Possibilities of revealing collective pion degrees of freedomin nuclei by means of quasielastic pion knockout by high-energy electrons

The kinematics of quasielastic pion knockout by longitudinal virtual photons in the electroproduction process is presented. The possibility of directly investigating pion momentum distributions in specific channels owing to pole-amplitude dominance is considered. It is shown that, taking into account the final-state interaction of the knock-on pion and the nucleus involved, one can reveal the existence of a pion condensate in nuclei, since the momentum distribution of collective pions has a pronounced maximum at a momentum in excess of 0.3 GeV/c and since the excitation spectrum of the final recoil nucleus is concentrated in the low-energy region E* ≈ K ²/(2AM _N ) ≤ 1 MeV. The picture of pion knockout from meson clouds of individual nucleons is totally different. The analogous rho-mesonmomentum distributions for the process ρ + γT* → π are also presented.     

Quark microscopic picture for vector and pseudoscalar mesons in the nucleon and their quasielastic knockout by high-energy electrons

A technique for projecting a multiquark wave function in the microscopic model of a ³P⁰ scalar fluctuation onto the virtual-decay channels N → N + ρ and N → N + π is formulated (at a more general level for the latter than previously). The amplitude for the electromagnetic transition ρ + γ _T ^* → π in electron-induced quasielastic rho-meson knockout followed by rho-meson conversion to a pion is considered. Theoretical results obtained in this way are contrasted against available experimental data, and reasonable agreement is found for cross-section values. This confirms a universal character of the ³P₀ model. The precision of relevant experiments is as yet insufficient for comparing the momentum distribution of the rho meson from the channel N → N + ρ with its theoretical counterpart.     

Potential of quasielastic meson knockout from a nucleon by high-energy electrons and pions for studying the structure of exotic excited states of a final-state baryon

The possibility of studying q ⁴ \(\bar q\) exotic baryon states by means of N(e, e’M)B reactions proceeding via an extremely simple mechanism and involving the quasielastic knockout of various mesons from a nucleon by electrons of energy in the few-GeV region is considered as a development of the previous investigations of our group. A quark microscopic formalism based on the cluster model of q ⁴ \(\bar q\) states, which makes it possible to determine momentum distributions of mesons in various channels of B → N + M virtual decays (in principle, these distributions can be compared with experimental data), is expounded by considering the example of the pentaquark (B = Θ⁺). The decay widths of the q ⁴ \(\bar q\) baryon states being discussed are governed by the degree of separation of quark clusters (this is a parameter of the model used). The electroproduction cross sections prove to be small because of kinematical constraints requiring that physically admissible values of the momentum ‖k‖ of the virtual meson M lie in the region where relevant amplitudes are suppressed substantially by form factors in pentaquark vertices. In particular, N (e, e′π ^±)B reactions involving pion knockout furnish direct information about nonstrange components of baryon B; however, the expected cross sections for such reactions are an order of magnitude smaller than their counterparts for analogous reactions leading to the production of a pentaquark Θ⁺. Because of the smallness of the electroproduction cross sections, it is reasonable to consider the production of a pentaquark and other q ⁴ \(\bar q\) exotic states in reactions characterized by quasielastic kinematics and initiated by pions of energy in the range between about 1 and 5 GeV and in similar stripping and pickup nuclear reactions.     

Pionic degrees of freedom in atomic nuclei and quasielastic knockout of pions by high-energy electrons

The microscopic mechanism of the identical bands in odd-odd nucleus ¹⁹⁴Tl and its neighbor odd-A nuclei ^193,195Tl is investigated using the particle-number-conserving (PNC) method for treating the cranked shell model with monopole and quadrupole pairing interactions. It is found that the blocking effect of the high-j intruder orbital plays an important role in the variation of moments of inertia (J⁽¹⁾ and J⁽²⁾) with rotational frequency for the superdeformed bands and identical bands. The variation of the occupation probability of each cranked orbital and the contributions to moment of inertia from each cranked orbital are presented.     

Quasielastic rho-meson knockout from nucleons by high-energy electrons

The electroproduction of charged rho mesons on a nucleon under conditions of quasielastic kinematics [Q ² > 2 (GeV/c)², W > 3 GeV, and ω _ρ ? q ⁰ in the laboratory frame] is considered. The main diagrams involved in this process are taken into account, and it is shown that the case of rho-meson knockout by a longitudinal photon is dominated by the t-channel pole diagram featuring a rho meson, so that direct rho-meson electroknockout makes a leading contribution to the process being considered. Questions concerning the gauge invariance of the electroproduction process are considered in detail, along with contact terms that inevitably arise in employing form factors in ρNN vertices and which are necessary for restoring gauge invariance. It is shown that the structure of these contact terms is not determined exclusively by the requirement that the electroproduction amplitude be gauge-invariant and may be such that the corresponding correction to the dominant diagram due to these terms proves to be suppressed in the quasielastic region (with increasing Q ²), with the result that the t-pole process featuring a rho meson remains dominant; at the same time, the contribution of this correction is significant at low Q ² or in the case of photoproduction, modifying substantially the electroproduction cross section.     

Effective Lagrangian Approach to pion photoproduction from the nucleon

We present a pion photoproduction model on the free nucleon based on an Effective Lagrangian Approach (ELA) which includes the nucleon resonances (Δ(1232), N(1440), N(1520), N(1535), Δ(1620), N(1650), and Δ(1700)), in addition to Born and vector meson exchange terms. The model incorporates a new theoretical treatment of spin-3/2 resonances, first introduced by Pascalutsa, avoiding pathologies present in previous models. Other main features of the model are chiral symmetry, gauge invariance, and crossing symmetry. We use the model combined with modern optimization techniques to assess the parameters of the nucleon resonances on the basis of world data on electromagnetic multipoles. We present results for electromagnetic multipoles, differential cross-sections, asymmetries, and total cross-sections for all one pion photoproduction processes on free nucleons. We find overall agreement with data from threshold up to 1 GeV in laboratory frame.     

s-and t-channel helicity conservation of pion induced nucleon diffraction dissociation at 3.9 GeV/c and the implications of a two-component model

A study of schannel and t-channel helicity conservation has been carried out on a prism plot separated sample of 3.9 GeV/cπ^±p → π (Nπ)_dd diffraction dissociation events. s-channel helicity conservation is found to be strongly violated while t-channel helicity conservation is found to be mildly violated. These results are similar to that found at 11.7 GeV/c. We explain these violations on the basis of a “two-component model” of which the first component is a single exchange amplitude. We postulate that this amplitude is the generalization of the pomeron exchange amplitude which may conserve s-channel helicity. The second component of the model is a double exchange mechanism which strongly violates s-channel helicity. The predictions of this model agree with the experimental data.     

Pseudoscalar and pseudovector coupling of a pion to a bound Dirac nucleon

The non-equivalence of pseudoscalar and pseudovector coupling of a pion to a Dirac nucleon in the presence of a scalar-vector potential is analysed and found to be strongly related to the magnitude of spin-orbit splitting. The concept of a Galilean invariant vertex operator turns out to be unjustified for a bound nucleon.     

Detection of high-energy electrons by a directional spectrometer

A method for the detection of 26- and 28-GeV electrons by a directional spectrometer based on oriented tungsten, tungstate, and garnet crystals has been reported. In spite of an anomalous character of the development of an electromagnetic shower at the initial stage in a crystal oriented along the 〈111〉 axis, the relative energy release of the shower in the spectrometer behind the crystal at the final stage is described by the standard dependence of the development of the electromagnetic shower.     

An application of the quasipotential method to pion nucleon scattering

The quasipotential method developed in Dubna Laboratory was applied to pion-nucleon scattering processes up to the energies 500 MeV, for obtaining the resonance behaviour of te P₃₃-phase shift. Although numerical calculations have not yet been completed, the existence of a resonance seems very probable.

     

Finite volume corrections to the electromagnetic current of the nucleon

We compute corrections to both the isovector anomalous magnetic moment and the isovector electromagnetic current of the nucleon to O(p³)ensuremath O(p^3) in the framework of covariant two-flavor Baryon Chiral Perturbation Theory. We then apply these corrections to lattice data for the anomalous magnetic moment from the LHPC, RBC & UKQCD and QCDSF Collaborations.     

Microscopics of mesonic degrees of freedom in nucleons and mesons in nuclei: Quasielastic meson knockout by high-energy electrons

The following questions are considered: (i) that of what quasielastic-knockout reactions are; (ii) that of what experience has been gained in measuring, in various channels, the momentum distributions and spectroscopic factors of nucleons and clusters in nuclei and of electrons in atoms, molecules, and solid-state bodies; (iii) that of how it is possible to introduce the concept of quasielastic knockout in the theory of meson-electroproduction processes p(e, e′m)B at beam energies of a few GeV and at moderate values of the square of the virtual-photon 4-momentum, Q ² = 2–4 (GeV/c)²; and (iv) that of how the momentum distributions of mesons in various channels of virtual proton decay, p → B + π, p → B + ρ, and p → Y + K, are predicted on the basis of the microscopic model of a fluctuation of the QCD vacuum in a nucleon. Proposals for relevant experiments are formulated. It is indicated that quasielastic-knockout processes like (e, e′π) provide the best way to study the problem of a scalar pion condensate in nuclei. In conclusion, it is emphasized that quasielastic processes ²H(e, e′p)B involving various spectator baryons B are of great value for determining the composition of multiquark configurations in nucleon-nucleon systems.     

Spectrum of two quasiequal-energy electrons ejected by a high-energy photon

We present numerical results for the photoelectron spectrum in double ionization by keV photons in the quasiequal-energy sharing region. In this region of the spectrum, the relevant ionizing mechanism is due to a mutual sharing of the photon momentum by both electrons, with small momentum transferred to the atomic nucleus. Calculations were performed for photon energies of 25 and 50 keV, where retardation effects are fundamental, while final-state correlations are of minor importance. The spectra present a two-peak structure, with maxima located at the photoelectron energies , with the photon energy in atomic units. We discuss the general features of the spectrum in terms of the picture of the photoionization of two free electrons, and we propose a way of detecting the contribution by experiments. Received 24 January 2000     

Center-of-mass corrections to overlap factors in the theory of nucleon transfer reactions

Form factors for direct nuclear reactions involving the transfer of one or more nucleons are examined for the consequences of the unphysical c.m. motion inherent in the shell model. The standard procedure employed in one-nucleon transfer is shown to be properly translation invariant. In the case of the transfer of two nucleons, it is pointed out that c.m. corrections are needed but that no tractable procedure exists for making them. Recent proposals of recoil corrections in transfer reactions are critically examined.     

Observations of high-energy electrons in the stratosphere

Summary To investigate the variation of the flux of cosmic radiation in the vicinity of the Earth, stratospheric balloon soundings are performed weekly at Campinas (Brazil). During some soundings, enhancements of the counting rates are detected. In this paper data from these excesses as well as their analysis are shown. It seems to indicate that possibly high-energy electrons are injected from the lower boundary of the radiation belt into the stratosphere.

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Riassunto Per ricercare la variazione del flusso delle radiazioni cosmiche in prossimità della Terra, sono fatti funzionare settimanalmente palloni sonda a Campinas (Brasile). In alcune sonde si registrano aumenti delle frequenze di conteggio. In questo lavoro si mostrano dati da questi eccessi nonché le loro analisi. Ciò sembra indicare che gli elettroni ad alta energia sono probabilmente immessi dallo strato inferiore della fascia di radiazioni nella stratosfera.

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Резюме для исследования изменения потока космического излучения в окрестности Земли еженедельно запускаются стратосферные шары-зонды в лаборатории Кампинас (Бразилия). Во время некоторых зондирований зарегистрированы увеличения интенсивности излучения. В этой статье приводятся данные измерений и их анализ. Проведенный анализ показывает, что эти случаи возможно связаны с электронами высоких энергий, инжектированными из нижней границы радиационного пояса в стратосферу.

     

Complete set of electromagnetic corrections to the nucleon mass in the Nambu-Jona-Lasinio model

We show how to derive the complete set of electromagnetic corrections to the Nambu-Jona-Lasinio3 (NJL) model of the nucleon. Our results enable an accurate estimate of the electromagnetic contribution to the neutron-proton mass difference within this model. At the same time, our procedure demonstrates the way to calculate the complete set of meson corrections to the NJL model that maintains chiral symmetry.     

Pauli principle and renormalization of the pion nucleon interaction in nuclei

We show that the exclusion principle provides an alternative mechanism to the Lorentz-Lorenz effect for the renormalization of the pion-nucleon coupling constant in nuclei. This mechanism proves to be less sensitive to the range of the π - N forces. The implications for pion-nucleus scattering are discussed.