Long-lived axion-like particles from electromagnetic cascades

TL;DR

The paper demonstrates that electromagnetic cascades in beam-dump targets substantially enhance long-lived axion-like particle production and decay yields, altering sensitivity estimates for SHiP and BDX. By developing ALPETITE (built on PETITE) to reweight SM showers into ALP fluxes and by incorporating Primakoff, bremsstrahlung, resonant annihilation, and Compton-like channels, the authors show order-of-magnitude (up to ~$10^4$ in some regimes) improvements, especially at low masses and couplings. They present updated SHiP and new BDX projections, highlighting that cascade-driven channels can dominate over primary production and extend reach to unexplored parameter space, including $g_{a\gamma\gamma} \lesssim 2\times10^{-7}\,\text{GeV}^{-1}$ for $m_a \gtrsim 70$ MeV. The work emphasizes that electromagnetic cascades are a critical component of accurate ALP sensitivity and suggests revisiting historical analyses and lowering energy thresholds to maximize discovery potential in upcoming experiments.

Abstract

We study axion-like particles (ALPs) in beam dump experiments, focusing on the Search for Hidden Particles (SHiP, at CERN) experiment and the Beam Dump eXperiment (BDX, at JLab). Many existing projections for sensitivity to ALPs in beam dump experiments have focused on production from either the primary proton/electron beam, or - in the case of SHiP - the secondary (high-energy) photons produced by neutral meson decays (e.g., $π^0\rightarrowγγ$). In this work, we study the subsequent production of axions from the full electromagnetic shower in the target, finding order-of-magnitude enhancements in the visible decay yields across a wide range of axion masses. We update SHiP's sensitivity curve and provide new projections for BDX. Both experiments will be able to reach currently unexplored regions of ALP parameter space.

Figures (9)

• Figure 1: Comparison to past literature -- PBC Beacham:2019nyx (green shaded region), ALPINIST Jerhot:2022chi (red shaded region) -- of our projections for the sensitivity of SHiP (solid black line) to axion-photon couplings, with updated geometry and nominal protons-on-target (POT) Aberle:2839677Albanese:2878604. For reference, we also plot our sensitivity projections with a lower POT, used before in the literature (dashed black line). We defined $g_{a\gamma\gamma}\equiv f^{-1}c_{a\gamma\gamma}\alpha/\pi$, and we set $c_{aee}=0$. Accounting for the full electromagnetic cascade substantially enhances the reach of SHiP at lower couplings and lighter masses. See \ref{['fig:ship_ecuts_sens']} for an illustration of how this result depends on the energy threshold used at SHiP.
• Figure 2: Event rate of axions per protons-on-target (POT), weighted by $f^4$, for an ALP with mass $m_a=100$ MeV and photon-dominated couplings. The setup simulates SHiP photons emerging from meson decays impinging on a molybdenum target, with the corresponding SHiP experimental geometry. The flux is calculated for ALPs directed towards the detector. The individual production channels are: Primakoff conversion from shower photons (green); bremsstrahlung from shower electrons and positrons (orange); $e^+e^-$ annihilation and Compton-like scattering of shower $e^\pm$ (blue); Primakoff conversion from the primary photon beam from neutral meson decays (red).
• Figure 3: Event rate of axions produced per electrons-on-target (EOT), weighted by $f^4$, for an ALP with mass $m_a=100$ MeV and electron-dominated couplings. The setup simulates a 10.6 GeV electron beam on an aluminum target, with a geometry representative of the BDX experiment. The flux is restricted to ALPs with trajectories pointing towards the forward detector. The contributions from different production mechanisms are shown separately: Primakoff conversion from shower photons (green); bremsstrahlung from shower electrons and positron (orange); $e^+e^-$ annihilation and Compton-like scattering from shower particles (blue); bremsstrahlung (red) and photon fusion (purple) from the primary electron beam.
• Figure 4: Comparison between the primary-only and the shower-induced coupling-independent weighted event rates in the LLP limit for different axion masses for SHiP (left) and BDX (right). As above, the axion count has been weighted by decay (in the LLP limit), geometry, and energy acceptance. In both panels, as the mass is increased, the ability of softer shower particles to produce axions is diminished, and production rates from the showers becomes smaller (in the case of SHiP) than the primary-only estimates for masses greater than $1~\text{GeV}$.
• Figure 5: Projected sensitivity for 5 signal events for the SHiP experiment for the electron-dominated (left) and photon-dominated (right) ALP coupling scenarios, assuming an integrated $6 \times 10^{20}$ POT. The green shaded region shows the sensitivity for ALPs generated by primary photons arising from neutral meson decays. The solid lines show the individual contributions from shower-induced processes: Primakoff (orange), bremsstrahlung (blue), and resonant annihilation/Compton scattering (purple). The total sensitivity, combining primary and shower contributions, is shown by the dashed black line, with the gray shaded area indicating the full parameter space covered. For the photon-dominated case, we do not show the bremsstrahlung, and resonant annihilation/Compton scattering lines since they are sub-dominant with respect to all the other processes.