Meson production in air showers and the search for light exotic particles

Abstract

Decays of mesons produced in cosmic ray induced air showers in Earth’s atmosphere can lead to a flux of light exotic particles which can be detected in underground experiments. We evaluate the energy spectra of the light neutral mesons ${\pi }^0$, $\eta$, ${\rho }^0$, $\omega$, $\varphi$ and $J∕\psi$ produced in interactions of cosmic ray protons and helium nuclei with air using QCD inspired event generators. Summing up the mesons produced in the individual hadronic interactions of air showers, we obtain the resulting fluxes of undecayed mesons. As an application, we re-consider the case of millicharged particles created in the electromagnetic decay channels of neutral mesons.

Introduction

While overwhelming evidence has been accumulated for the existence of dark matter (DM) from astrophysical and cosmological observations, the experimental searches for such particles in direct detection experiments have not been successful yet. Combined with the null results in searches for new physics at the LHC, this indicates that new particles with masses below the TeV scale are only weakly coupled to the standard model. The prime candidate for such a DM particle, a thermal relic with mass around the weak scale, has been constrained severely and is on the eve of being excluded: For instance, the upper limit on the annihilation cross section obtained by the Fermi-LAT collaboration using dwarf galaxies excludes thermal relics with masses below $\sim 100$  GeV [1], while model dependent limits from antiproton data are typically even more stringent [2], [3]. Therefore, both model building and experimental searches have expanded their phenomenological scope considerably the last decade, investigating e.g. light DM particles with masses in the sub-GeV range.

Traditionally, this mass range has been considered to be inaccessible to direct detection experiments, since the recoil energy of a DM particle with typical Galactic velocities, $v\sim 10^{-3}c$, is below the threshold energy of such experiments. However, Refs. [4], [5] recently pointed out that cosmic rays (CRs) colliding with DM can up-scatter them, leading to a significantly increased DM flux above the threshold energy of direct detection experiments. Another generic source of light DM particles are CR interactions in the atmosphere of the Earth [6], [7], [8], [9], [10], [11]. If mesons produced in these interactions decay partially into DM, an energetic DM flux that can be detected in underground experiments results. While the up-scattering mechanism relies on a sufficiently large abundance of the DM particle considered, CR interactions in the atmosphere depend only on the well-known flux of incident cosmic rays. This mechanism can moreover produce other long-lived exotic particles, thereby extending the reach of searches for new physics.

In this work, we re-evaluate the atmospheric fluxes of undecayed ${\pi }^0$, $\eta$, $\rho$, $\omega$, $\varphi$ and $J∕\psi$ mesons, which we denote collectively by $\mathcal{m}$. In a previous study by Plestid et al. [12], these fluxes were computed using parametrizations for the relevant production cross sections in $pp$ collisions. Here, we improve upon this in several aspects: First, we use QCD inspired event generators to model the particle production in single hadronic interactions. This allows us to account for the contribution of helium in the CR primary flux as well as for the effect of air as target nuclei. Comparing the results of different event generators we obtain an estimate for the uncertainties of their predictions. Moreover, we model the complete hadronic air shower by considering interactions of secondaries such as ${\pi }^{\pm }p\to \mathcal{m}X$ and $K^{\pm }p\to \mathcal{m}X$. Our main result is thus an improved description of the atmospheric flux of undecayed mesons produced in air showers. Our tabulated results can be used to evaluate the flux of exotic particles produced by atmospheric meson decays within generic extensions of the standard model.¹ Possible applications include, for instance, the decay of ${\pi }^0$ and $\eta$ mesons into a pair of DM particles through a bosonic mediator [11], and the case of millicharged DM that couples to the Standard Model (SM) via a photon [13]. As an illustration for the application of our atmospheric flux of undecayed mesons, we consider the production of a generic millicharged particle (mCP) and compare our results with those of Ref. [12]. Such particles arise naturally through, e.g., the kinetic mixing between the SM photon and a dark photon [14], [15], [16], [17], [18]. The possible mass-to-charge ratio $m∕\epsilon e$ of models in which the DM is charged are already strongly constrained by astrophysical processes as well as ground based experiments, see e.g. Refs. [19], [20], [21]. However, these limits can be avoided, if the charged component is unstable on cosmological time scales or constitutes only a small part of the total DM abundance. Therefore, DM theories with a sub-dominant charged component in a hidden sector have attracted attention, for recent reviews see Refs. [22], [23]. An additional motivation for such models is the EDGES anomaly which can be explained in a small window in parameter space close to the limits from direct detection experiments [24], [25].

This paper is structured as follows. In Section 2, we first compare the meson production cross sections calculated using various QCD inspired event generators to experimental data, and compute next the atmospheric flux of undecayed neutral mesons. As an example for the applicability of the tabulated fluxes, we re-evaluate the flux of mCPs from atmospheric meson decays in Section 3. Finally, a summary is given in Section 4.