Diffractive Contribution Research Articles

Diffraction, reflection, and interference during near-grazing and near-normal ocean surface backscattering

Theory for the impulse response of a spherical wave backscattered from a plane strip has been used with a computer model of a randomly rough long-crested ocean surface to reveal the relative contributions of diffraction, facet reflection, and interference at near-grazing and near-normal incidence and to incorporate acoustical shadowing in a natural way. The major findings for near-grazing incidence are that backscatter is due to diffraction at troughs and crests; the backscattering strength is −54 dB at 5 m/s and only 2 dB more at 20 m/s, both flat for grazing angles 1° to 10°; and it decreases about 3 dB for an octave increase in frequency, independent of wind speed. At normal incidence the decreasing value of the backscattered coherent intensity with increasing roughness is due to interference. It is related to the transform of the probability density function of the surface height until the roughness parameter g (=4k2σ2) reaches approximately 5, where there is a deep minimum due primarily to interference of the highly coherent diffracted contributions. At normal incidence the backscatter is dominated either by diffraction or reflection, depending on the sphericity of the wave front.

Real and imaginary parts of hadronic amplitudes, diffractive contribution and the Chew-Rosenzweig pomeron

We show that when the f′ and the diffractive elastic (and quasi-elastic) contributions are included within the Chew-Rosenzweig type solution for the pomeron, one can explain reasonably well the π and 2 K − π total cross sections as well as the ratios of the real over imaginary parts of the forward π and K amplitudes.

Relative contributions of diffraction, facet reflections, and interference to the field backscattered by a randomly rough surface

In a previous communication [J. C. Novarini and H. Medwin, J. Acoust. Soc. Am. 61, S37–S38(A) (1977)] near-grazing acoustical shadowing was discussed in terms of the results of a computer model study of boundary-diffracted waves. This work has been extended to clarify the contributions due to diffraction as well as facet reflections from long-crested surfaces having a Newmann-Pierson spectrum. The technique shows the importance of wave height and facet reflection with interference near normal incidence contrasted with wave slope and diffraction away from normal. Results are shown for backscattering strength as a function of frequency and angle of incidence for wind speeds of 5, 10, and 20 m/sec. [Work supported by ONR and the Research Council, Naval Postgraduate School.]

SU(3) breaking in a diffractive contribution to inelastic processes

The diffractive contribution to the production of various helicity states in an inelastic interaction is calculated. The formalism is applied to the production of $N(1688)$, $\ensuremath{\Lambda}(1815)$, and $\ensuremath{\Sigma}(1915)$ in nucleon-nucleon scattering, and an estimate is made of the diffractive portion of the helicity-flip amplitudes for these resonances. A nonsinglet Pomeron leads to finite asymptotic differences in the cross sections.

The Double Diffraction Model for LEED-Intensity Spectra of the Clean Pt(100) Surface

Abstract The intensity spectra of the superstructure diffraction spots of the clean platinum (100) surface are presented for normal or nearly normal incidence. Their gross features are interpreted in terms of multiple diffraction contributions. It is shown that most of the maxima in the spectra can be explained by double diffraction processes from an idealized pure hexagonal surface layer and quadratic bulk layers. Alternative models in which the surface layer exchanges already all occuring reciprocal lattice vectors would predict additional peaks which, however, do not arise. Therefore superstructure features of the surface layer are assumed to be of second order. Thus double dif­fraction involving only idealized hexagonal and quadratic structures seems to dominate the diffraction behaviour of clean Pt(100).

Quark description of currents and diffractive contributions to leptoproductions

Describing the weak and electromagnetic currents by their constituent quarks, we study the lepto-proton reactions in the small x region. Charm productions and their relation to the dimuon events observed in neutrino reactions are discussed.

Strange particle production in neutron-proton interactions at 10–24 GeV/ c

We present results of an experiment on strange particle production in np interactions below 24 GeV/ c. We derive cross sections in 25 exclusive channels, and compare them with lower energy data. We see evidence for a significant diffractive contributions to the lower multiplicity processes. We also obtain information about the inclusive production of Λ 0, Ξ −, Σ ±(1385) and K ±(890).

Structure of the reggeon-reggeon cut

The reggeon-reggeon cut amplitude is studied in a model which differs from the conventional Mandelstam graph model in that the component reggeon-particle scattering amplitude is given by the dual model. As DeGrand and DeTar have recently emphasized, it is not at all obvious that in such a model the relative proportions of the diffractive, absorbed multiperipheral, and polyperipheral contributions to the cross section should be the same as given by Abramovskii, Gribov and Kancheli for the conventional model. However, we find that while the singularity structure of the dual reggeon-particle amplitude is quite different from that of the Feynman amplitude, the asymptotic behavior is also quite different with the result that these proportions are the same. This unexpected result suggests that these proportions are very model-independent and are a general consequence of the structure of reggeon amplitudes. Our work also leads to some additional insight into how to define reggeon cut amplitudes in a way independent of the Feynman graph model.

A diffractive contribution to the single-particle inclusive neutron spectrum in pp collisions through low-mass resonance decay

The contribution of the two-step reaction, consisting in diffractive production of the N ∗(1470) resonance and its decay into a neutron, to the inclusive neutron spectrum in pp collisions, is studied in detail. It is found that, at low p⊥ and near x = 1, the effect gives rise to a peak which, in principle, can be detected in experiments with a good momentum resolution.

A new X-ray diffraction-fluorescence method for the study of films: preliminary results for CuxS on CdS

The structure of an ion-exchanged layer of CuxS on CdS has been studied by an X-ray method which yields diffraction and fluorescence results simultaneously. The method utilized a solid-state Ge crystal detector and a low incidence angle for the X-ray beam. From the fluorescence yield one can estimate the expected diffraction contribution of the surface film. The diffraction results indicated the presence of both chalcocite (Cu2S) and djurleite (Cu1.96S). A reversible phase transition near 100°C was observed.

Does the Pomeron have vacuum quantum numbers?

From general arguments, the imaginary part of every elastic scattering amplitude at sufficiently large impact parameter is governed by the two-pion exchange cut in the $t$ channel. Consequently, the large-impact-parameter tail of the Pomeron is not an SU(3) singlet. We calculate the contribution of the tail (typically about ${\ensuremath{\sigma}}_{\mathrm{tot}}$) approximately, and thereby predict observable differences between high-energy total cross sections for $\ensuremath{\pi}p$ and $Kp$, and between $pp$, $\ensuremath{\Lambda}p$, $\ensuremath{\Sigma}p$, and $\ensuremath{\Xi}p$. We estimate the asymptotic difference between $\ensuremath{\pi}p$ and $pp$ cross sections. We also obtain nonzero estimates for the diffractive helicity-flip component of elastic scattering and for diffractive contributions to processes such as $\ensuremath{\pi}p\ensuremath{\rightarrow}{A}_{2}p$ and $Kp\ensuremath{\rightarrow}{K}^{*}(890)p$. We predict cross sections for ${A}_{2}$ and ${K}^{*}$ at Fermilab, and elastic polarizations, e.g., in $pp\ensuremath{\rightarrow}pp$ and $\ensuremath{\Lambda}p\ensuremath{\rightarrow}\ensuremath{\Lambda}p$, at asymptotic energies.

Diffracted field computations by a series expansion

The possibility of a series expansion for the diffracted fields by a dielectric sphere or circular cylinder is considered by first expanding the Mie scattering coefficients into a geometric series (Debye expansion), and then applying a modified Watson transformation to each term of the series. The diffracted fields can then be obtained by finding poles of the Debye expansion, and evaluating the residues. The locations of the poles due to the Debye expansion as well as the Mie scattering coefficients themselves are discussed. Numerical values of the poles are tabulated. The diffracted field contributions associated with the first five terms of the Debye expansion are presented in the form of a radar scattering cross section. The geometrical optics contribution arising from each term of the Debye expansion is asymptotically evaluated. To compare the magnitude of a sum of the geometrical optics and diffracted field contributions, the radar scattering cross section of the total backscattered field, obtained from the exact Mie theory, is also plotted.

Multiperipheral operator factorization and the bootstrap idea

We discuss multiparticle production models including important direct channel unitarity effects in the light of the general multiperipheral dynamics proposed by Bassetto, Sertorio and Toller. We first solve the Finkelstein-Zachariasen model by means of the explicit t-channel operator factorization of the exclusive production amplitudes. We show that the absorption, which is introduced in the model according to the Gottfried-Jackson recipe, can be decomposed locally into factors along the multiperipheral chain. The model however does not treat diffraction in a self-consistent way as partial production cross sections and elastic cross sections have different asymptotic behaviour. We then propose a simple model which possesses diffractive contributions in the inelastic channels still exhibiting total cross section and multiplicity fluctuations with the same asymptotic behaviour as the ones of the Finkelstein-Zachariasen model. We solve it again by means of t-channel factorization. Finally we comment upon the relation between exclusive and inclusive factorization in our scheme and argue that it could provide room for a wider realization of the bootstrap idea.

Updated charged-particle multiplicity distribution from205−GeVcproton-proton interactions

The charged-particle multiplicity distribution in $205\ensuremath{-}\frac{\mathrm{GeV}}{c}$ proton-proton interactions is presented. In addition, the total diffractive contributions to each charged multiplicity are estimated assuming a factorizable Pomeron.

Multiperipheral and diffractive components of the total cross section in Gribov's Reggeon calculus

The usual classification of multiparticle processes into those with no large rapidity gaps (multiperipheral) and those with at least one large gap (diffractive) is made in the framework of Gribov's Reggeon calculus. (A rapidity gap is defined to be large if it exceeds a fixed, critical gap size ${\ensuremath{\xi}}_{c}$.) General formulas for the multiperipheral and diffractive contributions to the total cross section are obtained by cutting the leading Reggeon diagrams in the elastic amplitude and analyzing the intermediate states. The cutting procedure is always applied to physical Pomeranchukons. In obtaining our results, we assume that when a physical Pomeranchukon with rapidity span $\ensuremath{\xi}$ is cut, the probability $p(\ensuremath{\xi})$ of having a multiparticle intermediate state with no large rapidity gaps is given by $p(\ensuremath{\xi})={e}^{\ensuremath{-}\ensuremath{\gamma}\ensuremath{\xi}}$, $\ensuremath{\gamma}>0$. This form is shown to be reasonable if long-range correlation effects are not important. Our results depend on the new parameter $\ensuremath{\gamma}$, which is of course a function of the critical gap size ${\ensuremath{\xi}}_{c}$. At CERN ISR energies, the Pomeranchukon-cut contributions are found to be very important, and must be included along with the pole contribution.

Improved mode properties of unstable resonators with tapered reflectivity mirrors and shaped apertures

The use of tapered reflectivity mirrors and shaped apertures in unstable resonators is shown to be an effective method for improving the mode properties. Mode intensity and phase profiles are smoothed and the mode discrimination ratio is increased. Results are presented illustrating the importance of diffracted waves from sharp mirror edges in determining these mode properties. A simple expression for estimating the diffractive contribution is given and is used to determine optimum mirror designs. Shaped mirrors and mirrors having amplitude and phase reflectivity tapers are studied. The concept of equivalent Fresnel zones is used to gain physical insight into the mode properties.

High-mass diffraction and Regge behavior of total cross sections

We consider a modified two-component model of particle production in which diffractive excitation into high mass states gives a contribution to K ±p, π ±p and pp total cross sections of sufficient magnitude and with the proper associated threshold to be a possible explanation of deviations of σ tot from Regge pole model fits at Serpukhov energies. We present tests of the model. for K +p scattering by extrapolating σ tot to National Accelerator Laboratory energies and by calculating the high mass diffraction contribution to the forward peak in inclusive reactions. The consistency of this scheme is discussed in relation to the pomeron intercept and the form of the triple-pomeron coupling.

Test of the SRO hypothesis for two charged particle correlations and relative size of the diffractive and non-diffractive components

It is shown that the SRO hypothesis is well satisfied by the recent ISR data on two charged particle correlations at different energies. Moreover, within the framework of the two component model, we determine from the same data the relative amount of diffractive and non-diffractive contributions as a function of the cosmic ray variable η. This leads to a consistent description of the long range correlation.

Inclusive two-particle rapidity correlations at ISR energies and fireball production

ISR data on two-particle rapidity correlations in the region y 1 ≈ y 2 can be understood in terms of the multiperipheral fireball model and indicate an energy independent upper limit of the average fireball mass. The diffractive contribution to the fireball production leads to characteristic shapes of long range correlations. Data on long range rapidity correlations may be used to determine the fractions of non-diffractive and diffractive one- and two-fireball production.

The amount of diffraction dissociation in proton-proton scattering at 24 GeV/c

We estimate the amount of diffractive single and double dissociation in proton-proton scattering at 24 GeV/c in the context of the factorizable diffraction excitation model. We find that a realistic estimate for the maximal diffractive contribution is 1/3 of the total inelastic cross-section. The study of inclusive pion spectra shows that the possible early scaling cannot be attributed to production through diffractive excitations in any region of the phase space.