Searches for new physics phenomena with jet final states in the ATLAS detector

Collimated sprays of paticle, known as jets,are abundantly produced in high-energy proton-proton (pp) collisions, and are predicted by many theories of physics beyond the Standard Model. This thesis reports first on the calibration of jets with focus on the data-driven in situ calibration. Two searches for new physics phenomena in events with jets in the final state are then presented. The analyses use √s = 13 TeV pp collision data, collected by the ATLAS experiment at the LHC.The response to small- and large-radius jets is measured in situ using data collected between 2015 and 2017. The jet energy scales are measured in events where a jet recoild against a calibrated reference object. The mass scale of large-radius jets is measured with two methods: one uses track jets to provide an independent measurement of the jet mass, the other uses events with a pair of top quarks, that are boosted and decay hadronically. The first analysis is a search for massive coloured objects which are pair-produced and decay to two jets. The analysed data was recorded between 2015 and 2016, corresponding to an integrated luminosity of 36.7 fb-1. Two selections of four-jet events are defined; one with two b-tagged jets and one inclusive. No significant deviation from the expected background of Standard Model processes is observed in either event selection. The results are interpreted in a SUSY simplified model where the lightest supersymmetric particle is a stop squark, which decays hadronically to two quarks through R-parity violating couplings. Limits are derived on the stop squarks mass and on the mass of pair-produced colour-octet resonances. The scenario where the two squarks decay to a b-quark and a lighter quarks is considered separately using the b-tagged event selection.The second analysis is still ongoing and uses data collected between 2015 and 2018, which corresponds to an integrated luminosity of 139 fb-1. The search looks for a resonance, which could be a mediator between a confined hidden sector and the Standard Model sector. The mediator decays to two dark partons, which shower and hadronise according to the dynamics of the hidden sector. The search targets a selection of model parameters where all dark hadrons decay promptly to visible particles. The signature is a pair of "dark jets" with different characteristics than Standard Model QCD-jets. A mass-decorrelated tagger is defined based on the number of tracks associated with the jet. The tagger is designed to distinguish dark jets from the QCD background without distorting the smoothly falling background distribution

Searches for new physics phenomena with jet final states in the ATLAS detector

Collimated sprays of particles, known as $jets$, are abundantly produced in high-energy proton-proton ($pp$) collisions, and predicted by many theories of physics beyond the Standard Model. This thesis reports first on the calibration of jets with focus on the data-driven insitu calibration. Two searches for new physics phenomena in events with jets in the final state are then presented. The analyses use $\sqrt s = 13$ TeV $pp$ collision data collected by the ATLAS experiment at the LHC. The response to small- and large-radius jets is measured insitu using data collected between 2015 and 2017. The jet energy scales are measured in events where a jet recoils against a calibrated reference object. The mass scale of large-radius jets is measured with two methods: one uses track jets to provide an independent measurement of the jet mass, the other uses $t\overline{t}$-events with boosted, hadronically decaying $W$-bosons and top quarks. The first analysis is a search for massive coloured objects which are pair-produced and decay to two jets. The data was recorded between 2015 and 2016, corresponding to an integrated luminosity of 36.7 fb$^{-1}$ of data. Two selections of four-jet events are defined; one with two $b$-tagged jets and one inclusive. No significant deviation from the expected background of Standard Model processes is observed in either of the event selections. The results are interpreted in a SUSY simplified model where the lightest supersymmetric particle is a stop squark, which decays to two quarks through $R$-parity violating couplings. Limits are derived on the the stop squark mass and on the mass of pair -produced colour-octet resonances. The scenario where the two squarks decays to a $b$-quark and a lighter quark is considered separately using the $b$-tagged event selection. The second analysis is still ongoing and using data collected between 2015 and 2018, which corresponds to an integrated luminosity of 139 fb$^{-1}$. The search looks for a resonance, which could be a mediator between a confined hidden sector and the Standard Model sector. The mediator decays to two dark partons which shower and hadronise according to the dynamics of the hidden sector. The search targets a selection of model parameters where all dark hadrons decay promptly to visible particle. The signature is a pair of ``dark jets'' with different characteristics than Standard Model QCD-jets. An mass-decorrelated tagger is defined based on the number of tracks associated with the jet. The tagger is designed to distinguish dark jets from the QCD background, without distorting the smoothly falling background distribution

Search for flavor-changing neutral currents in top quark decays t →hc and t →hu in multilepton final states in proton-proton collisions at s =13 TeV with the ATLAS detector

Flavor-changing neutral currents are not present in the Standard Model at tree level and are suppressed in loop processes by the unitarity of the Cabibbo-Kobayashi-Maskawa matrix; the corresponding rates for top quark decay processes are experimentally unobservable. Extensions of the Standard Model can generate new flavor-changing neutral current processes, leading to signals which, if observed, would be unambiguous evidence of new interactions. A data set corresponding to an integrated luminosity of 36.1 fb-1 of pp collisions at a center-of-mass energy of s=13 TeV recorded with the ATLAS detector at the Large Hadron Collider is used to search for top quarks decaying to up or charm quarks with the emission of a Higgs boson, with subsequent Higgs boson decay to final states with at least one electron or muon. No signal is observed and limits on the branching fractions B(t→Hc

Search for the Higgs boson produced in association with a vector boson and decaying into two spin-zero particles in the H → aa → 4b channel in pp collisions at √s=13 TeV with the ATLAS detector

A search for exotic decays of the Higgs boson into a pair of spin-zero particles, H → aa, where the a-boson decay sin to b-quarks promptly or with a mean proper lifetime cτa up to 6 mm and has a mass in the range of 20–60 GeV, is presented. The search is performed in events where the Higgs boson is produced in association with a W or Z boson, giving rise to a signature of one or two charged leptons (electrons or muons) and multiple jets from b-quark decays. The analysis is based on the dataset of proton-proton collisions at s=13 TeV recorded in 2015 and 2016 by the ATLAS detector at the CERN Large Hadron Collider, corresponding to an integrated luminosity of 36.1 fb−1. No significant excess of events above the Standard Model background prediction is observed, and 95% confidence-level upper limits are derived for the production cross-sections for pp → WH, ZH and their combination, times the branching ratio of the decay chain H → aa → 4b. For a-bosons which decay promptly, the upper limit on the combination of cross-sections for WH and ZH times the branching ratio of H → aa → 4b ranges from 3.0 pb for ma = 20 GeV to 1.3 pb for ma = 60 GeV, assuming that the ratio of WH to ZH cross-sections follows the Standard Model prediction. For a-bosons with longer proper lifetimes, the most stringent limits are 1.8 pb and 0.68 pb, respectively, at cτa ∼ 0.4 mm.[Figure not available: see fulltext.]. © 2018, The Author(s)

Search for a heavy Higgs boson decaying into a Z boson and another heavy Higgs boson in the ℓℓbb final state in pp collisions at √s= 13 TeV with the ATLAS detector

A search for a heavy neutral Higgs boson, A, decaying into a Z boson and another heavy Higgs boson, H, is performed using a data sample corresponding to an integrated luminosity of 36.1 fb−1 from proton–proton collisions at s=13 TeV recorded in 2015 and 2016 by the ATLAS detector at the Large Hadron Collider. The search considers the Z boson decaying to electrons or muons and the H boson into a pair of b-quarks. No evidence for the production of an A boson is found. Considering each production process separately, the 95% confidence-level upper limits on the pp→A→ZH production cross-section times the branching ratio H→bb are in the range of 14–830 fb for the gluon–gluon fusion process and 26–570 fb for the b-associated process for the mass ranges 130–700 GeV of the H boson and 230–800 GeV of the A boson. The results are interpreted in the context of two-Higgs-doublet models. © 201

Search for Higgs bosons produced via vector-boson fusion and decaying into bottom quark pairs in s =13 TeV pp collisions with the ATLAS detector

A search for the bb decay of the Standard Model Higgs boson produced through vector-boson fusion is presented. Three mutually exclusive channels are considered: two all-hadronic channels and a photon-associated channel. Results are reported from the analysis of up to 30.6 fb-1 of pp data at s=13 TeV collected with the ATLAS detector at the LHC. The measured signal strength relative to the Standard Model prediction from the combined analysis is 2.5-1.3+1.4 for inclusive Higgs boson production and 3.0-1.6+1.7 for vector-boson fusion production only. © 2018 CERN

Search for resonances in the mass distribution of jet pairs with one or two jets identified as b -jets in proton-proton collisions at s =13 TeV with the ATLAS detector

A search for new resonances decaying into jets containing b-hadrons in pp collisions with the ATLAS detector at the LHC is presented in the dijet mass range from 0.57 to 7 TeV. The data set corresponds to an integrated luminosity of up to 36.1 fb-1 collected in 2015 and 2016 at s=13 TeV. No evidence of a significant excess of events above the smooth background shape is found. Upper cross-section limits and lower limits on the corresponding signal mass parameters for several types of signal hypotheses are provided at 95% C.L. In addition, 95% C.L. upper limits are set on the cross sections for new processes that would produce Gaussian-shaped signals in the di-b-jet mass distributions. © 2018 CERN

Prompt and non-prompt J/ ψ elliptic flow in Pb+Pb collisions at √sNN=5.02 Tev with the ATLAS detector

The elliptic flow of prompt and non-prompt J/ ψ was measured in the dimuon decay channel in Pb+Pb collisions at sNN=5.02 TeV with an integrated luminosity of 0.42nb-1 with the ATLAS detector at the LHC. The prompt and non-prompt signals are separated using a two-dimensional simultaneous fit of the invariant mass and pseudo-proper decay time of the dimuon system from the J/ ψ decay. The measurement is performed in the kinematic range of dimuon transverse momentum and rapidity 9 < pT< 30 GeV , | y| < 2 , and 0–60% collision centrality. The elliptic flow coefficient, v2, is evaluated relative to the event plane and the results are presented as a function of transverse momentum, rapidity and centrality. It is found that prompt and non-prompt J/ ψ mesons have non-zero elliptic flow. Prompt J/ ψv2 decreases as a function of pT, while for non-prompt J/ ψ it is, with limited statistical significance, consistent with a flat behaviour over the studied kinematic region. There is no observed dependence on rapidity or centrality. © 2018, CERN for the benefit of the ATLAS collaboration

Prompt and non-prompt J/ ψ and ψ(2 S) suppression at high transverse momentum in 5.02TeV Pb+Pb collisions with the ATLAS experiment

A measurement of J/ ψ and ψ(2 S) production is presented. It is based on a data sample from Pb+Pb collisions at sNN=5.02TeV and pp collisions at s=5.02TeV recorded by the ATLAS detector at the LHC in 2015, corresponding to an integrated luminosity of 0.42nb-1 and 25pb-1 in Pb+Pb and pp, respectively. The measurements of per-event yields, nuclear modification factors, and non-prompt fractions are performed in the dimuon decay channel for 9<pTμμ<40 GeV in dimuon transverse momentum, and - 2 < yμ μ< 2 in rapidity. Strong suppression is found in Pb+Pb collisions for both prompt and non-prompt J/ ψ, increasing with event centrality. The suppression of prompt ψ(2 S) is observed to be stronger than that of J/ ψ, while the suppression of non-prompt ψ(2 S) is equal to that of the non-prompt J/ ψ within uncertainties, consistent with the expectation that both arise from b-quarks propagating through the medium. Despite prompt and non-prompt J/ ψ arising from different mechanisms, the dependence of their nuclear modification factors on centrality is found to be quite similar. © 2018, CERN for the benefit of the ATLAS collaboration