W. Graves

Photoproduction of ϱ0 and ω on hydrogen at photon energies of 20 to 70 GeV

Abstract We present results on photoproduction of ϱ 0 and ω in the reactions γ p→ π + π − p and γ p→ π + π − π 0 p by tagged photons in the energy ranges 20 to 70 GeV and 20 to 45 GeV, respectively. The production of the ϱ 0 shows dominantly the characteristics of a diffractive process with respect to the E γ and t dependence of the cross section and the spin density matrix. The ϱ 0 photoproduction yields on average over the photon energy range a total cross section of σ ( γ p→ ϱ 0 p) = 9.4±0.1 μ b with an additional systematic error of ±1 μ b, and average slope parameters of the t distribution d σ /d t ≈exp(− b | t | + ct 2 ), of b =9.1±0.1 GeV −2 and c = 3.1 ±±0.2 GeV −4 . The shape of the ϱ 0 peak in the π + π − invariant spectra shows a skewing similar to that observed at lower energies. The photoproduction of ω is also consistent with a diffractive process and has a cross section of σ ( γ p→ ω p) = 1.2± 0.1 μ b with an additional systematic error of ±0.2 μ b. The average slope parameters of the t distribution are b =8.3 ± 1.3 GeV −2 and c = 3.4±2.6 GeV −4 .

The reaction γp → pπ+π−π+π− for photon energies from 25 to 70 GeV

Abstract Measurements of the reaction γ p → p π + π − π + π − are presented, in which π + π − π + π − systems with masses up to 3 GeV are produced from fragmentation of the incident photon. The reaction is dominated by production of the large peak of the ϱ′(1600) meson and, at higher masses ≳2 GeV, y production of jet-like 4 π systems. The ϱ′(1600) meson is produced by a predominantly s -channel helicity conserving mechanism. At higher masses there are also indications of ϱπ peaks, of masses 1.3 GeV (the A 2 meson) and 1.75 GeV, produced with a recoiling π meson by a mechanism consistent with the Deck effect.

Photoproduction of charmed D mesons at γ energies of 40–70 GeV

Inclusive D0 photoproduction is observed in Kπ+π− and K+π−π0. Evidence is presented for an associated (CD) production mechanism. Cross section estimates and limits are given for D and D mesons.

Observation of the ϱ′(1600) in the channel γp→π+π−p☆

Abstract A dipion enhancement of mass 1.59 GeV and width 0.23 GeV is observed in the channel γp→π + π − p. The spin-parity of the enhancement is consistent with being 1 − .

Observation of an ωπ0 state of mass 1.25 GeV produced by photons of energy 20–70 GeV☆

Abstract An analysis is presented of the channel γp→π + π − π 0 π 0 p. Evidence is found for an ωπ 0 enhancement of mass ≈1.25 GeV and width ≈0.3 GeV. The enhancement is a mixed spin-parity state with 1 − dominant.

Photoproduction of K+K− pairs on hydrogen at photon enegies of 20 to 36 GeV

The reaction γp→K+K−p has been investigated with photons in the energy range of 20<Eγ<36 GeV and with K+K− pairs in the mass range of MK+K−<2.0 GeV. The production of the φ(1019) contributes with a cross section σ(γp → φp) × BR(φ→K+K−) = 240±6 nb with an additional systematic error of ±20 nb. In the higher mass range of 1.05<MK+K−<2.0 GeV the production of K+K− pairs yields a cross section σ(γp→K+K−p) = 160±8 nb with an additional systematic error of +40−30 nb.

The reactions γp→ωπ+π−p and γp→ηπ+π−p at photon energies of 25–70 GeV

Measurements of ω and η production in the reaction γp→π+π−π+π−π0p are presented, showihg that the ωπ+π− channel is made up of an enhancement at ∼1.7 GeV of width ∼0.5 GeV, which is consistent with being a 1− state, together with a distribution at higher masses containing higher angular momentum terms. The ηπ+π− final state is produced with a similar cross section and also shows a low-mass enhancement.

Inclusive photoproduction of φ-mesons at photon energies of 25–70 GeV

Results are presented on inclusive φ photoproduction for incident energies between 25 and 70 GeV. Distributions in Feynman x, pT and decay angles are presented and interpreted in terms of quark fusion models. We demonstrate a similarity with the corresponding pion-induced reaction and obtain information on the hadronic nature of the photon.

Observation of a Charmed Neutral Meson Produced in a High-energy Photon Interaction

The production and decay of a D0 have been observed in a nuclear emulsion exposed to a photon beam from the CERN super proton synchroton in conjunction with the Omega spectrometer. All the decay products have been detected and measured in the emulsion and in the spectrometer. The mass of the charmed particle is 1866 ± 8 MeV/c2 and its decay time is (2.26 ± 0.05) × 10−14 s.

Jet-like structure in the reaction γp→π+π−π+π−π+π−p

Abstract We report on diffractive production of 6 π systems with a continuum of masses from 2 to 5 GeV/c 2 in the reaction γ p→ π + π − π + π − π + π - p. The event structure is compared in detail with Monte Carlo studies of p T limited phase space, and with the jets observed in hadronic systems produced by electron-positron annihilation. Strong similarities are found. For this jet-like structure of the 6 π system the axis is consistent with being along the incident photon direction.