Nonprompt direct-photon production in Au + Au collisions at s N N = 200 GeV

Résumé fourni par la source

The measurement of the direct-photon spectrum from $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{{s}_{{}_{NN}}}=200$ GeV is presented by the PHENIX Collaboration using the external-photon-conversion technique for 0%--93% central collisions in a transverse-momentum (${p}_{T}$) range of 0.8--10 $\text{GeV}/c$. An excess of direct photons, above prompt-photon production from hard-scattering processes, is observed for ${p}_{T}<6\phantom{\rule{4pt}{0ex}}\text{GeV}/c$. Nonprompt direct photons are measured by subtracting the prompt component, which is estimated as ${N}_{\mathrm{coll}}$-scaled direct photons from $p+p$ collisions at 200 GeV, from the direct-photon spectrum. Results are obtained for $0.8<{p}_{T}<6.0\phantom{\rule{4pt}{0ex}}\text{GeV}/c$ and suggest that the spectrum has an increasing inverse slope from $\ensuremath{\approx}0.2$ to 0.4 $\text{GeV}/c$ with increasing ${p}_{T}$, which indicates a possible sensitivity of the measurement to photons from earlier stages of the evolution of the collision. In addition, like the direct-photon production, the ${p}_{T}$-integrated nonprompt direct-photon yields also follow a power-law scaling behavior as a function of collision-system size. The exponent, $\ensuremath{\alpha}$, for the nonprompt component is found to be consistent with 1.1 with no apparent ${p}_{T}$ dependence.

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