F. Carbonara

Photofission of U and Th between 300 and 1000 MeV

Abstract Loaded nuclear emulsions have been exposed to the bremsstrahlung beam of the Frascati 1 GeV electron synchrotron to measure natural uranium and thorium fission cross-sections in the energy range 300–1000 MeV. The cross-sections “per photon” are constant within the experimental errors, and amount to 67±7 and 37±4 mb, for U and Th, respectively. These results are briefly discussed.

Analysis of γn→nπ+π− and γn→pπ−π0 reactions from threshold up to 1 GeV

SummaryThe final results of a bubble chamber experiment on double pion photoproduction in deuterium, from threshold up to 1 GeV, are reported. The total cross-sections of reactions γn→nπ−π+, γn→pπ−π0 and γp→pπ+π− are calculated, by using the spectator model. The squared-mass distributions have been fitted, and the cross-sections of reactions γn→Δ−π+, γn→Δ+π−→pπ0π− and γp→Δ++π− have been obtained. The general features of these cross-sections are consistent with a large nonresonant Δ production, accounted for in a gauge-invariant OPE model.RiassuntoSi riportano i risultati finali di un esperimento in camera a bolle di fotoproduzione doppia di pioni su deuterio, dalla soglia fino a 1 GeV. Si sono calcolate, facendo uso del modello del nucleone spettatore, le sezioni d’urto totali delle reazioni γn→nπ−π+, γn→pπ0π− e γp→pπ+π−. Sono state aggiustate le distribuzioni di massa quadrata e sono state ottenute le sezioni d’urto delle reazioni γn→Δ−π+, γn→Δ+π−→Pπ0π− e γp→Δ++π−. Le caratteristiche generali di queste sezioni d’urto sono consistenti con un’abbondante produzione di isobaro non risonante Δ, in accordo con un modello OPE invariante per trasformazioni di « gauge ».РеэюмеПриводятся реэультаты зксперимента на пуэырьковой камере по фоторождению двух пионов на дейтерии от порога до 1 ГзВ. Вычисляются полные поперечные сечения реакций γn→nπ−π+, γn→pπ0π− и γp→pπ+π−. Подгоняются распределения для квадратов масс. Определяются поперечные сечения реакций γn→Δ−π+, γn→Δ+π−→Pπ0π− и γp→Δ++π−. Обшие характеристики зтих поперечных сечений согласуются с интенсивным нереэонансным реждением Δ, которое общяс-няется в калибровочно инвариантной модели с однопионным обменом.

Associated hadronic production in μ-pair events at the CERN Intersecting Storage Rings

Abstract We have measured at √s=62GeV, at the CERN Intersecting Storage Rings, the associated charged hadrons with massive μ pairs, employing a vertex detector of ∼ 4π coverage. The experiment has provided the first data on hadron multiplicity, angular distributions, and correlations in lepton pair events at ISR energies. The average associated charged multiplicity 〈nch〉 is found to increase as the invariant mass of the hadronic system Mh increases, similarly to the observed √s dependence in e+e− annihilation into hadrons at high energies. The multiplicity also increases with pT in the hemisphere opposite to the transverse momentum pT of the dimuon. Small clusters appear to be produced, whose correlation with the μ-pair can be explained in a simple Monte Carlo model incorporating the production of a weakly collimated jet of low multiplicity, recoiling against the dimuon.

Associated multiplicities in μ-pair events at the ISR

SummaryA study of the dependence of the charged hadronic multiplicity 〈nch〉 in μ-pair events on the dynamic parameters of the dimuon (dimuon massm and transverse momentumpT, and total hadronic energyMh) is presented for proton-proton collisions at √s=63 and 44 GeV. The full data collected in the R209 experiment at the ISR is used. With respect to already published data at √s=63 GeV, the present work has about a factor of four more statistics and includes data at two energies. In addition, we have corrected the data for background and we present absolute multiplicities. J/ψ events (m<4.5 GeV/c2) and Drell-Yan events (m>4.5 GeV/c2) are studied separately, since the production dynamics is expected to be different in the two cases. 〈nch〉 is found to decrease slowly with increasingm and to be rather insensitive toMh. On the other hand, 〈nch〉 grows withpT in a different way in J/ψ and Drell-Yan events. The effect might be indicative of a low-multiplicity jet recoiling against the dimuon in J/ψ events. A model analysis of thepT distribution of the pair in the transverse event plane is reported, showing that a possible contamination of μ-pairs from cnn

PRODUCTION DYNAMICS OF HIGH-MASS MUON PAIRS

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Performance of the L3 Precision Muon Spectrometer in the First Year of Run at LEP

nn semileptonic decays should be small at all masses.RiassuntoSi presenta uno studio nell’urto protone-protone a √s=63 e 44 GeV della dipendenza della molteplicità carica adronica, in eventi di produzione di coppie di μ, da parametri dinamici del dimuone (massam e impulso trasversopT del dimuone, ed energia totale adronicaMh). Sono utilizzati tutti i dati raccolti nell’esperimento R209 agli ISR. In confronto ai dati già pubblicati a √s=63 GeV, questo lavoro ha una statistica circa quattro volte maggiore e comprende dati a due energie. In piú, abbiamo sottratto il fondo dai dati e presentiamo molteplicità assolute. Vengono studiati separatamente gli eventi J/ψ (m<4.5 GeV/c2), e gli eventi Drell-Yan (m>4.5 GeV/c2), poiché ci si aspetta che la dinamica di produzione sia diversa nei due casi. Si trova che 〈nch〉 diminuisce lentamente al crescere dim ed è sostanzialmente insensibile aMh. D’altro canto, 〈nch〉 aumenta conpT in modo diverso negli eventi J/ψ e Drell-Yan. L’effetto potrebbe stare ad indicare l’esistenza di un getto di bassa molteplicità rinculante contro il dimuone. Si descrive un’analisi modellistica della distribuzione inpT della coppia nel piano trasverso dell’evento, la quale mostra che una possibile contaminazione di coppie di μ da decadimento semileptonico di cnn

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