Searching for the light dark gauge boson in GeV-scale experiments

TL;DR

This work investigates a GeV-scale U(1)_d gauge boson that kinetically mixes with the Standard Model and couples to the EM current with small $\epsilon$. It combines collider, meson-decay, and fixed-target perspectives to map current constraints and future discovery potential, highlighting that fixed-target experiments can achieve the most comprehensive coverage of the $\epsilon$–$m_U$ parameter space, including regions relevant for dark matter self-interactions. The analysis provides concrete reach estimates across BaBar/Belle, KLOE, and meson decays, and outlines pragmatic fixed-target designs capable of reaching $\epsilon$ as small as $10^{-5}$–$10^{-6}$, depending on $m_U$. Overall, the paper argues for a targeted fixed-target program to exhaustively probe the U-boson hypothesis in the GeV range, complementing collider and meson-decay searches.

Abstract

We study current constraints and search prospects for a GeV scale vector boson at a range of low energy experiments. It couples to the Standard Model charged particles with a strength <= 10^-3 to 10^-4 of that of the photon. The possibility of such a particle mediating dark matter self-interactions has received much attention recently. We consider searches at low energy high luminosity colliders, meson decays, and fixed target experiments. Based on available data, searches both at colliders and in meson decays can discover or exclude such a scenario if the coupling strength is on the larger side. We emphasize that a dedicated fixed target experiment has a much better potential in searching for such a gauge boson, and outline the desired properties of such an experiment. Two different optimal designs should be implemented to cover the range of coupling strength 10^-3 to 10^-5, and < 10^-5 of the photon, respectively. We also briefly comment on other possible ways of searching for such a gauge boson.

Figures (5)

• Figure 1: Reach at BaBar, defined to be the value of $\epsilon$ at which $S/\sqrt{B} = 5$, as a function of the $U$ mass. At left, the reach for $e^+e^-\gamma$, $\mu^+\mu^-\gamma$, $\pi^+\pi^-\gamma$, channels; at right, the same figure with a zoom into the $m_U \leq 1.5$ GeV region. Note that muons are a much better search channel than electrons at larger $U$ masses, which is due to the large background from Bhabha scattering.
• Figure 2: Reach for $U \to e^+ e^-$ at KLOE in the process $\phi \to \eta U$. At left: reach in $e^+e^-$. The upper (purple) curve is for constant form factor $F_{\phi\eta\gamma^*}(q^2) = 1$, whereas the lower (blue) curve is for the single-pole fit $F_{\phi\eta\gamma^*}(q^2) = 1/(1 - 3.8 {\rm GeV}^{-2} q^2)$ from Ref. Achasov:2000ne. At right: The blue curve is the reach in $e^+ e^-$ (with single-pole form factor fit) and the gray curve is the corresponding reach in $\mu^+ \mu^-$.
• Figure 3: Schematic of a possible fixed-target experiment to search for the U-boson. An electron beam impacts a fixed target, with beam intensity and target thickness designed to produce about one $\mu^+ \mu^-$ pair per bunch. Tracks emerging from the target are measured in a magnetic field. An absorber stops electrons and photons emerging from the target, while a more distant detector tags the muons.
• Figure 4: The value of $\epsilon$ at which the decay length $c\tau$ becomes 1 millimeter, as a function of the U-boson mass $m_U$. The dashed horizontal line at 10$^{-5}$ indicates roughly the lowest $\epsilon$ we could hope to probe with a fixed-target experiment designed for prompt decays.
• Figure 5: Resolution in invariant mass at BaBar, as a function of $m(e^+ e^-)$ at left and of $m(\mu^+ \mu^-)$ at right.