CERN Accelerating science

Thesis
Report number	CERN-THESIS-2010-213
Title	New Physics Searches in the ZZ Sector with the ATLAS Experiment
Author(s)	Barber, Thomas (Cambridge U.)
Publication	Cambridge : Cavendish Laboratory, University of Cambridge, 2010 - 238.
Thesis note	PhD : Cambridge U. : 2010
Thesis supervisor(s)	Batley, Richard
Note	Presented 2010
Subject category	Particle Physics - Experiment
Accelerator/Facility, Experiment	CERN LHC ; ATLAS
Abstract	This thesis investigates the prospects of measuring anomalous triple gauge boson couplings in the ATLAS detector at the Large Hadron Collider (LHC). The most general (V ZZ, V = Z, γ) vertex is parametrised by four couplings, fV =Z, i=4,5 , all of which are zero in the Standard Model. Non-zero couplings would manifest themselves as an excess of events in ZZ diboson channels, and, if observed, would be a direct probe of new physics beyond the Standard Model A set of criteria are outlined to select events recorded by ATLAS in two such channels, ZZ → llll(l = e, μ) and ZZ → llν¯ν. With 1 fb−1 of integrated luminosity at a centre of mass energy of √s = 7 TeV, ATLAS can expect to observe 10±1 events in the ZZ → llll channel, with 0.5+0.9−0.2 background events. In the ZZ → llν¯ν channel, 6.2±0.7 signal events are expected, with a background of 1.9+2.0−0.2 events. The expected sensitivity of ATLAS to non-zero anomalous couplings is calculated by performing extended, unbinned maximum-likelihood fits to the Z boson transverse momentum spectrum. For 1 fb−1 of integrated luminosity at √s = 7 TeV, ATLAS has the potential to place constraints on the coupling parameters of |fZi | < 0.06 and |f gi | < 0.07 at the 95% confidence level. These limits assume a form factor with a cutoff of L_FF = 1.2 TeV. As a prelude to ZZ observation, criteria are defined to select Z → ll(l = e, μ) events in the first 315 nb−1 of ATLAS pp collision data at √s = 7 TeV. In total 57 events are observed in the electron channel, with 109 in the muon channel, leading to cross section measurements of σ(Z → ee) = 0.70±0.09 (stat)±0.10 (syst)±0.08 (lumi) pb and σ(Z → μμ) = 0.90±0.09 (stat)±0.07 (syst)±0.10 (lumi) pb, both of which are consistent with the Standard Model predictions. In addition, this thesis presents a summary of developments made to the Data Acquisition (DAQ) system of the ATLAS Semiconductor Tracker (SCT). These include the construction of a test system, involving a scaled-down version of the entire SCT readout chain. The test system was subsequently used to develop a number of new DAQ features, including a hardware-based event simulator and monitoring framework.

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