Photoproduction of π+ from9Be

The reaction⁹Be(γ, π ⁺)⁹Li has been studied with bremsstrahlung in the energy range 100–800 MeV employing the radioactivity method. The cross section curve deduced is compared with an impulse approximation calculation including a volume and a surface production model. In the energy region 200–300 MeV the experimental cross section, approximately 7 (μb, is best reproduced by the surface production model with a cut-off parameterr ₀ equal to 2.8 fm, i.e. somewhat larger than the r.m.s. radius of⁹Be.     

A scalar boson as a messenger of new physics

Quantum corrections generate a quadratically divergent mass term for the Higgs boson in the standard model. Thus, if the Higgs boson has a mass of order 100 GeV, it implies the presence of a cut-off of the theory around TeV scale, and some particles associated with the new physics may appear around the cut-off scale Λ. However, if Λ is several TeV, it may be difficult to find such particles at the LHC. In this Letter, we consider a situation in which the new physics provides relatively light particles compared with the scale Λ. In such a situation, we show that diphoton event and four lepton event by the decay of the Higgs and/or a new particle have naturally large cross section, and LHC may test the new physics in a considerably broad parameter region even if Λ is several TeV.     

Systematic trends in the analysis of photonuclear cross section ratios

Isomeric cross section ratios were measured for the photonuclear reactions¹⁰⁰Mo(γ,n) \({}^{99m_1 ,m_2 ,g}Mo\) and¹⁰² Ru(γ,p)^101m,g Tc Using the Huizenga-Vandenbosch-method spin cut-off parameters were deduced. The applicability of this statistical procedure is discussed. A systematic analysis of all known (γ, xn) isomeric ratio-measurements shows a linear correlation between derived spin cut-off parameters and the mean value of the spins of the isomeric pair.     

Partonic scattering cross sections in the QCD medium

A medium modified gluon propagator is used to evaluate the scattering cross section for the process gg→gg in the QCD medium by performing an explicit sum over the polarizations of the gluons. We incorporate a magnetic screening mass from a non-perturbative study. It is shown that the medium modified cross section is finite, divergence free, and is independent of any ad-hoc momentum transfer cut-off parameters. The medium modified finite cross sections are necessary for a realistic investigation of the production and equilibration of the minijet plasma expected at RHIC and LHC.     

Leading non-cancelling infrared divergences in perturbative QCD

In perturbative QCD, for the inclusive cross section for the scattering of two coloured particles, we identify graphs which contribute to the general leading order α_s(α_s lnλ)ⁿ of uncancelled IR divergences, and we sum these contributions (λ is the IR cut-off). The work is done in the Coulomb gauge; an appendix discusses the Feynman gauge.     

Antikaon production in nucleon- nucleon reactions near threshold

The antikaon production cross section from nucleon-nucleon reactions near threshold is studied in a meson exchange model. We include both pion and kaon exchange, but neglect the interference between the amplitudes. In case of pion exchange the antikaon production cross section can be expressed in terms of the antikaon production cross section from a pion-nucleon interaction, which we take from the experimental data if available. Otherwise, a K* -resonance exchange model is introduced to relate the different reaction cross sections. In case of kaon exchange the antikaon production cross section is related to the elastic KN and K?N cross sections, which are again taken from experimental measurements. We find that the one-meson exchange model gives a satisfactory fit to the available data for the NN → NN KK? cross section at high energies. We compare our predictions for the cross section near threshold with an earlier empirical parameterization and that from phase space models.     

Photoabsorption on nuclei

We calculate the total photoabsorption cross section on nuclei in the energy range from 300 MeV to 1 GeV within the framework of a semi-classical phase space model. Besides medium modifications like Fermi motion and Pauli blocking we focus on the collision broadening of the involved resonances. The resonance contributions to the elementary cross section are fixed by fits to partial wave amplitudes of pion photoproduction. The cross sections for NR → NN, needed for the calculation of collision broadening, are obtained by detailed balance from a fit to NN → NNπ cross sections. We show that a reasonable collision broadening is not able to explain the experimentally observed disappearance of the D₁₃(1520)-resonance in the photoabsorption cross section on nuclei.     

Elastic scattering of an electron on the Coulomb potential with a cut-off

The partial-wave method is used to study the elastic scattering of electrons on the Coulomb potential with a cut-off. There is considerable discrepancy between the present results and those obtained by means of the Born approximation and the classical method. At a given specific energy, the cross section exhibits a maximum. This due to the appearance of maxima in the differential cross section in both the forward and the backward direction.     

A comprehensive description of the photon structure function and photon-photon total cross section data

We fit the photon structure function and photon-photon total cross section obtained from two photone ⁺ e ^? experimental data. We show that a combination of the QPM point-like cross section and an hadronic cross section, given by a simple energy power expansion modified by a threshold factor, is in excellent agreement with the data.     

Influence of then-n cross section on the observables in dynamical process of heavy ion collisions

The influence of then- n cross section on the observables in dynamical process of heavy ion collisions was investigated by means of the BUU (Boltzmann-Uehling-Uhlenbeck) equation. The influence of then-n cross section upon transverse momentum, flow angle and quadrupole moment of momentum distribution with increasing incident energy was successfully illustrated according to the variation law ofn-n cross section with and without the medium effects with increasing bombarding energy. This investigation also shows a close relationship between the medium effects of then- n cross section and the nuclear equation of state (EOS).     

Optical model and DWBA analysis of the α-particle scattering for the nuclei26Mg and28,30Si including Hauser-Feshbach contributions

The elastic and inelastic α-scattering cross section has been measured for the nuclei²⁶Mg,^28,30Si at 15 bombarding energies between 15.0 and 16.4 MeV. The energy averaged scattering cross sections can be described by an incoherent sum of contributions from direct and compound reactions. The direct reaction part has been calculated by the optical model or DWBA, the compound reaction part by a simple Hauser-Feshbach formula. From the analyses deformation parameters and spin cut-off parameters are obtained.     

Autoionization cross section of potassium atom excited by electron impact

The autoionization cross section of potassium atoms excited by electron impact is measured in the energy range from the first autoionization threshold at 18.72 eV to 202 eV. The data are obtained by deter-mining the total intensity of electron spectra resulting from the decay of the 3p ⁵ n ₁ l ₁ n ₂ l ₂ autoionizing states. The cross section has two maxima, 1.8 × 10^?16 and 2.2 × 10^?16 cm², at 21 and 32 eV, respectively. The excitation dynamics of autoionization states suggests that the first maximum is associated with the resonance character of the near-threshold excitation. The second maximum, as well as the behavior of the cross section at energies above 50 eV, reflects the dynamics of electron excitation of quartet and doublet autoionizing states. The measured autoionization cross section is compared with known data for the total single ionization cross section of potassium atom by electron impact. The relative contribution of the autoionization cross section to the total single ionization cross section is found to reach 30% at 32 eV.     

Calculations of inelastic cross sections for rotational excitation

Cross sections for scattering of N₂ (j=0) molecules on He atoms have been calculated for relative energies below 5 · 10^?3 eV (58 °K). The time independent scattering formalism ofArthurs andDalgarno was used together with an assumed Lennard-Jones type potential with anisotropicP ₂(cos?) terms in the attractive and repulsive parts. The resulting system of coupled differential equations was solved in the distorted wave and close coupling approximations for the differential and integral cross sections for elastic and inelastic (j=0→j=2) scattering. In the integral inelastic cross section several sharp resonances were found to contribute 40% to the cross section at energies below 40 °K. The resonance peaks are attributed to orbiting or short-lived compound states since they are also observed in the elastic cross section at energies which are lower by the excitation energy of 1.5 · 10^?3 eV. Finally, the effect of varying the potential parameters on the integral inelastic cross section was studied at 50 °K and a rough formula for the cross section as a function of the parameters is obtained. The formula shows that a certain ratio of repulsive and attractive anisotropies leads to a small inelastic cross section indicating a mutual cancellation.     

Deep-inelastic one-particle inclusive processes in the parton model

We study the processes e + N → e′ + h + anything, e⁺ + e^? → h + anything, within the parton model of Landshoff, Polkinghorne and Short. Definite and peculiar predictions are obtained, also common to the Drell, Levy and Yan cut-off theory and to the λφ³ ladder model. These predictions can be summarized as follows: (i) in the photon fragmentation region the cross section factorizes after integration over transverse momentum of h; (ii) in the target fragmentation region Regge behaviour, valid at large ω, gives rise to double Regge exchange for small values of the Feynman variable y, but only up to $\text{y ??1/ω}$; (iii) the structure functions vanish at y = 0; (iv) total multiplicities in the first process increase like 1n ω for large ω, and in the second reach a finite limit at large q².In the case where only one SU(3) multiplet of partons is present (as in the quark parton model) unique relations, valid in the limit of exact SU(3), are found between the cross section and multiplicities in the first process and the analogous quantities for the forward region of the second process.     

Heavy ion induced transfer reactions on 148Nd

Transfer reactions induced by ¹⁶O and ¹⁸O beams on ¹⁴⁸Nd were measured with a time-of-flight setup at 72 MeV incident energy. The angular distributions are bell shapes having their maxima at angles somewhat below the grazing angle. The excitation in the final nuclei takes place (if possible) near the optimum Q-value and is spread over 5 MeV for the one-particle transfer reactions and up to 10 MeV for the multiparticle transfers. The cross sections for the individual channels are explained mostly by Q-window considerations. In spite of the differences in the individual channels the total transfer cross section integrated over excitation energy, angle, and all channels turns out to be the same same for both ¹⁶O and ¹⁸O beams. This cross section amounts to 20 % of the total reaction cross section and nicely fills the gap between the measured fusion cross section and the total reaction cross section obtained from optical model calculations based on elastic scattering data.     

The energy dependence of nuclear reaction cross sections

The reaction cross section σ_R^pA(E) for proton-nucleus collisions varies with energy as the total cross section σ_t^pN(E) does for proton-nucleon scattering in the energy regime 100 MeV ? E ?400 GeV. We propose several empirical relations, and test them by a χ² analysis of various data. Within the framework of Glauber theory, the parameters in the relations can be linked to the cross section for a single inelastic collision.     

Production and two-photon decay of the MSSM scalar Higgs bosons at the LHC

We consider the production and two-photon decay of theCP-even Higgs bosons (h ⁰ andH ⁰) of the Minimal Supersymmetric Standard Model (MSSM) at the Large Hadron Collider. We study in detail the dependence of the cross section on various parameters of the MSSM, especially the dependence on the mixing effects in the squark sector due to the Higgs bilinear parameterμ and the soft supersymmetry breaking parameterA. We find that the cross section for the production of these Higgs bosons has a significant dependence on the parameters which determine the chiral mixing in the squark sector. The cross section times the two-photon branching ratio ofh ⁰ is of the order of 15–25 fb in much of the parameter space that remains after imposing the present experimental constraints. For theH ⁰ the two-photon branching ratio is only significant if theH ⁰ is light, but then the cross section times the branching ratio may exceed 200 fb. The QCD corrections due to quark loop contributions are known to increase the cross section by 50%. We find the dependence of the cross section on the gluon distribution function used to be rather insignificant.     

Symmetric hadron pairs at large transverse momenta as a test of hard scattering models

The inclusive cross section of hadron pairs produced back-to-back with large transverse momenta is examined in the parton model. It is shown quantitatively that this cross section is determined directly by the hard scattering subprocesses, without being influenced by the internal momentum of the constituents, even for transverse momenta of the order of 2–3 GeV/c. The predictions of the phenomenological quark-quark scattering model and of the quantum chromodynamics model for the back-to-back cross section are compared with recent Fermi-lab data. Predictions are made for the corresponding cross section at ISR-energies.     

Role of barrier penetration in complete fusion of heavy ions

The relevance of tunneling in determining the cross section for complete fusion is explored. The classical model with frictional forces is used for the determination of the trajectory of two heavy ions up to the classical turning point. The penetrability of the barrier is calculated quantum mechanically. It is shown that the so calculated transmission coefficient differ from the sharp cut-off approximation, but yield cross section for complete fusion which are not significantly different from the cross section neglecting quantum mechanical effects.     

Analyzing power fluctuations in nuclear reactions

The joint analysis of differential analyzing power (AP) and differential cross section in the nuclear reactions of +J ₁→J ₂ +J ₃ type in the region of strongly overlapping resonances are presented. Consideration is valid in the absence of the channel-channel compound nuclear correlations. The fluctuations of AP and cross section are proved to be statistically correlated, the greater the direct contribution to the reaction cross section the stronger this correlation. For the purely statistical reactions the correlation is absent. It is shown that the joint investigation of the AP and cross section fluctuations gives possibility to extract the direct reaction AP and the separate the direct and compound nucleus contributions into the reaction cross section. In particular it can be done by means of the statistical correlation (SC) analysis. Furthermore, the SC analysis may serve as a test for revealing the intermediate structure involved in the reaction. Finally, the joint study of differential AP and differential cross section may be used to determine the spins of nuclear levels and polarization of incident beam. The results are employed to separate the compound and direct contributions in the inelastic scattering of polarized protons on²⁸Si. Comparison with experiment quantitatively supports the existence of the SC between the fluctuations of AP and cross section. Possible extension of the SC analysis to the case of the presence of channel-channel compound nuclear correlations are discussed.