Abstract Body

Superconducting detectors have been applied to observations in cosmology and astroparticle physics. In this talk, I will not cover all such projects, but instead, focus on two specific projects in which I have been involved.

First, I will discuss observations of the cosmic microwave background (CMB). These observations have provided profound insights into the nature of the universe, including the composition and age of the cosmos, by analyzing the anisotropy of intensity and E-mode polarization. Recent and upcoming CMB measurements are concentrating on detecting B-mode polarization. Large angular scale measurements of B-mode polarization could reveal signatures of primordial gravitational waves, which are believed to have been generated by quantum fluctuations of space during cosmic inflation, prior to the hot Big Bang. Current and future CMB experiments utilize arrays of superconducting detectors—ranging from several thousand to hundreds of thousands—to suppress the power fluctuations caused by the shot noise of incoming photons. These observations not only measure the CMB, originating from the farthest reaches of the universe, but also detect microwaves emitted by nearby galaxies due to synchrotron and dust emissions (foregrounds). To accurately measure CMB B-mode polarization, it is essential to remove these foregrounds, which can be done by exploiting their different frequency dependencies. Consequently, future B-mode polarization observations will involve wide frequency band measurements. I will describe a proposed space mission [1] designed to detect primordial gravitational waves with a precision capable of distinguishing between most major single-field inflation models.

Next, I will briefly introduce a smaller, future project aimed at detecting solar electron neutrinos. The Sun emits light through the fusion of hydrogen into helium at its core. This fusion process begins with the collision of two protons, producing a deuteron, a positron, and an electron neutrino (pp reaction). Neutrinos generated by this reaction are known as pp neutrinos. Measuring these pp neutrinos can provide invaluable information about the Sun's internal conditions, which cannot be observed by electromagnetic waves alone. Additionally, it could offer clues about new physics, including dark matter. Previous experiments have observed pp neutrinos using targets weighing between ten and one hundred tons. In 1976, Raghavan proposed using Indium-115 [2] as a target, which could significantly reduce background events through the delayed coincidence of de-excitation gamma rays (Fig. 1), allowing the target mass to be reduced to one ton. I will present an idea to use a target including indium combined with superconducting detector arrays.

References

[1] LiteBIRD Collaboration, Prog. Theor. Exp. Phys. 2023, 042F01 (2022).

[2] R. S. Raghavan, Phys. Rev. Lett. 37, 259 (1976).

Acknowledgment

The authors acknowledge the CRAVIT in AIST. The authors are supported by KAKENHI 23K25880 and 21H00077, and RECTOR in Okayama University

Figure 1. Solar electron neutrino reaction with In-115 produce the inverse beta rays and excited Sn which emits two gammas in the de-excitation process. The two gammas emitted with a delay of 3.3 microseconds can be used to have delayed coincidence to reduce background events.

Keywords: Cosmic Microwave Background, Solar neutrino