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Exploring Background Contributions in $H \to Z γ$ Decay

Aliaksei Kachanovich

TL;DR

Motivated by a Run 2 excess in $H \to Z\gamma$, the paper reexamines backgrounds in $H \to \ell\ell\gamma$ reconstruction and proposes a BSM-background explanation. It develops two realizations: a dimension-8 EFT operator with $\Lambda_R \sim 260$ GeV and a UV-complete model with a scalar $S$ and a vector-like fermion $\Psi$ that generate the same effect via loop diagrams. The EFT and UV scenarios can reproduce observed yields, including the latest ATLAS result using a narrow $m_{\ell\ell}$ window, while remaining consistent with collider and electroweak constraints; however, distinct differential $m_{\ell\ell}$ spectra are required to discriminate among SM, EFT, and UV explanations. The work underscores the importance of differential measurements and provides testable predictions for future analyses, with potential connections to dark matter phenomenology and precision observables.

Abstract

The rare decay $H \to Zγ$ has been investigated by both the ATLAS and CMS Collaborations, with each reporting an excess in Run~2 in 2023 characterized by $μ= 2.2 \pm 0.7$. This anomaly was initially attributed to possible modifications of the $HZγ$ vertex. However, because the $H \to Zγ$ signal is reconstructed via the $H \to \ell\ellγ$ channel, background effects -- particularly those from processes that mimic the same final state -- may have been underestimated. In this work, we re-examine these backgrounds in detail and propose that the observed excess could be explained by an additional BSM-induced contribution. We present both an effective field theory framework and a UV-complete model that provide the necessary rates and a consistent interpretation of the data. The proposed model reproduces the newest ATLAS result, $μ= 1.3^{+0.6}_{-0.5}$, obtained using a narrower dilepton invariant-mass window.

Exploring Background Contributions in $H \to Z γ$ Decay

TL;DR

Motivated by a Run 2 excess in , the paper reexamines backgrounds in reconstruction and proposes a BSM-background explanation. It develops two realizations: a dimension-8 EFT operator with GeV and a UV-complete model with a scalar and a vector-like fermion that generate the same effect via loop diagrams. The EFT and UV scenarios can reproduce observed yields, including the latest ATLAS result using a narrow window, while remaining consistent with collider and electroweak constraints; however, distinct differential spectra are required to discriminate among SM, EFT, and UV explanations. The work underscores the importance of differential measurements and provides testable predictions for future analyses, with potential connections to dark matter phenomenology and precision observables.

Abstract

The rare decay has been investigated by both the ATLAS and CMS Collaborations, with each reporting an excess in Run~2 in 2023 characterized by . This anomaly was initially attributed to possible modifications of the vertex. However, because the signal is reconstructed via the channel, background effects -- particularly those from processes that mimic the same final state -- may have been underestimated. In this work, we re-examine these backgrounds in detail and propose that the observed excess could be explained by an additional BSM-induced contribution. We present both an effective field theory framework and a UV-complete model that provide the necessary rates and a consistent interpretation of the data. The proposed model reproduces the newest ATLAS result, , obtained using a narrower dilepton invariant-mass window.

Paper Structure

This paper contains 9 sections, 8 equations, 3 figures, 1 table.

Table of Contents

  1. Introduction
  2. The Standard Model
  3. New Physics
  4. Effective Field Theory
  5. UV-complete model
  6. Phenomenology
  7. General results
  8. Phenomenological restriction
  9. Conclusions

Figures (3)

  • Figure 1: Solid red: total with modified $HZ\gamma$; blue dash–dotted: rescaled resonant SM; short–dashed red: SM; brown dashed: non-resonant; long dash–dotted green: tree level. No kinematic cuts were implemented.
  • Figure 2: (a) Contribution of the EFT operator (Eq. \ref{['eq:Lag_EFT']}) to $H \to \ell\ell\gamma$ with $\Lambda_R=260$ GeV, chosen to match the observed excess over the SM. (b) UV-complete benchmarks: $M=m_S=m_{\Psi}=62.5$ GeV (dot–dashed green; solid red) and $M=100$ GeV. The black long-dashed curve shows the EFT result from (a). No kinematic cuts were implemented.
  • Figure 3: Amplitudes for the UV-complete model. Bremsstrahlung from final-state leptons (b, c) is loop- and chirality-suppressed, while the virtual fermion emission (a) reproduces the structure of the effective operator in Eq. \ref{['eq:Lag_EFT']}.