Browsing by Subject "LHC"

Now showing 1 - 4 of 4
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    Search for New Heavy Resonances Decaying To t+t^- Pairs at the LHC with Square Root S= 7 TEV (L = 5.0 FB^-1)
    (2015-01-22) Suarez, Indara
    The Standard Model (SM) describes the known fundamental particles and their interactions due to the electromagnetic, weak, and strong forces through vector boson exchange. Although the SM has had major success in predicting a wealth of experimental measurements, astrophysical evidence for dark matter the observation of neutrino oscillations, and the matter-antimatter asymmetry in the universe indicate that the SM is not a complete theory. In addition to these experimental observations, problems stemming from the failure to incorporate the gravitational force and the quantum instability of the mass of the Higgs Boson have also contributed to the motivation to search for physics beyond the SM. Multiple theoretical scenarios, including those inspired by Grand Unified Theories (GUTs), models with extra spatial dimensions, and Supersymmetry (SUSY), have been proposed to address the shortcomings of the SM. In many of these models, the new symmetries that extend the SM gauge structure require the existence of new heavy neutral gauge bosons. Regardless of the exact nature or production mechanism of the hypothesized heavy bosons, they may be observed by studying dilepton final states at high energy colliders. As many models of physics beyond the SM predict enhanced couplings to third generation particles, searches for the new heavy bosons decaying into two ? -leptons are particularly well motivated. We present a direct search for high mass neutral resonances decaying into two opposite sign ?-leptons using data from proton-proton collisions at the LHC with center-of-mass energy square root of s = 7 TeV. The search has been conducted using data recorded by the Compact Muon Solenoid (CMS) experiment, corresponding to an integrated luminosity of 4.94 fb?1 and includes final states with leptonic and hadronic decays of the ?-lepton. The data has been found to be consistent with the background-only hypothesis within the sensitivity of the measurement. Using the Sequential Standard Model Z?-boson as a benchmark, we set a 95% confidence-level upper limit on the mass of Z?-bosons decaying to pairs of ?-leptons.
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    Search for Supersymmetry in the Jets + Met + TAUS Final State Using the CMS Detector at the LHC
    (2013-02-28) Montalvo, Roy Joaquin
    In this dissertation results are presented from a search for the pair production of heavy colored particles (gluinos, squarks) in R-parity conserving supersymmetric models, in which the lightest supersymmetric particle is a stable and neutral object. The search was performed for events with at least two tau leptons, two highly energetic jets and large missing transverse momentum in the final state on a data sample of proton-proton collisions at sqrt(s) = 7 TeV. The data sample was collected by the Compact Muon Solenoid detector at the Large Hadron Collider in 2011, and it corresponds to an integrated luminosity of 5fb^?1. The tau isolation variable was optimized for this search. The number of events corresponding to standard model processes in the final selection was estimated to be 7.49 ? 0.74 using background estimation techniques based on data. Nine observed events are found to be in agreement with the standard model prediction, and exclusion limits on gluino mass are obtained in the context of supersymmetric models at the 95% confidence level.
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    Search for Supersymmetry Using Weak Boson Fusion Processes in Proton-Proton Collisions at the Large Hadron Collider
    (2014-08-08) Flanagan, Will
    In 2012, the Large Hadron Collider at CERN (LHC) collided protons at an unprecedented center-of-mass energy of 8 TeV. With data corresponding to a total integrated luminosity of 19.7 fb^(?1), the Compact Muon Solenoid (CMS) collaboration is studying various Standard Model processes as well as searching for Beyond the Standard Model processes through a plethora of distinct projects. This dissertation searches for Supersymmetry using weak boson fusion processes. These processes provide a unique opportunity to access the electroweak sector of super-symmetry. Though well motivated, this search finds no excess above the Standard Model. Exclusion limits are obtained in the context of electroweakino masses at the 95% confidence level.
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    Searching for Top Squarks at the Large Hadron Collider
    (2014-08-01) Wang, Kechen
    This dissertation describes the search strategies we developed for the lighter top squark (called stop, or ?t) at the Large Hadron Collider (LHC). When the lighter top squarks are produced from the cascade decay of gluino and squark, the analysis is performed in the stop-neutralino coannihilation region where stop decays into a charm quark and the stable lightest supersymmetric particle (called lightest neutralino, or X ?_1^0). We develop observables through the endpoint measurements to determine the stop masses. When the lighter top squarks are produced from the direct production processes of stop pairs (?t?t*), three scenarios are investigated. In the fully hadronic final state scenario, we investigate the identification of stops which decay predominantly into a top quark and the stable lightest neutralino. A simple kinematical variable, M3, is used to reconstruct two top quarks which are pair-produced from the stops in the fully hadronic channel. We identify kinematical variables to reduce the standard model (SM) background. The expected mass reach of stop is shown at 8-TeV LHC (LHC8). In the Bino-Higgsino dark matter scenario, the lightest neutralino is a mixture of Bino and Higgsino, satisfying the thermal dark matter relic density. Stop can decay into a top quark plus the second or third lightest neutralino (called X ?_2^0, X ?_3^0), and the second or third lightest neutralino can decay into 2 leptons plus the lightest neutralino via an intermediate slepton (?light selpton? case) or Z boson (?heavy slepton? case). The final states have at least 2 jets, 2 opposite-sign same flavor leptons and missing energy. The opposite-sign same flavor dilepton mass distribution after subtracting the opposite-sign different flavor distribution shows a clear edge in the case of light slepton. We also calculate the significance at LHC8 for discovering such a scenario in both light slepton case and heavy slepton case. In the compressed scenario where the mass difference between stop and the lightest neutralino is approximately equal to the mass of the top quark, stop does either the two-body decay of a top quark, and the lightest neutralino (?two-body decay? case, when mass difference is slightly greater than the top quark mass), or the three-body decay of a bottom quark, a W boson and the lightest neutralino (?three-body decay? case, when the mass difference is smaller than the top quark mass). We perform the study for both two-body and three-body decay cases in the final state of two b-jets, one lepton, large missing energy, and two high energetic Vector Boson Fusion tagging jets with large separation in pseudo-rapidity, in opposite hemispheres, and with large dijet mass. The expected experiment discovery and exclusion limits of such a compressed scenario are shown at _(14)-TeV LHC (LHC_(14)) for both cases.