KU HepEx Group - Research

KU HepEx Group

As a member of the CMS collaboration, the following are the current research projects our group is involved in:

Physics analysis
- RPV SUSY search in 1-lepton final state
- Search for long-lived particles using HCAL timing
- Search for CP Violation in ttbar
- Search for SM 4top process in all hadronic final state
Detector and performance
- Development of LGAD sensor testing system
- Development of MTD FastSim

We are also interested in searches for millicharged particles:

SUB-Millicharge ExperimenT (SUBMET) at J-PARC
Other possibilities at Los Alamos National Lab, CERN, Fermilab

Please check below for the details!

R-Parity Violating (RPV) Supersymmetry (SUSY) Search

Most Supersymmetry(SUSY) searches require missing transverse energy(MET) because the conservation of R-parity requires that the LSP be stable, which gives MET. The high MET region is well covered by existing searches, but low MET regions are not. So, our team also searches for new physics without a MET requirement. This is well motivated in the context of R-parity-violating scenarios where SUSY particles can directly decay to SM particles without an LSP at the end of the decay chain. The model we focus on is gluino pair production, where the gluino decays to top, bottom, and strange quarks (gluino→ tbs), motivated by minimal flavor violating SUSY. The result with the 2016 data was published in Phys. Lett. B, where mgluino<1.6 TeV is excluded at 95% CL. We developed an analysis method to be less sensitive to systematic effects for the results with the full Run2 data. For Run 3, we plan to improve the analysis method further to be more inclusive both in signature and interpretation.

Team: Byeong Jin Hong, Chan Gi Jung, and Hoyong Jeong

Search for long-lived particles using HCAL timing

No evidence for new physics has been seen at the LHC so far, and the increase in the amount of data accumulated will be slower than before. This project aims to search for long-lived particles, which belong to a phase space that is not so thoroughly searched. If these particles are massive and weakly interacting, they can travel a considerable distance before decaying into visible particles and leave a signal of displaced non-prompt jets. A signal of this nature can be detected with the timing information and missing transverse momentum measurements.

We developed and employed the HCAL hit time using cross-correlation and used it to construct a jet time. Currently, we are working on an analysis strategy and understanding background sources.

Team: Jayashri Padmanaban, Changhyun Seo, and Jinseok Yoo in collaboration with Ryan Schmitz (Imperial)

CP Violation in ttbar events

The Standard Model predicts negligible CP violation (CPV) in the top-quark sector, making it a sensitive probe of new physics. We study the top-quark chromoelectric dipole moment (CEDM), which induces CPV in ttbar production. Sensitivity is achieved through the asymmetries of the CP-odd observables (O1) and (O3), constructed from the four-momenta of leptons, (b)-jets, and reconstructed top quarks. We are developing a machine learning–based top-quark reconstruction algorithm to improve the sensitivity to the CEDM.

Team: Haeyun Hwang and Seungkyu Ha in collaboration with Yonsei (Hwidong Yoo) and KNU (Sehwook Lee) groups

4-top production

The 4-top production is a rare Standard Model process, but its cross section needs to be measured precisely because it is a background in searches for BSM models with multiple top quarks. It also provides a promising probe of the top-quark Yukawa coupling.

We are interested in the all-hadronic channel where all W bosons from the top quarks decay to two quarks. We search for this process in the region of many jets and many b-jets with high energy requirements.

Team: Haeyun Hwang in collaboration with CMU group (Valentina Dutta and Manfred Paulini)

(figure: https://cds.cern.ch/images/ATLAS-PHOTO-2018-024-2)

MTD Sensor testing system

The planned high-luminosity upgrade of the Large Hadron Collider (HL-LHC) at CERN requires a phase-2 upgrade of the Compact Muon Solenoid (CMS) detector. To meet the demands of this environment, new high-granularity detectors with significantly improved timing and spatial resolution are being developed. In particular, the upgraded Endcap Timing Layer (ETL), a component of the CMS MIP Timing Detector (MTD), is expected to employ Low-Gain Avalanche Diode (LGAD) sensors. We developed a full chain of systems that allows us to test the full 16x16 sensor. It integrates custom hardware with control and monitoring software designed for automation, user convenience, and scalability. These features enable efficient testing, streamlined operation, and adaptability for future detector developments.

Team: Hoyong Jeong, Kyungmin Lee, Seokhyun Lee

MTD FastSim

FastSim (Fast Simulation) is a parametric Monte Carlo application within the CMS software framework that replicates CMS detector effects and event reconstruction without full tracking or GEANT4-based geometry. Our group is taking full responsibility for developing FastSim for MTD,

Team: Hoyong Jeong, Chan Gi Jung

Search for sub-millicharged particles at J-PARC

The existence of millicharged particles (mCPs) is not strictly excluded in the Grand Unified Theories, and mCPs may be a viable dark matter (DM) candidate (at least some fraction of DM). We are studying the feasibility of detecting mCPs with a scintillator-based detector at J-PARC. Our group led the design, assembly, and installation of the detector in 2024, and has been taking data since then. It is a collaborative work (spokesperson: Jae Hyeok Yoo) with UCSB, UC Davis, OSU, NYU, CERN, and Lebanese University.

We plan to continue data-taking by mid 2027 and probe the unexplored mass and charge region!

Team: Hoyong Jeong, Seungkyu Ha, Jinseok Yoo, and Sejun Kang

Search for sub-millicharged particles at LANL, Fermilab, and CERN

We are also interested in searching for higher-mass mCPs using other facilities with a higher beam energy. These include LANSCE at LANL, SPS and CERN, and NuMI at Fermilab.

Team: Hoyong Jeong, Seungkyu Ha, Jinseok Yoo, and Sejun Kang

Research projects prior to joining KU:

- SUSY search in 1L final state using MJ at CMS
- Evidence of H->WW->2l2v at CMS
- HCAL operations at CMS
- Search for millicharged particles using milliQan demonstrator

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