Showing papers in "Nuovo Cimento Della Societa Italiana Di Fisica A-nuclei Particles and Fields in 1990"

Relativistic corrections to the quark-antiquark potential and the quarkonium spectrum

Abstract: The theoretical derivation from QCD of the\(q\bar q\) potential with velocity-dependent corrections is reconsidered. In particular the problem of ordering between momenta and functions of position in the Hamiltonian is discussed and exact relations among the various potentials are derived. Explicit expressions for the long-range behaviour of the potentials are given, the effect of the velocity-dependent corrections on the quarkonium spectrum is evaluated and compared with the results obtained by different proposals based on more heuristic considerations.

Detecting «dark matter» using a liquid xenon scintillation detector

Abstract: In this note we point out the detectability of some «dark matter» candidates by means of a liquid xenon scintillation counter like that we are now assembling.

Interacting fields in finite QED

Abstract: In the framework of the causal approach to QED, interacting fields are introduced by functional differentiation of theS-matrix. The properties of these field operators are studied in detail. It is shown that appropriately defined quantum versions of the Dirac and wave equations hold true. As a by-product, the correct operator products for QED are obtained.

Gauge invariance in finite QED

Abstract: Gauge invariance is discussed in the causal approach to QED. It is proven that the iterative construction of theS-matrix by the method of Epstein and Glaser can be carried out in such a way that perturbative gauge invariance holds true. The proof rests on a careful analysis of the process of distribution splitting. In case of nontrivial distribution splitting gauge invariance implies the Ward-Takahashi identities.

Evidences for Associated Heat Generation and Nuclear Products Release in Palladium Heavy-Water Electrolysis.

Abstract: In a galvanostatic experiment of charging deuterium in a palladium cathode, nuclear and thermal effects were found. A sintered palladium electrode shaped as a parallelepiped was used. After six days of electrolysis at 200 mA/cm2, a simultaneous emission of neutrons, tritium excess in the electrolytic solution and temperature rapid increase were observed. During the event which lasted 4 minutes, we estimated an emission of 7.2·105 neutrons while the electrode temperature reached 150°C. Excess of tritium was evaluated to be (2.14±0.04)·1011 atoms. If the energy released as heat, 176 J, is compared with the energy associated to the d+d reactions, 0.1J, three orders of magnitude are still defective. Electrode characterization after the events is reported.

Excitation energy dependence of photofissility in preactinide nuclei

Abstract: The dependence on excitation energyE* of photofissilityPf of preactinide nuclei is examined. An accurate evaluation ofE*, and of mass and charge configurations of the compound nucleus for (100÷300) MeV photons is calculated by means of an intranuclear-cascade model. It is found that, at high excitation energy, is no longer a correct variable to describe the excitation energy dependence ofPf.

Symplectic projector in constrained systems

Abstract: In this work we develop a geometric formalism for constrained Hamiltonian systems. Using a symplectic projector, we show that the variables compatible with the constraints are written as functions of the arbitrary variables. Their Poisson bracket is just the Dirac bracket of the unconstrained variables. This approach yields in a natural way all properties associated to a generalized symplectic structure with a well-defined effective Hamiltonian.

An elementary particle constituent-quark model

Abstract: A comparison of mesonmasses with baryonexcitation energies reveals a common mass-band structure, in which particles with a variety of quantum numbers are grouped together. These quantized mass values seemingly mandate a constituent-quark (CQ) viewpoint. By studying the meson and baryonyrast states, we can deduce the masses and spin values of some of the constituent quarks. A comprehensive CQ model is presented that reproduces the observed spectrum of hadrons and leptons. These CQ quarks include the standard u, d, s, c, b set of quarks as a subset. The quark generation process, which features characteristic mass triplings, is a key ingredient of this CQ model. All of the constituent-quark masses are formed from a set of three basic masses, two spinning and one spinless, which are directly related to the masses of the electron, muon, and pion. The mass quantum X=420 MeV is a dominant excitation state that dictates meson, baryon, and lepton mass values. The quantum X occurs in bothS=0 andS=1 spin configurations, and it serves as a «window» for transformations between orbital and spin angular momenta. The fully relativistic spinning sphere, with its distinctive 3/2 mass increase, provides an isoergic link between spinless and spinning excitation quanta. The fine-structure constant α and the charge-splitting quantum e=9m e provide links between quarks and leptons. Particle lifetimes and decay modes reflect constituent-quark substructures.

On Two Coupled Abel-Type Differential Equations Arising in a Magnetostatic Problem.

Abstract: The question of polar instability of spherical shells due to core magnetic polarization examined briefly in a preceding communication is treated in more detail regarding Dirac's theory of magnetic charges and physical solutions of differential equations of Abel type resulting from the demand of rotational invariance. Both localized approximate solutions and several singular solutions or quasi-solutions with physical appeal are proposed, for the deformed spherical shell.

Nucleon polarizabilities in the constituent quark model

Abstract: The spectrum of the nonstrange baryons in the constituent quark model is critically analysed. The electromagnetic properties of the nucleon are calculated with particular emphasis on the electric and magnetic polarizabilities. Relativistic corrections up to orderm−2 are consistently considered.

An enlarged superfluid model of atomic nucleus

Abstract: The well-known superfluid model (or quasi-particle phonon nuclear model (QPNM) of atomic nucleus is enlarged by including an adequate four-nucleon effective interaction in addition to the pairing and long-range effective residual interactions. New experimental data can be explained without affecting those observable already described by the QPNM and in addition new features can be enumerated: 1) superfluidities of the neutron and proton systems may be generated by one another; 2) the phase structure is enriched by a new superfluid phase dominated by alpha-type correlations (ATC) and 3) superfluid isomers and their bands of elementary excitations are predicted. Unusual large two-nucleon and alpha transfer reactions cross-sections as well as some unusual large alpha-decay widths can be explained.

On the interactions between nucleons in the valence shells of trans-208Pb nuclei

Abstract: A lot of information concerning the interaction laws between nucleons in the valence shells of trans-208Pb nuclei can be obtained from experimental mass tables. The proton-neutron interaction energy can be expressed by\(E_{np} = N_p N_n \varepsilon '_{np} \), whereNpNn is the number of n-p couples in the valence shells, and\(\varepsilon '_{np} \) a weak, nearly constant interaction energy per n-p couple. Two methods are proposed for the calculation ofEnp. In even-even nuclei, the integral 2p-2p interaction energy is given by\(E_{2p - 2p} = - \mathcal{N}_p (\mathcal{N}_p - 1)\varepsilon _{2p - 2p} /2\), where\(\mathcal{N}_p \) is the number of proton pairs, and\(\varepsilon _{2p - 2p} \) a weak, nearly constant interaction energy per 2p-2p couple. A similar relation holds for valence neutron pairs. The energy binding all the valence proton pairs to the208Pb core is\(\mathcal{N}_p \bar \sigma _{2p} \), where\(\bar \sigma _{2p} \) is calculable in a simple way fromZ=84 toZ=92; its value is independent of the number of pairs of nucleons of the other kind in the valence shells. A similar relation holds for valence neutron pairs. Similar laws hold in odd nuclei. The weakness of all these valence shell interaction energies, and the fact that the energy binding an unpaired proton to the208Pb core of odd-Z nuclei decreases rapidly as\(\mathcal{N}_p \) increases, explain several properties, which are briefly discussed, of trans-208Pb nuclei: repulsion between valence neutrons, feasibility of Oganessian reactions, shell internal energy, and reactivity of the valence nucleons.

Comment on the prediction of the $$e^ + e^ - \to n\overline n $$ cross-section to be remarkably larger than the $$e^ + e^ - \to p\overline p $$

Abstract: The former modified six-resonance VMD model with the correct analytic properties of nucleon electromagnetic form factors and the asymptotic behaviour predicted by the quark model for baryons is improved, first by an addition of the \gr\s`(2150) meson contribution, then taking masses and widths of all considered resonances to be fixed at the world-averaged values and finally, differing also the effective inelastic threshold of the isoscalar (isovector) Dirac nucleon form factor from the effective inelastic threshold of the isoscalar (isovector) Pauli nucleon form factor. By means of the resultant model the analysis of all existing spacelike region nucleon form factor data is repeated. In the timelike region just the attainable data on\(e^ + e^ - \to p\overline p \) cross-section are employed and the assumption currently used by experimentalists about the equality of electric and magnetic proton form factors close above the proton-antiproton threshold is tested in this manner. The prediction of the\(e^ + e^ - \to n\overline n \) cross-section to be remarkably larger than the\(e^ + e^ - \to p\overline p \) one is established again.

Project GRAAL: the scientific case

Abstract: Backward Compton scattering of laser light against the high-energy electrons circulating in storage rings has produced tagged beams of fully polarized high-energy gamma-rays with an intensity useful for the study of photoreactions. At the ESRF this technique will produce a beam of 1.5 GeV and an intensity of 107 photons per second. We discuss the technical characteristics of such a project and the scientific goals which can be achieved in the study of the electromagnetic transitions of the hardrons and the photoproduction of isobars and strange particles in nucleons and nuclei.

Searching for the MSW effect with neutrino beams from next generation accelerators

Abstract: The possibility of carrying out an experiment to search for the predicted effect of resonant enhancement of neutrino oscillations in the Earth is discussed. The regions of the parameters sin22ϕ and Δm2 which can be explored in such experiments with next generation accelerators are obtained.

On some consequences of the interaction laws holding in the valence shells of trans-208Pb nuclei

Abstract: If the interactions between valence nucleons are taken into account, the true value of the pairing energy in the valence shells of trans-208Pb nuclei appears to be 1184.9 keV for proton pairs and 1249.1 keV for neutron pairs; and a new representation of theE(4+)/E(2+) ratio of the first two excited states of these nuclei can be proposed, in which a parameter related to the shell internal energy is used instead of theP parameter of Castenet al. New results concerning the 41+ level of214Po are reported. The nuclei212Po,214Po,214Rn,216Rn,216Ra and218Ra are of the «(j)2-like» type.

A new interpretation of asymmetric nuclear fission

Abstract: Twelve arguments are presented in favour of an interpretation of asymmetric nuclear fission as a two-step process. In the first step, a light cluster is formed within the nucleus from the valence nucleons of the fissioning nucleus. In the second step, the whole subshell of the208Pb core, or the major part of this subshell, is transferred to free valence states of the cluster.

Relativistic two-body plus potential problem: Compatible equations and double bound-state solutions (two-fermion case)

Abstract: We write a pair of compatible coupled Klein-Gordon equations, containing each a mutual interaction term ωV, an external attractive interaction termeVi (i=1,2) and a three-body termeωWi. These interaction terms can be chosen freely, within some restrictions imposed by the compatibility requirement. After eliminating the dependence on the relative time by performing a unitary transformation, we compute the double bound-state solutions by perturbations in the parameter ω, as in the corresponding nonrelativistic problem. The result suggests for the choice ofV + eWi an expression which is compact, easy to use (at least in the double bound-state problem) and which brings only minor modifications to the corresponding nonrelativistic theory.

The extended elastic model applied to the reaction3He(3He, 2p)4He

Abstract: The exteded elastic model is applied to the reaction3He(3He, 2p)4He, the cross-section factor is obtained, a comparison with the values reported in the literature is shown.

Prompt neutrinos and oscillations with mass differences of cosmological relevance

Abstract: Prompted by cosmological considerations, a scenario is considered in which the most massive neutrino has a massm ν≲25 eV while the other neutrinos are much lighter. The possibility of oscillations characterized by Δm 2≲m 2 ν is examined, with the conclusion thatν e↔ν x oscillations (x≠μ and possiblyx=τ) with sizeable mixing angles are not just allowed but in fact rather strongly indicated by the existing data on prompt neutrinos. The best values for the oscillation parameters Δm 2=(377±27±7) eV2, sin22α=0.48±0.10±0.05 are not in serious conflict with any of the existing limits.

Three-fermion relativistic bound states and cluster decomposition

Abstract: We write a system ofN coupled compatible Dirac equations. The energy eigenvalue equation is obtained by simply adding the free Hamiltonians and the mutual interaction potentials. We write also a system of three coupled compatible Dirac equations, which is more complicated but completely separable (we obtain three free equations if we «switch off» the three mutual interactions, but we obtain also two coupled and one free equations if we «switch off» two of the three mutual interactions only). As an example, we present a three-fermion symmetric relativistic harmonic-oscillator model.

Anisotropic chromodynamics and the QCD ground state

Abstract: Anisotropic chromodynamics (ACD), a color gauge theory on a postulated anisotropic space-time proposed long ago, is seen to emerge as the effective theory of quarks and gluons on what most likely is the QCD ground state, the chromo magnetic liquid (CML). A description of both the kinematics and dynamics of ACD is presented, and a possible strategy for a systematic calculation of hadronic physics is outlined.

Final results of a search for free neutron-antineutron oscillations

Abstract: The final analysis of the data collected in a search for free $$n - \bar n$$ oscillations is presented. The experiment has been carried out at the Triga Mark II reactor of the Pavia University. A thermal neutrons current of 3.2·1010s−1 propagates in a quasi-free condition for an average time of flight of 8ms and impinges on two thin targets. The possible antineutron annihilation products would be detected in the first target. The second target works as a monitor of the background. No evidence for $$n - \bar n$$ transitions is found. A lower limit of 4.9·105s (90% c.l.) has been reached, with «zero» signal and with «zero» background events.

Fission of Al, Ti, Co, Zr, Nb, Ag, In, Nd, Sm and Ta nuclei induced by (0.8÷1.8) GeV photons

Abstract: Samples of Al, Ti, Co, Zr, Nb, Ag, In, Nd, Sm and Ta elements in contact with solid-state nuclear track detectors were exposed to (0.8÷1.8) GeV bremsstrahlung beams at the 2.5 GeV electron synchrotron of the Bonn University. The detectors were processed to produce visible fission tracks for track analysis with optical microscopes. Absolute mean cross-sections per photon and fissility were evaluated. Results are discussed and compared with other photofission data as well as with estimates from the current fission models. A broad minimum found for nuclear fissility of 10−4÷10−3 covering the range 15≲Z 2/A≲25 seems to confirm the predictions from the models. For Al and Ti nuclei the probability of fission amounts to ∼ 10−1.

Generator coordinate method calculations of natural orbitals and single-particle occupation numbers in4He,16O and40Ca

Abstract: Single-particle occupation numbers and natural orbitals in4He,16O and40Ca are calculated diagonalizing the one-body density matrix obtained in an approach within the generator coordinate method. Square-well and harmonic-oscillator construction potentials and Skyrme-like forces are used. The natural orbitals (single-particle wave functions) are presented in both coordinate and momentum spaces. Together with the nucleon momentum distributionn(k) they are compared with results of Hartree-Fock as well as of correlation methods and with available experimental data forn(k) in4He. The realistic high-momentum behaviour ofn(k) and of the natural orbitals in the case of square-well construction potential shows that the nucleon-nucleon correlations at small distances are taken into account. It is pointed out that the correlations strongly affect the natural orbitals but not the occupation numbers.

The nuclear fusion for the reactions2H(d,n)3He,2H(d,γ)4He at low deuterons energy and «cold» nuclear fusion

Abstract: The measurements of (d+d)-fusion on palladium target saturated with gaseous deuterium were taken at the energy of incident deuterons from 6 keV to 20 keV. The results are in good agreement with the known data about (d+d) cross-sections, extrapolated to the low-energy region. The possible «cold» nuclear fusion contribution to (d+d) cross-sections at low energy were not observed.

Phase space model of hard-photon production in heavy-ion collisions

Abstract: We propose a semi-classical model to study the characteristics of the observed hard-photon production in heavy-ion collisions. For this we divide the nucleons in two categories on a geometrical basis: participant nucleons in the overlapping region which can suffer two-body collisions thus producing photons through n-p bremsstrahlung and spectator nucleons. We investigate the relative importance of the gain and loss term of the Uehling-Uhlenbeck collision integral. The elementary pn→pnγ process is included for two different prescriptions: i) classical soft limit, ii) quantum calculation including meson exchange. We also study the influence of the Δ-resonance in hard-photon production. We show that for heavy colliding nuclei and high beam energies a bump should be visible.

Search for cold fusion induced by electrolysis in palladium

Abstract: A search for electrochemically induced cold fusion in palladium has been carried out in the Gran Sasso Underground Laboratory in order to suppress the background due to environmental neutrons. Both p-d and d-d fusion processes have been searched for. A characteristic of our experiment is the attempt to induce abnormal local concentrations of hydrogen isotopes in the cathode by artificially applying mechanical stresses. No evidence has been found for any signal of cold fusion; namely for excess of tritium,4He, heat and particularly for emissions of neutrons and gamma-rays, for whose determination our experiment has been specially conceived.

Dirac optical-model analysis of polarized protons elastic scattering from12C,16,18O,20Ne and24,26Mg at 800 MeV

Abstract: Proton elastic-scattering cross-sections and analysing powers at 800 MeV have been used to derive, within the Dirac formalism, optical-model parameters for six elements from12C to26Mg. The best set of Dirac optical parameters has been obtained by searching on all nine parameters which showed a large negative real scalar potential, a large positive real vector potential and a large negative imaginary vector potential. These calculations have been found to be consistent with an imaginary scalar potential set to zero (V s i =0). Considerable improvements were obtained by using the Dirac formalism compared to the traditional Schrodinger one with a similar number of parameters.

Pseudopotential iteration of propagator equations

Abstract: A new iterative procedure for propagator equations describing systems with singular interactions is suggested. The standard perturbation theory for such systems beginning with the Hartree or Hartree-Fock approximations looses its sense because of the divergence of the corresponding approximate expressions for the mass operator. To define the latter a regular procedure containing no divergences is constructed. A new notion is introduced: that of the doubling function transforming the one-particle propagator to the two-particle one. The solution of the equation for this function makes the basis of the suggested approach. The divergences in all orders of an iterative procedure are eliminated owing to the multiplicative renormalization of the interaction potential by a smoothing function taking account of short-range correlations. As a result, the iterative approximations for the mass operator include solely the powers of the renormalized potential called, for brevity, the pseudopotential.