Potential for the discovery of the protophobic boson at the STCF

TL;DR

The paper tackles the search for a 17 MeV protophobic X-boson motivated by ATOMKI anomalies. It uses the TrackEff framework to model STCF's main drift chamber and to assess displaced-vertex sensitivity across the X-boson mass–coupling parameter space, including both visible and dark-sector decay scenarios. A 5σ discovery reach is mapped by combining detector efficiency with luminosity across CoM energies, showing that STCF could observe the X-boson in certain regions while tolerating substantial backgrounds (up to ~$10^4$ events) for favorable widths; more stringent background control is needed if the dark width dominates. This work constitutes a first feasibility assessment for displaced light-boson searches at STCF and motivates a full Geant-4 detector simulation to validate the results and inform detector design choices.

Abstract

We study the morphology of the main drift chamber (MDC) proposed to be built around the collision point at the upcoming Super tau-charm facility (STCF), to check for its suitability in discovering the 17 MeV protophobic boson (X17 boson), hypothesised as a solution to the persistent ATOMKI nuclear-transition anomalies. These anomalies, observed in the excited $^8$Be, $^4$He, $^{12}$C, $^{16}$O nuclear transitions, have been interpreted as evidence for a $\sim$17 MeV, protophobic vector boson. Using the TrackEff framework, we perform detector-level simulations of the STCF MDC, and evaluate displaced-vertex sensitivities towards the protophobic boson, across the relevant mass-coupling parameter space. We study benchmark scenarios with visible and dark decay channels to perform likelihood-based significance estimates in order to determine the 5$σ$ discovery reach for the protophobic boson. We find that STCF can discover the protophobic boson while tolerating $\sim 10^4$ background events for specific regions of the parameter space around the 17 MeV peak. Our analysis establishes the first feasibility study of displaced light-boson searches at the STCF, motivating a full Geant-4 simulation.

Figures (11)

• Figure 1: A compilation of the different results in support of the $X$-boson. The different experiments are indicated by the year of publication and the nucleus undergoing transition. Note, for the data points where error-bars are not given, we have taken the error to be larger than our scope of analysis. Details are given in the text.
• Figure 2: We plot the partial widths and the branching ratios of the $X$ boson, for $\Gamma_\mathrm{DS}=0$. The top panel shows the partial widths and the bottom panel shows the branching ratios. The separate channels we consider are to taus, hadrons, muons, and electrons. We note, for $\Gamma_\mathrm{DS}=0$, the width is completely saturated by the electron channel below the muon mass.
• Figure 3: The $t$- and $u$-channel processes contributing to the production of the $X$-boson.
• Figure 4: Left: We plot the differential cross section as a function of the scattering angle, $\theta$ for $m_X=$ 17MeV, $g^\prime=10^{-5}, \sqrt{s}=$ 4GeV. We plot the cross sections generated from MC simulations in MG5_aMC, and calculated from \ref{['eq:CS1']}. We have indicated the lower and upper angular cuts of $\theta=20^\circ$ and $\theta=160^\circ$ that we impose at analysis level. Right: We plot the total cross section as a function of CoM energies.
• Figure 5: Cross-sectional (right) and longitudinal (left) views of the STCF main drift chamber. We show the relevant lengths in both the views (all lengths in mm). In the cross-sectional view we show the different superlayers in the MDC with both axial (A) and stereo (U,V) arrangements.