Search for the Axion-Like-Particles in the $\eta\to\pi^{+}\pi^{-}e^{+}e^{-}$ decay with HADES detector

Abstract

The dark matter and existence of new particles are now a possible explanation of several physics phenomena which evade the predictions of the Standard Model. In this context Axion-Like-Particles (ALP) with masses in the MeV to GeV range with additional Peccei-Quinn breaking contribution, and which are coupled to the Standard Model have been postulated. To search for the existence of such new particles, we have launched dedicated analysis of a high statistics data sample collected by High-Acceptance Di-Electron Spectrometer (HADES) operating at GSI in Darmstadt. In particular, we study $\eta$ meson decays into $\pi^{+}\pi^{-}e^{+}e^{-}$, where hypothesized isoscalar gauge boson $a$ could be produced in the intermediate state $\eta\to\pi^{+}\pi^{-}a$ decaying predominantly to $e^{+}e^{-}$. In this report we describe the analysis strategy we applied to search for a resonant peak in the dilepton invariant mass spectrum $\eta\to\pi^{+}\pi^{-}a\to \pi^{+}\pi^{-}e^{+}e^{-}$ and present the method for event selection and particle identification.