Limit on the production of a light vector gauge boson in phi meson decays with the KLOE detector

Abstract: We present a new limit on the production of a light dark-force mediator with the KLOE detector at DAPHNE. This boson, called U, has been searched for in the decay phi --> eta U, U --> e+ e-, analyzing the decay eta --> pi0 pi0 pi0 in a data sample of 1.7 fb-1. No structures are observed in the e+e- invariant mass distribution over the background. This search is combined with a previous result obtained from the decay eta --> pi+ pi- pi0, increasing the sensitivity. We set an upper limit at 90% C.L. on the ratio between the U boson coupling constant and the fine structure constant of alpha'/alpha < 1.7x10^-5 for 30<M_U<400 MeV and alpha'/alpha < 8x10^-6 for the sub-region 50<M_U<210 MeV. This result assumes the Vector Meson Dominance expectations for the phietagamma^* transition form factor. The dependence of this limit on the transition form factor has also been studied.

Limit on Light Vector Gauge Boson Production in $\phi$ Meson Decays

The paper presented by the KLOE-2 Collaboration explores the potential production of a light vector gauge boson, designated as $U$, within the decay processes of $\phi$ mesons. This investigation is conducted at the DA$\Phi$NE $\phi$-factory using the KLOE detector, focusing on the decay $\phi \rightarrow \eta U$, where $U$ decays into electron-positron pairs ($U \rightarrow e^+ e^-$).

Objectives and Context

The theoretical framework behind this study is rooted in hypotheses extending beyond the Standard Model (SM), which posit the existence of new mediators linked to hidden gauge symmetries. These theories propose weakly interacting bosons that could account for anomalies in astrophysical observations. The $U$ boson is hypothesized to interact through kinetic mixing with photons, characterized by a small coupling parameter $\epsilon$. Prior searches at various $e^+ e^-$ and $e$-$p$ facilities including studies at the SND experiment have yielded no conclusive evidence, necessitating further exploration through meson decay channels.

Methodology

The research utilizes a comprehensive data sample of 1.7 fb$^{-1}$, corresponding to $6 \times 10^9$ produced $\phi$ mesons. Two primary decay processes were examined: $\eta \rightarrow \gamma \gamma$ and $\eta \rightarrow \pi^+ \pi^- \pi^0$. These processes were analyzed through simulated Monte Carlo models, incorporating a Vector Meson Dominance (VMD) framework to account for the $\phi \eta \gamma^*$ transition form factor. Emphasis was placed on refining background rejection techniques and enhancing the sensitivity of the search compared to previous results.

Results

No discernible structures were observed within the $e^+ e^-$ invariant mass distribution across the investigated range of 5 to 470 MeV. The analysis yields an upper limit at the 90% confidence level on the boson coupling ratio $\alpha'/\alpha$ to the fine structure constant. Specifically, constraints were placed at less than $1.7 \times 10^{-5}$ for $30 < M_U < 400$ MeV and refined to $8 \times 10^{-6}$ within the $50 < M_U < 210$ MeV sub-region. These results align with theoretical expectations under VMD predictions and notably enhance previous limits attained in such experimental setups.

Implications and Future Work

This research contributes significant findings towards constraining the parametric space for the existence of the $U$ boson, particularly in relation to discrepancies in the muon anomalous magnetic moment ($a_\mu$) measurements, disfavoring $U$ boson masses in the range of 60–435 MeV. Future endeavors may expand upon these results by exploring additional decay channels or enhancing detector capabilities to further refine statistical significance. The work sets a precedent for investigations into hidden sector physics and offers a pivotal baseline for theoretical considerations in extending SM predictions.

By leveraging methodologies and frameworks demonstrated herein, subsequent research could deepen understanding of dark matter interactions and underlying symmetry-breaking phenomena. Continued exploration in this domain holds potential for unveiling novel insights into particle physics and contributing to the broader quest of understanding fundamental forces governing matter.