The H1 and ZEUS collaborations have recently reported a significant excess of
e
+
p →
e
+ jet events at high-
Q
2. While there exists insufficient data to conclusively determine the origin of this ...excess, one possibility is that it is due to a new leptoquark at mass scale around 200 GeV. We examine the type of leptoquark states that exists in superstring-derived standard-like models, and show that, while these models may contain the standard leptoquark states which exist in Grand Unified Theories, they also generically contain new and exotic leptoquark states with fractional lepton number,
±
1
2
. In contrast to the traditional GUT-type leptoquark states, the couplings of the exotic leptoquarks to the Standard Model states are generated after the breaking of
U(1)
B−
L
. This important feature of the exotic leptoquark states may result in local discrete symmetries which forbid some of the undesired leptoquark couplings. We examine these couplings in several models and study the phenomenological implications. The flavor symmetries of the superstring models are found to naturally suppress leptoquark flavor-changing processes.
Leptoquark searches in TESLA Żarnecki, Aleksander Filip
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
10/2001, Volume:
472, Issue:
1
Journal Article
Peer reviewed
Leptoquark searches are considered for possible future e
+e
− and e
±γ experiments at TESLA. Expected exclusion limits from direct leptoquark searches are compared with indirect limits expected from ...the contact interaction analysis. If the leptoquark signal suggested by the existing data is detected at LHC, with mass below
1
TeV
, TESLA will be an ideal place to study its properties, provided that the leptoquark Yukawa coupling is not too small.
A review of the experimental searches for physics beyond the Standard Model performed at collider facilities is presented. The emphasis is put on topics of interest for future data-taking at the ...upgraded Tevatron pp and HERA ep colliders. The status and discovery prospects for leptoquarks, lepton flavour violation, technicolour, R-parity violating supersymmetry, flavour-changing neutral currents, excited fermions and contact interactions is discussed.
L'expérience CMS (Compact Muon Solenoid), construite sur l'anneau du LHC (Large Hadron Collider) a enregistré une quantité colossale de données provenant des collisions proton-proton de 2009 à 2012 à ...des énergies dans le centre de masse de 7 TeVet 8 TeV. Cette expérience est consacrée aux mesures de précision des paramètres du modèle standard, à l'étude du boson de Higgs ainsi qu'à la mise à l'épreuve de théories au-delà du modèle standard. Un certain nombre de ces théories prévoient l'existence de particules appelées leptoquarks, de nouveaux bosons se couplant à la fois aux leptons et au quarks. La première partie de cette thèse est consacrée à la présentation du Modèle Standard des particules et à une introduction théorique aux leptoquarks. La seconde présente brièvement le fonctionnement du LHC et de l'expérience CMS. La troisième partie est dédiée au travail effectué dans le groupe générateur de CMS au cours de la première année de ma thèse. Cette mission consistait principalement à générer et à valider massivement des évènements Monte-Carlo avec le logiciel Madgraph pour l'ensemble de la collaboration CMS. Enfin, la dernière partie porte sur l'analyse de données et la recherche de paires de leptoquarks scalaires de troisième génération. Trois canaux de désintégration des LQ sont envisagés : en quark top et lepton tau, en quark top et neutrino et enfin en quark top et muon. Cette étude est basée sur l'ensemble des données enregistrées par l 'expérience CMS au cours de l'année 2012. Ce qui représente une luminosité intégrée totale de 19,7 fb. Différentes sélections basées sur au moins un lepton et de nombreux jets sont examinées afin de définir les régions présentant les meilleures efficacités de signal par rapport au fond du Modèle Standard. Enfin, une optimisation des coupures sur des variables très discriminantes est réalisée afin d'obtenir les meilleures limites à 95% de confiance sur les sections efficaces de production des leptoquarks. Aucun excès significatif par rapport aux prévisions du modèle standard n'a pu être mis en évidence ce qui a permis de poser des limites sur les sections efficaces de production des leptoquarks. Les leptoquarks de troisième génération sont exclus jusqu'à des masses de 320 GeV à 95 % de niveau de confiance pour le canal top tau, jusqu'à 240 GeV pour le canal top nu et jusqu'à 910 GeV pour le canal top mu
Installed on the LHC (Large Hadron Collider) ring, the CMS experiment (Compact Muon Solenoid) recorded a great amount of proton-proton collisions data from 2009 to 2012 at center of mass energies of 7 TeV and 8 TeV. This experiment was designed for precise measurements of the Standard Model parameters, for Higgs boson studies and to test theories beyond the standard model. Some of these theories foresee the existence of leptoquarks which are new bosons with couplings to both leptons and quarks. The first part of this thesis is dedicated to a presentation of the Standard Model and to a leptoquarks theoretical introducion. The second one briefly explains how the LHC and the CMS experiment work. The third part details the work accomplished during the first year of my thesis in the CMS generator group. Finaly, the last one deals with data analysis and the search of third generation leptoquarks pair production. Three LQ desintegration channels are investigated : to a top quark and a tau lepton, to a top quark and a neutrino and at last to a top quark and a muon. This study is based on the full integrated luminosity recorded during 2012, namely 19.7 fb. Various selections requiring at least one lepton and several jets are scrutinized in order to identify the regions with best signal efficiency regarding to Standard model backgrounds. Finally, a selection optimisation on discriminative variables is performed to extract the best 95 No significative excess with respect to Standard Model expectations was found. Thus we were able to set limits on the leptoquarks pair production cross sections. Third generation leptoquarks with masses below 320 GeV, 240 GeV and 910 GeV are excluded for the top tau, top nu and top mu channels, respectively
Non-vanishing leptoquark-Higgs couplings result in mixing of different leptoquark (LQ)
SU(2)
L
multiplets leading to new low energy four fermion vertices. The rare process
K
L
→
μ
±
e
∓ is explored ...in order to derive bounds on the LQ parameter space when LQ mixing-matrices are taken into account.
Results are presented from the ATLAS and CMS searches for charged Higgs bosons, supersymmetry, and exotica with tau leptons in the final state. The analyses discussed use a dataset corresponding to 5 ...fb−1 of integrated luminosity from s=7TeV proton–proton collisions at the LHC in 2011.
We calculate the CP-violating electric and weak dipole form factors of the top quark and the tau lepton in models with scalar leptoquarks coupling only to the third generation of quarks and leptons. ...We obtain numerical values of the real and imaginary parts of these form factors at various energies for different values of leptoquark masses and couplings. The existing limits on the tau electric and weak dipole form factors allow us to put a limit on the masses and couplings of such leptoquarks and therefore on the top electric and weak dipole form factors. We also discuss constraints on the form factors coming from indirect limits on leptoquark masses and couplings deduced from LEP results on Z properties.
A search for pair production of third-generation scalar leptoquarks and supersymmetric top quark partners, top squarks, in final states involving tau leptons and bottom quarks is presented. The ...search uses events from a data sample of proton–proton collisions corresponding to an integrated luminosity of 19.7 fb−1, collected with the CMS detector at the LHC with s=8 TeV. The number of observed events is found to be in agreement with the expected standard model background. Third-generation scalar leptoquarks with masses below 740 GeV are excluded at 95% confidence level, assuming a 100% branching fraction for the leptoquark decay to a tau lepton and a bottom quark. In addition, this mass limit applies directly to top squarks decaying via an R-parity violating coupling λ333′. The search also considers a similar signature from top squarks undergoing a chargino-mediated decay involving the R-parity violating coupling λ3jk′. Each top squark decays to a tau lepton, a bottom quark, and two light quarks. Top squarks in this model with masses below 580 GeV are excluded at 95% confidence level. The constraint on the leptoquark mass is the most stringent to date, and this is the first search for top squarks decaying via λ3jk′.
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