M. Le Bellac

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.

The Role of Soft Gluons in Lepton Pair Production

Abstract We derive expressions for the exponentiation of soft gluon effects in lepton pair production and discuss their phenomenological implications for the rapidity and Q 2 dependence of the so-called “K- factor ” . Predictions are given for scaling violations to be seen in the determination of the pion structure function.

Structure of two-body correlations in high energy hadron production

Abstract Experimental information on diffraction is used to estimate long range effects in multiparticle production. Diffractively produced secondaries are found to produce a plateau in the central region. Diffraction accounts for 30% of the observed correlations and the remainder, consistent with being short range, corresponds to a cluster size of 2.4 charged particles.

POMERON DOMINANCE AND INCLUSIVE MULTIPARTICLE CROSS-SECTIONS.

Abstract We show that it is not possible to assume that both the N -particle inlusive cross-sections and the two-body elastic cross-section are dominated by the exchange of an isolated, factorizable Pomeron.

Mass singularities at finite temperature in a scalar field theory

Using as a model the φ3 field theory in space-time dimensionD=6, we show the validity of the Kinoshita-Lee-Nauenberg theorem at finite temperature to first order in the coupling constant.

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.

Renormalization of external lines in relativistic field theories at finite temperature

We try to generalize the notion of wave-function renormalization at finite temperature. In some limiting, but physically important cases, it is indeed possible to define a (momentum-dependent) wave-function renormalization “constant”, to be associated with external lines in physical process. We examine the limitations of this approach in other cases.

Inclusive spectra in single- and double-excitation models

SummaryThe inclusive pion spectra in p-p collisions predicted by the single-excitation (or Nova) model and the double-excitation model are discussed. The description is given in terms of the rapidityy, which facilitates the comparison of the two models. Whereas in the limit of infinite energies the two models may be shown to have the same form, their predictions at accelerator and even at ISR energies differ appreciably. In particular we discuss the question of how the scaling limit in the central region is reached, and which factors in the model govern the approach. The problem of enforcing a cut-off in momentum transfer is examined, and it is found that the effect of such a cut-off is to cause a dip in the central plateau. However the effect is much more severe for the double-excitation model than for the single-excitation one. Furthermore the contribution of the two-fireball model to the inelastic cross-section is found to increase considerably between accelerator and ISR energies, to such an extent that it should be the dominant process at very high energies.RiassuntoSi discutono gli spettri pionici inclusivi nelle collisioni p-p predetti dal modello di singola eccitazione (o Nova) e dal modello di doppia eccitazione. Si espone la discussione in termini della rapiditày, che facilita il confronto dei due modelli. Mentra si può dimostrare che nel limite delle energie infinite i due modelli hanno la stessa forma, le loro predizioni alle energie degli acceleratori ed anche degli ISR differiscono in modo apprezzabile. In particolare si discute la questione di come si raggiunga il limite di scala nella regione centrale e quali fattori del modello regolino l’approccio. Si esamina il problema di imporre un taglio nell’impulso trasferito, e si trova che l’effetto di questo taglio è di causare un abbassamento nel ripiano centrale. Però l’effetto è molto più forte nel modello di doppia eccitazione che in quello di singola eccitazione. Inoltra si trova che il contributo del modello delle due sfere di fuoco alla sezione d’urto anelastica aumenta considerevolmente tra le energie degli acceleratori e quelle degli ISR, per un tale importo che esso dovrebbe essere il processo predominante alle alte energie.--РеэюмеОбсуждаются включаюшие пионные спектры в р-р соударениях, которые предскаэываются на основе модели синглетного воэбуждения и модели дублетного воэбуждения. Формалиэм эаписывается в терминах скорости у, что облегчает сравнение зтих двух моделей. Тогда как можно покаэать, что в пределе бесконечных знергий зти две модели имеют одинаковую форму, но их пред-скаэания при ускорительных знергиях, и даже при ISR эаметно отли-чаются. В частности, мы обсуждаем вопрос, как достигается предел подобия в центральной области и какие факторы модели определяют зтот подход. Исследуется проблема введения обреэания по передаваемому импульсу. Получается, что зффект такого обреэания должен приводить к провалу в центральном плато. Однако зтот зффект является более сушественным для модели дублетного воэбуждения, чем для модели синглетного воэбуждения. Кроме того, получается, что вклад модели двух файрболов в неупругое поперечное сечение увеличивается между ускоритель-ными знергиями и ISR знергиями до такого раэмера, что должен стать домини-руюшим процессом при очень высоких знергиях.

Diffractive models and high multiplicities.

Abstract We show that diffractive models in their present form cannot fit simultaneously the multiplicity distribution at high energy and the inclusive one-particle cross-action in the central region.