Production of Z bosons in proton-proton collisions at root out s = 10 TeV: Expectations for early measurements at the ATLAS experiment

The production of the Z boson in proton-proton collisions at the LHC serves as a standard candle at the ATLAS experiment during early data-taking. The decay of the Z into an electron-positron pair gives a clean signature in the detector that allows for calibration and performance studies. The cross-section of ~ 1 nb allows first LHC measurements of parton density functions. In this thesis, simulations of 10 TeV collisions at the ATLAS detector are studied. The challenges for an experimental measurement of the cross-section with an integrated luminositiy of 100 pb−1 are discussed. In preparation for the cross-section determination, the single-electron efficiencies are determined via a simulation based method and in a test of a data-driven ansatz. The two methods show a very good agreement and differ by ~ 3% at most. The ingredients of an inclusive and a differential Z production cross-section measurement at ATLAS are discussed and their possible contributions to systematic uncertainties are presented. For a combined sample of signal and background the expected uncertainty on the inclusive cross-section for an integrated luminosity of 100 pb−1 is determined to 1.5% (stat) +/- 4.2% (syst) +/- 10% (lumi). The possibilities for single-differential cross-section measurements in rapidity and transverse momentum of the Z boson, which are important quantities because of the impact on parton density functions and the capability to check for non-pertubative effects in pQCD, are outlined. The issues of an efficiency correction based on electron efficiencies as function of the electron’s transverse momentum and pseudorapidity are studied. A possible alternative is demonstrated by expanding the two-dimensional efficiencies with the additional dimension of the invariant mass of the two leptons of the Z decay.

On the geometry of the spin-statistics connection in quantum mechanics

The Spin-Statistics theorem states that the statistics of a system of identical particles is determined by their spin: Particles of integer spin are Bosons (i.e. obey Bose-Einstein statistics), whereas particles of half-integer spin are Fermions (i.e. obey Fermi-Dirac statistics). Since the original proof by Fierz and Pauli, it has been known that the connection between Spin and Statistics follows from the general principles of relativistic Quantum Field Theory. In spite of this, there are different approaches to Spin-Statistics and it is not clear whether the theorem holds under assumptions that are different, and even less restrictive, than the usual ones (e.g. Lorentz-covariance). Additionally, in Quantum Mechanics there is a deep relation between indistinguishabilty and the geometry of the configuration space. This is clearly illustrated by Gibbs' paradox. Therefore, for many years efforts have been made in order to find a geometric proof of the connection between Spin and Statistics. Recently, various proposals have been put forward, in which an attempt is made to derive the Spin-Statistics connection from assumptions different from the ones used in the relativistic, quantum field theoretic proofs. Among these, there is the one due to Berry and Robbins (BR), based on the postulation of a certain single-valuedness condition, that has caused a renewed interest in the problem. In the present thesis, we consider the problem of indistinguishability in Quantum Mechanics from a geometric-algebraic point of view. An approach is developed to study configuration spaces Q having a finite fundamental group, that allows us to describe different geometric structures of Q in terms of spaces of functions on the universal cover of Q. In particular, it is shown that the space of complex continuous functions over the universal cover of Q admits a decomposition into C(Q)-submodules, labelled by the irreducible representations of the fundamental group of Q, that can be interpreted as the spaces of sections of certain flat vector bundles over Q. With this technique, various results pertaining to the problem of quantum indistinguishability are reproduced in a clear and systematic way. Our method is also used in order to give a global formulation of the BR construction. As a result of this analysis, it is found that the single-valuedness condition of BR is inconsistent. Additionally, a proposal aiming at establishing the Fermi-Bose alternative, within our approach, is made.

Ontogenetic studies on the determination of the apical meristem in racemose inflorescences

This thesis presents a comparative developmental study of inflorescences and focuses on the production of the terminal flower (TF). Morphometric attributes of inflorescence meristems (IM) were obtained throughout the ontogeny of inflorescence buds with the aim of describing possible spatial constraints that could explain the failure in developing the TF. The study exposes the inflorescence ontogeny of 20 species from five families of the Eudicots (Berberidaceae, Papaveraceae-Fumarioideae, Rosaceae, Campanulaceae and Apiaceae) in which 745 buds of open (i.e. without TF) and closed (i.e. with TF) inflorescences were observed under the scanning electron microscope.rnThe study shows that TFs appear on IMs which are 2,75 (se = 0,38) times larger than the youngest lateral reproductive primordium. The shape of these IMs is characterized by a leaf arc (phyllotactic attribute) of 91,84° (se = 7,32) and a meristematic elevation of 27,93° (se = 5,42). IMs of open inflorescences show a significant lower relative surface, averaging 1,09 (se=0,26) times the youngest primordium size, which suggests their incapacity for producing TFs. The relative lower size of open IMs is either a condition throughout the complete ontogeny (‘open I’) or a result from the drastic reduction of the meristematic surface after flower segregation (‘open II’). rnIt is concluded that a suitable bulge configuration of the IM is a prerequisite for TF formation. Observations in the TF-facultative species Daucus carota support this view, as the absence of the TF in certain umbellets is correlated with a reduction of their IM dimensions. A review of literature regarding histological development of IMs and genetic regulation of inflorescences suggests that in ‘open I’ inflorescences, the histological composition and molecular activity at the tip of the IM could impede the TF differentiation. On the other side, in ‘open II’ inflorescences, the small final IM bulge could represent a spatial constraint that hinders the differentiation of the TF. The existence of two distinct kinds of ontogenies of open inflorescences suggests two ways in which the loss of the TF could have occurred in the course of evolution.rn

Measurement of the ZZ production cross section and limits to the anomalous triple gauge couplings with forward electrons with the ATLAS detector

Measurements of the self coupling between bosons are important to test the electroweak sector of the Standard Model (SM). The production of pairs of Z bosons through the s-channel is forbidden in the SM. The presence of physics, beyond the SM, could lead to a deviation of the expected production cross section of pairs of Z bosons due to the so called anomalous Triple Gauge Couplings (aTGC). Proton-proton data collisions at the Large Hadron Collider (LHC) recorded by the ATLAS detector at a center of mass energy of 8 TeV were analyzed corresponding to an integrated luminosity of 20.3 fb-1. Pairs of Z bosons decaying into two electron-positron pairs are searched for in the data sample. The effect of the inclusion of detector regions corresponding to high values of the pseudorapidity was studied to enlarge the phase space available for the measurement of the ZZ production. The number of ZZ candidates was determined and the ZZ production cross section was measured to be: rn7.3±1.0(Stat.)±0.4(Sys.)±0.2(lumi.)pb, which is consistent with the SM expectation value of 7.2±0.3pb. Limits on the aTGCs were derived using the observed yield, which are twice as stringent as previous limits obtained by ATLAS at a center of mass energy of 7 TeV.