A recent study published in Physical Review Letters reports on a successful proof-of-principle experiment conducted by an international team of researchers, including the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), aimed at verifying strong-field quantum electrodynamics with exotic atoms. The researchers achieved this by conducting high-precision measurements of the energy spectrum of muonic characteristic X-rays emitted from muonic atoms using a state-of-the-art X-ray detector. The results of the experiment represent a significant step towards verifying fundamental physical laws under strong electric fields, which have not yet been artificially created by humans. The highly efficient and accurate X-ray energy determination method used in this research is expected to have applications in various research fields, including non-destructive elemental analysis methods using muonic atoms.
Discovering physical laws has always been a goal of scientists, as they provide explanations for observed phenomena that existing theories cannot account for. In many cases, the discovery of new physics requires the development of new experimental techniques and improved measurement accuracy. Quantum ElectroDynamics (QED) is the most precisely tested theory of physical laws, describing the microscopic interactions between charged particles and light. Scientists continue to push the limits of QED's accuracy in describing our physical reality.
The team injected a low-velocity negative muon beam from the J-PARC facility into neon gas, and the energy of characteristic X-rays emitted from resulting muonic neon (Ne) atoms was precisely measured using a superconducting Transition-Edge Sensor (TES) detector. The energy of the muonic characteristic X-rays was determined with an absolute uncertainty of less than 1/10,000, and contributions from vacuum polarization in strong-field quantum electrodynamics were verified with a high precision of 5.8%.
The TES detector was originally developed for space X-ray observation. The current project at Kavli IPMU involves carrying out cross-disciplinary research using this detector. The team includes Kavli IPMU Project Assistant Professor Shin'ichiro Takeda, Project Researcher Miho Katsuragawa, and graduate student Kairi Mine, who participated in the muon experiments.
A new experimental technique utilizing muonic atoms has been demonstrated in collaboration, with expectations of significant advancements in the study of QED verification under strong electric fields. The details of the study have been published in Physical Review Letters.
QED effects are more prominent in strong electric field environments, but theoretical calculations become increasingly challenging in such scenarios. Consequently, strong electric field environments are critical for QED verification. To realize such environments, experiments using highly charged ions (HCIs) have been conducted for years. HCIs are atoms that have had multiple electrons stripped from them, resulting in a stronger electric field experienced by the bound electrons as atomic number increases and shielding is reduced. Despite their efficacy, the finite size effect of the nucleus on HCIs cannot be ignored, leading to compromised accuracy of QED verification.
To overcome this limitation, international research groups have turned to exotic atoms, such as muonic atoms, in which a negatively charged particle is bound to the nucleus instead of an electron. Muonic atoms are made up of negative muons, which are elementary particles that are 200 times heavier than electrons, and nuclei.
Negative muons can be extracted as beams from large accelerators. Muonic atoms are characterized by the extremely close proximity of the negative muon to the nucleus, resulting in an electric field that is 40,000 times stronger than that felt by a bound electron of the same quantum level in an HCI, leading to a significant QED effect. Furthermore, experiments using negative muons can suppress the finite size effect of the nucleus by occupying high angular momentum quantum levels with minimal overlap. By measuring the energy of muonic characteristic X-rays emitted when muonic atoms deexcite from a particular level to lower levels, QED can be verified under strong electric fields (as illustrated in Figure 1).
Although muonic atoms present a promising avenue for strong-field QED verification, there are several challenges to be addressed. The primary issue is the need to prepare a sufficient number of isolated muonic atoms, as the presence of nearby atoms or molecules can cause rapid electron transfer and alter the energy of the muonic characteristic X-rays. To mitigate this, researchers must use dilute gas targets with low pressure, which reduces both the number of muonic atoms produced and the resulting intensity of the X-rays.
To address these challenges, an international research group conducted experiments at the Japan Proton Accelerator Research Complex (J-PARC) in Tokai-mura, Ibaraki, which has the world's most intense low-velocity muon beam. To accurately measure the energy of the low-intensity muonic characteristic X-rays, the experiment utilized a superconducting transition edge sensor (TES) microcalorimeter – a highly efficient and high-resolution X-ray detector.
The research team utilized rare gas neon (10Ne) atoms as the target and achieved an impressive energy resolution that is one order of magnitude higher than conventional semiconductor detectors (FWHM : 5.2 eV) under low-pressure conditions of 0.1 atm. They successfully measured the muonic characteristic X-rays, as shown in Figure 2. The peaks observed were primarily due to the overlap of muonic characteristic X-rays from six distinct transitions. By carefully analyzing contributions from each of these transitions, the team was able to determine the energy of the muonic characteristic X-rays with an exceptional accuracy of 0.002%.
To ensure the muonic neon atoms were in an isolated environment, the researchers conducted additional measurements while varying the pressure of the neon gas target (as shown in Figure 3). The results showed that the energy of the muonic X-rays remained constant within experimental error, regardless of the neon gas target's pressure. By comparing the latest theoretical calculations with the experimental results, the team confirmed that they were in agreement within the experimental error. They were able to verify the effect of vacuum polarization under a strong electric field with an unprecedented accuracy of 5.8%, which is comparable to the accuracy obtained using the multiply charged uranium ion U91+. This achievement marks a significant milestone in the field of strong-field QED verification.
Source: University of Tokyo