Novel Search for Light Dark Photon in the Forward Experiments at the LHC

We propose a novel approach for discovering a light dark photon in the forward experiments at the LHC, including the SND@LHC and the FASER experiments. Assuming the dark photon is lighter than twice the electron mass and feebly interacts with ordinary matter, it is long-lived enough to pass through 100 m of rock in front of the forward experiments and also through the detector targets. However, some portion of them could be converted into an electron-positron pair inside the detector through their interaction with the detector target, leaving an isolated electromagnetic shower as a clear new physics signature of the dark photon. With copiously produced dark photons from neutral pion decays in the forward region of the LHC, we expect to observe sizable events inside the detector. Our estimation shows that more than 10 signal events of the dark photon could be observed in the range of kinetic mixing parameter, $6.2\times10^{-5} \lesssim \epsilon \lesssim 2\times10^{-1}$ and $3\times10^{-5} \lesssim \epsilon \lesssim 2\times10^{-1}$ for dark photon mass $m_{A^\prime} \lesssim$ 1 MeV with integrated luminosities of 150 fb$^{-1}$ and 3 ab$^{-1}$, respectively.