From the sublime to the ridiculous: top physics and minimum bias events in the ATLAS detector at the LHC

This thesis is comprised of two separate physics themes, both of which involve the ATLAS detector situated at the LHC at CERN. The first constituent is a study of the top quark signal in the fully-leptonic channel for proton-proton collisions at a centre-of-mass energy of 10 TeV. Here an event counting analysis is performed based on Monte Carlo simulation. This is supplemented by a study into one of the sources of systematic error. The second component is forward-backward correlations in minimum bias events. For this, there is a Monte Carlo hadron-level comparison of the correlation for 900 GeV centre-of-mass collisions, followed by a comparison of Monte Carlo predictions to data for 900 GeV and 7 TeV collisions. Top Physics A measurement of the fully-leptonic ttbar cross-section in the three decay channels ee, mumu, and emu is performed on ATLAS produced fully simulated pseudo-event data-samples. Selection rates for signal and background events consistent with ATLAS results are found along with the kinematic distributions of selected events. A calculation of the non-hadronic ttbar cross-section, based on the measured cross-sections, will then return the theoretical value of 217:06pb used to generate the original samples, showing the closure of the pseudo-analysis process. A more detailed study is made of the systematic uncertainty arising from variations in the initial (ISR) and final (FSR) state showering models, based on the Pythia event generator. A fast simulation of the ATLAS detector is used with similar object and event selection to the fully simulated case. The effect of ISR variations on the signal is found to be negligible as it is washed out in the subsequent decays of the ttbar system. However, the effect of FSR is found to cause 5% uncertainty in the selected signal events. In addition, in the main background of each of the selection channels the effect of FSR is found to produce variations of up to 30% in well populated channels. The variations in signal and background measurements will then be used to calculate a new estimate of the systematics on the measured ttbar cross-section for each channel. Minimum Bias A detailed study of the forward-backward (FB) correlation and event shapes of a selection of Pythia tunes for pp collisions with CoM = 900 GeV is performed. This includes an investigation into the sources of particle production in generated minimum bias events as well as the component sub-processes in generated minimum bias events. The tunes are found to be practically degenerate (within 10 - 20% variation) for the 'standard' distributions. The inclusion of a new observable, namely the forward-backward correlation, to the standard set is recommended. The study finds that the FB-correlation and its pT and dependent variations are able to discern differences between the selected tunes to a greater degree than the usual inclusive distributions. Further, the FB-correlation is found to be sensitive to the particle production processes within the tunes, an invaluable property for the purposes of generator tuning. A measurement of the forward-backward correlation for pp collision of CoM = 900 GeV and 7 TeV at the LHC using the ATLAS detector is made. The measured correlation is compared to the predicted correlation of several ATLAS centrally produced generator tunes. A correction procedure is developed and validated on the generator samples to correct the generated correlation to the hadron-level correlation. This is then applied to the measured correlation and a comparison of corrected data to the hadron-level predictions of the generated tunes made. The corrected correlations at the two collision energies are compared as well as the calculation of a global correlation at both energies. The measured and corrected correlations are found to lie above the predicted distributions at both energies and across the eta-range. Further investigation of measured correlation using augmented FB-correlations is recommended

Forward-Backward Correlations and Event Shapes as probes of Minimum-Bias Event Properties

Measurements of inclusive observables, such as particle multiplicities and momentum spectra, have already delivered important information on soft-inclusive ("minimum-bias") physics at the Large Hadron Collider. In order to gain a more complete understanding, however, it is necessary to include also observables that probe the structure of the studied events. We argue that forward-backward (FB) correlations and event-shape observables may be particulary useful first steps in this respect. We study the sensitivity of several different types of FB correlations and two event shape variables - transverse thrust and transverse thrust minor - to various sources of theoretical uncertainty: multiple parton interactions, parton showers, colour (re)connections, and hadronization. The power of each observable to furnish constraints on Monte Carlo models is illustrated by including comparisons between several recent, and qualitatively different, PYTHIA 6 tunes, for pp collisions at sqrt(s) = 900 GeV.Comment: 13 page

From the sublime to the ridiculous : top physics and minimum bias events in the ATLAS detector at the LHC

This thesis is comprised of two separate physics themes, both of which involve the ATLAS detector situated at the LHC at CERN. The first constituent is a study of the top quark signal in the fully-leptonic channel for proton-proton collisions at a centre-of-mass energy of 10 TeV. Here an event counting analysis is performed based on Monte Carlo simulation. This is supplemented by a study into one of the sources of systematic error. The second component is forward-backward correlations in minimum bias events. For this, there is a Monte Carlo hadron-level comparison of the correlation for 900 GeV centre-of-mass collisions, followed by a comparison of Monte Carlo predictions to data for 900 GeV and 7 TeV collisions. Top Physics A measurement of the fully-leptonic ttbar cross-section in the three decay channels ee, mumu, and emu is performed on ATLAS produced fully simulated pseudo-event data-samples. Selection rates for signal and background events consistent with ATLAS results are found along with the kinematic distributions of selected events. A calculation of the non-hadronic ttbar cross-section, based on the measured cross-sections, will then return the theoretical value of 217:06pb used to generate the original samples, showing the closure of the pseudo-analysis process. A more detailed study is made of the systematic uncertainty arising from variations in the initial (ISR) and final (FSR) state showering models, based on the Pythia event generator. A fast simulation of the ATLAS detector is used with similar object and event selection to the fully simulated case. The effect of ISR variations on the signal is found to be negligible as it is washed out in the subsequent decays of the ttbar system. However, the effect of FSR is found to cause 5% uncertainty in the selected signal events. In addition, in the main background of each of the selection channels the effect of FSR is found to produce variations of up to 30% in well populated channels. The variations in signal and background measurements will then be used to calculate a new estimate of the systematics on the measured ttbar cross-section for each channel. Minimum Bias A detailed study of the forward-backward (FB) correlation and event shapes of a selection of Pythia tunes for pp collisions with CoM = 900 GeV is performed. This includes an investigation into the sources of particle production in generated minimum bias events as well as the component sub-processes in generated minimum bias events. The tunes are found to be practically degenerate (within 10 - 20% variation) for the 'standard' distributions. The inclusion of a new observable, namely the forward-backward correlation, to the standard set is recommended. The study finds that the FB-correlation and its pT and dependent variations are able to discern differences between the selected tunes to a greater degree than the usual inclusive distributions. Further, the FB-correlation is found to be sensitive to the particle production processes within the tunes, an invaluable property for the purposes of generator tuning. A measurement of the forward-backward correlation for pp collision of CoM = 900 GeV and 7 TeV at the LHC using the ATLAS detector is made. The measured correlation is compared to the predicted correlation of several ATLAS centrally produced generator tunes. A correction procedure is developed and validated on the generator samples to correct the generated correlation to the hadron-level correlation. This is then applied to the measured correlation and a comparison of corrected data to the hadron-level predictions of the generated tunes made. The corrected correlations at the two collision energies are compared as well as the calculation of a global correlation at both energies. The measured and corrected correlations are found to lie above the predicted distributions at both energies and across the eta-range. Further investigation of measured correlation using augmented FB-correlations is recommended.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Measurement of the forward Z boson production cross-section in pp collisions at s=13\sqrt{s} = 13 TeV

A measurement of the production cross-section of Z bosons in pp collisions at $\sqrt{s} = 13$ TeV is presented using dimuon and dielectron final states in LHCb data. The cross-section is measured for leptons with pseudorapidities in the range $2.0 \eta 4.5$, transverse momenta $p_\text{T} 20$ GeV and dilepton invariant mass in the range $60 m(\ell\ell) 120$ GeV. The integrated cross-section from averaging the two final states is \begin{equation*}\sigma_{\text{Z}}^{\ell\ell} = 194.3 \pm 0.9 \pm 3.3 \pm 7.6\text{ pb,}\end{equation*} where the first uncertainty is statistical, the second is due to systematic effects, and the third is due to the luminosity determination. In addition, differential cross-sections are measured as functions of the Z boson rapidity, transverse momentum and the angular variable $\phi^*_\eta$

Les droits disciplinaires des fonctions publiques : « unification », « harmonisation » ou « distanciation ». A propos de la loi du 26 avril 2016 relative à la déontologie et aux droits et obligations des fonctionnaires

The production of tt‾ , W+bb‾ and W+cc‾ is studied in the forward region of proton–proton collisions collected at a centre-of-mass energy of 8 TeV by the LHCb experiment, corresponding to an integrated luminosity of 1.98±0.02 fb−1 . The W bosons are reconstructed in the decays W→ℓν , where ℓ denotes muon or electron, while the b and c quarks are reconstructed as jets. All measured cross-sections are in agreement with next-to-leading-order Standard Model predictions.The production of $t\overline{t}$, $W+b\overline{b}$ and $W+c\overline{c}$ is studied in the forward region of proton-proton collisions collected at a centre-of-mass energy of 8 TeV by the LHCb experiment, corresponding to an integrated luminosity of 1.98 $\pm$ 0.02 \mbox{fb}^{-1}. The $W$ bosons are reconstructed in the decays $W\rightarrow\ell\nu$, where $\ell$ denotes muon or electron, while the $b$ and $c$ quarks are reconstructed as jets. All measured cross-sections are in agreement with next-to-leading-order Standard Model predictions

Testbeam analysis of biasing structures for irradiated hybrid pixel detectors

Journal of Instrumentation The International School for Advanced Studies (SISSA), find out more. paper • The following article isOpen access Testbeam analysis of biasing structures for irradiated hybrid pixel detectors A.G. Rennie, C.M. Buttar, Y. Gao, R. González López, D. Maneuski, E. Pender, Q. Qin, M. Sullivan, J.T. Taylor and K. Wraight Published 31 January 2025 • © 2025 The Author(s) Journal of Instrumentation, Volume 20, January 2025 Citation A.G. Rennie et al 2025 JINST 20 P01029 DOI 10.1088/1748-0221/20/01/P01029 DownloadArticle PDF Authors References Article metrics 21 Total downloads Share this article Article information Abstract Following the Phase-II upgrade during Long Shutdown (LS3), the LHC aims to reach a peak instantaneous luminosity of 7.5× 1034 cm-2 s-1, which corresponds to an average of around 200 inelastic proton-proton collisions per beam-crossing (every 25 ns). To cope with these conditions, the ATLAS Inner Detector will be replaced by a new all-silicon system — the Inner Tracker (ITk). The ITk will be operational for more than ten years, during which time ATLAS is expected to record approximately 4000 fb-1 of data. The ITk's pixel sub-system is based on hybrid pixel modules with new silicon sensors and readout chips. These studies focus on testbeam campaigns undertaken to study the spatial resolution and efficiencies of hybrid pixel detector modules based on the first large-structure prototype front-end readout chip — the RD53A — using planar silicon sensors. These devices have been irradiated to replicate the effect of the high radiation environment present during operation in the ATLAS detector. Results for devices using sensors with different punch-through bias structures and using different readout modes are summarised. Those with sensors incorporating a punch-through bias structure are found to exhibit systematically lower efficiency than those without, as a result of local areas of relative inefficiency around the punch-through dots. Despite this, all devices measured are found to satisfy the requirement of 97% efficiency at Vbias = 400 V after being irradiated to end-of-life fluence

Further evidence for increased macrophage migration inhibitory factor expression in prostate cancer

BACKGROUND: Macrophage migration inhibitory factor (MIF) is a cytokine associated with prostate cancer, based on histologic evidence and circulating (serum) levels. Recent studies from another laboratory failed to document these results. This study's aims were to extend and confirm our previous data, as well as to define possible mechanisms for the discrepant results. Additional aims were to examine MIF expression, as well as the location of MIF's receptor, CD74, in human prostatic adenocarcinoma compared to matched benign prostate. METHODS: MIF amounts were determined in random serum samples remaining following routine PSA screening by ELISA. Native, denaturing and reducing polyacrylamide gels and Western blot analyses determined the MIF form in serum. Prostate tissue arrays were processed for MIF in situ hybridization and immunohistochemistry for MIF and CD74. MIF released into culture medium from normal epithelial, LNCaP and PC-3 cells was detected by Western blot analysis. RESULTS: Median serum MIF amounts were significantly elevated in prostate cancer patients (5.87 ± 3.91 ng/ml; ± interquartile range; n = 115) compared with patients with no documented diagnosis of prostate cancer (2.19 ± 2.65 ng/ml; n = 158). ELISA diluent reagents that included bovine serum albumin (BSA) significantly reduced MIF serum detection (p < 0.01). MIF mRNA was localized to prostatic epithelium in all samples, but cancer showed statistically greater MIF expression. MIF and its receptor (CD74) were localized to prostatic epithelium. Increased secreted MIF was detected in culture medium from prostate cancer cell lines (LNCaP and PC-3). CONCLUSION: Increased serum MIF was associated with prostate cancer. Diluent reagents that included BSA resulted in MIF serum immunoassay interference. In addition, significant amounts of complexed MIF (180 kDa under denaturing conditions by Western blot) found in the serum do not bind to the MIF capture antibody. Increased MIF mRNA expression was observed in prostatic adenocarcinoma compared to benign tissue from matched samples, supporting our earlier finding of increased MIF gene expression in prostate cancer

Inclusive and differential cross-section measurements of t\bartZ production in pp collisions at √s=13 TeV with the ATLAS detector, including EFT and spin-correlation interpretations

Measurements of both the inclusive and differential production cross sections of a top-quark-top-antiquark pair in association with a Z boson (tt¯Z) are presented. Final states with two, three or four isolated leptons (electrons or muons) are targeted. The measurements use the data recorded by the ATLAS detector in pp collisions at s√=13 TeV at the Large Hadron Collider during the years 2015-2018, corresponding to an integrated luminosity of 140 fb−1. The inclusive cross section is measured to be σtt¯Z=0.86±0.04 (stat.)±0.04 (syst.) pb and found to be in agreement with the most advanced Standard Model predictions. The differential measurements are presented as a function of a number of observables that probe the kinematics of the tt¯Z system. Both the absolute and normalised differential cross-section measurements are performed at particle level and parton level for specific fiducial volumes, and are compared with NLO+NNLL theoretical predictions. The results are interpreted in the framework of Standard Model effective field theory and used to set limits on a large number of dimension-6 operators involving the top quark. The first measurement of spin correlations in tt¯Z events is presented: the results are in agreement with the Standard Model expectations, and the null hypothesis of no spin correlations is disfavoured with a significance of 1.8 standard deviations

Observation of quantum entanglement in top-quark pairs using the ATLAS detector

We report the highest-energy observation of entanglement, in top−antitop quark events produced at the Large Hadron Collider, using a proton−proton collision data set with a center-of-mass energy of s√=13 TeV and an integrated luminosity of 140 fb−1 recorded with the ATLAS experiment. Spin entanglement is detected from the measurement of a single observable D, inferred from the angle between the charged leptons in their parent top- and antitop-quark rest frames. The observable is measured in a narrow interval around the top−antitop quark production threshold, where the entanglement detection is expected to be significant. It is reported in a fiducial phase space defined with stable particles to minimize the uncertainties that stem from limitations of the Monte Carlo event generators and the parton shower model in modelling top-quark pair production. The entanglement marker is measured to be D=−0.547±0.002 (stat.)±0.021 (syst.) for 340&lt;mtt¯&lt;380 GeV. The observed result is more than five standard deviations from a scenario without entanglement and hence constitutes both the first observation of entanglement in a pair of quarks and the highest-energy observation of entanglement to date

New algorithms for identifying the flavour of B⁰mesons using pions and protons

Two new algorithms for use in the analysis of pp collision are developed to identify the flavour of B0mesons at production using pions and protons from the hadronization process. The algorithms are optimized and calibrated on data, using B0→D-π+ decays from pp collision data collected by LHCb at centre-of-mass energies of 7 and 8 TeV . The tagging power of the new pion algorithm is 60% greater than the previously available one; the algorithm using protons to identify the flavour of a B0meson is the first of its kind.</p