Two-photon decay

Gidley, D.W., Nico, J.S. and Skalsey, M. (1991). Direct search for two-photon decay modes of orthopositronium. Phys. Rev. Lett. 66 1302-1305. 

Parapositronium (p-Ps) is the spin 0 state of Ps, which decays with rate As into an even number of photons due to charge conjugation invariance. The decay into four photons is significantly suppressed [21] and can be ignored at the current experimental level. The two photon decay rate, A2, is calculated using perturbation theory and has been recently calculated [4] through order (n)2 to be1 7989.50 0.02 ps-1. 

We will detect the 2S-3S transition by observing fluorescence from the 3S-2P-1S decay cascade, see Fig. 7. The 30 nm radiation may be detected using a channel electron multiplier. The dominant source of background is expected to be from the spontaneous two-photon decay of the metastable state, giving the characteristic broad, symmetrical frequency spectrum centred on 60.8 nm. A thin aluminium filter will be used to suppress this background and ehminate the scattered 328 nm light. 

For the 11 So ground state of para-Ps a complete calculation of the two photon decay up to the order R a5 has recently been given in [9]. The most precise theoretical value for the branching ratio of the four photon decay is given in [10]. 

However, in these experiments it can be shown that the signal for a given laser intensity scales as Z 6 while the background, from the two-photon decay of the... 

The two-photon decay of 2S /2 produces a continuous spectrum peaked near 250 eV. Sensitivity to the high energy part of this spectrum is suppressed in our detector by the K-absorbtion edge of carbon in the polypropylene window, while the lower part is discriminated electronically by a suitable threshold setting. The observed background rate was 4 kHz/particle-nA. Thus the signal-to-background ratio in the resonance peak was 25. 

Many other attempts to observe this new excited state of positronium have been made. Indications of correlated, equal energy, two-photon decay with a summed energy of 1062 keV have been found by DANZMANN et al. [22] in the same reaction as produced the electron-positron pairs. [15, 18, 19] The authors believe that this line, at a new, fourth energy, may belong to the same neutral system which produced the correlated electron-positron pairs. [22]... 

Experimentally, the two-photon decay of neutral pions from the charge-exchange reaction following n capture in proton ... 

Interest in the polarization correlation of photons goes back to the early measurements of the linear polarization correlation of the two photons produced in the annihilation of para-positronium which were carried out as a result of a suggestion by Wheeler that these photons, when detected, have orthogonal polarizations. Yang subsequently pointed out that such measurements are capable of giving information on the parity state of nuclear particles that decay into two photons. In addition, the polarization correlation observed in the two-photon decay of atoms is considered to be one of the few phenomena where semiclassical theories of radiation are inadequate and it is necessary to invoke a full quantum theory of radiation. The effect has also been used to demonstrate the phenomenon of quantum interference. ... 

In a real experiment, such as shown in Figure 3, the radiation emitted by the source is collected and collimated by a pair of lenses with a finite aperture before being analyzed by the polarizers. The various experiments that have been carried out differ mainly in their choice of source and type of polarizer used. Because of the angular correlation of the photon pairs emitted in two-photon decay processes from an atom, the finite solid angle of detection, and the low detection efSciency of the photodetectors, in practice, only a very small proportion of the photon pairs emitted by the source is actually detected. In this situation, in order to have any hope of violating the BCHSH inequality it is necessary to take N in Eq. (4) for E a,b) to be the total number of photon pairs that would have been detected in the absence of the polarizers, rather than the total number actually emitted by the source. In evaluating (a, fe) we therefore take... 

So far the results of 12 experimental measurements of the polarization correlation in the two-photon decay of atoms have been published. All but two of these have been carried out in order to test the BCHSH inequality in one form or another. 

While two-photon absorption spectroscopy has been widely applied for precision measurements of atomic structure, the polarization correlation of the simultaneous two-photon emission from the metastable Is state of atomic hydrogen has only been measured very recently. The emission of the coincident two photons can be described by a single state vector which determines the circular and linear two-photon polarization. Compared to the two-photon cascade experiments the polarization correlation of the simultaneous two-photon decay of metastable hydrogen is conceptually closer to the original proposals by Bell and Bohm for tests of the foundation of quantum mechanics. More than SO years have elapsed since the famous Einstein-Bohr debate on microphysical reality and quantum formalism. The present and future outcome of the hydrogen two-photon correlation experiment is considered to be a most crucial test with regard to the rivalry between quantum mechanics and local realistic theories. 

Fig. 6.1. The approximate partial decay widths of H to all two-body partonic decay modes. The dashed curves show decays to charged leptons, the solid curves the decay into hadrons as computed from the contributions to each quark separately. The dot-dashed curves illustrate the two-gluon and two-photon decays. See text.

Fig.7.4. Experimental apparatus for the investigation of the two-photon decay of the metastable 2 level of He. (After Novick (1969).)...

Two-photon decay of helium-like ions. The two-photon... 

In neutral hydrogen the restrictions on E] are very much more severe owing to the smaller values of both the Lamb shift and the two-photon decay rate. 

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