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Sanghoon Lim

Publications and source records attributed to Sanghoon Lim.

At least 19 recordsLinked to original sources
arXiv2026

Development and demonstration of the Korea ALICE Telescope using electron beams at KEK PF-AR

The development of ultra-low-mass, high-precision vertex detectors is a key requirement for future collider experiments and motivates extensive research and development of novel silicon tracking technologies. In this work, we present the development and beam-test demonstration of the Korea ALICE Telescope (KATS), a silicon-tracking telescope designed to support R&D on next-generation cylindrical vertex detectors, such as the proposed ALICE ITS3 upgrade. The telescope consists of six ALPIDE Monolithic Active Pixel Sensors (MAPS) used as reference tracking planes, a bent ALPIDE sensor serving as the device under test, and a scintillating-fiber-based trigger system, all housed in a light-tight modular enclosure. This setup enables precise track reconstruction and detailed performance studies of both planar and curved silicon sensors. Beam tests were carried out using high-energy electron beams at the KEK Photon Factory Advanced Ring (PF-AR). The telescope system operated stably under realistic beam conditions, and its tracking performance was successfully validated. The bent ALPIDE sensor was operated at a bending radius of approximately 18 mm, consistent with ITS3's design goals, without any observable degradation in detection performance. The measured results confirm that the KATS provides a versatile and reliable platform for studies of curved MAPS technologies, alignment precision, and tracking performance. These results provide important experimental validation of key technologies for future low-mass cylindrical silicon vertex detectors and establish KATS as a valuable facility for ongoing and future detector R&D.

physics.ins-det↗
arXiv2026

Investigation of hadronic effects on resonance productions in small collision systems using the EPOS4 model

Recent experimental results in high-multiplicity proton-proton (pp) collisions have suggested the possible emergence of collective behavior and medium-like effects previously considered characteristic of heavy-ion collisions. Resonance production provides a sensitive probe of such effects, as resonance yields and transverse-momentum distributions can be modified by hadronic interactions occurring between chemical and kinetic freeze-out. In this study, these effects are investigated using the EPOS4 event generator, in which hadronic final-state interactions are modeled through the UrQMD transport approach. By comparing calculations performed with and without UrQMD, the impact of hadronic interactions on resonance production is evaluated. In addition, the UrQMD contributions are separated into regeneration and rescattering, enabling a detailed investigation of both resonance production enhancement and the loss of reconstructible resonance signals. The analysis is performed for various mesonic and baryonic resonances with different lifetimes in pp collisions at LHC energies and is extended to p-O, O-O, and Pb-Pb collisions to study the system-size dependence of hadronic-phase effects. The results show that resonance production is governed by the competition between regeneration and rescattering, whose relative importance depends strongly on the resonance species, transverse momentum, and collision system. While rescattering suppresses reconstructible short-lived resonance signals, regeneration can significantly enhance the yields of several resonance species, particularly baryonic resonances. These findings demonstrate that hadronic interactions can play an important role even in small collision systems and highlight the need to measure resonances with different lifetimes and quantum numbers to constrain the dynamics and lifetime of the hadronic phase across collision systems.

nucl-ex↗
arXiv2026

Test-Beam Performance of the AstroPix Silicon Sensor for Imaging Calorimetry

AstroPix is a high-voltage CMOS HVCMOS monolithic active pixel sensor MAPS developed for future space-based gamma-ray missions. It is also a candidate technology for the imaging layer of the Barrel Imaging Calorimeter BIC in the ePIC experiment at the future Electron-Ion Collider EIC. We report the first AstroPix test-beam results obtained at the KEK Photon Factory Advanced Ring PF-AR and the CERN Proton Synchrotron PS T10 beam line in 2025, using the third prototype AstroPix-v3. AstroPix-v3 sensors were operated as both standalone tracking layers and imaging layers interleaved with prototype lead/scintillating-fiber Pb/SciFi calorimeter modules, using electron and hadron beams in the few-GeV/c momentum range. Event synchronization between the continuous readout of AstroPix-v3 and the trigger-based readout of the Pb/SciFi calorimeter was achieved using a common timestamp. The AstroPix-v3 sensors exhibit stable performance, reaching a maximum hit efficiency of 68 percent at a bias voltage of -400 V under pion-dominated beam conditions. When combined with the Pb/SciFi calorimeter, the AstroPix layers successfully capture the development of electromagnetic showers. Using Cherenkov-based particle identification, electron-induced events exhibit significantly higher hit multiplicities and broader spatial distributions than pion-induced events, thereby providing clear discrimination between electromagnetic and hadronic showers. These results demonstrate that AstroPix-v3 provides effective, high-granularity imaging of shower development and is well suited as an imaging layer in future calorimeter systems for both collider and space-based experiments.

physics.ins-det↗
arXiv2026

Beam test of a Pb/SciFi prototype for the Barrel Imaging Calorimeter at the Electron-Ion Collider

A Lead-Scintillating Fiber (Pb/SciFi) prototype for the Barrel Imaging Calorimeter (BIC) at the Electron--Ion Collider (EIC) was tested with electron beams at the CERN PS T10 beam line in August 2024. The prototype consisted of unit modules with a sampling structure of lead sheets and scintillating fibers, corresponding to a total depth of approximately $10.9\,X_{0}$. Beam tests were performed with electron momenta between 0.5 and 3~GeV/$c$ to evaluate the energy and timing performance of the prototype. This study characterizes the performance of a Pb/SciFi prototype and provides input for future beam tests, calibration and readout optimization, and the development of larger-scale prototypes.

physics.ins-det↗
arXiv2026

$f_2(1270)\to\pi+\pi$ as a probe of spin and vorticity in heavy-ion collisions

The correlation between vorticity and spin alignment in heavy-ion collisions can be probed through polarization measurements of hadrons, whose total spin originates from both constituent-quark spins and orbital angular momentum in the quark-model framework. To motivate such experimental studies, we calculate the general angular distribution of produced pion in $f_2(1270)\to\pi+\pi$ using interaction Lagrangian and helicity formalism and check that both methods yield the same result. The distribution is given as a function of angle between pion and initial quantization axis of $f_2$ and the spin density matrix element of $f_2$. Its diagonal entries and $\rho_{20}$ component were computed assuming local thermal equilibrium and blast wave model for different centrality classes, hence given as a function of azimuthal angle with respect to the impact parameter.

nucl-th↗
arXiv2025

Exploring an image-based $b$-jet tagging method using convolution neural networks

Jet flavor tagging, the identification of jets originating from $c$-quarks, $b$-quarks, and other quarks (light quarks and gluons), is a crucial task in high-energy heavy-ion physics, as it enables the investigation of flavor-dependent responses within the hot and dense nuclear medium produced in heavy-ion collisions. Recently, several methods based on deep learning techniques, such as deep neural networks and graph neural networks, have been developed. These deep-learning-based methods demonstrate significantly improved performance compared to traditional methods that rely on track impact parameters and secondary vertices. In the tagging algorithms, various properties of jets and constituent charged particles are used as input parameters. We explore a new method based on images surrounding the primary vertex, utilizing charged particles within the jet cone, which can be measured using a silicon tracking system. For this initial experimental study, we assume the ideal performance of the tracking system. To analyze these images, we employed convolutional neural networks. The image-based flavor tagging method shows an 80-90% $b$-jet tagging efficiency for jets in the transverse momentum range from 20 to 100 GeV/$c$. This approach has the potential to significantly improve the accuracy of jet flavor tagging in high-energy nuclear physics experiments.

physics.ins-det↗
arXiv2025

Activities of the Korea ALICE group for the development and production of the next-generation silicon tracker

ALICE 3 is the proposed next-generation heavy-ion experiment at the CERN Large Hadron Collider (LHC), envisioned for operation during Run 5. The tracking system of ALICE 3 will consist of a high-precision vertex detector integrated into a retractable structure inside the beam pipe, complemented by a large-area outer tracker covering a broad pseudorapidity range. Both systems will be based on the Monolithic Active Pixel Sensor (MAPS) technology, building upon the developments realized for the recently upgraded ALICE Inner Tracking System (ITS2) and the future ITS3 upgrade. The total silicon area of the ALICE 3 tracking system is expected to be approximately five times larger than that of ITS2, presenting significant challenges in terms of large-scale sensor testing and module production. To address these challenges, research and development activities have been initiated in Korea, including the adaptation of an automated die-attach machine, commonly used in the semiconductor packaging industry, for efficient sensor-to-substrate assembly. This contribution presents the ongoing efforts of the Korea ALICE group toward the development and production of the ALICE 3 silicon tracker. The scope includes sensor evaluation, automated assembly techniques, and prototype module construction, with the aim of establishing scalable procedures for future mass production.

physics.ins-det↗
arXiv2025

Characterisation of the first wafer-scale prototype for the ALICE ITS3 upgrade: the monolithic stitched sensor (MOSS)

This paper presents the characterisation and testing of the first wafer-scale monolithic stitched sensor (MOSS) prototype developed for the ALICE ITS3 upgrade that is to be installed during the LHC Long Shutdown 3 (2026-2030). The MOSS chip design is driven by the truly cylindrical detector geometry that imposes that each layer is built out of two wafer-sized, bent silicon chips. The stitching technique is employed to fabricate sensors with dimensions of 1.4 $\times$ 25.9 cm, thinned to 50 $\mu$m. The chip architecture, in-pixel front-end, laboratory and in-beam characterisation, susceptibility to single-event effects, and series testing are discussed. The testing campaign validates the design of a wafer-scale stitched sensor and the performance of the pixel matrix to be within the ITS3 requirements. The MOSS chip demonstrates the feasibility of the ITS3 detector concept and provides insights for further optimisation and development.

physics.ins-det↗
arXiv2025

Selection bias effects on high-$p_\mathrm{T}$ yield and correlation measurements in Oxygen+Oxygen collisions

Oxygen+Oxygen (O+O) collisions at RHIC and the LHC offer a unique experimental opportunity to observe the onset of jet quenching in intermediate relativistic collision systems. As with the smaller proton-nucleus or larger nucleus-nucleus systems, measurements of centrality-selected high-$p_\mathrm{T}$ processes in O+O collisions are expected to be sensitive to selection bias effects, which will be necessary to quantify or mitigate before a definitive conclusion on the presence of jet quenching. Using two Monte Carlo heavy-ion event generators, we provide a survey of centrality bias effects on high-$p_\mathrm{T}$ yield and correlation measurements. Some highlights of our findings include that (1) bias factors for the accessible kinematic range at RHIC show a non-trivial $p_\mathrm{T}$ dependence, compared to a negligible one at the LHC given the smaller accessible Bjorken-$x$ range, (2) centrality definitions based on multiplicity are less sensitive to bias effects than those based on the transverse energy, (3) the Angantyr generator gives qualitatively similar but larger-magnitude bias factors than HIJING, and (4) correlation measurements have a much smaller sensitivity to bias effects than do yield measurements. The findings here are intended to guide the experimental design and interpretation of O+O jet quenching and other hard-process measurements.

nucl-ex↗
arXiv2025

Investigation of the performance of a GNN-based b-jet tagging method in heavy-ion collisions

Beauty-tagged jets (b-jets)-collimated sprays of particles originating from the fragmentation of beauty quarks produced in the initial hard scatterings-provide a unique probe of parton dynamics in the quark-gluon plasma (QGP) created in ultrarelativistic heavy-ion collisions. In particular, energy loss patterns of low-$p_T$ b-jets traversing the QGP offer valuable insight into the strong interaction in its nonperturbative regime. CMS and ATLAS Collaborations at the LHC have studied b-jet production in Pb-Pb collisions. The results were limited to a high-$p_T$ region, because a major challenge at low-$p_T$ is the overwhelming number of background particles from QGP hadronisation, which severely hinders the effectiveness of conventional b-jet tagging techniques. To enable precise measurements in such complex environments, advanced tagging methods are required. Graph Neural Networks (GNNs), capable of learning relational structures among jet constituents, represent a promising deep learning approach for b-jet identification. In this study, we adopt and adapt the GN1 model, initially developed by ATLAS, for use in Pb-Pb collision environments. We investigate the model's performance by applying it to jets embedded with Pb-Pb background particles, evaluating both tagging decisions and robustness against background contamination. This work presents a comprehensive evaluation of GNN-based b-jet tagging under heavy-ion collision conditions, aiming to advance future precision studies of QGP-induced partonic energy loss.

physics.data-an↗
arXiv2025

Exploring hadronic rescattering effects on resonance productions in pp and p-Pb collisions with PYTHIA8

In relativistic heavy-ion collisions, the quark-gluon plasma is created, and as the medium cools down, the system transitions into a hadronic phase. While such interactions are well established for large systems, such as Pb-Pb collisions, their relevance in smaller collision systems remains unclear. Consequently, hadronic interactions during the hadronic phase are studied in pp collisions at $\sqrt{s}=13$ TeV and p-Pb collisions at $\sqrt{s_{\rm{NN}}}=5.02$ TeV with the PYTHIA8 event generator. The interaction is studied via the yield ratios between resonances and stable particles with similar quark contents, which are obtained as a function of transverse momentum ($p_{\rm{T}}$) using $\mathrm{\rho(770)^0}$, $\mathrm{K^*(892)^0}$, and $\mathrm{\phi(1020)}$ mesons and their stable particles, $\mathrm{\pi^\pm}$ and $\mathrm{K^\pm}$ at midrapidity ($|\rm{y}|<0.5$). Yield ratios are calculated in five multiplicity classes for pp and six for p-Pb collisions, using the 60-100% multiplicity class in pp as a reference. Although rescattering leads to stronger suppression at low $p_{\rm{T}} < 2$ GeV/$c$, a visible suppression remains even when rescattering is turned off. To isolate the rescattering effect, double ratios between the rescattering on and off configurations are obtained. These are then integrated in the full $p_{\rm{T}}$ range ($0<p_{\rm{T}}<6.0$ GeV/$c$). The normalized double ratios show a decreasing trend with increasing multiplicity, independent of the collision system. The lower limit of the hadronic phase lifetimes extracted in the integrated-$p_{\rm{T}}$ region increases with multiplicity in both systems, but with a notable discrepancy between pp and p-Pb collisions.

nucl-th↗
arXiv2025

Further Characterisation of Digital Pixel Test Structures Implemented in a 65 nm CMOS Process

The next generation of MAPS for future tracking detectors will have to meet stringent requirements placed on them. One such detector is the ALICE ITS3 that aims to be very light at 0.07% X/X$_{0}$ per layer and have a low power consumption in the active area of 40 mW/cm$^{2}$ by implementing wafer-scale MAPS bent into cylindrical half layers. To address these challenging requirements, the ALICE ITS3 project, in conjunction with the CERN EP R&D on monolithic pixel sensors, proposed the Tower Partners Semiconductor Co. 65 nm CMOS process as the starting point for the sensor. After the initial results confirmed the detection efficiency and radiation hardness, the choice of the technology was solidified by demonstrating the feasibility of operating MAPS in low-power consumption regimes, < 50 mW/cm$^{2}$, while maintaining high-quality performance. This was shown through a detailed characterisation of the Digital Pixel Test Structure (DPTS) prototype exposed to X-rays and ionising beams, and the results are presented in this article. Additionally, the sensor was further investigated through studies of the fake-hit rate, the linearity of the front-end in the range 1.7-28 keV, the performance after ionising irradiation, and the detection efficiency of inclined tracks in the range 0-45$^\circ$.

physics.ins-det↗
arXiv2025

Proposal of the KOTO II experiment

The KOTO II experiment is proposed to measure the branching ratio of the decay $K_L\to\pi^0\nu\bar{\nu}$ at J-PARC. With a beamline to extract long-lived neutral kaons at 5 degrees from a production target, the single event sensitivity of the decay is $8.5\times 10^{-13}$, which is much smaller than the Standard Model prediction $3\times 10^{-11}$. This allows searches for new physics beyond the Standard Model and the first discovery of the decay with a significance exceeding $5\sigma$. As the only experiment proposed in the world dedicated to rare kaon decays, KOTO II will be indispensable in the quest for a complete understanding of flavor dynamics in the quark sector. Moreover, by combining efforts from the kaon community worldwide, we plan to develop the KOTO II detector further and expand the physics reach of the experiment to include measurements of the branching ratio of the $K_L\to\pi^0\ell^+\ell^-$ decays, studies of other $K_L$ decays, and searches for dark photons, axions, and axion-like particles. KOTO II will therefore obtain a comprehensive understanding of $K_L$ decays, providing further constraints on new physics scenarios with existing $K^+$ results.

hep-ex↗
arXiv2024

Investigation of suppression of $\Upsilon(nS)$ in relativistic heavy-ion collisions at RHIC and LHC energies

The primary purpose of studying quarkonium production in relativistic heavy-ion collisions is to understand the properties of the quark-gluon plasma. At various collision systems, measurements of quarkonium states of different binding energies, such as $\Upsilon(nS)$, can provide comprehensive information. A model study has been performed to investigate the modification of $\Upsilon(nS)$ production in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=$ 5.02 TeV and Au-Au collisions at $\sqrt{s_{\mathrm{NN}}}=$ 200 GeV. The Monte-Carlo simulation study is performed with a publicly available hydrodynamic simulation package for the quark-gluon plasma medium and a theoretical calculation of temperature-dependent thermal width of $\Upsilon(nS)$ considering the gluo-dissociation and inelastic parton scattering for dissociation inside the medium. In addition, we perform a systematic study with different descriptions of initial collision geometry and formation time of $\Upsilon(nS)$ to investigate their impacts on yield modification. The model calculation with a varied parameter set can describe the experimental data of $\Upsilon(nS)$ in Pb-Pb collisions at 5.02 TeV and $\Upsilon(2S)$ in Au-Au collisions at 200 GeV but underestimates the modification of $\Upsilon(1S)$ at the lower collision energy. The nuclear absorption mechanism is explored to understand the discrepancy between the data and simulation.

nucl-th↗
arXiv2023

Is $K_{1}/K^{*}$ enhancement in heavy ion collisions a signature of chiral symmetry restoration?

We extend the recent study of $K_{1}/K^{*}$ enhancement as a signature of chiral symmetry restoration in heavy ion collisions at the Large Hadron Collider (LHC) via the kinetic approach to include the effects due to non-unity hadron fugacities during the evolution of produced hadronic matter and the temperature-dependent $K_1$ mass. Although the effect of non-unity fugacity only slightly reduces the $K_1/K^*$ enhancement due to chiral symmetry restoration, the inclusion of the temperature-dependent $K_1$ mass leads to a substantial reduction in the $K_1/K^*$ enhancement. However, the final $K_1/K^*$ ratio in peripheral collisions still shows a more than factor of two enhancement compared to the case without chiral symmetry restoration and thus remains a good signature for chiral symmetry restoration in the hot dense matter produced in relativistic heavy ion collisions.

nucl-th↗
arXiv2023

Diquarks and the production of charmed baryons

Utilizing a quark model characterized by parameters that effectively replicate the masses of ground state hadrons, we illustrate that $(us)$ or $(ds)$ diquarks exhibit greater compactness in comparison to $(ud)$ diquarks. Concretely, the binding energy of the $(us)$ diquark - defined as the diquark's mass minus the combined masses of its individual quarks - is found to be stronger than that of the $(ud)$ diquark. This heightened attraction present in $(us)$ diquarks could lead to enhanced production of $\Xi_c/D$ particles in high-energy pp or ultrarelativistic heavy-ion collisions.

hep-ph↗
arXiv2022

Highlights from the PHENIX experiment

PHENIX has performed an extensive study on the evolution of medium effects from small to large systems. PHENIX has continued searching for Quark-Gluon Plasma (QGP) in small systems by measuring collectivity, modification of light hadron and quarkonia production, and jet substructure. In large systems, detailed studies on the property of the QGP have been done using direct photon, $π^{0}$-hadron correlation, heavy-flavor electron, and $J/ψ$ flow with a large statistics of data collected in 2014. This report covers new results from the PHENIX experiment in various collision systems.

nucl-ex↗
arXiv2022

$X(3872)$ and $T_{cc}$: structures and productions in heavy ion collisions

We argue why the recently observed $T_{cc}$ could either be a compact multiquark configuration or a loosely bound molecular configuration composed of charmed mesons, whereas the $X(3872)$ is most likely a molecular configuration. The argument is based on different short range interactions for these tetraquark states coming from the color-color and color-spin interaction in a quark model, and the presence of a common strong D-wave mixing at larger distance similar to the deuteron case, which for the molecular configurations lead to large sizes. Such an analogy at large distance allows us to calculate the transverse momentum dependence of the loosely bound molecular configuration of tetraquarks produced in heavy ion collisions using the coalescence model that successfully reproduces the deutron data using the proton spectra. The ratio of the integrated $X(3872)$ yield obtained from our method to the $ψ(2S)$ yield obtained from statistical hadronization model method is calculated to be $0.806 \pm 0.234$, which is a factor of 2.47 larger than that obtained by using statistical model predictions for both particles and in line with the data from the CMS experiment. As the previously calculated transverse momentum distribution of the $T_{cc}$ assuming the structure to be a compact multiquark configuration is markedly different, experimental measurements of the transverse distribution of the tetraquark states will discriminate between their two possible structures.

hep-ph↗