Sergio Lupia

Unified QCD description of hadron and jet multiplicities

Abstract The evolution equation for parton multiplicities in quark and gluon jets which takes into account the soft gluon interference and energy conservation is solved numerically using the initial conditions at threshold. If the k⊥-cutoff Qc is lowered towards the hadronic scale Q0 of a few hundred MeV, the jets are fully resolved into hadrons. Both hadron and jet multiplicities in e+e− annihilation are well described with a common normalization. Evidence is presented within this perturbative approach that the coupling αs(k⊥) rises by an order of magnitude when approaching the low energy region. The ratio of hadron multiplicities in gluon and quark jets is found smaller than in previous approximate solutions of the evolution equation.

Properties of factorial cumulant to factorial moment ratio

Abstract It is shown that the ratio of factorial cumulant moments to factorial moments for a multiplicity distribution truncated in the tail reveals oscillations in sign similar to those observed in experimental data. It is suggested that this effect be taken into account in the analysis of data in order to obtain correct physical information on the multiplicity distributions.

Common origin of the shoulder in multiplicity distributions and of oscillations in the factorial cumulants to factorial moments ratio

Abstract The shoulder structure of charged particles multiplicity distributions (MDs) in full phase space in e + e − annihilation at the Z 0 peak and the quasi-oscillatory behavior of the ratio of factorial cumulants over factorial moments, H q , as a function of the order q , are quantitatively reproduced within a simple parametrization of the MD in terms of a weighted superposition of two Negative Binomial Distributions, associated to two- and multi-jet production, i.e., to hard gluons radiation.

QCD coherence and the soft limit of the particle spectrum

Abstract Perturbative QCD predicts an energy independent limit for the density of partons at small momenta in a jet which follows from the colour coherence in soft gluon production. The existing data on the charged and identified particle inclusive spectra in e + e − annihilation follow closely the perturbative prediction for partons which is in support of the local parton hadron duality picture also for low momentum particles. Further tests of this interpretation of the data are proposed.

POISSONIAN LIMIT OF SOFT GLUON MULTIPLICITY

Abstract It is shown that the gluons produced with small transverse momenta k ⊥ inside a jet ( k ⊥ k ⊥ cut , k ⊥ cut → 0) are independently emitted from the primary parton, as QCD coherence suppresses their showering. Consequently, the low k ⊥ gluons follow a Poisson distribution, very much like the soft photons radiated by a charged particle in QED. On the contrary, the distribution of gluons with limited absolute momenta remains non-Poissonian even for small k cut . It 0750 V 3 will be interesting to find out to what extent this perturbative prediction for partons survives the hadronization process.

Testing the running of the QCD coupling in particle energy spectra

The spectra of charged particles in the energy variable ξ = log 1x (“the hump backed plateau”) are analyzed in terms of moments in order to investigate the sensitivity to the running of αs in particular towards small scales, and to further test thereby the local parton hadron duality picture. The modified leading log approximation (MLLA) of QCD provides a very satisfying description of the e+e− data in the full range of c.m. energies from about 1 up to 91 GeV. The same calculation but with fixed coupling αs yields moments which are inconsistent with the data already close to threshold. In particular the soft particles (E≤1 GeV) reflect directly the running of αs.

The negative binomial distribution in quark jets with fixed flavour

Abstract We show that both the multiplicity distribution and the ratio of factorial cumulants over factorial moments for 2-jet events in e+e− annihilation at the Z0 peak can be well reproduced by the weighted superposition of two negative binomial distributions, associated to the contribution of b b and light flavoured events respectively. The negative binomial distribution is then suggested to describe the multiplicity distribution of 2-jet events with fixed flavour.

Perturbative description of particle spectra at LEP-1.5

Abstract The recent data from LEP-1.5 on charged particle spectra are analyzed within the analytical QCD approach.

Multiplicity distributions in a thermodynamical model of hadron production ine + e − collisions

Predictions of a thermodynamical model of hadron production for multiplicity distribution ine+e− annihilations at LEP and PEP-PETRA centre of mass energies are shown. The production process is described as a two-step process in which primary hadrons emitted from the thermal source decay into final observable particles. The final charged tracks multiplicity distributions turn out to be of Negative Binomial type and are in quite good agreement with experimental observations. The average number of clans calculated from fitted Negative Binomial coincides with the average number of primary hadrons predicted by the thermodynamical model, suggesting that clans should be identified with primary hadrons.

Low and High Energy Limits of Particle Spectra in QCD Jets

Abstract. Charged particle energy spectra in