arXiv subjects
Publications and source records attributed to Daniel Wicke.
The Tevatron experiments CDF and D0 investigated the top quark decay in search of deviations from the Standard Model. Rare processes expected in the Standard Model as well as decays including hypothetical particles of models beyond the Standard Model were investigated. This contribution gives an overview of the studies that were performed with datasets of up to 7.5fb^-1 of integrated luminosity.
The top quark was discoverd at the CDF and D0 experiments in 1995. As the partner of the bottom quark its properties within the Standard Model are fully defined. Only the mass is a free parameter. The measurement of the top quark mass and the verification of the expected properties have been an important topic of experimental top quark physics since. In this review the recent results on top quark properties obtained by the Tevatron experiments CDF and D0 are summarised. At the advent of the LHC special emphasis is given to the basic measurement methods and the dominating systematic uncertainties.
The CDF and D0 experiments have measured single and double top quark production in ppbar collisions at the Tevatron at a centre-of-mass energy of 1.96TeV. The applied methods are used to constrain properties of the top quark and to search for new physics. Several methods of signal to background separation and of the estimation of the background contributions are discussed. Experimental results using an integraged luminosity up to 5.3fb^-1 are presented.
The top quark is the heaviest elementary particle observed to date. Its large mass of about 173 GeV/c^2 makes the top quark act differently than other elementary fermions, as it decays before it hadronises, passing its spin information on to its decay products. In addition, the top quark plays an important role in higher-order loop corrections to standard model processes, which makes the top quark mass a crucial parameter for precision tests of the electroweak theory. The top quark is also a powerful probe for new phenomena beyond the standard model. During the time of discovery at the Tevatron in 1995 only a few properties of the top quark could be measured. In recent years, since the start of Tevatron Run II, the field of top-quark physics has changed and entered a precision era. This report summarises the latest measurements and studies of top-quark properties and gives prospects for future measurements at the Large Hadron Collider (LHC).
The modelling of non-perturbative effects is an important part of modern collider physics simulations. In hadron collisions there is some indication that the modelling of the interactions of the beam remnants, the underlying event, may require non-trivial colour reconnection effects to be present. We recently introduced a universally applicable toy model of such reconnections, based on hadronising strings. This model, which has one free parameter, has been implemented in the Pythia event generator. We then considered several parameter sets (`tunes'), constrained by fits to Tevatron minimum-bias data, and determined the sensitivity of a simplified top mass analysis to these effects, in exclusive semi-leptonic top events at the Tevatron. A first attempt at isolating the genuine non-perturbative effects gave an estimate of order +-0.5GeV from non-perturbative uncertainties. The results presented here are an update to the original study and include recent bug fixes of Pythia that influenced the tunings investigated.
The D0 experiment has searched for phenomena beyond the standard model in top quark events. The methods and results of four analyses covering various possible deviations from the standard model behaviour are discussed. With data sets covering up to 2.1fb^-1 no deviation from the standard model expectations could be found.
We present a new, universally applicable toy model of colour reconnections in hadronic final states. The model is based on hadronising strings and has one free parameter. We next present an implementation of this model in the Pythia event generator and provide several parameter sets ("tunes"), constrained by fits to Tevatron minimum-bias data. Finally, we consider the sensitivity of a simplified top mass analysis to these effects, in exclusive semi-leptonic top events at the Tevatron. A first attempt at isolating the genuine non-perturbative effects gives an estimate of order delta(m_top) ~ 0.5 GeV from non-perturbative uncertainties, and a further delta(m_top) ~ 1 GeV from shower effects.
With its discovery in 1995 by the CDF and D0 collaborations the top quark completed the set of quarks expected by the Standard Model. It is predicted to have the same quantum numbers and couplings as the other up-type quarks. Albeit, only very few of these properties have been verified so far. This article summarises the existing measurements of top quark properties in the pair production mode.
The heaviest known Fermion particle -- the top quark -- was discovered at Fermilab in the first run of the Tevatron in 1995. However, besides its mere existence one needs to study its properties precisely in order to verify or falsify the predictions of the Standard Model. With the top quark's extremely high mass and short lifetime such measurements probe yet unexplored regions of the theory and bring us closer to solving the open fundamental questions of our universe of elementary particles such as why three families of quarks and leptons exist and why their masses differ so dramatically. To perform these measurements hundreds of millions of recorded proton-antiproton collisions must be reconstructed and filtered to extract the few top quarks produced. Simulated background and signal events with full detector response need to be generated and reconstructed to validate and understand the results. Since the start of the second run of the Tevatron the D0 collaboration has brought Grid computing to its aid for the production of simulated events. Data processing on the Grid has recently been added and thereby enabled us to effectively triple the amount of data available with the highest quality reconstruction methods. We will present recent top quark results D0 obtained from these improved data and explain how they benefited from the availability of computing resources on the Grid.
Measurements of the ttbar production cross-section at \sqrt{s} = 1.96TeV in proton-antiproton collisions were performed by the CDF and Dzero collaborations using ttbar final states where both W's decay hadronically. Each experiment uses luminosity of about 160pb^-1 of data collected during Run II of the Fermilab Tevatron collider. Beside kinematical observables both experiments take advantage of identifying b-jets based on lifetime tagging. While CDF counts the number of b-tags over background after a cut based analysis, Dzero counts the excess of events after a neural network based kinematical selection and requiring a b-tagged jet. CDF obtains $σ_{ttbar}=7.8+-2.5{stat}^{+4.7}_{-2.3}{syst}; Dzero $σ_{ttbar}=7.7^{+3.4}_{-3.3}{stat}^{+4.7}_{-3.8}{syst}+-0.5{lumi}.
The precision of measurements of the strong coupling constant using event-shape variables in e+e- annihilations is limited by theoretical systematic uncertainties. The uncertainties are related to missing higher orders in the perturbative predictions for the event-shape distributions. A new method is presented for the assessment of theoretical uncertainties in alpha_s. This method evaluates the systematic uncertainty of the parameter alpha_s from the uncertainty of the prediction for the distributions from which it is extracted. The perturbative uncertainties are calculated on a purely theoretical basis, without accessing measured distributions. The method is therefore especially suited for an unbiased combination of results from different observables or experiments. It is universal and can be applied to other processes like jet production in deep-inelastic ep scattering or in hadron collisions.
The D0 experiment faces many challenges enabling access to large datasets for physicists on four continents. The new concepts for distributed large scale computing implemented in D0 aim for an optimal use of the available computing resources while minimising the person-power needed for operation. The real live test of these concepts is of special interest for the LHC Computing GRID, LCG, which follows a similar strategy.
To measure the strong coupling alpha_s from event shape observables two ingredients are necessary. A perturbative prediction containing the dependence of observables on alpha_s and a description of the hadronisation process to match the perturbative prediction with the hadronic data. As perturbative prediction O(alpha_s^2), NLLA and combined calculations are available. Beside the well known Monte-Carlo based models also analytical predictions, so called power corrections, exist to describe the hadronisation. Advantages and disadvantages of the different resulting methods for determining the strong coupling and its energy dependence will be discussed, the newest DELPHI results will be presented, and an overview of the LEP results will be included.
The size of non-perturbative corrections to event shape observables is predicted to fall like powers of the inverse centre of mass energy. These power corrections are investigated for different observables from $e^+e^-$-annihilation measured at LEP as well as previous experiments. The obtained corrections are compared to other approaches and theoretical predictions. Measurements of $α_s$ using power corrections are compared to conventional methods.
The size of non-perturbative corrections to event shape observables is predicted to fall like a power of the inverse centre of mass energy. These power corrections are investigated for different observables from e+e-annihilation and compared to the theoretical predictions. Using the latest DELPHI high energy data advantages of determining α_s from these predictions are discussed and compared to conventional methods.