Showing papers on "Meson published in 1995"

Semileptonic meson decays in the quark model: An update.

Abstract: The authors present the predictions of ISGW2, an update of the ISGW quark model for semileptonic meson decays. The updated model incorporates a number of features which should make it more reliable, including the constraints imposed by Heavy Quark Symmetry, hyperfine distortions of wave-functions, and form factors with more realistic high recoil behaviors.

Dispersion-Theoretical Analysis of the Nucleon Electromagnetic Formfactors

Abstract: Dispersion relations allow for a coherent description of the nucleon electromagnetic form factors measured over a large range of momentum transfer, $Q^2 \simeq 0 \ldots 35$ GeV$^2$. Including constraints from unitarity and perturbative QCD, we present a novel parametrisation of the absorptive parts of the various isoscalar and isovector nucleon form factors. Using the current world data, we obtain results for the electromagnetic form factors, nucleon radii and meson couplings. We stress the importance of measurements at large momentum transfer to test the predictions of perturbative QCD.

Understanding the scalar meson $$q\bar q$$ nonet

Abstract: It is shown that one can fit the available data on thea o(980),f o(1300) andK o * (1430) mesons as a distorted 0++ $$q\bar q$$ nonet using very few (5–6) parameters and an improved version of the unitarized quark model. This includes all light two-pseudoscalar thresholds, constraints from Adler zeroes, flavour symmetric couplings, unitarity and physically acceptable analyticity. The parameters include a bare $$u\bar u$$ or $$d\bar d$$ mass, an over-all coupling constant, a cutoff and a strange quark mass of 100 MeV, which is in accord with expectations from the quark model. It is found that in particular for thea 0(980) andf 0(980) the $$K\bar K$$ component, in the wave function is large, i.e., for a large fraction of the time the $$q\bar q$$ state is transformed into a virtual $$K\bar K$$ pair. This $$K\bar K$$ component, together with a similar component of η′π for thea 0(980), and ηη, ηη′ and η′η′ components for thef 0(980), causes the substantial shift to a lower mass than what is naively expected from the $$q\bar q$$ component alone. Mass, width and mixing parameters, including sheet and pole positions, of the four resonances are given, with a detailed pedagogical discussion of their meaning.

Numerical evidence for the observation of a scalar glueball

Abstract: We compute from lattice QCD in the valence (quenched) approximation the partial decay widths of the lightest scalar glueball to pairs of pseudoscalar quark-antiquark states. These predictions and values obtained earlier for the scalar glueball's mass are in good agreement with the observed properties of ${f}_{J}(1710)$ and inconsistent with all other observed meson resonances.

Extraction of V(ub) from decay B ---> pi lepton neutrino

Abstract: We develop the perturbative QCD formalism for semileptonic {ital B} meson decays, which includes Sudakov suppression on the spatial extent of a heavy meson containing a light valence quark. We show that the perturbative calculation for the spectrum of the decay {ital B}{r_arrow}{pi}{ital l}{nu} is reliable for the energy fraction of the pion above 0.3. Combining predictions from soft pion theorems, we obtain an upper limit of the matrix element {vert_bar}{ital V}{sub {ital ub}}{vert_bar} of roughly 3.5{times}10{sup {minus}3}.

Heavy mesons in a relativistic model

Abstract: Motivated by the present interest in the heavy quark effective theory, the authors use the spectator equations to treat the mesonic bound states of heavy quarks. The kernel they use is based on scalar confining and vector Coulomb potentials. Wave functions are treated to leading order and energies to order 1/m{sub Q} in the heavy-light systems, and order 1/m{sub Q{sup 2}} in heavy-heavy systems. Their results are in reasonable agreement with experimental measurements. They estimate two of the parameters of the heavy quark effective theory, and propose further calculations that may be undertaken in the future.

Strangeness in hadronic stellar matter

Abstract: We examine the presence of strangeness-bearing components, hyperons and kaons, in dense neutron star matter. Calculations are performed using relativistic mean field models, in which both the baryon-baryon and kaon-baryon interactions are mediated by meson exchange. Results of kaon condensation are found to be qualitatively similar to previous work with chiral models, if compatibility of the kaon optical potentials is required. The presence of strangeness, be it in the form of hyperons or kaons, implies a reduction in the maximum mass and a relatively large number of protons, sufficient to allow rapid cooling to take place. The need to improve upon the poorly known couplings of the strange particles, which determine the composition and structure of neutron stars, is stressed. We also discuss generic problems with effective masses in mean field theories.

Leptonic and semileptonic decays of charm and bottom hadrons

Abstract: The authors review the experimental measurements and theoretical descriptions of leptonic and semileptonic decays of particles containing a single heavy quark, either charm or bottom. Measurements of bottom semileptonic decays are used to determine the magnitudes of two fundamental parameters of the standard model, the Cabibbo-Kobayashi-Maskawa matrix elements ${V}_{\mathrm{cb}}$ and ${V}_{\mathrm{ub}}$. These parameters are connected with the physics of quark flavor and mass, and they have important implications for the breakdown of $\mathrm{CP}$ symmetry. To extract precise values of $|{V}_{\mathrm{cb}}|$ and $|{V}_{\mathrm{ub}}|$ from measurements, however, requires a good understanding of the decay dynamics. Measurements of both charm and bottom decay distributions provide information on the interactions governing these processes. The underlying weak transition in each case is relatively simple, but the strong interactions that bind the quarks into hadrons introduce complications. The authors also discuss new theoretical approaches, especially heavy-quark effective theory and lattice QCD, which are providing insights and predictions now being tested by experiment. An international effort at many laboratories will rapidly advance knowledge of this area of physics during the next decade.

Decays of B mesons to two pseudoscalars in broken SU(3) symmetry.

Abstract: The decays of {ital B} mesons to two-body hadronic final states are analyzed within the context of broken flavor SU(3) symmetry, extending a previous analysis involving pairs of light pseudoscalars to decays involving one or two charmed quarks in the final state. A systematic program is described for learning information from decay rates regarding (i) SU(3)-violating contributions, (ii) the magnitude of exchange and annihilation diagrams (effects involving the spectator quark), and (iii) strong final-state interactions. The implication of SU(3)-breaking effects for the extraction of weak phases is also examined. The present status of data on these questions is reviewed and suggestions for further experimental study are made.

Physics of B c -mesons

Abstract: The mass spectrum for the system (c) is considered in the framework of potential models for the heavy quarkonium. Spin-dependent splittings, with account taken of the change of a constant representing the effective Coulomb interaction between the quarks, and widths of radiative transitions between the (c) levels are calculated. In the framework of QCD sum rules, the masses of the lightest vector Bc* and pseudoscalar Bc states are estimated, the scaling relation for leptonic constants of heavy quarkonia is derived, and the leptonic constant fBc is evaluated. The Bc decays are considered in the framework both of the potential models and of the QCD sum rules. The relations, following from the approximate spin symmetry for the heavy quarks in the heavy quarkonium, are analysed for the form factors of the semileptonic weak exclusive decays of Bc. The Bc lifetime is evaluated with account taken of the corrections to the spectator mechanism of the decay, because of the quark binding into the meson. The total and differential cross sections of the Bc production in different interactions are calculated. The analytic expressions for the fragmentational production cross sections of Bc are derived. The possibility of the practical search for Bc in current and planned experiments at electron–positron and hadron colliders is analysed.

Angular distributions and lifetime differences in $B_s \to J/\psi \phi$ decays

Abstract: The strange $B$ meson $B_s \equiv \bar b s$ and its charge-conjugate $\bar B_s \equiv b \bar s$ are expected to mix with one another in such a way that the mass eigenstates $B_s^H$ (``heavy'') and $B_s^L$ (``light'') may have a perceptible lifetime difference of up to 40\%, with the CP-even eigenstate being shorter-lived. A simple transversity analysis permits one to separate the CP-even and CP-odd components of $B_s \to J/\psi \phi$, and thus to determine the lifetime difference. The utility of a similar analysis for $B^0 \to J/\psi K^{*0}$ is noted.

Fast hadronization of supercooled quark-gluon plasma.

Abstract: A new rapid hadronization scenario is proposed based on the dynamical chiral model including quarks interacting with background meson fields. We estimate time scales and spatial characteristics of chiral-symmetry breaking instabilities in expanding, nonthermal quark-gluon plasma. The transition from the chirally symmetric to broken state proceeds through the formation of multiquark-antiquark clusters, surrounded by domains of the coherent chiral field.

Isospin structure of penguin diagrams and their consequences in B meson physics.

Abstract: Isospin structure of electroweak penguin diagrams is different from the gluon-mediated or strong penguin. Given the large top mass, these electroweak contributions are significant. This has the consequence that some previous analyses which relied on a simple isospin structure in charmless [ital B] meson decays become inapplicable. We present the general Hamiltonian in next-to-leading order QCD, and illustrate our conclusion quantitatively for [ital B][r arrow][pi][pi] and [ital B][r arrow][ital K][pi] decays in the factorization approximation. Some remarks on [ital CP] asymmetries in [ital B] meson decays are also made.

Photo- and electroproduction of eta mesons

Abstract: Eta photo- and electroproduction off the nucleon is investigated in an effective lagrangian approach that contains Born terms and both vector meson and nucleon resonance contributions. In particular, we review and develop the formalism for coincidence experiments with polarization degrees of freedom. The different response functions appearing in single and double polarization experiments have been studied. We will present calculations for structure functions and kinematical conditions that are most sensitive to details of the lagrangian, in particular with regard to contributions of nucleon resonances beyond the dominantS 11(1535) resonance.

Low- and High-Mass Components of the Photon Distribution Functions *

Abstract: The structure of the general solution of the inhomogeneous evolution equations allows the separation of a photon structure function into perturbative (“anomalous”) and non-perturbative contributions. The former part is fully calculable, and can be identified with the high-mass contributions to the dispersion integral in the photon mass. Properly normalized “state” distributions can be defined, where the $$\gamma \to q\bar q$$ splitting probability is factored out. These state distributions are shown to be useful in the description of the hadronic event properties, and necessary for a proper eikonalization of jet cross sections. Convenient parametrizations are provided both for the state and for the full anomalous parton distributions. The non-perturbative parts of the parton distribution functions of the photon are identified with the low-mass contributions to the dispersion integral. Their normalizations, as well as the value of the scaleQ 0 at which the perturbative parts vanish, are fixed by approximating the low-mass contributions by a discrete, finite sum of vector mesons. The shapes of these hadronic distributions are fitted to the available data onF 2 γ (x, Q 2). Parametrizations are provided forQ 0=0.6 GeV andQ 0=2 GeV, both in the DIS and the $$\overline {MS}$$ factorization schemes. The full parametrizations are extended towards virtual photons. Finally, the often-used “FKP-plus-TPC/2γ” solution forF 2 γ (x, Q 2) is commented upon.

Radiative weak decays of charm mesons

Abstract: We address standard model predictions for flavor-changing radiative transitions of the pseudoscalar charm mesons. Short-distance contributions in D radiative transitions are contrasted with those in B decays. A full analysis is presented of the c\ensuremath{\rightarrow}u+\ensuremath{\gamma} electromagnetic penguin amplitude with QCD radiative corrections included. Given the importance of long-range effects for the charm sector, special attention is paid to such contributions as the vector dominance and pole amplitudes. A number of two-body final states in exclusive charm radiative decays is considered and the corresponding branching ratio predictions are given.

Variations of hadron masses and matter properties in dense nuclear matter.

Abstract: Using a self-consistent quark model for nuclear matter we investigate variations of the masses of the nonstrange vector mesons, the hyperons, and the nucleon in dense nuclear matter (up to four times the normal nuclear density). We find that the changes in the hadron masses can be described in terms of the value of the scalar mean field in matter. The model is then used to calculate the density dependence of the quark condensate in-medium, which turns out to be well approximated by a linear function of the nuclear density. Some relations among the hadron properties and the in-medium quark condensate are discussed.

Physics of B_c mesons

Abstract: In the framework of potential models for heavy quarkonium the mass spectrum for the system ($\bar b c$) is considered. Spin-dependent splittings, taking into account a change of a constant for effective coulomb interaction between the quarks, and widths of radiative transitions between the ($\bar b c$) levels are calculated. In the framework of QCD sum rules, masses of the lightest vector $B_c^*$ and pseudoscalar $B_c$ states are estimated, scaling relation for leptonic constants of heavy quarkonia is derived, and the leptonic constant $f_{B_c}$ is evaluated. The $B_c$ decays are considered in the framework of both the potential models and the QCD sum rules, where the significance of Coulomb-like corrections is shown. The relations, following from the approximate spin symmetry for the heavy quarks in the heavy quarkonium, are analysed for the form factors of the semileptonic weak exclusive decays of $B_c$. The $B_c$ lifetime is evaluated with the account of the corrections to the spectator mechanism of the decay, because of the quark binding into the meson. The total and differential cross sections of the $B_c$ production in different interactions are calculated. The analytic expressions for the fragmentational production cross sections of $B_c$ are derived. The possibility of the practical $B_c$ search in the current and future experiments at electron-positron and hadron colliders is analysed.

Chiral perturbation theory for vector mesons.

Abstract: We derive a heavy vector-meson chiral Lagrangian in which the vector mesons are treated as heavy static matter fields. The unknown couplings of the chiral Lagrangian are further related using the 1/N_c expansion. Chiral perturbation theory is applied to the vector-meson mass matrix. At one loop there are large corrections to the individual vector-meson masses, but the singlet-octet mixing angle remains almost unchanged. The parity-violating s-wave φ→ρπ weak decay amplitude is derived in the combined chiral and large N_c limits. Rare φ decays provide a sensitive test of nonleptonic neutral current structure.

Dynamical generation of the gauged su(2) linear sigma model

Abstract: The fermion and meson sectors of the quark-level SU(2) linear sigma model are dynamically generated from a meson–quark Lagrangian, with the quark (q) and meson (σ, π) fields all treated as elementary, having neither bare masses nor expectation values. In the chiral limit, the masses are predicted to be mq = fπg, mπ = 0, mσ = 2mq, and we also find that the quark–meson coupling is $g =2\pi /\sqrt{N_c}$, the three-meson coupling is $g' =m_\sigma^2 /2f_\pi =2gm_q$ and the four-meson coupling is λ = 2g2 = g′/fπ, where fπ ≃ 90 MeV is the pion decay constant and Nc = 3 is the color number. By gauging this model one can generate the couplings to the vector mesons ρ and A1, including the quark–vector coupling constant gρ = 2π, gρππ, gA1ρπ and the masses mρ ~ 700 MeV, $m_{A_1}\simeq \sqrt{3} m_\rho$; of course the vector and axial currents remain conserved throughout.