Searching for a new force at VEPP-3

TL;DR

This work proposes a missing-mass search for a light gauge boson $A'$ in $e^+e^-$ annihilation on an internal hydrogen target at VEPP-3, using a 500 MeV positron beam to access $m_{A'}$ in the $5{-}20$ MeV range. By detecting a single forward photon from $e^+e^-\to \gamma A'$ and reconstructing the missing mass, the method is independent of the $A'$ decay channels or lifetime. With a target luminosity of $L\sim10^{32}$ cm$^{-2}$s$^{-1}$ and a six-month run, the experiment aims to reach $|f_{eA'}|^2$ at the level of $\mathcal{O}(10^{-9})$–$\mathcal{O}(10^{-8})$ at $m_{A'}\approx15$ MeV, improving current constraints by up to three orders of magnitude. The approach combines a dedicated beam-chicane, an internal hydrogen target, a forward calorimeter (PbWO$_4$/CsI(Tl) hybrid), and a positron-veto system to suppress QED backgrounds by factors of 50–100, enabling a feasible, low-luminosity search with significant discovery potential for light dark-sector gauge bosons.

Abstract

We propose an experiment to search for a new gauge boson, A$^\prime$, in $e^+e^-$ annihilation by means of a positron beam incident on a gas hydrogen target internal to the VEPP-3 storage ring. The search method is based on a missing mass spectra in the reaction $e^+e^-\rightarrow γ$ A$^\prime$. It allows observation of the A$^\prime$ signal independently of its decay modes and life time. The projected result of this experiment corresponds to an upper limit on the square of coupling constant $|f_{e\text{A}{^\prime}}|^2=1\cdot 10^{-8}$ with a signal-to-noise ratio of five to one at an A$^\prime$ mass of 15 MeV.

Figures (9)

• Figure 1: Two-dimensional distribution of the photon events in the scattering angle and the photon energy for a 500 MeV positron beam incident on a hydrogen gas target. The black band shows the location of A$^\prime$-boson~events of 15 MeV mass.
• Figure 2: The diagrams of a) two-photon annihilation, b) the A$^\prime$-boson$-\gamma$ production. and the kinematics of a) $e^+e^- \rightarrow \gamma + \gamma$ and b) $e^+e^- \rightarrow {\text{A}}^\prime + \gamma$ reactions.
• Figure 3: Kinematic correlations for positron-electron annihilation at E$_+=500$ MeV. Left panel: photon polar angle in CM frame vs. that in Lab frame for two--photon annihilation. Right panel: photon energy vs. its polar angle for $e^+e^-\rightarrow \gamma\gamma$ and for $e^+e^-\rightarrow \gamma \text{A}^\prime$. Dotted vertical lines indicate the range covered in the proposed measurements.
• Figure 4: Expected background photon rates at beam energy E$_+=500$ MeV and luminosity $L=10^{32} cm^{-2}s^{-1}$. Left panel: from positron-electron annihilation. Dotted vertical lines indicate the range covered in the proposed measurements. Right panel: from positron bremsstrahlung on hydrogen. At a 50 MeV threshold, the expected rate for the proposed detector configuration is $1.5\cdot 10^4~s^{-1}$.
• Figure 5: The layout of the proposed experiment at VEPP--3