exotic atoms and nuclei

antiprotonic helium

Antiprotonic Helium Recent results are summarized in, R. S. Hayano, "Antiprotonic helium and CPT invariance", Reports on Progress in Physics 70 (2007) 1995, http://dx.doi.org/10.1088/0034-4885/70/12/R01 timespectrum

Serendipitous discovery at KEK- When an antiproton (or any negatively-charged hadron) is brought to rest in a target, it gets instantly (~10-12s) absorbed by the nucleus. We however found an exception. In helium, about 3% of antiprotons miraculously survive for several microseconds. This serendipitous discovery we made at the KEK proton synchrotron M. Iwasaki et al., "Discovery of antiproton trapping by long-lived metastable states in liquid helium", Phys. Rev. Lett. 67 (1991) 1246, http://dx.doi.org/10.1103/PhysRevLett.67.1246 started our decades-long study. This figure shows a time distribution (log scale) of antiproton annihilation in helium. In addition to the "prompt" annihilation at t=0, there is a tail due to "anomalously long-lived antiprotons".

Further study at CERN - To understand what causes this anomalous behavior, we continued experiments at CERN's antiproton facilities (initially at the low-energy antiproton ring - LEAR and now at the antiproton decelerator - AD), and established that the longevity is due to the formation of antiprotonic helium atoms (helium nucleus + electron + antiproton) T. Yamazaki et al., "Antiprotonic helium", Physics Reports 366 (2002) 183, http://www.sciencedirect.com/science/article/B6TVP-44J1N6J-1/2/ summarizes our LEAR results. . Antiprotonic helium is produced when a stopping antiproton replaces one of the two electrons of helium. It is a very exotic, man-made, half-matter-half-antimatter system.

resonance Laser spectroscopy - When the antiprotonic helium is initially formed, the radius of the antiproton orbit is similar to that of the electron. In terms of the antiproton's principal quantum number, this is about 40 (very highly excited). We succeeded to irradiate antiprotonic helium atoms with laser photons to resonantly change the antiproton orbit (e.g., from n=40 to n=39).
This is very different from all other laser spectroscopic studies, in which electronic states are modified.
The photo to the right taken in ASACUSA's counting room at CERN shows an example of the laser resonance profile (signal intensity plotted against laser frequency).

pbarmass Weighing the antiproton - From high-precision laser spectroscopy of antiprotonic helium, we can deduce the antiproton-to-electron mass ratio. How this is possible can be understood by examining how the antiprotonic-helium transition frequencies depend on the antiproton mass Here, R is the Rydberg constant, c is the light velocity, Zeff is the charge of the helium nucleus. Zeff is in fact less than 2 due to the charge screening by the remaining electron, and high-precision three-body quantum electrodynamical calculations are necessary to obtain the Zeff values for each laser transition we study. . frequency
Over the years, we have improved the frequency-measurement precisions, and have recently determined the antiproton-to-electron mass ratio "almost as precise as" the proton-to-electron mass ratio(the figure compares the proton-to-electron mass ratio(●) to the antiproton-to-electron mass ratio(●).).
We are now starting a new series of measurements which will (in principle) improve the precision of the antiproton-to-electron mass ratio by an order of magnitude. In several years, we may be able to claim that we know the antiproton mass better than the proton mass!