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Studies of Gauge Boson production with a gamma-gamma collider at TESLA

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Abstract (Summary)
In absence of the Standard Model Higgs boson the interaction among the gauge bosons becomes strong at high energies (? 1TeV) and influences the couplings between them. Each trilinear and quartic gauge boson vertex is characterized by a set of couplings which are expected to deviate from their Standard Model values already at energies lower than the energy scale of the New Physics. The precise measurement of gauge boson couplings can provide clues to the mechanism of the electroweak symmetry breaking and their anomalous values can be a sign of a New Physics effect beyond the Standard Model. Estimation of the precision with which we can measure the deviations of the charged trilinear gauge boson couplings (TGCs) with a photon collider at TESLA is one of the two topics covered by this theses. Deviations are denoted as ??? and ??? (anomalous TGCs) and represent the strength of the gauge boson couplings at the W W ? vertices. The single W boson production in ?e collisions is studied via ?e? ? W ??e in two different operating ?e modes - real and parasitic. While the first assumes the electron and photon beam in the collision, the second one is considered as a background to ?? collisions: the interaction between photons (from each side) and unconverted electrons. The W boson pair production is studied in ?? collisions via ?? ? W +W ? in two different initial polarization states, JZ = 0 and |JZ| = 2. While the first polarization state assumes the colliding photons have the same helicities, the second one assumes opposite photon helicities. The W bosons from both channels are reconstructed from hadronic final states, as two jets in ?e collisions and as four jets in ?? collisions. The study includes the influence of low-energy events ?? ? hadrons (pileup) on the signal and estimated background in both channels. The error estimation of the measurement of the TGCs is performed using a binned ?2 and binned maximum Likelihood fit of reweighted event distributions of variables which are sensitive to the anomalous couplings. It was found that the error of the ?? and ?? measurement at a photon collider at TESLA is of O(10?3 ? 10?4), depending on the ??/?e channel, mode and the coupling under the consideration. Compared to the measurements at Tevatron and LEP experiments this is about one to two orders of magnitude higher accuracy. The influence of some systematic errors such those from the W mass measurement, uncertainties due to the beam energy and polarization and the effects of the background is also estimated. Another part of this theses covers the optimization of the ??-detector in the forward region. Detailed ’incoherent particle-particle’ and ’coherent particle-beam’ interactions have been simulated in order to estimate the lowenergy background contribution to the tracking devices. The ??-detector is optimized in the way to minimize the direct and the backscattered background in its forward region. For that purpose, the beam pipes in the region between the interaction point and the electromagnetic calorimeter are surrounded by a tungsten mask. The space around the beam pipes in the region where the electromagnetic and hadronic calorimeters are positioned is filled by graphite and tungsten, while the outgoing electron beam pipes are made of graphite. With this design of the forward region the estimated background that enters into the main tracking system, the time projection chamber (TPC) and the vertex detector (VTX) is brought to the level that provides small occupancies in TPC and VTX.
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School:Oberlin College

School Location:USA - Ohio

Source Type:Master's Thesis

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