474 research outputs found
Strange particle production in final states in collisions at TeV with the ATLAS detector at the LHC
No full textThe LHC has opened up a new era in particle physics in as much it paved the way for the discovery of the Higgs boson, the missing piece in the so called standard model (SM) , pushing the lower limits for particles predicted in many beyond the standard model (BSM) scenarios to values well above the TeV scale, allowing precision measurements at unprecedented energies. This ambitious scientific program has been carried out with the construction of two big multipurpose detectors: ATLAS and CMS. The discovery of the Higgs boson is, no doubt, one of the most important milestones in the history of particle physics. Another field where one can reach unprecedented precision is in the study of the top quark. With the advent of unprecedented statistics for top quark pair production, one can think of directly measuring the Cabibbo-Kobayashi-Maskawa matrix elements where the top quark is involved, in particular . Ali et al. [Phys. Lett. B 693 (2010) 44-51] made the proposal to determine this matrix element by discerning the signal final state from the dominant background process . This could be done by looking at neutral strange particle production which turns out to be leading in -quark jets while non-leading in -quark jets as -quarks decay sequentially . This proposal makes use of MVA techniques, with the possible existence of jet secondary vertices and soft leptons as additional variables with discriminant power. Therefore this proposal is dependent on the good agreement between data and Monte Carlo generators used in the training phase. However the proposal we refer to is qualitatively right, but quantitatively is somewhat overoptimistic as it is based on generator level data. The aim of the present thesis is twofold: on the one hand, as a continuation of the thesis presented in 2015 by Dr. Javier Llorente, who measured - and light-jet shapes in events, we have investigated the improvements in the performance of the MVA discussed above when adding the jet shapes as a discriminant variable. This was presented at ICHEP 2014 in Valencia, [Nucl. Part. Phys. Proc. 273-275 (2016) 2761-2763]. This is the basis for Chapter 4 in this thesis. On the other hand, we made use of the ATLAS data at 7 TeV taken in 2011, to study neutral strange particle production in the dileptonic final states. This study has been published in Eur. Phys. J. C 79 (2019) 1017. It is found that neutral strange particle production embedded in jets, -tagged or not, is well described by current MC models like the following generators: POWHEG+PYTHIA6, POWHEG+PYTHIA8, MC@NLO+HERWIG, POWHEG+HERWIG7, aMC@NLO+HERWIG7 or SHERPA. This lends support to the idea put forward by ABL that one could attempt to determine with a TMVA. Furthermore, neutral strange particles produced outside jets, the dominant contribution, in underestimated by current MC generators by 30 %. Thus, these data have the potential to help tune models for MPI. This is summarised in Chapter 5. For the sake of completeness and without any pretension to originality, the first three chapters are devoted to present the most relevant aspects of the SM, Chapter 1, the ATLAS detector and the LHC, Chapter 2, and the objects (leptons, jets, missing energy) reconstruction, Chapter 3
Measurements of and production in dileptonic events in collisions at with the ATLAS detector.
No full textMeasurements of and production in final states have been performed. They are based on a data sample with integrated luminosity of 4.6 from proton--proton collisions at a centre-of-mass energy of 7 TeV, collected in 2011 with the ATLAS detector at the Large Hadron Collider. Neutral strange particles are separated into three classes, depending on whether they are contained in a jet, with or without a -tag, or not associated with a selected jet. The aim is to look for differences in their main kinematic distributions. A comparison of data with several Monte Carlo simulations using different hadronisation and fragmentation schemes, colour reconnection models and different tunes for the underlying event has been made. The production of neutral strange particles in dileptonic events is found to be well described by current Monte Carlo models for and production within jets, but not for those produced outside jets
Fragmentation studies with the ATLAS detector
No full textGluon splitting to b-quark pairs is a unique probe into the properties of gluon fragmentation, as identified b-tagged jets provide a proxy for the quark daughters of the initial gluon. If available, we present a measurement of key differential distributions related to g→b bbar using data collected with the ATLAS detector at √s=13 TeV. Track jets are used to probe angular scales below the standard R=0.4 jet radius. The observables are unfolded to particle level in order to facilitate direct comparison with predictions from simulations and provide an important constraint to hadronization models. If available, a measurement of the properties of jet fragmentation performed with pp data collected with the ATLAS detector at √s=13 TeV is presented. Charged particle tracks are used to measure charged particle multiplicity, the jet charge, the summed fragmentation function, the momentum transverse to the jet axis, and the radial profile of the jet. Each observable is unfolded to correct for acceptance and detector effects. Exclusive interpretations in terms of quarks and gluons are provided in order to directly compare with state-of-the-art calculations
Search for heavy diboson resonances in semileptonic final states in pp collisions at s=13 TeV with the ATLAS detector
No full textThis article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/
Search for dark matter produced in association with a hadronically decaying vector boson in pp collisions at =13 TeV with the ATLAS detector
No full textSee paper for full list of authors - 10 pages plus author list + cover pages (30 pages total), 5 figures, 2 tables, submitted to PLB, All figures including auxiliary figures are available at http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/EXOT-2015-08/International audienceA search is presented for dark matter produced in association with a hadronically decaying W or Z boson using 3.2 /fb of pp collisions at =13 TeV recorded by the ATLAS detector at the Large Hadron Collider. Events with a hadronic jet compatible with a W or Z boson and with large missing transverse momentum are analysed. The data are consistent with the Standard Model predictions and are interpreted in terms of both an effective field theory and a simplified model containing dark matter
Higgs boson production cross-section measurements and their EFT interpretation in the 4 ℓ decay channel at √s= 13 TeV with the ATLAS detector
No full textHiggs boson properties are studied in the four-lepton decay channel (where lepton = e, μ) using 139 fb - 1 of proton–proton collision data recorded at s=13 TeV by the ATLAS experiment at the Large Hadron Collider. The inclusive cross-section times branching ratio for H→ ZZ∗ decay is measured to be 1.34 ± 0.12 pb for a Higgs boson with absolute rapidity below 2.5, in good agreement with the Standard Model prediction of 1.33 ± 0.08 pb. Cross-sections times branching ratio are measured for the main Higgs boson production modes in several exclusive phase-space regions. The measurements are interpreted in terms of coupling modifiers and of the tensor structure of Higgs boson interactions using an effective field theory approach. Exclusion limits are set on the CP-even and CP-odd ‘beyond the Standard Model’ couplings of the Higgs boson to vector bosons, gluons and top quarks. © 2020, The Author(s)
Measurement of the and production cross sections in multilepton final states using 3.2 fb of collisions at = 13 TeV with the ATLAS detector
No full textSee paper for full list of authors - 22 pages plus author list + cover page (40 pages total), 8 figures, 5 tables. Submitted to Eur. Phys. J. C. All figures including auxiliary figures are available at http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/TOPQ-2015-22/International audienceA measurement of the and production cross sections in final states with either two same-charge muons, or three or four leptons (electrons or muons) is presented. The analysis uses a data sample of proton-proton collisions at TeV recorded with the ATLAS detector at the Large Hadron Collider in 2015, corresponding to a total integrated luminosity of 3.2 fb. The inclusive cross sections are extracted using likelihood fits to signal and control regions, resulting in pb and pb, in agreement with the Standard Model predictions
Search for bottom squark pair production in proton--proton collisions at 13 TeV with the ATLAS detector
No full textSee paper for full list of authors - 20 pages (plus author list: 37 pages total), 4 figures, 7 tables. Submitted to Eur. Phys. J. C. All figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/SUSY-2015-01International audienceThe result of a search for pair production of the supersymmetric partner of the Standard Model bottom quark () is reported. The search uses 3.2 fb of collisions at 13 TeV collected by the ATLAS experiment at the Large Hadron Collider in 2015. Bottom squarks are searched for in events containing large missing transverse momentum and exactly two jets identified as originating from -quarks. No excess above the expected Standard Model background yield is observed. Exclusion limits at 95% confidence level on the mass of the bottom squark are derived in phenomenological supersymmetric -parity-conserving models in which the is the lightest squark and is assumed to decay exclusively via , where is the lightest neutralino. The limits significantly extend previous results; bottom squark masses up to 800 (840) GeV are excluded for the mass below 360 (100) GeV whilst differences in mass above 100 GeV between the and the are excluded up to a mass of 500 GeV
Measurement of top quark pair differential cross-sections in the dilepton channel in collisions at = 7 and 8 TeV with ATLAS
No full textSee paper for full list of authors - 33 pages plus author list (50 pages total), 11 figures, 10 tables, submitted to Phys. Rev. D, all figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/TOPQ-2015-07/International audienceMeasurements of normalized differential cross-sections of top quark pair () production are presented as a function of the mass, the transverse momentum and the rapidity of the system in proton-proton collisions at center-of-mass energies of = 7 TeV and 8 TeV. The dataset corresponds to an integrated luminosity of 4.6 fb at 7 TeV and 20.2 fb 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 -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 production. The results are consistent with the majority of predictions in a wide kinematic range
Search for new resonances decaying to a or boson and a Higgs boson in the , , and channels with collisions at TeV with the ATLAS detector
No full textSee paper for full list of authors, 18 pages (plus author list + cover pages: 36 pages total), 13 figures, 1 table. Submitted to PLB. All figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/EXOT-2015-18/International audienceA search is presented for new resonances decaying to a or boson and a Higgs boson in the , , and channels in collisions at TeV with the ATLAS detector at the Large Hadron Collider using a total integrated luminosity of 3.2 fb. The search is conducted by looking for a localized excess in the / invariant or transverse mass distribution. No significant excess is observed, and the results are interpreted in terms of constraints on a simplified model based on a phenomenological Lagrangian of heavy vector triplets
- …
