Experimental Investigation of High-Energy Photon Splitting in Atomic Fields

Data analysis of an experiment in which photon splitting in atomic fields was observed is presented. The experiment was performed at the tagged photon beam of the ROKK-1M facility at the VEPP-4M collider. In the energy region of 120–450 MeV, statistics of $1.6\times {10}^9$ photons incident on the BGO target was collected. About 400 candidate photon-splitting events were reconstructed. Within the attained experimental accuracy, the experimental results are consistent with the calculated exact atomic-field cross section. The predictions obtained in the Born approximation differ significantly from the experimental results.

Figure 1

Feynman diagram for photon splitting. Solid lines correspond to electron propagators in the atomic electric field.

Figure 2

Principal scheme of setup: LKr calorimeter (1); scintillation veto-counter (2); beam dump (BGO) (3); He-filled tube (4 m length) (4); target (BGO) (5); active collimator (BGO) (6); lead absorber (7); guiding tube for the gamma-quanta beam (8); cleaning magnet (9); passive lead collimator (10); TS scintillation counter (11); Nd:YAG is the laser; TS1–TS4 are tagging system hodoscopes; M1 and M2 are bending magnets; L1 and L2 are quadrupole lenses.

Figure 3

Energy profile in the calorimeter strip structure (left) and reconstructed kinematics (right) for a typical candidate to the photon splitting event.

Figure 4

(a) The photon energy spectrum measured in the tagging system (TS). (b) The number of reconstructed PS events as a function of the tagged photon energy $E_{\mathrm{T}\mathrm{S}}$. In plot (b) solid circles present the experimental results, and the histogram is the result of MC simulation using exact formulas.

Figure 5

The number of the selected PS events as a function of the azimuth angle between momenta of the outgoing photons (a), the polar angle ${\theta }_{\mathrm{m}\mathrm{i}\mathrm{n}}=\mathrm{m}\mathrm{i}\mathrm{n}\{{\theta }_2,{\theta }_3\}$ (b), the polar angle ${\theta }_{\mathrm{m}\mathrm{a}\mathrm{x}}=\mathrm{m}\mathrm{a}\mathrm{x}\{{\theta }_2,{\theta }_3\}$ (c), and the variable $\tilde{x}={\theta }_{\mathrm{m}\mathrm{i}\mathrm{n}}/({\theta }_{\mathrm{m}\mathrm{i}\mathrm{n}}+{\theta }_{\mathrm{m}\mathrm{a}\mathrm{x}})$ (d). In (b), (c), and (d) only events satisfying the complanarity cut $\phi \ge 150°$ [see (a)] are included. Solid circles present the experimental results, and the histograms are the results of MC simulation using exact formulas.