J/ψ spin-alignment measurement in pp collisions at √s = 8 TeV with the ATLAS detector at the LHC

The J/ψ and Υ mesons were discovered about forty years ago. Since then, the study of qq bound states provide significant input for the understanding of quantum chromodynamics (QCD). Although the J/ψ is one of the simplest systems in QCD, it is difficult to describe in detail its production mechanism. A measurement of the J/ψ spin-alignment could shed light on our understanding, and help to distinguish between the different proposed theoretical models. A precise measurement of the spin-alignment of J/ψs decaying into two muons in pp collisions at a centre of mass energy of √s = 8 TeV at the LHC is presented. This is the first measurement of this quantity at this energy regime. The current study is based on integrated luminosity of 14/fb of data collected by the ATLAS experiment in 2012. As an input for this analysis, a measurement of the production ratio of prompt to non-prompt J/ψ was done. From this measurement we extract the fractions of three physical processes in our signal region, the fraction of prompt J/ψs, non-prompt J/ψs (coming from B-decays) and non-J/ψ background. It was found that the fraction of prompt J/ψs decreases with the increase of pT, where at low pT it starts from 60% and decreases to 30% in the highest pT. The fraction of the non-prompt J/ψ has the opposite behaviour, and the background is below 20% over the whole region. The measurement of the prompt J/ψ spin-alignment was preformed in the two dimensional angular distribution (cosθ∗ and φ∗) in bins of pT and rapidity of the J/ψ. All three spin-alignment parameters, λθ∗ , λφ∗ and λθ∗φ∗ where measured. The λθ∗ was found to be consistent with zero (with respect to the total uncertainties) in the low pT region, and positive (≈ 0.2) with the increase of J/ψ pT. The other two parameters λφ∗ and λθ ∗φ∗ were found to be very small, with small uncertainties, where the λφ∗ slightly increases with pT, and λθ∗φ∗ is consistent with zero in most of the pT spectrum. The last part of this thesis describes the development of the small Thin Gap Chambers (sTGC) for the Phase-I ATLAS upgrade. It was developed and proposed to replace the small wheel of the ATLAS detector. This technology is based on the TGCs that are operated today in the ATLAS Muon Spectrometer and are used for triggering on high pT muons in the ATLAS endcaps region. The new detector design was approved by the collaboration this year. This new detectors design will allow to maintain the full trigger acceptance and precise muon tracking at the highest LHC luminosities expected after the LHC upgrades.