Speaker
Description
In March 2025 the muon system of the SND@LHC detector was upgraded with Mini Drift Tubes modules. The contribution will describe the new detectors, smaller versions of CMS Drift Tubes, and the electronics for the triggerless time-to-digital conversion and readout, based on the OBDT-theta board for the Phase2-upgrade of CMS Drift Tubes and on a VCU118 board as a backend system. The integration into the control system and DAQ of the SND@LHC experiment will be shown, together with performance results, from the initial commissioning to muon reconstruction in LHC data.
Summary (500 words)
SND@LHC is a stand-alone experiment for the detection of neutrinos produced in pp collisions at the LHC. Precise muon tracking is crucial for the detector operations, to estimate the background in neutrino physics analysis, and for searches of rare events with muons in the final state.
To improve muon identification, which was only based on planes of 1-cm wide scintillating bars, two Mini Drift Tubes (MiniDT) modules, smaller versions of CMS DTs, were added to SND@LHC in March 2025. These modules provide additional x-y (transverse plane) position and direction measurements, with resolutions of about 150 um and 10 mrad, respectively.
MiniDTs are equipped with a custom time-to-digital conversion board, called OBDT-theta, designed and produced for the Phase-2 upgrade of CMS DT. The OBDT-theta measures hit times in steps of 0.78 ns and in triggerless mode, referencing time to the beginning of LHC orbits. A backend system based on a Xilinx VCU118 evaluation board was developed. Thanks to a custom-designed mezzanine and a CERN HPTC board, the backend receives the Timing, Trigger and Control (TTC) LHC signals, and its firmware, developed in CIEMAT, handles the OBDT-theta board lpGBT links and the buffering and readout of hits. The firmware also performs online muon reconstruction, measuring the track crossing time, position and impact angle. The net available bandwidth through a gigabit ethernet interface was measured to be up to 750 Mbps, corresponding to a hit rate of up to 11 MHz. The electronics set up is completed by a BeagleBone module for remote reprogramming of the FPGA and configuration of the MiniDT frontend.
The large drift time within MiniDT cells, up to ~400 ns, prevents online event building within the SND@LHC DAQ, which is based on 25 ns time windows. A separate control and readout server independently stores raw data, while full event building, merging MiniDT hits into SND@LHC events, is performed at the offline stage. A control and monitoring software is interfaced with the SND@LHC Experiment Control System, while the mutual synchronization of the two DAQs streams is based on a bunch-crossing-zero signal issued at the start of each run.
The system was very stable and efficient during operations. Excluding external events such as power cuts or network connection issues, the system down time was negligible and no data losses due to readout issues were detected across 2025 and 2026 data taking. Hit rates during LHC stable beams are about 35 kHz, well below the readout bandwidth limit. The detector hit efficiency, measured with tracks reconstructed using the legacy muon detectors, was found to be above 97%, in line with expectations and providing an efficiency for standalone tracking of about 95%.
The contribution explains how the improvement of the muon system enhances the SND@LHC physics potential and describes MiniDTs control and readout electronics and its integration in the detector, DAQ and control systems. Finally, selected performance results will be shown, from the initial system commissioning to muon track reconstruction from LHC data.