arXiv subjects

J. Speth

Publications and source records attributed to J. Speth.

At least 91 records · Page 5Linked to original sources

arXiv1999

The piNN vertex function in a meson-theoretical model

The piNN vertex function is calculated within a dispersion theoretical approach, including both pi-rho and pi-sigma intermediate states, where the sigma meson is an abbreviation for a correlated pion pair in a relative S-wave with isospin I=0. A strong coupling between the pi-sigma and the pi-rho states is found. This leads to a softening of the piNN form factor. In a monopole parameterization, a cut-off Lambda=800 MeV is obtained as compared to Lambda=1000 MeV using pi-rho intermediate states only.

nucl-th↗

arXiv1999

What does `rho-exchange' in piN scattering mean?

We present an alternative method for calculating amplitudes for correlated pi pi exchange in the ``sigma'' and rho channel in piN scattering. Starting from a fixed mass meson exchange potential, we introduce the width of the exchanged particles by integrating over a mass spectral function. The spectral functions are constructed from the pseudoempirical N\bar{N} -> pi pi data. Using this approach we develop a prescription for resolving ambiguities of the correlated pi pi exchange in the rho channel that occur when different dispersion theoretical formulations of rho exchange are used to construct piN potentials.

nucl-th↗

arXiv1999

Eta and eta-prime meson production in the reaction pn -> dM near threshold

The two-step model with pi-, rho- and omega- exchanges taken into account is applied to investigate the reactions pn -> d eta and pn -> d eta-prime. The existing experimental data on the reaction pn -> d eta are analysed and predictions for the cross section of the reaction pn -> d eta-prime are presented. It is found that pi- as well as rho exchange yield significant contributions in both reactions. The effect of the final state interaction is also studied.

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arXiv1999

Imprints of log-periodic self-similarity in the stock market

Detailed analysis of the log-periodic structures as precursors of the financial crashes is presented. The study is mainly based on the German Stock Index (DAX) variation over the 1998 period which includes both, a spectacular boom and a large decline, in magnitude only comparable to the so-called Black Monday of October 1987. The present example provides further arguments in favour of a discrete scale-invariance governing the dynamics of the stock market. A related clear log-periodic structure prior to the crash and consistent with its onset extends over the period of a few months. Furthermore, on smaller time-scales the data seems to indicate the appearance of analogous log-periodic oscillations as precursors of the smaller, intermediate decreases. Even the frequencies of such oscillations are similar on various levels of resolution. The related value $λ\approx 2$ of preferred scaling ratios is amazingly consistent with those found for a wide variety of other complex systems. Similar analysis of the major American indices between September 1998 and February 1999 also provides some evidence supporting this concept but, at the same time, illustrates a possible splitting of the dynamics that a large market may experience.

cond-mat↗

arXiv1999

$ϕ$-meson production in proton-proton collisions

The production of $ϕ$-mesons in proton-proton collisions is investigated within a relativistic meson-exchange model of hadronic interactions. The experimental prerequisites for extracting the $NNϕ$ coupling strength from this reaction are discussed. In the absence of a sufficient set of data, which would enable an accurate determination of the $NNϕ$ coupling strength, we perform a combined analysis, based on some reasonable assumptions, of the existing data for both $ω$- and $ϕ$-meson production. We find that the recent data from the DISTO collaboration on the angular distribution of the $ϕ$ meson indicate that the $NNϕ$ coupling constant is small. The analysis yields values for $g_{NNϕ}$ that are compatible with the OZI rule.

nucl-th↗

arXiv1998

Do the E866 Drell Yan data change our picture of the chiral structure of the nucleon ?

We revisit the evaluation of the pionic mechanism of the $\bar{u}-\bar{d}$-asymmetry in the proton structure function. Our analysis is based on the extended AGK unitarity relation between contributions of different mechanisms to the inclusive particle production and the total photoabsorption cross-section (i.e. the proton structure function). We reanalyze the role of isovector reggeons in inclusive production of nucleons and Delta isobars in hadronic reactions. We find rather large contribution of reggeon-exchange induced production of Delta isobars. This leaves much less room for the pion-exchange induced mechanism of $Δ$ production and provides a constraint on the $πN Δ$ form factor. The production of leading pions in proton-proton collisions at ISR puts additional constraints on the $πNN$ vertex form factors. An extension of the AGK-rules to reggeon exchange suggests a negligible contribution to the proton structure function from DIS off the exchanged $ρ,a_2$ reggeons. All these constraints are used then to estimate the pion content of the nucleon and allow to calculate parameter-free the $x$-dependence of $\bar d - \bar u$. We discuss the violation of the Gottfried Sum Rule and $\bar d$-$\bar u$ asymmetry and compare to the one obtained from the E866 experiment at Fermilab. We estimate the background to the pion structure function being determined by the ZEUS and H1 collaborations at HERA from leading neutron experiments.

hep-ph↗

arXiv1998

The Reaction $pp \to ppϕ$ and the Validity of the OZI Rule

The reaction $pp \to pp ϕ$ is investigated within a relativistic meson-exchange model of hadronic interactions. The possibility of extracting the $NNϕ$ coupling constant from a study of this process is explored. A combined analysis of $ω-$ and $ϕ$-meson production in proton-proton collisions utilizing recent data on these reactions is carried out and yields values for $g_{NNϕ}$ that are compatible with the OZI rule.

nucl-th↗

arXiv1998

Dynamics of Light Antiquarks in the Proton

We present a comprehensive analysis of the recent data from the E866 experiment at Fermilab on Drell-Yan production in pD and pp collisions, which indicates a non-trivial x-dependence for the asymmetry between u-bar and d-bar quark distributions in the proton. The relatively fast decrease of the asymmetry at large x suggests the important role played by the chiral structure of the nucleon, in particular the pi-N and pi-Delta components of the nucleon wave function. At small x the data require an additional non-chiral component, which may be attributed to the Pauli exclusion principle as first suggested by Field and Feynman.

hep-ph↗

arXiv1998

Enhancement of $πA \to ππA$ Threshold Cross Sections by In-Medium $ππ$ Final State Interactions

We address the problem of pion production in low energy $π$-nucleus collisions. For the production mechanism we assume a simple model consisting of a coherent sum of single pion exchange and the excitation---followed by the decay into two pions and a nucleon---of the $N^*(1440)$ resonance. The production amplitude is modified by the final state interaction between the pions calculated using the chirally improved Jülich meson exchange model including the polarization of the nuclear medium by the pions. The model reproduces well the experimentally observed $πA \to ππA$ cross sections, especially the enhancement with increasing $A$ of the $π^+π^-$ mass distribution in the threshold region.

nucl-th↗

arXiv1998

Role of the $Δ$ isobar in the reaction $NN \to NNπ$ near threshold

A model calculation for pion production in nucleon-nucleon collisions is presented. Direct production, pion rescattering and contributions from pair diagrams are taken into account. The amplitudes for the elementary processes are based on well established microscopic models of nucleon-nucleon and pion-nucleon scattering. The $Δ(1232)$ is included explicitly and is found to play an important role even at energies close to production threshold. A good overall agreement with existing data from the pion production threshold up to the Delta resonance region is achieved.

nucl-th↗

arXiv1998

Semi-inclusive Pion Production and the d/u Ratio

We explore the feasibility of directly extracting the large-x valence d/u ratio through a measurement of pions in the current fragmentation region of semi-inclusive deep-inelastic scattering from protons.

hep-ph↗

arXiv1998

Meson-Exchange Model for the YN Interaction

We report on progress in the development of a microscopic model for the hyperon-nucleon interaction within the meson-exchange framework, which incorporates both one-meson as well as correlated two-meson exchange contributions. The main new feature of the model is the exchange of two correlated pions or kaons, both in the scalar-isoscalar and the vector-isovector channels. In the scalar channel it provides the main part of the intermediate-range interaction between two baryons, thereby eliminating the need for the fictitious sigma meson, which has been an unavoidable element of effective one-boson-exchange descriptions.

nucl-th↗

arXiv1998

The reactions $pp\to ppπ^0$ and $pp\to dπ^+$ at threshold: The role of the isoscalar $πN$ scattering amplitude

We examine the role of the elementary isoscalar pion--nucleon scattering amplitude in the description of the processes $pp \to ppπ^0$ and $pp\to dπ^+$ at threshold. We argue that the presently used tree level dimension two approximation used in chiral perturbation theory is insufficient as input by direct comparison with the $π$N scattering data. We also show that a successful semi--phenomenological boson--exchange model does better in the description of these data. The influence of the violation of crossing symmetry in the meson--exchange model has to be studied in more detail. We stress that further investigations of the process $pp\to dπ^+$ can pave the way to a deeper understanding of the pion dominated part of the transition operator.

nucl-th↗

arXiv1998

Production of $ω$-mesons in proton-proton collisions

The production of $ω$-mesons in proton-proton collisions for proton incident energies up to 2.2 GeV is investigated within a meson-exchange model of hadronic interactions. We find a large cancellation between the dominant $πρω$ meson-exchange current and nucleonic current contributions. A comparison with preliminary data from SATURNE calls for the inclusion of off-shell form factors at the $NNω$ and $πρω$ production vertices. Due to the present lack of knowledge of these form factors, together with the destructive interference mentioned above, the relative magnitude of the nucleonic and meson-exchange current contributions cannot be determined from existing total cross section data. However, it is shown that the angular distribution of the produced $ω$-mesons provides an unique and clear signature of the magnitude of these currents, thus allowing one to disentangle these two basic reaction mechanisms.

nucl-th↗

arXiv1997

Collectivity Embedded in Complex Spectra of Finite Interacting Fermi Systems: Nuclear Example

The mechanism of collectivity coexisting with chaos in a finite system of strongly interacting fermions is investigated. The complex spectra are represented in the basis of two-particle two-hole states describing the nuclear double-charge exchange modes in $^{48}$Ca. An example of $J^π=0^-$ excitations shows that the residual interaction, which generically implies chaotic behavior, under certain specific and well identified conditions may create strong transitions, even much stronger than those corresponding to a pure mean-field picture. Such an effect results from correlations among the off-diagonal matrix elements, is connected with locally reduced density of states and a local minimum in the information entropy.

chao-dyn↗

arXiv1997

Leading proton spectrum from DIS at HERA

The QCD hardness scale for secondary particles ($h$) production in semi-inclusive deep inelastic scattering (DIS), $e p \to e' X h$, gradually decreases from $Q^{2}$, the photon virtuality which determines the hard scale in the virtual photon (current) fragmentation region to a soft, hadronic, scale in the proton fragmentation region. This suggests similarity of the inclusive spectra of leading protons and neutrons, $h=p,n$, in high energy hadron-proton and virtual photon-proton collisions. We explore this similarity extending to the DIS regime the nonperturbative peripheral mechanisms of inelastic scattering traditionally used in hadronic interactions to explain fast nucleons production. While the production of leading neutrons is known to be exhausted by DIS off charged pions, the production of leading protons by DIS off neutral pions must be supplemented by a substantial contribution from isoscalar reggeon ($f_{0}$) exchange extrapolated down to moderate values of $x_{L}$. We comment on the $x$ and $Q^{2}$ dependence of leading proton production as a probe of a universal pattern of the $x,Q^{2}$ evolution of the nucleon and meson (reggeon) structure functions at small $x$.

hep-ph↗

arXiv1997

Absorptive corrections to the one pion exchange and measurablity of the small-x pion structure function at HERA

We compaWe compare the absorptive corrections to the one pion exchange in $pp\to Xn$ and $ep\to e'Xn$ reactions. It is shown that the absorption is stronger in the case of $pp$ collision. The difference in strength of the absorption for the $pp\to Xn$ and $ep\to e'Xn$ reactions breaks the factorization peculiar to the pure one pion exchange. We evaluate the emerging model dependence of extraction of the small-$x$ pion structure function from an analysis of the HERA data on the neutron production at physical values of $t$.

hep-ph↗

arXiv1997

Collectivity Embedded in Complex Spectra: Example of Nuclear Double-Charge Exchange Modes

The mechanism of collectivity coexisting with chaos is investigated on the quantum level. The complex spectra are represented in the basis of two-particle two-hole states describing the nuclear double-charge exchange modes in $^{48}$Ca. An example of $J^π=0^-$ excitations shows that the residual interaction, which generically implies chaotic behavior, under certain specific and well identified conditions may create transitions stronger than those corresponding to the pure mean-field picture. Therefore, for this type of excitations such an effect is not generic and in most cases the strength of transitions is likely to take much lower values, even close to the Porter-Thomas distributed.

nucl-th↗