Logo: to the web site of Uppsala University

This paper presents studies and measurements of the Lorentz angle and cluster size for the ATLAS Semiconductor Tracker (SCT). The performance of the SCT detector has been studied with cosmic rays during the commissioning period in 2008. The Lorentz angle and the cluster size are important monitors of the SCT performance in the 2 T magnetic field. The Lorentz angle of the SCT barrel has been measured to be θ=3.93±0.03(stat.)±0.10(syst.) degrees which is in agreement with the model prediction of  θ=3.69±0.26(syst.) degrees.

The tau identification in two distinct high-multiplicity environments, pile-up and ttbar events, is investigated. The tau ID performance in such environments is known to be degraded. It is shown that it is necessary to adjust the tau ID quality cut in order to reach a working point in terms of tau efficiency versus jet rejection comparable to more frequently investigated and cleaner types of events like Z->tautau and W->taunu. Three different ways of recovering performance and robustness are investigated: Firstly, the usage of dedicated probability density functions as input to the tau likelihood method is tested, obtained from Z->tautau events with pile-up and ttbar events. Effects are generally small, except for high-pT tau jets in ttbar events. Secondly, it is investigated whether likelihood input variables can be removed without reducing performance (to increase robustness and reduce systematic uncertainties), and thirdly, whether adding new variables or replacing existing variables with new variables can lead to increased performance in terms of jet rejection at fixed tau efficiency. It is shown that several variables are redundant for tau identification in high-multiplicity environments. At least one variable is identified which can efficiently replace one of the existing variables in high-multiplicity events, namely the fraction of energy deposited in a wider isolation cone than used by the default tau identification.

The discovery of a charged Higgs boson would be tangible proof of physics beyond the Standard Model. This note presents the ATLAS potential for discovering a charged Higgs boson, utilizing five different final states of the signal arising from the three dominating fermionic decay modes of the charged Higgs boson. The search covers the region below the top quark mass, taking into account the present experimental constraints, the transition region with a charged Higgs boson mass of the order of the top quark mass, and the high-mass region with a charged Higgs boson mass up to 600 GeV. All studies are performed with a realistic simulation of the detector response including all three trigger levels and taking into account all dominant systematic uncertainties. Results are given in terms of discovery and exclusion contours for each channel individually and for all channels combined, showing that the ATLAS experiment is capable of detecting the charged Higgs boson in a significant fraction of the (tanb, mH±) parameter space with its first 10 fb−1 of data. The so-called intermediate tanb region (around tanb = 7) is experimentally hard to reach but exclusion sensitivity is given in this area.

In the theoretical framework of the minimal supersymmetric extension of the standard model (MSSM), we study the pair production of heavy charged and neutral Higgs bosons in e+e- collisions with a center-of-mass energy of 3 TeV, at the Compact Linear Collider (CLIC). The high-energy beam-beam effects at the interaction point are taken into account in our simulations. With an integrated luminosity of 3000fb-1, the hadronic cascade decays of H+H- and A0H0 pairs can be observed over a wide range of Higgs boson masses, extending beyond 1 TeV. The Higgs mass parameter mA can be derived from a χ2-analysis, and the corresponding statistical error was found to be smaller than 1%. In addition, by comparing the signal rates of different decay channels, one can determine the ratio between the vacuum expectation values tan β with a good accuracy in the intermediate region, i.e. when tan β lies between 6 and 11 typically.