#### Research Activities

##### Research Interests

Nowadays, the improvement of technologies in quantum science is so rapid that various new approaches to problems are becoming available in a short period. These new approaches include $\textbf{quantum sensing}$ techniques to detect a faint signal and $\textbf{quantum computation}$ that allows us to simulate the dynamics of quantum systems by directly manipulating quantum states. Given this situation, it is incumbent on theoretical particle physicists to stay in touch with new technologies and to develop ways to use them in the search for beyond the standard model physics. I have been working along this line; I mainly study the phenomenology of the standard model and beyond, including the $\textbf{dark matter}$ search and important quantum corrections to the $\textbf{parton shower}$ process using various quantum technologies. My main research directions are categorized as $\textit{(i) direct detection of light dark matter with quantum sensing techniques}$ and $\textit{(ii) construction of quantum algorithms applicable to systems such as parton shower}$. In addition, as $\textit{(iii) other directions}$, I work on several different approaches to further explore beyond the standard model physics, including $\textbf{false vacuum decay}$ rate calculation and $\textbf{collider phenomenology}$.

###### Direct detection of light dark matter with quantum sensing techniques

###### Construction of quantum algorithms applicable to systems such as parton shower

###### Other directions

###### Conclusion

##### Publications

###### Quantum Parton Shower with Kinematics [arXiv: 2310.19881]

###### Christian W. Bauer, So Chigusa, Masahito Yamazaki (2023/10/30)

###### Abstract

Parton showers which can efficiently incorporate quantum interference effects have been shown to be run efficiently on quantum computers. However, so far these quantum parton showers did not include the full kinematical information required to reconstruct an event, which in classical parton showers requires the use of a veto algorithm. In this work, we show that adding one extra assumption about the discretization of the evolution variable allows to construct a quantum veto algorithm, which reproduces the full quantum interference in the event, and allows to include kinematical effects. We finally show that for certain initial states the quantum interference effects generated in this veto algorithm are classically tractable, such that an efficient classical algorithm can be devised.

###### Effects of Finite Material Size On Axion-magnon Conversion [arXiv: 2310.17704]

###### So Chigusa, Asuka Ito, Kazunori Nakayama, Volodymyr Takhistov (2023/10/26)

###### Abstract

Magnetic materials are particularly favorable targets for detecting axions interacting with electrons because the collective excitation of electron spins, the magnon, can be excited through the axion-magnon conversion process. It is often assumed that only the zero-momentum uniformly precessing magnetostatic (Kittel) mode of the magnon is excited. This is justified if the de Broglie wavelength of the axion is much longer than the size of the target magnetic material. However, if the de Broglie wavelength is shorter, finite-momentum magnon modes can also be excited. We systematically analyze the target material size dependence of the axion-magnon conversion rate. We discuss the importance of these effects in the detection of relativistic axions as well as in the detection of axion dark matter of relatively heavy mass with large material size.

###### Aiming for Tops of ALPs with a Muon Collider [arXiv: 2310.11018]

###### So Chigusa, Sudhakantha Girmohanta, Yuichiro Nakai, Yufei Zhang (2023/10/17)

###### Abstract

Future muon colliders with center-of-mass energy of \(\mathcal{O}(1-10)\) TeV can provide a clean high-energy environment with advantages in searches for TeV-scale axion-like particles (ALPs), pseudo-Nambu-Goldstone bosons associated with spontaneously broken global symmetries, which are widely predicted in physics beyond the Standard Model (SM). We exploit ALP couplings to SM fermions, and guided by unitarity constraints, build a search strategy focusing on the ALP decay to top quark pairs at muon colliders. It is found that a large parameter space of TeV-scale ALPs with TeV-scale decay constants can be probed by utilizing the ALP-top quark coupling.

###### Axion detection via superfluid \(^3\)He ferromagnetic phase and quantum measurement techniques [arXiv: 2309.09160]

###### So Chigusa, Dan Kondo, Hitoshi Murayama, Risshin Okabe, Hiroyuki Sudo (2023/09/17)

###### Abstract

We propose to use the nuclear spin excitation in the ferromagnetic A1 phase of the superfluid \(^3\)He for the axion dark matter detection. This approach is striking in that it is sensitive to the axion-nucleon coupling, one of the most important features of the QCD axion introduced to solve the strong CP problem. We review a quantum mechanical description of the nuclear spin excitation and apply it to the estimation of the axion-induced spin excitation rate. We also describe a possible detection method of the spin excitation in detail and show that the combination of the squeezing of the final state with the Josephson parametric amplifier and the homodyne measurement can enhance the sensitivity. It turns out that this approach gives good sensitivity to the axion dark matter with the mass of \(O(1) \, \mu \mathrm{eV}\) depending on the size of the external magnetic field. We estimate the parameters of experimental setups, e.g., the detector volume and the amplitude of squeezing, required to reach the QCD axion parameter space.

###### Dark matter detection using nuclear magnetization in magnet with hyperfine interaction [arXiv: 2307.08577]

###### So Chigusa, Takeo Moroi, Kazunori Nakayama, Thanaporn Sichanugrist (2023/07/14)

###### Abstract

We consider the possibility to detect cosmic light dark matter (DM), i.e., axions and dark photons, of mass \(\sim 10^{-6}\) eV and \(\sim 10^{-4}\) eV, by magnetic excitation in a magnet with strong hyperfine interaction. In particular, we consider a canted anti-ferromagnet, MnCO\(_3\), as a concrete candidate material. With spin transfer between nuclear and electron spins allowed by the hyperfine interaction, nuclear spins become naturally highly polarized due to an effective (electron-spin-induced) magnetic field, and have long-range interactions with each other. The collective precession of nuclear spins, i.e., a nuclear magnon, can be generated by the DM field through the nucleon-DM interaction, while they are also sensitive to the electron-DM interaction through the electron-nuclear spin mixing. Compared to conventional nuclear-spin precession experiments, this system as a DM sensor is sensitive to higher frequency needing only a small static magnetic field applied. The system also has collective precession of electron spins, mixed with nuclear spins, as the additional channels that can be used for DM probes. We estimate the sensitivity under appropriate readout setups such as an inductive pick-up loop associated with an LC resonant circuit, or a photon cavity with a photon counting device. We show that this method covers an unexplored parameter region of light bosonic DM.

###### Stability of Electroweak Vacuum and Supersymmetric Contribution to Muon g-2 [arXiv: 2306.16596]

###### So Chigusa, Takeo Moroi, Yutaro Shoji (2023/06/28)

###### Abstract

We study the stability of the electroweak vacuum in the supersymmetric (SUSY) standard model (SM), paying particular attention to its relation to the SUSY contribution to the muon anomalous magnetic moment \(a_\mu\). If the SUSY contribution to \(a_\mu\) is sizable, the electroweak vacuum may become unstable because of enhanced trilinear scalar interactions in particular when the sleptons are heavy. Consequently, assuming enhanced SUSY contribution to \(a_\mu\), an upper bound on the slepton masses is obtained. We give a detailed prescription to perform a full one-loop calculation of the decay rate of the electroweak vacuum for the case that the SUSY contribution to \(a_\mu\) is enhanced. We also give an upper bound on the slepton masses as a function of the SUSY contribution to \(a_\mu\).

##### Invited Seminars

##### Talks

- Light Dark Matter Search with Nitrogen-Vacancy Centers in Diamonds (Invited)
PNU-IBS workshop on Axion Physics : Search for axions @ Busan, Korea (2023/12/06)

- LHC Run 3 と高輝度 LHC で探る新物理模型（シンポジウム講演）
JPS 2023 Fall @ Tohoku University (2023/09/18)

- Light Dark Matter Search with Nitrogen-Vacancy Centers in Diamonds
PASCOS 2023 @ UC Irvine (2023/06/27)

##### Awards

- Best presentation award for young scientists @ Unraveling the History of the Universe 2020
2020/06/02

- Best Poster Award @ HPNP 2019
2019/02/22