440 research outputs found
Robustness of the ATLAS pixel clustering neural network algorithm
No full textProton-proton collisions at the energy frontier puts strong constraints on track reconstruction algorithms. The algorithms depend heavily on accurate estimation of the position of particles as they traverse the inner detector elements. An artificial neural network algorithm is utilised to identify and split clusters of neighbouring read-out elements in the ATLAS pixel detector created by multiple charged particles. The method recovers otherwise lost tracks in dense environments where particles are separated by distances comparable to the size of the detector read-out elements. Such environments are highly relevant for LHC run 2, e.g. in searches for heavy resonances. Within the scope of run 2 track reconstruction performance and upgrades, the robustness of the neural network algorithm will be presented. The robustness has been studied by evaluating the stability of the algorithm’s performance under a range of variations in the pixel detector conditions
Spin studies of the discovered 125 GeV Higgs boson in theH → W W ∗→ £ν£ν channel using the ATLAS detector
No full textCharged particle distributions and robustness of the neural network pixel clustering in ATLAS
No full textThis thesis contains a study of the robustness of the artificial neural network used in the ATLAS track reconstruction algorithm as a tool to recover tracks in dense environments. Different variations, motivated by potential discrepancies between data and simulation, are performed to the neural network’s input while monitoring the corresponding change in the output. Within reasonable variation magnitudes, the neural networks prove to be robust to most variations. In addition, a measurement of charged particle distributions is summarised. This is one of the first such measurements carried out for proton-proton colli- sions at √s = 13 TeV, limited to a phase space defined by transverse momentum pT > 100 MeV and absolute pseudorapidity |η| < 2.5. Tracks are corrected for de- tector inefficiencies and unfolded to particle-level. The result is compared to the prediction of different models. Overall, the EPOS and Pythia 8 A2 models show the best agreement with the data. Spår från elektriskt laddade partiklar rekonstrueras i ATLAS genom att kombinera mätningar från de innersta subdetektorerna. I de extrema miljöer som skapas i proton-proton-kollisionerna i Large Hadron Collider vid CERN är det av yttersta vikt att algoritmen för att rekonstruera spår är högpresterande. Uppgiften är särskilt svår i partikelrika miljöer där flera partiklar färdas nära varandra, åtskilda av avstånd jämförbara med storleken på detektorns utläsningselement. Ett artificiellt neuralt nätverk används i algoritmen för att klassificera mätdata från pixeldetektorn, belägen närmast interaktionspunkten, för att lyckas identifiera spår i partikelrika miljöer som annars hade gått förlorade. I denna avhandling utreds det neurala nätverkets stabilitet. Dess känslighet studeras genom att manuellt manipulera dess indata och därefter utvärdera dess resultat. Nätverket tränas med simulerad data. Variationerna i indata är utformade för att undersöka skillnader mellan data och simulering, orsakade av osäkerheter i simuleringsmodellen eller osäkerheter i pixeldetektorns kalibrering. Av de undersökta variationerna har en osäkerhet i skalan eller utläsningströskeln för pixeldetektorns kalibrering den största effekten på nätverkets resultat. Andra variationer har en betydligt mindre påverkan. Avhandlingen presenterar också en studie av distributioner av elektriskt laddade partiklar producerade i proton-proton-kollisioner. Det är en av de första studierna av partikeldistributioner för Large Hadron Colliders andra körning med mass-centrum-energi √s = 13 TeV. Mätningen är begränsad till fasrymden definierad av en transversell rörelsemängd pT > 100 MeV, och absolut rapiditet |η| < 2.5. Spår av partiklar rekonstrueras och korrigeras för detektorns ineffektiviteter för att presenteras på partikelnivå. Dessa jämförs sedan med förutsägelser från olika modeller. Modellerna EPOS och Pythia 8 A2 är generellt de som bäst överensstämmer med data. Författaren har undersökt partiklar som migrerar in och ut ur fasrymden. Andelen spår associerade till partiklar som migrerat utifrån uppskattas med simulerad data, till som mest 10% nära fasrymdens gränser. Osäkerheten på denna andel uppskattas till att vara som mest 4.5%, huvudsakligen orsakad av osäkerheten på mängden material i de innersta subdetektorerna.QC 20160817</p
Measurements of the Standard Model Higgs boson cross sections in the WW* decay mode with the ATLAS experiment
No full textThis thesis summarises measurements of the Standard Model Higgs boson production cross sections based on proton–proton collision data at √s = 13 TeV produced by the Large Hadron Collider at CERN. By analysing data collected during 2015 and 2016 by the ATLAS experiment, corresponding to an integrated luminosity of 36 fb−1, the Higgs boson gluon–gluon fusion and vector boson fusion production cross sections are measured in the WW* decay mode. To obtain a high signal to background ratio, the data is filtered for final states with one electron (positron) and one anti-muon (muon) and missing transverse momentum. A major part of the thesis concerns the estimation of backgrounds with misidentified leptons. These backgrounds originate from the production of a W boson and an associated object mistakenly identified as an (anti-)electron or (anti-)muon, and are estimated with data driven techniques. A maximum likelihood fit is performed and the cross sections times branching ratios are simultaneously measured to be σ ·B = 12.6+2.3/-2.1 pb and σ ·B = 0.50+0.30/-0.29 pb for the gluon–gluon fusion and vector boson fusion modes, respectively. Both systematic and statistical uncertainties are taken into account in the confidence intervals. The corresponding Standard Model predictions are 10.4 ± 0.6 pb and 0.81 ± 0.02 pb. The observed (expected) significance of the gluon–gluon fusion mode is 6.3 (5.2) standard deviations above the Standard Model background. For the vector boson fusion mode, the observed and expected significances are 1.9 and 2.7 standard deviations, respectively. A smaller part of the thesis investigates the prospects for measuring the luminosity in the high-luminosity phase of the Large Hadron Collider, to begin in 2026. ATLAS will build and insert a timing detector with silicon pixel technology into the forward region, to cope with the harsh pileup environment present at high luminosity. The capabilities of this detector to provide luminosity measurements are investigated. The number of detector hits is observed to scale linearly with collision multiplicity across the full range of expected multiplicities.QC 20180903</p
Spektralanalys av gammautbrott: En studie i ett av Universums mest energirika fenomen
No full textKosmologiska gammautbrott är bland de mest energirika fenomen som observerats i universum. Ursprunget för dessa kraftiga blixtar av gammastrålning är idag relativt okänt, men tros komma ur kollisionereller kollapser av tunga stjärnor. Efter analys av data från detektorerna ombord på Fermi Gamma Ray Space Telescope visas i detta arbete att spektrumet från GRB 080916C beskrivs väl av den enkla Bandfunktionen (två sammanlänkade potensfunktioner) och därför troligtvis kommer från en enkel strålningsprocess. Den starkaste kandidaten är synkotonstrålning, även om modellen idag inte till fullo kan förklara spektrumet på ett tillfredsställande sätt. GRB 080916C:s isotropa energi mäts till den extremt höga nivån ~ 9 x 10 (upphöjt till 54) erg. vilket är bland de högsta som uppmätts för gammautbrott
Measurements of the Standard Model Higgs boson cross sections in the WW* decay mode with the ATLAS experiment [Elektronisk resurs]
No full textThis thesis summarises measurements of the Standard Model Higgs boson production cross sections based on proton–proton collision data at √s = 13 TeV produced by the Large Hadron Collider at CERN. By analysing data collected during 2015 and 2016 by the ATLAS experiment, corresponding to an integrated luminosity of 36 fb−1, the Higgs boson gluon–gluon fusion and vector boson fusion production cross sections are measured in the WW* decay mode. To obtain a high signal to background ratio, the data is filtered for final states with one electron (positron) and one anti-muon (muon) and missing transverse momentum. A major part of the thesis concerns the estimation of backgrounds with misidentified leptons. These backgrounds originate from the production of a W boson and an associated object mistakenly identified as an (anti-)electron or (anti-)muon, and are estimated with data driven techniques. A maximum likelihood fit is performed and the cross sections times branching ratios are simultaneously measured to be σ ·B = 12.6+2.3/-2.1 pb and σ ·B = 0.50+0.30/-0.29 pb for the gluon–gluon fusion and vector boson fusion modes, respectively. Both systematic and statistical uncertainties are taken into account in the confidence intervals. The corresponding Standard Model predictions are 10.4 ± 0.6 pb and 0.81 ± 0.02 pb. The observed (expected) significance of the gluon–gluon fusion mode is 6.3 (5.2) standard deviations above the Standard Model background. For the vector boson fusion mode, the observed and expected significances are 1.9 and 2.7 standard deviations, respectively.A smaller part of the thesis investigates the prospects for measuring the luminosity in the high-luminosity phase of the Large Hadron Collider, to begin in 2026. ATLAS will build and insert a timing detector with silicon pixel technology into the forward region, to cope with the harsh pileup environment present at high luminosity. The capabilities of this detector to provide luminosity measurements are investigated. The number of detector hits is observed to scale linearly with collision multiplicity across the full range of expected multiplicities.</p
Search for new resonances in events with one lepton and missing transverse momentum in pp collisions at root s=13 TeV with the ATLAS detector
No full textA search for W' bosons in events with one lepton (electron or muon) and missing transverse momentum is presented. The search uses 3.2 fb(-1) of pp collision data collected at root s = 13 TeV by the ATLAS experiment at the LHC in 2015. The transverse mass distribution is examined and no significant excess of events above the level expected from Standard Model processes is observed. Upper limits on the W' boson cross-section times branching ratio to leptons are set as a function of the W' mass. Within the Sequential Standard Model W' masses below 4.07 TeV are excluded at the 95% confidence level. This extends the limit set using LHC data at root s = 8 TeV by around 800 GeV. (C) 2016 The Author(s). Published by Elsevier B.V.</p
High-E-T isolated-photon plus jets production in pp collisions at root s=8 TeV with the ATLAS detector
No full textThe dynamics of isolated-photon plus one-, two- and three-jet production in pp collisions at a centre-of-mass energy of 8 TeV are studied with the ATLAS detector at the LHC using a data set with an integrated luminosity of 20.2 fb(-1). Measurements of isolated-photon plus jets cross sections are presented as functions of the photon and jet transverse momenta. The cross sections as functions of the azimuthal angle between the photon and the jets, the azimuthal angle between the jets, the photon-jet invariant mass and the scattering angle in the photon-jet centre-of-mass system are presented. The pattern of QCD radiation around the photon and the leading jet is investigated by measuring jet production in an annular region centred on each object; enhancements are observed around the leading jet with respect to the photon in the directions towards the beams. The experimental measurements are compared to several different theoretical calculations, and overall a good description of the data is found. (C) 2017 The Author(s). Published by Elsevier B.V.</p
Measurement of top quark pair differential cross sections in the dilepton channel in pp collisions at root s=7 and 8 TeV with ATLAS
No full textMeasurements of normalized differential cross sections of top quark pair (t (t) over bar) production are presented as a function of the mass, the transverse momentum and the rapidity of the t (t) over bar system in proton-proton collisions at center-of-mass energies of root s = 7 and 8 TeV. The data set corresponds to an integrated luminosity of 4.6 fb(-1) at 7 TeV and 20.2 fb(-1) at 8 TeV, recorded with the ATLAS detector at the Large Hadron Collider. Events with top quark pair signatures are selected in the dilepton final state, requiring exactly two charged leptons and at least two jets with at least one of the jets identified as likely to contain a b hadron. The measured distributions are corrected for detector effects and selection efficiency to cross sections at the parton level. The differential cross sections are compared with different Monte Carlo generators and theoretical calculations of t (t) over bar production. The results are consistent with the majority of predictions in a wide kinematic range.</p
A measurement of material in the ATLAS tracker using secondary hadronic interactions in 7 TeV p p collisions
No full textKnowledge of the material in the ATLAS inner tracking detector is crucial in under-standing the reconstruction of charged-particle tracks, the performance of algorithms that identify jets containing b-hadrons and is also essential to reduce background in searches for exotic particles that can decay within the inner detector volume. Interactions of primary hadrons produced in pp collisions with the material in the inner detector are used to map the location and amount of this material. The hadronic interactions of primary particles may result in secondary vertices, which in this analysis are reconstructed by an inclusive vertex-finding algorithm. Data were collected using minimum-bias triggers by the ATLAS detector operating at the LHC during 2010 at centre-of-mass energy root s = 7 TeV, and correspond to an integrated luminosity of 19 nb(-1). Kinematic properties of these secondary vertices are used to study the validity of the modelling of hadronic interactions in simulation. Secondary-vertex yields are compared between data and simulation over a volume of about 0.7m(3) around the interaction point, and agreement is found within overall uncertainties.</p
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