Total Photoabsorption Cross Section Research Articles

Total4Hephotoabsorption cross section reexamined: Correlated versus effective interaction hyperspherical harmonics

Two conceptually different hyperspherical harmonics expansions are used for the calculation of the total 4He photoabsorption cross section. Besides the well known method of CHH the recently introduced effective interaction approach for the hyperspherical formalism is applied. Semi-realistic NN potentials are employed and final state interaction is fully taken into account via the Lorentz integral transform method. The results show that the effective interaction leads to a very good convergence, while the correlation method exhibits a less rapid convergence in the giant dipole resonance region. The rather strong discrepancy with the experimental photodisintegration cross sections is confirmed by the present calculations.

Inner-shell photoabsorption ofFe14+:Unimportance of correlation and relativistic effects

We have performed a series of calculations of the photoabsorption cross section of Fe{sup 14+} near the 2p{sup -1} and 2s{sup -1} edges, within the framework of R-matrix theory. We find the unexpected result that the inclusion in the calculation of such effects as the spin-orbit interaction and higher-order correlations leads to a mere redistribution of flux among the various final channels, while the total convoluted photoabsorption cross section remains virtually unchanged. Hence, an appropriate minimal configuration LS coupling representation can adequately describe inner-shell total photoabsorption cross sections, leading to a saving in computational effort by orders of magnitude compared to more elaborate calculations.

Transverse dynamics of hard partons in nuclear media and the QCD dipole

We derive the non-abelian generalization of the Furry approximation which describes the transverse dynamical evolution of a hard projectile parton inside a spatially extended colour target field. This provides a unified starting point for the target rest frame description of the nuclear dependence of a large class of observables. For the case of the virtual gamma -> q-qbar photoabsorption cross section, we investigate then in detail under which conditions the nuclear dependence encoded in the Furry wavefunctions can be parametrized by a q-qbar QCD dipole cross section. The important condition is colour triviality, i.e., the property that for arbitrary N-fold rescattering contributions the only non-vanishing colour trace is N_c C_F^N. We give proofs for the colour triviality of the inelastic, diffractive and total photoabsorption cross section measured inclusively or with one jet resolved in the final state. Also, we list examples for which colour interference effects remain. Colour triviality allows us to write the gamma -> q qbar contribution to the DIS nuclear structure function F_2 for small Bjorken x in terms of a path integral which describes the transverse size evolution of the q-qbar pair in the nuclear colour field. This expression reduces in an opacity expansion to the N=1 result of Nikolaev and Zakharov, and in the eikonal approximation to the Glauber-type rescattering formulas first derived by Mueller. In the harmonic oscillator approximation of the path integral, we quantify deviations from the eikonal limit. Their onset is characterized by the scales L/l_f and E_t L which relate the longitudinal extension L of the nuclear target to the coherence length l_f and the total transverse energy E_t accumulated by the q-qbar pair.

Comment on: “Proton polarization shifts in electronic and muonic hydrogen”: [Phys. Lett. B 463 (1999) 317-322

The starting point of the article by Rosenfelder [Phys. Lett. B 463 (1999) 317; hep-ph/9903352] is that the proton polarization shifts are expressed through the total photoabsorption cross-section σ γ ( ν). In fact, it is not so, even under the conditions formulated by Rosenfelder [Phys. Lett. B 463 (1999) 317; hep-ph/9903352].

Molecular size effect on the site-specific fragmentation of N and O K shell excited CH3OCOCN and CH3OCOCH2CN molecules

The effect of molecular size on the site-specific fragmentation was investigated by exciting the N and O K shells of methyl cyanoformate (CH3OCOCN) and methyl cyanoacetate (CH3OCOCH2CN). The fragmentation patterns were essentially identical in the N and O core excitations of CH3OCOCN. Site and state dependent fragmentation was clearly observed in the core hole CH3OCOCH2CN: The fragment ions mainly observed at the O K edge excitation were CH3+, HCO+, CH2CN+, CH3OCO+, and COCH2CN+. At the N(1s) excitation the large CH3OCO+ and COCH2CN+ fragments were extremely depressed, and alternatively the small CH3+, HCO+, and CH2CN+ ions were dominant. Especially at the π*CN←N(1s) resonance transition the N+ and CO+ fragment ions were exclusively produced with yields of 40 and 15%, respectively. The total photoabsorption cross sections were measured in the N and O K regions and the observed peaks were tentatively assigned. Reflectron type time-of-flight mass spectrometry was employed to attain the mass separation M/ΔM≥100 at M/e≈100 for the core excited molecules ejecting fragments with considerable kinetic energies.

Time-dependent quantum mechanical study of photodissociation of molecular oxygen in the Schumann–Runge continuum

We present a time-dependent quantum mechanical investigation of predissociation of molecular oxygen following absorption in the Schumann–Runge continuum. We calculate the total and partial photoabsorption cross sections in a wave packet formalism in which the predissociation of the B 3Σu− molecular state is attributed to spin-orbit interactions with the Πu1, a 3Πu, Πu5, and 2 3Σu+ repulsive states. We calculate the branching ratio, describing the relative rates of production of excited (1D) and ground-state (3P) oxygen atoms, and find close agreement with a recent measurement.

Helicity dependence of γp→nπ+ results from the GDH experiment at mamiresults from the GDH experiment at mami

The helicity dependence of the total photoabsorption cross section and partial channels for polarized protons have been measured for incident photon energies from 120 to 800 MeV. We used the large acceptance detector DAPHNE with additional forward components in conjunction with a newly developed frozen spin target and the tagged polarized photon beam facility of the MAMI accelerator in Mainz (Glasgow-Mainz tagger). First preliminary results for the polarized total cross section of the γp→nπ+ channel and its contribution to the GDH integral and the forward spin polarizability are compared with partial wave analysis of DHT and SAID.

A new frozen-spin target for 4π particle detection

Abstract A new frozen-spin target has been developed, that allows the detection of emitted particles in an angular acceptance of almost 4π in the laboratory frame. The central part of this new target represents a 3He/4He dilution refrigerator that is installed horizontally along the beam axis. The refrigerator includes an internal superconducting holding coil to maintain the nucleon polarization in the frozen-spin mode longitudinally to the beam. The design of the dilution refrigerator and the use of an internal holding coil enabled for the first time the measurement of a spin-dependent total cross section in combination with a polarized solid state target. This new frozen-spin target was used successfully to measure the helicity asymmetry of the total photoabsorption cross-section at the Mainz accelerator facility MAMI. This experiment has been performed in order to verify for the first time the GDH sum rule.

Low χbj DIS, QCD and generalized vector dominance

We give a brief overview on the present status of Generalized Vector Dominance as applied to vector-meson production and the total photoabsorption cross section in the region of small χ bj . We comment on how GVD originates from QCD notions such as color transparency.

Proton polarization shifts in electronic and muonic hydrogen

The contribution of virtual excitations to the energy levels of electronic and muonic hydrogen is investigated combining a model-independent approach for the main part with quark model predictions for the remaining corrections. Precise values for the polarization shifts are obtained in the long-wavelength dipole approximation by numerically integrating over measured total photoabsorption cross sections. These unretarded results are considerably reduced by including retardation effects in an approximate way since the average momentum transfer (together with the mean excitation energy) turns out to be larger than usually assumed. Transverse and seagull contributions are estimated in a simple harmonic oscillator quark model and found to be non-negligible. Possible uncertainties and improvements of the final results are discussed.

Electromagnetic Few-Body Response Functions with the Lorentz IntegralTransform Method

The inclusive reactions \(\) and \(\) are studied with the Lorentz integral transform method. The method allows the inclusion of the full final-state interaction without explicit knowledge of the continuum states. Features of the inversion procedure are discussed in this work. The technique for the calculation of the matrix elements of the Hamiltonian is described. The total photoabsorption cross section of 3H is calculated with a realistic super-soft-core NN force. In the threshold region the cross section is rather similar to that obtained with central forces only, while in the peak the realistic NN interaction leads to more strength. For the longitudinal (e, e′) response we extend our calculation with a semirealistic force to \(\) MeV/c. Rather good agreement with experimental data is found. The accuracy of the quasielastic response approximation is discussed.

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 $\Delta$ production and provides a constraint on the $\pi N \Delta$ form factor. The production of leading pions in proton-proton collisions at ISR puts additional constraints on the $\pi NN$ 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 $\rho,a_2$ 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.

Determination of the forward Compton scattering amplitudes for C and Pb

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 Δ region. Difference between carbon and lead is also evident. The forward Compton scattering cross-sections have been calculated and compared with the available data. The Weise sum rule is discussed together with the predictions of a recent theoretical model.

Inner-shell photoionization of sodium: Experiment and theory

Recent measurements of the photoionization of Na by ejection of a $3s,$ $2p,$ or $2s$ electron give branching ratios, \ensuremath{\beta} parameters, and absolute partial cross sections for a number of excitation processes, allowing one to perform the full partition of the total photoabsorption cross section into its components. In parallel, recent developments of the $R$-matrix code, together with extensive configuration and close-coupling expansions, make it possible to calculate the relevant quantities over the continuum energy range (40--140 eV) as well as in the resonant energy region corresponding to the excitation of a $2p$ or $2s$ inner-shell electron onto empty optical orbitals. A critical comparison is made of our experimental results with previous and present theoretical calculations. Valuable information is obtained for the ionization energies and relative cross sections. The main limitation in testing the validity of the various theoretical approximations is the lack of accuracy of the experimental photoabsorption cross section measured only to about 25%.

Study of nucleon spin-dependent properties in the resonance region

In this paper, the spin-dependent structure functions of nucleon g1, and photoabsorption cross sections σ1/2, σ3/2 and σT in the resonance region are estimated based on the constituent quark model and the properties of the five phenomenological Breit-Wigner resonances P33(1232), S11(1535), D13(1520), P11(1440), and F15(1680). Our results are compared to the recent E143 data of the polarized structure functions g1(W2, Q2) at points Q2=0.5 GeV2 and Q2=1.2 GeV2 and the data of the total inclusive photoabsorption cross sections.

Breit-Wigner resonances and nucleon properties in the resonance region

In this paper, constituent quark model and the phenomenological Breit-Wigner resonances P 33 (1232), S 11 (1535), D 13 (1520), P 11 (1440), and F 15 (1680) are utilized to calculate the nucleon spin structure function g 1 ( x , Q 2 ) in the resonance region at points Q 2 =0.5 GeV 2 and Q 2 =1.2 GeV 2 . Moreover, the total real photoabsorption cross sections are also estimated. The results are compared with recent experimental data.

Calculation of total photoabsorption cross sections of Ar, Kr, N 2 and CO

We present a new inversion procedure for the dispersion integral equation relating the total photoabsorption cross section to the frequency-dependent (dynamic) polarizability of an atom or molecule. Together with a reliable method to calculate the latter, this opens a way to the calculation of photoabsorption cross sections for complex molecules. We assess the reliability of the new inversion method by inverting the response functions for Ar, Kr, N 2 and CO as obtained by the time-dependent second-order Møller-Plesset method and comparing with other theoretical and experimental results.

Photoabsorption spectrum of ozone in the K-edge region

The K-shell (1s) photoabsorption spectrum of ozone has been studied for the first time by using synchrotron radiation as a light source. The total photoabsorption cross section exhibits two distinct peaks at 529.1 and 535.4 eV attributed to the π ∗ ← O(1 s) transitions of the terminal and the central oxygen atoms in ozone, respectively. The σ ∗ ← O(1 s) transition bands were also observed.

One-electron versus multielectron effects in the near-threshold C 1s photoionization of acetylene

The C 1s partial photoionization cross section and asymmetry parameter β of the C2H2 molecule have been determined with high-energy resolution between threshold and 360 eV. The C 1s shake-up satellite spectrum is richly structured; the cross section of several satellites increases strongly near threshold, which indicates that there is a conjugate contribution to their intensity. It is shown that the large enhancement in the total photoabsorption cross section between 305 and 320 eV, previously attributed to a σ* shape resonance, is largely due to this photon energy dependence of the shake-up transitions.

Total Photoabsorption Cross Section and Excitation Function in the Quasi-Deuteron Energy Region

In photofission a lot of studies have been made to analyze the energy of the nuclei to understand their inner structure. The relevance of the excitation function has been pointed out. The Hamiltonian of interaction for the photofission was deduced and connected to the excitation function.