W.M. Evans

A search for glueballs and a study of doouble pomeron exchange at the CERN intersecting storage rings

We present results from a study of centrally produced mesons in 3 × 106 events with two small-angle protons at the CERN Intersecting Storage Rings. A high-statistics sample of exclusive pp → ppπ+π− events at √s = 63 GeV has been obtained, where the reaction mechanism is dominated by double pomeron exchange. Scalar or tensor glueballs may be produced by this process. The π+π− mass spectrum has a distinctive structure, and analysis shows that the data are dominantly S-wave up to 1600 MeV. The behaviour of the D-wave provides evidence for a 2++ resonance (M = 1480 ± 50 MeV, Γ = 150 ± 50 MeV) in addition to the f(1270). We also show data on exclusive K+K−, pp, and π+π−π+π− production, and on the analogous reaction αα → ααπ+π− at √s = 126 GeV. Flavour independence is suggested by the observation of approximately equal numbers of K+K− and π+π− pairs for mass above 1 GeV. The mass spectra are also apparently independent of √s (45, 63, 126 GeV) and incident particle type (p, α).

Direct evidence for the emergence of jets in events triggered on large transverse energy in pp collisions at s = 63 GeV

We present data obtained from a 1.7 sr hadron calorimeter, triggered on transverse energy, in pp collisions at s=63 GeV at the CERN-ISR. From the change in the distribution of energy in the calorimeter, we extract the cross section for two-constituent hard scattering for pT between 6 and 14 GeV at y = 0. The decrease of this jet cross section over this pT range is consistent with exp (−bpT), with b = (1.02 ± 0.09) GeV−1. The slope and normalization of the cross section agree well with a QCD motivated Monte Carlo model. The ratio between jet and single particle cross sections [dσJET/dpT)/(dσπ0/dpT)]| y = 0 changes from about 200 at 6 GeV to about 1500 at 14 GeV.

Bose-Einstein correlations in αα, pp and pp interactions

Abstract Bose-Einstein correlations between pions produced in the central region of rapidity in αα, pp and p p interactions have been studied at the ISR. The parameters r and τ c -often interpreted as the radius and depth of the π-emitting region - are found to be independent of incident particle type but to depend on the mean charged multiplicity. In high multiplicity events, the pions appear to originate from a larger space-time region.

Further evidence for jet production from the charged particles produced in pp collisions at triggered on large transverse energy

Abstract The detailed shape of events triggered by a 1.7 sr hadron calorimeter is studied using charged track information in the Axial Field Spectrometer at the CERN ISR. With increasing transverse energy (ET) a large fraction of the events are seen to have a limited transverse momentum relative to the trigger direction. A comparison of the circularity of the events with a prediction of a QCD-motivated Monte Carlo model shows that at high ET the events originate predominantly from two-constituent scattering.

The dominance of jets at large transverse energy in a full-azimuth hadron calorimeter at ISR energies

Abstract Events with a large transverse energy in a calorimeter with full azimuthal coverage and |y|

A study of exclusive central hadron production at the ISR as a search for gluonium states

Abstract We present first results from a study of the exclusive reaction pp→pπ + π − p at √s=63 GeV with π + and π − produced in the central region. This reaction has been suggested as a method of searching for gluonium states. The π + π − mass distribution shows rapid order of magnitude decreases in cross section at both 1000 MeV/ c 2 and 1500 MeV/ c 2 . Further structure is observed above 2000 MeV/ c 2 .

Inclusive vector-meson production in the central region of pp collisions at √s = 63 GeV

Production of ϱ0, K∗0, K∗0, and φ have been measured in proton-proton collisions at the CERN Intersecting Storage Rings (ISR) at low pT in the central region. The transverse mass spectra are well described by exp (−aμT), with a = −6.4±0.2 GeV−1. The cross sections for dσ/dy|y=0are 6.5±0.8±1.2 mb for ϱ0, 1.9±0.3±0.3 mb for K∗), 1.9±03±03 mb for K∗0, and 60.±0.12±0.13 mb for φ, where the first error is statistical and the second is sytematic.

Antibaryon production in the central region at the ISR

Using annihilation in a calorimeter as a trigger on antiprotons, we have measured the relative production cross sections of p, Λ and Ξ at y∼0 in the transverse momentum range 1 to 2 GeV/c in pp collisions at √s=63 GeV. We investigate correlations between the antibaryons and associated produced particles, and find evidence for local baryon number conservation.

Multiplicity distributions in p-α and α-α collisions in the CERN ISR

Abstract Measurements of charged particle multiplicity distributions in the central rapidity region in p-p and p-α, and α-α collisions are reported. They are better fitted to the “wounded nucleon” than to the “gluon string” model. The average transverse momenta, for all three reactions, are identical (and almost independent of multiplicity) up to very high multiplicities.

Production of prompt positrons at low transverse momentum in 63 GeV pp collisions at the CERN intersecting storage rings

Abstract The production of inclusive prompt positrons in pp collisions at √ s = 63 GeV and θ cm = 90° has been measured over the p T interval 0.08 T GeV c . The e / π ratio rises from ∼ 2 × 10 −4 at p T = 0.9 GeV c to ∼ 1 × 10 −3 at p T = 0.1 GeV c . This is a factor of 6 higher than would be expected from the sum of hadronic bremsstrahlung and the semileptonic decay of charm.