This paper reports studies of the reactions γp→ρ π π πp and γp→ρ ρ πp. In particular a peak is reported in the ρ ρ π mass spectrum with closely similar mass and width to those of the ω ρ π peak previously reported in the reaction γp→ω ρ πp. The ratio of production cross sections is found to be ρ ρ π/ω ρ π=0.96±0.19, in serious disagreement with the expectation from Vector Meson Dominance. A possible explanation is indicated.

The η ω mass spectrum resulting from diffractive photoproduction, by photons of 25–50 GeV, shows a peak at a mass of 1.61±0.04 GeV and width of 0.23±0.08 GeV. Comparison with the corresponding η ρ0 mass spectrum suggests that a resonance interpretation is favoured.

It is shown that a likely interpretation of theb1± p± mass spectrum from diffractive photoproduction is that it is due to contributions from the ?3(1670), in accord with corresponding production of the ?3(1690), and from a further peak with a mass of 1.88±0.04 GeV and a width ?0.3 GeV. The possibility that this latter peak can be due to a group of <img src="/fulltext-image.asp?format=htmlnonpaginated&src=V416L04434Q72752_html\10052_2005_Article_BF01566756_TeX2GIFIE1.gif" border="0" alt=" $$q\bar qg$$ " /> hybrid states, as proposed by Isgur and Paton, is explored.

It is shown that a likely interpretation of theb 1 ± π∓ mass spectrum from diffractive photoproduction is that it is due to contributions from the ω3(1670), in accord with corresponding production of the ρ3(1690), and from a further peak with a mass of 1.88±0.04 GeV and a width ≦0.3 GeV. The possibility that this latter peak can be due to a group of\(q\bar qg\) hybrid states, as proposed by Isgur and Paton, is explored.

A study of photoproduction ofK+K−π+π− shows a broad threshold enhancement, about 30% of which is due toK*Kπ production. ThisK*Kπ component shows no dynamical features indicative of the existence of a ϕ′ vector meson in this channel.

In diffractive photoproduction ofηπ+π−, the two-body substatesηρ0 andA2π are found to contribute significantly to the cross-section forηπ+π− masses below 2.4 GeV. From a spin-parity analysis the branching ratio, ρ′(1600)→ηρ/ρ′(1600)→, is determined to be <0.02 at the 68.3% confidence level. TheA2π component shows an enhancement around 1.7 GeV. The spin-parity analysis indicates a probable contribution to this signal from exclusive photoproduction of theg(1690).

Inclusive photoproduction of ϕ,K*0(890),\(\bar K*^0 \) andK*0(1420),\(\bar K*^0 \) has been studied in γp collisions with photons of energy 20 to 70 GeV and in the range 0.1≦x F ≦0.95 [0.4≦x F ≦0.8 for the\(\mathop K\limits^{( - )} *\)(1420)],x F being the Feynman variable of the vector/tensor meson. The cross-sections for these processes, averaged over the photon energy range and integrated overx F are given. The inclusive ϕ production in the forward direction can be described quantitatively by a triple-Regge model calculation. The remaining ϕ production and the totalK*0(890) and\(\bar K*^0 \) production are consistent with a quark fusion picture.

A study of ϕ-meson photoproduction by partially polarized photons of energy 20–40 GeV is reported. The production mechanism is found to conserves-channel helicity and to proceed via natural-parity exchange in thet channel. In the photoproduction of high-massK+K− states with photons of energy 20–70 GeV, there is evidence for an enhancement at a mass of 1.76 GeV with width 0.08 GeV.

Abstract The reactionγp→π + π − π 0 π 0 p (excludingωπ 0 production) has been studied for photon energies in the range 20–70 GeV. A peak is seen in the 4π mass spectrum at ∼1.66 GeV with a width of ∼0.3 GeV which is identified with theρ′(1600). Maximum likelihood fits show that the peak is dominantly inρ ± π ∓ π 0 withB(ρ′→ρ 0 π 0 π 0)/B(ρ′→ρ ± π ∓ π 0)<0.1. This indicates anI=1ρπ final-state interaction. However, no evidence is found for any resonant ρπ state such as anA 1 orπ′.

The photoproduction of the final state ω π+π−π0 has been studied as part of a survey of photoproduction in the energy range 20–70 GeV in the Omega spectrometer at CERN. The π+π−π0 system produced with the ω meson has a strongρ±π± component which is predominantlyI=1 andJπ=1+. For theωρ±π∓ state a spin-parity analysis favoursJπ=1−, and the mass spectrum peaks at 2.28±0.05 GeV. The fitted width is Γ=0.44±0.11 GeV. The photoproduction cross-section of theωρ±π∓ state, averaged over the energy range 25–60 GeV, is 150±50 nb.