N -N Annihilation intoKK from atomicS-states

We investigate the initial state interaction for the antiprotonic hydrogen (protonium) annihilation intoKK. We calculate thepp-n¯n mixing in the annihilation region and underline that the initial state interaction drastically changes the results of the isospin analysis of theN¯NK¯K amplitude.On leave of absence from ITEP, Moscow     

Production of slow protonium in vacuum

We descrbe how protonium, the quasi-stable antiproton-proton bound system, has been synthesized following the interaction of antiprotons with the molecular ion in a nested Penning trap environment. From a careful analysis of the spatial distributions of antiproton annihilation events in the ATHENA experiment, evidence is presented for protonium production with sub-eV kinetic energies in states around n = 70, with low angular momenta. This work provides a new two-body system for studies using laser spectroscopic techniques.      

Search for the 70 keV resonance in17O(p, α)14N

A direct search for theE _R =70 keV resonance in¹⁷O(p, )¹⁴N was carried out using¹⁷O enriched Ta₂O₅ targets in combination with a 1 mA proton beam and suitable detectors of large solid angle. The observed upper limit for the resonance strength is8×10^–10 eV.Work supported in part by Deutsche Forschungsgemeinschaft (Ro429/21-2) and the Science Program SC1-0065 of the European Economic Community     

The decay of spin-isospin modes in a semi-infinite slab

The surface response for charge exchange (p, n) and (³He, T) reactions is studied in the-region using the semi-infinite slab model. The contribution to the total response from different decay channels, (NN, N, ), is calculated. These decay channels corresponds to the exclusive channels, (pp, p ⁺, ⁺), measured in recent (p, n) and (³He, T) experiments. The in-medium properties of the-resonance is taken into account by using microscopic calculations of the-width in nuclear matter. From the-width in nuclear matter a non-local imaginary-potential, as well as a local potential, is constructed for the semi-infinite slab model. The results in the semi-infinite slab model gives a qualitative understanding of the exclusive experiments. The exclusive (NN, N, ) channels are more sensitive, than the total response, tog-correlation parameters,-width and the absorption function used at the external vertex. Our calculations suggest low values of theg-correlation parametersg _N andg , (0.3). The results with the-width represented as a non-local or a local-potential are very similar, with only minor differences in the exclusive channels.Supported in part by the Swedish Natural Science Research Council and by the Director, Office of Energy Research, Office of High Energy and Nuclear Physics, Nuclear Physics Division of the U.S. Department of Energy under Contract No. DE-ACO3-76SF00098     

The\bar pp \to e^ + e^ - reaction at rest and the hierarchy of the annihilation ranges

A new procedure is devised to evaluate the proton form-factor at the ¯pp threshold from experimental data on ¯ppe ⁺ e ^– annihilation at rest. Comparison of the ¯ppe ⁺ e ^– and¯pp ^+– processes reveals that annihilation ranges for various channels are ranked in line with quark dynamics.This work has been funded in part by the German Federal Minister for Research and Technology (BMFT) under the contract number 06HD756     

B c decays

Theoretical predictions on decay properties of the simplest particle containing more than one heavy quark, the pseudoscalar mesonB _c ⁺ , are presented. Some channels that should present a distinctive signature are discussed in more detail.     

How effective is the weak mixing angle?

The usefulness of an effective weak mixing angle to parameterise the electroweak couplings of quarks and leptons measured at LEP is discussed. After a review of the theoretical formalism used to handle the electroweak radiative corrections to LEP processes, some suggestions for procedures to be used to extract the mixing angle are given and the various definitions of the effective weak mixing angle used by the experiments are examined. This is followed by an attempt to extract a coherent set of mixing angle values from the four experiments with the published 1989/90 (and some 1991) measurements of the Z⁰ line-shapes, asymmetries and polarisation. These results are updated with preliminary results from the 1991 data presented at the XXVI International Conference on High Energy Physics, in Dallas, USA, in August 1992, including the preliminary measurement ofA _LR at the SLC. The average value found for the effective weak mixing angle, defined for leptons, is sin² _eff ^lept =02322±0.0010. The error on sin² _eff ^lept can be further reduced by imposing the constraints between the parameters implicit in the MSM. With the additional constraint _s=0.123±0.006 imposed in the fit, the result is sin² _eff ^lept , where the central value is form _H =300 GeV, and the second error is the variation ifm _H is changed to 50 GeV (lower error) and 1000 GeV (upper error). Within the MSM, sin² _lept ^eff is directly related to the top-quark mass, and the corresponding value ism _t =153 _–25–23 ⁺²²⁺¹⁹ GeV, where the errors have the same meaning as those for sin² _eff ^lept . Finally, the prospects for improved precision are discussed.     

Higher twist and higher order contributions to the pion electromagnetic form factor

The andO ( _S ² ) corrections to the leading term of the perturbative QCD calculation of the pion electromagnetic form factor are examined numerically. Both sets of terms provide significant corrections for values ofQ ² between 1 and 15 GeV²/c².Parts of this work were done while one of us (JP) was a visitor at the national INT, the University of Maryland and TRIUMF. He is grateful to those institutions for their hospitality. We have benefitted from discussions with A. Mueller and G. Sterman. This work is supported in part by the U.S. Department of Energy.     

Solving the Gross-Neveu model with relativistic many-body methods

The Gross-Neveu model provides a unique opportunity to apply relativistic many-body techniques (Dirac-Hartree approximation, RPA) in a context where all calculations can be done analytically and — in the largeN limit — yield the exact results. The physical fermion as well as multifermion (baryon) and fermion-antifermion (meson) bound states are discussed in this spirit, with special emphasis on the role of the Dirac sea.Supported by the Bundesministerium für Forschung und Technologie     

Spatial dependence of meson correlation functions at high temperature

The spatial dependence of meson correlation functions at high temperature is studied in perturbative QCD, keeping only the lowest order term. We obtain analytic results for the static correlation function in this approximation. Problems connected with the regularization of the divergent expressions are discussed in detail. The meson screening mass is determined from the form of the correlation function at large distances. We obtain , which agrees with the results of Eletskii and Ioffe, for massless quarks (M=0). Finally, the correlation function in the temporal direction is briefly discussed. For massless quarks, we find the corresponding screening massm _scr=2T.Research supported in part by the Polish State Committee for Scientific Research under Grant No. 2.0204.91.01     

Complete spectroscopy of51,52V via the50,51V(n, γ) reactions

The gamma-ray spectra emitted after thermal neutron capture in highly enriched⁵⁰V and^natV targets have been studied using in-pile targets at the ILL high flux reactor and pair and germanium detectors. The neutron binding energies in⁵¹V and⁵²V were determined to beB _n (⁵¹V)=11051.11(17) keV andB _n (⁵²V)=7311.22(26) keV. The thermal neutron capture cross-section in⁵⁰V was measured to be 21 _–2 ⁺⁴ b. From 724 lines attributed to⁵¹V, 330 transitions, comprising 90% of the-ray flux, were placed into the level scheme. Fifty-nine primary dipole transitions from the 11/2⁺ or 13/2⁺ capture states in⁵¹V were established from which the E1 strength distribution was deduced. The energy scaling of these primary transitions was found to follow the E1-giant dipole resonance dominance. Many new levels were established; a number of states proposed in previous (d,p) and (n,) work were confirmed from their primary population and decaying secondary radiations. The density of levels in the high spin (9/2I15/2) region was parametrized with the Fermi gas model. The spin distributions of⁵¹V were analyzed and a spin cut-off parameter =2.8 (3) was deduced. — A nearly complete level scheme of⁵²V up to 3.5 MeV excitation and similar results concerning the level density and the primary -ray spectrum were obtained in the⁵¹V (n, )⁵²V study.Supported by Deutsches BMFT under contract 06GÖ141     

Deuterons from 300 GeV proton collisions calculated from the Lund model

We present an alternative approach to describe deuteron production in high energy particle collisions. The phenomenological Lund-model has been used to give the number and momentum distribution of secondary nucleons produced in 300 GeV proton-proton collisions. Deuterons are assumed to be produced in a final state interaction between the nucleons inNN d reactions inside a volume of 1 F³. The results are compared to experimental data.Data became available for antideuteron production in e⁺e^– annihilation. * A Lund Monte Carlo calculation for the production of secondary nucleons in e⁺e^– collisions exist. The results from a final state interaction were too small by a factor 10³. A calculation for a coalescent model which uses the overlap of the proton-neutron momentum distribution with a distribution from a Hulthen wave function is in good agreement with the data. The coalescent model works well in this case. It is because the relative momentum of the coalescent nucleons produced for 10 GeV cm energy in e⁺e^– collisions is very much smaller than the relative momentum of the nucleons produced in 300 GeV fixed target energy ofpp collisions.     

Bivariate distributions in statistical spectroscopy studies: IV. Interacting particle Gamow-Teller strength densities and β-decay rates offp-shell nuclei for presupernova stars

A method to calculate temperature dependent-decay rates is developed by writing the expression for the rates explicitly in terms of bivariateGT strength densities (I _O ^H _(GT)) for a given hamiltonianH=h+V and state densities of the parent nucleus besides having the usual phase space factors. The theory developed in the preceding paper (III) for constructing NIP strength densities is applied for generatingI _O ^h _(GT) and thenI _O ^H _(GT) is constructed using the bivariate convolution formI _O ^H _(GT)= _S I _O(GT) ^h,S _O(GT) ^V,S ; _BIV-G . The spreading bivariate Gaussian _O(GT) ^V ;BIV-G, forfp-shell nuclei, is constructed by assuming that the marginal centroids are zero, the marginal variances are same as the corresponding state density variances and fixing the bivariate correlation coefficient using experimental-decay half lifes. With the deduced values of 0.67,-S-decay rates for^61,62Fe and^62–64Co isotopes are calculated at presupernova matter densities=10⁷–10⁹ gm/cc, temperaturesT=(3–5)×10⁹ K and electron fractions Y_e=0.43–0.5. The convolution form forI _O(GT) ^H led to a simple expression for calculatingGT non-energy weighted sum rule strength and it describes (within 10%) the shell model results offp-shell nuclei.The work presented in this paper has grown out of many discussions and correspondence one of the authors (VKBK) has with J.B. French in the last five years and also out of an ongoing project one of the authors (VKBK) is carrying out with J.B. French and R.U. Haq. Thanks are due to K. Kar and S. Sarkar for supplying some of their results before publishing. The authors thank V. Potbhare for his interest in the work presented in this paper.     

Dispersion approach to quark-binding effects in weak decays of heavy mesons

The dispersion approach based on the constituent quark picture and its applications to weak decays of heavy mesons are reviewed. Meson interaction amplitudes are represented within this approach as relativistic spectral integrals over the mass variables in terms of the meson wave functions and spectral densities of the corresponding Feynman diagrams. Various applications of this approach are discussed:Relativistic spectral representations for meson elastic and transition form factors at spacelike momentum transfers are constructed. Form factors at q² > 0 are obtained by the analytical continuation. As a result of this procedure, form factors are given in the full q² range of the weak decay in terms of the wave functions of the participating mesons.The 1/m_Q expansion of the obtained spectral representations for the form factors for the particular limits of the heavy-to-heavy and heavy-to-light transitions are analysed. Their full consistency with the constraints provided by QCD for these limits is demonstrated.Predictions for form factors for B_(s) and D_(s) decays to light mesons are given.The B → γℓν decay and the weak annihilation in rare radiative decays are considered. Nonfactorizable corrections to the B⁰ mixing are calculated.Inclusive weak B decays are analysed and the differential distributions are obtained in terms of the B meson wave function.     

Model-independent separation of structure functions over an extended kinematical region

A method for the separation of structure functions in (e, e p) experiments is proposed, which is an extension of the traditional Rosenbluth-type techniques of [1,2]. In our approach, we use a very flexible Ansatz to describe the structure functions within an extended kinematical regionG and determine its free parameters with a x² minimization. The procedure is tested by pseudo data (¹²C(e, ep)¹¹B_g.s.) in the quasi-free region.Part of the doctorate-thesis of Thomas Veit     

Refractive scattering and reactions,comparison of two systems:16O+16O and20Ne+12C

Elastic, inelastic scattering as well as one-neutron transfer channels have been measured over a wide angular range for systems¹⁶O+¹⁶O at the incident energy of 350 MeV and²⁰Ne+¹²C at 390 MeV, respectively, using the Q3D magnetic spectrometer. In both cases differential cross sections have been measured down to about 50 nb/sr (or d/d _R10^–4) at large angles. For the¹⁶O+¹⁶O system refractive contributions are found at the level of these cross sections, whereas in the²⁰Ne+¹²C case a steeper decrease of the differential cross section with the angle is observed and the refractive contribution can not be determined. The elastic scattering data have been analyzed using standard Woods-Saxon potentials and potentials calculated in different versions of the double-folding model. Some properties of these potentials are tested in the calculations for inelastic scattering and one-neutron transfer within the DWBA. With the refractive pattern observed for the¹⁶O+¹⁶O system, the scattering and transfer data are found to be sensitive to the interaction potential at small internuclear distances down to about 2.5 fm.It should be acknowledged that part of the folding analysis reported here was done while one of the authors (D.T.K.) was staying at the Institute for Theoretical Physics, University of Tübingen. We also thank Prof. H. Clement and H. Abele for numerous discussions and contributions and Prof. G.R. Satchler for helpful comments on the use of the DWBA code PTOLEMY.     

Contribution of high-order twists to the inclusive photocreation process of mesons with large PT

The contributions of high twists to the inclusive photocreation of a meson is treated within QCD in the present study. It is shown that the subprocessesq Mq of a high order twist provide important contributions to the inclusive effective cross section at largeP _T . The effective cross section of a high order twist has been determined for various wave functions as a function of the transverse momentum of the mesonP _T for differents values.M. E. Rasuldaze Baku State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 88–93, May, 1995.     

Selectively dopedn-AlxGa1−xAs/GaAs heterostructures with high-mobility two-dimensional electron gas for field effect transistors

The transport properties of warm and hot electrons in selectively dopedn-Al _x Ga_1–x As/GaAs heterostructures created by electric fields up to 500 V/cm were studied by Hall effect, conductivity, and Shubnikov-de Haas measurements at lattice temperatures from 4.2 to 300 K. Hall measurements revealed a substantial decrease of electron mobility and also of sheet electron concentration at 77 K with enhanced electric field. The accelerated 2D electrons are partly scattered into the low-mobility first excited (E ₁) subband, and they are partly trapped in immobile states located in the Al_xGa_1–xAs near the interface. Consequently, two differentv(E) characteristics were obtained at 77 K. The 2D electrons populating only the lowest (E ₀) subband exhibit a velocity of v-2×10⁷ cm/s at 500 V/cm, while the averaged velocity due to all electrons reaches a value of v-1.5×10⁷cm/s at 500 V/cm. The analysis of the Shubnikov-de Haas oscillations and Fast Fourier transformation of the data manifested that the 2D electrons are very rapidly accelerated at 4.2 K and achieve electron temperatures much higher than the lattice temperature at electric fields as low as 1 V/cm. The major cooling process for these electrons is scattering into the low-mobilityE ₁ subband.