Since FASER is installed in a side tunnel of the LHC (about 500m along the LHC tunnel from ATLAS) access to the area is only possible when there is no other work going on in that sector of the LHC. To take into account the access constraints two month-long windows for the FASER installation were agreed with the LHC coordination – the first, for the installation of the FASER magnets and other infrastructure, in November 2020, and the second for the installation and commissioning of the detector components in March 2021.
During the November access period, the three FASER permanent magnets (two of 1m-long/1-tonne and the third 1.5m-long/1.5-tonne) were transported into the LHC tunnel to the FASER location. The magnets needed to be carried over the LHC machine and then installed into the FASER trench in TI12 using a dedicated crane that was installed specifically for the FASER installation. Once they were installed, the magnets were precisely aligned with the beam collision axis with help of the CERN survey team. During this access the FASER cooling unit (designed and constructed by the CERN cooling and ventilation group (EN-CV)) was also installed and tested in the tunnel. Finally, this access was used to lay the cables from the FASER electrical racks at the end of TI12 to the detector location – this cabling was done by colleagues from the EN-EA group. Figure 1 shows the TI12 tunnel at the end of the November 2020 installation window.
Fig.2: Commissioning of the FASER detector in TI12 during the March 2021 access window.
The FASER collaboration has detected the first candidate particle interactions for neutrinos produced in LHC collisions. The result, described in a paper posted online, paves the way for studies of high-energy neutrinos at current and future colliders.
After analysing the pilot detector data and estimating a background of particle events that could mimic the signal from neutrino interactions, the FASER team found several candidate events for collider neutrinos. The result has a statistical significance of 2.7 standard deviations, a little below the 3 standard deviations required to claim evidence of a particle or process in particle physics. “The goal of the pilot detector was to demonstrate the feasibility of neutrino measurements in the experimental environment of the LHC,” says FASER co-spokesperson Jamie Boyd. “So we are very excited that this small detector, which is only about 1% of the final detector, allowed us to see the first candidate events for neutrino interactions at a collider.”
The team expects to observe about 20 000 collider neutrino interactions with the full-fledged FASERν detector in the next LHC run, from 2022 to 2024.
Image: Two candidate events for neutrinos produced in LHC collisions and interacting in the FASERν pilot detector. The neutrinos enter the detector from the left, and interact with the detector material to produce a number of charged particles. The different lines in each event show tracks from these charged particles, originating from the neutrino interaction point. (Image: FASER/CERN)