Search for new physics effects in $ν\barνγ$ production at a Tera-Z factory

TL;DR

The paper addresses probing new physics in rare Z decays, specifically $Z\to\nu\bar{\nu}\gamma$, using the forthcoming Tera-Z runs at FCC-ee/CEPC. It employs an EFT framework with dimension-6 and dimension-8 operators to parameterize non-standard $Z\nu\bar{\nu}\gamma$ couplings and performs a full detector-level study with a pipeline of UFO models, MadGraph5_aMC@NLO, Pythia, and Delphes to simulate $e^+e^-\to Z\to\nu\bar{\nu}\gamma$ and relevant backgrounds at the $Z$-pole. The analysis, focused on kinematic handles such as $E_\gamma$, missing $E_T$, and $S_{\slashed{E}_T}$, projects upper limits on the anomalous couplings $\kappa/\Lambda^2$ and $\alpha_8/\Lambda^4$ translating to BR$(Z\to\nu\bar{\nu}\gamma)$ of order $10^{-9}$—improving LEP bounds by several orders of magnitude. The results highlight the strong discovery potential of the Tera-Z program for loop-level SM tests and for constraining or revealing new neutrino–photon interactions, with significance sensitive to systematic uncertainties.

Abstract

Rare decays of the Z boson provide a sensitive probe for physics beyond the Standard Model (SM). This study investigates the $e^{+}e^{-} \to Z \to ν\barνγ$ process within the context of the Tera-Z programmes at future colliders such as the FCC-ee and CEPC. The SM predicts a one-loop branching ratio of $7.16 \times 10^{-10}$ for $Z \to ν\barνγ$, a value four times smaller than the current experimental limit from the LEP. To explore this window for new physics, we parameterize anomalous $Zν\barνγ$ interactions using an Effective Field Theory framework, considering both dimension-6 and dimension-8 operators. A detailed simulation is performed by generating signal and background events with MadGraph, modeling particle showers with Pythia, and simulating detector effects with Delphes. The analysis employs key kinematic variables-including the photon energy ($E_γ$), missing transverse energy ($\not{E}_T$), and the missing transverse energy significance ($S_{\not{E}_T}$) to isolate the signal. The results yield upper limits on the anomalous couplings, from which we infer branching ratios for $Z \to ν\barνγ$ on the order of $10^{-9}$. This represents a significant improvement of several orders of magnitude over the LEP sensitivity. Consequently, this study demonstrates the unique potential of the Tera-Z runs not only to test the SM loop-level predictions with unprecedented precision but also to tightly constrain or reveal new anomalous interactions.

Figures (9)

• Figure 1: Representation of $Z\nu \bar{\nu}\gamma$ vertex (red dot) in the Effective Lagrangian approach.
• Figure 2: The dimension-6 ($\kappa/\Lambda^2=\kappa_1/\Lambda^2=\kappa_2/\Lambda^2$) and dimension-8 ($\alpha_8/\Lambda^4$) couplings couplings to the minimum energy distribution of a single photon for different values of the branching ratio of the $Z\to\nu \bar{\nu}\gamma$ decay
• Figure 3: BR($Z \to \nu \bar{\nu}\gamma$) decay as a function of dimension-6 ($\kappa_1/\Lambda^2$ and $\kappa_2/\Lambda^2$) and dimension-8 ($\alpha_8/\Lambda^4$) couplings. Here, $\Lambda$ is taken equal to 1 TeV.
• Figure 4: Representative Feynman diagrams for the $e^+e^- \to \nu \bar{\nu}\gamma$ process: (a) represents the new physics signal contribution, while diagrams (b)–(d) correspond to the SM background processes contributing to the same final state.
• Figure 5: The total cross sections of the process $e^+e^-\to\nu\bar{\nu}\gamma$ as a function of center-of-mass energy for different photon energies.