Implementation of the ROD Crate DAQ Software for the ATLAS Tile Calorimeter and a Search for a MSSM Higgs Boson decaying into Tau pairs

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The ATLAS experiment of the LHC at CERN started its operation in 2009. The ROD Crate DAQ (RCD) as a part of the Trigger and Data Acquisition system (TDAQ) is responsible for the operation of the sub-­‐detector specific hardware at the level of the back-­‐end electronics crates. The RCD for the Tile Calorimeter is fully implemented. In first place, the specific Tile hardware and software is described in the configuration database. In second place, the various plug-­‐ins for the RCD that make use of sophisticated programming techniques are characterized. The RCD performance can be evaluated in terms of global DAQ efficiency, which is over 94% for the considered period in 2010 and in terms of the average transition time of operation during collisions where the Tile Calorimeter has not been the slowest sub-­‐system to change state. Other functionalities included in the Tile online software are the Detector Verification System (DVS) tests, which provide high precision measurements without making use of the Data-­‐Flow infrastructure. Results from these tests are structured in a hardware oriented layout with a user friendly display. A review is done of the full Tile Calorimeter read-­‐out chain from the front-­‐end electronics to the data-­‐flow system to introduce the Optimal Filtering algorithm responsible for the energy and time reconstruction of the signal. The performance of the signal reconstruction is evaluated by comparing the results provided by the online reconstruction done inside the ROD Digital Signal Processors (DSPs), and those provided by the offline reconstruction. Two different implementations are used in the offline reconstruction. The first one, mimics completely the implementation in the DSP. The maximum energy differences in this case are far below any physics cut for energy (1MeV in high gain and 40 MeV in low gain) and within the expected resolution for the Tile Calorimeter for time (0.3 ns in both gains). The second one, iteratively applies the algorithm three times, selecting the proper weights for the time computed in the previous iteration. As the algorithm is affected by the difference between the expected and the received time of the pulse, the relative energy difference shows a parabolic deviation as a function of the reconstructed time, which can be corrected offline. Phase corrected online energy shows relative differences smaller than 10% for low energy region and 1-­‐2% for higher energy deposits. Finally, time calibration is assessed by means of physics quantities, where a technique to evaluate out of time cells from jets that pass quality cuts and have an associated track is described. Cells that contribute to out of time deposits are identified and their expected time of the signal is corrected. Time based cuts may be used to develop algorithms to search for the Higgs boson. One of the most interesting channels for the search for Higgs bosons is the tau pair decay channel within the MSSM, where the production cross section is enhanced by a factor tan2β with respect to the Standard Model. The semi-­‐leptonic decay channel has been explored where the tau that decays leptonically is identified by the final lepton, and the tau that decays hadronically is identified by the tau reconstruction algorithm. The event selection criteria incorporates a cut on the transverse mass plane of the lepton plus missing ET system versus the hadron plus missing ET system that allows strong rejection of W+jets background. Visible mass is introduced to increase the statistics in the observables, as opposed to the invariant mass approximation through means of the collinear approximation for which back-­‐to-­‐ back events have to be removed. Bayesian statistics are used where the posterior probability is obtained through the profiling technique. Incorporation of energy scale uncertainties is done through means of the template morphing technique developed at CDF. Nuisance parameters describe normalization errors and morphing intensities as Gaussian PDFs. Simulation studies are conducted at √s = 14 TeV to compute exclusion limits and discovery significances for neutral MSSM Higgs bosons decaying to tau pairs. Expected results are very promising for ATLAS already at 1 fb-­‐1 where the Higgs could be excluded as low as tan2β = 10 or claimed for discovery at tan2β = 15 for mA = 150 GeV. Also expected results for 10 and 30 fb-­‐1 have been computed although the cuts have been optimized for 1 fb-­‐1 and no b-­‐tagging.
El detector ATLAS del LHC en el CERN ha empezado a operar en 2009. Para ello ha hecho uso del sistema de Trigger y Adquisición de Datos de ATLAS, conocido como TDAQ. Como parte del mismo, se encuentra el ROD Crate DAQ que tiene como objetivo el control del hardware específico de los sub-­‐detectores, donde se encuentra el caso del ROD del Calorímetro Hadrónico TileCal. Este dispositivo es responsable del cálculo de la energía depositada en las celdas del calorímetro. Así como del cálculo de la fase a la que se ha depositado la señal. Un estudio de las fases depositadas en el calorímetro puede permitir el desarrollo de algoritmos de selección de eventos para la búsqueda del Bosón de Higgs. Uno de los canales más interesantes para esta búsqueda se encuentra en el modelo MSSM. En este modelo, la sección eficaz de producción de Higgs se encuentra favorecida cuadráticamente por un factor propio del modelo, tangente de beta, con respecto al modelo estándar. Es posible hacer una búsqueda del Bosón de Higgs estableciendo un límite superior a su sección eficaz en caso de una no-­‐ observación o estableciendo la significancia estadística en caso de un posible exceso en las observaciones. Se describe la implementación el software necesario para la integración de los RODs del Calorímetro Hadrónico TileCal dentro del TDAQ de ATLAS. Esto supone en primer lugar la descripción del hardware y software específico de TileCal en la base de datos de configuración. En segundo lugar, la implementación de varios plugins para el ROD Crate DAQ. Destacan el plugin del ROD y del TBM. Se presentan los resultados de rendimiento y tiempo de reacción. Finalmente se muestran los resultados de las tareas de validación del algoritmo de Optimal Filtering y puesta a punto del timing de los canales del TileCal. Se ha importado a ATLAS la técnica bayesiana desarrollada en CDF para establecer límites superiores a la sección eficaz de producción del bosón de Higgs y la significancia de una posible observación. Este método tiene en cuenta errores sistemáticos que describen nuestro conocimiento sobre parámetros físicos como la luminosidad y la sección eficaz de los procesos de fondo que modifican la normalización de las distribuciones, además de otras medidas como la precisión en la determinación de la escala electromagnética o la incertidumbre en la calibración de la energía de los jets que modifican la forma de las distribuciones. Se presentan resultados esperados para los límites de exclusión y significancias de descubrimiento para Bosones de Higgs neutros decayendo a pares de taus. Estos resultados esperados son muy prometedores para ATLAS durante los primeros dos años de toma de datos.
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