Forward Cross Section Research Articles

Correlation effects in 11B (π −, π +) 11Li reactions

We have studied the effect of the spatial correlation between the valence neutrons in 11Li on the double-charge exchange reaction 11B(π −, π +) 11Li. The spatial correlation enhances the forward angle cross section by a factor of 2–3 compared to the prediction of an independent particle description.

The forward, backward and 90° deuteron photodisintegration between 7 and 19 MeV photon energy

The 0° and 180°, as well as the 90° differential cross sections for the 2H(γ, p)n reaction have been measured at lab photon energies between 7 and 19 MeV. Special attention was paid to the accuracy, in particular by measuring the forward Compton electron yield. For the extreme angles, the statistical error on our results amounts to 4–5% (0°) and 7–8% (180°), respectively, while the systematic uncertainty is at most 3%. The data confirm the existence of a minimum in the forward cross section and indicate beyond any doubt that the fore/aft ratio is larger than unity. Comparison shows reasonable agreement with the results from recent “conventional” theoretical approaches, including meson exchange and relativistic corrections, although the description of the c-coefficient, appearing in the Partovi expansion of the differential cross section, remains unsatisfactory. A possible source for this discrepancy could be the inadequate description of the El and/or the E2 transition operator.

Effective interactions for nucleon-nucleus scattering above 300 MeV.

A nonrelativistic nucleon-nucleon (NN) coupled channels isobar model is used to calculate nuclear medium corrections to the first-order Watson optical potential for nucleon-nucleus scattering. The NN interaction includes \ensuremath{\pi}, 2\ensuremath{\pi}, \ensuremath{\rho}, \ensuremath{\eta}, and \ensuremath{\omega} exchange for the pure nucleonic sector, one-pion exchange for nucleon-isobar coupling, and phenomenological short-range interactions for both the diagonal and coupling terms. Good fits to NN elastic phase shifts from 0--1 GeV have been obtained with this model. The specific nuclear medium effects studied here include Pauli blocking of intermediate nucleon scattering states and off-shell binding energy shifts. On-momentum-shell matrix elements of the density dependent, effective NN t matrix are used to calculate the local proton-nucleus optical potential assuming the local density approximation. The model is applied to proton elastic scattering from $^{16}\mathrm{O}$, $^{40}\mathrm{Ca}$, $^{48}\mathrm{Ca}$, and $^{208}\mathrm{Pb}$ at 320, 400, 500, 650, and 800 MeV and the predictions are compared with available data. In general, the medium modifications significantly affect the nonrelativistic predictions, even at 800 MeV, and bring about a substantial, overall improvement in the description of differential cross section and spin observable data. The improvements occur for each case and at all angles studied, but the agreement is most successful for forward angle differential cross sections and for spin observable data from 1 to 3 ${\mathrm{fm}}^{\mathrm{\ensuremath{-}}1}$. Deficiencies remain in the density dependent, nonrelativistic predictions for the forward angle spin observables where relativistic models are quite successful. The relation of this work to other nonrelativistic models which include nuclear medium corrections, off-shell effects, and full-folding contributions is also discussed.

Coupled channels analysis of 12C

Existing data for the ${0}_{2}^{+}$ state in $^{12}\mathrm{C}$ at 50 and 67.5 MeV are compared with coupled channels calculations. Calculations are done with a realistic potential which includes the Lorentz-Lorenz-Ericson-Ericson p-wave correction and terms second order in the nuclear density. These are compared to results obtained using a phenomenological Kisslinger potential to investigate the sensitivity to the details of the \ensuremath{\pi}A interaction. The suppression of the forward angle cross sections by the Lorentz-Lorenz-Ericson-Ericson effect is insufficient to bring the direct transition into agreement at both energies. The interference between the one- and two-step excitation mechanisms is required to reproduce the forward angle data. The dependence of the cross section on the choice of monopole transition density is examined.

Nuclear-target effects in J/ ψ production in p̄ and π − interactions

The cross sections for J/ ψ hadroproduction in Be, Cu, and W targets are presented as functions of Feynman-x (x F) and transverse momentum ( pT). A suppression of the forward cross section per nucleon in heavy targets is observed; the suppression increases with increasing x F and decreases with increasing pT. The data are compared with other J/ ψ hadroproduction data and data from the Drell-Yan production of muon pairs, J/ ψ photoproduction and open charm hadroproduction.

Absorptive effects in K+ Lambda photoproduction on nucleons and nuclei.

Incorporating final-state correlation effects, we have reinvestigated the ${\ensuremath{\gamma}}^{1}\mathrm{H}\ensuremath{\rightarrow}{K}^{+}\ensuremath{\Lambda}$ elementary process. Our model not only resolves the persisting trouble of the small $\mathrm{KN}\ensuremath{\Lambda}$ coupling constant, but also yields agreement with cross-section data at higher energies. Using our amplitudes, we calculate angular distributions for the reaction $^{16}\mathrm{O}(\ensuremath{\gamma}, {K}^{+})_{\ensuremath{\Lambda}}^{16}\mathrm{N}$. While the forward cross section increases with increasing energy, the total cross section is almost constant from ${E}_{\ensuremath{\gamma}}=1.2$ through 2 GeV, suggesting that rather low photon energies hold promise for exploration of hypernuclear excitations.

Eikonal model description of forward energy distribution in oxygen-induced reactions at 60 A GeV and 200 A GeV

The forward energy flow cross section is described in terms of the projectile-nucleon abrasion probability which is calculated in the impact parameter representation within the framework of the Glauber multiple scattering formalism. Realistic nuclear densities and the nucleon-nucleon inelastic cross section have been used in the calculation. The recent WA80 data are in reasonable agreement with the calculation. A simple relationship has been obtained between the abrased nucleons from the target and projectile and the zero degree energy.

Contribution of the second kind of meson exchange current to pion-nucleus double-charge-exchange reactions 18O( pi +, pi -) 18Ne(g.s) and 14C( pi +, pi -)14Og.s.

Based on the Lagrangian of Olsson and Turner, contributions of the second kind of meson exchange current to pion-nucleus double-charge-exchange reactions $^{18}\mathrm{O}$(${\mathrm{\ensuremath{\pi}}}^{+}$,${\mathrm{\ensuremath{\pi}}}^{\mathrm{\ensuremath{-}}}$${)}^{18}$${\mathrm{N}}_{\mathit{e}(\mathrm{g}.\mathrm{s}))}$ and $^{14}\mathrm{C}$(${\mathrm{\ensuremath{\pi}}}^{+}$,${\mathrm{\ensuremath{\pi}}}^{\mathrm{\ensuremath{-}}}$${)}^{14}$${\mathrm{O}}_{\mathrm{g}.\mathrm{s}.}$ have been evaluated. The calculated results show that the second kind of meson exchange current makes the forward theoretical cross section closer to the experimental data but the long-standing puzzling problem of the location of the minimum of the angular distribution at the energy of 164 MeV has not been solved.

E+e-pairs from pi-p interactions at 0.3 GeV/c

The production of wide-angle e+e- pairs by pi -p at 0.3 GeV/c has been studied using a magnetic spectrometer. The forward cross section obtained for pi -p to ne+e- agrees with theoretical calculations. The rare decay pi 0 to e+e- is shown to have a branching ratio B<5.3*10-7=11 times the unitarity lower bound and it is argued that there is as yet no convincing evidence for an anomalously large branching ratio.

Measurement with a free neutron beam of absolute neutron-proton forward elastic-scattering differential cross section at intermediate energies.

The differential cross section in free n-p forward elastic scattering has been measured for incident neutron energies of 378, 481, 582, 683, 784, 884, and 1085 MeV and for momentum transfer 0.01l\ensuremath{\Vert}t\ensuremath{\Vert}l0.08 (GeV/c${)}^{2}$. The experiment used a recoil-detector ionization chamber which served at the same time as a gas target. Special care has been taken to obtain a precise absolute normalization.

Alpha-transfer study of 23Na

Abstract The reaction 19 F( 6 Li, d) 23 Na has been studied at a beam energy of 34 MeV using a high resolution magnetic spectrometer. A total of 36 states in 23 Na have been observed in the range E x = 4–13MeV and deuteron angular distributions obtained for 26 states. Compound processes accounted for less than 10% of the forward angle cross section in most instances. Finite-range distorted-wave Born approximation calculations have been performed and compared with the data. The calculations reproduced the observed angular distributions well, allowing relative α-particle spectroscopic factors to be extracted and several new or improved spin assignments to be made. The 6 Li- 19 F optical potential was derived from elastic scattering data obtained in the present work. The results are discussed with reference to the rotational structure of 23 Na.

Low-frequency reflection and scattering by ellipsoidally embossed surfaces

Earlier low-frequency results for scattering and reflection by random distributions of relatively arbitrary bosses on rigid or free base planes are applied numerically to completely rigid or free ellipsoidally embossed isotropic and anisotropic surfaces. Illustrative curves exhibit the effects of boss shape and orientation, and of their statistical distribution (pair correlated as well as uncorrelated) on the coherent power reflection coefficient and the reflected phase, and on the incoherent differential scattering cross section per unit area versus angles. In addition, for rigid surfaces, simple approximations are included for the location and magnitude of the pseudo-Brewster minimum in the reflected power and for the extrema of the forward (specular) and backscattered cross sections, as well as for the location of the zero of the phase change on reflection. We emphasize anisotropic forward and backscattered differential cross sections near grazing for applications to sea–air, sea–bottom, and sea–ice programs.

Charge-exchange processes of hydrogen ions with Hg atoms at keV energies.

A search for the production of the 2${p}^{2}$ ${\mathrm{}}^{3}$${P}^{e}$ state of ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ has been made by crossing a ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$(1${\mathrm{s}}^{2}$) ion beam of energies E&lt;140 keV with a thermal Hg beam inside a static electric field. The formation of ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$(2${\mathrm{p}}^{2}$) ions and their subsequent decay to neutral H(1s) results in a feature on the angular distribution of the neutral particles produced in electron loss of the ground-state ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$(1${\mathrm{s}}^{2}$) ions. The absence of such a feature sets an upper limit of ${10}^{\mathrm{\ensuremath{-}}20}$ ${\mathrm{cm}}^{2}$ to the cross section for formation of the 2${p}^{2}$ ${\mathrm{}}^{3}$${P}^{e}$ state. Differential (with respect to angle) charge-transfer cross sections for scattering in the angular range 3\ifmmode^\circ\else\textdegree\fi{}&lt;\ensuremath{\theta}&lt;15\ifmmode^\circ\else\textdegree\fi{} as well as the small-angle forward cross sections ${\ensuremath{\sigma}}_{10}$ and ${\ensuremath{\sigma}}_{\mathrm{\ensuremath{-}}10}$ for hydrogen ions passing through Hg vapor are presented.

Searching for Higgs Bosons in Weinberg-Salam Model by Longitudinal Polarized Reactions

Using the Primakoff effect of longitudinal polarized reactions which would have extremely sharp peak both in the forward and backward cross sections, one could hopefully observe Higgs bosons in W–S model and determine its mass experimentally.

Small-angle p̄p elastic scattering and the real-to-imaginary ratio of the forward amplitude between 413 715

Abstract We have measured the p p forward elastic differential cross section between 413 and 715 MeV c . The real-to-imaginary ratio of the p p forward-scattering amplitude was determined, and was analyzed in terms of a forward dispersion relation. The data suggests that there should be significant contributions to the dispersion integral from the high-mass region of the unphysical cut. The data are also compared with the predictions of N N potential models.

Theoretical calculation of the forward cross section for deuteron photodisintegration up to 120 MeV

The forward differential cross section for γd → pn in the c.m. frame is calculated for laboratory photon energies up to 120 MeV, using the Paris potential and including relativistic corrections to the impulse approximation and the effect of one-pion exchange, with and without Δ(1231) intermediate states. We give a full treatment of one-pion exchange effects and show that the correction due to them is small (except at energies below 20 MeV) and that it can be accurately calculated. We give a consistent relativistic treatment of the two-nucleon system and correct errors in the well-known work of Partovi which arise from his nonrelativistic treatment. It is shown that the calculated cross section is very sensitive to charge-dependent effects in the one-pion exchange part of the nucleon-nucleon potential and that it is possible to obtain reasonable agreement with the present experimental data when the potential used to calculate the T = 1 np wave functions has a one-pion exchange part which takes account of the masses and coupling constants of both neutral and charged pions.

State selected charge transfer reactions in the BC++Ar⇄BC+Ar+ systems: Comparison among BC=H2, O2, and NO

The relative state selected cross sections for the low-energy charge transfer (CT) reactions in the systems H+2(X2 Σ+g,v)+Ar, Ar+(2PJ)+H2, O+2(a 4Πu,v) +Ar, Ar+(2PJ)+O2, NO+(a 3Σ+,v)+Ar, and Ar+(2PJ)+NO have been determined using the threshold electron-secondary ion coincidence (TESICO) technique. In the (H2+Ar)+ system, the cross sections for both the forward and backward reactions were found to show a characteristic internal-state dependence which can be consistently interpreted in terms of a single model based on energy defects (ΔE) and Franck–Condon (FC) factors. In contrast, in the (O2+Ar)+ and (NO+Ar)+ systems, the strong dependence of the cross sections on the selected internal states were observed only for the forward (starting from the diatomic ions) reaction, and not for the backward (starting from the Ar+ ion) reaction. The results for the forward reactions were again interpreted, at least partially, by the energy defects and FC factors between the reactant and product states. These features of the internal-state dependence of cross sections have been discussed in conjunction with the characteristics of the relevant potential energy surfaces of each system. The discrepancy between the (H2+Ar)+ system and the (O2+Ar)+ and (NO+Ar)+ systems in the behavior of the forward and backward cross sections was ascribed to the difference in the number of potential surfaces involved; in the former system only two surfaces are involved in both forward and backward reactions allowing exactly the same mechanism for both reactions, whereas in the latter two systems, the occurrence of more than two surfaces causes different mechanisms for the forward and backward reactions. In the (O2+Ar)+ system, the doublet and quartet surfaces participate in the backward reaction, leading to two different states (X 2Πg and a 4Πu) of the O+2 product ion, while only quartet surfaces are involved in the forward reaction. In the (NO+Ar)+ system, different reaction paths arise between the [NO+(a 3Σ+)+Ar] and (Ar++NO) states due to the anisotropy in these interactions. This allows the different behavior of the forward and backward reactions, in spite of the fact that the product state of the backward reaction is predominantly NO+(a 3Σ+)+Ar. An ab initio calculation of partial potential energy surfaces for the triplet states of the (NO+Ar)+ system supported this view. From these considerations, low-energy charge transfer reactions in the (BC+Ar)+ systems have been classified into two groups according to the possible types of nonadiabatic transitions. These groups are considered to correspond respectively to the phenomenological ‘‘direct’’ and ‘‘intimate’’ reaction mechanisms.

Spin-flip versus non-spin-flip interaction in forward (p, n) reactions

The ratio of the probabilities for triggering Gamow-Teller or Fermi transitions in forward (p, n) reactions is evaluated, using total and forward nucleon-nucleon cross sections. Our results indicate that the ratio extracted from the (p, n) reaction is not consistent with the free nucleon-nucleon data.

Spin effects in low energy proton-antiproton forward elastic scattering

Predictions of the p̄p forward elastic cross section and of the real to imaginary ratio ϱ given by the NN̄ interaction we proposed recently are reported. Our cross sections compare very well with recent measurements at 400 < p L < 730 MeV/ c and 700 MeV/ c. It is also shown that spin effects cannot be neglected in the determination of ϱ as usually assumed.

On the formation and spectroscopy of ΛΛ hypernuclei

We use the distorted wave Born approximation (DWBA) to estimate formation cross sections in the (K -, K +) reaction for discrete states of doubly strange ΛΛ hypernuclei. Forward (K -,K +) cross sections of a few nanobarn/sr are predicted for the high spin states of ΛΛ 16C and ΛΛ 40Ar.