126082272
Measurement of hard double-parton interactions with the ATLAS detector
The production of a W boson in association with two jets was studied in proton-proton collisions at a centre-of-mass energy of √ s = 7 TeV with the ATLAS detector at the LHC. Data corresponding to an integrated luminosity of 36 pb−1 were analysed for the presence of doubleparton interactions. The fraction of events arising from double-parton interactions was extracted using the momentum balance of the two jets, and was evaluated to be f Detector DP = 0.08± 0.01(stat.)± 0.02(syst.) for jets with transverse momentum pT > 20 GeV and rapidity |y| < 2. This is expressed as a measure of the effective area parameter for hard double-parton interactions, σeff = 15±3(stat.)+5 −3(syst.) mb.
59363644
Towards a Level-1 tracking trigger for the ATLAS experiment at the High Luminosity LHC
At the high luminosity HL-LHC, upwards of 140 individual proton-proton interactions (pileup) are expected per bunch-crossing at luminosities of around 5×1034 cm-2s-1. A proposal by the ATLAS collaboration to split the ATLAS first level trigger in to two stages is briefly detailed. The use of fast track finding in the new first level trigger is explored as a method to provide the discrimination required to reduce the event rate to acceptable levels for the read out system while maintaining high efficiency on the selection of the decay products of electroweak bosons along with other high pT physics signatures at HL-LHC luminosities. It is shown that the available bandwidth in the proposed new strip tracker is sufficient for a region of interest based track trigger given certain optimisations. Further methods for improving upon the proposal are discussed.
123039432
Development and online operation of minimum bias triggers in ATLAS
Since the restart of the LHC in November 2009, ATLAS has recorded pp collisions to investigate new phenomena at the high energy frontier. One early measurement is of charged particle multiplicities in inelastic interactions, which was performed on data sets of about 360,000 and 10,000,000 minimum bias event at (s)1/2 = 900 GeV and 7 TeV respectively. This measurements will help to constrain various phenomenological models describing inelastic interactions. Understanding trigger efficiencies for different event types is crucial to minimize any possible bias in the event selection. ATLAS has a three level trigger system and utilises two main minimum bias triggers. A primary scintillator based trigger situated in the forward regions of 2.1 < |η| < 3.8 was proven to select pp collisions with high efficiency. A complementary control trigger runs on unbiased filled proton bunches and utilizes space points reconstructed in the silicon tracking volume to trigger on inelastic interactions. Both triggers were essential for the analysis of charged particle multiplicities. Their performance and trigger efficiency measurements as well as studies on possible bias sources are presented. The triggers are shown to form an effective and inclusive triggering solution for early LHC physics.
118764684
Analyzing new neutral gauge bosons at the LHC using third generation fermions
This thesis reports on techniques and the corresponding phenomenology for distinguishing between models in the analysis of new neutral gauge boson production at the LHC. Specifically, third generation final states are reviewed and determined to be effective channels for analysis. Transverse momentum cuts of pxf > 0.3M z > to hadronic final states are employed to reduce QCD backgrounds and distinguish t and b signals. An analysis of the ratio of the t and b cross sections to the fi cross section, within an invariant mass window of 0.965Mz > < < 1.035M z >, is performed. For final state fermion detection efficiencies of = 0.96, et = 0.10, and e*, = 0.50, most models considered can be distinguished by at least one standard deviation for Mz > = 1500 GeV, with 10 fb_1 integrated luminosity. Furthermore, the ratio of the t to cross sections is analyzed and used to understand models with preferential couplings to third generation particles in the lepton family. Assuming an efficiency of er = 0.2, generation universality can be determined up to approximately a 5% uncertainty for a 1000 GeV Z' with 10 fb_1 integrated luminosity.
118575499
Discovery and Identification of Extra Neutral Gauge Bosons at the LHC
We present the discovery prospects for a Z' resonance and some expected results for Z' diagnostic measurements at the LHC. The discovery reach is highly dependent on the energy and luminosities that may be attained at the LHC, and a number of energy and integrated luminosity scenarios are presented. In addition, the use of third generation quark final states for distinguishing between models and for measuring a forward-backward asymmetry is explored.
118404330
Unravelling an extra neutral gauge boson at the LHC using third generation fermions
We study the potential to use measurements of extra neutral gauge bosons (Z') properties in pp collisions at the Large Hadron Collider to unravel the underlying physics. We focus on the usefulness of third generation final states (tau, b, t) in distinguishing between models with non-universal Z'-fermion couplings. We present an update of discovery limits of Z's including the 2010-2011 LHC run and include models with non-universal couplings. We show how ratios of sigma(pp -> Z' -> ttbar), sigma(pp -> Z' -> bbbar), and sigma(pp -> Z' -> tau^+tau^-) to sigma(pp -> Z' -> mu^+mu^-) can be used to distinguish between models and measure parameters of the models. Of specific interest are models with preferential couplings, such as models with generation dependent couplings. We also find that forward-backward asymmetry measurements with third generation fermions in the final state could provide important input to understanding the nature of the Z'. Understanding detector resolution and efficiencies will be crucial for extracting results.