D. Treille

Elastic scattering and two-body annihilations at 5 GeV/c

We present results of measurements of K±p and pp elastic scattering and of the annihilation reactions pp→π+π− and pp→K+K− at an incident laboratory momentum of 5 GeV/c. Nearly complete angular distributions were obtained. Results are also presented for π-meson proton elastic scattering in the momentum transfer ranges 2 < −t < 8 (GeV/c)2 (for π+) and 0.16 < −t < 7 (GeV/c)2 (for π−). All measurements were done in one experimental geometry. The measured differential cross sections range from 10 to 10−5 mb/(GeV/c)2. For the first time, backward peaks have been observed in K−p and pp elastic scattering. In K+p elastic scattering the forward peak shows a break at −t ≈ 0.8 (GeV/c)2 and the backward peak shows a structure at −u ≈ 1.2 (GeV/c)2. Large angle πp scattering oscillates around an average differential cross section of 0.1 μb/(GeV/c)2. In addition to the well-known dip at −t ≈ 3 (GeV/c)2, we observe further dips at −t ≈ 4.6 and 5.7 (GeV/c)2. The annihilation reactions show both forward and backward peaks.

Experimental comparison of Jψ production by π±, K±, p and p̄ beams at 39.5 GeV/c

Abstract Measurements have been made of relative production cross sections of the J ψ by π±, K±, p and p at 39.5 GeV/c incident on copper. J ψ production rates from π−, K− and p are similar. The J ψ relative particle/anti-particle production cross sections for x>0 are σ(π + ) σ(π − )=(0.87±0.14) , σ(K+)/σ(K−)=(0.85±0.5) and σ( p ) σ( p )=(0.15 ±0.08) . The small p/p cross section ratio disagrees with models of J/ψ production by gluon amalgamation.

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.

K±p elastic scattering at 5 GeV/c. Evidence for a K−p backward peak

The elastic scattering of K± mesons on protons has been studied at 5 GeV/c. A total of about 500 000 events have been measured in the c.m. angular range 17° < θcm < 165° corresponding to 0.2 < −t < (GeV/c)2. We observed a K−p backward peak which we have parametrized as dσ/du = (0.6 ± 0.2) exp [(3.3 ± 0.6)u] μb/(GeV/c)2, while for the K+p backward peak we find dσ/du = (17.5 ± 1) exp [(3.6 ± 0.2)u] μb/(GeV/c)2. The K−p cross-section falls to about 0.03 μb(GeV/c)2 around −t = 5 (GeV/c)2, while the K+p cross-section stays in the vicinity of 0.3 μb(GeV/c)2 in the same t-region. The K+p and K−p differential cross-sections have cross-over points at −t = 0.2, 1.1 and about 3.5 (GeV/c)2.

Measurement of Deep Inelastic Compton Scattering of High-energy Photons

Abstract We present the first results on inclusive photo-production of prompt photons at high transverse momenta. The data were taken in an open spectrometer at CERN using a high intensity photon beam energy between 50 and 150 GeV. After subtracting the yield of photons from indirect sources, a clear excess is observed for transverse momenta above 2.5 GeV c Deep inelastic Compton scattering with appropriate QCD corrections account for this excess. The data disfavour the gauge integer charge quark models so far proposed.

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 − .

High energy photoproduction of large transverse-momentum π0 mesons: A quantitative test of QCD

Abstract The first results on inclusive photoproduction of π 0 at transverse momenta up to 4 GeV/ c , using incident γ energies between 50 and 150 GeV are presented. A comparison is made with inclusive π 0 production obtained, in the same experiment, with incident π − . Using the π − data to parametrize the hadronic behaviour of the photon, significant differences are observed in quantitative agreement with QCD Compton scattering and corrections thereof.

Elastic scattering of 10 GeV/c π+ and K+ mesons and of 9 GeV/c protons on protons

Abstract Angular distributions of π+ and K+p elastic scattering have been measured for an incident beam momentum of 10.0 GeV/c. For π+p elastic scattering almost the complete angular distribution was measured. The angular distribution of proton-proton elastic scattering was measured for an incident momentum of 9.0 GeV/c in the interval of the four-momentum transfer squared from 0.7 (GeV/c)2 to 5.0 (GeV/v)2. For π+p elastic scattering the structures at −t = 2.8 (GeV/c)2 and −t = 4.8 (GeV/c)2 are less pronounced than at lower momenta. The cross section for scattering at 90° in the c.m. system is of the order of 1 nb/GeV/c)2. For K+p elastic scattering is a break in the angular distribution around −t = 3 (GeV/c)2. The differential cross sections for proton-proton elastic scattering decrease smoothly with increasing momentum transfers.