Search

Within the framework of a nonrelativistic quark model we evaluate the six form factors associated to the \Lambda_b^0 \to \Lambda_c^+ l^- \bar{\nu}_l semileptonic decay. The baryon wave functions were evaluated using a variational approach applied to a family of trial functions constrained by Heavy Quark Symmetry (HQS). We use a spectator model with only one-body current operators. For these operators we keep up to first order terms on the internal (small) heavy quark momentum, but all orders on the transferred (large) momentum. Our result for the partially integrated decay width is in good agreement with lattice calculations. Comparison of our total decay width to experiment allows us to extract the V_{cb} Cabbibo-Kobayashi-Maskawa matrix element for which we obtain a value of |V_{cb}|=0.047\pm 0.005 in agreement with a recent determination by the DELPHI Collaboration. Furthermore, we obtain the universal Isgur-Wise function with a slope parameter \rho^2=0.98 in agreement with lattice results., Comment: 4 pages, 3 figures. Presented at 6th International Conference on Hyperons, Charm and Beauty Hadrons (BEACH 2004), Chicago, Illinois, 27 Jun - 3 Jul 2004

The Quasi-Elastic (QE) contribution of the nuclear inclusive electron model developed in reference \cite{GNO97} is extended to the study of electroweak Charged Current (CC) induced nuclear reactions at intermediate energies of interest for future neutrino oscillation experiments. Long range nuclear (RPA) correlations, Final State Interaction (FSI) and Coulomb corrections are included within the model. RPA correlations are shown to play a crucial role in the whole range of neutrino energies, up to 500 MeV, studied in this work. Predictions for inclusive muon capture for different nuclei through the Periodic Table and for the reactions $^{12}$C $(\nu_\mu,\mu^-)X$ and $^{12}$C $(\nu_e,e^-)X$ near threshold are also given., Comment: Talk given by J. Nieves at NUINT04, Gran Sasso, March 2004

The Quasi-Elastic (QE) contribution of the nuclear inclusive electron scattering model developed in Nucl. Phys. A627 (1997) 543 is extended to the study of electroweak Charged Current (CC) induced nuclear reactions, at intermediate energies of interest for future neutrino oscillation experiments. The model accounts for, among other nuclear effects, long range nuclear (RPA) correlations, Final State Interaction (FSI) and Coulomb corrections. Predictions for the inclusive muon capture in $^{12}$C and the reaction $^{12}$C $(\nu_\mu,\mu^-)X$ near threshold are also given. RPA correlations are shown to play a crucial role and their inclusion leads to one of the best existing simultaneous description of both processes, with accuracies of the order of 10-15% per cent for the muon capture rate and even better for the LSND measurement., Comment: 31 pages and 14 figures, accepted for publication as a regular article in Physical Review C

We study the quark-mass dependence of J^P = \frac12^- s-wave and J^P = \frac32^- d-wave baryon resonances. Parameter-free results are obtained in terms of the leading order chiral Lagrangian. In the 'heavy' SU(3) limit with m_\pi =m_K \simeq 500 MeV the s-wave resonances turn into bound states forming two octets plus a singlet representations of the SU(3) group. Similarly the d-wave resonances turn into bound states forming an octet and a decuplet in this limit. A contrasted result is obtained in the 'light' SU(3) limit with m_\pi =m_K \simeq 140 MeV for which no resonances exist., Comment: 8 pages, three figures, talk presented at HYP2003

We extend our previous analysis of the unitarized pion-nucleon scattering amplitude including up to fourth order terms in Heavy Baryon Chiral Perturbation Theory. We pay special attention to the stability of the generated Delta(1232 resonance, the convergence problems and the power counting of the chiral parameters., Comment: 9 pages, 4 figures

The use of Heavy Quark Symmetry to study bottom and charmed baryons leads to important simplifications of the non-relativistic three body problem, which turns out to be easily solved by a simple variational ansatz. Our simple scheme reproduces previous results (baryon masses, charge and mass radii, $...$) obtained by solving the Faddeev equations with simple non-relativistic quark--quark potentials, adjusted to the light and heavy--light meson spectra. Wave functions, parameterized in a simple manner, are also given and thus they can be easily used to compute further observables. Our method has been also used to find the predictions for strangeness-less baryons of the SU(2) chirally inspired quark-quark interactions. We find that the one pion exchange term of the chirally inspired interactions leads to relative changes of the $\Lambda_b$ and $\Lambda_c$ binding energies as large as 90%., Comment: 20 pages, 5 figures. Revised version to be published in Nucl. Phys. A

We evaluate masses of bottom and charmed baryons using several non-relativistic quark potentials which parameters have been adjusted to the meson spectra. Heavy Quark Symmetry leads to important simplifications of the three body problem, which turns out to be easily solved by a simple variational ansatz. The wave functions obtained can be readily used to compute further observables as mass densities or form factors. The quark-quark potentials explored so far, show an overall good agreement with the experimental masses., Comment: Contribution to the Hadron Spectroscopy (Hadron'03) conference. 5 pages, 2 figures, 5 tables

We study the quark mass dependence of $J^P = \frac12^-$ s-wave baryon resonances. Parameter free results are obtained in terms of the leading order chiral Lagrangian. In the 'heavy' SU(3) limit with $m_\pi =m_K \simeq $ 500 MeV the resonances turn into bound states forming two octets plus a singlet representations of the SU(3) group. A contrasted result is obtained in the 'light' SU(3) limit with $m_\pi =m_K \simeq $ 140 MeV for which no resonances exist. Using physical quark masses our analysis suggests to assign to the $S=-2$ resonances $\Xi(1690)$ and $\Xi(1620)$ the quantum numbers $J^P=1/2^-$., Comment: 4 pages, 2 figure

The $s-$wave meson-baryon scattering is analyzed for the isospin-strangeness $I=1/2, S=0$ and $I=0,S=-1$ sectors, in a Bethe-Salpeter coupled channel formalism incorporating Chiral Symmetry. For both sectors, four channels have been considered: $\pi N$, $\eta N$, $K \Lambda$, $K \Sigma$ and $\pi \Sigma$, $\bar K N$, $\eta \Lambda$, $K \Xi$, respectively. The needed two particle irreducible matrix amplitudes are taken from lowest order Chiral Perturbation Theory in a relativistic formalism. There appear undetermined low energy constants, as a consequence of the renormalization of the amplitudes, which are obtained from fits to the available data: elastic $\pi N $ phase-shifts, $\pi^- p \to \eta n$ and $\pi^- p \to K^0 \Lambda$ cross sections and to $\pi\Sigma\to\pi\Sigma$ mass-spectrum, the elastic $\bar K N \to \bar K N$ and $ \bar K N\to \pi \Sigma$ $t$--matrices and to the $ K^- p \to \eta \Lambda$ cross section data. The position and residues of the complex poles in the second Riemann sheet of the scattering amplitude determine masses, widths and branching ratios of the $S_{11}-$ $N$(1535) and $-N$(1650) and $S_{01}-$ $\Lambda$(1405) and $-\Lambda$(1670) resonances, in reasonable agreement with experiment. A good overall description of data, from threshold up to around 2 GeV is achieved despite the fact that three-body channels have not been explicitly included., Comment: 5 Pages, 2 figures, invited contribution to Focus Session on Nature of Threshold N*, to be published in Proceedings of Nstar 2002, Pittsburgh, USA, October 9-12, 2002 (World Scientific)

We use standard distorted wave theory techniques and dimensional regularization to find out solutions of the nucleon-nucleon Lippman--Schwinger equation with a kernel determined by the Weinberg's next-to-leading potential, which consists of one--pion exchange and additional contact terms with derivatives. Though for simplicity, we restrict the discussion to the $^1S_0$ channel and to contact terms containing up to two derivatives, the generalization to higher waves and/or number of derivatives is straightforward. The undetermined low energy constants emerging out of the renormalization procedure are fitted to data., Comment: 7 pages, revtex4, Phys. Lett. B in print

Meson-Baryon Interactions can be successfully described using both Chiral Symmetry and Unitarity. The $s-$wave meson-baryon scattering amplitude is analyzed in a Bethe-Salpeter coupled channel formalism incorporating Chiral Symmetry in the potential. Two body coupled channel unitarity is exactly preserved. The needed two particle irreducible matrix amplitude is taken from lowest order Chiral Perturbation Theory in a relativistic formalism. Off-shell behavior is parameterized in terms of low energy constants. The relation to the heavy baryon limit is discussed. The position of the complex poles in the second Riemann sheet of the scattering amplitude determine masses and widths baryonic resonances of the N(1535), N(1670), $\Lambda (1405)$ and $\Lambda(1670)$ resonances which compare well with accepted numbers., Comment: 14 pages, 3 figures; Talk at 2nd International Workshop On Hadron Physics: Effective Theories Of Low-Energy QCD, Coimbra, Portugal, 25-29 Sep 2002. (To appear in the proceedings.) Presented by E. Ruiz Arriola

The $s-$wave meson-baryon scattering is analyzed for the strangeness $S=-1$ and isospin I=0 sector in a Bethe-Salpeter coupled channel formalism incorporating Chiral Symmetry. Four channels have been considered: $\pi \Sigma$, $\bar K N$, $\eta \Lambda$ and $K \Xi$. The required input to solve the Bethe-Salpeter equation is taken from lowest order Chiral Perturbation Theory in a relativistic formalism. There appear undetermined low energy constants, as a consequence of the renormalization of the amplitudes, which are obtained from fits to the $\pi\Sigma\to\pi\Sigma$ mass-spectrum, to the elastic $\bar K N \to \bar K N$ and $ \bar K N\to \pi \Sigma$ $t$--matrices and to the $ K^- p \to \eta \Lambda$ cross section data. The position and residues of the complex poles in the second Riemann Sheet of the scattering amplitude determine masses, widths and branching ratios of the $S_{01}-$ $\Lambda$(1405) and $-\Lambda$(1670) resonances, in reasonable agreement with experiment. A good overall description of data, from $\pi \Sigma$ threshold up to 1.75 GeV, is achieved despite the fact that three-body channels have not been explicitly included., Comment: 23 pages, Latex, 10 Figures. In this revised version a new subsection 3.6 on Heavy Baryon Expansion and new references have been added

We calculate $K^-$-nucleus elastic differential, reaction and total cross sections for different nuclei ($^{12}$C,$^{40}$Ca and $^{208}$Pb) at several laboratory antikaon momenta, ranging from 127 MeV to 800 MeV. We use different antikaon-nucleus optical potentials, some of them fitted to kaonic atom data, and study the sensitivity of the cross sections to the considered antikaon-nucleus dynamics., Comment: Only 4 pages, Latex, 3 Figures. This version is much shorter than the previous one. Some details and references have been omitted

We evaluate pionic decay widths of Lambda hypernuclei using a shell model for, both the nuclear bound and the continuum nucleon wave functions in the final state, and distorted waves for the outgoing pion. An excellent agreement with the recent KEK measurement of $\pi^-$-decay widths of $^{12}_\Lambda$C and $^{28}_\Lambda$Si is found. Besides, results for $^{56}_\Lambda$Fe are consistent with the existing upper bound., Comment: 19 pages, 4 figures, 7 tables, revised version to be published in Phys. Rev. C

The $s-$wave meson-baryon scattering amplitude is analyzed for the strangeness $S=-1$ and isospin I=0 sector in a Bethe-Salpeter coupled channel formalism incorporating Chiral Symmetry. Four two-body channels have been considered: $\bar K N$, $\pi \Sigma $, $\eta \Lambda $, $ K \Xi$. The needed two particle irreducible matrix amplitude is taken from lowest order Chiral Perturbation Theory in a relativistic formalism. Off-shell behaviour is parameterized in terms of low energy constants, which outnumber those assumed in previous works and provide a better fit to the data. The position of the complex poles in the second Riemann sheet of the scattering amplitude determine masses and widths of the $\Lambda (1405)$ and $\Lambda(1670)$ resonances which compare well with accepted numbers., Comment: 3 pages, 2 figures; contribution to the proceedings of Quark Nuclear Physics (QNP 2002), Juelich, Germany, 9-14 Jun 2002. Presented by E. Ruiz Arriola

The energies and widths of bound states of the $\eta$ meson in different nuclei are obtained using the results for its selfenergy in a nuclear medium, which is evaluated in a selfconsistent manner using techniques of unitarized chiral perturbation theory. We find bound states in all studied nuclei (from $^{12}{C}$ on) and the half widths obtained are larger than the separation of the levels, what makes the experimental observation of peaks unlikely. We have paid a special attention to the region of nuclei where only the $1s$ state appears and the binding energies are of the order of magnitude of the half width, which would magnify the chances that some broad peak could be observed. This is found in the region of $^{24}{Mg}$ with a binding energy around 12.6 MeV and half width of 16.7 MeV. In heavy nuclei like $^{208}{Pb}$ there are many bound states which would be difficult to disentangle and the deepest state has a binding energy about 21 MeV and half width around 16 MeV. Such an overlapping accumulation of states could be seen as an extension of the continuum of $\eta$ strength into the bound region in $\eta$ production experiments., Comment: 9 pages, Latex, 2 Figures

We evaluate widths and shifts of pionic atoms using a theoretical microscopical potential in which the pion decay constant $f_\pi$ is changed by an in--medium density dependent one ($f_\pi(\rho)$), predicted by different partial Chiral restoration calculations. We show that the results obtained for shifts and widths are worse than if this modification were not implemented. On the other hand, we argue that in microscopic many body approaches for the pion selfenergy, based on effective Lagrangians, the mechanisms responsible for the change of $f_\pi$ in the medium should be automatically incorporated. Therefore, the replacement of $f_\pi$ by $f_\pi(\rho)$ in the many body derivation of the microscopic potential would be inappropriate., Comment: 10 pages, new comments and references added

Data on Lambda-Lambda hypernuclei provide a unique method to learn details on the strangeness S =-2 sector of the baryon-baryon interaction. From the free space Bonn-Julich potentials, determined from data on baryon-baryon scattering in the S=0,-1 channels, we construct an interaction in the S =-2 sector to describe the experimentally known Lambda-Lambda hypernuclei. After including short--range (Jastrow) and RPA correlations, we find masses for these Lambda-Lambda hypernuclei in a reasonable agreement with data, taking into account theoretical and experimental uncertainties. Thus, we provide a natural extension, at low energies, of the Bonn-Julich OBE potentials to the S =-2 channel., Comment: 4 pages, 2 figures, revtex4 style. Minor changes in conclusions. References updated. Accepted in Phys. Rev. Lett

A microscopic optical potential based on a chiral model is used as a starting point for studying kaonic atoms levels. We add to this potential a phenomenological part fitted to the experimentally known shifts and widths of kaonic levels. This fitted potential is used to predict deeply bound atomic levels, as well as nuclear levels. Comparison with the predictions of other optical models found in the literature is done. Also the effects on the kaonic atoms levels of certain known non-local contributions to the optical potential are also analyzed., Comment: Corrected misprints on fitted parameters. 4 pag., 2 fig., 1 table. Contribution to "Mesons and Light Nuclei 01", 2001, Prague, Czech Republic

The inverse amplitude method is used to unitarize the two loop $\pi\pi$ scattering amplitudes of SU(2) Chiral Perturbation Theory in the $I=0,J=0$, $I=1,J=1$ and $I=2,J=0$ channels. An error analysis in terms of the low energy one-loop parameters $\bar l_{1,2,3,4,}$ and existing experimental data is undertaken. A comparison to standard resonance saturation values for the two loop coefficients $\bar b_{1,2,3,4,5,6} $ is also carried out. Crossing violations are quantified and the convergence of the expansion is discussed., Comment: (Latex, epsfig) 30 pages, 13 figures, 8 tables

In this talk we briefly describe some basic elements of chiral perturbation theory, $\chi PT$, and how the implementation of unitarity and other novel elements lead to a better expansion of the $T$ matrix for meson meson and meson baryon interactions. Applications are then done to the $ \pi \pi $ interaction in nuclear matter in the scalar and vector channels, antikaons in nuclei and $K^-$ atoms, and how the $\phi$ meson properties are changed in a nuclear medium., Comment: 15 pages, 9 figures, Invited talk in the International Symposium on Nuclear Physics, Bombay, december 2000

We evaluate the p-wave $K^-N$ amplitudes from the chiral Lagrangians and from there construct the p-wave part of the $K^-$ nucleus optical potential plus a small s-wave part induced from the elementary p-wave amplitude and the nuclear Fermi motion. Simultaneously, the momentum and energy dependence of the s-wave optical potential, previously developed, are taken into account and shown to generate a small p-wave correction to the optical potential. All the corrections considered are small compared to the leading s-wave potential, and lead to changes in the shifts and widths which are smaller than the experimental errors. A thorough study of the threshold region and low densities is conducted, revealing mathematical problems for which a physical solution is given., Comment: revised version, 28 pages, Latex, 8 postscript figures. Submitted to Nucl. Phys. A

We review our recent work on unitarization and chiral perturbation theory both in the $\pi\pi$ and the $\pi N$ sectors. We pay particular attention to the Bethe-Salpeter and Inverse Amplitude unitarization methods and their recent applications to $\pi\pi$ and $\pi N$ scattering., Comment: (LaTeX) 10 pages, 5 figures, Talk given by E.R.A. at the Miniworkshop on "Few Quark Problems'', Bled, Slovenia, July-2000

Heavy Baryon Chiral Perturbation Theory (HBChPT) to leading order provides a kernel to solve the Bethe-Salpeter equation for the $P_{33}$ ($\Delta(1232)$-channel) $\pi-N$ system, in the infinite nucleon mass limit. Crossed Born terms include, when iterated within the Bethe-Salpeter equation, both {\it all} one- and {\it some} two-pion intermediate states, hence preserving elastic unitarity below the two-pion production threshold. This suggests searching for a solution with the help of dispersion relations and suitable subtraction constants, when all in-elasticities are explicitly neglected. The solution allows for a successful description of the experimental phase shift from threshold up to $\sqrt{s}=1500$ MeV in terms of four subtraction constants. Next-to-leading order HBChPT calculations are also used to estimate the unknown subtraction constants which appear in the solution. Large discrepancies are encountered which can be traced to the slow convergence rate of HBChPT., Comment: 11 pages, 3 figures

We use the Omnes representation to obtain the q-squared dependence of the form factors f+ and f0 for semileptonic H -> pi decays from the elastic pi H -> pi H scattering amplitudes, where H denotes a B or D meson. The scattering amplitudes used satisfy elastic unitarity and are calculated from two-particle irreducible amplitudes obtained using tree-level heavy meson chiral perturbation theory (HMChPT). The q-squared dependences for the form factors agree with lattice QCD results when the HMChPT coupling constant, g, takes values smaller than 0.32, and confirm the milder dependence of f0 on q-squared found in sumrule calculations., Comment: 9 pages latex, 5 figures, uses graphicx package. Version accepted for publication (discussion extended, one extra figure)

We show how the unitarized description of pion nucleon scattering within Heavy Baryon Chiral Perturbation Theory can be considerably improved, by a suitable reordering of the expansion over the nucleon mass. Within this framework, the $\Delta$ resonance and its associated pole can be recovered from the chiral parameters obtained from low-energy determinations. In addition, we can obtain a good description of the six $S$ and $P$ wave phase shifts in terms of chiral parameters with a natural size and compatible with the Resonance Saturation Hypothesis., Comment: 10 pages, 3 tables, 2 figures

Using a microscopic antikaon-nucleus optical potential recently developed by Ramos and Oset (nucl-th/9906016, in print in Nuclear Physics A) from a chiral model, we calculate strong interaction shifts and widths for $K^-$ atoms. This purely theoretical potential gives an acceptable description of the measured data ($\chi^2/{\rm num.data}= 3.8$), though it turns out to be less attractive than what can be inferred from the existing kaon atomic data. We also use a modified potential, obtained by adding to the latter theoretical one a s-wave term which is fitted to known experimental kaonic data ($\chi^2/{\rm degree of freedom}= 1.6$), to predict deeply bound $K^-$ atomic levels, not detected yet. This improved potential predicts, in general, states even narrower than those recently reported by Friedman and Gal. (Phys.Let.B459, 1999, 43). This reinforces the idea that these deeply atomic states can be detected and resolved by using suitable nuclear reactions. Besides, we also study $K^-$ and $\bar K^0$ nuclear bound states and compute binding energies and widths, for both species of antikaons, in $^{12}$C, $^{40}$Ca and $^{208}$Pb. Despite of restricting our study only to potentials obtained from best fits to the known kaonic atom data, the dynamics of these nuclear bound states depend dramatically on the particular optical potential used., Comment: 22 pages, Latex, 6 postscript figures. References and figures added. In print in Nuclear Physics A

Based on the recent phenomenological analysis done by M. Mojzis, Eur. Phys. Jour. {\bf C2} (1998) 182, of $\pi N $ scattering within the framework of Heavy Baryon Chiral Perturbation Theory we predict, after a suitable unitarization of the amplitude, the $\Delta$ resonance at $\sqrt{s} = M_\Delta = 1238 \er{22}{18}{\rm MeV} $ and with a width of $ \Gamma_\Delta = 150 \er{43}{31} {\rm MeV} $ in excellent agreement with the experimental numbers. The error bars reflect only the uncertainties in the low energy parameters as determined from an extrapolation to threshold of experimental data. We also describe satisfactorily, within errors, the phase shifts up to $\sqrt{s} \sim 1300 {\rm MeV} $., Comment: (RevTeX) 12 pages, 2 figures

We present the results of a lattice computation of the form factors for B^0 ->pi^- l^+nu_l decays near zero-recoil. These results will allow a determination of the CKM matrix element |Vub| when measurements of the differential decay rate become available. We also provide models for extrapolation of the form factors and rate to the full recoil range. Our computation is performed in the quenched approximation to QCD on a 24^3x48 lattice at beta=6.2, using a non-perturbatively O(a)-improved action. The masses of all light valence quarks involved are extrapolated to their physical values., Comment: 8 Pages, 4 colour figures, Accepted for Publication in Phys.Lett.B. Typset in REVTeX 3.1 added figure and table with minor text modification

A lattice determination of the form factor and decay constants for the semileptonic decay of heavy pseudoscalar (PS) mesons at zero recoil is presented from which the soft pion relation is satisfied. Chiral extrapolation of the form factor is performed at constant $q^2$. Pole dominance is used to extrapolate the form factor in heavy quark mass. At the B mass, the form factor at zero recoil lies somewhat below the ratio of decay constants; the relation remains satisfied within error., Comment: 3 pages, 2 figures

The Bethe-Salpeter equation restores exact elastic unitarity in the $s-$ channel by summing up an infinite set of chiral loops. We use this equation to show how a chiral expansion can be undertaken in the two particle irreducible amplitude and the propagators accomplishing exact elastic unitarity at any step. Renormalizability of the amplitudes can be achieved by allowing for an infinite set of counter-terms as it is the case in ordinary Chiral Perturbation Theory. Crossing constraints can be imposed on the parameters to a given order. Within this framework, we calculate the leading and next-to-leading contributions to the elastic $\pi \pi$ scattering amplitudes, for all isospin channels, and to the vector and scalar pion form factors in several renormalization schemes. A satisfactory description of amplitudes and form factors is obtained. In this latter case, Watson's theorem is automatically satisfied. From such studies we obtain a quite accurate determination of some of the ChPT $SU(2)-$low energy parameters (${\bar l}_1 - {\bar l}_2 = -6.1\er{0.1}{0.3} $ and ${\bar l}_6= 19.14 \pm 0.19$). We also compare the two loop piece of our amplitudes to recent two--loop calculations., Comment: 63 pages, 9 figures. Some discussions on off-shell ambiguities and convergence of the expansion added

Using the analysis of ChPT to two loops, we perform an error analysis of the threshold and low energy parameters, based on the uncertainties for the one loop low energy parameters and the resonance saturation mechanism. Different sets of one loop low energy constants have been considered.Thus, the predictive power of the effective field theory is quantified on the basis of the present experimental uncertainties., Comment: 12 pages, 2 tables, 3 figures. Numerics upgraded to v2 of ref.(8). New reference added. Typos corrected. New figure for SU(3) parameters included

We fit the $^1 S_0 \Lambda\Lambda$ interaction in the nuclear medium to the masses of the experimentally known double-$\Lambda$ hypernuclei: $^{\phantom{6}6}_{\Lambda\Lambda}$He, $^{10}_{\Lambda\Lambda}$Be and $^{13}_{\Lambda\Lambda}$B. We derive this effective interaction from OBE J\"ulich $\Lambda\Lambda$-type potentials and using both Hartree-Fock and variational approaches. We find that the inclusion of $\Lambda\Lambda$ correlations in the variational scheme leads to significant differences and a better understanding of the dynamical features of the system. We investigate the sensitivity of the binding energies and the mesonic decay widths of the above double-$\Lambda$ hypernuclei to the $\omega \Lambda\Lambda $ coupling and the form factor at the $\sigma \Lambda\Lambda$ vertex. We also use this effective interaction to predict binding energies and pionic decay widths of heavier double-$\Lambda$ hypernuclei, not discovered yet. Finally, we discard the existence of $^1 S_0$ $\Lambda\Lambda$ bound states provided the $\Lambda\Lambda-\Xi N$ coupling can be neglected., Comment: 49 pages, LaTeX + 8 ps-figure files

The $\Lambda$-$\Lambda$ effective interaction, in the channel $L=S=0$, in the nuclear medium is fitted to the available binding energies, $B_{\Lambda\Lambda}$, of double $\Lambda$ hypernuclei: $^{6}_{\Lambda\Lambda}$He, $^{10}_{\Lambda\Lambda}$Be and $^{13}_{\Lambda\Lambda}$B. The mesonic decay of these hypernuclei is also investigated. Finally, this effective interaction is used to predict the binding energies and mesonic decays widths of heavier double $\Lambda$ hypernuclei., Comment: 4 pages, (latex file, postscript-file and 3 Postscript-figures included)

We have performed a many body calculation of the inclusive (e,e') cross section which runs over the three traditional regions at intermediate energies: the quasielastic peak, the dip region and the delta region. The longitudinal and transverse response functions in the quasielastic peak have also been evaluated. Traditional effects like polarization, meson exchange currents, final state interaction and delta renormalization in the nuclear medium have been included. Meson exchange currents are generated from a model of pion electroproduction on the nucleon which reproduces accurately the experimental data. The inclusive cross section accounts for 1N,2N,3N mechanisms of virtual photon absorption and one pion production. Meson exchange currents associated to the (\gamma^*,2\pi) reaction are also accounted for. We obtain good results for the (e,e') cross sections in the whole energy range and for different nuclei. The response functions are also in good agreement with the latest experimental analysis. On the other hand, the method provides the separation of the contribution to the inclusive cross section from different physical channels which is a necessary input to evaluate cross sections like (e,e' N), (e,e' NN), (e,e' \pi) etc., Comment: 65 pages (LaTeX + ps-figure files)

We study the inclusive $(e,e^\prime N)$, $(e,e^\prime NN)$, $(e,e^\prime \pi)$, $(e,e^\prime \pi N)$ reactions in nuclei using a Monte Carlo simulation method to treat the multichannel problem of the final state. The input consists of reaction probabilities for the different steps evaluated using microscopical many body methods. We obtain a good agreement with experiment in some channels where there is data and make predictions for other channels which are presently under investigation in several electron laboratories. The comparison of the theoretical results with experiment for several kinematical conditions and diverse channels can serve to learn about different physical processes ocurring in the reaction. The potential of this theoretical tool to make prospections for possible experiments, aiming at pinning down certain reaction probabilities, is also emphasized., Comment: 21 pages (LaTeX + figure files)

We present the results of the first lattice study of semileptonic decays of baryons containing a b-quark. Predictions for the decay distributions are given and the Isgur-Wise functions for heavy baryons are computed, for values of the velocity transfer up to about omega=1.2. The computations are performed on a 24^3 x 48 lattice at beta=6.2 using the Sheikholeslami-Wohlert action in the quenched approximation., Comment: 55 pages, 17 ps figures, Latex 2.09, uses epsf

We describe the form factors for semileptonic B to rho l nu and radiative B to K* gamma decays with just two parameters and the two form factors for semileptonic B to pi l nu decays with three parameters. The parametrizations are constrained by lattice results and are consistent with heavy quark symmetry, kinematic constraints and light cone sum rule scaling relations., Comment: 3 pages, latex, 2 eps files, uses epsf.sty and espcrc2.sty, poster presented at Lattice 97, Edinburgh, 22-26 July 1997

The radiative pion capture process in nuclei is approached by using a continuum shell-model description of the nucleus, together with a phenomenological treatment of the two particle-two hole effects. It is found that these effects play an important role to reproduce the observed experimental photon energy distribution. This distribution as well as the integrated one depends significantly on the details of the mean field potential. This makes this process interesting to investigate the nuclear structure dynamics., Comment: 21 pages, LateX file + 5 figures, epsf.sty

We report on a lattice calculation of the form factors A_0 and A_3 for the pseudoscalar to vector meson semileptonic decay Bbar^0 to rho^+ l^- nubar_l. We find that resonant (or pole-type) contributions alone are unable to describe these two form factors simultaneously. For the quantity A_0(q^2=0), which is important phenomenologically for the determination of |Vub|, we extract a range of values, A_0(q^2=0) = (0.16--0.35) +0.09-0.06, where the range is due to systematic uncertainty and the quoted error is statistical. We have also determined A_2(q^2=0) = 0.28 +0.09-0.06 +0.04-0.05., Comment: 14 pages LaTeX209 with 5 figures from 11 encapsulated PostScript files, using epsf.sty. Also available from http://wwwhep.phys.soton.ac.uk/hepwww/papers/shep9601/

We present results of a lattice computation of the vector and axial-vector current matrix elements relevant for the semileptonic decay B^0-bar -> rho^+ l^- nu_l-bar. The computations are performed in the quenched approximation of lattice QCD on a 24^3 x 48 lattice at beta = 6.2, using an O(a) improved fermionic action. Our principal result is for the differential decay rate, dGamma/dq^2, for the decay B^0-bar -> rho^+ l^- nu_l-bar in a region beyond the charm threshold, allowing a model-independent extraction of |V_{ub}| from experimental measurements. Heavy quark symmetry relations between radiative and semileptonic decays of B-bar mesons into light vector mesons are also discussed., Comment: 22 pages LaTeX-209 (dependent on settings in a4.sty), 23 PostScript figures included with epsf.sty. Complete PostScript file including figures available at http://wwwhep.phys.soton.ac.uk/hepwww/papers/shep9518/

We present a unified method for analysing form factors in B -> pi l nu-bar_l and B -> K* gamma decays. The analysis provides consistency checks on the q^2 and 1/M extrapolations necessary to obtain the physical decay rates. For the first time the q^2 dependence of the form factors is obtained at the B scale. In the B -> pi l nu-bar_l case, we show that pole fits to f^+ may not be consistent with the q^2 behaviour of f^0, leading to a possible factor of two uncertainty in the decay rate and hence in the value of |V_{ub}|^2 deduced from it. For B -> K* gamma, from the combined analysis of form factors T_1 and T_2, we find the hadronisation ratio R_{K^*} of the exclusive B -> K* gamma to the inclusive b -> s gamma rates is of order 35% or 15% for constant and pole-type behaviour of T_2 respectively., Comment: 13 pages, uuencoded compressed postscript file (including five figures). Also available from http://wwwhep.phys.soton.ac.uk/hepwww/papers/shep9509/

We use the method of finite temperature field theory to examine the propagation of mixed neutrinos through dense media, putting the emphasis in those situations in which the neutrinos themselves are in the background. The evolution equation for the flavor amplitudes is deduced, and the expressions for the corresponding hamiltonian matrix are given explicitly. We find that, in order to include the nonlinear effects due to the $\nu$-$\nu$ interactions, the neutrino propagator that must be used in the calculation of the neutrino self-energy diagrams that contain neutrinos in the internal lines is the propagator for the neutrino modes in the medium instead of the thermal free-field propagator. We also show how the absorptive contributions are included in terms of a non-hermitian part of the hamiltonian matrix, which we indicate how it is calculated. Our treatment provides a consistent generalization of a method that has been successfully applied to the study of neutrino oscillations in matter., Comment: 31 pages, latex, 1 figure available upon request