Radiative Decay Width Measurements of Neutral Kaon Excitations Using the Primakoff Effect

We use $K_L$’s in the 100–200 GeV energy range to produce 147 candidate events of the axial vector pair $K_1(1270)\text{−}{K}_1(1400)$ in the nuclear Coulomb field of a Pb target and determine the radiative widths $\Gamma \mathbf{(}{K}_1(1400)\to K^0+\gamma \mathbf{)}=280.8\pm 23.2(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t})\pm 40.4(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t})\mathrm{k}\mathrm{e}\mathrm{V}$ and $\Gamma \mathbf{(}{K}_1(1270)\to K^0+\gamma \mathbf{)}=73.2\pm 6.1(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t})\pm 28.3(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t})\mathrm{k}\mathrm{e}\mathrm{V}$. These first measurements appear to be lower than the quark-model predictions. We also place upper limits on the radiative widths for $K^{*}(1410)$ and $K_2^{*}(1430)$ and find that the latter is vanishingly small in accord with SU(3) invariance in the naive quark model.

Figure 1

Mass vs angular momentum and parity ($J^P$) for neutral kaon resonances. Arrows indicate resonances accessible by Primakoff excitation.

Figure 2

(a) $K_S{\pi }^0$ invariant mass in the four-body channel showing $K^{*}(892)\to K_S{\pi }^0$ decays. (b) The same $K_S{\pi }^0$ invariant mass in the $1.4\mathrm{G}\mathrm{e}\mathrm{V}/c^2$ region. $K_2^{*}(1430)$ and $K^{*}(1410)$ Monte Carlo simulations are also shown to arbitrary scale. No $K_2^{*}(1430)$ or $K^{*}(1410)$ resonance is apparent.

Figure 3

(a) [$K^{*}(892){\pi }^0$] $p_t^2$ vs invariant mass after all other cuts. (b) Projection onto the abscissa after the $p_t^2$ cut. Decomposition of the observed signal into $K_1(1270)$ and $K_1(1400)$ is also shown. (c) Projection onto the ordinate after the mass cut. Note the sharply forward nature of Primakoff production. We discard events to the right of the arrow.

Figure 4

Data/MC comparisons for the $K_S{\pi }^0$ invariant mass (a) and the $p_t^2$ for the observed $K_1(1270)-K_1(1400)\to K^{*}(892){\pi }^0$ signal (b). A Jacobian distribution in $p_t^2$ indicates the recoil of the daughter $K^{*}(892)$ against the ${\pi }^0$. We discard events to the left of the arrow.