1,721,174 research outputs found
A search for massive top quark resonances with the ATLAS detector at the Large Hadron Collider
No full textThis thesis presents a search for resonant production of top-antitop quark pairs in final states containing at least one electron or muon. A number of beyond the standard model (BSM) theories incorporate a specific role for the top quark, resulting in resonances that preferentially decay to t(bar over t) pairs. The data sample analysed corresponds to an integrated luminosity of 2.05 fb-1 recorded during 2011 using the ATLAS detector at the Large Hadron Collider. The proton-proton centre-of-mass energy was 7 TeV. The search is tailored towards heavy resonances at the TeV-scale which therefore decay to top quarks with high transverse momentum. Large hadronic jets are used to reconstruct the energy carried by the hadrons and the substructure of these jets is studied in order to identify hadronically decaying top quarks. The reconstruction can therefore proceed even if the decay products of the top quark are highly collimated. This study represents the first use of jet substructure techniques in a search for t(bar over t) resonances using hadron-hadron collision data. The invariant mass of the reconstructed t(bar over t) pair is used to test compatibility of the data with the standard model prediction. No evidence for t(bar over t) resonances is found. Upper limits are derived on the production cross-section times branching ratio for narrow and wide resonant states, at the 95 % credibility level. An upper limit of 0.61 (0.65) pb is set for a narrow (wide) resonance with a mass of 1 TeV. Two specific BSM models are excluded within certain mass ranges: the narrow leptophobic Z' boson with mass 600 - 1150 GeV and the wide Kaluza-Klein gluon with a mass below 1.5 TeV. These results represent a significant improvement on those of previous searches performed at the ATLAS experiment, which did not use jet substructure techniques. In addition, the possibility of using jets which decrease in size as their transverse momentum increases is investigated using simulated data. The yield of events due to resonant t(bar over t) production increases by approximately 20 % compared to when using jets of fixed size. Furthermore, the resolution of the invariant mass of the reconstructed t(bar over t) pair is found to improve by almost one fifth. It is recommended that a calibration scheme be developed for these variable-sized jets, so that their potential to improve the sensitivity to t(bar over t) resonances can be investigated further
A search for massive resonances in final states with boosted top-antitop pairs decaying into a lepton and jets with the ATLAS detector at the Large Hadron Collider
Full text linkIn this thesis, a search for new elementary particles decaying to a top-antitop pair (ttbar) is presented. Massive new particles that preferentially decay to top quarks are predicted by a number of theoretical models that have been proposed to address various open questions in the currently established Standard Model of Particle Physics, in particular those related to the Higgs mechanism through which elementary particles acquire mass. The search is conducted in proton-proton collision data collected by the ATLAS experiment at the Large Hadron Collider, located at CERN, the European Laboratory for Particle Physics. The integrated luminosity of the dataset, which was collected in 2012, is 20.3 fb-1 . This is the first and only search for new particles in ttbar final states that uses the full ATLAS dataset collected in collisions at the centre-of-mass energy √ = 8 TeV. The search focuses on ttbar-->(Wb)(Wbbar) final states in which one W boson decays into an electron or muon and the corresponding (anti)neutrino and the other W boson decays into a quark and an antiquark. The detector signature of these events is characterised by the presence of an electron or muon, a certain number of hadronic jets and large missing transverse energy. Two independent reconstruction strategies for top quarks with small and large transverse momenta, respectively, are combined to optimise the sensitivity of the search over the mass range between 0.4 TeV and 3.0 TeV. In particular, jet substructure techniques as well as a transverse- momentum dependent treatment of lepton isolation and lepton-jet overlap are used to account for the collimation of the decay products of highly energetic (boosted) top quarks. The distributions of the reconstructed ttbar invariant mass in data are analysed for local excesses or deficits with respect to the Standard Model prediction. No evidence for the existence of new particles is found in the analysed mass range. Upper limits on the cross-section times branching ratio to ttbar , at the 95% confidence level, are derived for four benchmark models. For a narrow Z' resonance, these limits range from 4.2 pb for a resonance mass of 0.4 TeV to 0.03 pb for a mass of 3.0 TeV. A topcolor-assisted technicolor Z'TC2 boson with a relative width of 1.2% is excluded for masses between 0.4 TeV and 1.8 TeV. Upper limits on the cross-section times branching ratio of 4.8 pb (0.09 pb) at 0.4 TeV (3.0 TeV) are also derived for a broad colour-octet state with a width of 15.3%. A Bulk RS Kaluza-Klein gluon with the above width is excluded in the mass range between 0.4 TeV and 2.2 TeV. For the first time in ATLAS, the results of a resonance search in ttbar final states are also interpreted in the context of models with a narrow Bulk RS Kaluza-Klein graviton and a generic scalar resonance, respectively. The upper limits on the cross-section times branching ratio for the Bulk RS Kaluza-Klein graviton (scalar resonance) range from 2.5 pb (3.0 pb) at 0.4 TeV to 0.03 pb at 2.5 TeV (3.0 TeV)
A Study Of Longitudinal Hadronic Shower Leakage and the Development of a Correction for its Associated Effects at = 8 TeV with the ATLAS Detector
Full text linkIn the high energy environment of the Large Hadron Collider, there is a finite probability for the longitudinal tail of the hadronic shower represented by a jet to leak out of the calorimeter, commonly referred to as longitudinal hadronic shower leakage, or jet “punchthrough”. This thesis prescribes a method for identifying such “punch-through” jets via the use of muon activity found behind a jet in the ATLAS muon spectrometer, finding an occurance rate of up to 18% in the worst affected regions. “Punch-through” jets were found to degrade the measured jet energy scale by up to 30%, and jet energy resolution by a factor of 3. A correction to remove these effects was developed in Monte Carlo and validated in data, with associated systematic uncertainties derived. The correction was found to negate the degredation of the measured jet energy scale, improving the jet energy resolution by up to 10% in the worst affected regions, and up to 1.6% overall. The correction was integrated into the final 2012 ATLAS jet energy calibration scheme as the fifth step of the Global Sequential corrections. The prescription developed in this thesis to derive the correction is currently being used by ATLAS in Run II of the Large Hadron Collider
The jet energy scale uncertainty derived from Gamma-jet events for small and large radius jets and the calibration and performance of variable R Jets with the ATLAS Detector
Full text linkIn this thesis the jet energy scale uncertainty of small and large radius jets at the ATLAS detector is evaluated in-situ using gamma-jet events. The well calibrated photon in the gamma-jet events is used to probe the energy scale of the jets. The studies of the jet energy scale of small radius jets are performed using 4.7 fb-1 of data collected at sqrt{s} = 7 TeV in 2011. The gamma-jet methods which were developed are then adapted and applied to large radius jets, using 20.3 fb^-1 of data collected at sqrt{s} = 8 TeV in 2012. The new jet energy scale uncertainties are found to be ~1 % for |eta| 0.8. These uncertainties are significantly lower than the 3-6 % precision which has previously been achieved at ATLAS using track jets as a reference object. Due to the increase in precision, uncertainties due to pile-up and the topology of the jet also had to be evaluated. The total energy scale uncertainties for large radius jets are reduced by ~1-2 % (0.5-1 %) for |eta| 0.8). This reduction will be beneficial to analyses using large radius jets and it is specifically shown to benefit the t-tbar resonance search in the semi-leptonic channel. The t-tbar search looks for events with two top quarks in the final state, where one decays leptonically and the other hadronically. The hadronically decaying top quark is reconstructed using a large radius jet, and the jet energy scale uncertainty is a dominant source of uncertainty in the analysis. In addition to the studies of the jet energy scale of large radius jets, the first derivation of a calibration, and jet energy scale uncertainties derived with gamma-jet events, are shown for Variable R jets. The Variable R jet algorithm is a new type of jet algorithm with a radius that is inversely proportional to the size of the jet, making it useful for the study of high momentum top quarks. It is shown that similar methods can be used to calibrate and assess the uncertainties of Variable R jets as are used for standard, fixed radius jets at the ATLAS detector, although some adaptations will be necessary. The studies provide a basis for the calibration of Variable R jets in the future
Search for resonant Higgs boson pair production in the semi-leptonic bb ÌWW* decay channel with the ATLAS detector at the Large Hadron Collider
Full text linkThis thesis presents the search for resonant Higgs boson pair production with the bbWW*
decay mode in the semi-leptonic final state containing one electron or muon with multiple
jets and missing transverse momentum. The Standard Model of particle physics predicts
only non-resonant pair production of Higgs bosons but a number of theoretical models predict
massive new particles that decay to two Higgs bosons. The search is performed using
ps = 13 TeV proton-proton collision data collected in 2015 and 2016, corresponding to an
integrated luminosity of 36:1 fb-1, recorded by the ATLAS detector at the Large Hadron
Collider.
The search focuses on TeV-scale resonances which decay to two Higgs bosons with large
transverse momenta. Jets with large radius parameter are used to reconstruct the kinematics
of one of the Higgs bosons which decays hadronically into a pair of b-jets. The invariant
mass of the reconstructed Higgs boson pair is used to test the compatibility of the observed
data with the Standard Model background prediction. No statistically significant excess is
observed in data over the expected Standard Model background and upper limits are placed
with 95% confidence level on the resonant Higgs boson pair production cross section times
branching ratio for narrow spin-0 and wide spin-2 resonant states.
Techniques which use advanced subjet reconstruction algorithms to improve identification
of high momentum Higgs boson decays to b-jets are also presented. The three algorithms
demonstrate significant performance improvements in simulation compared to the standard
method currently used by the ATLAS collaboration.</p
Searches for new physics using Dijet Angular Distributions in proton-proton collisions at √s = 7 TeV collected with the ATLAS detector
Full text linkAngular distributions of jet pairs (dijets) produced in proton-proton collisions at a centre-of- mass energy √s = 7 TeV have been studied with the ATLAS detector at the Large Hadron Collider using the full 2011 data set with an integrated luminosity of 4.8 fb−1, and reaching dijet masses up to 4.5 TeV. All angular distributions are consistent with QCD predictions. Analysis of the dijet angular distribution, using a novel technique simultaneously employing the dijet mass, is employed. This analysis is sensitive to both resonant new physics and phenomena with a slow-onset in mass. Using this technique, new exclusion limits have been set at 95% credibility level for several hypotheses of physics beyond the standard model including: quantum gravity scales, with 6 extra dimensions, below 4.11 TeV, quark contact interactions below a compositeness scale of 7.6 TeV, and excited quarks with a mass below 2.75 TeV. In a large and complex scientific experiment, such as ATLAS, the collection, management and usability of coherent data and metadata is a challenging operation. The availability of these data to physicists within the experiment is essential to all analysis efforts. A new web-based interface called “RunBrowser”, which makes ATLAS and LHC operations data available to the ATLAS Collaboration, is introduced
Performance of a soft secondary vertex tagger usingproton-proton collisions collected at 13.6 TeV withthe ATLAS detector
No full textThis thesis evaluates a b-tagging algorithm optimized to identify low pT (soft) b-hadrons in theATLAS experiment at the LHC. The algorithm, called the NewVrtSecInclusiveTool, reconstructssoft secondary vertices (SSVs), which can be connected to the decay of a soft b-hadron. Theanalysis evaluates the properties of these soft secondary vertices and compares them with theproperties of b-hadrons using a dileptonic t¯t sample from the Monte Carlo campaign denotedas mc23a which corresponds to the ATLAS data taking in 2022. An acceptance definition isintroduced to specifically test the identification ability of the algorithm outside of jets. Furthermore,a ΔR matching procedure is developed to assess if a soft secondary vertex can be associated tothe decay of a b-hadron. This procedure divides the SSVs into true SSVs and fake SSVs. Acomparison of these objects is done and an explanation for the origin of the fake SSVs is given.The b-hadrons and SSVs in acceptance, the matched and fake SSVs and the matched b-hadronsare used to develop an efficiency definition for the algorithm. Moreover, the average number offake SSVs nF is introduced to assess how often the algorithm makes a wrong tagging decision.The efficiency and the average number of fake SSVs are then analysed as a function of b-hadronproperties and event variables. Additionally, they are used to evaluate the 3 working points ofthe algorithm. The overall efficiency is in the order of 0.2 to 0.25 depending on the working point.The overall average number of fake SSVs is between 0.01 and 0.04 depending on the workingpoint. Furthermore, regions which are enhanced in matched and fake SSVs are constructed and afurther splitting of these regions is discussed in an effort to enable a calibration of the algorithmin the future. Finally, Monte Carlo to data comparisons are performed using data from 2022 and2023 corresponding to the Monte Carlo campaigns denoted as mc23a and mc23d respectiv
Searches for dijet resonances using âs = 13 TeV proton-proton collision data recorded by the ATLAS detector at the Large Hadron Collider
Full text linkThis thesis presents three searches for new resonances in dijet invariant mass spectra. The spectra are produced using âs = 13 TeV proton-proton collision data recorded by the ATLAS detector. New dijet resonances are searched for in the mass range 200 GeV to 6.9 TeV in mass. Heavy new resonances, with masses above 1.1 TeV, are targeted by a high mass dijet search. Light new resonances, with masses down to 200 GeV, are searched for in dijet events with an associated high momentum object (a photon or a jet) arising from initial state radiation. The associated object is used to efficiently trigger the recording of low mass dijet events. All of the analyses presented in this thesis search for an excess of events, localised in mass, above a data-derived estimate of the smoothly falling QCD background. In each search no evidence for new resonances is observed, and the data are used to set 95% C.L. limits on the production cross-section times acceptance times branching ratio for model-independent Gaussian resonance shapes, as well as benchmark signals. One particular benchmark signal which is considered in all of the searches is an axial-vector Z' dark matter mediator model whose parameter space is reduced due to the results presented in this thesis.</p
Measurements of simplified template cross sections in the HâWW* decay and interpretations with effective field theory
Full text linkThis thesis discusses the measurement of the Higgs boson production cross-sections
in the HâWW*â eÏÎ¼Ï decay channel and the Effective Field Theory (EFT)
interpretations of combined Higgs boson measurements. The results are obtained
using 36 fb-1 proton-proton collision data produced at the Large Hadron Collider
at a centre-of-mass energy of 13 TeV, and recorded by the ATLAS detector in 2015
and 2016. The measured HâWW* cross-sections are defined within the Simplified
Template Cross Sections (STXS) framework, where the gluon-gluon fusion (ggF) and
vector boson fusion production modes are studied. In the case of the ggF production
mode the cross-sections are measured as a function of transverse momentum and
number of jets. With the HâWW* measurements high transverse momentum can
be accessed, which is sensitive to new physics. Seven cross-section measurements
with the STXS framework are perfomed using the HâWW* decay. This thesis
also presents the first ATLAS EFT interpretation of combined Higgs boson data in
Hâγγ and HâÎÎ* â4l channels. Finally, the EFT interpretation is also
performed including measurements from the Hâγγ, HâÎÎ* â 4l, and HâWW* channels. The constraints are obtained on the EFT parameters sensitive to modifications of the Higgs boson couplings to strong and electroweak gauge bosons,
and to the top quark.</p
Automated Optimization of an ATLAS Search for Higgs Boson Pair Production at the LHC
No full textParticle physics analyses are inherently complex and need to process large datasets through a chain of many interdependent steps. These analyses typically aim to optimize their sensitivity to potential signals. However, due to the stepwise nature, optimizations at several stages often need to rely on approximations to the final analysis sensitivity. This approach leads to extensive re-optimizations that require a careful balance between optimizing selection criteria, accounting for uncertainties, and maintaining a meaningful statistical analysis. This work explores neural end-to-end-optimized summary statistics (NEOS) as a novel, automated alternative to this procedure. For the first time, it is applied to a full-featured analysis in a search for boosted Higgs boson pair production via vector boson fusion, decaying into a four b-quark final state. A dedicated optimization framework, auTOMATed Optimization of Sensitivity (TOMATOS), is introduced, enabling a unified analysis optimization that targets the analysis sensitivity directly. Its performance is benchmarked against traditional methods, demonstrating comparable results. This thesis presents the latest expected ATLAS constraints on the coupling using Run 2 data:
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