Search for the Axion-Like-Particles in the $η\toπ^{+}π^{-}e^{+}e^{-}$ decay with HADES detector

TL;DR

This work outlines a motivated search for Axion-Like Particles (ALPs) in rare η decays using the HADES detector at GSI. It targets a resonant ALP signature in the dilepton channel $η \rightarrow π^{+}π^{-}a \rightarrow π^{+}π^{-}e^{+}e^{-}$, exploiting high-statistics data and Resonance Chiral Theory guidance to model expected rates and backgrounds. The analysis develops robust particle identification and background suppression methods, including like-sign combinatorial subtraction and event-mixing, yielding thousands of reconstructed η events and establishing a groundwork to set upper limits on ALP decays in the $0–200$ MeV mass range. Ongoing optimization with extended data aims to improve signal sensitivity and constrain ALP scenarios that couple to the SM in this mass window.

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 $η$ meson decays into $π^{+}π^{-}e^{+}e^{-}$, where hypothesized isoscalar gauge boson $a$ could be produced in the intermediate state $η\toπ^{+}π^{-}a$ 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 $η\toπ^{+}π^{-}a\to π^{+}π^{-}e^{+}e^{-}$ and present the method for event selection and particle identification.

Figures (3)

• Figure 1: Base criteria for particle selection and identification. (left) Distribution of particle velocity $\beta$ as a dependence of momentum $p\cdot q$. The superimposed red and blue lines indicate the selection cuts for pions and protons, respectively. (right) Example of the angular distribution of $\Delta\theta$ as a function of momentum for leptons. The selection cuts are indicated by superimposed lines: purple for electrons and magenta for positrons.
• Figure 2: The dependence of the missing mass and invariant mass for all reconstructed and identified particles fulfilling the hypothesis of $\pi^+ \pi^- e^+ e^-$: (left) for the Monte Carlo simulation of the $\eta \to \pi^+\pi^- e^+ e^-$ decay, and (right) for the experimental data. Superimposed red line on both panels indicates the chosen constrain.
• Figure 3: Distribution of the invariant mass for the identified $\pi^+\pi^-e^+e^-$ hypothesis. (left) Result of consecutive kinematical cuts leading to the reduction of the multipion background. (right) Comparison after all selection cuts: black points represent the experimental distribution after the subtraction of the combinatorial background from like-sign lepton pairs; blue points show the background estimated with the event–mixing method; red points correspond to the difference between the experimental data (black) and the mixed-event background estimation (blue).