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Study of the rare B-meson decays with the ATLAS experiment
Biroš, Marek ; Řezníček, Pavel (advisor) ; Scheirich, Daniel (referee)
Rare B0 d → K∗ µ− µ+ decay is one of physics channels testing Stan- dard Model predictions at the ATLAS experiment. This work provides input to the future analysis of this rare decay in the data-taking period of Run 2. Speci- fically, in the work we were searching for the descriptions of the distributions of reconstructed invariant masses of the B and K∗ mesons and of the distribution of the decay angles for the signal decay and expected background processes. For the procedure of finding optimal description, a maximal likelihood method of fit was used. Search for the probability density functions was performed on simulated data containing signal decay and main the background channels (decays, which are misinterpreted by the detector like our signal decay). The background decays included Bs → φ(K+ K− )µ+ µ− , Λb → Λ(1520)(pK− )µ+ µ− and Λb → pK− µ+ µ− . The signal data were contamined with events, when detector misidentified the kaon and pion from the K∗ meson decay, which lead to modifications of the fit- ted parameters. These cases were separated and treated as special background process. In addition to the simulations, we analyzed the combinatorial background from the real data of Run 1, though only the angular distribution. Invariant mas- ses were not analyzed for their simplicity. We also show...
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Studium rozpadů Higgsova bosonu na páry tau leptonů s použitím detektoru ATLAS na LHC
Kello, Tomáš ; Scheirich, Daniel (advisor) ; Řezníček, Pavel (referee)
The analysis of the Higgs boson decay into the pairs of tau leptons H → ττ is presented. In the scope of this thesis, only the tau leptons decaying purely lep- tonically are considered. This analysis is performed using data recorded from the proton-proton collisions at a center-of-mass energy √ s = 13 TeV by the ATLAS detector at the Large Hadron Collider and correspond to the integrated lumi- nosity of 36.1 fb−1 . The Boosted Decision Trees (BDT) multivariate technique was used in order to provide a classification of events. Datasets considered in this analysis consisted of the triggered, reconstructed and calibrated events. In addition, events had to pass the pre-selection criteria and were separated into four categories. Our main contribution to this analysis is based on developing and incorporating techniques used in the process of BDT training and evaluation. The performed analysis presented in this thesis is blinded. Therefore, the Monte Carlo simulation was used instead of real data in the Maximum Likelihood Fit in order to determine the expected significance. For the H → ττ leptonic channel, the expected signal significance was measured to be 1.16 σ. 1
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Study of the lepton flavor violating decays of the Higgs boson at the ATLAS experiment
Kouba, Petr ; Scheirich, Daniel (advisor) ; Mlynáriková, Michaela (referee)
Title: Study of the Lepton Flavor Violating Decays of the Higgs Boson at the ATLAS Experiment Author: Petr Kouba Institute: Institute of Particle and Nuclear Physics Supervisor: Mgr. Daniel Scheirich, Ph.D., Institute of Particle and Nuclear Physics Abstract: The thesis contains an analysis of the potential lepton flavor violating Higgs boson decays performed on the data acquired by the ATLAS experiment during the Run II period of the LHC. The analysis comprised of an event selection aiming at the reduction of the backgrounds, while keeping the highest possible amount of the signal events. An estimate of the leptons misidentified by the reconstruction algorithms of the ATLAS experiment was included in the anal- ysis. For the selected events, invariant mass was reconstructed in the collinear approximation. The distribution of the invariant mass according to the MMC was provided for reference purposes. Keywords: Higgs boson, ATLAS, collinear approximation ii
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Study of proton-proton and heavy-ion collisions with the ATLAS experiment at the LHC
Balek, Petr ; Dolejší, Jiří (advisor) ; Scheirich, Daniel (referee) ; Šumbera, Michal (referee)
Title: Study of proton-proton and heavy-ion collisions with the ATLAS experi- ment at the LHC Author: Mgr. Petr Balek Department: Institute of Particle and Nuclear Physics Supervisor: Doc. RNDr. Jiří Dolejší, CSc. Abstract: The presented results show a suppression in a production of charged hadrons in heavy-ion collisions with respect to the production measured in proton- proton collisions. Lead-lead data at √ sNN = 2.76 TeV were recorded by the AT- LAS experiment at the LHC in 2010 and 2011 and they have a total integrated luminosity of 0.15 nb−1 , and proton-proton data at √ s = 2.76 TeV were recorded in 2011 and 2013 having an integrated luminosity of 4.2 nb−1 . The results are presented in wide transverse momentum (0.5 < pT < 150 GeV) and pseudorapid- ity (|η| < 2) ranges, and also in different centrality intervals of Pb+Pb collisions. Nuclear modification factor RAA shows a clear pT-dependence with a minimum at around 7 GeV, however above 60 GeV, it reaches a plateau with no significant pT-dependence. As a function of |η|, the RAA is flat over whole pT region. Keywords: ATLAS detector, heavy-ion collisions, charged-hadron spectra, nuclear modification factor RAA
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Measurement of the Λb baryon mass in the ATLAS experiment
Macko, Vladimír ; Scheirich, Daniel (advisor) ; Faltová, Jana (referee)
The mass of the Λb baryon has been studied in proton-proton collisions using experimental data from the ATLAS detector at the LHC in CERN. The mass is measured by reconstructing momenta of products in the studied decay. Decay channel Λb → J/ψ(µ+ µ− )Λ(pπ− ) is chosen for the mass reconstruction in this work. The selection criteria are applied in order to minimize background effect and, therefore, obtain the value of the mass of the Λb baryon with the lower statistical error. The systematic error is obtained from a comparison of the three statistical models describing experimental data. The result of measurement of the Λb baryon mass in the ATLAS experiment performed in this work is: mΛb = (5619.1 ± 1.0) MeV. 1
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Study of hard processes in heavy ion collisions at ATLAS
Štefko, Pavol ; Spousta, Martin (advisor) ; Scheirich, Daniel (referee)
Jet production in PbPb collisions at a per-nucleon center-of-mass energy of 2.76 TeV has been studied using the ATLAS detector at the LHC. Interactions between the high- pT partons and the hot, dense medium, produced in these ultrarelativistic collisions, are expected to cause the loss of the jet energy (jet quenching). This thesis presents results of the jet analysis done on the data taken during the 2011 heavy-ion run at the LHC as well as PYTHIA Monte Carlo reference. Jets are reconstructed using the anti-kt jet clustering algorithm and studied as a function of collision centrality and dijet energy imbalance. With increasing centrality, dijets are observed to be increasingly asymmetric, consistent with the theory of jet quenching. The study of charged particle tracks indicates the increase of the low-pT tracks in the strongly quenched jets. 1
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Development of optimal algorithms for the selection of rare semimuonic B decays in ATLAS
Scheirich, Daniel ; Doležal, Zdeněk (advisor) ; Mašík, Jiří (referee)
In this thesis we propose the trigger algorithm for the semimuonic rare B decay selection in ATLAS experiment and we study its performance using the Monte Carlo sample of b 0u+u. The cut optimisation have been performed in order to obtain the optimal cut values and the signal rate for the process was estimated. The feasibility and performance of the tag-and-probe method for the calibration of di-muon triggers is investigated, too. Eects of the systematic uncertainties of the method on the measurements of J/ polarisation and the forward-backward asymmetry of b 0u+u were studied.
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