ALP and $Z^\prime$ boson at the Electron-Ion collider

TL;DR

This work investigates the sensitivity of an electron–ion collider to electrophilic beyond-Standard-Model states in the GeV range, focusing on an axion-like particle (ALP) and a heavy neutral gauge boson $Z'$ that couple only to electrons within an EFT framework. Using $e^-p$ collisions at $\sqrt{s}=141$ GeV and $\mathcal{L}=100\,\text{fb}^{-1}$, the authors analyze multiple final states, with tri-electron events providing the strongest ALP constraints and the $3e$ channel offering competitive bounds on a leptophilic $Z'$. They perform detector smearing, background modeling, and Crystal Ball fits to extract mass-resonance signals, translating cross-section limits into limits on $g_{aee}$ and $g_{Z'}$. The results show that the EIC can probe electrophilic ALP and $Z'$ parameter spaces that are weakly constrained by current experiments, particularly in the $m_a$ and $m_{Z'}$ ranges of 10–100 GeV and 10–30 GeV respectively. Overall, the EIC provides a clean, high-luminosity environment capable of significantly extending the reach for GeV-scale electrophilic BSM physics.

Abstract

We study the sensitivity of the upcoming electron-ion (EIC) collider to purely electrophilic new physics in the GeV mass range. Within an effective field theory framework, we consider two different scenarios: an axion-like particle (ALP) and a new heavy neutral vector gauge boson $Z^\prime $, each couples to electrons only. We analyze electron-proton collisions at $\sqrt{s}= 141$ GeV with an integrated luminosity of $100~{\rm fb}^{-1}$, focusing primarily on the tri-electron final state. Additionally, loop-induced ALP-photon couplings driven photon final states are also explored. Incorporating realistic detector effects and systematic uncertainties, we obtain projected exclusion limits on the relevant cross-sections and couplings. We find that the results from EIC can significantly extend the sensitivity to electrophilic axion-like particles and $Z^\prime $ bosons in regions of parameter space that remain weakly constrained by existing experiments.

Figures (17)

• Figure 1: The Feynman diagrams for the signal process, $e^-p\to e^+e^-e^+j$, where ALP is produced (a) off-shell and (b) on-shell in the chosen ALP mass range. The Feynman diagrams for the background process are also shown in (c) and (d).
• Figure 2: The $\mathrm{m}_{ee}$ distribution for the ALP signal and Crystal Ball fits, in blue and red color, respectively. Fits are shown for signal events with $m_a=$ 20, 40, 60, 80, and 100 GeV. As example, $a_{\rm 20~GeV}$ refers to an ALP with mass $m_a=$ 20 GeV.
• Figure 3: Upper limit at $95\%$ CL on (a) signal production cross-section, $\sigma(e^- p\to e^-e^+e^-j)$ and (b) ALP-electron coupling, $g_{aee}$ as a function of ALP mass, $m_a$. The solid blue line corresponds to adding null systematic uncertainty, while dashed and dotted lines include $1\%$ and $5\%$ systematic uncertainties, respectively.
• Figure 4: (a) and (b) The Feynman diagrams for the signal process, $e^-p\to e^-e^+e^-j$ that include the ALP-photon coupling, $g_{a\gamma\gamma}$. (c) The ALP can decay to a photon pair with an electron in the loop.
• Figure 5: The $\mathrm{m}_{ee}$ distribution for the ALP signal and Crystal Ball fits, in blue and red color, respectively. Fits are shown for signal events with $m_a=$ 20, 40, 60, 80, and 100 GeV. As example, $a_{\rm 20~GeV}$ refers to an ALP with mass $m_a=$ 20 GeV.