Main results of a search on multiplicity distributions in pp collisions is anybody afraid o.pdf

a r X i v : h e p - p h / 0 4 1 0 2 0 2 v 1 1 4 O c t 2 0 0 4 Main results of a search on multiplicity distributions in pp collisions: is anybody afraid of a new class of hard events?? R. Ugoccioni and A. Giovannini Dipartimento di Fisica Teorica and INFN - Sezione di Torino Via P. Giuria 1, 10125 Torino, Italy After an introduction on possible scenarios in the TeV energy region in pp collisions, extrapolated from the knowledge of the GeV energy region, attention is focused on the onset of a third class of events, harder than the semi-hard and soft ones identified at SpS and Tevatron. The expected features and signatures in multiplicity fluctuations, forward-backward cor- relations and collision energy density are discussed. PACS numbers: 13.85.Hd 1. Introduction The weighted superposition mechanism (WSM) of two properly defined classes of events explains several experimental facts [1] (shoulder structure in Pn vs n, quasi-oscillatory behaviour of Hq ≡ Kq/Fq vs q, energy dependence of forward-backward multiplicity correlations) which altogether characterise collective variables properties in high energy pp collisions and e+e? annihi- lation. In pp collisions the two classes of events are the soft one (without mini-jets) and the semi-hard one (with mini-jets); in e+e? annihilation one distinguishes between two-jet and three-jet samples of events. It should be pointed out that the qualifying assumption of the WSM is that the multiplicity distribution (MD) is described for each class of events in terms of the Pascal, i.e., negative binomial (NB), MD; its characteris- tic parameters are n?, i.e., the average charged particle multiplicity, and k (linked to the variance D2 ≡ 〈n2〉 ? n?2 by the relation k = n?2/(D2 ? n?).) This assumption leads to a sound description of the experimental data. The NB (Pascal) MD is well known in high energy physics and has been justified in the framework of QCD. ? Presented at XXXIV INTERNATIONAL SYMPOSIUM ON MULTIPARTICLE DY- NAMICS, S