Antiprotonic helium

In the last method the antiproton forms an exotic metastable antiprotonic helium atom, which then reacts with deliberately introduced positrons (or positronium atoms). 

Ito, Y., Widmann, E. and Yamazaki, T. (1993). Possible formation of antihydrogen atoms from metastable antiprotonic helium atoms and positrons/positroniums. Hyperfine Interactions 76 163-173. 

Kartavstev, O.I. (1996). Variational calculations of antiprotonic helium atoms. Russian J. Nucl. Phys. 59 1541—1550. 

Morita, N., Kumakura, M., Yamazaki, T., Widmann, E., Masuda, H., Sugai, I., Hayano, R.S., Maas, F.E., Torii, H.A., Hartmann, F.J., Daniel, H., von Egidy, T., Ketzer, B., Muller, W., Schmidt, W., Horvath, D. and Eades, J. (1994). First observation of laser-induced resonant annihilation in metastable antiprotonic helium atoms. Phys. Rev. Lett. 72 1180-1183. 

Yamazaki, T. (1992). A possible way to promote antihydrogen formation via metastable antiprotonic helium atoms. Z. Phys. A 341 223-225. 

Abstract. The antiprotonic helium,pe He2+ (= pHe+), is a peculiar metastable atom, interfacing between matter and antimatter. A series of metastable states axe composed of the He nucleus, one electron in the ground Is configuration and one antiproton orbiting with large quantum numbers (n, l), where n l e 38. They possess... 

The cause of the p longevity is the formation of various metastable states of e pHe2+ (= pHe+) in helium media. They are collectively called Antiprotonic Helium. 

Formation When an p slows down in He, its kinetic energy eventually falls below the He ionization threshold (Jo = 24.6 eV), at which point it replaces one of the electrons in a He atom to form pHe+. The antiprotonic helium atom thus formed with an initial kinetic energy around 5 eV reaches thermal equilibrium within nanosecond without suffering destruction. 

Antiprotonic Helium has many interesting facets because of its unique three-body character involving one p and thus provides playgrounds of physics and chemistry. 

Primordial exotic atom The metastable states are located in the primordial zone (n no = /M /me), where the exotic particle and the atomic electron coexist in the same spatial region. With the exception of antiprotonic helium, the primordial zone of exotic atoms has never been identified and remains an untouched object of investigation. 

The p and He2+ are thus regarded as two atomic centers in a diatomic molecule. Because of the dual character as an exotic atom and an exotic molecule Antiprotonic Helium is often called antiprotonic helium atom-molecule, or for short, atomcule. Since the Is electron motion, coupled to a large-(n, l) p orbital, is faster by a factor of 40 than the p motion, the three-body system pHe+ is solved by using the Born-Oppenheimer approximation, as fully discussed by Shimamura [6]. 

Unique interface between matter and antimatter Whereas particles and antiparticles cannot coexist stably, Antiprotonic Helium is an exceptional case, where an intruder antiparticle (p) coexists with the normal matter (helium medium) for microseconds. Here, the property of the orbiting p (charge, mass, magnetic moment and other QED characteristics) can be probed. It is an interesting irony that the property of the proton cannot always be studied so precisely, because there is no atomic system in which a proton is orbiting. 

Fig. 2. Atomic and molecular views of Antiprotonic Helium. The large (n, l) states in the atomic yrast region in the atomic model axe also assigned as the molecular states of corresponding rotational and vibrational quantum numbers (J,v) = (l,n — l — 1) in the one-dimensional potential for each J. The radiative transitions with Av = 0, as shown by arrows, are favoured because of the maximum overlapping of the radial densities. In this sense, the atomcule system has a dual character by itself...

Fig. 3. First successful observation of laser resonance of antiprotonic helium, now attributed to the (n, l) = (39,35) —> (38,34) transition. (Left) Observed time spectra of delayed annihilation of antiprotons with laser irradiation of various vacuum wavelengths near 597.2nm. Spikes due to forced annihilation through the resonance transitions are seen. (Upper right) Enlarged time profile of the resonance spike. (Lower right) Normalized peak count versus vacuum wavelength in the resonance region. From Morita et al. [11]...

The excellent agreement between experiment and theory is used to deduce a constraint on the assumed mass Mp and charge Qp of antiproton. While the cyclotron frequency of p measured by Gabrielse et al. [22] sets a severe constraint on the ratio Mp/Qp, the antiprotonic helium gives a constrant on the p Rydberg constant MpQ a la Hughes and Deutch [23]. Combining these two physical quantities we obtain Mp and Qp independently. The constraints thus obtained are shown in Fig. 7. 

The metastablity of antiprotonic helium is known to be affected when foreign atoms and molecules are added to the helium media, as revealed from delayed annihilation time spectra (DATS) in the early stage [2,24,25], However, DATS alone is a macroscopic quantity in which all the microscopic informations cannot be differentiated. Laser resonance techniques have made it possible to investigate microscopically the (n, /[-dependent lifetime shortening effects on the surrounding physico-chemical conditions of antiprotonic helium. 

Bakalov et al. treated the trajectories of the helium atom in collision with pHe+ in a semiclassical way, and calculated the pressure shifts and broadening. They obtained numerical values for a numer of transitions, as presented in Table 2. For the precisely known transitions (39,35) —> (38,34) and (37,34) —> (36,33) their theoretical values with realistic collision trajectories (not the linear approximation) turned out to be in excellent agreement with the experimental values. The theoretical treatment of Bakalov et al. was the first quantum chemistry type calculation on the interaction of antiprotonic helium with other atoms and molecules. 

A method to observe microwave resonances in antiprotonic helium has been proposed and is being prepared for the coming antiproton decelerator (AD) ring at CERN [36]. It is called 2-laser-microwave triple resonance method, which has the following steps. 

A new antiproton facility AD (antiproton decelerator) has been completed at CERN and a series of experimental programs are in progress. These include more systematic studies of the structure and formation of pHe+, higher precision laser spectroscopy, microwave spectroscopy [36] and search for type-II antiprotonic helium based on the excited helium [37]. 

Abstract. We present theoretical calculations for the (36,35) —> (34,33) transition between metastable states in the antiprotonic helium 4He+p, which is supposed to be measured in the two-photon high-precision spectroscopy experiment at CER.N. 

Hyperfine Structure Measurements of Antiprotonic Helium and Antihydrogen... 

Fig. 1 shows the level diagram of antiprotonic helium which was experimentally established by observing several laser-induced transitions of the antiproton (see talk by T. Yamazaki [4]). Each level in Fig. 1 is split due to the presence of three angular momenta the orbital angular momentum L (mainly carried by the p), and the spins of the electron Se and the antiproton Sp. These momenta couple according to the following scheme ...

Fig. 3. Observed hyperfine splitting in the (n, L) = (37,35) —> (38,34) transition of antiprotonic helium. Plotted here is the area under the laser-induced annihilation peak normalized to the total delayed annihilations vs. the laser wavelength...

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