Background estimation, search for heavy resonances and off-shell Higgs boson signal strength measurement in the 4 lepton final state with the ATLAS detector

Το 2012 η ανακάλυψη του μποζονίου Higgs από τα δύο μεγάλα πειράματα στο CERN, ATLAS και CMS, επιβεβαίωσε το Καθιερωμένο Πρότυπο (ΚΠ), την θεωρία η οποία περγράφει τα στοιχειώδη σωμάτια και τις αλληλεπιδράσεις τους. Καθώς με το μποζόνιο Higgs ολοκληρώθηκε η αναζήτηση των σωματιδίων τα οποία προβλέπονται από το ΚΠ, η έρευνα εστιάζεται πλέον στην μελέτη των ιδιοτήτων του, καθώς και στην αναζήτηση σωματιδίων τα οποία προβλέπονται από θεωρίες πέραν του ΚΠ. Κατά την 2η περίοδο λειτουργίας του LHC, τα έτη 2015-2018, σε ενέργεια κέντρου μάζας 13 TeV, το πείραμα ATLAS συvέλλεξε δεδομένα τα οποία αντιστοιχούν σε ολοκληρωμένη φωτεινότητα περίπου 140 /fb συμβάλλοντας σε αυτές τις μέλετες. Η ικανότητα του πειράματος να συλλέγει δεδομένα θα βελτιωθεί μέσω μιας σειράς αναβαθμίσεων. Στην 2η φάση αναβάθμισης θα αντικατασταθεί ο εσωτερικός ανιχνευτής τροχιών από τον new Inner Tracker (ITk). Ο ΙΤk θα καλλύπτει περιοχή ψευδοωκύτητας μέχρι 4, δίνοντας για πρώτη φορά την δυνατότητα ανακατασκευής τροχιών για ψευδοωκύτητα μεγαλύτερη από 2.5. Σε αυτή την περιοχή (2.5-4.0) η πληροφορία για τις τροχιές των σωματιδίων αναμένεται να συμβάλλει καθοριστικά στην δυνατότητα του ηλεκτρομαγνητικού καλοριμέτρου να αναγνωρίζει ηλεκτρόνια. Στην συγκεκριμένη μελέτη, η οποία εξετάζει αυτή την δυνατότητα, αρχικά μελετάται η γωνιακή απόσταση των clusters που δημιουργούνται στο καλορίμετρο και των τροχιών στον ΙTk. Έπειτα εκπαιδεύται ένα Νευρωνικό Δίκτυο (ΝΔ) με τις μεταβλητές που περιγράφουν το σχήμα των clusters. Συνδυάζοντας την διακρίνουσα του ΝΔ με κριτήρια γωνιακής απόστασης cluster-τροχίας ορίζονται τα 3 working points των ηλεκτρονίων με απόδοση 90%, 80% και 70%. Τέλος εξετάζεται και η προοπτική συμβολής στην αναγνώριση κάποιων επιπλέον μεταβλητών. Στην τελική κατάσταση 4 λεπτονίων το μειώσιμο υπόβαθρο απορρίπτεται σε μέγαλο βαθμό μέσω των κριτηρίων επιλογής γεγονότων. Η εκτίμησή του όμως γίνεται από τα πραγματικά δεδομένα, για να αποφευχθούν οι όποιες αβεβαιότητες. Για την εκτίμηση στην llμμ τελική κατάσταση χρησιμοποίουνται 4 περιοχές ελέγχου. Αυτές κατασκευάζονται αντιστρέφοντας κάποια από τα κριτήρια επιλογής, έτσι ώστε να είναι εμπλουτισμένες στις διάφορες διαδικασίες υποβάθρου. Η αναλλοίωτη μάζα του πρωτεύοντος ζεύγους λεπτονίων χρησιμοποιείται για να κατασκευαστεί ένα μοντέλο, το οποίο προσαρμόζεται στα πραγματικά δεδομένα, στις 4 περιοχές ελέγχου, ταυτόχρονα. Οι περιοχές επικοινωνούν μέσω μια 5ης περιοχής. Τα αποτελέσματα εκφράζονται αρχικά σε αυτήν την 5η περιοχή και έπειτα μεταφέρονται στην περιοχή όπου όλα τα κριτήρια εφαρμόζονται κανονικά μέσω κάποιων παραγόντων μεταφοράς. Αυτοί οι παράγοντες ελέγχονται επίσης από τα πραγματικά δεδομένα χρησιμοποιώντας ένα κατάλληλο σύνολο γεγονότων. Στην ανάζητηση βαρέων συντονισμών η μέλετη επικεντρώνεται στην διαδικασία της προσαρμογής στα πραγματικά δεδομένα. Η προσαρμογή πραγματοποιείται με μια συνάρτηση μέγιστης πιθανοφάνειας. Από αυτήν προκύπτει μία test statistic μεταβλητή, η οποία δείχνει την συμφωνία των δεδομένων με κάποια θεωρητική υπόθεση. Μέσω αυτής της μεταβλητής ορίζονται τα ανώτερα όρια στην παράγωγη κάποιου νέου σωματιδίου με την μέθοδο CLs. Τέλος παρουσιάζονται τα όρια για την παραγωγή ενός Higgs-like συντονισμού. Στην τελευταία μελέτη γίνεται προσπάθεια μέτρησης του πλάτους διάσπασης του μποζονίου Higgs. Για να πραγματοποιηθεί αυτή η μέτρηση απαιτείται μέτρηση της ενεργού διατομής off-shell παραγωγής του μποζονίου. Για να αυξηθεί η ευαισθησία του πειράματος, εκτός από την βασική επιλογή γεγονότων, χρησιμοποιείται και η μέθοδος του στοιχείου πίνακα. Με την διακρίνουσα που πάραγεται από αυτήν την μέθοδο γίνεται η τελική προσαρμογή στα πραγματικά δεδομένα και εν τέλει εκτιμάται ένα ανώτερο όριο στο πλάτος διάσπασης.The discovery of Higgs boson in 2012 by the ATLAS and CMS experiments marked a key milestone in the history of particle physics. It confirmed a long-standing prediction of the Standard Model (SM), the theory that describes our present understanding of elementary particles and their interactions. The Higgs boson was the last particle of the SM which was observed. After its observation the research in in the ATLAS experiment focused on the measurement of its properties and also on the effort to observe particles beyond the SM. During Run II (2015-2018) it collected data corresponding to about 140 /fb, having a key role in these studies. This thesis presents a study for search of new high mass Higgs-like bosons as well a study for the measurement of Higgs boson decay width. It presents also the data driven estimation of the reducible background for these studies. The first chapter (1) is a short theoretical introduction to the SM. The particles and the way that their interactions emerge are described. The Higgs mechanism is introduced as a way to retain the invariance of a Lagrangian with massive bosons. The mechanism can be extended to give mass to the fermions too. Afterwards the Higgs boson production and decay modes are described. Except the SM Higgs boson, additional Higgs-like bosons are predicted by beyond the SM models. Lastly an innovative way to measure Higgs boson decay width is presented. In the second chapter (2) the ATLAS experiment is described, after a short introduction about about CERN and LHC. The ATLAS experiment consists of 3 main sub-detector systems, the Inner Detector (ID), the Calorimeters and the Muon Spectrometer (MS). The ID goal is to reconstruct charge particle tracks close to the interaction point. The Calorimeters measure the energy which is deposit in them by electrons/photons and hadrons. Lastly the MS reconstructs muon tracks. A trigger and a data acquisition system are used in order to choose and store the interesting event collisions. The ATLAS detector will be undergone two major upgrades. Soon the inner wheel of the end-cap MS will be replaced by the New Small Wheel. Also during Upgrade phase II the inner detector will be replaced by the New Inner Tracker (ITk). In the third chapter (3) a study for electron identification in the forward region is described. The new ITk will extend pseudorapidity coverage up to |η| = 4. An identification method for the electrons in the region 2.5 < |η| < 4.0 was developed. It uses track-cluster matching criteria and a Artificial Neural Network to separate the true electrons from the fakes, which come either from pile-up or jets. Lastly the potential of improving identification by using an Isolation variable is studied. In the forth chapter (4) the event selection and the background estimation in the 4-lepton channel are described. The description focuses on the Data Driven Estimation of the llμμ reducible background. The estimation is performed in 4 Control Regions (CRs). Each of them is enriched in a specific background component. The 4 CRs are linked via a fifth region, called Relaxed Region (RR). A simultaneous likelihood fit is performed in the 4 CRs and the results are extrapolated to the RR via Fractions. The results from the RR are subsequently extrapolated to the Signal Region via Transfer Factors (TFs). The TFs are estimated and controlled using a sample containing Z+μ events. In the fifth chapter (5) the search for a high mass ZZ resonance is described. In this analysis a cut based selection is used to categorized the events. The signal and the ZZ backgrounds are modeled with analytic functions, while the reducible background is data driven estimated. The methodology of the Likelihood fit and the extraction of CLs limits is described in detail. Lastly the final upper limits for the production of a higgs-like particle are presented. In the sixth chapter (6) the measurement of the Higgs boson decay width is described. For this measurement, a measurement of the Higgs boson off-shell cross section is required. For this study the MC samples are corrected with the latest k-factors. Then in additional to the nominal event selection a Matrix Element based discriminant is used to suppress the ZZ background. The reducible background is estimated in a Data Driven way. Finally the results are combined with these of the llμμ channel to set limits on the decay width

Diagnosis and management of intradiverticular bladder tumours: A pooled analysis of 498 cases

Objective: Intradiverticular bladder tumors (IDBT) are uncommon clinical entities. We reviewed the literature for clinical presentation, diagnosis and therapeutic options to establish recommendations for diagnostic and therapeutic management. Methods: Bibliographic research was performed using PubMed from database inception until October 15, 2022. A pooled analysis was performed of 498 patients with IDBT presented in the literature. The evaluation included patient sex, age, diagnostic methods, symptoms, localization of the tumor, tumor staging, tumor histopathology, treatment, and the presence of recurrence. To express results, descriptive statistics were used appropriately. Results: The mean age at diagnosis was 64.81 years (range 49 days to 84 years). The ratio between men and women was ≈ 24:1, suggesting a male predominance (85% male, 3.6% female). The most common presenting symptom was gross hematuria (60.88%). Most of the patients had cystoscopy (56.85%) and intravenous or computed tomography urography (52.01%). Regarding tumor staging, most of the patients were diagnosed with pT1 tumors. For the histopathology of IDBT, 87.95% of the specimens were transitional cell carcinomas and in 10.84% there were concomitant CIS. Regarding the treatment, radical cystectomy was chosen in 34.34%, partial cystectomy in 26.66%, diverticulectomy in 15.95% and transurethral resection of bladder tumour (TURBT) in 16.36% of the patients. Conclusions: Most common diagnostic tool for IDBT seems to be cystoscopy followed by computerized tomography urogram. Due to the absence of muscle layer in the diverticulum and the highgrade histology of most of them at diagnosis, cystectomy is the first therapeutic choice. However, for patients that are not considered appropriate candidates or for those presenting with lowgrade and low volume tumors, TURBT is a good option

Search for light long-lived neutral particles that decay to collimated pairs of leptons or light hadrons in pp collisions at √s = 13 TeV with the ATLAS detector

A search for light long-lived neutral particles with masses in the O(MeV-GeV) range is presented. The analysis targets the production of long-lived dark photons in the decay of a Higgs boson produced via gluon-gluon fusion or in association with a W boson. Events that contain displaced collimated Standard Model fermions reconstructed in the calorimeter or muon spectrometer are selected in 139 fb−1 of √s = 13 TeV pp collision data collected by the ATLAS detector at the LHC. Background estimates for contributions from Standard Model processes and instrumental effects are extracted from data. The observed event yields are consistent with the expected background. Exclusion limits are reported on the production cross-section times branching fraction as a function of the mean proper decay length cτ of the dark photon, or as a function of the dark-photon mass and kinetic mixing parameter that quantifies the coupling between the Standard Model and potential hidden (dark) sectors. A Higgs boson branching fraction above 1% is excluded at 95% CL for a Higgs boson decaying into two dark photons for dark-photon mean proper decay lengths between 10 mm and 250 mm and dark photons with masses between 0.4 GeV and 2 GeV

A search for R-parity-violating supersymmetry in final states containing many jets in pp collisions at √s=13TeV with the ATLAS detector

A search for R-parity-violating supersymmetry in final states with high jet multiplicity is presented. The search uses 140fb−1 of proton-proton collision data at √s=13TeV collected by the ATLAS experiment during Run 2 of the Large Hadron Collider. The results are interpreted in the context of R-parity-violating supersymmetry models that feature prompt gluino-pair production decaying directly to three jets each or decaying to two jets and a neutralino which subsequently decays promptly to three jets. No significant excess over the Standard Model expectation is observed and exclusion limits at the 95% confidence level are extracted. Gluinos with masses up to 1800 GeV are excluded when decaying directly to three jets. In the cascade scenario, gluinos with masses up to 2340 GeV are excluded for a neutralino with mass up to 1250 GeV

Searches for resonances decaying to boson pairs in ATLAS

Many new physics models predict the existence of resonances decaying into two bosons (W, Z, photon, or Higgs bosons) making these important signatures in the search for new physics. Searches for Vy, VV, and VH resonances have been performed in various final states. In some of these searches, jet substructure techniques are used to disentangle the hadronic decay products in highly boosted configurations. This talk summarises recent ATLAS searches with Run 2 data collected at the LHC and explains the experimental methods used, including vector- and Higgs-boson-tagging techniques

Event selection and differential cross section measurements in H->4l decays

This talk presents the event selection and background estimation used in the analysis of the H -> ZZ* -> 4l channel, where l = e or μ. Subsequently, it presents measurements of the Higgs boson inclusive fiducial and differential cross sections performed in this channel, using proton-proton collision data corresponding to an integrated luminosity of 139 /fb at a center-of-mass energy of 13TeV, as recorded by the ATLAS detector at the Large Hadron Collider (LHC) from 2015-2018

Searches for resonances decaying to boson pairs in ATLAS

Many new physics models predict the existence of new particles decaying into two bosons ($W$, $Z$, photons, or Higgs bosons) making these important signatures in the search for new physics. Searches for $\gamma\gamma$, $V\gamma$, $VV$, and $VH$ resonances have been performed in various final states. In some of these searches, jet substructure techniques are used to disentangle the hadronic decay products in highly boosted configurations. This talk summarises recent ATLAS searches with Run 2 data collected at the LHC and explains the experimental methods used, including vector- and Higgs-boson-tagging techniques

Background estimation, search for heavy resonances and off-shell Higgs boson signal strength measurement in the 4 lepton final state with the ATLAS detector

The discovery of Higgs boson in 2012 by the ATLAS and CMS experiments marked a key milestone in the history of particle physics. It confirmed a long-standing prediction of the Standard Model (SM), the theory that describes our present understanding of elementary particles and their interactions. The Higgs boson was the last particle of the SM which was observed. After its observation the research in the ATLAS experiment focused on the measurement of its properties and also on the effort to observe particles beyond the SM. During Run II (2015-2018) it collected data corresponding to about 140 fb$^{-1}$, having a key role in these studies. Firstly a study for electron identification in the forward region is described. The new ITk will extend pseudorapidity coverage up to |$\eta$| = 4. An identification method for the electrons in the region 2.5 < |$\eta$| < 4.0 was developed. It uses track-cluster matching criteria and a Artificial Neural Network to separate the true electrons from the fakes, which come either from pile-up or jets. Lastly the potential of improving identification by using an Isolation variable is studied. Next the event selection and the background estimation in the 4-lepton channel are described. The description focuses on the Data Driven Estimation of the $ll\mu\mu$ reducible background. The estimation is performed in 4 Control Regions (CRs). Each of them is enriched in a specific background component. The 4 CRs are linked via a fifth region, called Relaxed Region (RR). A simultaneous likelihood fit is performed in the 4 CRs and the results are extrapolated to the RR via Fractions. The results from the RR are subsequently extrapolated to the Signal Region via Transfer Factors (TFs). The TFs are estimated and controlled using a sample containing $Z+\mu$ events. Using the above selection and estimation several physics studies are performed. The search for a high mass $ZZ$ resonance is one of them. In this analysis a cut based selection is used to categorize the events. The signal and the $ZZ$ backgrounds are modeled with analytic functions, while the reducible background is data Driven estimated. The methodology of the Likelihood fit and the extraction of CLs limits is described in detail. Lastly the final limits are presented. At last the measurement of the Higgs boson decay width is described. In order to achieve this measurement, a measurement of the Higgs boson off-shell cross section is required. For this study the MC samples are corrected with the latest k-factors. Αfterward ιn additional to the nominal event selection a Matrix Element based discriminant is used to suppress the $ZZ$ background. The reducible background is estimated in a Data Driven way. Finally the results are combined with these of the $ll\nu\nu$ channel to set limits on the decay width