Title: Study of jet production with the ALICE experiment at the LHC Author: Vít Kučera Department: Institute of Particle and Nuclear Physics Supervisor: RNDr. Jana Bielčíková, Ph.D., Nuclear Physics Institute of the ASCR, v.v.i. Abstract: The theory of strong interaction, quantum chromodynamics, predicts a phase transition between hadronic matter and the quark-gluon plasma where quarks and gluons are deconfined. This state of matter is expected to be created in ultra-relativistic collisions of heavy ions. Jets from partons which interact with the strongly interacting matter created in heavy-ion collisions can be used as probes to study this medium. The ALICE experiment installed at the Large Hadron Collider at CERN enables using jets to study medium in heavy-ion collisions. An analysis of data measured with the ALICE experiment in 2010 in the lead nuclei collisions at energy √ sNN = 2.76 TeV is presented in the thesis. The results of this analysis are presented in a study of jets reconstructed with the kt and anti-kt algorithms. The study includes an analysis of tracks, an analysis of background and focuses on an analysis of inclusive jet spectra. Various aspects of jet spectra are studied including modification of jets in the central collisions expressed by the ratio RCP. Also a summary of author's...

Este trabalho consistiu em analisar a possibilidade de verificação da violação de sabor em correntes neutras intermediadas por glúons de Kaluza-Klein (GKK) através do modelo de dimensões extras compactas e curvas. Este introduz a possibilidade de violação de sabores em correntes neutras através de processos do tipo próton/próton em top/anti-quark c. Inicialmente estudamos o comportamento de jatos com altos momentos transversos (da ordem de 300 GeV) e suas subestruturas. Após isto, estudamos a detecção do quark top através dos algoritmos HEPTopTagger e Johns Hopkins (fizemos uma breve comparação entre eles). Por último, propusemos duas maneiras de encontrar os GKK. Uma delas, através da procura de somente quarks top. A outra, através da procura de um quark top e um jato sem subestrutura (algoritmo do tipo TJ). Pudemos ver que a possibilidade de encontrar o fenômeno de violação de sabores é acessível (a razão S/B da ordem de 0,1) para ambos os casos de massa analisados através do algoritmo do tipo TJ. / This study was to examine the possibility of verifying the violation of flavor in neutral currents mediated by Kaluza-Klein gluons (GKK), in which it was introduced by compact warped extra dimensions. The possibility of violation of flavors in neutral currents occurs through processes like proton/proton into quark top/ antiquark charm. Initially we studied the behavior of jets with high transverse momentum (the order of 300 GeV) and its substructures. After this, we studied the detection of the top quark via HEPTopTagger and Johns Hopkins algorithms (we did a brief comparison between them). Finally, we proposed two ways to find the GKK. One of them, by searching for only quark top. The other, by looking for a quark top and a jet without substructure (algorithm of TJ type). We could see that the possibility of encountering the phenomenon of violation of flavors is accessible (the ratio S/B is the order of 0.1) in both cases of mass analyzed by TJ type algorithm.

Este trabalho consistiu em analisar a possibilidade de verificação da violação de sabor em correntes neutras intermediadas por glúons de Kaluza-Klein (GKK) através do modelo de dimensões extras compactas e curvas. Este introduz a possibilidade de violação de sabores em correntes neutras através de processos do tipo próton/próton em top/anti-quark c. Inicialmente estudamos o comportamento de jatos com altos momentos transversos (da ordem de 300 GeV) e suas subestruturas. Após isto, estudamos a detecção do quark top através dos algoritmos HEPTopTagger e Johns Hopkins (fizemos uma breve comparação entre eles). Por último, propusemos duas maneiras de encontrar os GKK. Uma delas, através da procura de somente quarks top. A outra, através da procura de um quark top e um jato sem subestrutura (algoritmo do tipo TJ). Pudemos ver que a possibilidade de encontrar o fenômeno de violação de sabores é acessível (a razão S/B da ordem de 0,1) para ambos os casos de massa analisados através do algoritmo do tipo TJ. / This study was to examine the possibility of verifying the violation of flavor in neutral currents mediated by Kaluza-Klein gluons (GKK), in which it was introduced by compact warped extra dimensions. The possibility of violation of flavors in neutral currents occurs through processes like proton/proton into quark top/ antiquark charm. Initially we studied the behavior of jets with high transverse momentum (the order of 300 GeV) and its substructures. After this, we studied the detection of the top quark via HEPTopTagger and Johns Hopkins algorithms (we did a brief comparison between them). Finally, we proposed two ways to find the GKK. One of them, by searching for only quark top. The other, by looking for a quark top and a jet without substructure (algorithm of TJ type). We could see that the possibility of encountering the phenomenon of violation of flavors is accessible (the ratio S/B is the order of 0.1) in both cases of mass analyzed by TJ type algorithm.

This thesis presents a search for resonant production of top-antitop quark pairs in final states containing at least one electron or muon. A number of beyond the standard model (BSM) theories incorporate a specific role for the top quark, resulting in resonances that preferentially decay to t(bar over t) pairs. The data sample analysed corresponds to an integrated luminosity of 2.05 fb^-1 recorded during 2011 using the ATLAS detector at the Large Hadron Collider. The proton-proton centre-of-mass energy was 7 TeV. The search is tailored towards heavy resonances at the TeV-scale which therefore decay to top quarks with high transverse momentum. Large hadronic jets are used to reconstruct the energy carried by the hadrons and the substructure of these jets is studied in order to identify hadronically decaying top quarks. The reconstruction can therefore proceed even if the decay products of the top quark are highly collimated. This study represents the first use of jet substructure techniques in a search for t(bar over t) resonances using hadron-hadron collision data. The invariant mass of the reconstructed t(bar over t) pair is used to test compatibility of the data with the standard model prediction. No evidence for t(bar over t) resonances is found. Upper limits are derived on the production cross-section times branching ratio for narrow and wide resonant states, at the 95 % credibility level. An upper limit of 0.61 (0.65) pb is set for a narrow (wide) resonance with a mass of 1 TeV. Two specific BSM models are excluded within certain mass ranges: the narrow leptophobic Z' boson with mass 600 - 1150 GeV and the wide Kaluza-Klein gluon with a mass below 1.5 TeV. These results represent a significant improvement on those of previous searches performed at the ATLAS experiment, which did not use jet substructure techniques. In addition, the possibility of using jets which decrease in size as their transverse momentum increases is investigated using simulated data. The yield of events due to resonant t(bar over t) production increases by approximately 20 % compared to when using jets of fixed size. Furthermore, the resolution of the invariant mass of the reconstructed t(bar over t) pair is found to improve by almost one fifth. It is recommended that a calibration scheme be developed for these variable-sized jets, so that their potential to improve the sensitivity to t(bar over t) resonances can be investigated further.