Particle Rates

In the current picture of hadron production, factorisation plays a predominant role in the different evolution stages of the process: `preparation' of the primary partonic configuration, additional parton production described e.g. by (angular-ordered) parton showers (pQCD), hadron formation described e.g. by string or cluster fragmentation (non-perturbative QCD), secondary hadronic cascade-decays (QFD and non-perturbative QCD). To test such an ansatz, a comparative study of jet properties--including particle rates and spectra--in different reactions is required. At present this has not yet become a topic of primary interest at hadron-hadron colliders and HERA in spite of its importance for QCD.

How much remains to be done is illustrated in [4] where the impressive results from the LEP experiments are updated and reviewed. As far as hadron production is concerned, 38 different inclusive production rates of mesons and baryons are now measured at the Z⁰ and, for many of these, inclusive spectra are available.

In all, good agreement is observed for the rates with tuned versions of JETSET 7.4 [5] and HERWIG 5.9 [6]. A noteworthy exception is the baryon sector which remains an embarrassment for HERWIG. Either a better retuning or a critical re-evaluation of the cluster decay model seems required. The Lund JETSET string approach fares better but contains a large number of parameters related to flavour and spin. Since this number is increasing with time, little real predictive power is left.

In e⁺e^- annihilation at LEP, evidence for breaking of `jet universality' and factorisation may have been found from excess $\eta$ production--above JETSET expectations--at large momentum in three-jet events [7] (glueball production or surplus iso-singlet hadrons?) while no anomaly is seen for $\pi^0$ production. It could be that the long-awaited direct manifestation of gluon jet fragmentation has finally been found [8]. If so, even larger discrepancies could be expected for the $\eta'$ [9]. The f₀(975) and a₀(980) mesons could also play a special role in the dynamics of quark confinement  [10]. A comparative study of these and other hadrons in quark and in gluon jets is called for.

Problems also appear with strangeness production (mainly K and $\Lambda$)where DELPHI notes a deficit of strange particles in extreme two-jet events [11]: the production of strangeness depends on the event topology in a manner that is not quantitatively described by JETSET.

Although HERA experiments have only started to investigate the field so thoroughly explored at LEP, and information on identified particles is still scarce, first evidence has been found that the level of strangeness production in DIS and photoproduction, translated into a s/u relative production rate is close to 0.2, to be compared with 0.3 in e⁺e^- [12].

Lessons to be learned from the vast amount of data in e⁺e^- annihilations at the Z⁰ are that deviations from `universal fragmentation' may well have been observed and that the topology of the confining QCD fields is likely to play a role in hadroproduction. The rich variety of such topological configurations possible in ep collisions poses a real challenge for the experimentalists.