SND@LHC, or Scattering and Neutrino Detector at the LHC, is designed to study neutrinos –fundamental particles with no electric charge and vanishingly small mass.
Although particle colliders produce neutrinos in huge numbers, no neutrino has ever been directly observed at a collider, because they interact very weakly with matter and are missed by typical collider detectors. Most of the LHC neutrinos lie in an energy range where their interactions have not been studied, which makes their investigation even more interesting.
SND@LHC consists of a neutrino target followed downstream by a device to measure the neutrino energy and to detect muons (the heavier cousins of electrons) that are produced when neutrinos interact with the target.
The apparatus is located underground close to the ATLAS experiment, in an unused tunnel that links the LHC to the Super Proton Synchrotron. Positioned slightly off the LHC’s beamline, it will be able to detect neutrinos produced in the LHC collisions at small angles with respect to the beamline. These angles are larger than those covered by FASERν, a subdetector of the FASER experiment that is also designed to study neutrino interactions at high energies but is located along the beamline.
A large fraction of the SND@LHC neutrinos will come from the decays of particles made of heavy quarks. SND@LHC is therefore also studying heavy-quark particle production in an angular range that the other LHC experiments cannot access.
Moreover, the experiment also searches for weakly interacting particles that are not predicted by the Standard Model and could make up dark matter.
SND@LHC was approved in 2021, constructed, installed and commissioned underground in about one year, and began taking data in Run 3 of the LHC, which started in July 2022.