18 research outputs found
LUCID Upgrade for ATLAS Luminosity Measurement in Run II.
No full textThe main ATLAS luminosity monitor LUCID and its read-out electronics has been completely rebuilt for the 2015 LHC run in order to cope with a higher center of mass energy (13 TeV) and with 25 ns bunch-spacing. The LUCID detector is measuring Cherenkov light produced in photomultiplier quartz windows and in quartz optical fibers. It has a novel calibration system that uses radioactive Bi sources that produces internal conversion electrons above the Cherenkov threshold in quartz. The new electronics can count particle hits above a threshold but also the integrated pulseheight of the signals from the particles which makes it possible to measure luminosity with new methods. The new detector, calibration system and electronics will be covered by the contribution as well as the results of the luminosity measurements with the detector in 2015
Data-driven methods for misreconstructed objects estimation in lepton final states with ATLAS at TeV
No full textNeutrino mass generation is needed to explain neutrino oscillations. Several new physics models, such as left-right symmetric models or seesaw mechanisms, offer a solution and motivate searches for heavy bosons or heavy neutral leptons in either opposite-charge or same-charge leptons final states. This contribution discusses the challenging backgrounds for same-charge final states, either due to incorrectly identified jets as leptons or due to mismeasurements of the electron charge. The results presented here use the data collected by the ATLAS experiment at the LHC with a center-of-mass energy of TeV
Search for doubly charged Higgs bosons with the ATLAS detector
No full textThis contribution will focus on two complementary searches for doubly charged Higgs bosons () in ATLAS using collisions at TeV. This new particle allows to explain the lightness of neutrino masses through a type-II seesaw mechanism and appears in many BSM theories such as left-right symmetric models or Higgs triplets
Study of multi-lepton final states with the ATLAS experiment: from Standard Model to Beyond Standard Model Higgs.
No full textThis thesis exploits the features of final states containing light leptons with
same electric charge, produced in LHC proton-proton collisions at 13 TeV and collected by the ATLAS detector during the 2015 and 2016 data-taking.
Events containing leptons with same electric charge allow to reject a
substantial fraction of the background from the Standard Model processes.
New physics is searched both indirectly and directly through two processes:
the production of a Higgs boson in association with a pair of top quarks
(ttH) and the production of new doubly charged Higgs bosons (H++). The
measurement of the ttH signal strength (mu) into leptonic final states provides
mu = 2.5 (+1.3 - 1.1), compatible at 95% confidence level with the Standard Model
prediction mu(ttH) = 1. Regarding direct searches, no evidence for new physics
is found in the search for H++ boson production: lower limits on its mass
are set and vary from 770 GeV to 870 GeV at 95% confidence level
Multivariate analysis methods in the search for the Higgs boson produced in association with top pairs at the ATLAS experiment at LHC.
No full textThe Large Hadron Collider, located at the CERN laboratories in Geneva, is the largest particle accelerator in the world. One of the main research fields at LHC is the study of the Higgs boson, the latest particle discovered at the ATLAS and CMS experiments.
Due to the small production cross section for the Higgs boson, only a substantial statistics can offer the chance to study this particle properties. In order to perform these searches it is desirable to avoid the contamination of the signal signature by the number and variety of the background processes produced in pp collisions at LHC.
Much account assumes the study of multivariate methods which, compared to
the standard cut-based analysis, can enhance the signal selection of a Higgs
boson produced in association with a top quark pair through a dileptonic
final state (ttH channel).
The statistics collected up to 2012 is not sufficient to supply a significant
number of ttH events; however, the methods applied in this thesis will provide a powerful tool for the increasing statistics that will be collected during the next LHC data taking
ATLAS Standard Model Measurements Using Jet Grooming and Substructure
No full textBoosted topologies allow to explore Standard Model processes in kinematical regimes never tested before. In such LHC challenging environments, standard reconstruction techniques quickly hit the wall. Targeting hadronic final states means to properly reconstruct energy and multiplicity of the jets in the event. In order to be able to identify the decay product of boosted objects, i.e. W bosons, pairs or Higgs produced in association with pairs, ATLAS experiment is currently exploiting several algorithms using jet grooming and jet substructure. This contribution will mainly cover the following ATLAS measurements: differential cross section production and jet mass using the soft drop procedure. Standard Model measurements offer the perfect field to test the performances of new jet tagging techniques which will become even more important in the search for new physics in highly boosted topologies.
Data-driven methods for misreconstructed objects estimation in lepton final states using collisions at =13 TeV with the ATLAS detector at the LHC
No full textNeutrinos mass generation is needed to explain neutrino oscillations. Seesaw mechanisms offer a solution and motivate searches for heavy neutral leptons in either opposite-sign or same-sign leptons final states. This contribution discusses the challenging backgrounds for same-sign final states, either due to incorrectly identified jets as leptons or to mismesureaments of the electron charge
Search for doubly-charged Higgs boson in multi-lepton final states at = 13 TeV with the ATLAS detector
No full textA search for new high mass resonances decaying to two high- leptons with same-charge is presented. The results reported here use the collision data sample corresponding to 36.1 fb of integrated luminosity collected in 2015 and 2016 by the ATLAS detector at the LHC with a centre-of-mass energy of 13 TeV. The benchmark model is doubly charged Higgs boson (DCH) production via Drell--Yan with subsequent exclusive decay into leptons. Leptonic final states are very interesting since they provide a good sensitivity and small systematic uncertainties. In addition, requiring same-sign leptonic final states reduces the background contamination from Standard Model processes while providing a large sensitivity to Beyond Standard Model phenomena. No significant evidence of a signal was observed and corresponding limits on the production cross-section and a lower limit on were derived at 95% confidence level. The results at = 13 TeV complement and improve former ones obtained by the ATLAS Collaboration with a LHC centre-of-mass energy of = 8 Te
Search for doubly-charged Higgs boson in multi-lepton final states using 36.1 fb with ATLAS at = 13 TeV
No full textA search for new high mass resonances which decay to two high- leptons with same-sign charge is presented. The results reported here use the \textit{pp} collision data sample corresponding to 36.5 fb of integrated luminosity collected in 2015 and 2016 by the ATLAS detector at the LHC with a centre-of-mass energy of 13 TeV. We choose the pair production of the doubly charged Higgs boson (DCH) via the Drell-Yan process as a benchmark model, because we regard it as the main production mode for this new particle at the LHC. The decay mode of the DCH was assumed to be exclusively into leptons. \\ Leptonic final states are very interesting since they provide a good sensitivity and small systematic uncertainties. In addition, requiring same-sign leptonic final states reduces the background contamination from Standard Model processes while providing a large sensitivity to BSM phenomena, such as production. This BSM particle is motivated in the picture of left-right symmetric models, which are appealing because they are able to give an explanation on two points left unanswered by the Standard Model: parity violation and neutrino mass origins. The hypothesis assumes the existence of a higher energy scale symmetry of the type , subsequently broken into the Standard Model electroweak symmetry . The Higgs scalar sector needs to be extended by the addition of a Higgs triplet. The initial symmetry is spontaneously broken by the non-vanishing vacuum expectation value acquired by the new Higgs field, which generates both Dirac and Majorana neutrino mass terms.\\ A model independent search will also be performed on distinct event categories, defined by the number of same-sign lepton pairs. This search will focus on signal regions definition and on the strategy for the estimation of the main backgrounds: mis-identified (fake) prompt leptons, originating from either hadronic jets or secondary weak hadron decays, and electrons with mis-identified charge. The search will be performed in all light lepton flavor channels, allowing also the doubly charged Higgs to decay to mixed lepton flavor final states via lepton number violation. A limit on the mass of the doubly charged Higgs boson, for both left-handed and right-handed particle states, will be set
Search for doubly-charged Higgs boson in multi-lepton final states at =13 TeV with the ATLAS detector
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