Scale-free networks constitute a fast-developing field that has already provided us with importan... more Scale-free networks constitute a fast-developing field that has already provided us with important tools to understand natural and social phenomena. From biological systems to environmental modifications, from quantum fields to high energy collisions, or from the number of contacts one person has, on average, to the flux of vehicles in the streets of urban centres, all these complex, non-linear problems are better understood under the light of the scale-free network's properties. A few mechanisms have been found to explain the emergence of scale invariance in complex networks, and here we discuss a mechanism based on the way information is locally spread among agents in a scale-free network. We show that the correct description of the information dynamics is given in terms of the q-exponential function, with the power-law behaviour arising in the asymptotic limit. This result shows that the best statistical approach to the information dynamics is given by Tsallis Statistics. We discuss the main properties of the information spreading process in the network and analyse the role and behaviour of some of the parameters as the number of agents increases. The different mechanisms for optimization of the information spread are discussed.
This work presents an analysis of fractional derivatives and fractal derivatives, discussing thei... more This work presents an analysis of fractional derivatives and fractal derivatives, discussing their differences and similarities. The fractal derivative is closely connected to Haussdorff’s concepts of fractional dimension geometry. The paper distinguishes between the derivative of a function on a fractal domain and the derivative of a fractal function, where the image is a fractal space. Different continuous approximations for the fractal derivative are discussed, and it is shown that the q-calculus derivative is a continuous approximation of the fractal derivative of a fractal function. A similar version can be obtained for the derivative of a function on a fractal space. Caputo’s derivative is also proportional to a continuous approximation of the fractal derivative, and the corresponding approximation of the derivative of a fractional function leads to a Caputo-like derivative. This work has implications for studies of fractional differential equations, anomalous diffusion, infor...
The forward Compton scattering amplitudes for carbon and lead have been calculated from total pho... more The forward Compton scattering amplitudes for carbon and lead have been calculated from total photoabsorption cross section data by using dispersion relations. The results show a large difference between the scattering amplitudes for nuclei and the free nucleon, above the ∆ region. Difference
We have evaluated the nonmesonic weak decay spectra within the independent-particle shellmodel, a... more We have evaluated the nonmesonic weak decay spectra within the independent-particle shellmodel, and compared them with the recent measurements of: i) the single and double coincidence nucleon spectra in 12 Λ C performed at KEK, and ii) proton kinetic energy spectra in 5 Λ He, 7 Λ Li, 9 Λ Be, 11 Λ B, 12 Λ C, 13 Λ C, 15 Λ N and 16 Λ O done by FINUDA. Based on this comparison we argue that the extraction from the data of the three-body ΛN N → nN N induced decay rate, as done in these works, could be questionable.
The interest in the Klein–Gordon equation with different potentials has increased in recent years... more The interest in the Klein–Gordon equation with different potentials has increased in recent years due to its possible applications in Cosmology, Hadron Physics and High-Energy Physics. In this work, we investigate the solutions of the Klein–Gordon equation for bosons under the influence of an external potential by using the Feshbach–Villars method. We present detailed results for two cases: the Coulombic potential and the harmonic potential. For the latter case, we studied the effects of self-interacting particles by adopting a mean-field approach. We show that our results converge smoothly to the solution of the Schrödinger equation for the same systems as the relativistic effects diminish.
Scale-free networks constitute a fast-developing field that has already provided us with importan... more Scale-free networks constitute a fast-developing field that has already provided us with important tools to understand natural and social phenomena. From biological systems to environmental modifications, from quantum fields to high energy collisions, or from the number of contacts one person has, on average, to the flux of vehicles in the streets of urban centres, all these complex, non-linear problems are better understood under the light of the scale-free network’s properties. A few mechanisms have been found to explain the emergence of scale invariance in complex networks, and here we discuss a mechanism based on the way information is locally spread among agents in a scale-free network. We show that the correct description of the information dynamics is given in terms of the q-exponential function, with the power-law behaviour arising in the asymptotic limit. This result shows that the best statistical approach to the information dynamics is given by Tsallis Statistics. We disc...
Scale-free networks constitute a fast-developing field that has already provided us with importan... more Scale-free networks constitute a fast-developing field that has already provided us with important tools to understand natural and social phenomena. From biological systems to environmental modifications, from quantum fields to high energy collisions, or from the number of contacts one person has, on average, to the flux of vehicles in the streets of urban centres, all these complex, non-linear problems are better understood under the light of the scale-free network’s properties. A few mechanisms have been found to explain the emergence of scale invariance in complex networks, and here we discuss a mechanism based on the way information is locally spread among agents in a scale-free network. We show that the correct description of the information dynamics is given in terms of the q-exponential function, with the power-law behaviour arising in the asymptotic limit. This result shows that the best statistical approach to the information dynamics is given by Tsallis Statistics. We disc...
Mathematical models can provide insights into the control of pandemic COVID-19, which remains a g... more Mathematical models can provide insights into the control of pandemic COVID-19, which remains a global priority. The dynamics of directly-transmitted infectious diseases, such as COVID-19, are usually described by compartmental models where individuals are classified as susceptible, infected and removed. These SIR models typically assume homogenous transmission of infection, even in large populations, a simplification that is convenient but inconsistent with observations. Here we use original data on the dynamics of COVID-19 spread in a Brazilian city to investigate the structure of the transmission network. We find that transmission can be described by a network in which each infectious individual has a small number of susceptible contacts, of the order of 2-5, which is independent of total population size. Compared with standard models of homogenous mixing, this scale-free, fractal infection process gives a better description of COVID-19 dynamics through time. In addition, the con...
The main properties of a fractal model to describe the epidemic spread of diseases are discussed.... more The main properties of a fractal model to describe the epidemic spread of diseases are discussed. The most important features of the epidemic process that leads to the conclusion of an underlying fractal structure are analyzed, and the meaning of the parameters of the model are investigated. It is shown that the existence of a fractal mechanism in the epidemic evolution leads to a simple relation between time interval for the epidemic spread and the population size. It is shown that the contamination rate varies with the population size. The relevant differences between the SIR model and the fractal model are discussed.
Nonadditive Tsallis q-statistics has successfully been applied for a plethora of systems in natur... more Nonadditive Tsallis q-statistics has successfully been applied for a plethora of systems in natural sciences and other branches of knowledge. Nevertheless, its foundations have been severely criticised by some authors based on the standard additive Boltzmann-Gibbs approach thereby remaining a quite controversial subject. In order to clarify some polemical concepts, the distribution function for an ideal gas with a finite number of point particles and its q-index are analytically determined. The two-particle correlation function is also derived. The degree of correlation diminishes continuously with the growth of the number of particles. The ideal finite gas system is usually correlated, becomes less correlated when the number of particles grows, and is finally, fully uncorrelated when the molecular chaos regime is reached. It is also advocated that both approaches can be confronted through a careful kinetic spectroscopic experiment. The analytical results derived here suggest that Tsallis q-statistics may play a physical role more fundamental than usually discussed in the literature.
The cross sections of 110 radioactive nuclide with mass numbers 22 ≤ A ≤ 198 amu from the interac... more The cross sections of 110 radioactive nuclide with mass numbers 22 ≤ A ≤ 198 amu from the interaction of 2.2 GeV/nucleon deuterons from the Nuclotron of the Laboratory of High Energies (LHE), Joint Institute for Nuclear Research (JINR) at Dubna with a 197 Au target are investigated using induced activity method. The results including charge and mass distributions are parameterized in terms of 3-parameter equation in order to complete the real isobaric distribution. Using data from charge distribution total mass-yield distribution was obtained. The analysis of the mass-yield distribution allows to suppose existence of different channels of the interaction such as spallation, deep spallation, fission-like and multifragmentation processes.
We investigate the transition from the nucleon-meson to quark-gluon description of the strong int... more We investigate the transition from the nucleon-meson to quark-gluon description of the strong interaction using the photon energy dependence of the d(γ, p)n differential cross section for photon energies above 0.5 GeV and center-of-mass proton angles between 30 • and 150 •. A possible signature for this transition is the onset of cross section s −11 scaling with the total energy squared, s, at some proton transverse momentum, PT. The results show that the scaling has been reached for proton transverse momentum above about 1.1 GeV/c. This may indicate that the quark-gluon regime is reached above this momentum.
This work is devoted to the application of highresolution gammaray spectrometry in the study of t... more This work is devoted to the application of highresolution gammaray spectrometry in the study of the effective dose coming from naturally occurring radionuclides, namely 40 K, 232 Th and 238 U, present in building materials such as sand, cement, bricks and granitic gravel. Four calculation models were applied to estimate the effective dose and the hazard indices. The maximum estimated effective dose coming from the three reference rooms considered is 0.90(22) mSv/yr, and maximum internal hazard index is 0.77(14), both for the compact clay brick reference room. The principal gamma radiation sources are cement, sand and bricks.
Simple and coincidence spectra of the NM weak decay of light hypernuclei have been evaluated in a... more Simple and coincidence spectra of the NM weak decay of light hypernuclei have been evaluated in a systematic way for the first time. We have only considered 1N induced processes, neglecting entirely the events induced by 2N emission, as well as the effects of the FSIt's. As the theoretical framework we have used the IPSM with three different parametrizations for the transition potential. The comparison with data strongly suggests that the soft π + K exchange model could be a good starting point to describe the dynamics in the NM weak decays of sand p-shell hypernuclei.
We present results from a joint theoretical and experimental study of inclusive quasifree electro... more We present results from a joint theoretical and experimental study of inclusive quasifree electrofission of 238 U. The off-shell cross sections for the quasifree reaction stage have been calculated within the plane wave impulse approximation with distortion corrections included in the effective momentum approximation. Proton and neutron single-particle momentum distributions were calculated in the macroscopic-microscopic approach. The fissility for proton and neutron single hole excited states of the residual nuclei 237 Pa and 237,238 U was calculated within the compound nucleus model. Final state interaction corrections to residual nucleus excitation energy were calculated using the imaginary part of the optical potential. The total inclusive electrofission cross section was measured with high absolute precision, and all principal partial contributions are analyzed, in particular, the quasifree one.
The investigation of statistical and direct aspects related to the (␥,n) and (␥,np) decay channel... more The investigation of statistical and direct aspects related to the (␥,n) and (␥,np) decay channels of 64 Zn in the giant dipole resonance ͑GDR͒ and quasideuteron ͑QD͒ energy regions was performed by a trial function fitting to the respective (e,n) and (e,np) electrodisintegration yields measured by residual activity. The trial function incorporated the GDR and QD models to describe the initial photoabsorption mechanism and the geometry dependent hybrid exciton model used in the ALICE/LIVERMORE-82 code to calculate the relevant branching ratios, with the E1 virtual photon spectra being calculated in the distorted wave Born approximation. We compared our results for the (␥,n) cross section with other existing experimental measurements, and the long-standing normalization issue among different laboratories was revisited and addressed. We obtained for the first time the absolute (␥,np) cross section from threshold to 60 MeV. We succeeded in separating statistical and direct contributions of the (␥,np) process, the latter being remarkably well described by the QD model in the interval 40-60 MeV. A possible direct contribution for the (␥,n) decay in the GDR is also addressed. Finally, the total photoabsorption cross section of 64 Zn was reevaluated up to 21 MeV, and the results were compared with previous estimates performed by other groups.
The reaction ep → e ′ K + Λ(1520) with Λ(1520) → p ′ K − was studied at electron beam energies of... more The reaction ep → e ′ K + Λ(1520) with Λ(1520) → p ′ K − was studied at electron beam energies of 4.05, 4.25, and 4.46 GeV, using the CLAS detector at the Thomas Jefferson National Accelerator Facility. The cosθ K + , φ K + , Q 2 , and W dependencies of Λ(1520) electroproduction are presented for the kinematic region 0.9 < Q 2 < 2.4 GeV 2 and 1.95 < W < 2.65 GeV. Also, the Q 2 dependence of the Λ(1520) decay angular distribution is presented for the first time. The cosθ K + angular distributions suggest t-channel diagrams dominate the production process. Fits to the Λ(1520) t-channel helicity frame decay angular distributions indicate the mz = ± 1 2 parentage accounts for about 60% of the total yield, which suggests this reaction has a significant contribution from t-channel processes with either K + exchange or longitudinal coupling to an exchanged K *. The Q 2 dependence of the Λ(1520) production cross section is the same as that observed for Λ(1116) photo-and electroproduction.
The energy and nuclear mass dependences of the total hadronic cross section in the energy range 0... more The energy and nuclear mass dependences of the total hadronic cross section in the energy range 0.5-2.6 GeV have been measured at Bonn using the SAPHIR tagged photon beam. The measurement, performed on C, Al, Cu, Sn, and Pb, provides the first photoabsorption data in the region 1.2-1.7 GeV. The results show a significant reduction of the photoabsorption strength on the bound nucleon compared to the free-nucleon case in the whole energy region. Above 1.2 GeV this reduction decreases with the average nuclear density. ͓S0556-2813͑99͒01512-5͔
Scale-free networks constitute a fast-developing field that has already provided us with importan... more Scale-free networks constitute a fast-developing field that has already provided us with important tools to understand natural and social phenomena. From biological systems to environmental modifications, from quantum fields to high energy collisions, or from the number of contacts one person has, on average, to the flux of vehicles in the streets of urban centres, all these complex, non-linear problems are better understood under the light of the scale-free network's properties. A few mechanisms have been found to explain the emergence of scale invariance in complex networks, and here we discuss a mechanism based on the way information is locally spread among agents in a scale-free network. We show that the correct description of the information dynamics is given in terms of the q-exponential function, with the power-law behaviour arising in the asymptotic limit. This result shows that the best statistical approach to the information dynamics is given by Tsallis Statistics. We discuss the main properties of the information spreading process in the network and analyse the role and behaviour of some of the parameters as the number of agents increases. The different mechanisms for optimization of the information spread are discussed.
This work presents an analysis of fractional derivatives and fractal derivatives, discussing thei... more This work presents an analysis of fractional derivatives and fractal derivatives, discussing their differences and similarities. The fractal derivative is closely connected to Haussdorff’s concepts of fractional dimension geometry. The paper distinguishes between the derivative of a function on a fractal domain and the derivative of a fractal function, where the image is a fractal space. Different continuous approximations for the fractal derivative are discussed, and it is shown that the q-calculus derivative is a continuous approximation of the fractal derivative of a fractal function. A similar version can be obtained for the derivative of a function on a fractal space. Caputo’s derivative is also proportional to a continuous approximation of the fractal derivative, and the corresponding approximation of the derivative of a fractional function leads to a Caputo-like derivative. This work has implications for studies of fractional differential equations, anomalous diffusion, infor...
The forward Compton scattering amplitudes for carbon and lead have been calculated from total pho... more The forward Compton scattering amplitudes for carbon and lead have been calculated from total photoabsorption cross section data by using dispersion relations. The results show a large difference between the scattering amplitudes for nuclei and the free nucleon, above the ∆ region. Difference
We have evaluated the nonmesonic weak decay spectra within the independent-particle shellmodel, a... more We have evaluated the nonmesonic weak decay spectra within the independent-particle shellmodel, and compared them with the recent measurements of: i) the single and double coincidence nucleon spectra in 12 Λ C performed at KEK, and ii) proton kinetic energy spectra in 5 Λ He, 7 Λ Li, 9 Λ Be, 11 Λ B, 12 Λ C, 13 Λ C, 15 Λ N and 16 Λ O done by FINUDA. Based on this comparison we argue that the extraction from the data of the three-body ΛN N → nN N induced decay rate, as done in these works, could be questionable.
The interest in the Klein–Gordon equation with different potentials has increased in recent years... more The interest in the Klein–Gordon equation with different potentials has increased in recent years due to its possible applications in Cosmology, Hadron Physics and High-Energy Physics. In this work, we investigate the solutions of the Klein–Gordon equation for bosons under the influence of an external potential by using the Feshbach–Villars method. We present detailed results for two cases: the Coulombic potential and the harmonic potential. For the latter case, we studied the effects of self-interacting particles by adopting a mean-field approach. We show that our results converge smoothly to the solution of the Schrödinger equation for the same systems as the relativistic effects diminish.
Scale-free networks constitute a fast-developing field that has already provided us with importan... more Scale-free networks constitute a fast-developing field that has already provided us with important tools to understand natural and social phenomena. From biological systems to environmental modifications, from quantum fields to high energy collisions, or from the number of contacts one person has, on average, to the flux of vehicles in the streets of urban centres, all these complex, non-linear problems are better understood under the light of the scale-free network’s properties. A few mechanisms have been found to explain the emergence of scale invariance in complex networks, and here we discuss a mechanism based on the way information is locally spread among agents in a scale-free network. We show that the correct description of the information dynamics is given in terms of the q-exponential function, with the power-law behaviour arising in the asymptotic limit. This result shows that the best statistical approach to the information dynamics is given by Tsallis Statistics. We disc...
Scale-free networks constitute a fast-developing field that has already provided us with importan... more Scale-free networks constitute a fast-developing field that has already provided us with important tools to understand natural and social phenomena. From biological systems to environmental modifications, from quantum fields to high energy collisions, or from the number of contacts one person has, on average, to the flux of vehicles in the streets of urban centres, all these complex, non-linear problems are better understood under the light of the scale-free network’s properties. A few mechanisms have been found to explain the emergence of scale invariance in complex networks, and here we discuss a mechanism based on the way information is locally spread among agents in a scale-free network. We show that the correct description of the information dynamics is given in terms of the q-exponential function, with the power-law behaviour arising in the asymptotic limit. This result shows that the best statistical approach to the information dynamics is given by Tsallis Statistics. We disc...
Mathematical models can provide insights into the control of pandemic COVID-19, which remains a g... more Mathematical models can provide insights into the control of pandemic COVID-19, which remains a global priority. The dynamics of directly-transmitted infectious diseases, such as COVID-19, are usually described by compartmental models where individuals are classified as susceptible, infected and removed. These SIR models typically assume homogenous transmission of infection, even in large populations, a simplification that is convenient but inconsistent with observations. Here we use original data on the dynamics of COVID-19 spread in a Brazilian city to investigate the structure of the transmission network. We find that transmission can be described by a network in which each infectious individual has a small number of susceptible contacts, of the order of 2-5, which is independent of total population size. Compared with standard models of homogenous mixing, this scale-free, fractal infection process gives a better description of COVID-19 dynamics through time. In addition, the con...
The main properties of a fractal model to describe the epidemic spread of diseases are discussed.... more The main properties of a fractal model to describe the epidemic spread of diseases are discussed. The most important features of the epidemic process that leads to the conclusion of an underlying fractal structure are analyzed, and the meaning of the parameters of the model are investigated. It is shown that the existence of a fractal mechanism in the epidemic evolution leads to a simple relation between time interval for the epidemic spread and the population size. It is shown that the contamination rate varies with the population size. The relevant differences between the SIR model and the fractal model are discussed.
Nonadditive Tsallis q-statistics has successfully been applied for a plethora of systems in natur... more Nonadditive Tsallis q-statistics has successfully been applied for a plethora of systems in natural sciences and other branches of knowledge. Nevertheless, its foundations have been severely criticised by some authors based on the standard additive Boltzmann-Gibbs approach thereby remaining a quite controversial subject. In order to clarify some polemical concepts, the distribution function for an ideal gas with a finite number of point particles and its q-index are analytically determined. The two-particle correlation function is also derived. The degree of correlation diminishes continuously with the growth of the number of particles. The ideal finite gas system is usually correlated, becomes less correlated when the number of particles grows, and is finally, fully uncorrelated when the molecular chaos regime is reached. It is also advocated that both approaches can be confronted through a careful kinetic spectroscopic experiment. The analytical results derived here suggest that Tsallis q-statistics may play a physical role more fundamental than usually discussed in the literature.
The cross sections of 110 radioactive nuclide with mass numbers 22 ≤ A ≤ 198 amu from the interac... more The cross sections of 110 radioactive nuclide with mass numbers 22 ≤ A ≤ 198 amu from the interaction of 2.2 GeV/nucleon deuterons from the Nuclotron of the Laboratory of High Energies (LHE), Joint Institute for Nuclear Research (JINR) at Dubna with a 197 Au target are investigated using induced activity method. The results including charge and mass distributions are parameterized in terms of 3-parameter equation in order to complete the real isobaric distribution. Using data from charge distribution total mass-yield distribution was obtained. The analysis of the mass-yield distribution allows to suppose existence of different channels of the interaction such as spallation, deep spallation, fission-like and multifragmentation processes.
We investigate the transition from the nucleon-meson to quark-gluon description of the strong int... more We investigate the transition from the nucleon-meson to quark-gluon description of the strong interaction using the photon energy dependence of the d(γ, p)n differential cross section for photon energies above 0.5 GeV and center-of-mass proton angles between 30 • and 150 •. A possible signature for this transition is the onset of cross section s −11 scaling with the total energy squared, s, at some proton transverse momentum, PT. The results show that the scaling has been reached for proton transverse momentum above about 1.1 GeV/c. This may indicate that the quark-gluon regime is reached above this momentum.
This work is devoted to the application of highresolution gammaray spectrometry in the study of t... more This work is devoted to the application of highresolution gammaray spectrometry in the study of the effective dose coming from naturally occurring radionuclides, namely 40 K, 232 Th and 238 U, present in building materials such as sand, cement, bricks and granitic gravel. Four calculation models were applied to estimate the effective dose and the hazard indices. The maximum estimated effective dose coming from the three reference rooms considered is 0.90(22) mSv/yr, and maximum internal hazard index is 0.77(14), both for the compact clay brick reference room. The principal gamma radiation sources are cement, sand and bricks.
Simple and coincidence spectra of the NM weak decay of light hypernuclei have been evaluated in a... more Simple and coincidence spectra of the NM weak decay of light hypernuclei have been evaluated in a systematic way for the first time. We have only considered 1N induced processes, neglecting entirely the events induced by 2N emission, as well as the effects of the FSIt's. As the theoretical framework we have used the IPSM with three different parametrizations for the transition potential. The comparison with data strongly suggests that the soft π + K exchange model could be a good starting point to describe the dynamics in the NM weak decays of sand p-shell hypernuclei.
We present results from a joint theoretical and experimental study of inclusive quasifree electro... more We present results from a joint theoretical and experimental study of inclusive quasifree electrofission of 238 U. The off-shell cross sections for the quasifree reaction stage have been calculated within the plane wave impulse approximation with distortion corrections included in the effective momentum approximation. Proton and neutron single-particle momentum distributions were calculated in the macroscopic-microscopic approach. The fissility for proton and neutron single hole excited states of the residual nuclei 237 Pa and 237,238 U was calculated within the compound nucleus model. Final state interaction corrections to residual nucleus excitation energy were calculated using the imaginary part of the optical potential. The total inclusive electrofission cross section was measured with high absolute precision, and all principal partial contributions are analyzed, in particular, the quasifree one.
The investigation of statistical and direct aspects related to the (␥,n) and (␥,np) decay channel... more The investigation of statistical and direct aspects related to the (␥,n) and (␥,np) decay channels of 64 Zn in the giant dipole resonance ͑GDR͒ and quasideuteron ͑QD͒ energy regions was performed by a trial function fitting to the respective (e,n) and (e,np) electrodisintegration yields measured by residual activity. The trial function incorporated the GDR and QD models to describe the initial photoabsorption mechanism and the geometry dependent hybrid exciton model used in the ALICE/LIVERMORE-82 code to calculate the relevant branching ratios, with the E1 virtual photon spectra being calculated in the distorted wave Born approximation. We compared our results for the (␥,n) cross section with other existing experimental measurements, and the long-standing normalization issue among different laboratories was revisited and addressed. We obtained for the first time the absolute (␥,np) cross section from threshold to 60 MeV. We succeeded in separating statistical and direct contributions of the (␥,np) process, the latter being remarkably well described by the QD model in the interval 40-60 MeV. A possible direct contribution for the (␥,n) decay in the GDR is also addressed. Finally, the total photoabsorption cross section of 64 Zn was reevaluated up to 21 MeV, and the results were compared with previous estimates performed by other groups.
The reaction ep → e ′ K + Λ(1520) with Λ(1520) → p ′ K − was studied at electron beam energies of... more The reaction ep → e ′ K + Λ(1520) with Λ(1520) → p ′ K − was studied at electron beam energies of 4.05, 4.25, and 4.46 GeV, using the CLAS detector at the Thomas Jefferson National Accelerator Facility. The cosθ K + , φ K + , Q 2 , and W dependencies of Λ(1520) electroproduction are presented for the kinematic region 0.9 < Q 2 < 2.4 GeV 2 and 1.95 < W < 2.65 GeV. Also, the Q 2 dependence of the Λ(1520) decay angular distribution is presented for the first time. The cosθ K + angular distributions suggest t-channel diagrams dominate the production process. Fits to the Λ(1520) t-channel helicity frame decay angular distributions indicate the mz = ± 1 2 parentage accounts for about 60% of the total yield, which suggests this reaction has a significant contribution from t-channel processes with either K + exchange or longitudinal coupling to an exchanged K *. The Q 2 dependence of the Λ(1520) production cross section is the same as that observed for Λ(1116) photo-and electroproduction.
The energy and nuclear mass dependences of the total hadronic cross section in the energy range 0... more The energy and nuclear mass dependences of the total hadronic cross section in the energy range 0.5-2.6 GeV have been measured at Bonn using the SAPHIR tagged photon beam. The measurement, performed on C, Al, Cu, Sn, and Pb, provides the first photoabsorption data in the region 1.2-1.7 GeV. The results show a significant reduction of the photoabsorption strength on the bound nucleon compared to the free-nucleon case in the whole energy region. Above 1.2 GeV this reduction decreases with the average nuclear density. ͓S0556-2813͑99͒01512-5͔
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