Gian Piero Siroli

We have searched for lepton flavour violating Z0→eμ, Z0→eτ and Z0→μτ decays in a sample of 4.0×106 visible Z0 decays collected with the OPAL detector at LEP during 1991 to 1994. No candidates are found for Z0→eμ. The samples of selected Z0→eτ and Z0→μτ candidates are consistent with the expected background. The following limits are set at 95% confidence level: $$\begin{gathered} BR(Z^0 \to e\mu ){\text{ }}< {\text{ 1}}{\text{.7 }} \times {\text{ 10}}^{ - {\text{6}}} \hfill \\ BR(Z^0 \to e\tau ){\text{ }}< {\text{ 9}}{\text{.8 }} \times {\text{ 10}}^{ - {\text{6}}} \hfill \\ BR(Z^0 \to \mu \tau ){\text{ }}< {\text{ 17}}{\text{. }} \times {\text{ 10}}^{ - {\text{6}}} . \hfill \\ \end{gathered}$$

The relative yield of high transverse momentum protons measured inpp interctions at the CERN ISR is calculated in the framework of simple parton models. Whereas models based exclusively on quark and gluon scattering fail to describe the data, the hypothesis of diquark scattering provides a quantitative understanding of the measured dependences on transverse momentum, polar angle, and centre-of-mass energy. Different assumptions on diquark structure functions are tested.

In a high-statistics experiment at the CERN-ISR we analyze the variation of the average transverse momentum in fixed rapidity intervals as a function of the multiplicity density of charged particles produced in proton-proton collisions at √s = 31, 44 and 62 GeV. The average transverse momentum depends on both rapidity y and charged multiplicity density, ρ = Δn/Δy. The dependence on ρ changes gradually from an increase at low y, constancy at intermediate y and decrease at large y.

Events obtained by triggering on a single particle with high transverse momentump T show a four-jet structure. The two transverse jets are due to the fragmentation of point-like scattered partons. Experimental evidence is presented that highp T K − mesons and their associated jets are produced by hard scattering and subsequent fragmentation of flavour neutral partons with a rather soft structure function. Hence, gluons are a natural source of highp T K − mesons which do not share any valence quark with the incoming protons. The analysis is based upon measurements of short-range quantum number correlations within the trigger jet and of long-range correlations between different jets. The data were obtained at $$\sqrt s$$ =62 GeV GeV with the Split Field Magnet Detector (SFM) at the CERN ISR.

Two drift tubes (DTs) chambers of the CMS muon barrel system were exposed to a 40 MHz bunched muon beam at the CERN SPS, and for the first time the whole CMS Level-1 DTs-based trigger system chain was tested. Data at different energies and inclination angles of the incident muon beam were collected, as well as data with and without an iron absorber placed between the two chambers, to simulate the electromagnetic shower development in CMS. Special data-taking runs were dedicated to test for the first time the Track Finder system, which reconstructs track trigger candidates by performing a proper matching of the muon segments delivered by the two chambers. The present paper describes the results of these measurements.

The convergence of telecommunication and computer technologies that has evolved in the field of information and communication technologies (ICT) in the last two decades has had very important effects on new war technologies and the ongoing process of battlefield digitisation. The Stuxnet worm, uncovered in 2010 and responsible for the sabotaging of a uranium enrichment infrastructure in Iran, is a clear example of a digital weapon. The incident shows what is meant by cyber war and what the particular features of this new warfare dimension are compared to the conventional domains of land, sea, air and space, with relevance both at the operational and strategic levels. But cyberspace also extends to the semantic level, within the complimentary field of information warfare involving the content of messages flowing through the Internet for the purposes of propaganda, information, disinformation, consensus building, etc. The overall cyber warfare domain needs to be put into perspective internationally as many countries are developing strong cyber capabilities and an ‘arms race’ is already taking place, showing that these technologies can potentially be used to undermine international stability and security. What is needed is a public debate on the topic and its impact on global stability, and some kind of regulation or international agreement on this new warfare domain, including an approach involving confidence building measures (CBMs).

Cross-sections and angular distributions for the production of events with single and multiple photons are measured from data recorded with the OPAL detector at the recently upgraded LEP collider. The measured cross-sections are generally consistent with Standard Model expectations for the e+e−→ νvγ(γ) and e+e− → γγ(γ) processes. Six events with an acoplanar photon pair and large missing mass are found. The observed number of events is larger than expected from e+e−→ ννγγ; however, the missing mass distribution is compatible with the Z0 resonance. Deviations from QED are constrained by the data on e+e− → γγ(γ). Lower limits are set at 95% confidence level on the QED cut-off parameters Λ+ and Λ− of 152 GeV and 142 GeV, respectively, and also on the mass of an excited electron of 147 GeV.

The contribution of single diffraction dissociation to highp T particle production has been determined with the split field magnet detector. For transverse momentap T >2 GeV/c and production angles θ>10° it is found to be less than 5% for all types of trigger particles. Specifically, a diffractive origin of the large production cross section for highp T protons can be excluded. The contributions found to highp T production are found to be in good agreement with extrapolations of the relative particle yields from single diffraction dissociation observed in inelastic minimum bias events.

The production of ρ(770)0,K *(892)0, $$\bar K^* (892)^0 $$ andf 2(1270) mesons as leading particles in hadron jets (<z>∼0.7) has been measured in deep inelasticpp interactions at the CERN ISR at $$\sqrt s = 62$$ GeV. The comparison of vector and pseudoscalar meson production at the same transverse momentum provides a rather model independent way to determine the fragmentation parameterV/P. We determine(V/P) u,d =1.66±0.18±0.63 from the ρ/π ratio and(V/P) s =0.90±0.13±0.27 from theK */K ratio. For thef 2(1270) production we findf 2/ρ0=0.30±0.08.

Production of jets with fast leading fragments has been studied in deep inelastic proton-proton interactions using the Split Field Magnet detector at the CERN ISR. The kinematics of the underlying parton processes is determined on an event-by-event basis. Parton scattering amplitudes are extracted for scattering angles, Θ∗, in the interval −0.4 < cos Θ∗ < 0.9. The data which are known to be dominated by quark gluon scattering, agree with predictions from lowest order QCD, while e.g. an abelian theory can be excluded.

The CMS muon barrel drift tubes system has been recently fully installed and commissioned in the experiment. The performance and the current status of the detector are briefly presented and discussed.

Ratios of inclusive cross sections σ(π+)/σ(π++K ++p) and $$\sigma (\pi ^ - )/\sigma (\pi ^ - + K^ - + \bar p)$$ were measured for proton-proton interactions with a highp T hadron in the final state around c.m.s. scattering angles θ≅20°, 20° and 45° at two ISR energies $$\sqrt s = 31$$ Gev and 62 GeV. Results are shown as functions of transverse and longitudinal momentum and are compared with parton model predictions. The different dependences of positive and negative pion fractions atp T ≅2–3 GeV/c on longitudinal momenta is similar to that observed in soft hadronic interactions at low values ofp T where the leading proton effect (diquark fragmentation) is known to contribute. The quantitative agreement of the data with diquark model predictions indicates the presence of diquark fragmentation also in highp T jets.