Top quark pair production cross-section in proton-proton collisions at √s = 7 TeV

With proton beams colliding at an energy of 7 TeV in the center-of-mass (8 TeV in 2012), the Large Hadron Collider allows physicists to experimentally access regions of the phase-space that were previously unexplored.In this thesis, the cross-section of the top-antitop quark pair was measured, namely how often the top-antitop quark pair is produced, by identifying those topologies with a single-lepton in their final state.The top quark is the heaviest elementary particle known to date, and due to its mass it is expected to play an important role in understanding the electroweak symmetry breaking mechanism, the procedure with which particles gain mass. The top-antitop quark pair cross-section is the first step towards gaining a better understanding of the top quark physics at the energies provided by the LHC. To achieve the goal, the thesis also addresses the issue of selecting the proper events real-time and subsequently further identifying them offline over the huge background of other processes. The result presented in this thesis, using data of only 35 pb-1 of integrated luminosity, is found to be: 178.1 (stat.: +- 13.5) (syst.: +23.0 -24.6) (lumi.: +6.2 -5.9) pico-barns, which is considered to be in good agreement with the theoretical expectations

Exploring spatial and temporal resilience in socio‐ecological systems: evidence from sacred forests in Epirus, Greece

Socio-ecological resilience is the capacity of a system to adapt to changing eco-logical and social disturbances. Its assessment is extremely important to integrate long-term management of ecological and social features of natural ecosystems. This is especially true for Sacred Natural Sites, such as sacred forests and groves, where it can reveal the influence of social processes in ecosystem recovery or degradation. Using tree ages determined through dendrochronology and tree population size- class distributions collected in five sacred forests in Epirus (NW Greece), we explore spatial and temporal dynamics of resilience in a socio-ecological system, identifying which cultural and social elements characterize resilience in space and time. Our main results show that over past centuries sacred forests in Epirus underwent periods of varying tree establishment rate, depending on the intensity of human activities and historical disturbance events. We also identified strong evidence of the role of the social component (i.e. the church and associated cultural praxis) in determining the spatial extent of the forests' current recovery phase, and thus the overall resilience of the system. Policy implications. Appreciation of the ways sacred forests' ecological resilience is linked to changing socio-cultural praxis over both temporal and spatial scales is crucial for guiding conservation and restoration strategies. We argue that greater attention should be paid to the role of the social component of socio-ecological systems and specifically for sacred natural sites that provide both a nucleus of established forest habitat and the conditions necessary for forest recovery and restoration

Charged-particle multiplicities in pp interactions at √s = 900 GeV measured with the ATLAS detector at the LHC

The first measurements from proton-proton collisions recorded with the ATLAS detector at the LHC are presented. Data were collected in December 2009 using a minimum-bias trigger during collisions at a centre-of-mass energy of 900 GeV. The charged-particle multiplicity, its dependence on transverse momentum and pseudorapidity, and the relationship between mean transverse momentum and charged-particle multiplicity are measured for events with at least one charged particle in the kinematic range |η|500 MeV. The measurements are compared to Monte Carlo models of proton-proton collisions and to results from other experiments at the same centre-of-mass energy. The charged-particle multiplicity per event and unit of pseudorapidity at η=0 is measured to be 1.333 ± 0.003(stat.) ± 0.040(syst.), which is 5-15% higher than the Monte Carlo models predict