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Probing MeV to 90 GeV axion-like particles with LEP and LHC

Joerg Jaeckel, Michael Spannowsky

TL;DR

This work addresses the gap in ALP sensitivity for masses from the MeV to tens of GeV by leveraging LEP $Z$-pole data to constrain ALP couplings to two gauge bosons, including on-shell $Z\to a\gamma$ with $a\to 2\gamma$ and $Z\to 3\gamma$ decays, as well as production via virtual photons. It provides new LEP constraints on the couplings $g_{aBB}$ and $g_{a\gamma\gamma}$ across three ALP-mass regions and demonstrates that LHC searches in the $pp\to Z\to a\gamma\to 3\gamma$ channel can probe $4\lesssim m_a\lesssim 60$ GeV, offering a complementary and strong test of hypercharge- or photon-only couplings. The paper also projects substantial sensitivity improvements at future $e^+e^-$ colliders (FCC-ee, ILC, CEPC), where enormous $Z$-pole statistics could enhance coupling limits by factors of about $30$–$100$, effectively closing the MeV–GeV gap. Overall, the study highlights a concrete path to test ALPs with two-gauge-boson couplings through LEP data, future precision $Z$-factories, and LHC multi-photon final states, underscoring the value of dedicated multi-photon analyses in collider experiments.

Abstract

Axion-like particles (ALPs), relatively light (pseudo-)scalars coupled to two gauge bosons, are a common feature of many extensions of the Standard Model. Up to now there has been a gap in the sensitivity to such particles in the MeV to 10 GeV range. In this note we show that LEP data on $Z\toγγ$ decays provides significant constraints in this range (and indeed up to the $Z$-mass). We also discuss the sensitivities of LHC and future colliders. Particularly the LHC shows promising sensitivity in searching for a pseudo-scalar with $4 \lesssim m_a \lesssim 60$ GeV in the channel $pp \to 3 γ$ with $m_{3γ}\approx m_{Z}$.

Probing MeV to 90 GeV axion-like particles with LEP and LHC

TL;DR

This work addresses the gap in ALP sensitivity for masses from the MeV to tens of GeV by leveraging LEP -pole data to constrain ALP couplings to two gauge bosons, including on-shell with and decays, as well as production via virtual photons. It provides new LEP constraints on the couplings and across three ALP-mass regions and demonstrates that LHC searches in the channel can probe GeV, offering a complementary and strong test of hypercharge- or photon-only couplings. The paper also projects substantial sensitivity improvements at future colliders (FCC-ee, ILC, CEPC), where enormous -pole statistics could enhance coupling limits by factors of about –, effectively closing the MeV–GeV gap. Overall, the study highlights a concrete path to test ALPs with two-gauge-boson couplings through LEP data, future precision -factories, and LHC multi-photon final states, underscoring the value of dedicated multi-photon analyses in collider experiments.

Abstract

Axion-like particles (ALPs), relatively light (pseudo-)scalars coupled to two gauge bosons, are a common feature of many extensions of the Standard Model. Up to now there has been a gap in the sensitivity to such particles in the MeV to 10 GeV range. In this note we show that LEP data on decays provides significant constraints in this range (and indeed up to the -mass). We also discuss the sensitivities of LHC and future colliders. Particularly the LHC shows promising sensitivity in searching for a pseudo-scalar with GeV in the channel with .

Paper Structure

This paper contains 8 sections, 12 equations, 5 figures, 1 table.

Table of Contents

  1. Introduction
  2. Searching ALPs at LEP with $Z\to2\gamma$ and $Z\to 3\gamma$
  3. ALPs from $Z$ decays
  4. ALPs from production via virtual photons
  5. ALPs at LHC and Future Colliders
  6. Future electron-positron machines
  7. Testing the hypercharge coupling with LHC
  8. Conclusions

Figures (5)

  • Figure 1: Limits on the axion-like particle to two photon coupling. Figure slightly adapted from Alekhin:2015byh which is a compilation adapted from Redondo:2008enJaeckel:2010ni updated with Cadamuro:2011fdHewett:2012nsJaeckel:2012yzMimasu:2014neaPayez:2014xsaMillea:2015qra. Note the gap in the MeV to 10 GeV region.
  • Figure 2: Production of ALPs with subsequent decay into two photons. Left panel:$a+\gamma$ production via a virtual photon and subsequent decay to $3\gamma$. Right panel: production of an on-shell $Z$ and subsequent decays into $a+\gamma$ and then $3\gamma$.
  • Figure 3: $\Delta R$ separation of the two closest photons for different values of $m_a$ in the process $e^+e^- \to Z \to a \gamma \to 3 \gamma$. The black vertical lines correspond to $\Delta R = 4m_a/m_Z$.
  • Figure 4: Left panel: Limits on a coupling to two hypercharge bosons. Right panel: Limits on a coupling only to photons. The new LEP limits from 2 and 3 photon signatures are shaded in green and enclosed by dashed and solid black lines, respectively. The future FCC-ee limit is indicated by the red solid line. Our projected LHC sensitivity for 13 TeV and 100 fb$^{-1}$ by the blue line (only applicable to the coupling to hypercharge bosons). The rest of the figure is adapted from Alekhin:2015byhRedondo:2008enJaeckel:2010niCadamuro:2011fdHewett:2012nsJaeckel:2012yzMimasu:2014neaPayez:2014xsaMillea:2015qra.
  • Figure 5: Transverse momentum distributions for the hardest, second and third hardest photons (left). $\Delta R$ separations of the three photons (right). We choose $m_a = 4$ GeV (upper panels), $m_a = 20$ GeV (middle panels) and $m_a=60$ GeV (lower panels).