Two-nucleon production of hyperons and the search for strange dibaryons

The double charge exchange reaction³He(K^?,π ⁺)Xn was studied at 870 MeV/c. In the X missing mass range below the sigma-nucleon production threshold (2130 MeV/c²), events were detected which can be attributed to the two-nucleon process pp(K^?,π ⁺)λn. This reaction and mass range also offers good prospects for finding theI=1/2,l=1 (¹ P₁) spin-singlet dibaryon D_s suggested as the lowest massS=?1 dibaryon in the MIT Bag Model. Although the existence of the D_s is not ruled out by the present data, there is no need to invoke such an object to account for the observed events below σ production threshold. We show that the cross section level for these events is compatible with a dominant two-nucleon mechanism K^?p→π ⁰λ,π ⁰p →π ⁺n. We also offer an interpretation of the recent (K^?,K⁺) data on nuclear targets from Iijimaet al., which display a broad peak centered around a K⁺ momentum of 600 MeV/c. We find that the two-nucleon mechanism K^?N →πY,πN→K⁺Y produces cross sections which are at least an order of magnitude smaller than those observed, and we suggest that the one nucleon process K^?p →Φλ, followed by the decayΦ → K⁺K^?, accounts for the data.     

Chiral anomaly in Kγ → Kπ reactions

Implications of the chiral anomaly for the cross sections of the reactions K ⁺γ → K ⁺π⁰, K ⁰γ → K ⁰π⁰, K ⁺γ → K ⁰π⁺, and K ⁰γ → K ⁺π^? are investigated. Near the threshold, the contribution of the chiral anomaly dominates the cross sections for the first and the second reaction. These cross sections are compared with the cross sections for the third and the fourth reaction, which receive no contribution from the chiral anomaly. Prospects for an experimental study of the KKπγ vertex in the Coulomb production of π mesons by a K-meson beam are discussed.     

Subthreshold and near-threshold K +-meson photoproduction on nuclei

The inclusive K ⁺-meson production in photon-induced reactions in the near-threshold and subthreshold energy regimes is analyzed for the one-step (γN → K ⁺ Y, Y=Λ, Σ) incoherent production processes on the basis of an appropriate new folding model that takes properly into account the struck-target nucleonremoval energy and the internal momentum distribution (nucleon spectral function), extracted from recent quasielastic-electron-scattering experiments and from many-body calculations based on realistic models of NN interaction. Simple parametrizations of the total and differential cross sections for K ⁺ production in photon-nucleon collisions are presented. A comparison of the model calculations of the K ⁺ differential cross sections for γ¹²C interactions in the threshold region with existing experimental data is given, which displays the contributions to K ⁺ production at considered incident energies from the use of the single-particle part, as well as high momentum and high removal energy part, of the nucleon spectral function. Detailed predictions for the K ⁺ total and differential cross sections for γ²H, γ¹²C, and γ²⁰⁸Pb interactions at subthreshold and near-threshold energies are provided. The effect of the uncertainties in the elementary K ⁺-production cross sections on the K ⁺ yield is explored.     

Subthreshold and near-threshold K +-meson production on light nuclei by protons

Inclusive K ⁺-meson production in proton-nucleus collisions in the near-threshold and subthreshold energy regimes is analyzed with respect to the one-step (pN → K ⁺ YN, Y=Λ, Σ) and two-step (pN → NNπ, NN2π; πN → K ⁺ Y) incoherent production processes. An appropriate folding model is used that properly takes into account the struck-target-nucleon-removal energy and the momentum distribution (nucleon spectral function), novel elementary cross sections for proton-nucleon reaction channel close to threshold, as well as nuclear-mean-field-potential effects on the one-step and two-step kaon-creation processes. A detailed comparison of the model calculations of the K ⁺ total and differential cross sections for the p+⁹Be and p+¹²C interactions with the existing experimental data is given, which displays both the relative role of the primary and secondary production channels at considered incident energies and those features of the cross sections that are sensitive to the high-momentum and high-removal-energy parts of the nucleon spectral function. It is found that, contrary to previous studies known in the literature, the pion-nucleon production channels do not necessarily dominate in pA collisions at subthreshold energies and that the relative strength of the proton-and pion-induced reaction channels for light target nuclei in the subthreshold energy regime is governed by the kinematics of the experiment under consideration.     

K + production in proton and deuteron induced reactions at subthreshold energies

The production ofK ⁺ mesons in proton-nucleus and deuteron-nucleus collisions is analyzed with respect to one-step nucleon-nucleon (NN → N ΛK ⁺),Δ- nucleon (ΔN → N Λ K⁺) and two-step pion-nucleon (πN → K ⁺ Λ) production channels on the basis of experimental ground state momentum distributions and free on-shell production processes. Whereas forK ⁺ production in proton-nucleus reactions the secondary channelπN clearly dominates at subthreshold energies, meson and nucleon induced channels are of similar magnitude in deuteron-nucleus reactions. Contrary to nucleus-nucleus collisions theΔ induced reaction channels are found to be of minor importance. The experimental differentiation of the underlying microscopic reaction channels appears possible via differential proton —K ⁺ coincidence measurements as shown in detail by the microscopic simulations including proton rescattering.     

Exclusive decays of upsilons

The leading order contributions to decay rates of two-meson decays of an upsilon, in particular Y → K⁺K^?, D⁺D^?, are calculated using perturbative quantum chromodynamics. An interesting enhancement of Y → D⁺D^? is found. The branching ratio of Y → D⁺D^? to the upsilon's total hadronic width is larger than that of Y → K⁺K^? by two orders of magnitude.     

Spectroscopy of the heavy resonances ρ′, ρ″, ω′ and ω″

The spectroscopy of the isovector resonances ρ′ and ρ″ and the isoscalar resonances ω′ and ω″ is considered on the basis of their masses and coupling constants extracted from data on e ⁺ e ^? annihilation, τ-lepton decays, and the reaction K ^? p → π ⁺ π ^?Λ. The total widths of these resonances and the branching fractions for their decay modes are calculated. It is argued that the shift of the apparent position of the peak in the energy dependence of the cross section with respect to the bare mass is significant for broad resonances. The results are compared with the predictions of available models for the ρ′, ρ″, ω′, and ω″ resonances.     

Energy dependence of the cross sections for the e<Superscript>+</Superscript>e<Superscript>–</Superscript> → hadrons process near the production threshold for the <Emphasis Type="Italic">N</Emphasis><Emphasis Type="Italic">N̄</Emphasis> pair

The energy dependence of the cross sections for the processes e⁺e^? → π⁺π^?π⁰, K⁺K^?, ηπ⁺π^?, ωπ₀, and K⁺K^?π⁺π^? is studied near the production threshold for a nucleon–antinucleon pair. It is shown that, within the measurement errors, the cross sections in question behave smoothly in this energy region.     

To search for four-quark states in γγ-collisions

It is shown that the reactions γγ into hadrons is a rather effective means of searching for fourquark states. We predict the suppression ofS ^*(980,0⁺) and δ(980,0⁺) in γγ→ππ and γγ→π⁰ η respectively, if they are four-quark states from (9.0⁺). The well-known anomaly in γγ→ρ ⁰ ρ ⁰ is interpreted as the production of the tensor four-quark resonances with the isospin 0 and 2. We predict the absence of similar enhancement in γγ→ρ ⁺ ρ ^?. The bright anomalies are predicted near thresholds of γγ→ρ ⁰ ω and γγ→ρ ⁰ ?     

Experimental search for radiative decays of the pentaquark baryon Θ+(1540)

The data on the reactions K ⁺Xe → K ⁰ γX and K ⁺Xe → K ⁺ γX, obtained with the bubble chamber DIANA, have been analyzed for possible radiative decays of the Θ⁺(1540) baryon: Θ⁺ → K ⁰ pγ and Θ⁺ → K ⁺ nγ. No signals have been observed, and we derive the upper limits Γ(Θ⁺ → K ⁰ pγ)/Γ(Θ⁺ → K ⁰ p) < 0.032 and Γ(Θ⁺ → K ⁺ nγ)/Γ(Θ⁺ → K ⁺ nγ) < 0.041 which, using our previous measurement of Γ(Θ⁺ → KN) = 0.39 ± 0.10 MeV, translate to Γ(Θ⁺ → K ⁰ pγ) < 8 keV and Γ(Θ⁺ → K ⁺ nγ) < 11 keV at 90% confidence level. We have also measured the cross sections of K ⁺-induced reactions involving emission of a neutral pion: σ(K ⁺ n → K ⁰ pπ ⁰) = 68 ± 18 µb and σ(K ⁺ N → K ⁺ Nπ ⁰) = 30 ± 8 µb for incident K ⁺ momentum of 640 MeV.     

Phase-shift analysis of π N → K Σ

We have performed an energy-independent phase-shift analysis of πN → KΣ by analyzing differential cross-section and polarization data for the reactions π⁺p → K⁺∑⁺ and

at fifteen energies from threshold to 2137 MeV invariant mass. The single-energy solutions were linked by shortest-path methods with and without constraints implied by the Odorico zeros for the reaction π^?p → K⁺Σ^?. Similar to π^?p → KΛ it has been found that both isostates are dominated below 1900 MeV by resonances in the spin $\text{1}\text{2}$ waves. The F₃₇ resonance couples to KΣ with √x_πNx_KΣ= 0.04.     

t-channel factorization description of γγ→V 1 V 2

At-channel factorization model is used to estimate cross sections for the processes γγ→V ₁ V ₂. WheneverV=ρ, the width of the ρ has been included in the calculations. The channels γγ→ρ⁰ρ⁰, ρ⁰?, ??, ωω, ρ⁰ω and ρ⁺ρ^? are calculated for two quasi-real photons. Predictions are also given for the process γ^*γ→ρ⁰ρ⁰ for virtual photon mass squaredQ ²<5 GeV². Our results are consistent with all available experimental data.     

A study of K−p interactions in deuterium in the c.m. energy range from 1.60 to 1.74 GeV

We present results of a K^?d bubble-chamber experiment at four K^? momenta between 680 and 840 MeV/c. The validity of the spectator model for the analysisof KN interactions in deuterium is discussed. Cross sections are determined for the reactions K^?p → Σ^?π⁺, K^?p → Σ⁺π^? and K^?p → Λπ⁺π^? and are compared to those obtain ed on free protons.     

Rates for the reactions\bar pp \to \pi \phi andγ φ

We study ¯pp annihilation at rest intoπφ andγφ. Rescattering by ¯K^*K+K^*¯K and ρ⁺ρ^? for ¯pp→πφ states is sizable, of order (0.90 to 2.6) × 10^?4 in the branching ratio, but smaller than experiment. For ¯pp→γφ the rescattering contributions are negligible, but theγφ channel is well explained by aρφ intermediate state combined with vector meson dominance.     

Broadening of ρ(→ e+e−) meson produced coherently in the photonuclear reaction

The e ⁺ e ^? invariant mass distribution spectra are calculated to estimate the hadron parameters of the ρ meson produced coherently in the photonuclear reaction. The elementary reaction occurring in the nucleus is considered to proceed as γN → ρ ⁰ N; ρ ⁰ → e ⁺ e ^?. We describe the elementary ρ meson photoproduction by the experimentally determined reaction amplitude $f_{\gamma {\rm N} \to \rho ^0 {\rm N}}$ . The ρ meson propagator is presented by the eikonal form, and the ρ meson nucleus optical potential V _Oρ appearing in it is estimated using the “t?” approximation. The ρ meson nucleon scattering amplitude f _ρN extracted from the measurements is used to generate this potential. The calculated e ⁺ e ^? invariant mass distribution spectra are compared with those measured at Jefferson Laboratory. The calculated results for the transparency ratio are also presented.     

Analysis of the φ → γπη and φ → γπ 0 π 0 decays

We study interference patterns in the φ → (γa ₀+π ⁰ ρ) → γπη and φ → (γf ₀+π ⁰ ρ) → γπ ⁰ π ⁰ reactions. Taking into account the interference, we fit the experimental data and show that the background reaction does not distort the π ⁰ η spectrum in the decay φ → γπη everywhere over the energy region and does not distort the π ⁰ π ⁰ spectrum in the decay φ → γπ ⁰ π ⁰ in the wide region of the π ⁰ π ⁰-system invariant mass, π ^ππ > 670 MeV, or when the photon energy is less than 300 MeV. We discuss the details of the scalar meson production in the radiative decays and note that there are reasonable arguments in favor of the one-loop mechanism φ → K ⁺ K ^? → γa ₀ and φ → K ⁺ K ^? → γf ₀. We also discuss distinctions between the four-quark, molecular, and two-quark models and argue that the Novosibirsk data give evidence in favor of the four-quark nature of the scalar a ₀(980) and f ₀(980) mesons.     

Measurement of the reactions K+d → K+d and K+d → p+pn and an investigation of elastic K+-nucleon scattering at 4.6 GeV/c

The coherent elastic reaction K⁺d → K⁺d and the break-up reaction K⁺d → K⁺pn are studied in a K⁺d experiment at 4.6 GeV/c which the CERN 2 m bubble chamber. Partial and differential cross sections are given and the slopes of the differential cross sections are determined. The results for the reaction K⁺d → K⁺p(n_s), where n_s denotes the spectator neutron, are compared with those of the reaction K⁺p → K⁺p on free protons. Combining our data with existing data on the reactions K⁺d → K⁰pp and K⁺p → K⁺p, parameters of the elastic K⁺-nucleon scattering at 4.6 GeV/c are determined in the framework of the Glauber model. The D-wave of the deuteron and spin-flip effects are taken into account.     

Diffraction dissociation of the proton into ΛK+ and (ΣK)+in π+ p interactions at 10.3 GeV/c

We observe a threshold ΛK⁺ enhancement in the reaction π⁺p → ΛK⁺π⁺ at 10.3 GeV/c. The production characteristics are very similar to those of the diffractive ΛK⁺ enhancement observed at the CERN ISR. The ΣKπ channels show no similar (ΣK)⁺ enhancement, as predicted by SU(6).     

Phenomenological Studies of the Low Energy Dynamics in the ppK + K − System

In this article we review studies of the near threshold pp → ppK ⁺ K ^? reaction done with the COSY-11 and ANKE detectors. We discuss phenomenological studies of the ppK ⁺ K ^? dynamics, in particular the recent investigations on the K ⁺ K ^? final state interaction are presented.     

The controversy in theγγ →ρρ process: potential scattering or qq\bar q};\bar q}; resonance?; resonance?

Theγγ →ρ ⁰ ρ ⁰ → 4π reaction shows a broad peak at 1.5 GeV in the (J ^P ,J _z )=(2⁺,2) channel which has no counterpart in theρ ⁺ ρ ^? channel. This “resonance” is considered as a candidate for a qq $\bar q$ ; $\bar q$ state in the “s-channel”. We show, however, that it can also be explained by potential scattering ofρ ⁰ ρ ⁰ via theσ- exchange in the “t-channel”.