Toward new frontiers : Encounter and synergy of state-of-the-art astronomical detectors and exotic quantum beams

Program A01-7	Study of dynamical processes toward the observation of parity non-conservation effects in muonic atoms
Principal Investigator	KANDA, Sohtaro (High Energy Accelerator Research Organization (KEK))

When a nuclear Coulomb potential captures a negative muon, the muon forms an exotic bound-state called a muonic atom. The nature of muonic atoms enable precise validation of bound-state quantum electrodynamics (QED) and search for physics beyond the Standard Model [1]. In particular, a measurement of atomic parity violation (APV) in muonic atoms has been proposed for a long time [2], but has not been realized due to experimental difficulties [3]. To revisit this topic, we propose a new experiment using a high-intensity pulsed muon beam and a segmented crystal calorimeter.

Muonic atoms form in a highly excited state with a large principle quantum number n ∼ 14. The atoms are then de-excited by radiative and Auger transition, leading to the ground-state within a short time. In a low-density gas, where the muonic atoms can be considered isolated from their surroundings, some of the atoms remain in the 2S metastable state [4]. Atoms in the 2S state mix with the 2P state in a parity-violating weak interaction between the muon and the nucleus. This Parity non-conservation process leads to a one-photon transition from the 2S to the 1S state, which is suppressed by the selection rule. Since this transition is parity-violating, photons are emitted asymmetrically with respect to the muon spin direction. By measuring the asymmetry of the emitted photons, the APV effect in muonic atoms can be observed and quantified. Since the APV gives the weak charge of the nucleus, the Weinberg angle, which is a parameter of electroweak interaction, can be determined. Measurements of the Weinberg angle at various energy scales are essentially important as a precision test of the Standard model and search for new physics.

In order for the experiment to work, a 2S metastable state of the muonic atom is essential, which can only be achieved with a muonic atom that has lost all its orbital electrons. The muonic atom loses electrons via the Auger transition in the cascade process and gains electrons by filling from surrounding atoms. The electronic state of the muonic atom is determined by the history of these complicated dynamical processes. The study of dynamical processes of muonic atoms connects a wide range of interdisciplinary fields from nuclear, atomic, and molecular physics to radiochemistry, and has applications in materials science, chemical reaction research, and elemental analysis.

Although various experimental results and theoretical calculations are known about cascades and atomic collisions of muonic atoms, there are still many aspects that remain to be clarified. For example, the residual electrons of muonic carbon atoms in hydrocarbon gases cannot be reproduced by theoretical calculations of cascades [5].

The proposed experiment aims to reveal the electronic states of muonic carbon atoms in a low-density gas. Figure 1 illustrates the experimental setup. A pulsed negative muon beam irradiates the gas target contained in a vessel. Typical target pressure is 0.1 atm, which provide both sufficient muon-stopping efficiency and muonic atom isolation. Inside the vessel, a calorimeter consisting of LYSO:Ce scintillator crystals and silicon photomultipliers (SiPMs) are placed to detect both muonic X-rays and decay electrons. The muon stopping distribution in the gas is obtained by a fiber hodoscope. Combining the intensity of X-rays emitted upon cascade de-excitation with the residual polarization and electrons in the ground-state, we estimate the distribution of the principal quantum number and the number of electrons at the moment of muonic atom formation using the Bayesian method, and construct a new model of the cascade with predictive capability.


Fig. 1. Experimental schematic. References
[1] B. Batell et al., Phys. Rev. Lett. 107, 011803 (2011).
[2] J. Missimer and L. M. Simons, Phys. Rep. 118, 179 (1985).
[3] K. Kirch et al., Phys. Rev. Lett. 78, 4363 (1997).
[4] H. P. von Arb et al., Phys. Lett. B 136, 232 (1984).
[5] K. Kirch et al., Phys. Rev. A 59, 3375 (1999).

Members

Reference Materials

• S. Kanda and K. Ishida, “Development of a calorimeter for muonic X-ray detection,” RIKEN Accel. Prog. Rep. 54, to be published (2021).