Search

GroundBIRD is a ground-based cosmic microwave background (CMB) experiment for observing the polarization pattern imprinted on large angular scales ($\ell > 6$ ) from the Teide Observatory in Tenerife, Spain. Our primary scientific objective is a precise measurement of the optical depth $\tau$ ($\sigma(\tau) \sim 0.01$) to the reionization epoch of the Universe to cross-check systematic effects in the measurements made by previous experiments. GroundBIRD observes a wide sky area in the Northern Hemisphere ($\sim 40\%$ of the full sky) while continuously rotating the telescope at a high speed of up to 20 rotations per minute (rpm) to overcome the fluctuations of atmospheric radiation. We have adopted the NbTiN/Al hybrid microwave kinetic inductance detectors (MKIDs) as focal plane detectors. We observe two frequency bands centered at 145 GHz and 220 GHz. The 145 GHz band picks up the peak frequency of the CMB spectrum. The 220 GHz band helps accurate removal of the contamination of thermal emission from the Galactic interstellar dust. The MKID arrays (138 MKIDs for 145GHz and 23 MKIDs for 220GHz) were designed and optimized so as to minimize the contamination of the two-level-system noise and maximize the sensitivity. The MKID arrays were successfully installed in May 2023 after the performance verification tests were performed at a laboratory. GroundBIRD has been upgraded to use the full MKID arrays, and scientific observations are now underway. The telescope is automated, so that all observations are performed remotely. Initial validations, including polarization response tests and observations of Jupiter and the moon, have been completed successfully. We are now running scientific observations., Comment: Event: SPIE Astronomical Telescopes + Instrumentation, 2024, Yokohama, Japan; paper number 13102-7, Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy XII

We report an extensive high-sensitivity search for axion dark matter above 1\,GHz at the Center for Axion and Precision Physics Research (CAPP). The cavity resonant search, exploiting the coupling between axions and photons, explored the frequency (mass) range of 1.025\,GHz (4.24\,$\mu$eV) to 1.185\,GHz (4.91\,$\mu$eV). We have introduced a number of innovations in this field, demonstrating the practical approach of optimizing all the relevant parameters of axion haloscopes, extending presently available technology. The CAPP 12\,T magnet with an aperture of 320\,mm made of Nb$_3$Sn and NbTi superconductors surrounding a 37-liter ultralight-weight copper cavity is expected to convert DFSZ axions into approximately $10^2$ microwave photons per second. A powerful dilution refrigerator, capable of keeping the core system below 40\,mK, combined with quantum-noise limited readout electronics, achieved a total system noise of about 200\,mK or below, which corresponds to a background of roughly $4\times 10^3$ photons per second within the axion bandwidth. The combination of all those improvements provides unprecedented search performance, imposing the most stringent exclusion limits on axion--photon coupling in this frequency range to date. These results also suggest an experimental capability suitable for highly-sensitive searches for axion dark matter above 1\,GHz., Comment: A detailed axion dark matter article with 27 pages, 22 figures

We report the first search for the Sagittarius tidal stream of axion dark matter around 4.55 $\mu$eV using CAPP-12TB haloscope data acquired in March of 2022. Our result excluded the Sagittarius tidal stream of Dine-Fischler-Srednicki-Zhitnitskii and Kim-Shifman-Vainshtein-Zakharov axion dark matter densities of $\rho_a\gtrsim0.184$ and $\gtrsim0.025$ GeV/cm$^{3}$, respectively, over a mass range from 4.51 to 4.59 $\mu$eV at a 90% confidence level., Comment: 6 pages, 7 Figures, PRD Letter accepted

We report an axion dark matter search at Dine-Fischler-Srednicki-Zhitnitskii sensitivity with the CAPP-12TB haloscope, assuming axions contribute 100\% of the local dark matter density. The search excluded the axion--photon coupling $g_{a\gamma\gamma}$ down to about $6.2\times10^{-16}$ GeV$^{-1}$ over the axion mass range between 4.51 and 4.59 $\mu$eV at a 90\% confidence level. The achieved experimental sensitivity can also exclude Kim-Shifman-Vainshtein-Zakharov axion dark matter that makes up just 13\% of the local dark matter density. The CAPP-12TB haloscope will continue the search over a wide range of axion masses., Comment: 7 pages, 6 figures

The Center for Axion and Precision Physics research at the Institute for Basic Science is searching for axion dark matter using ultra-low temperature microwave resonators. We report the exclusion of the axion mass range 10.7126$-$10.7186 $\mu$eV with near Kim-Shifman-Vainshtein-Zakharov (KSVZ) coupling sensitivity and the range 10.16$-$11.37 $\mu$eV with about 9 times larger coupling at 90$\%$ confidence level. This is the first axion search result in these ranges. It is also the first with a resonator physical temperature of less than 40 mK.

GroundBIRD is a ground-based experiment for the precise observation of the polarization of the cosmic microwave background (CMB). To achieve high sensitivity at large angular scale, we adopt three features in this experiment: fast rotation scanning, microwave kinetic inductance detector (MKID) and cold optics. The rotation scanning strategy has the advantage to suppress $1/f$ noise. It also provides a large sky coverage of 40\%, which corresponds to the large angular scales of $l \sim 6$. This allows us to constrain the tensor-to-scalar ratio by using low $l$ B-mode spectrum. The focal plane consists of 7 MKID arrays for two target frequencies, 145 GHz and 220 GHz band. There are 161 pixels in total, of which 138 are for 144 GHz and 23 are for 220 GHz. This array is currently under development and the prototype will soon be evaluated in telescope. The GroundBIRD telescope will observe the CMB at the Teide observatory. The telescope was moved from Japan to Tenerife and is now under test. We present the status and plan of the GroundBIRD experiment., Comment: 7 pages, 5 figures, LTD18 proceeding, Published in JLTP

The axion, a consequence of the PQ mechanism, has been considered as the most elegant solution to the strong-CP problem and a compelling candidate for cold dark matter. The Center for Axion and Precision Physics Research (CAPP) of the Institute for Basic Science (IBS) was established on 16 October 2013 with a main objective to launch state of the art axion experiments in South Korea. Relying on the haloscope technique, our strategy is to run several experiments in parallel to explore a wide range of axion masses with sensitivities better than the QCD axion models. We utilize not only the advanced technologies, such as high-field large-volume superconducting (SC) magnets, ultra low temperature dilution refrigerators, and nearly quantum-limited noise amplifiers, but also some unique features solely developed at the Center, including high-quality SC resonant cavities surviving high magnetic fields and efficient cavity geometries to reach high-frequency regions. Our goal is to probe axion dark matter in the frequency range of 1-10 GHz in the first phase and then ultimately up to 25 GHz, even in a scenario where axions constitute only 10% of the local dark matter halo. In this report, the current status and future prospects of the experiments and R&D activities at IBS/CAPP are described., Comment: 156 pages, 135 figures, conceptual/technical design report

The axion is a hypothetical particle proposed to solve the strong $CP$ problem, and also a candidate for dark matter. This non-relativistic particle in the galactic halo can be converted into a photon under a strong magnetic field and detected with a microwave resonant cavity. Relying on this detection method, many experiments have excluded some mass regions with certain sensitivities in terms of axion-photon coupling ($g_{a\gamma\gamma}$) for decades, but no axion dark matter has been discovered to date. CAPP-8TB is an axion haloscope experiment at IBS/CAPP designed to search for the axion in a mass range of 6.62 to 7.04 $\mu$eV. The experiment aims for the most sensitive axion dark matter search in this particular mass range with its first-phase sensitivity reaching the QCD axion band. In this presentation, we discuss the overview of the experiment, and present the first result. We also discuss an upgrade of the experiment to achieve higher sensitivity., Comment: Talk presented at the 2019 Meeting of the Division of Particles and Fields of the American Physical Society (DPF2019), July 29 - August 2, 2019, Northeastern University, Boston, C1907293

This work demonstrates that two systematic errors, coherent betatron oscillations (CBO) and muon losses can be reduced through application of radio frequency (RF) electric fields, which ultimately increases the sensitivity of the muon $g-2$ experiments. As the ensemble of polarized muons goes around a weak focusing storage ring, their spin precesses, and when they decay through the weak interaction, $\mu^+ \rightarrow e^+ \nu_e \bar{\nu_\mu}$, the decay positrons are detected by electromagnetic calorimeters. In addition to the expected exponential decay in the positron time spectrum, the weak decay asymmetry causes a modulation in the number of positrons in a selected energy range at the difference frequency between the spin and cyclotron frequencies, $\omega_\text{a}$. This frequency is directly proportional to the magnetic anomaly $a_\mu =(g-2)/2$, where $g$ is the g-factor of the muon, which is slightly greater than 2. The detector acceptance depends on the radial position of the muon decay, so the CBO of the muon bunch following injection into the storage ring modulate the measured muon signal with the frequency $\omega_\text{CBO}$. In addition, the muon populations at the edge of the beam hit the walls of the vacuum chamber before decaying, which also affects the signal. Thus, reduction of CBO and unwanted muon loss increases the $a_\mu$ measurement sensitivity. Numerical and experimental studies with RF electric fields yield more than a magnitude reduction of the CBO, with muon losses comparable to the conventional method., Comment: 14 pages, 25 figures

In this paper, we completely characterize the niche graphs of bipartite tournaments and find their interesting properties.

In this paper, we present a minimal chordal completion $G^*$ of a graph $G$ satisfying the inequality $\omega(G^*) - \omega(G) \le i(G)$ for the non-chordality index $i(G)$ of $G$. In terms of our chordal completions, we partially settle the Hadwiger conjecture and the Erd\H{o}s-Faber-Lov\'{a}sz Conjecture, and extend the known $\chi$-bounded class by adding to it the family of graphs with bounded non-chordality indices.

In this paper, we use the Erd\"{o}s-Szekeres lemma to show that there exists a graph with partial order competition dimension greater than five.

The \emph{graph grabbing game} is a two-player game on a weighted connected graph in which two players, Alice and Bob, alternatively remove non-cut vertices one by one to gain the weights on them. Alice wins the game if she gains at least half of the total weights. In this paper, we show that on every connected even graph which does contain a fully spiked cycle as an induced subgraph, Alice always has a winning strategy with an arbitrary weight function whose codomain is $\{0,1\}$. In addition, we give a list of forbidden subgraph for the family of graphs on which Alice has a winning strategy with an arbitrary weight function whose codomain is $\{0,1\}$., Comment: 9 pages, 4 figures

In this paper, we study $(1,2)$-step competition graphs of bipartite tournaments. A bipartite tournament means an orientation of a complete bipartite graph. We show that the $(1,2)$-step competition graph of a bipartite tournament has at most one non-trivial component or consists of exactly two complete components of size at least three and, especially in the former, the diameter of the nontrivial component is at most three if it exists. Based on this result, we show that, among the connected non-complete graphs which are triangle-free or the cycles of which are edge-disjoint, $K_{1,4}$ is the only graph that can be represented as the $(1,2)$-step competition graph of a bipartite tournament. We also completely characterize a complete graph and the disjoint union of two complete graphs, respectively, which can be represented as the $(1,2)$-step competition graph of a bipartite tournament. Finally we present the maximum number of edges and the minimum number of edges which the $(1,2)$-step competition graph of a bipartite tournament might have., Comment: 16 pages, 2 figures

Hefner [K. A. S. Hefner, K. F. Jones, S. -R. Kim, R. J. Lundgren and F. S. Roberts: $(i,j)$ competition graphs, Discrete Applied Mathematics, 32, (1991) 241-262] characterized acyclic digraphs each vertex of which has inderee and outdegree at most two and whose competition graphs are interval. They called acyclic digraphs each vertex of which has inderee and outdegree at most two $(2,2)$ digraphs. In this paper, we study the phylogeny graphs of $(2,2)$ digraphs. Especially, we give a sufficient condition and necessary conditions for $(2,2)$ digraphs having chordal phylogeny graphs. Phylogeny graphs are also called moral graphs in Bayesian network theory. Our work is motivated by problems related to evidence propagation in a Bayesian network for which it is useful to know which acyclic digraphs have their moral graphs being chordal., Comment: 15 pages, 14 figures

In this paper, we relate the competition number of a graph to its edge clique cover number by presenting a tight inequality $k(G) \ge \theta_e(G)-|V(G)|+\widetilde{k}(G)$ where $\theta_e(G)$, $k(G)$, and $\widetilde{k}(G)$ are the edge clique cover number, the competition number, and the co-competition number of a graph $G$, respectively. By utilizing this inequality and a notion of competition-effective edge clique cover, we obtain some meaningful results on competition numbers of planar graphs., Comment: 15 pages, 3 figures

Choi {\it et al.} [{J.~Choi, K.~S.~Kim, S.~-R.~Kim, J.~Y.~Lee, and Y.~Sano}: {On the competition graphs of $d$-partial orders}, \emph{Discrete Applied Mathematics} (2015), \texttt{http://dx.doi.org/10.1016/j.dam.2015.11.004}] introduced the notion of the partial order competition dimension of a graph. It was shown that complete graphs, interval graphs, and trees, which are chordal graphs, have partial order competition dimensions at most three. In this paper, we study the partial order competition dimensions of chordal graphs. We show that chordal graphs have partial order competition dimensions at most three if the graphs are diamond-free. Moreover, we also show the existence of chordal graphs containing diamonds whose partial order competition dimensions are greater than three., Comment: This paper has been withdrawn by the author due to some overlapping content with arXiv:1505.00204. Instead, we will update arXiv:1505.00204

Choi {\it et al.} [{J.~Choi, K.~S.~Kim, S.~-R.~Kim, J.~Y.~Lee, and Y.~Sano}: {On the competition graphs of $d$-partial orders}, \emph{Discrete Applied Mathematics} (2015), \texttt{http://dx.doi.org/10.1016/j.dam.2015.11.004}] introduced the notion of the partial order competition dimension of a graph. It was shown that complete graphs, interval graphs, and trees, which are chordal graphs, have partial order competition dimensions at most three. In this paper, we study the partial order competition dimensions of chordal graphs. We show that chordal graphs have partial order competition dimensions at most three if the graphs are diamond-free. Moreover, we also show the existence of chordal graphs containing diamonds whose partial order competition dimensions are greater than three., Comment: 13 pages, 4 figures

In this paper, we study the competition graphs of $d$-partial orders and obtain their characterization which extends results given by Cho and Kim \cite{chokim} in 2005. We also show that any graph can be made into the competition graph of a $d$-partial order for some positive integer $d$ as long as adding isolated vertices is allowed. We then introduce the notion of the partial order competition dimension of a graph and study graphs whose partial order competition dimensions are at most three., Comment: This paper has been withdrawn by the author due to some overlapping content with arXiv:1601.01188v1. Instead, we will update arXiv:1601.01188

In this paper, we study a matrix equation $X^n = aI$. We factorize $X^n - aI$ based upon the factorization of $x^n - a$ and then give a necessary and sufficient condition for one of the factors to be the zero matrix., Comment: 9 pages