A Search For Exotic Particles Decaying To Wz Pairs In Pp Collisions At S
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| Total Pages | : 0 |
| Release | : 2012 |
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A search for exotic particles decaying via WZ to final states with electrons and muons is performed using a data sample of pp collisions collected at 7 TeV center-of-mass energy by the CMS experiment at the LHC, corresponding to an integrated luminosity of 4.98 fb-1. A cross section measurement for the production of WZ is also performed on a subset of the collision data. No significant excess is observed over the Standard Model background, so lower bounds at the 95% confidence level are set on the production cross sections of hypothetical particles decaying to WZ in several theoretical scenarios. Assuming the Sequential Standard Model, W' bosons with masses below 1143 GeV/c2 are excluded. New limits are also set for several configurations of Low-Scale Technicolor.
| Author | : Weifeng Ji |
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| Total Pages | : 112 |
| Release | : 2018 |
| Genre | : Large Hadron Collider (France and Switzerland) |
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In this dissertation, I present the searches looking for decays of stopped massive long-lived exotic particles, which are pair produced in the proton-proton collisions at 13 TeV and come to rest afterwards in the CMS detector at the CERN LHC. The decays are most likely to be observed when there are no collisions. Two specific decay scenarios are studied: a long-lived particle (LLP) decaying hadronically or semi- leptonically in the calorimeter, and a LLP decaying semi-leptonically or leptonically to a pair of muons in the muon detector. The calorimeter-based search is performed using 2.7 fb-1 of 13 TeV collision run data collected in 2015 and 35.9 fb-1 of 13 TeV collision run data collected in 2016, corresponding to a total trigger livetime of 721 hours. The muon search is performed using 2.8 fb-1 of 13 TeV collision run data collected in 2015 and 36.2 fb-1 of 13 TeV collision run data collected in 2016, corresponding to a total trigger livetime of 744 hours. Search results are interpreted in a couple of simplified models. The limits on the production cross sections and masses of LLPs are calculated as a function of particle lifetimes, which vary from 100 ns to 10 days, assuming the corresponding decaying branching fractions to be 100%. The results are the most stringent ones on the stopped LLPs so far.
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| Release | : 2015 |
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A search is performed for long-lived massive neutral particles decaying to quark-antiquark pairs. The experimental signature is a distinctive topology of a pair of jets, originating at a secondary vertex. Events were collected with the CMS detector at the CERN LHC in proton-proton collisions at a center-of-mass energy of 8 TeV. The data analyzed correspond to an integrated luminosity of 18.5 fb–1. No significant excess is observed above standard model expectations. Upper limits at 95% confidence level are set on the production cross section of a heavy neutral scalar particle, H, in the mass range of 200 to 1000 GeV, decaying promptly into a pair of long-lived neutral X particles in the mass range of 50 to 350 GeV, each in turn decaying into a quark-antiquark pair. For X with mean proper decay lengths of 0.4 to 200 cm, the upper limits are typically 0.5–200 fb. The results are also interpreted in the context of an R-parity-violating supersymmetric model with long-lived neutralinos decaying into a quark-antiquark pair and a muon. For pair production of squarks that promptly decay to neutralinos with mean proper decay lengths of 2–40 cm, the upper limits on the cross section are typically 0.5–3 fb. As a result, the above limits are the most stringent on these channels to date.
| Author | : Matthias U. Mozer |
| Publisher | : Springer |
| Total Pages | : 121 |
| Release | : 2016-03-24 |
| Genre | : Science |
| ISBN | : 3319303813 |
The book discusses the recent experimental results obtained at the LHC that involve electroweak bosons. The results are placed into an appropriate theoretical and historical context. The work pays special attention to the rising subject of hadronically decaying bosons with high boosts, documenting the state-of-the-art identification techniques and highlighting typical results. The text is not limited to electroweak physics in the strict sense, but also discusses the use of electroweak vector-bosons as tool in the study of other subjects in particle physics, such as determinations of the proton structure or the search for new exotic particles. The book is particularly well suited for graduate students, starting their thesis work on topics that involve electroweak bosons, as the book provides a comprehensive description of phenomena observable at current accelerators as well as a summary of the most relevant experimental techniques.
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| Release | : 2005 |
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| Release | : 2013 |
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We present a search for a new particle T decaying to a top quark via T = t + X, where X goes undetected. We use a data sample corresponding to 5.7 fb-1 of integrated luminosity of pp collisions with √s = 1.96 TeV, collected at Fermilab Tevatron by the CDF II detector. Our search for pair production of T' is focused on the hadronic decay channel, pp = TT= tt+XX=bqqbqq + XX. We interpret our results in terms of a model where T is an exotic fourth generation quark and X is a dark matter particle. The data are consistent with standard model expectations. We set a limit on the generic production of TT = tt =+ XX, excluding the fourth generation exotic quarks T at 95% confidence level up to mT = 400 GeV/c2 for mX ≤ 70 GeV/c2.
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| Release | : 2013 |
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We present a measurement of the production cross section for ZW and ZZ boson pairs in final states with a pair of charged leptons, from the decay of a Z boson, and at least two jets, from the decay of a W or Z boson, using the full sample of proton-antiproton collisions recorded with the CDF II detector at the Tevatron, corresponding to 8.9 fb(̂-1) of integrated luminosity. We increase the sensitivity to vector boson decays into pairs of quarks using a neural network discriminant that exploits the differences between the spatial spread of energy depositions and charged-particle momenta contained within the jet of particles originating from quarks and gluons. Additionally, we employ new jet energy corrections to Monte Carlo simulations that account for differences in the observed energy scales for quark and gluon jets. The number of signal events is extracted through a simultaneous fit to the dijet mass spectrum in three classes of events: events likely to contain jets with a heavy-quark decay, events likely to contain jets originating from light quarks, and events that fail these identification criteria. We determine the production cross section to be 2.5 +2.0 -1.0 pb (
| Author | : Ewan Chin Hill |
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| Release | : 2017 |
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The ATLAS Experiment at the CERN Large Hadron Collider is a particle physics experiment to study fundamental particles and their interactions at very high energies. Supersymmetry is a theory of new physics beyond the Standard Model of particle physics. A search for directly produced pairs of the supersymmetric partner of the top quark was performed using 20 fb^-1 of proton--proton collision data at a centre of mass energy of 8 TeV taken in 2012. The search targeted a model where the supersymmetric partner of the top quark (``scalar top'') decays via the supersymmetric partner of the tau lepton (``scalar tau'') into the supersymmetric partner of the graviton (``gravitino''). Scalar top candidates were searched for in pp collision events with either two hadronically decaying taus, two light leptons (electrons or muons), or one hadronically decaying tau and one light lepton. The numbers of events passing the analysis selection criteria agree with the Standard Model expectations. Exclusion limits at the 95% confidence level were set as a function of the scalar top and scalar tau masses. Depending on the scalar tau mass, ranging from the 87 GeV limit set by the LEP experiments to a few GeV below the scalar top mass, lower limits between 490 GeV and 640 GeV were placed on the scalar top mass within the model considered.
