High-precision measurement of the space-like ${\eta }^{\prime }$ transition form factor

Abstract

Using a data sample corresponding to an integrated luminosity of $20.3{\text{fb}}^{-1}$, collected with the BESIII detector at a center-of-mass energy of $3.773\text{GeV}$ at the BEPCII collider, we report a precision measurement of the product $Q^2|F(Q^2)|$, where $F(Q^2)$ is the single-virtual space-like transition form factor of the ${\eta }^{\prime }$ meson and $Q^2$ is the squared momentum transfer of the tagged virtual photon. The transition form factor is extracted from the differential Born cross section of the two-photon fusion processes $e^{+}{e}^{-}\to e^{+}{e}^{-}\gamma {\gamma }^{\ast }\to e^{+}{e}^{-}{\eta }^{\prime }$ using a single-tag technique, where only one scattered lepton is detected. The measurement covers $Q^2\in [0.1,6.0]$ GeV${}^2$, achieving unprecedented precision, better than $3.0\%$ for $Q^2<1.5$ GeV${}^2$, and providing the first direct determination at $Q^2<0.3$ GeV${}^2$.