J.R. Campbell

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

Abstract 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 ΔO(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 fO-exchanges are possible, they dominate over pion-exchange; pion exchange dominates over ϱ or A2-exchange in ΔQ = 1 or ΔI = 1 reactions, at least up to 10 GeV/c. (vi) Differential cross sections dσ/dt′ 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.

Observation of t-channel helicity conservation in diffractive reactions producing A1 and Q-mesons

Abstract It is shown that helicity is conserved in the t-channel for A 1 and Q-meson production by pions of 8 and 16 GeV/ c and kaons of 10 GeV/ c , respectively. This is in contrast to evidence for s-channel helicity conservation reported for elastic scattering and rho photoproduction. This difference in behaviour of diffractive processes may be due to the fact that there is a change in spin in the former reactions but not in the latter, or, alternatively, to different mechanisms operating in the two cases.

Study of lambda production in 10 GeV/c K−p interactions

Lambda production is studied in K−p interactions at 10.1 GeV/c, where the dominant reaction is K−p → Λ + pions. General characteristics such as the distributions of the double differential cross section in the lab system, of the variable x = pL∗pmax∗, of p⊥2 and of the missing mass to the lambda are presented. Total cross sections for Λ production and for the various channels are given. Differential cross sections dσdt, dσdt′ and dσdu′ are presented. Forward and backward peaks are observed in the dσdt′ and dσdu′ distributions, respectively. It is found that the exponential slope of these distributions decreases with increasing missing mass to the lambda and, for dσdt′, also for increasing multiplicity in the final state. The polarization of the lambdas is studied as a function of multiplicity, pL∗, (Λπ±) effective mass, t′ and u′. The forward lambdas show

Measurement of the real part of the k- p elastic scattering amplitude at 10 gev/c

Abstract Bubble chamber film of 10 GeV/ c K − p interactions was scanned automatically by an H.P.D. to look for small angle scatters in the | t |-range from 0.008 to 0.1 GeV 2 . Combining the 1800 events so obtained with 22 000 elastic events obtained from normal scanning (| t | > 0.06 GeV 2 ), the real part of the elastic scattering amplitude was found to be (+25 ± 10)% of the imaginary part. Evidence is found for a change in slope in the differential cross-section distribution, from 9.8 ± 0.6 GeV −2 in the | t |-range below 0.1 GeV 2 to 7.1 ± 0.2 GeV −2 in the range 0.12 t | ⩽ 0.4 GeV 2 .

Separation of isospin 12 and 32 contributions in single pion production in π+p interactions at 8 and 16 GeV/c

The isospin I = 12 state of the nucleon pion system in π+p reactions leading to single pion production has been separated from the I = 32 state. The contribution of the former to the total ππN cross section is found to decrease much less with increasing energy than that of the latter. This is interpreted in terms of pomeron-exchange (or diffractive) processes responsible for at least 70% of the I = 12 contribution at 16 GeV/c and 50 % of the ππN cross section. For I = 12, the (Nπ) mass spectrum has a broad unresolved enhancement which reaches its maximum value at ≈ 1300 MeV, well below the first known I = 12 isobar. The production characteristics of this low mass (Nπ) system are different from those of the Δ (1236).

How well does an LPS analysis separate production mechanisms in π+p interactions at 8 and 16 GeV/c?

Abstract A longitudinal phase space (LPS) analysis has been performed on the reaction π+p→π+π+π−p at 8 and 16 GeV/c. Special attention has been given to the question of the separation of the various reaction mechanisms. The conclusions are: (i) Dissociation of the pion can be clearly isolated, both at 8 and 16 GeV/c. Baryon dissociation is fairly well separated at 16 GeV/c, but at 8 GeV/c it is strongly contaminated by the reactions δ++ϱ0 and even more by δ++f; (ii) The cross section for diffraction dissociation dominated regions of LPS is constant or even slightly increasing between 8 and 16 GeV/c.

Tests of s-and t-channel helicity conservation in pπ+ π−, K−π+π− and π±π+π− production in 16 GeV/c π±p and 10 GeV/c K−p interactions

Abstract Necessary conditions for s-and t-channel helicity conservation are tested for pπ+π−, K−π+π− and π±π+π− states produced in 16 GeV/c π±p and 10 GeV/c K−p interactions by means of a method which is independent of specific spin-parity assignments to these states. It is found that the production of pπ+π− states is generally consistent with t-channel helicity conservation for masses up to 2.4 GeV, but that for π±π+π− and K−π+π− some inconsistency with t-channel helicity conservation is found, primarily in the A1± and Q− mass regions. s-channel helicity conservation is excluded for all states.

Interference between (P, f0…) and (ϱ0, B0,…) exchanges in pion diffraction dissociation at 16 GeV/c

Abstract It is found in the reactions π±p→(π±π+π−)p, believed to be dominated by diffraction dissociation, that the d σ d t′ distributions show a “cross-over” effect at t′ ≈ 0.15, similar to the effect observed in elastic scattering. This gives evidence for the interference of (ϱ0, B0,…)-exchanges with ( P , f 0 , …) -exchanges in pion diffraction dissociation reactions. No such evidence is found for baryon dissociation, π±p→π±(pπ+π−), at the same energy.

s-Wave interference in the f° resonance region

Abstract The anomalous shape of the decay angular distribution of the f° resonance can be explained by an interference of the resonating d-wave with an isospin zero s-wave, whose phase stays near 90° in the f° region.

ππ scattering effects in π+p interactions at 8 GeV/c

Abstract The reaction π + p → Δ ++ π + π − at 8 GeV/c has been used to study ππ scattering in the ϱ-meson mass region. Two different methods have been used. In the first, it is assumed that ππ scattering in this mass range is entirely elastic and confined to S-, P- and D-waves. The P- and D-waves are assumed to be given by the ϱ0 and f0 resonant amplitudes extrapolated off-shell using Durr-Pilkuhn form factors. The remaining cross section (which by our assumption is S-wave) fits a Breit-Wigner distribution with mS = 680 ± 30 MeV and ΓS0 = 310 ± 70 MeV. In the second method a ππ phase-shift analysis has been used to obtain the S-wave phase shifts. There are two possible solutions here, one of which resonates at mS = 710 ±a 80 MeV and ΓS0 = 140 ± 20 MeV.