Study of Diquark Fragmentation at the CERN Intersecting Storage Rings

Abstract

Forward particle production in pp interactions triggered by a 30° pion of momentum exceeding 5 GeV/c has been studied at cm. energy of 63 GeV. Quantum-chromodynamic model calculations show that in a majority of such interactions the incident proton loses a valence quark. Results indicate that hadronization of diquarks is primarily a recombination process leading to the formation of a high-* baryon.PACS numbers: 13.80.Kf, 12.40.CcThe fragmentation of quarks has been extensively studied in e*e~ annihilation as well as in hadron collisions and deep-elastic lepton production experiments. Data on the "diquark" system which remains when a valence quark is ejected from a nucleon is, however, considerably more limited. 1 In order to investigate the mechanism of diquark hadronization we have undertaken a study of small-angle hadron production in 63-GeV (cm. energy) pp collisions triggered by a high-momentum pion at 30° in the same hemisphere. In this experiment the forward fragments were identified by Cerenkov counters. Previous studies along this line 2 ' 3 have been handicapped by the absence of forward-particle identification.In the naive quark-par ton model, 4 the trigger pion is a fragment of a quark ejected in a hardscattering process, as shown in Fig. 1(a). For a pion ofp T <; 2.5 GeV/c, the parent is likely to be a valence quark with x Bj^0 .2, implying the production of a forward diquark. Moreover, the charge of the trigger pion should reflect the flavor of the quark {u for 7r + and d for 7r"), and thereby give a clue to the flavor content of the diquark (ud or uu). A calculation based on the more realistic quantum-chromodynamics-(QCD-) inspired model of Feynman, Field, and Fox 5 indicates that hard gluons are a sizable source of triggers. Table I shows the fractional parton composition for 2ip T =3 GeV/c TT* trigger given by this calculation. 6 a) TRIGGER p= \/s/2 MESONS FIG. 1. Parton diagrams for (a) quark-quark elastic scattering, (b) diquark hadronization by fast baryon formation, and (c) diquark hadronization by fast meson formation. 398