Pionic atoms

Keywords Pionic atoms, non-relativistic Hamiltonian, scattering length, energy shift. 

Beane S. R. et al. The S-Wave Pion-Nucleon Scattering Lengths from Pionic Atoms using Effective Field Theory, Nucl.Phys. A 720, 399, (2003)... 

The experiment on the observation of 7r+7r- atoms was carried out at the 70 GeV proton synchrotron (U-70) at Serpukhov [31,32]. Pionic atoms and 7r+7r pairs ( free pairs) were produced in a 8pm thick tantalum target ( thick target) inserted into the internal proton beam. The atoms can either annihilate into 7r°7r° pairs or break up (ionize) into 7r+7r pairs ( atomic pairs) inside the same target. The free and atomic pairs get into the 40 m long vacuum channel (the acceptance is 3.8 10-5 sr) at 8.4° to the proton beam and are detected by the setup in the 0.8 4- 2.4GeV/c pion momentum interval. 

Abstract. Recent experiments are aiming at an accuracy of 1 ppm for the mass of the charged pion using the characteristic X-rays from exotic atoms. Once the pion mass is established with that precision, the narrow lines from medium Z pionic atoms can be used as a calibration standard in the few keV range. The precision of this new standard is not limited by the large natural line width of fluorescence X-rays and their complex structure due to multi-hole excitations. 

At the ttE5 beam of the Paul-Scherrer-Institut (PSI), about 2% of the incoming pions (> 109/s) are stopped in the gas cell with a degrader set-up optimized for pionic atoms. Muons originating from pions decaying shortly before capture are slow enough to be stopped in the gas cell as well. With a set-up optimized for muons, the count rate for muonic atoms is about 4% of the one for pions. 

Having once established a precise value for the pion mass, the X-ray transitions from pionic atoms can be used to produce a set of calibration lines in the few keV range. 

In conclusion it can be stated that the results from scattering data and atom experiments are still contradictory and therefore need further investigation. From the side of the atom experiments it should be clarified whether the shift and the width values of pionic hydrogen and deuterium are true strong interaction effects and are not spoiled by the interaction of the pionic atom with the surrounding molecules. In other words the shift and the width measurements for pionic hydrogen and deuterium should be extrapolated to zero pressure. In a second step state of the art electromagnetic corrections should be applied. 

The still necessary increase in accuracy requires an additional effort. A simultaneous spectroscopy of pionic and muonic hydrogen atoms is planned as the muonic X-rays do not show any strong interaction broadening, but exhibit Doppler broadening similar to pionic atoms. A method was found to measure pionic and muonic X-rays simultaneously. The reduced masses of pionic and muonic hydrogen exhibit almost the same ratio as two lattice plane differences of quartz. With a two crystal set up the pionic and muonic X-rays can be Bragg reflected to the same CCD detector. 

For instance, the Bohr radius of in muonic Pb is only about 4 fm, whereas the radius of the nucleus is about 7 fm. Finally, the muon may be captured by the nucleus or it may decay as a free particle. The influence of the charge distribution in nuclei on muons is also greater than that on electrons, and X rays emitted by muonic atoms, in particular from inner orbitals, give information about the charge distribution and surface structure of nuclei. The influence of electron densities and chemical bonds has been studied by use of pionic atoms, such as p 7r. ... 

The evaluation of the spectroscopic quadrupole moment from the measured B-factors has to rely to a large extent on theoretical calculations of the electronic part of the quadrupole interaction. From the study of muonic and pionic atoms, a number of accurate quadrupole moments have been determined [1]. These values serve as reference data in the same way as for the dipole interaction. 

Figure 12 shows a schematic model of muonic and pionic atoms. Their orbitals are much smaller than those of electrons. Characteristic transitions between various levels are shown in Fig. 13. Due to the strong interaction of a pion with a nucleus, Lyman series pionic X-rays (n - 1) cannot be observed.

Fig. 12. Bohr radii of muonic and pionic atoms. Those for p p and ic p are indicated...

D.F. Anagnostopoulos, D. Gotta, P. IndeUcato, L.M. Simons, Low-energy X-ray standards from hydrogenUke pionic atoms, Phys. Rev. Lett. 91 (24) (2003) 240801. 

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