#### Research Activities

##### Publications

###### Light Dark Matter Search with Nitrogen-Vacancy Centers in Diamonds [arXiv: 2302.12756]

###### So Chigusa, Masashi Hazumi, Ernst David Herbschleb, Norikazu Mizuochi, Kazunori Nakayama (2023/02/24)

###### Abstract

We propose new ideas to directly search for light dark matter, such as the axion or the dark photon, by using magnetometry with nitrogen-vacancy centers in diamonds. If the dark matter couples to the electron spin, it affects the evolution of the Bloch vectors consisting of the spin triplet states, which may be detected through several magnetometry techniques. We give several concrete examples with the use of dc and ac magnetometry and estimate the sensitivity on dark matter couplings.

###### Quantum simulations of dark sector showers [arXiv: 2204.12500]

###### So Chigusa, Masahito Yamazaki (2022/04/26)

###### Abstract

We consider dark sector scenarios where dark matter is accompanied by a dark photon and multiple-flavor dark fermions charged under the dark gauge group. We study quantum interference effects in dark sector jets, where multiple dark photons are emitted from high-energy dark fermions. We perform fully quantum simulations of dark sector showers and compare the results against those of the classical Monte-Carlo simulations. We find important differences in probability distributions of dark photon countings between quantum and classical computations. When the number of dark-fermion flavors is large, we find significant enhancements in large numbers of dark photon emissions. Such enhancements can provide distinguishing signals for our scenarios at particle colliders.

###### Upper bound on the smuon mass from vacuum stability in the light of muon g-2 anomaly [arXiv: 2203.08062]

###### So Chigusa, Takeo Moroi, Yutaro Shoji (2022/03/15)

###### Abstract

We derive an upper bound on the smuon mass assuming that the muon \(g-2\) anomaly is explained by the supersymmetric (SUSY) contribution. In the minimal SUSY standard model, the SUSY contribution to the muon \(g-2\) is enhanced when the Higgsino mass parameter is large. Then, the smuon-smuon-Higgs trilinear coupling is enhanced, which may destabilize the electroweak vacuum. We calculate precisely the decay rate of the electroweak vacuum in such a case. We include one-loop effects which are crucial to determine the overall normalization of the decay rate. Requiring that the theoretical prediction of the muon anomalous magnetic moment is consistent with the observed value at the \(1\) and \(2\sigma\) levels (equal to the central value of the observed value), we found that the lightest smuon mass should be smaller than \(1.38\) and \(1.68\ {\rm TeV}\) (\(1.20\ {\rm TeV}\)) for \(\tan\beta=10\) (with \(\tan\beta\) being the ratio of the vacuum expectation values of the two Higgs bosons), respectively, and the bound is insensitive to the value of \(\tan\beta\).

###### Deeply learned preselection of Higgs dijet decays at future lepton colliders [arXiv: 2202.02534]

###### So Chigusa, Shu Li, Yuichiro Nakai, Wenxing Zhang, Yufei Zhang, Jiaming Zheng (2022/02/05)

###### Abstract

Future electron-positron colliders will play a leading role in the precision measurement of Higgs boson couplings which is one of the central interests in particle physics. Aiming at maximizing the performance to measure the Higgs couplings to the bottom, charm and strange quarks, we develop machine learning methods to improve the selection of events with a Higgs decaying to dijets. Our methods are based on the Boosted Decision Tree (BDT), Fully-Connected Neural Network (FCNN) and Convolutional Neural Network (CNN). We find that the BDT and FCNN-based algorithms outperform the conventional cut-based method. With our improved selection of Higgs decaying to dijet events using the FCNN, the charm quark signal strength is measured with a \(16\%\) error, which is roughly a factor of two better than the \(34\%\) precision obtained by the cut-based analysis. Also, the strange quark signal strength is constrained as \(\mu_{ss} \lesssim 35\) at the \(95\%\) C.L. with the FCNN, which is to be compared with \(\mu_{ss} \lesssim 70\) obtained by the cut-based method.

###### Studying squark mass spectrum through gluino decay at 100 TeV future hadron colliders [arXiv: 2102.07910]

###### So Chigusa, Koichi Hamaguchi, Takeo Moroi, Atsuya Niki, Kosaku Ono (2021/02/16)

###### Abstract

We study the prospect of determining the decay properties of the gluino in the supersymmetric (SUSY) standard model at a 100 TeV future hadron collider. We consider the case where the neutral Wino is the lightest superparticle. In this case, the long-lived charged Wino can be used to eliminate standard model backgrounds, which enables us to study the details of superparticles. We show that, based on the analysis of the numbers of high \(p_T\) leptons, boosted \(W\)-jets, and \(b\)-tagged jets, we may determine the gaugino species and the quark flavors in the gluino decay. With such determinations, we can obtain information about the mass spectrum of squarks even if squarks are out of the kinematical reach.

##### Invited Seminars

##### Talks

- Topical theory talk: Vacuum stability
Workshop for Tera-Scale Physics and Beyond @ Hakata (2023/06/23)

- Axion detection with spin dynamics: magnons and axions (Invited)
Joint IQ Initiative & PITT PACC Workshop: Axions, Fundamental and Synthetic @ The University of Pittsburgh (2023/04/06)

- 高エネルギー反応におけるパートンシャワーへの量子計算の応用（シンポジウム講演）
JPS 2023 Spring @ Online (2023/03/25)

##### 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