Search for vector-like quarks using jet substructure techniques with the CMS experiment

In this thesis, a search for pair produced vector-like $T$ quarks in $pp$ collision data from the LHC at $\sqrt{s} = 13\,\text{Te}\kern-0.06667em\text{V}$ is presented. The data were collected with the CMS detector and correspond to an integrated luminosity of up to $2.6\,\mathrm{fb}^{-1}$. Vector-like quarks are hypothetical new particles predicted by many extensions of the Standard Model in which the Higgs boson is a composite state of an unknown strong interaction.Vector-like $T$ quarks are assumed to decay via three different decay modes to either $bW$, $tZ$ or $tH$, with branching fractions that are not fixed and can vary depending on the particular model featuring vector-like quarks.This search focuses on decays of the $T{\overline T}$ system in which at least one muon or electron is present in the final state, and in which at least one of the $T$ quarks decays to a top quark and a Higgs boson.As the $T$ quarks are expected to be quite heavy, their decay products are significantly Lorentz-boosted in the reference frame of the $T{\overline T}$ system. The subsequent decay products of the Higgs boson are then emitted with a very small angle between them. This search is optimised for the main decay channel of the Higgs boson to two bottom quarks and attempts to reconstruct the two $b$ quarks within a single jet with a large cone size. Dedicated jet substructure techniques, in combination with algorithms to identify jets originating from the fragmentation of a $b$ quark, are then used to reconstruct the entire $H \rightarrow b{\overline b}$ decay.The event categories of this search are also combined with the categories of a search for pair-produced $T$ quarks that focuses on the $T \rightarrow bW$ decay. This approach provides a high sensitivity to $T{\overline T}$ production for many different combinations of branching fractions to the three possible decay modes.No excess of the data above the expected background contribution from the Standard Model is observed in any of the final event categories. Upper limits on the $T{\overline T}$ production cross section are calculated at 95% confidence level and lower mass limits for vector-like $T$ quarks are set for various assumptions of their decays: for branching fractions corresponding to an electroweak isospin singlet $T$ quark, masses below 860$\,\text{Ge}\kern-0.06667em\text{V}$ are excluded, while for an electroweak isospin doublet, $T$ quarks are excluded with masses up to 830$\,\text{Ge}\kern-0.06667em\text{V}$. For other branching fraction combinations, lower mass limits are derived ranging from 710$\,\text{Ge}\kern-0.06667em\text{V}$ to 940$\,\text{Ge}\kern-0.06667em\text{V}$.These exclusion limits are comparable to and for some branching fraction assumptions even higher than exclusion limits set by the CMS experiment using a data set corresponding to $19.7\,\mathrm{fb}^{-1}$ at $\sqrt{s} = 8\,\text{Te}\kern-0.06667em\text{V}$

Demographic and clinical characteristics of the patients.

<p>Demographic and clinical characteristics of the patients.</p

Metabolites with significant difference between the control and the disease activity of NMOSD (C, healthy control; RL, relapse; RM, Remission).

<p>Metabolites with significant difference between the control and the disease activity of NMOSD (C, healthy control; RL, relapse; RM, Remission).</p

Metabolic profiling in MS.

<p>Metabolic profiling in MS.</p

Metabolic profiling in NMOSD.

<p>Metabolic profiling in NMOSD.</p

The ROC curve analysis for the composite metabolites.

<p>ROC curves of each group comparison were created by 7-fold cross validated predicted y-values from OPLS-DA model. The top 2, 3, 5, 10, 20, 32 (max) important variables based on the VIP values from OPLS-DA model were used to build classification models. The AUC values were obtained from OPLS-DA models of NMOSD-others, NMOSD-MS, and MS-others with combination of metabolites (A). The combination of five metabolites that showed high importance in the group comparison of NMOSD-others, NMOSD-MS, and MS-others provided the AUC value, 0.872, 0.856, and 0.835, respectively (B).</p

Metabolites with significant difference between the control and patients groups.

<p>Metabolites with significant difference between the control and patients groups.</p

Schematic representation of relevant metabolism involved in MS and NMOSD.

<p>The perturbed metabolites in patients are depicted in red and blue. The colored dotted arrows represent up (red)- or down (blue)-regulation of metabolites. The black dotted arrows means not direct but abbreviated pathways. Lower glucose and higher acetone together with lower citrate in MS patients and higher lactate in NMOSD patients may suggest an impaired TCA cycle in mitochondria. Up-regulation of 2-hydroxybutyrate and pyroglutamate in patients indicates that an elevated oxidative stress may be caused by impaired GSH metabolism in neurological diseases such as MS and NMOSD. The elevated level of formate might reflect a malfunction of brain mitochondria as well as oxidative stress. Down-regulation of acetate in the disease groups and down-regulation of citrate in MS patients may indicate a decrease of fatty acid metabolism, particularly in myelin synthesis. γ-GCT: γ-glutamyl cyclotransferase; LDH: lactate dehydrogenase; CoA: coenzyme A.</p

Metabolites with significant difference between the control and the disease activity of MS.

<p>Metabolites with significant difference between the control and the disease activity of MS.</p

Chronic cystic changes of brain lesions on MRI.

<p>At the time of an acute brain attack, brain MRI showed multiple T2-hyperintense lesions with subtle T1 hypointensity in the frontal white matter (Aa), corticospinal tract (Ba, Ca), occipital white matter (Da) and corpus callosum (Ea). On follow-up MRI, all T2 hyperintense lesions were markedly decreased in size but revealed focal T1-hypointensity with cystic changes (Ab, Bb, Cb, Db, Eb).</p