A. Di Ciaccio

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.

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.

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.

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.

A comparison of pp and pp interactions in the central region at √s=53 GeV

Abstract The inclusive production cross sections of pions, kaons, protons, and antiprotons in p p and pp interactions at √ s =53 GeV are compared in the kinematic range | y | p T c . We observe differences in the K + /K − and p /p ratios for the two data samples. Although the bulk of the particles are produced with the same momentum and rapidity distributions in p p and pp collisions, we observe difference in these distributions for produced protons and antiprotons.

A study of charged particles associated with high-pT photons and pions

We report on the distributions of charged particles produced in association with high p/sub T/ prompt photons and neutral pions in pp collisions at ..sqrt.. s = 63 GeV and theta = 90/sup 0/ in the centre of mass. We find photons to be relatively unaccompained by other particles at small angles with respect to the photon, and thereby extract a measurement of the fraction of prompt photons due to bremsstrahlung production. The relative density of positive to negative particles in the system recoiling from the photon has been measured and compared with QCD motivated calculations.

Hadron and hadron-pair production at large transverse momentum in pp and pp interactions at the ISR

We have measured the production of one and two large transverse momentum hadrons in pp and pp interactions in the range 2 < pT < 6 GeV/c for the central rapidity region |y| < 0.9 at s = 63 and 31 GeV. No statistically significant difference between pp and pp collisions is observed. The results are in accordance with lowest order QCS perturbative calculations and rule out a large contribution of Constituent Interchange Model (CIM), di-quark of quark-fusion subprocesses in this kinematic range.

High-pT direct photon production at 11° in pp collisions at s = 63 GeV

Abstract The production of direct photons has been measured relative to π 0 s in the rapidity range 2.00 y s = 63 GeV at the CERN Intersecting Storage Rings. The γ/π 0 ratio increases from ⪅2% at p T = 1.5 GeV/ c to ∼8% at p T = 4.25 GeV/ c , similar to the value observed near 90°. The results indicate no strong enhancement of single-photon production due to quark bremsstrahlung in this kinematic region.

Production of π0 andη0 at 11° inpp Collisions at\(\sqrt s = 63 GeV\)

AbstractThe inclusive π0 production cross-section and the η/π0 ratio have been measured inpp collisions atn