Exotic antiproton

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

The list of simple atoms accessible now includes a broad range of very different natural and artificial systems hydrogen, helium, muonium, positronium, various few-electron ions, muonic atoms and exotic atomic systems containing a pion, antiproton etc. While hydrogen atoms form the essential part of our universe, the unstable atoms like muonium do not exist in nature at all. The investigation of simple atoms has provided us with important knowledge on fundamental interactions between the particles these atoms consist of. 

There is a kind of atom where the nuclear effects are very large - exotic atoms, containing hadrons, i.e. particles that can interact strongly pions, antiprotons, kaons etc. In such atoms any advanced high-accurate QED theory is not necessary and a goal to study such atoms is to measure these nuclear parameters. An important feature of any spectroscopic measurement is its high accuracy in respect to non-spectroscopic methods. That is very important for exotic atoms, because some, like e.g. pionium (7r+7r -system or bound 7rp-system), are available in very small quantities (a few hundreds) [35],... 

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]. 

Exotic atoms are produced by stopping a beam of negatively charged particles like muons, pions, or antiprotons in a target, where they are captured in the Coulomb potential of the atoms at high principal quantum numbers n. These systems deexcite mainly by fast Auger emission of electrons in the upper part of the atomic cascade and more and more by X-radiation for lower-lying states. 

Exotic ground-state hydrogen atom Antiprotonic Helium is an exotic... 

Some possible mechanisms for the quenching of pHe+ states were discussed in [29]. First, it is to be noted that the antiprotonic helium resembles a hydrogenlike atom from the physico-chemical point of view, since the pHe+ system has only one electron. The proton in this system is a high-lying state [pHe +]( q with a net charge - -1, but an effective charge around 1.6, depending on The other view of antiprotonic helium is that it is a kind of diatomic molecule with the two centers p and He +. One of the plausible processes is exotic molecule formation ... 

Antiprotons and positrons. Antiprotons and positrons are of special interest because an excess over what is expected from production by protons during propagation could reflect an exotic process such as evaporation of primordial black holes or decay of exotic relic particles (Bottino et al., 1998). At a more practical level, they are important because they are secondaries of the dominant proton component of the cosmic radiation. As a consequence their spectra and abundances provide an independent constraint on models of cosmic-ray propagation (Moskalenko et al., 1998). 

Secondary antiprotons have a kinematic feature analogous to that in 7r°-decay gamma rays but at a higher energy related to the nucleon mass. In this case the feature is related to the high threshold for production of a nucleon-antinucleon pair in a proton-proton collision. This kinematic feature is observed in the data (Orito et al., 2000), and suggests that an exotic component of antiprotons is not required. Antiproton fluxes are consistent with the basic model of cosmic-ray propagation described in the Introduction. 

There are several reasons that makes it interesting to study three-body atomic and molecular systems. The recent experimental and theoretical studies on antiprotonic helium[3,4] is an example of an exotic atomic system which was studied in order to find possible differences between antimatter and matter. 

Let us discuss now how these exotic nuclear states can be produced in the laboratory. We believe that the most direct way is to use antiproton beams of multi-GeV energy. This high energy is needed to suppress annihilation on the nuclear surface which dominates at low energies. To form a deeply bound state, the fast antiproton must transfer its energy and momentum to one of the surrounding nucleons. This can be achieved through reactions of the type pN BB in the nucleus. 

There are several cascade codes developed for reproducing the cascade decays of exotic atoms (see, e.g., Borie and Leon 1980 Markushin 1999 Jensen and Markushin 2002). The cascade process is studied by detecting X-rays and Auger electrons from transitions in all exotic atoms and via laser spectroscopy in metastable antiprotonic helium (see O Sect. 28.6.3.2). It was experimentally observed that medium-heavy muonic atoms such as p Ar lose all atomic electrons via Auger effect by the time the muon reaches the ground state (Bacher et al. 1988). 

Using exotic atoms, one can separately study the distributions of protons and neutrons in the nucleus by comparing muon and pion absorption. Muons are absorbed by the protons only, whereas the strong interaction is independent of the electric charge, so hadrons (pions, kaons, and antiprotons) interact equally with protons and neutrons. 

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