Charge-exchange processes in the quasi-elastic (e,e′p) and (e,e′n) reactions

Calculations of charge-exchange processes in the final state of the quasi-elastic (e, e′N) reaction have been performed employing the Lane formalism. The Lane equations were decoupled by means of a transformation to the isospin representation. It is found that charge exchange has a small effect on the (e, e′p) cross section, but may play an appreciable role in the (e, e′n) reaction.     

Multi-scattering effects in (e,e′p) knockout

Multistep direct scattering of the ejectile proton is considered in (e,e′p) reactions in the quasi-free region as an improvement over the usual treatment of final-state interactions by means of an optical potential. The theory is applied to ⁴⁰Ca(e,e′p) as a case example. Important contributions of two- and three-step processes are found at high missing energy and momentum in agreement with the experimental trend.     

Joint production of ϕ mesons andπ ±,π 0,p,\bar p,K s 0 andK ± in hadronic interactions

The joint production of ? mesons andπ ^±,π ⁰,p, \(\bar p\) ,K _s ⁰ andK ^± is investigated using a sample of 600,000 inclusive ? meson events obtained in hadron Be interactions with incidentπ ^±,p, \(\bar p\) andK ^± beams. Evidence is presented for the joint production of ? mesons and strange particles produced with non-strange incident beams. With incidentK ^± beam the number of additional strange particles is suppressed. The results are found to be in agreement with the qualitative predictions of a parton fusion model. The comparison with the Lund model for lowp _T processes is fair.     

Towards spin degrees of freedom in (e,e′p)

The expeprimental (e, ep) program at Mainz is presented. The reaction (e, ep) without spin degrees of freedom is compared with the case when polarization observables are considered. It is demonstrated on some examples how the new information can be used to extend the knowledge of the reaction mechanism and nucleon/nuclear structure. Construction and operation of a focal-plane proton polarimeter at Mainz is described.Lectures held at the Indian-Summer School on Electromagnetic and Weak Interactions of Particles with Nuclei, Sázava, Czechoslovakia, 6–11 September 1992.     

Final-state interactions in (e,e′p) reactions with polarized nuclei

The cross section for coincidence, quasielastic proton knock-out by electrons from a polarized ³⁹K nucleus is computed in DWIA using an optical potential in describing the wave function of the ejected nucleon. The dependence of the FSI on the initial polarization angles of the nucleus is analyzed and explained in a new, semi-classical picture of the reaction in which the nuclear transparency decreases as a function of the amount of nuclear matter that the proton has to cross, thus providing a method for obtaining detailed information on its mean free path in finite nuclei. We propose a procedure to find the best initial kinematical conditions for minimizing the FSI which will be useful as a guide for future experiments with polarized nuclei.     

Comparison of Monte Carlo simulation and direct multistep scattering theory in (e,e′p) nuclear reactions

Two methods to deal with final state interactions in (e,e′p) reactions in nuclei are compared. One of them uses a Monte Carlo semiclassical approach while the other uses a statistical quantum mechanical approach. The comparison serves to give support to both approaches, showing at the same time their limitations.     

Elementary muon, pion, and kaon particles as resonators for neutrino quanta. Calculations of mass ratios for e, μ, π0, π±, K 0, K ±, and ν e

Masses of a number of elementary particles are calculated on the basis of the model suggested in [1] with the use of one parameter. In this model, an electron is considered as an electric cloud enclosed inside an elastic lepton shell, electron neutrino ν _е is considered as an elastic lepton shell contracted to a minimal size, and muon, pion and kaon are considered as resonators for quanta of virtual neutrinos excited inside the elastic lepton shell. The number and type of these quanta are determined from the decay scheme for μ, π, and K: 2 for the muon (ν _е and ), 3 for the pion (ν _е , ν _μ, and ), and at least 21 for the kaon. The model allows mass ratios approximating the experimental data for these particles to be obtained for the first time, with the ratio of μ and е masses equal to (6πℏс/е ²)^2/3 ≅ 188, the ratio of π ⁰ and μ masses equal to (3/2)^2/3, and the ratio of K ⁰ and π ⁰ equal masses to 7^2/3. The calculated e, μ , π ⁰, and K ⁰ masses are in the 0.547:105.707:134.963:493.87 (MeV) ratios (normalized by the neutral pion mass). This is in good agreement with the available experimental data. The mass ν _е (≅ 0.02 eV) is also estimated in this model, and the variety of K-meson decay schemes is naturally explained as a result of the variety of excited intrinsic neutrino field structures with the same energy. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 23–29, January, 2009.     

Partial wave analysis for p̄ p and e+ e− annihilation processes

A new highly modular and generic Partial Wave Analysis (PWA) software package including a general framework for future hadron spectroscopy experiments like PANDA is currently under development. The aims and present status of the software package are summarized and the first PWA results obtained with the already existing tools are presented. These analyses are mainly focused on important aspects of antiproton proton annihilation processes with relevance for the PANDA experiment. For this purpose Crystal Barrel LEAR data are currently under investigation in order to gain a deeper insight to the production mechanisms of vector mesons. In addition analyses for the identification of resonances in radiative J / ψ decays from BESIII data are summarized.     

Longitudinal-transverse interference structure function in the reaction2H(e,e′ p)

The longitudinal-transverse interference structure functionf ₀₁ has been measured in deuterium electrodisintegration experiments at various values of the four-momentum transferQ ². Data taken at different laboratories and at roughly the same value ofQ ² are shown to be consistent. When compared to non-relativistic impulse-approximation calculations mostf ₀₁ data show an enhancement. A relativistic treatment of the reaction²H(e, e p) removes the discrepancy. More definite evidence for relativistic effects in deuterium electrodisintegration can be obtained at higher values of the recoil momentum. The experimental implications for such²H(e, e p) experiments are discussed as well.Dedicated to Prof. Werner Sandhas on the occasion of his 60th birthday     

Computation of resonance energies and widths in e ± H collisions using the stabilization method

We report a calculation of S-wave resonances (energies and widths) in positron-hydrogen scattering below the N=2 hydrogen and the N=2 positronium excitation thresholds with the stabilization method using Hylleraastype wavefunctions. Three resonances have been extracted out of which one is associated with the hydrogen n=2 threshold and the other two are found lying below the Ps(n=2) formation threshold. The present results agree with the most accurate calculations obtained by other methods.     

The process\bar p p→e − e + with polarized initial particles and proton form factors in time-like region

The process pe ^– e ⁺ is considered in the general case of polarized initial particles. A relation between the difference of the phases of the electromagnetic form factorsG _M andG _E in the time-like region and measurable asymmetries is derived. It is shown that the moduli of the form factors can be determined from measurements of the total unpolarized cross section and of the integral asymmetry for longitudinally polarized (or, transversely polarized) andp. The behaviour of the proton form factors at highq ² in the time-like region is also discussed. From the Phragmén-Lindelöf's theorem it follows that the asymptotical behaviour of the form factors in the space-like and time-like regions must be the same. An analysis of experimental data in both regions based on perturbative QCD is presented.     

Study of relativistic bound state wave functions in quasielastic (e,e′ p) reactions

The unpolarized response functions of the quasielastic ¹⁶O(e,e′ p)¹⁵N reaction are calculated for three different types of relativistic bound-state wave functions. The wave functions are obtained from relativistic Hartree, relativistic Hartree-Fock and density-dependent relativistic Hartree calculations that reproduce the experimental rms charge radius of ¹⁶O. The sensitivity of the unpolarized response functions to the single-particle structure of the different models is investigated in the relativistic plane-wave impulse approximation. Redistributions of the momentum dependence in the longitudinal and transverse response function can be related to the binding energy of the single-particle states. The interference responses R_LT and R_TT reveal a strong sensitivity to the small component of the relativistic bound-state wave function.     

Measurement of the exclusive D(p,e+e−)3He and D(p, γ)3He reactions at 98 and 176 MeV

A high resolution magnetic pair spectrometer for medium energy photons has been used for studies of the exclusive (p, e⁺e⁻) and (p, γ) reactions on deuterium at 98 and 176 MeV. The (p, e⁺e⁻) cross sections, obtained at 40° and 80°, are compared with the corresponding data for the (p, γ) reaction measured with the same apparatus. At small virtual photon masses the (p, e⁺e⁻) cross sections show significant enhancement over what is expected from model calculations, a behaviour which has earlier been observed also for the ¹¹B(p, e⁺e⁻)¹²C reaction.     

Correlations and the cross section of exclusive (e,e′p) reactions for 16O

The reduced cross section for exclusive (e, e′p) reactions has been studied in DWIA for the example of the nucleus ¹⁶O using a spectral function containing effects of correlations. The spectral function is evaluated directly for the finite nucleus starting from a realistic nucleon-nucleon interaction within the framework of the Green's function approach. The emphasis is focused on the correlations induced by excitation modes at low energies described within a model-space of shell-model configurations including states up to the sdg shell. Cross sections for the p-wave quasi-hole transitions at low missing energies are presented and compared with the most recent experimental data. In the case of the so-called perpendicular kinematics the reduced cross section derived in DWIA shows an enhancement at high missing momenta as compared to the PWIA result. Furthermore the cross sections for the s- and d-wave quasi-hole transitions are presented and compared to available data at low missing momenta. Also in these cases, which cannot be described in a model without correlations, a good agreement with the experiment is obtained.     

Anomalies in(semi)-leptonic B decays B~±→τ~±ν,B~±→Dτ~±ν and B~±→D~*τ~±ν,and possible resolution with sterile neutrino

The universality of the weak interactions can be tested in semileptonic b→c transitions,and in particular in the ratios R(D~(*)) ≡Γ(B → D(*)τν)/Γ(B → D~(*)lν)(where 1 = μ or e).Due to the recent differences between the experimental measurements of these observables by Ba Bar,Belle and LHCb on the one hand and the Standard Model predicted values on the other hand,we study the predicted ratios R(D~(*))=Γ(B→D~(*)τ+"missing")/Γ(B→D(*)lν)in scenarios with an additional sterile heavy neutrino of mass ～1 Ge V.Further,we evaluate the newly defined ratio R(0)≡Γ(B→τ+"missing")/Γ(B→μν) in such scenarios,in view of the future possibilities of measuring the quantity at Belle-Ⅱ.     

The quasi-free 3He(e,e′p) reaction

The proton spectral function has experimentally been determined with the ³He(e, e′p) reaction for missing energies, 0<E_m<70MeV, and recoil momenta, 0<P_B<310 MeV/c. Experimental results are obtained for both the two-body, ³He(e, e′p)²H, and three-body breakup processes. Proton momentum density distributions, obtained in a PWIA analysis, are compared with theoretical calculations: Faddeev solutions with the RSC and Paris potentials, and variational calculations with various potentials, including those with a three-nucleon interaction term. Energy-weighted sum rule results are presented and compared with theoretical predictions. The coincidence cross sections are also compared with calculations which include the effects of final state interactions and meson exchange currents. Consistency of the results with PWIA is investigated in the framework of the Chew-Low extrapolation procedure.     

Post-collision interactions and the polarization effect in （e, 2e） collisions of helium

A modified distorted-wave Born approximation (DWBA) method is used to calculate the triple differential cross sections (TDCSs) in a coplanar asymmetric geometry for the electron impact single ionization of a He (1s2) atom at intermediate and lower energies. The post-collision interaction and the polarization effect in (e, 2e) collisions of helium are considered in the calculations. The polarization potentials from the damping method and density functional theory (DFT) are compared. Theoretical results are compared with the recent experimental data.     

Study of inclusive production of π± and protons in\bar p p interactions at 32 GeV/c

Results are presented on the inclusive production of π^± mesons and protons in \(\bar p\) p interactions at an incident antiproton momentum of 32 GeV/c in the MIRABELLE bubble chamber, based on a sample of 55,000 inelastic events. The spectra of π⁺(π^?) andp( \(\bar p\) ) are separated in the whole kinematically admitted region of phase space by a statistical method. The semi-inclusive and inclusive cross sections and the main average characteristics of π andp production are calculated. The invariant differential cross sections are studied as functions of the Fevnman scaling variablex, the rapidity and the transverse momentum. Thex distribution of π^± mesons in the proton fragmentation region does not show any significant contribution from quark exchange or annihilation processes. A possible contribution of hard quark scattering on mesons or baryons is observed for large transverse momenta.     

Determination of the neutron electric form factor in the D(e,e′n)p reaction and the influence of nuclear binding

The electric form factor of the neutron G _E,n has been determined at the Mainz Microtron MAMI at the low momentum transfer Q ²= 0.15 (GeV/c)² in a measurement of the recoil polarisation ratio P _x/P_z in the quasifree reaction D(e,e′n)p. At this Q ² the influence of the nuclear binding is strong. A purely kinematical model is used to get some insight into the effect of the initial Fermi momentum distribution of the neutron. The influence of the final state interaction is determined quantitatively by a model of Arenh?vel et al.. After the corresponding corrections a value of G _E,n(0.15 (GeV/c)²) = 0.0481±0.0065_stat±0.0053_syst is obtained. Received: 12 April 1999