Search for a muonic dark force at BABAR

TL;DR

The paper searches for a muon-specific Z' boson via direct production in $e^+e^- \to \mu^+\mu^- Z', Z' \to \mu^+\mu^-$ using $514 fb^{-1}$ of data from BABAR, spanning $0.212-10$ GeV in mass. Signal candidates are defined by two oppositely charged muon pairs and the reduced dimuon mass $m_R = \sqrt{m_{\mu^+\mu^-}^2 - 4 m_\mu^2}$ scanned over 2219 hypotheses; the dominant background is $e^+e^- \to \mu^+\mu^-\mu^+\mu^-$. An unbinned maximum-likelihood fit is used to extract yields; no significant Z' signal is observed, with the largest local significance $4.3\sigma$ and a global significance of $1.6\sigma$. 90% CL upper limits on the cross section are translated into bounds on the muon-sector coupling $g'$ for models with equal vector couplings to $\mu$, $\tau$, and $\nu$, reaching down to $7\times 10^{-4}$ near threshold, thereby improving neutrino-based constraints and excluding most of the parameter space above the dimuon threshold.

Abstract

Many models of physics beyond the Standard Model predict the existence of new Abelian forces with new gauge bosons mediating interactions between "dark sectors" and the Standard Model. We report a search for a dark boson Z' coupling only to the second and third generations of leptons in the reaction e+e- -> mu+mu- Z', Z' -> mu+mu- using 514 fb-1 of data collected by the BABAR experiment. No significant signal is observed for Z' masses in the range 0.212 - 10 GeV. Limits on the coupling parameter g' as low as 7x10^-4 are derived, leading to improvements in the bounds compared to those previously derived from neutrino experiments.

Figures (5)

• Figure 1: The distribution of the four-muon invariant mass, $m(4\mu)$, for data taken at the $焇{(4S)}\xspace$ peak together with Monte Carlo predictions of various processes normalized to data luminosity. The $e^+e^- \rightarrow \mu^+\mu^- \mu^+\mu^-$ Monte Carlo does not include ISR corrections.
• Figure 2: The distribution of the reduced dimuon mass, $m_R$, together with Monte Carlo predictions of various processes normalized to data luminosity. Four combinations per event are included. The fit of the ratio between reconstructed and simulated events is shown as a light blue dashed line. The $e^+e^- \rightarrow \mu^+\mu^- \mu^+\mu^-$ Monte Carlo does not include ISR or other efficiency corrections (see text).
• Figure 3: The measured $e^+e^- \rightarrow \mu^+\mu^- Z', Z' \rightarrow \mu^+\mu^-$ cross-section together with its statistical significance, $S_S$ (see text for definition), as a function of the $Z'$ mass. The uncertainty on each point is shown as light gray error bars. The dark gray band indicates the region excluded from the analysis.
• Figure 4: The 90% CL upper limits on the cross-section $\sigma(e^+e^- \rightarrow \mu^+\mu^- Z', Z' \rightarrow \mu^+\mu^-)$ as a function of the $Z'$ mass. The dark gray band indicates the region excluded from the analysis.
• Figure 5: The 90% CL upper limits on the new gauge coupling $g'$ as a function of the $Z'$ mass, together with the constraints derived from the production of a $\mu^+\mu^-$pair in $\nu_\mu$ scattering ("Trident" production) Altmannshofer:2014pbaKamada:2015era. The region consistent with the discrepancy between the calculated and measured anomalous magnetic moment of the muon within $2\sigma$ is shaded in red.