Unnatural Parity Exchange Research Articles

The production of Regge recurrences

For resonance states lying on a given Regge trajectory, the two-body production mechanism as a function of the excitation of the recurrence state is discussed. A dual resonance model suggests general features for the Regge-Regge-particle coupling involved in such production. An application is made to the high energy production in πN→ ππN of ϱ, f 0 and g mesons with emphasis on the relative production cross sections, the relative t-dependences, the ratio of natural to unnatural parity exchange and the helicity dependence.

How to analyse vector-meson production in inelastic lepton scattering

The kinematics of the process ℓN → ℓNV, is studied in the one-photon approximation for unpolarized as well as polarized leptons ℓ. The vector-meson spin density matrix is expressed in terms of the s-channel helicity amplitudes in the hadron c.m.s. and the vector-meson decay angular distribution is discussed. The use of longitudinally polarized lepton beams is found to increase considerably the amount of information that can be deduced from the decay distribution. With longitudinal beam polarization it is possible to separate all 26 observable independent density matrix elements into contributions from natural and unnatural parity exchange in the t-channel, respectively.

Polarization Effects and Analysis in Meson + Nucleon → Meson + Meson + Nucleon

A general framework is developed for the description of nucleon polarization effects in reactions of the kind meson (${0}^{\ensuremath{-}}$)+neuclon (${\mathrm{\textonehalf{}}}^{+}$)\ensuremath{\rightarrow} meson (${0}^{\ensuremath{-}}$)+meson (${0}^{\ensuremath{-}}$)+ nucleon (${\mathrm{\textonehalf{}}}^{+}$). The new information which can be obtained from target- (or recoil-) nucleon polarization measurements is delineated and high-energy predictions are given based on pion dominance of the possible unnatural-parity exchange poles. Minimum measurements required for a unique amplitude analysis are determined from a graphical technique, and specific high-energy predictions of a $\ensuremath{\pi}+{A}_{2}$ model are given for the target polarization left-right asymmetry in ${\ensuremath{\pi}}^{\ensuremath{-}}p\ensuremath{\rightarrow}{\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{+}n$. Finally, we also discuss the simplifications that occur for general amplitude analyses if transversity amplitudes are used.

Experimental determination of parity-doublet contributions in baryon-exchange reactions

Very simple relations between differential cross sections and polarizations in backward scattering (baryon exchange) which allow an experimental separation of parity-doublet contributions are pointed out. We show, for instance, that in πN elastic scattering, the cross sections with nucleon polarization parallel or antiparallel to the normal of the scattering plane are, respectively, proportional to unnatural or natural parity exchange. A rapid survey of available experimental data in πN elastic scattering and π −p→K 0 Λ exhibits unexpected features.

The photoproduction processes {fx377-1}, KΛ, KΣ in a dual model with fixed poles, KΛ, KΣ in a dual model with fixed poles

The photoproduction of pseudoscalar mesons is investigated in the framework of a dualB5-model with fixed and Regge poles. A reasonable fit to the available high-energy forward scattering data is presented which is characterized by essentially three parameters. The values of the Regge residues for natural- and unnatural-parity exchanges agree with the values obtained in other photoproduction models. The photoexcitation features of the low-lying baryon resonances derived from our model are in qualitative agreement with experimental results.

Non-diffractive production of meson resonances

We study the peripheral cross sections of resonances that cannot be produced by π-exchange. In particular, we concentrate on the four meson nonets expected as L = 1 quark states (i.e., the J P = 0 + π N(980); J P = 1 + A 1, B; J P = 2 + A 2). We use SU(3), Regge poles, factorization, exchange degeneracy, pole extrapolation, and the vector-meson-photon analogy. We predict the cross sections in both photoproduction and non-diffractive hadronic reactions. In passing, we discuss the large unnatural-parity (B, K−Q B) exchange contributions and even the possibility of studying ππ → πω while avoiding the B production background.

Some remarks on the analysis of the decay density matrix of mixed scalar-vector-tensor boson systems

The asymptotically valid rules which enable one to separate the t-channel components of natural and unnatural parity in quasi-two-body reactions are applied to the case where a mixed scalar-vector-tensor-two-boson system is produced. Some model-independent tests for the absence of unnatural-parity exchange are derived. Further tests are devised which are valid when more specific assumption are made concerning the t-channel exchanges.

The reaction π+p → ωΔ++ (1236) at 11.7 GeV/ c

For the reaction π +p → ωΔ ++ data on the total cross section ( σ = 61 ± 12 μb), differential cross sections, spin density matrix elements and statistical tensor elements are given. We observe natural and unnatural parity exchange contributions to the total cross section. We note that the value of ϱ 00 is not zero and in the helicity frame exhibits a dip at t ≈ −0.25 (GeV/ c) 2. A qualitative theoretical discussion of our results is presented.

Model-independent analysis of density-matrix element measurements in multiparticle reactions

We present a general study of polarization measurement in multiparticle production processes, including a definition of experimentally observable quantities, the splitting into natural- and unnatural parity exchanges and a study of their relative phase, and information concerning the quantum numbers of a packet of particles. When only one single particle is detected in the final state (inclusive reactions for instance) we prove that the usual formalism for two-body reactions is applicable. Very interesting new information on multi-particle production can be reached when it can be asserted that another particle has been emitted above or below a given plane, parallel or perpendicular to the scattering plane.

Spin structure of high-energy multiple scattering

A multichannel K-matrix formalism is used to unitarize the amplitudes for elastic scattering and diffraction dissociation written down in the impact-parameter representation. The inelastic intermediate states in the unitarity relations are effectively represented by a set of quasi-two-particle states. The complications due to nonvanishing spins are considered and the constraints resulting from definite (natural or unnatural) parity in the s- and t-channel are investigated. It is found that the Pomeranchuk contribution can be built up from multiple exchanges of lower lying trajectories. The Pomeranchuk coupling in helicity flip transitions turns out in this model to be logarithmically suppressed with increasing energy compared to the corresponding helicity nonflip couplings of the “vacuum trajectory”. The coupling determining the imaginary parts of the helicity nonflip elastic scattering and diffraction dissociation amplitudes is found to correspond to natural parity exchanged in the t-channel only. Contrary to this the real parts of the amplitudes for helicity nonflip elastic scattering and diffraction dissociation receive in this model at large energies contributions from both natural and unnatural parity exchange in the t-channel. The same is true for the Pomeranchuk contributions to helicity flip elastic scattering and diffraction dissociation processes which are, already in lowest order rescattering, determined by a mixture of natural and unnatural parity exchanged in the t-channel.

Two-Body Strange-Particle Final States inπ−pInteractions at 4.5 and 6 GeV/c

Results on the following ${\ensuremath{\pi}}^{\ensuremath{-}}p$ reactions involving a hyperon are studied at 4.5 and 6.0 GeV/c from a high-statistics bubble-chamber experiment. (1) ${\ensuremath{\pi}}^{\ensuremath{-}}p\ensuremath{\rightarrow}(\ensuremath{\Lambda}, {\ensuremath{\Sigma}}^{0}){K}^{0}$: Differential cross sections and hyperon polarizations are presented. Comparison with the line-reversed reactions $\overline{K}N\ensuremath{\rightarrow}(\ensuremath{\Lambda}, {\ensuremath{\Sigma}}^{0})\ensuremath{\pi}$ indicates the failure of the predictions of ${K}^{*}(890)$ and ${K}^{*}(1420)$ exchange degeneracy. Effective trajectories for these two reactions are compared. Shrinkage is observed in $\overline{K}N\ensuremath{\rightarrow}\ensuremath{\Lambda}\ensuremath{\pi}$ and not in ${\ensuremath{\pi}}^{\ensuremath{-}}p\ensuremath{\rightarrow}\ensuremath{\Lambda}{K}^{0}$. (2) ${\ensuremath{\pi}}^{\ensuremath{-}}p\ensuremath{\rightarrow}(\ensuremath{\Lambda}, {\ensuremath{\Sigma}}^{0}){K}^{*}{(890)}^{0}$: Differential cross sections, hyperon polarizations, and ${K}^{*}{(890)}^{0}$ density-matrix elements are determined. $\ensuremath{\Lambda}{K}^{*}{(890)}^{0}$ decay correlations are found to impose strong constraints on the scattering amplitudes. The data indicate that both natural- and unnatural-parity exchanges contribute large, but opposite, $\ensuremath{\Lambda}$ polarizations. This behavior cannot be explained by a simple exchange model utilizing $K$ and the exchange-degenerate ${K}^{*}(890)$ and ${K}^{*}(1420)$ only. Additional trajectories or absorption effects are required to obtain the observed $\ensuremath{\Lambda}$-polarization effects. Comparison of $\ensuremath{\Lambda}{K}^{*}{(890)}^{0}$ and ${\ensuremath{\Sigma}}^{0}{K}^{*}{(890)}^{0}$ indicates the greater importance of unnatural-parity exchange in the former reaction. We observe no evidence for deviations from isospin predictions in $\ensuremath{\Lambda}{K}^{*}{(890)}^{0}$ production where ${K}^{*}{(890)}^{0}\ensuremath{\rightarrow}{K}^{+}{\ensuremath{\pi}}^{\ensuremath{-}} \mathrm{and} {K}_{S}^{0}{\ensuremath{\pi}}^{0}$. (3) ${\ensuremath{\pi}}^{\ensuremath{-}}p\ensuremath{\rightarrow}\ensuremath{\Lambda}{K}^{*}{(1420)}^{0} \mathrm{and} \ensuremath{\Lambda}{K}^{*}{(1300)}^{0}: {K}^{*}{(1420)}^{0}$ density-matrix elements satisfying positivity constraints are determined allowing for $s$-wave interference effects. Evidence of the existence of a narrow ${K}^{*}{(1300)}^{0}\ensuremath{\rightarrow}K\ensuremath{\pi}\ensuremath{\pi}$ with a dominant ${K}^{+}{\ensuremath{\rho}}^{\ensuremath{-}}$ decay mode is observed in the 4.5- and 6-GeV/c data. (4) $\ensuremath{\Sigma}(1385)$, $\ensuremath{\Lambda}(1405)$, $\ensuremath{\Lambda}(1520)$ production: Differential cross sections for the quasi-two-body reactions ${\ensuremath{\pi}}^{\ensuremath{-}}p\ensuremath{\rightarrow}{Y}^{0}{K}^{0}$, where ${Y}^{0}$ is $\ensuremath{\Lambda}(1405)$, $\ensuremath{\Lambda}(1520)$, or $\ensuremath{\Sigma}{(1385)}^{0}$, are presented and found to have a very similar flat slope in the forward direction. Data for forward ${K}^{+}$ scattering in the reaction ${\ensuremath{\pi}}^{\ensuremath{-}}p\ensuremath{\rightarrow}\ensuremath{\Sigma}{(1385)}^{\ensuremath{-}}{K}^{+}$ are presented and discussed. It is argued that this forward peak cannot be explained by kinematic reflection or an $s$-channel effect and therefore must be due to either two-particle exchange or a single exotic exchange in the $t$ channel.

On the properties of the pion exchange contribution to two and quasi two body reactions

The structure of the t-channel unnatural parity exchange contribution to two and quasi two body reactions is examined. A method in which the absorbed pion exchange contribution is strongly suppressed for − t⩾ 0.2 GeV 2 is discussed. A hybrid model is presented which enables us to concurrently provide a consistent description of reactions in which the pion exchange plays an important role. This is achieved without recourse to additional parameters.

Exchange interference effect on Reϱ 10 in K ∗ production

The interference between exchanges, in vector meson production, that can contribute to Re ϱ 10 are: (i) interference between unnatural parity exchanges with the same CP quantum numbers, (ii) interference between exchanges of natural parity and those of unnatural parity with CP = +1. The application of this property and line reversal to K ∗ production data on Re ϱ 10 gives an indication of possible unnatural parity exchange with CP = +1 and C = −1, in addition to the π-B and π-H interferences. Features and distinction of these models are discussed.

Polarization and unnatural parity exchange in K − p → (ω, ϕ) Λ and π − p → K ∗0 Λ

We use newly obtained joint density matrix elements for K −p → ( ω, ϕ) Λ and π − p → K ∗0Λ to isolate three interference terms contributing to the polarization. The data show large interference between natural parity exchanges (K ∗∗, K ∗) and substantial interference between unnatural parity exchanges (K-K A). We use Su(3) to compare ω, ϕ and K ∗0 production.

The reactions [formula omitted] and [formula omitted] at 14.3 GeV/ c

Preliminary results are presented on K ∗(890) and K ∗(1420) production in three-body final states of 14.3 GeV/ c K −p interactions. For K ∗ō(890) production, the total and differential cross-sections and the density matrix elements are determined. The K ∗− reaction is found to be dominated by natural parity exchange and the charge exchange reaction by unnatural parity exchange. The total cross sections are given for K ∗ō(1420) production.

Inelastic scattering of 61 mev protons from 90Zr and 89Y

Abstract Differential cross sections for the excitation by 61.2 MeV protons of the first seven resolved states in 90 Zr have been measured over the angular range 18°–105°. The results are analyzed using a microscopic model including knock-on exchange and core polarization, with central interactions of Yukawa and Hamada-Johnston type. The analogous excitations of the 5 − state in 90 Zr and the 9 2 + state in 89 Y are compared; it is shown that core polarization of the E5 type is more important for the former, but there may be additional core polarization of M4 type in the latter transition. It is also shown that unnatural-parity exchange may be important for the excitation of the ( g 9 2 ) 2 sequence of states in 90 Zr. Both microscopic and collective models were used for the 3 − state in 90 Zr.

K ∗(890) and K ∗(1420) production in quasi two-body K −p reactions at 10 GeV/c

Results are presented for six reactions induced by K −p collisions at 10 GeV/ c, where K ∗(890) and K ∗(1420) resonances are produced in association with a proton, a neutron, a Δ +(1236) or Δ O(1236) isobar. Partial and differential cross sections as well as density matrix elements are compared to one another and to previous results at lower energies. The following observations are made: (i) There is a first indication of a difference in the energy dependence of the pK ∗− and nK ∗O cross sections. (ii) In the pK ∗−(890) channel the ratio of π to ω exchange decreases with increasing energy. (iii) The ratio of π to ω exchange increases with the mass produced, the pion exchange contribution to the pK ∗−(1420) channel being larger than that to the pK ∗−(890) channel. (iv) Natural spin-parity exchange dominates in the pK ∗−(890) channel, but there are significant contributions of both natural and unnatural parity exchanges in the nK ∗O(890) channel. (v) There exists a clear hierarchy of exchanges such that when ω or f O-exchanges are possible, they dominate over pion-exchange; pion exchange dominates over ϱ or A 2-exchange in ΔQ = 1 or ΔI = 1 reactions, at least up to 10 GeV/ c. (vi) Differential cross sections d σ/d t′ of reactions dominated by pion exchange are better fitted by a two-exponential formula of the type P[exp (− Qt′)+ R exp (− St′)], than by the pion propagator. (vii) At least ≈ 25% of the unnatural exchange in the nK ∗O(890) channel at 4–10 GeV/ c must be due to mechanisms other than simple pion exchange.

Determination of theω0Spin—Density-Matrix Elements in the Reactionπ−p→ω0n

We report measurements of the ${\ensuremath{\omega}}^{0}$ spin-density matrix elements in the reaction ${\ensuremath{\pi}}^{\ensuremath{-}}p\ensuremath{\rightarrow}{\ensuremath{\omega}}^{0}n$ at incident momenta of 3.65, 4.5, and 5.5 $\frac{\mathrm{GeV}}{c}$. In the Gottfried-Jackson frame the data exhibit nonzero values of ${\ensuremath{\rho}}_{00}$, indicating the presence of unnatural-parity exchange as well as a large value of ${\ensuremath{\rho}}_{11}+{\ensuremath{\rho}}_{1\ensuremath{-}1}$ which does not show a dip in the momentum transfer distribution. The unnatural-parity contribution decreases with respect to the natural-parity contribution as the momentum increases.

Natural- versus Unnatural-Parity Exchange in Forward Scattering

The assumption of one-particle (pole) exchange at the quark level for $s\ensuremath{\rightarrow}\ensuremath{\infty}$ leads to the decoupling, in the forward direction, of natural-parity particles (leading Regge trajectories) from octet-decuplet baryonic vertices. In the differential cross section of the reaction $\ensuremath{\pi}p\ensuremath{\rightarrow}\ensuremath{\pi}\ensuremath{\Delta}$ a dip is predicted at $t\ensuremath{\simeq}0$, for high $s$. The study of this dip can lead to an estimate of the upper limit of the quark mass.

Comparison of the high energy models for neutral meson photoproduction and the related hadronic processes

To critically compare the Michigan and Argonne models we perform simultaneus fits to the high-energy data for π 0 and ν photoproduction together with that for the related hadronic vector meson production reactions, in particular that for π +n → ωp. Thus all constraints on these models are considered simultaneously. Particular attention is given to the strength of absoptive cuts in both models, and to the role of the B-meson exchange in filling dips in the Argonne model. We find that the ϱ-Pomeron cuts (i.e. cuts associated with large helicity flip at the N N vertex) are essential to introducing unnatural parity exchange in the Michigan model, and that the B-meson contribution is incapable of filling unwanted dips which appear in any model with nonsense wrong signature zeros as required by exchange degeneracy. The flip and non-flip ϱ and ω coupling strengths at the N N vertex are determined. Predictions are made for various polarization asymmetries for γp → π 0p and γp → ηp. The differential cross section for K L 0p → K S 0p is calculated and compared with the existing data.