Measurement of the Charged-Particle Pseudorapidity Distribution in Proton-Antiproton Collisions at 630 GEV Center-Of Energy.

The pseudorapidity density of the charged particles (dN_{ch}/deta) produced in proton-antiproton collisions at a center-of-mass energy of 630 GeV is determined with precision comparable to that of the best previous measurement (UA5 collaboration) in the range 1.5 <eta< 5. The data were collected by the P238 collaboration at the CERN SPS -Collider using a silicon microvertex detector. The UA5 result, obtained by a different method, is confirmed for pseudorapidity values between 2 and 4. For eta> 4 P238 finds a larger charged-particle multiplicity per unit of pseudorapidity than UA5 and for eta< 2 the P238 dN_{ch}/d{ eta} curve lies below that of UA5. Although dN_{ch}/d{eta} cannot be predicted from QCD first principles, the measured pseudorapidity density is in good agreement with that given by the popular Lund model of high-energy hadronic interactions.