String theory and Field theory of strongly interacting system.

Modern string theory has dual purpose: one is as a theory for quantum gravity and the other is as a theory for strongly interacting system. During the 60’s particle physicists found that the spectrum of hadrons had strange property that their mass squared is linear to spin. Soon it was found that such behavior can be explained if one assumes that hadrons are spinning string, which is the birth of string theory. In modern days many interesting condensed matter phenomena as well as nuclear physics are unexplained due to the lack of calculational tools. In these days of gauge-gravity duality, it is extremely interesting and promising area since one can fit the data known as hard to fit in terms of simple string theory calculation.

A reasonable quantum gravity

The very final frontier that we want to apply quantum mechanical methodology is gravity. Already, they have found and confirmed that gravitational waves exist. What about graviton? May the well established quantum gravity theory(ies) exist? There have been many of attempts to construct such quantum gravities, but they realized that there are two kinds of obstructions that are usually coming when the gravity is quantized: lost of unitarity and(or) non-renormalizability. How can we overcome these problems? Still there is no good answer to this question. Some of the candidates for well designed quantum gravity are considered these days: (partially) massive gravity, higher derivative gravity, non-local gravity and so on. Each theory has specific features to overcome the two problems but some of those cause the other matters.

Belle II experiment

The Belle II, a successor to the Belle experiment, is one of the leading experiments on the intensity frontier of particle physics. It focuses on discovering evidence of New Physics (NP) beyond the Standard Model, which is the core theory that explains most of the observed phenomena related to elementary particles. Belle II will measure new CP violating phenomena (matter-antimatter asymmetries) and rare decays of particles containing beauty quarks, charm quarks and tau leptons. In addition, we plan unique studies of the dark sector (weakly coupled new particles). Rather than addressing NP via the direct production of new particles as is the case for most ATLAS and CMS studies at the LHC, Belle II looks for the presence of new couplings that are due to NP. In the end when this complementary approach is successful, the new weak interaction couplings would be, in fact, due to new particles and NP. We have been working on the Belle and the Belle II upgrade experiments for more than 20 years starting from the initial stage of the Belle experimental proposal. In particular, our hardware and software contributions to the ECL calorimeter trigger system were critical to operating the experiment stably and with good efficiency for more than 10 years. In the Belle II/SuperKEKB upgrade program there were physics commissioning starting from February 2018. We have already contributed on the ECL calorimeter trigger system, trigger simulation and monitoring software. The entire ECL trigger system was designed and constructed by the Hanyang group starting from the global design, through prototype R&D, mass production, installation and calibration. This trigger system is quite important and required to operate the Belle II experiment and collect physics events with ~100% trigger efficiency at the online level. Furthermore, the entire ECL readout electronics was constructed by the Notice company in Korea, which was managed by Hanyang group. Therefore, the Belle II ECL calorimeter system would not have been possible without our enormous effort and contributions; we expect this trigger to contribute to beam background rejection and reduction of the trigger rate from dangerous backgrounds in the coming physics run. From early of 2019 Belle II started luminosity run to collect real data so that we are on the stage of deeper physics analyses of rare B meson decays and charmed baryon decays that we have done in Belle.

CMS experiment

The Compact Muon Solenoid (CMS) experiment is one of two particle detectors that are built on the Large Hadron Collider (LHC) at CERN located in Switzerland and France. Main purpose of the CMS experiment is to detect elementary particles and search for new physics phenomenology. CMS is an international collaboration where the high energy group in Hanyang University has an important participation. Our group is interested in top quark physics and searching for beyond standard model through top quark decay and production using CMS detector at the LHC. Discovering Higgs boson allows us to open new windows towards solving many unanswered questions such as the unification of forces or dark energy in the Universe. Top quark is still believed to be the key which will give us hints in this journey. Data at 13 TeV proton-proton collision energy which is the highest energy ever has been collected from 2015 to 2018. With this big data, exciting time is just ahead of us.

Telescope Array experiment

The Telescope Array experiment at the Utah desert in the US is for the study of the ultra-high-energy cosmic-ray (UHECR), and studies the energy spectrum, composition and arrival direction of the UHECRs with three fluorescence detector (FD) stations and approximately 500 surface detectors, e.g., observation coverage of 700 km2 in total. We operate the electron linear accelerator and analyze the FD data by using the Electron Light Source (ELS) for the calibration of FD energy measurement.