Enhanced Production of Low-Mass Electron-Positron Pairs in 40-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>A</mml:mi></mml:math>GeV Pb-Au Collisions at the CERN SPS

Abstract

We report on first measurements of low-mass electron-positron pairs in Pb-Au collisions at the CERN SPS beam energy of $40\text{ }\text{ }A\mathrm{G}\mathrm{e}\mathrm{V}$. The observed pair yield integrated over the range of invariant masses $0.2&lt;m\ensuremath{\le}1\text{ }\text{ }\mathrm{G}\mathrm{e}\mathrm{V}/{c}^{2}$ is enhanced over the expectation from neutral meson decays by a factor of $5.9\ifmmode\pm\else\textpm\fi{}1.5(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t})\ifmmode\pm\else\textpm\fi{}1.2(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\text{ }\mathrm{d}\mathrm{a}\mathrm{t}\mathrm{a})\ifmmode\pm\else\textpm\fi{}1.8(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\text{ }\text{ }\mathrm{m}\mathrm{e}\mathrm{s}\mathrm{o}\mathrm{n}\text{ }\mathrm{d}\mathrm{e}\mathrm{c}\mathrm{a}\mathrm{y}\mathrm{s})$, somewhat larger than previously observed at the higher energy of $158\text{ }\text{ }A\mathrm{G}\mathrm{e}\mathrm{V}$. The results are discussed with reference to model calculations based on ${\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}\ensuremath{\rightarrow}{e}^{+}{e}^{\ensuremath{-}}$ annihilation with a modified $\ensuremath{\rho}$ propagator. They may be linked to chiral symmetry restoration and support the notion that the in-medium modifications of the $\ensuremath{\rho}$ are more driven by baryon density than by temperature.