Interferometry of Direct Photons in Central ${}^{208}\mathrm{P}\mathrm{b}+{}^{208}\mathrm{P}\mathrm{b}$ Collisions at $158A\mathrm{G}\mathrm{e}\mathrm{V}$

Two-particle correlations of direct photons were measured in central ${}^{208}\mathrm{P}\mathrm{b}+{}^{208}\mathrm{P}\mathrm{b}$ collisions at $158A\mathrm{G}\mathrm{e}\mathrm{V}$. The invariant interferometric radii were extracted for $100<K_T<300\mathrm{M}\mathrm{e}\mathrm{V}/c$ and compared to radii extracted from charged pion correlations. The yield of soft direct photons, $K_T<300\mathrm{M}\mathrm{e}\mathrm{V}/c$, was extracted from the correlation strength and compared to theoretical calculations.

Figure 1

The two-photon correlation function for narrow showers with $L_{\min }>20\mathrm{c}\mathrm{m}$ (diamonds) and average photon momenta $100<K_T<200\mathrm{M}\mathrm{e}\mathrm{V}/c$ (top) and $200<K_T<300\mathrm{M}\mathrm{e}\mathrm{V}/c$ (bottom) fitted with Eq. (1). The solid line shows the fit result in the fit region used (excluding the ${\pi }^0$ peak at $Q_{\mathrm{i}\mathrm{n}\mathrm{v}}\approx m_{{\pi }^0}$) and the dotted line shows the extrapolation into the low $Q_{\mathrm{i}\mathrm{n}\mathrm{v}}$ region where backgrounds are large.

Figure 2

Comparison of ${\lambda }_{\mathrm{i}\mathrm{n}\mathrm{v}}$ parameter fit results for different fit regions for $100<K_T<200\mathrm{M}\mathrm{e}\mathrm{V}/c$ (top) and $200<K_T<300\mathrm{M}\mathrm{e}\mathrm{V}/c$ (bottom), calculated for narrow showers with different cuts on the minimum shower separation distance: $L_{\min }=20\mathrm{c}\mathrm{m}$, ■; 25 cm, ▲; 30 cm, ▼; 35 cm, ○ (same $Q_{\min }$ for each).

Figure 3

Comparison of parameters of correlation functions with different particle identification criteria: △, all clusters; ▼, narrow electromagnetic; □, all neutral; ○, narrow neutral electromagnetic (no significant result for high $K_T$).

Figure 4

Yield of direct photons extracted from the strength of the two-photon correlation ($R_O=0\mathrm{f}\mathrm{m}$, closed circles; $R_O=6\mathrm{f}\mathrm{m}$, closed triangles) and by the statistical subtraction method (open circles, or arrows indicating upper limits) [8]. Total statistical plus systematical errors are shown. The calculations are described in the text.