature phase transition with concomitant changes in the bonding configuration. Both the room-temperature and low-temperature phases of this system were investigated by surface x-ray diffraction using synchrotron radiation. The room-temperature Ge(001)-(2 1 0 3) phase is best described by a model with dynamically flipping germanium dimers underneath a distorted Pb(111) overlayer with predominantly metallic properties. In the low-temperature Ge(001)-(2 1 0 6) phase the dimers are static and the interaction between adsorbate and substrate and within the adsorbate is stronger than at room temperature. These results suggest that the phase transition is of order-disorder type.
We have applied the meson cloud model (MCM) to calculate the asymmetries in $D$ and $D_s$ meson production in high energy $Σ^-$-nucleus and $π^-$-nucleus collisions. We find a good agreement with recent data. Our results suggest that the asymmetries may decrease at large $x_F$.
We perform a non-perturbative calculation of the $J/Ψ-π$ cross section using a $SU(2)\times SU(2)$ effective Lagrangian. Our results differ from those of previous calculations, specially in the description of vertices involving pions.
The $NDΛ_c$ and $ND^*Λ_c$ form factors are evaluated in a full QCD sum rule calculation. We study the double Borel sum rule for the three point function of one meson one nucleon and one $Λ_c$ current up to order six in the operator product expansion. The double Borel transform is performed with respect to the nucleon and $Λ_c$ momenta, and the form factor is evaluated as a function of the momentum $Q^2$ of the heavy meson. These form factors are relevant to evaluate the charmonium absorption cross section by hadrons. Our results are compatible with constant form factors in these vertices.
The $H^*Hπ$ form factor for H = B and D mesons is evaluated in a QCD sum rule calculation. We study the Borel sum rule for the three point function of two pseudoscalar and one vector meson currents up to order four in the operator product expansion. The double Borel transform is performed with respect to the heavy meson momenta. We discuss the momentum dependence of the form factors and two different approaches to extract the $H^*Hπ$ coupling constant.
We apply the Meson Cloud Model to the calculation of nonsinglet parton distributions in the nucleon sea, including the octet and the decuplet cloud baryon contributions. We give special attention to the differences between nonstrange and strange sea quarks, trying to identify possible sources of SU(3) flavor breaking. A analysis in terms of the $κ$ parameter is presented, and we find that the existing SU(3) flavor asymmetry in the nucleon sea can be quantitatively explained by the meson cloud. We also consider the $Σ^+$ baryon, finding similar conclusions.
The quark-meson-coupling model is used to study droplet formation from the liquid-gas phase transition in cold asymmetric nuclear matter. The critical density and proton fraction for the phase transition are determined in the mean field approximation. Droplet properties are calculated in the Thomas-Fermi approximation. The electromagnetic field is explicitly included and its effects on droplet properties are studied. The results are compared with the ones obtained with the NL1 parametrization of the non-linear Walecka model.
A detailed structural model for the indium-induced Si(111)-(4x1) surface reconstruction has been determined by analyzing an extensive set of x-ray-diffraction data recorded with monochromatic ($\hbarω$ = 9.1 keV) synchrotron radiation. The reconstruction is quasi-one-dimensional. The main features in the structure are chains of silicon atoms alternating with zigzag chains of indium atoms on top of an essentially unperturbed silicon lattice. The indium coverage corresponds to one monolayer. The structural model consistently explains all previously published experimental data.
Accurate atomic coordinates of the room-temperature (root3xroot3)R30degree and low-temperature (3x3) phases of 1/3 ML Sn on Ge(111) have been established by grazing-incidence x-ray diffraction with synchrotron radiation. The Sn atoms are located solely at T4-sites in the (root3xroot3)R30degree structure. In the low temperature phase one of the three Sn atoms per (3x3) unit cell is displaced outwards by 0.26 +/- 0.04 A relative to the other two. This displacement is accompanied by an increase in the first to second double-layer spacing in the Ge substrate.
We calculate the nonperturbative charmonium - nucleon cross sections with the model of the stochastic vacuum, which has been succesfully applied in many high energy reactions. We also give a quantitative discussion of pre-resonance formation and medium effects.
The NKY coupling constant for $Y = Λ$ and $Σ$ is evaluated in a QCD sum rule calculation. We discuss and extend the result of a previous analysis in the $\rlap{/}{q}iγ_5$ structure and compare it with the result obtained with the use of the $γ_5 σ_{μν}$ structure. We find a huge violation of the SU(3) symmetry in the $γ_5 σ_{μν}$ structure.
We calculate the nonperturbative $J/Ψ-N$ and $Ψ'-N$ cross sections with the model of the stochastic vacuum which has been succesfully applied in many high energy reactions. We also give a quantitative discussion of pre-resonance formation and medium effects.
We have applied the Meson Cloud Model to calculate inclusive momentum spectra of pions and kaons produced in high energy proton-proton and proton-nucleus collisions. For the first time these data are used to constrain the cloud cut-off parameters. We show that it is possible to obtain a reasonable description of data, especially the large $x_F$ ($x_F \geq 0.2$) part of the spectrum and at the same time describe (partially) the E866 data on $\bar d - \bar u$ and $\bar d / \bar u$. We also discuss the relative strength of the $πN$ and $πΔ$ vertices. We find out that the corresponding cut-off parameters should be both soft and should not differ by more than 200 MeV from each other. An additional source (other than the meson cloud) of sea antiquark asymmetry, seems to be necessary to completely explain the data. A first extension of the MCM to proton nucleus collisions is discussed.
QCD sum rules for the determination of form factors of $Λ_b$ and $Λ_c$ semileptonic decays are investigated. With a form for the baryonic current appropriate for the limits of the heavy quark symmetries, the different tensor structures occurring in the two- and three-point functions are separately studied, and in each case general relations are written for the form factors. Particular attention is given to the treatment of the kinematical region ascribed to the continuum. The $t$-dependence of the form factors and the decay rates are numerically evaluated and compared to experimental information.