arXiv subjects

Tae Jeong Kim

Publications and source records attributed to Tae Jeong Kim.

10 recordsLinked to original sources

arXiv2025

Develoment of thin high-pressure-laminate RPC electrodes for future high-energy experiments

In this R&D, an innovative method for producing thin high-pressure laminate (HPL) electrodes for resistive plate chambers (RPC) for future high-energy experiments is introduced. Instead of using thick phenolic HPL (2-mm thick Bakelite), which has been used for conventional RPC triggers, the RPC electrodes in the present study are constructed by bonding 500 μm-thick melamine-based HPL to a graphite-coated polycarbonate plate. A double-gap RPC prototype to demostrate the present technology has been constructed and tested for cosmic muons. Furthermore, the uniform detector characteristrics shown in the test result allows us to explore the present technology in future high-energy experiments.

physics.ins-det↗

arXiv2023

Measurements of the Cross-Section for the $t\bar{t}$ + Heavy-Flavor Production at the LHC

At the LHC, the process of a Higgs boson decaying into bottom or charm quarks produced in association with a pair of top quarks, ttbarH , allows for an empirical exploration of the heavy-flavor quark Yukawa couplings to the Higgs boson. Accordingly, the cross-sections for the $t\bar{t}$ + heavy-flavor production without the appearance of the Higgs boson have been measured at the LHC in various phase spaces using data samples collected in pp collisions at $\sqrt{s}$ = 7, 8 and 13 TeV with the ATLAS and CMS experiments. Flavor ratios of cross-sections of $t\bar{t}$ + heavy-flavors to $t\bar{t}$ + additional jets processes are also measured. In this paper, the measured cross-sections and ratios are reviewed and the prospects with more data are presented.

hep-ex↗

arXiv2021

Learning to increase matching efficiency in identifying additional b-jets in the $\text{t}\bar{\text{t}}\text{b}\bar{\text{b}}$ process

The $\text{t}\bar{\text{t}}\text{H}(\text{b}\bar{\text{b}})$ process is an essential channel to reveal the Higgs properties but has an irreducible background from the $\text{t}\bar{\text{t}}\text{b}\bar{\text{b}}$ process, which produces a top quark pair in association with a b quark pair. Therefore, understanding the $\text{t}\bar{\text{t}}\text{b}\bar{\text{b}}$ process is crucial for improving the sensitivity of a search for the $\text{t}\bar{\text{t}}\text{H}(\text{b}\bar{\text{b}})$ process. To this end, when measuring the differential cross-section of the $\text{t}\bar{\text{t}}\text{b}\bar{\text{b}}$ process, we need to distinguish the b-jets originated from top quark decays, and additional b-jets originated from gluon splitting. Since there are no simple identification rules, we adopt deep learning methods to learn from data to identify the additional b-jets from the $\text{t}\bar{\text{t}}\text{b}\bar{\text{b}}$ events. Specifically, by exploiting the special structure of the $\text{t}\bar{\text{t}}\text{b}\bar{\text{b}}$ event data, we propose several loss functions that can be minimized to directly increase the matching efficiency, the accuracy of identifying additional b-jets. We discuss the difference between our method and another deep learning-based approach based on binary classification arXiv:1910.14535 using synthetic data. We then verify that additional b-jets can be identified more accurately by increasing matching efficiency directly rather than the binary classification accuracy, using simulated $\text{t}\bar{\text{t}}\text{b}\bar{\text{b}}$ event data in the lepton+jets channel from pp collision at $\sqrt{s}$ = 13 TeV.

hep-ex↗

arXiv2020

Identification of additional jets in the t-tbar b-bbar events using a deep neural network

In the top quark pair production in association with the Higgs boson decaying to a b quark pair t-tbar H (b-bbar), the final state has an irreducible nonresonant background from the production of a top quark pair in association with a b quark pair t-tbar b-bbar. Therefore, understanding of the t-tbar b-bbar process precisely in particular differential cross-section as functions of the properties of the additional b jets not from the top quark decay is essential for improving the sensitivity of a search for the t-tbar H b-bbar process. The two additional b jets can be identified by using various approaches. In this paper, the performances are compared quantitatively in the lepton+jets decay channel in terms of the matching efficiency of assigning two additional b jets as a figure of merit. This study provides valuable information towards the precise measurement of differential cross-sections as a function of properties of the additional b jets in the t-tbar b-bbar events. We showed that a matching efficiency of around 40% could be achieved using a deep neural network method. In the events with at least 4 b jets, this performance is 8% better than that achieved using minimum Delta R(b,bbar) method. This is consistent with the boosted decision tree method within its statistical uncertainty.

hep-ex↗

arXiv2019

Correlation between $R_{D^{(*)}}$ and top quark FCNC decays in leptoquark models

Some interpretations of $R_{D^{(*)}}$ anomaly in $B$ meson decay using leptoquark (LQ) models can also generate top quark decays through Flavor Changing Neutral Current (FCNC). In this work we focus on two LQs, i.e. scalar $S_1$ and vector $U_1$ which are both singlet under the $SU(2)_L$ gauge group in the Standard Model (SM). We investigate their implications on the 3-body top FCNC decays $t\to c \ell_i \ell_j$ at tree level and the 2-body $t\to c V$ at one-loop level, with $\ell$ being the SM leptons and $V=γ, Z, g$ being the SM gauge bosons. We utilize the $2σ$ parameter fitting ranges of the LQ models and find that $Br(t\to c \ell_i \ell_j)$ at tree level can reach $\mathcal{O}(10^{-6})$ and $Br(t\to c V)$ at one-loop level can reach $\mathcal{O}(10^{-10})$. Some quick collider search prospects are also analyzed.

hep-ph↗

arXiv2017

Differential cross sections of global event variables of ttbar

During the Run 2 period of the LHC, the ATLAS and CMS experiments have accumulated proton-proton collision data corresponding to an integrated luminosity of around 3 fb$^{-1}$ in 2015 and 36 fb$^{-1}$ in 2016 at a center of mass energy of 13 TeV. In the journey of finding new physics, it is of importance to understand the standard model which seems to be complete after the Higgs discovery. Precision tests must be performed in every corner of the phase space since new physics can appear in any different places. In this proceedings, measurements of the differential cross sections of global event variables from the top quark pair productions at the both experiments using the data collected during the Run 2 period by the time of the year 2016 are presented.

hep-ex↗

arXiv2017

A study of top-quark mass measurement using the lepton energy distribution at the Large Hadron Collider

We present a feasibility study of top-quark mass measurement using the energy distribution of a lepton from a W boson in a top quark decay in pp collisions at the LHC. The proposed method requires only the lepton energy distribution at the parton level. The analysis is performed in the lepton + jets final state by using fast simulation data corresponding to an integrated luminosity of approximately 20 fb-1 at 14 TeV. Events with exactly one lepton, at least 3 jets and at least 1 b jet are selected. The lepton energy distribution at the parton level is obtained by applying the bin-by-bin unfolding technique. The study shows that the pole mass of the top quark can be measured within an uncertainty of 1 GeV.

hep-ex↗

arXiv2016

Search for Flavor Changing Neutral Current in Top Production and Decays

Searches for flavor changing neutral currents in top production and decay using data collected by the Compact Muon Solenoid (CMS) experiment at $\sqrt{s}$ = 7 and 8 TeV are presented, corresponding to an integrated luminosity of around 5 fb$^{-1}$ and 20 fb$^{-1}$. FCNC searches are conducted to probe $tqZ$, $tqγ$, $tqH$, and $tgq$ interactions in various channels. By the time of the 38$^{th}$ ICHEP conference in 2016, the upper limits on $\mathcal{B}(t \to uγ)$ $<$ 0.013%, $\mathcal{B}(t \to ug)$ $<$ 0.036%, $\mathcal{B}(t \to uZ)$ $<$ 0.05% and $\mathcal{B}(t \to uH)$ $<$ 0.42% at the 95% confidence level had been obtained by the CMS collaboration.

hep-ex↗

arXiv2015

Study of the Top-quark Pair Production in Association with a Bottom-quark Pair from Fast Simulations at the LHC

A large number of top quarks will be produced at the Large Hadron Collider (LHC) for Run II period. This will allow us to measure the rare processes from the top sector in great details. We present the study of the top-quark pair production in association with a bottom-quark pair (ttbb) from fast simulations for the Compact Muon Solenoid (CMS) experiment. The differential distributions of ttbb are compared with the top-quark pair production with two additional jets (ttjj) and with the production in association with the Higgs (ttH), where the Higgs decays to a bottom-quark pair. The significances of ttbb process in the dileptonic and semileptonic decay mode are calculated with the data corresponding to an integrated luminosity of 10 fb-1, which is foreseen to be collected in the early Run II period. This study will be an important input in searching for new physics beyond the standard model as well as in searching for ttH process where the Yukawa coupling with the top quark can be directly measured.

hep-ex↗

arXiv2012

t-tbar Pair production cross section measurement at the LHC

Measurement of $t\bar{t}$ pair production cross sections with an integrated luminosity of around 1 fb$^{-1}$ at $\sqrt{s}$ = 7 TeV obtained with the ATLAS and CMS detectors are reported. The inclusive cross sections in dilepton (ee, $eμ$, $μμ$ and $μτ$), lepton+jets (e, $μ$) and all hadronic decay modes are measured. In addition to inclusive cross section measurement, the study of jet multiplicity with additional jets are also presented, which is important to constrain the initial state radiation. Measurement of the charge asymmetry at the LHC is also presented. All measurements are compatible with Standard Model predictions.

hep-ex↗