Bárbara Chazin Quero

The CMS drift tubes (DT) muon detector, built for withstanding the LHC expected integrated and instantaneous luminosities, will be used also in the High Luminosity LHC (HL-LHC) at a 5 times larger instantaneous luminosity and, consequently, much higher levels of radiation, reaching about 10 times the LHC integrated luminosity. Initial irradiation tests of a spare DT chamber at the CERN gamma irradiation facility (GIF++), at large (∼ O(100)) acceleration factor, showed ageing effects resulting in a degradation of the DT cell performance. However, full CMS simulations have shown almost no impact in the muon reconstruction efficiency over the full barrel acceptance and for the full integrated luminosity. A second spare DT chamber was moved inside the GIF++ bunker in October 2017. The chamber was being irradiated at lower acceleration factors, and only 2 out of the 12 layers of the chamber were switched at working voltage when the radioactive source was active, being the other layers in standby. In this way the other non-aged layers are used as reference and as a precise and unbiased telescope of muon tracks for the efficiency computation of the aged layers of the chamber, when set at working voltage for measurements. An integrated dose equivalent to two times the expected integrated luminosity of the HL-LHC run has been absorbed by this second spare DT chamber and the final impact on the muon reconstruction efficiency is under study. Direct inspection of some extracted aged anode wires presented a melted resistive deposition of materials. Investigation on the outgassing of cell materials and of the gas components used at the GIF++ are underway. Strategies to mitigate the ageing effects are also being developed. From the long irradiation measurements of the second spare DT chamber, the effects of radiation in the performance of the DTs expected during the HL-LHC run will be presented.

During the High Luminosity LHC, the Drift Tube chambers installed in the CMS detector need to operate with an integrated dose ten times higher than expected at the LHC due to the increase in integrated luminosity from 300 fb-1 to 3000 fb-1. Irradiations have been performed to assess the performance of the detector under such conditions and to characterize the radiation aging of the detector. The presented analysis focuses on the behaviour of the high voltage currents and the dose measurements needed to extrapolate the results to High Luminosity conditions, using data from the photon irradiation campaign at GIF++ in 2016 as well as the efficiency analysis from the irradiation campaign started in 2017. Although the single-wire loss of high voltage gain observed of 70% is very high, the muon reconstruction efficiency is expected to decrease less than 20% during the full duration of High Luminosity LHC in the areas under highest irradiation.

Reliable and performant heavy flavour identification is of prime importance for the physics program of the CMS experiment.During the last years the CMS collaboration has dedicated a considerable effort to improve and expand its capabilities in this sector by applying several machine learning techniques well established in industry, but still experimental in HEP.These proceedings describe a poster focused on a selection of these techniques and the description of the implementation details as well as the resulting gains.

espanolTras el descubrimiento en 2012 del boson de Higgs, el Modelo Estandar se ha establecido firmemente como una teoria efectiva, valida hasta energias del orden de la escala electrodebil ( ∼ TeV). Las ultimas evidencias experimentales demuestran que su actual formulacion es incompleta y existen numerosas cuestiones por resolver. Entre ellas, se encuentran la existencia de la materia oscura, la asimetria entre materia y antimateria o el problema de la jerarquia entre la masa del boson de Higgs y la masa de Planck. Parece entonces plausible pensar modelos de nueva fisica que describan la naturaleza a escalas mas altas de energia en un rango fenomenologico mas amplio. Una de las alternativas mas prometedoras al Modelo Estandar se encuentra en la supersimetria. El principio de supersimetria asigna a cada particula del Modelo Estandar un companero supersimetrico a traves de una transformacion de simetria espacio-temporal que cambia en 1/2 el momento angular de espin de la particula original. La extension supersimetrica mas simple del Modelo Estandar se denomina, en ingles, the Minimal Supersymmetric Standard Model (MSSM). Este modelo puede resolver algunos de los problemas mas importantes del actual Modelo Estandar como proveer de un candidato a materia oscura o solucionar el problema de la jerarquia. Esta tesis doctoral presenta la busqueda de dos particulas supersimetricas producidas en colisiones proton-proton en el LHC con una energia de centro de masas de √s = 13 TeV. La muestra de datos analizada ha sido recogida por el detector CMS en 2016 y corresponde a una luminosidad integrada de 35.9 fb − 1 . El analisis se centra en la produccion del chargino y del top squark mas ligeros ( χ±_1 , t_1 ) en pares de particula-antiparticula decayendo en estados finales con dos leptones y momento transverso faltante. Los resultados son interpretados en terminos de varios modelos de senal simplificados asumiendo conservacion de la simetria R-parity y asumiendo como la particula supersimetrica mas ligera, en ingles lightest supersymmetric particle (LSP), el neutralino mas ligero ( χ0_1) llamado a partir de aqui simplemente neutralino. La busqueda de parejas de chargino se realiza en todo el plano de masas, considerando como principal referencia un modelo donde el chargino decae en un lepton (l), un neutrino (ν) y el neutralino a traves de la desintegracion de un slepton ( χ±_1→ lν → lν χ0_1) o de un sneutrino ( χ±_1 → lν → lν χ0_1 ). Se asume que las tres generaciones de sleptones estan degeneradas con una masa igual al promedio entre las entre las masas del chargino y del neutralino. Tambien se asume que la fraccion de desintegracion de un chargino en un slepton cargado o un neutrino es la misma. Los resultados son asi mismo interpretados en terminos un segundo modelo donde cada chargino decae en un neutralino y un boson W. Otras busquedas de pares de chargino han sido previamente publicadas por la Colaboracion CMS en el contexto del primer escenario de senal usando datos de colisiones proton-proton a 8 TeV y por la Colaboracion ATLAS en el contexto de ambos escenarios, usando datos de colisiones a 8 TeV y a 13 TeV. En el caso de los pares de top squarks, se considera como referencia un modelo donde el squark top decae en quark top (t) y un neutralino. La busqueda se realiza en la zona comprimida del espectro de masas donde la diferencia de masas (∆m) entre el squark top y el neutralino se encuentra entre la masa del quark top y del boson W, mW mt donde los quarks top son producidos con mayor momento. Con respecto a ese analisis, esta busqueda gana sensibilidad en la region de masas comprimida al suavizar las exigencias sobre los jets provenientes de la hadronizacion del quark bottom y mediante la optimizacion de la seleccion de eventos de senal enfocandola a la produccion de neutralinos con bajo momento. La estrategia del analisis emplea las mismas tecnicas para la estimacion de fondos y la extraccion de senal en ambas busquedas, en tanto que el espacio de fases especifico para cada hipotesis varia ligeramente en funcion de sus propiedades cinematicas. La sensibilidad del analisis se basa en la combinacion de dos variables discriminatorias que permiten la separacion de los principales procesos del Modelo Estandar y la senal. Para la estimacion efectiva de la contribucion de senal en la region de interes se realiza un ajuste de maxima verosimilitud, en ingles maximum likelihood, a los datos. Se establece, asi mismo, un limite superior en la seccion eficaz de produccion para los diferentes modelos considerados. El procedimiento para hacerlo se basa una aproximacion asintotica del criterio CLs que fija dicho limite con un nivel de confianza del 95%. En los modelos donde el chargino decae en un slepton o un sneutrino se han logrado excluir masas con valores proximos a los 800 y 320 GeV para el chargino y el neutralino respectivamente, siendo los limites mas altos que se han obtenido hasta la fecha. Por otro lado, se ha encontrado una sensibilidad limitada en modelos donde el chargino decae en un boson W y un neutralino debido a la fraccion de desintegracion relativamente baja de la desintegracion leptonica de un boson W, lograndose asi excluir masas de chargino en el rango de los 170 a 200 GeV. En el caso de la produccion de una pareja squarks top, mientras que la estrategia del analisis ha sido optimizada para la zona de masa comprimida, los limites superiores de la seccion eficaz se presentan, por completitud, en todo el plano de masas. Cuando consideramos la zona comprimida, donde ∆m esta entre la masa del quark top y del boson W, y la desintegracion del squark top en un quark top y en un neutralino los valores de masa excluidos alcanzan los 420 y 360 GeV respectivamente. En el caso del segundo modo de decaimiento considerado la masa del quark top es excluida en el rango de 225-325 GeV. En resumen, con el estudio de los datos recogidos por el experimento CMS en 2016, esta busqueda extiende los actuales limites de exclusion en la produccion de parejas de charginos decayendo en sleptones, mejorando en aproximadamente 70 GeV el limite en la masa del chargino para un neutralino sin masa. Los limites de exclusion de la produccion de parejas de squark top extienden los resultados obtenidos por la Colaboracion CMS en estados finales con dos leptones de carga opuesta en la zona de masa comprimida, siendo estos competitivos con los resultados obtenidos por la Colaboracion ATLAS en el mismo canal de desintegracion. EnglishThis PhD thesis presents a search for pair production of supersymmetric particles in events with two oppositely charged leptons (electrons or muons) and missing transverse momentum. The data sample corresponds to an integrated luminosity of 35.9 fb −1 of proton-proton collisions at √s = 13 TeV collected with the CMS detector during the 2016 data taking period at the LHC. No significant deviation is observed from the predicted standard model background. The results are interpreted in terms of several simplified models for chargino and top squark pair production, assuming R-parity conservation and with the neutralino as the lightest supersymmetric particle. When the chargino is assumed to undergo a cascade decay through sleptons, with a slepton mass equal to the average of the chargino and neutralino masses, exclusion limits at 95% confidence level are set on the masses of the chargino and neutralino up to 800 and 320 GeV, respectively. For top squark pair production, the search focuses on models with a small mass difference between the top squark and the lightest neutralino. When the top squark decays into an off-shell top quark and a neutralino, the limits extend up to 420 and 360 GeV for the top squark and neutralino masses, respectively.

To sustain and extend its discovery potential, the Large Hadron Collider (LHC) will undergo a major upgrade in the coming years, referred to as High Luminosity LHC (HLLHC), aimed to increase its instantaneous luminosity, 5 times larger than the designed limit, and, consequently leading to high levels of radiation, with the goal to collect 10 times larger the original designed integrated luminosity. The drift tube chambers (DT) of CMS muon detector system is built to proficiently measure and trigger on muons in the harsh radiation environment expected during the HL-LHC era. Ageing studies are performed at the CERNs gamma ray irradiation facility (GIF++) by measuring the muon hit efficiency of these detectors at various LHC operation conditions. One such irradiation campaign was started in October 2017, when a spare MB2 chamber moved inside the bunker and irradiated at lower acceleration factors. Two out of twelve layers of the DT chamber were operated while being irradiated with the radioactive source and then their muon hit efficiency was calculated in coincidence with other ten layers which were kept on the standby. The chamber absorbed an integrated dose equivalent to two times the expected integrated luminosity of the HL-LHC. Investigation on the outgassing of cell materials and of the gas components used at the GIF++ are underway and strategies to mitigate the aging effects are also being developed. The effect of radiation on the performance of DT chamber and its impact on the overall muon reconstruction efficiency expected during the HL-LHC are presented.