De hoge energie fysica, die de wereld van de elementaire deeltjes bestudeert, heeft met experimenten in de loop der jaren veel nieuwe inzichten verschaft. Hierdoor is het zogenoemde Standaard Model ...opgesteld, dat de elementaire deeltjes en hun wisselwerkingen beschrijft. Ondanks hetsucces van dit model blijven er nog veel vragen onbeantwoord. Zo wordt het bestaan van een vooralsnog onontdekt Higgs deeltje voorspelt en beschrijft het model niet alle aspecten van de elementaire deeltjes. Het Atlas experiment bij de LHC deeltjesversneller nabij Geneve, Zwitserland, zal binnenkort opstarten. Dit experiment zal antwoord geven op de vraag of het Higgs deeltje bestaat - of niet. Mogelijk worden er nog veel meer nieuwe vormen van materie gevonden en krijgen we grip op nog een aantal onbeantwoorde vragen van het Standaard Model.
Jet clustering algorithms are widely used to analyse hadronic events in high energy collisions. Recently a new clustering method, known as ‘Cambridge’, has been introduced. In this article we present ...an algorithm to determine the transition values of for this clustering scheme, which allows to resolve any event to a definite number of jets in the final state. We discuss some particularities of the Cambridge clustering method and compare its performance to the Durham clustering scheme for Monte Carlo generated annihilation events.
Gluon jets are identified in hadronic Z decays as all the particles in a hemisphere opposite to a hemisphere containing two tagged quark jets. Gluon jets defined in this manner are equivalent to ...gluon jets produced from a color singlet point source and thus correspond to the definition employed for most theoretical calculations. In a separate stage of the analysis, we select quark jets in a manner to correspond to calculations, as the particles in hemispheres of flavor tagged light quark (uds) events. We present the distributions of rapidity, scaled energy, the logarithm of the momentum, and transverse momentum with respect to the jet axes, for charged particles in these gluon and quark jets. We also examine the charged particle multiplicity distributions of the jets in restricted intervals of rapidity. For soft particles at large , we observe the charged particle multiplicity ratio of gluon to quark jets to be , in agreement with the prediction that this ratio should approximately equal the ratio of QCD color factors, . The intervals used to define soft particles and large for this result, GeV/ and GeV/, are motivated by the predictions of the Herwig Monte Carlo multihadronic event generator. Additionally, our gluon jet data allow a sensitive test of the phenomenon of non-leading QCD terms known as color reconnection. We test the model of color reconnection implemented in the Ariadne Monte Carlo multihadronic event generator and find it to be disfavored by our data.
The energy distribution and type of the particle with the highest momentum in quark jets are determined for each of the five quark flavours making only minimal model assumptions. The analysis is ...based on a large statistics sample of hadronic \(\rm Z^0\) decays collected with the OPAL detector at the LEP \(\rm e^+e^-\) collider. These results provide a basis for future studies of light flavour production at other centre-of-mass energies. We use our results to study the hadronisation mechanism in light flavour jets and compare the data to the QCD models JETSET and HERWIG. Within the JETSET model we also directly determine the suppression of strange quarks to be by comparing the production of charged and neutral kaons in strange and non-strange light quark events. Finally we study the features of baryon production.
In order to facilitate access to the large volumes of data (multiple petabytes per year) which will be produced during data taking and Monte Carlo production at ATLAS, work has proceeded on building ...a system of event-level metadata to allow selections of a subset of events to use as input to an analysis. This was included in the ATLAS Computing Model and was first studied and implemented by the Physics Analysis Tools group based on the decisions of the ESD/AOD Task Force. They used tools developed and supported by the CERN IT group and the ATLAS Database group. During 2005 this structure was put through various tests and evaluations. Also, work by physicists on reconstruction and analysis led to an improved understanding of the requirements on the TAG. This report addresses the effect of these new inputs on the previous work with regard to content and the infrastructure needed to support it.
The total hadronic cross-section \(\sigma_{\gamma \gamma }(W)\) for the interaction of real photons, \(\gamma\gamma\rightarrow\) hadrons, is measured for \(\gamma\gamma\) centre-of-mass energies ...\(10\le W \le 110\) GeV. The cross-section is extracted from a measurement of the process \({\rm e}^+{\rm e}^-\rightarrow{\rm e}^+{\rm e}^-\gamma^*\gamma^*\rightarrow{\rm e}^+{\rm e}^-+\) hadrons, using a luminosity function for the photon flux together with form factors for extrapolating to real photons (\(Q^2=0\) GeV\(^2\)). The data were taken with the OPAL detector at LEP at \({\rm e}^+{\rm e}^-\) centre-of-mass energies \(\sqrt{s}_{\rm ee}=161\), 172$ and 183 GeV. The cross-section \(\sigma_{\gamma\gamma}(W)\) is compared with Regge factorisation and with the energy dependence observed in \(\gamma\)p and pp interactions. The data are also compared to models which predict a faster rise of \(\sigma_{\gamma\gamma}(W)\) compared to \(\gamma\)p and pp interactions due to additional hard \(\gamma\gamma\) interactions not present in hadronic collisions.
Cross-sections for hadronic, and lepton pair final states in collisions at =183 GeV, measured with the OPAL detector at LEP, are presented and compared with the predictions of the Standard Model. ...Forward-backward asymmetries for the leptonic final states have also been measured. Cross-sections and asymmetries are also presented for data recorded in 1997 at =130 and 136 GeV. The results are used to measure the energy dependence of the electromagnetic coupling constant , and to place limits on new physics as described by four-fermion contact interactions or by the exchange of a new heavy particle such as a leptoquark, or of a squark or sneutrino in supersymmetric theories with R-parity violation.
Cross-sections and angular distributions for hadronic and lepton pair final states in \({\rm e^+e^-}\) collisions at a centre-of-mass energy near 189 GeV, measured with the OPAL detector at LEP, are ...presented and compared with the predictions of the Standard Model. The results are used to measure the energy dependence of the electromagnetic coupling constant \(\alpha_{\rm em}\), and to place limits on new physics as described by four-fermion contact interactions or by the exchange of a new heavy particle such as a sneutrino in supersymmetric theories with R-parity violation. A search for the indirect effects of the gravitational interaction in extra dimensions on the \(\mu^+\mu^-\) and \(\tau^+\tau^-\) final states is also presented.
Bose-Einstein correlations in pairs of identical charged pions produced in a sample of 4.3 million Z\(^0\) hadronic decays are studied as a function of the three components of the momentum ...difference, transverse (“out” and “side”) and longitudinal with respect to the thrust direction of the event. A significant difference between the transverse, r\(_{t_{side}}\), and longitudinal, r\(_l\), dimensions is observed, indicating that the emitting source of identical pions, as observed in the Longitudinally CoMoving System, has an elongated shape. This is observed with a variety of selection techniques. Specifically, the values of the parameters obtained by fitting the extended Goldhaber parametrisation to the correlation function ${\mathrm C'} ={\mathrm C^{DATA}}/{\mathrm C^{MC}}$ for two-jet events, selected with the Durham algorithm and resolution parameter y\(_{cut}\) = 0.04, are r\(_{t_{side}}\) = (0.809 \(\pm\) 0.009 (stat) \(^{+0.019}_{-0.032}\) (syst)) fm, r\(_l\) = (0.989 \(\pm\) 0.011 (stat) \(^{+0.030}_{-0.015}\) ({\it syst})) fm and r\(_l\)/r\(_{t_{side}}\) = 1.222 \(\pm\) 0.027 (stat) \(^{+0.075}_{-0.012}\) (syst). The results are discussed in the context of a recent model of Bose-Einstein correlations based on string fragmentation. The results of a unidimensional analysis are also presented.
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