Low-$p_{T}$ $\mu^{+}\mu^{-}$ pair production in Au+Au collisions at $\sqrt{s_\mathrm{NN}}$ = 200 GeV at STAR

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In high energy heavy-ion collisions, the strong electromagnetic (EM) fields of the nuclei can produce energetic, high-density photon fluxes, leading to photon-induced interactions. Recently, significant enhancements of $e^{+}e^{-}$ pair and J/$\psi$ production at very low transverse momentum ($p_{T}$) were observed by the STAR [1, 2] and ALICE [3] collaborations in peripheral hadronic A+A collisions. The excess yields exhibit a much weaker centrality dependence compared to the expectation for hadronic production, and are consistent with coherent photon-photon and photon-nucleus interactions. The measured $p_{T}$ broadening for $e^{+}e^{-}$ pairs may indicate the existence of a strong magnetic field in the medium. Measurements with $\mu^{+}\mu^{-}$ pairs provide a complementary channel to investigate these phenomena.

In 2014 and 2016, the STAR experiment at RHIC recorded large samples of Au+Au collisions at $\sqrt{s_{_{\rm NN}}}$ = 200 GeV with di-muon triggers utilizing the Muon Telescope Detector. In this talk, we will present invariant mass and yield distributions as a function of centrality for inclusive $\mu^{+}\mu^{-}$ pair production at \mbox{$p_{T} < 0.15$ GeV/$c$} in the mass range between 2.6 and 10 GeV/$c^{2}$. The $p_{T}^{2}$ distribution of the excess yields for these very low $p_{T}$ $\mu^{+}\mu^{-}$ pairs will also be shown. Physics implications will be discussed together with model comparisons.

References: [1] J. Adam et al. (STAR Collaboration), Low-$p_{T}$ $e^{+}e^{-}$ Pair Production in Au + Au Collisions at $\sqrt{s_{_{\rm NN}}}$ = 200 GeV and U + U Collisions at $\sqrt{s_{_{\rm NN}}}$ = 193 GeV at STAR, Phys. Rev. Lett. 121, 132301 (2018). [2] J. Adam et al. (STAR Collaboration), Observation of excess J/$\psi$ yield at very low transverse momenta in Au + Au Collisions at $\sqrt{s_{_{\rm NN}}}$ = 200 GeV and U + U Collisions at $\sqrt{s_{_{\rm NN}}}$ = 193 GeV at STAR, arXiv: 1904.11658. [3] J. Adam et al. (ALICE Collaboration), Measurement of…