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A dedicated search is presented for new phenomena in inclusive eight- and ten-jet final states with low missing transverse momentum, with and without identification of jets originating from b quarks. The analysis is based on data from proton-proton collisions corresponding to an integrated luminosity of 19.6 inverse femtobarns collected with the CMS detector at the LHC at sqrt(s) = 8 TeV. The dominant multijet background expectations are obtained from low jet multiplicity control samples. Data agree well with the standard model background predictions, and limits are set in several benchmark models. Colorons (axigluons) with masses between 0.6 and 0.75 (up to 1.15) TeV are excluded at 95% confidence level. Similar exclusion limits for gluinos in R-parity violating supersymmetric scenarios are from 0.6 up to 1.1TeV. These results comprise the first experimental probe of the coloron and axigluon models in multijet final states.
Many interesting physical processes produce non-interacting particles that could only be measured using the missing transverse momentum. The increase of the proton beam intensity in the Large Hadron Collider (LHC) provides sensitivity to rare physics processes while inevitably increasing the number of simultaneous proton collisions in each event. The missing transverse momentum (MET) is a variable of great interest, defined as the negative sum of the transverse momentum of all visible particles. The precision of the MET determination deteriorates as the complexity of the recorded data escalates. Given the current complexity of data analysis, a new algorithm is developed to effectively determine the MET. Several well-understood physics processes were used to test the effectiveness of the newly designed algorithm. The performance of the new algorithm is also compared to that of the standard algorithm used in the ATLAS experiment.
In this book, the anomaly mediated supersymmetry breaking (AMSB) model is explored by searching for charged winos with their subsequent decays collected with the ATLAS detector at the Large Hadron Collider (LHC). The author develops a new method, called “re-tracking,” to detect charged winos that decay before reaching the Semiconductor Tracker (SCT) detector. Because the nominal tracking algorithm at the ATLAS experiment requires at least seven successive hits in the inner tracking system, the sensitivity to charged winos having a fraction of a nanosecond in the past analysis was therefore limited. However, re-tracking requires a minimum of three pixel hits and provides a fully efficient tracking capability for charged winos traversing the pixel detector, resulting in around about 100 times greater efficiency for charged winos with a lifetime ~0.2 ns longer than that in past searches. Signal topology is characterized by a jet with large transverse momentum (pT), large missing transverse energy, and a high-pT disappearing track. There are three types of back ground tracks: interacting hadron tracks, charged leptons, and tracks with mismeasured pT. A background estimation based on the Monte Carlo (MC) simulation suffers from large uncertainties due to poor statistics and has difficulty simulating the properties of background tracks. Therefore, a data-driven approach has been developed by the author of the book to estimate the background track-pT spectrum. No significant excess above the background expectation is observed for candidate tracks with large transverse momentum, and constraints on the AMSB model are obtained. The author shows that in the AMSB model, a charged wino mass below 270 GeV is excluded at 95 % confidence level, which also directly constrains the mass of wino dark matter.
A measurement of the underlying event activity is performed for proton-proton collisions at a center of mass energy of 13 TeV using the Compact Muon Solenoid detector. The azimuthal angle of the highest transverse-momentum track is used to split the azimuthal plane into three regions of equal area: Toward, Away, and Transverse. The Transverse region is further subdivided into a region of maximal activity and a region of minimal activity. The activity is quantified by the number of tracks and the sum of their transverse momenta in each region. Comparisons are made with previous results obtained at 0.3, 0.9, 1.96, and 7 TeV. These combined results can be used to refine and improve Monte Carlo predictions of the underlying event activity at current and higher energies which will lead to the improved modelling of proton-proton collisions leading to more precise measurements at the Large Hadron Collider.
Distributions of transverse momentum pTll and the related angular variable $\phi ^*_{\eta }$ of Drell-Yan lepton pairs are measured in 20.3 fb-1 of proton-proton collisions at √s=8 TeV with the ATLAS detector at the LHC. Measurements in electron-pair and muon-pair final states are corrected for detector effects and combined. Compared to previous measurements in proton[Gamma]Çôproton collisions at √s=7 TeV these new measurements benefit from a larger data sample and improved control of systematic uncertainties. Measurements are performed in bins of lepton-pair mass above, around and below the Z -boson mass peak. The data are compared to predictions from perturbative and resummed QCD calculations. For values of $\phi ^*_{\eta }$
Zusammenfassung: In this thesis, measurements of the W boson pair production cross section in proton-proton collisions at a centre-of-mass energy of 8 TeV are presented. A dataset corresponding to an integrated luminosity of 22.3 /fb recorded by the ATLAS experiment in the year 2012 is used. The W bosons are identified by their decay products of exactly two leptons with opposite electrical charge, electrons or muons, and missing transverse momentum induced by their corresponding neutrinos. First, a measurement restricted to final states without hadronic jets is presented and differential fiducial cross sections, total cross sections and limits on anomalous triple gauge boson couplings are obtained. A second measurement of the fiducial cross section in association with exactly one hadronic jet is presented. The ratio of fiducial cross sections for different jet multiplicities is reported and the combined fiducial cross section is extrapolated to the total phase space.