G. Plaut

QCD corrections to the Drell-Yan mechanism and the pion structure function

Abstract We compute the differential cross section d σ /d Q 2 d y for lepton pair production up to first order in the strong coupling constant α s . We show that, in the region where the theory applies and where the statistical accuracy of the experiments is good, the ratio of the QCD correction to the Drell-Yan cross section is approximately constant, and that its magnitude is in good agreement with the results of the CERN-NA3 experiment. We thus show that, within the framework of this first-order calculation, the shape of the pion structure function extracted from dilepton production in πp collisions is approximately correct. We also comment on the average transverse momentum of the dilepton due to gluon or quark recoil.

Cluster models against new and old experimental data on multiparticle production

Abstract We analyze in the cluster model framework not only recent data on zones, rapidity gap distributions and charge transfer correlations in multiparticle production, but also data on “older” quantities, in particular on inclusive and semi-inclusive longitudinal correlations. We show that a global analysis of experimental results strongly constrains the cluster characteristics. High intracluster multiplicities are definitely excluded while about 50 to 60% of clusters have to carry an electric charge; these conclusions show that well-known mesonic resonance production may indeed account for a large part of clustering.

Azimuthal correlations in P- or Q-factorized cluster production

Abstract The study of the mean value 〈 p ·1 · p ·2 〉 ( y 1 , y 2 ) in P- or Q-factorized cluster proceduction indicates the presence of mean range azimuthal correlations in multiperipheral (Q-factorized) cluster production, and gives test for the size of the mean transverse momentum of the clusters.

Exchange-degeneracy and absorption in pp → YY

Abstract We study Λ Λ, Λ Σo + cc, and Σ oΣo production in p p scattering with the help of absorbed K∗−K∗∗ Regge pole model with exchange degeneracy, We show that the model fits rather well the cross sections and the polarization data in p p → Λ Λ. A reliable measurement of polarization correlation coefficients should provide a crucial test of the model.

A study of QCD scaling violations by direct resolution of Altarelli-Parisi type equations

Abstract We present a detailed study of scaling violations for non-singlet, singlet and glue distribution functions in the framework of several approximation schemes of QCD. Our formalism consists of direct resolution of the Altarelli-Parisi type equations and leads to a simple exponential form for the function q ( x , Q 2 ). This form is very suitable for the analysis of experimental data and for the exploration of different evolution schemes. In particular, we examine the implications on the QCD scaling parameter Λ and the gluon parameter n G .

Compound poisson distributions and KNO scaling

Abstract We propose to represent the multiplicity distribution by a compound Poisson distribution suggested by unitary or absorptive multiperipheral models. In this framework, we discuss the scaling law proposed recently by Koba, Nielsen and Olesen, and in particular the behaviour of non-asymptotic terms. We show that our model is in very good agreement with the present experimental data.

What semi-inclusive data say about clusters

Abstract Our global analysis of inclusive high-energy multi-particle production data in the cluster model framework is extended to semi-inclusive data. Our cluster model embodies leading particle effects and kinematical constraints, which are shown to be of great importance. It appears that models with light clusters decaying on average into 〈n c 〉 ⋍ 2 charged particles, with a rapidity width δ ⋍ 0.6–0.7 and a distribution much narrower than a Poisson-type, allow one to fit in a nice way both inclusive and semi-inclusive data. It is pointed out that the most constraining semi-inclusive data are those regarding longitudinal correlations, which definitely exclude heavy cluster models, whereas the data on zone characteristics only bear out that a non-negligible percentage of clusters have to carry an electric charge.

QCD versus recent experimental data on lepton pair production at accelerator energies

New and precise experimental results on Drell-Yan pair production at accelerator energies are now available, both in π− andp-nucleus interactions. These data are analyzed within the QCD framework including soft gluon resummation. It is shown that a quantitative agreement of the theory with experimental data can be reasonably managed. In particular, the soft gluon resummation is shown to build up nicely the so-calledK-factor. The pion formfactor is deduced from experimental data, but is rather sensitive to the nuclear structure functions we use. Definite nuclear structure functions are needed.

Charge dependent effects in azimuthal two-particle correlations and cluster production

Abstract It is shown that neutral heavy cluster models provide a natural explanation of the recently observed charged dependent effects in azimuthal correlations between produced pions. This may be interpreted as evidence for identical particle effect and π + π − resonance production in clusters.

Double resonance production and the equal-phase hypothesis

Abstract If all the helicity amplitudes for the reaction 0 − + 1 2 + → J 1 + J 2 , J 2 ⩾ 3 2 have the same phase, it follows that certain bilinear forms in the statistical tensors t L 1 L 2 M 1 M 2 , with L 1 and L 2 even, vanish. An intuitive discussion is given of how these tests of the equal-phase hypothesis arise, their relation to positivity constraints on the joint-density-matrix is studied, and their behavior in the limit of forward production is examined. In order to illustrate the sensitivity of these tests, calculations of helicity amplitudes and the corresponding bilinear forms were made, using reggeized absorption-model calculations for π p → ϱΔ . Some of the available data on π p → ϱΔ , π p → ωΔ and K + p → K ∗ Δ are studied using our tests. Some recent high statistics data on ϱΔ indicate that the equal-phase hypothesis is incorrect for this reaction. A by-product of the investigation is the interesting result that if all the helicity amplitudes are in phase, and the absence of a polarized target, one arbitrarily chosen helicity amplitude can be taken to be zero at an arbitrary value of the kinematic variables, without affecting any observable results.