arXiv subjects

B. Kim

Publications and source records attributed to B. Kim.

At least 37 records · Page 2Linked to original sources

arXiv2020

Phenomenological constraints on the transport properties of QCD matter with data-driven model averaging

Using combined data from the Relativistic Heavy Ion and Large Hadron Colliders, we constrain the shear and bulk viscosities of quark-gluon plasma (QGP) at temperatures of ${\sim\,}150{-}350$ MeV. We use Bayesian inference to translate experimental and theoretical uncertainties into probabilistic constraints for the viscosities. With Bayesian Model Averaging we account for the irreducible model ambiguities in the transition from a fluid description of the QGP to hadronic transport in the final evolution stage, providing the most reliable phenomenological constraints to date on the QGP viscosities.

hep-ph↗

arXiv2020

Constraints on jet quenching from a multi-stage energy-loss approach

We present a multi-stage model for jet evolution through a quark-gluon plasma within the JETSCAPE framework. The multi-stage approach in JETSCAPE provides a unified description of distinct phases in jet shower contingent on the virtuality. We demonstrate a simultaneous description of leading hadron and integrated jet observables as well as jet $v_n$ using tuned parameters. Medium response to the jet quenching is implemented based on a weakly-coupled recoil prescription. We also explore the cone-size dependence of jet energy loss inside the plasma.

nucl-th↗

arXiv2020

First results from Hybrid Hadronization in small and large systems

"Hybrid Hadronization" is a new Monte Carlo package to hadronize systems of partons. It smoothly combines quark recombination applicable when distances between partons in phase space are small, and string fragmentation appropriate for dilute parton systems, following the picture outlined by Han et al. [PRC 93, 045207 (2016)]. Hybrid Hadronization integrates with PYTHIA 8 and can be applied to a variety of systems from $e^++e^-$ to $A+A$ collisions. It takes systems of partons and their color flow information, for example from a Monte Carlo parton shower generator, as input. In addition, if for $A+A$ collisions a thermal background medium is provided, the package allows sampling thermal partons that contribute to hadronization. Hybrid Hadronization is available for use as a standalone code and is also part of JETSCAPE since the 2.0 release. In these proceedings we review the physics concepts underlying Hybrid Hadronization and demonstrate how users can use the code with various parton shower Monte Carlos. We present calculations of hadron chemistry and fragmentation functions in small and large systems when Hybrid Hadronization is combined with parton shower Monte Carlos MATTER and LBT. In particular, we discuss observable effects of the recombination of shower partons with thermal partons.

nucl-th↗

arXiv2020

Photon-jet correlations in p-p and Pb-Pb collisions using JETSCAPE framework

It is now well established that jet modification is a multistage effect; hence a single model alone cannot describe all facets of jet modification. The JETSCAPE framework is a multistage framework that uses several modules to simulate different stages of jet propagation through the QGP medium. These simulations require a set of parameters to ensure a smooth transition between stages. We fine tune these parameters to successfully describe a variety of observables, such as the nuclear modification factors of leading hadrons and jets, jet shape, and jet fragmentation function. Photons can be produced in the hard scattering or as radiation from quarks inside jets. In this work, we study photon-jet transverse momentum imbalance and azimuthal correlation for both $p-p$ and $Pb-Pb$ collision systems. All the photons produced in each event, including the photons from hard scattering, radiation from the parton shower, and radiation from hadronization are considered with an isolation cut to directly compare with experimental data. The simulations are conducted using the same set of tuned parameters as used for the jet analysis. No new parameters are introduced or tuned. We demonstrate a significantly improved agreement with photons from $Pb-Pb$ collisions compared to prior efforts. This work provides an independent, parameter free verification of the multistage evolution framework.

nucl-th↗

arXiv2020

AI-optimized detector design for the future Electron-Ion Collider: the dual-radiator RICH case

Advanced detector R&D requires performing computationally intensive and detailed simulations as part of the detector-design optimization process. We propose a general approach to this process based on Bayesian optimization and machine learning that encodes detector requirements. As a case study, we focus on the design of the dual-radiator Ring Imaging Cherenkov (dRICH) detector under development as part of the particle-identification system at the future Electron-Ion Collider (EIC). The EIC is a US-led frontier accelerator project for nuclear physics, which has been proposed to further explore the structure and interactions of nuclear matter at the scale of sea quarks and gluons. We show that the detector design obtained with our automated and highly parallelized framework outperforms the baseline dRICH design within the assumptions of the current model. Our approach can be applied to any detector R&D, provided that realistic simulations are available.

physics.ins-det↗

arXiv2020

Ready Student One: Exploring the predictors of student learning in virtual reality

Immersive virtual reality (VR) has enormous potential for education, but classroom resources are limited. Thus, it is important to identify whether and when VR provides sufficient advantages over other modes of learning to justify its deployment. In a between-subjects experiment, we compared three methods of teaching Moon phases (a hands-on activity, VR, and a desktop simulation) and measured student improvement on existing learning and attitudinal measures. While a substantial majority of students preferred the VR experience, we found no significant differences in learning between conditions. However, we found differences between conditions based on gender, which was highly correlated with experience with video games. These differences may indicate certain groups have an advantage in the VR setting.

physics.ed-ph↗

arXiv2020

Detection of the Chiral Spin Structure in Ferromagnetic SrRuO$_3$ Thin Film

A SrRuO$_3$ (SRO) thin film and its heterostructure have brought much attention because of the recently demonstrated fascinating properties, such as topological Hall effect and skyrmions. Critical to the understanding of those SRO properties is the study of the spin configuration. Here, we conduct resonant soft x-ray scattering (RSXS) at oxygen K-edge to investigate the spin configuration of a 4 unit-cell SRO film that was grown epitaxially on a single crystal SrTiO$_3$. The RSXS signal under a magnetic field (~0.4 Tesla) clearly shows a magnetic dichroism pattern around the specular reflection. Model calculations on the RSXS signal demonstrate that the magnetic dichroism pattern originates from a Néel-type chiral spin structure in this SRO thin film. We believe that the observed spin structure of the SRO system is a critical piece of information for understanding its intriguing magnetic and transport properties.

cond-mat.mtrl-sci↗

arXiv2019

CTLearn: Deep Learning for Gamma-ray Astronomy

CTLearn is a new Python package under development that uses the deep learning technique to analyze data from imaging atmospheric Cherenkov telescope (IACT) arrays. IACTs use the Cherenkov light emitted from air showers, initiated by very-high-energy gamma rays, to form an image of the longitudinal development of the air shower on the camera plane. The spatial, temporal, and calorimetric information of the originating high-energy particle is then recorded electronically. The sensitivity of IACTs to astrophysical sources depends strongly on the efficient rejection of the background of much more numerous cosmic-ray showers. CTLearn includes modules for running machine learning models with TensorFlow, using pixel-wise camera data as input. Its high-level interface provides a configuration-file-based workflow to drive reproducible training and prediction. We illustrate the capabilities of CTLearn by presenting some results using IACT simulated data.

astro-ph.IM↗

arXiv2019

Studying deep convolutional neural networks with hexagonal lattices for imaging atmospheric Cherenkov telescope event reconstruction

Deep convolutional neural networks (DCNs) are a promising machine learning technique to reconstruct events recorded by imaging atmospheric Cherenkov telescopes (IACTs), but require optimization to reach full performance. One of the most pressing challenges is processing raw images captured by cameras made of hexagonal lattices of photo-multipliers, a common layout among IACT cameras which topologically differs from the square lattices conventionally expected, as their input data, by DCN models. Strategies directed to tackle this challenge range from the conversion of the hexagonal lattices onto square lattices by means of oversampling or interpolation to the implementation of hexagonal convolutional kernels. In this contribution we present a comparison of several of those strategies, using DCN models trained on simulated IACT data.

astro-ph.IM↗

arXiv2019

Camera design and performance of the prototype Schwarzschild-Couder Telescope for the Cherenkov Telescope Array

The Schwarzschild-Couder Telescope (SCT) is a candidate technology for a medium-sized telescope within the Cherenkov Telescope Array, the next generation ground based observatory for very high energy gamma ray astronomy. The SCT uses a novel two-mirror design and is expected to yield improvements in field of view and image resolution compared to traditional Cherenkov telescopes based on single-mirror-dish optics. To match the improved optical resolution, challenging requirements of high channel count and density at low power consumption must be overcome by the camera. The prototype camera, currently commissioned and tested on the prototype SCT, has been developed based on millimeter scale SiPM pixels and a custom high density digitizer ASIC, TARGET, to provide 1600 pixels spanning a 2.7 degree field of view while being able to sample nanosecond photon pulses. It is mechanically designed to allow for an upgrade to 11,328 pixels covering a field of view of 8 degrees and demonstrating the full potential of the technology. The camera was installed on the telescope in 2018. We will present its design and performance including first light data.

astro-ph.IM↗

arXiv2019

Prototype Schwarzschild-Couder Telescope for the Cherenkov Telescope Array: Commissioning Status of the Optical System

The Cherenkov Telescope Array (CTA), with more than 100 telescopes, will be the largest ever ground-based gamma-ray observatory and is expected to greatly improve on both gamma-ray detection sensitivity and energy coverage compared to current-generation detectors. The 9.7-m Schwarzschild-Couder telescope (SCT) is one of the two candidates for the medium size telescope (MST) design for CTA. The novel aplanatic dual-mirror SCT design offers a wide field-of-view with a compact plate scale, allowing for a large number of camera pixels that improves the angular resolution and reduce the night sky background noise per pixel compared to the traditional single-mirror Davies-Cotton (DC) design of ground-based gamma-ray telescopes. The production, installation, and the alignment of the segmented aspherical mirrors are the main challenges for the realization of the SCT optical system. In this contribution, we report on the commissioning status, the alignment procedures, and initial alignment results during the initial commissioning phase of the optical system of the prototype SCT.

astro-ph.IM↗

arXiv2019

Development and operations of INFN optical modules for the SCT Telescope camera proposed for the Cherenkov Telescope Array Observatory

The Schwarzschild-Couder Telescope (SCT) is a proposal for the Medium Size Telescopes of the Cherenkov Telescope Array. Its concept is based on a two-mirror optical system designed to improve the telescope field of view and image resolution with respect to the single mirror Davies-Cotton solution. The SCT camera is planned to be instrumented with 177 photodetection modules, each composed of 64 Silicon Photomultiplier (SiPM) pixels. The third generation of $6 x 6~mm^2$ high density NUV SiPMs (NUV-HD3) produced by Fondazione Bruno Kessler (FBK) in collaboration with INFN has been used to equip optical units to be integrated on the upgrade of the camera of the SCT prototype (pSCT). Each optical unit is composed of an array of 16 NUV-HD3 SiPMs coupled with the front-end electronics, which is designed for full-waveform nanosecond readout and digitization using the TARGET-7 ASIC. Several optical units have been assembled and tested in the laboratories of INFN and have been integrated on the camera of the pSCT telescope, that is currently operating at the Fred Lawrence Whipple Observatory. In this contribution we report on the development, assembly and calibration of the optical units that are currently taking data on the pSCT camera.

astro-ph.IM↗

arXiv2019

A New Approach for Measuring the Muon Anomalous Magnetic Moment and Electric Dipole Moment

This paper introduces a new approach to measure the muon magnetic moment anomaly $a_μ = (g-2)/2$, and the muon electric dipole moment (EDM) $d_μ$ at the J-PARC muon facility. The goal of our experiment is to measure $a_μ$ and $d_μ$ using an independent method with a factor of 10 lower muon momentum, and a factor of 20 smaller diameter storage-ring solenoid compared with previous and ongoing muon $g-2$ experiments with unprecedented quality of the storage magnetic field. Additional significant differences from the present experimental method include a factor of 1,000 smaller transverse emittance of the muon beam (reaccelerated thermal muon beam), its efficient vertical injection into the solenoid, and tracking each decay positron from muon decay to obtain its momentum vector. The precision goal for $a_μ$ is statistical uncertainty of 450 part per billion (ppb), similar to the present experimental uncertainty, and a systematic uncertainty less than 70 ppb. The goal for EDM is a sensitivity of $1.5\times 10^{-21}~e\cdot\mbox{cm}$.

physics.ins-det↗

arXiv2018

Virtual Reality as a Teaching Tool for Moon Phases and Beyond

A ball on a stick is a common and simple activity for teaching the phases of the Moon. This activity, like many others in physics and astronomy, gives students a perspective they otherwise could only imagine. For Moon phases, a third person view and control over time allows students to rapidly build a mental model that connects all the moving parts. Computer simulations of many traditional physics and astronomy activities provide new features, controls, or vantage points to enhance learning beyond a hands-on activity. Virtual reality provides the capabilities of computer simulations and embodied cognition experiences through a hands-on activity making it a natural step to improve learning. We recreated the traditional ball-and-stick moon phases activity in virtual reality and compared participant learning using this simulation with using traditional methods. We found a strong participant preference for VR relative to the traditional methods. However, we observed no difference across conditions in average levels of performance on a pre/post knowledge test.

physics.ed-ph↗

arXiv2018

Tests of a dual-readout fiber calorimeter with SiPM light sensors

In this paper, we describe the first tests of a dual-readout fiber calorimeter in which silicon photomultipliers are used to sense the (scintillation and Cherenkov) light signals. The main challenge in this detector is implementing a design that minimizes the optical crosstalk between the two types of fibers, which are located very close to each other and carry light signals that differ in intensity by about a factor of 60. The experimental data, which were obtained with beams of high-energy electrons and muons as well as in lab tests, illustrate to what extent this challenge was met. The Cherenkov light yield, a limiting factor for the energy resolution of this type of calorimeter, was measured to be about twice that of the previously tested configurations based on photomultiplier tubes. The lateral profiles of electromagnetic showers were measured on a scale of millimeters from the shower axis and significant differences were found between the profiles measured with the scintillating and the Cherenkov fibers.

physics.ins-det↗

arXiv2018

Hundredfold Enhancement of Light Emission via Defect Control in Monolayer Transition-Metal Dichalcogenides

Two dimensional (2D) transition-metal dichalcogenide (TMD) based semiconductors have generated intense recent interest due to their novel optical and electronic properties, and potential for applications. In this work, we characterize the atomic and electronic nature of intrinsic point defects found in single crystals of these materials synthesized by two different methods - chemical vapor transport and self-flux growth. Using a combination of scanning tunneling microscopy (STM) and scanning transmission electron microscopy (STEM), we show that the two major intrinsic defects in these materials are metal vacancies and chalcogen antisites. We show that by control of the synthetic conditions, we can reduce the defect concentration from above $10^{13} /cm^2$ to below $10^{11} /cm^2$. Because these point defects act as centers for non-radiative recombination of excitons, this improvement in material quality leads to a hundred-fold increase in the radiative recombination efficiency.

cond-mat.mtrl-sci↗

arXiv2018

First muon acceleration using a radio frequency accelerator

Muons have been accelerated by using a radio frequency accelerator for the first time. Negative muonium atoms (Mu$^-$), which are bound states of positive muons ($μ^+$) and two electrons, are generated from $μ^+$'s through the electron capture process in an aluminum degrader. The generated Mu$^-$'s are initially electrostatically accelerated and injected into a radio frequency quadrupole linac (RFQ). In the RFQ, the Mu$^-$'s are accelerated to 89 keV. The accelerated Mu$^-$'s are identified by momentum measurement and time of flight. This compact muon linac opens the door to various muon accelerator applications including particle physics measurements and the construction of a transmission muon microscope.

physics.acc-ph↗

arXiv2017

Exploring deep learning as an event classification method for the Cherenkov Telescope Array

Telescopes based on the imaging atmospheric Cherenkov technique (IACTs) detect images of the atmospheric showers generated by gamma rays and cosmic rays as they are absorbed by the atmosphere. The much more frequent cosmic-ray events form the main background when looking for gamma-ray sources, and therefore IACT sensitivity is significantly driven by the capability to distinguish between these two types of events. Supervised learning algorithms, like random forests and boosted decision trees, have been shown to effectively classify IACT events. In this contribution we present results from exploratory work using deep learning as an event classification method for the Cherenkov Telescope Array (CTA). CTA, conceived as an array of tens of IACTs, is an international project for a next-generation ground-based gamma-ray observatory, aiming to improve on the sensitivity of current-generation experiments by an order of magnitude and provide energy coverage from 20 GeV to more than 300 TeV.

astro-ph.IM↗