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A search for physics beyond the standard model in events with at least three leptons is presented. The data sample, corresponding to an integrated luminosity of 19.5 inverse femtobarns of proton-proton collisions with center-of-mass energy sqrt(s) = 8 TeV, was collected by the CMS experiment at the LHC during 2012. The data are divided into exclusive categories based on the number of leptons and their flavor, the presence or absence of an opposite-sign, same-flavor lepton pair (OSSF), the invariant mass of the OSSF pair, the presence or absence of a tagged bottom-quark jet, the number of identified hadronically decaying tau leptons, and the magnitude of the missing transverse energy and of the scalar sum of jet transverse momenta. The numbers of observed events are found to be consistent with the expected numbers from standard model processes, and limits are placed on new-physics scenarios that yield multilepton final states. In particular, scenarios that predict Higgs boson production in the context of supersymmetric decay chains are examined. We also place a 95% confidence level upper limit of 1.3% on the branching fraction for the decay of a top quark to a charm quark and a Higgs boson (t to c H), which translates to a bound on the left- and right-handed top-charm flavor-violating Higgs Yukawa couplings, lambda[H, tc] and lambda[H, ct], respectively, of sqrt(abs(lambda[H, tc])2̂ + abs(lambda[H, ct])2̂)
The authors report a search for the anomalous production of events with multiple charged leptons in p{bar p} collisions at √s = 1.96 TeV using a data sample corresponding to an integrated luminosity of 346 pb−1 collected by the CDF II detector at the Fermilab Tevatron. The search is divided into three-lepton and four-or-more-lepton data samples. They observe six events in the three-lepton sample and zero events in the ≥4-lepton sample. Both numbers of events are consistent with standard model background expectations. Within the framework of an R-parity violating supergravity model, the results are interpreted as mass limits on the lightest neutralino ({bar {chi}}1°) and chargino ({bar {chi}}1{sup {+-}}) particles. For one particular choice of model parameters, the limits are M({bar {chi}}1°) > 110 GeV/c2 and M({bar {chi}}1{sup {+-}}) > 203 GeV/c2 at 95% confidence level; the variation of these mass limits with model parameters is presented.
The authors report a search for the anomalous production of events with multiple charged leptons in p{bar p} collisions at √s = 1.96 TeV using a data sample corresponding to an integrated luminosity of 346 pb−1 collected by the CDF II detector at the Fermilab Tevatron. The search is divided into three-lepton and four-or-more-lepton data samples. They observe six events in the three-lepton sample and zero events in the ≥4-lepton sample. Both numbers of events are consistent with standard model background expectations. Within the framework of an R-parity violating supergravity model, the results are interpreted as mass limits on the lightest neutralino ({bar {chi}}1°) and chargino ({bar {chi}}1{sup {+-}}) particles. For one particular choice of model parameters, the limits are M({bar {chi}}1°)> 110 GeV/c2 and M({bar {chi}}1{sup {+-}})> 203 GeV/c2 at 95% confidence level; the variation of these mass limits with model parameters is presented.
This dissertation presents two searches for phenomena beyond the Standard Model using events with three or more charged leptons. The searches are based on 20.3 fb-1 of proton- proton collision data with a center-of-mass energy of √s = 8 TeV collected by the ATLAS detector at the CERN Large Hadron Collider in 2012. The first is a model-independent search for excesses beyond Standard Model expectations in many signal regions. The events are required to have least three charged leptons, of which at least two are electrons or muons, and at most one is a hadronically decaying [tau] lepton. The selected events are categorized based on the flavor and charge of the leptons, and the signal regions are defined using several kinematic variables sensitive to beyond the Standard Model phenomena. The second search looks for new heavy leptons decaying resonantly to three electrons or muons, two of which are produced through an intermediate Z boson. The resonant decay produces a narrowly- peaked excess in the trilepton mass spectrum. In both cases, no significant excess beyond Standard Model expectations is observed, and the data are used to set limits on models of new physics. The model-independent trilepton search is used to confront a model of doubly charged scalar particles decaying to e[tau] or [mu][tau], excluding masses below 400 GeV at 95% confidence level. The trilepton resonance search is used to test models of vector-like leptons and the type III neutrino seesaw mechanism. The vector-like lepton model is excluded for most of the mass range 114 GeV - 176 GeV, while the type III seesaw model is excluded for most the mass range 100 GeV - 468 GeV. Both searches also present tools to facilitate reinterpretations in the context of other models predicting the production of three or more charged leptons.
A search is presented for anomalous production of events containing multiple low-energy leptons produced in association with a W or Z boson using the Collider Detector at Fermilab. The leptons are not required to be isolated, and the signature is therefore sensitive to a wide range of so-called "lepton jet" topologies. The search uses data corresponding to 5.1 fb--1 of integrated luminosity from pp ̄ collisions at a center-of-mass energy of s = 1.96 TeV. The observed events are compared with the standard model predictions in bins of additional electron and muon multiplicity. No indications are found of phenomena beyond the standard model. A 95% confidence level limit is presented on the production cross section for an example benchmark point of a dark-matter inspired model of supersymmetric hidden valley Higgs production. Object identification efficiencies are also provided in order to enable the calculation of limits on additional models.
This paper presents a search for anomalous production of multiple low-energy leptons in association with a $W$ or $Z$ boson using events collected at the CDF experiment corresponding to $5.1$ fb${̂-1}$ of integrated luminosity. This search is sensitive to a wide range of topologies with low-momentum leptons, including those with the leptons near one another. The observed rates of production of additional electrons and muons are compared with the standard model predictions. No indications of phenomena beyond the standard model are found. A 95\% confidence level limit is presented on the production cross section for a benchmark model of supersymmetric hidden-valley Higgs production. Particle identification efficiencies are also provided to enable the calculation of limits on additional models.
We present a search for anomalous production of the signature lepton + photon + b-quark + missing transverse energy (`[gamma]6ETb) produced in $p\bar{p}$ collisions at √ s = 1.96 TeV using 6.0 fb-1 of data taken with the CDF detector in Run II at the Tevatron. In addition to this signature-based search, we present a closely-related search for top pair production with an additional radiated photon, t$\bar{t}$[gamma]. We find 85 `[gamma]6ETb events versus an expectation of 99.1±9.3 events. High-statistics control samples are used to evaluate a low-energy photon [chi] 2 cut. Additionally requiring the events to contain at least three jets, have a total transverse energy of 200 GeV, and that the photon candidate passes the [chi] 2 cut, we observe 30 t$\bar{t}$[gamma] candidate events versus an expectation from non-top Standard Model (SM) sources of 13.0 ± 2.1 Assuming the difference between the observed number and the predicted non-top SM total is due to top production, we measure the tt$\bar{[gamma]}$ cross section to be 0.18 ± 0.08 pb. We also measure the ratio of the t$\bar{t}$[gamma] cross section to the t$\bar{t}$cross section to be 0.024 ± 0.009, which compares well with a SM prediction of 0.024 ± 0.013.
Abstract: A search with minimal model dependence for physics beyond the Standard Model in events featuring three or four charged leptons (3l and 4l, ) is presented. The analysis aims to be sensitive to a wide range of potential new-physics theories simultaneously. This analysis uses data from pp collisions delivered by the Large Hadron Collider at a centre-of-mass energy of TeV and recorded with the ATLAS detector, corresponding to the full Run 2 dataset of 139 fb−1. The 3l and 4l phase space is divided into 22 event categories according to the number of leptons in the event, the missing transverse momentum, the invariant mass of the leptons, and the presence of leptons originating from a Z-boson candidate. These event categories are analysed independently for the presence of deviations from the Standard Model. No statistically significant deviations from the Standard Model predictions are observed. Upper limits for all signal regions are reported in terms of the visible cross-section
Abstract: The results presented in this thesis use the full dataset collected in 2012 by the ATLAS experiment from the LHC proton-proton collisions at sqrt(s) = 8 TeV, corresponding to an integrated luminosity of 20.3 fb^(−1). The first part is devoted to the search for the WWW production in the decay channel where each W boson decays leptonically (excluding leptonic tau decays). Upper limits are placed on the production cross-section in the fiducial phase space. The results are also interpreted in terms of the total cross-section when combined together with the semi-leptonic decay channel. In addition, limits on anomalous quartic gauge boson couplings in an effective field theory approach are extracted. The second part presents a search for supersymmetry in events with multiple jets, two leptons of identical charges or three leptons (electrons or muons). The absence of any excess in the signal regions with respect to the Standard Model expectations is interpreted in terms of limits in the parameter space of 14 supersymmetric models and one mUED model
A search for excited leptons is carried out with the CMS detector at the LHC, using 36 inverse picobarns of pp collision data recorded at sqrt(s) = 7 TeV. The search is performed for associated production of a lepton and an oppositely charged excited lepton pp to l l*, followed by the decay l* to l gamma, resulting in the l l gamma final state, where l = electron or muon. No excess of events above the standard model expectation is observed. Interpreting the findings in the context of l* production through four-fermion contact interactions and subsequent decay via electroweak processes, first upper limits are reported for l* production at this collision energy. The exclusion region in the compositeness scale Lambda and excited lepton mass M(l*) parameter space is extended beyond previously established limits. For Lambda = M(l*), excited lepton masses are excluded below 1070 GeV/c2̂ for e* and 1090 GeV/c2̂ for mu* at the 95% confidence level.