Short-Baseline Near Detector

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Short-Baseline Near Detector (SBND) is a liquid argon time projection chamber (LArTPC) experiment at Fermilab in Batavia, Illinois. SBND is the near detector for the Short-Baseline Neutrino (SBN) program, which includes the ICARUS experiment as the far detector and MicroBooNE lies in between. Like ICARUS and MicroBooNE, SBND lies along the Booster Neutrino Beam (BNB), which uses proton collisions on a beryllium target to produce a concentrated neutrino beam that is then sent through each detector. By comparing measurement results at the various detectors along the beamline, detailed studies of
ν
_e appearance and
ν
_μ disappearance can be done through the SBN program. SBND is the newest detector in the SBN program, coming online in December 2024, with plans to take data until early 2028.

Design

File:RealSchematicTPC.png

Schematic of LArTPC charge collection principle

SBND is a LArTPC with an active volume of 4 × 4 × 5 meters and active mass 112 tons, lying along the BNB beamline 110 m from the target. The SBND active volume is split into two drift volumes, with two electronics readout anode plane assemblies (APAs) separated by 200 cm from the cathode plane in between. Each APA has two induction planes and one collection plane at a wire-pitch of 3 mm, contributing to a total of 11,264 wires. Since SBND is a surface level detector, the rate of crossing cosmic ray muons will be much higher than similar experiments run underground. In order to understand this cosmic muon background better, a Cosmic Ray Tagger (CRT) system consisting of plastic scintillator panels with optical fibers inside, which are then coupled to silicon photo multipliers (SiPM) for light detection.

File:Groeeser.Hamamatsu.Photomultiplier.jpg

Example of Hamamatsu PMT

Along with charge readout, SBND also includes a sophisticated photon detection system (PDS). This PDS includes 120 8-inch Hamamatsu cryogenic photo multiplier tubes (PMTs) and 192 X-ARAPUCA modules, which are a SiPM-based photon trap. Standard light detectors (like PMTs and SiPMs) are most sensitive in the visible region, which makes them poorly suited for detecting argon scintillation photons, which are emitted in the vacuum ultraviolet (VUV) region with a peak intensity of 128 nm. To make detection of these VUV photons possible, a wavelength shifting coating of either tetraphenyl-butadiene (TPB) or paraterphenyl (pTP) was applied directly to the optical detector windows as well as a series coated foils mounted on the cathode plane. Putting everything together, this system provides a high light yield, precise timing resolution, and even makes it possible to build an independent position reconstruction only based on scintillation light signals.

Physics

Some of the SBND physics goals include:

• The search for eV-scale sterile neutrinos by performing high-precision measurements of neutrino flux close to the BNB target. This plays a key role in understanding the MiniBooNE and Liquid Scintillator Neutrino Detector (LSND) low energy excess observed for election-like events.
• Performing precision measurements of the neutrino-argon scattering cross sections, which is helped by the high event rate expected for SBND. SBND expects around 7,000 neutrino events to be collected everyday, meaning it will easily collect multiple times the entire six-year MicroBooNE dataset on a yearly basis.
• Searches for other Beyond the Standard Model (BSM) physics, such as heavy neutral leptons, dark photons, heavy QCD axions, among others.

SBND serves as an important precursor to the Deep Underground Neutrino Experiment (DUNE), the next-generation LArTPC neutrino experiment also hosted by Fermilab and currently under construction. The design and many of the technologies used in SBND will also be used in DUNE, which will benefit from the lessons learned from the construction and operational experience.

Operations Timeline

SBND started commissioning Feb 2024, and the detector was filled with liquid argon until April 2024. Commissioning continued in early summer 2024 with the high voltage ramp up of the TPC, leading to the first events being detected in early July. The rest of the detector was installed, including the CRT plane on top of the detector in early September. After the summer shutdown of the beams at Fermilab ended, SBND saw its first events fully assembled on September 10, 2024. The experiment started its data-taking period on December 2024, with plans to continue taking data until early 2028, corresponding to the long shutdown of the Fermilab accelerator facilities for the PIP-II upgrades.

The SBND collaboration is exploring options to continue taking data after the long shutdown and accelerator facilities come back online in 2029 and beyond. One proposed plan for SBND after the restart includes taking data with the BNB in an antineutrino beam configuration, allowing for data collection of antineutrino-argon scatterings, which currently does not have large statistics compared to neutrino-argon scattering. Another proposal includes running in a dedicated beam-dump mode, enabling SBND to significantly enhance sensitivity to many new physics scenarios. The SBND Collaboration is conducting sensitivity studies to better understand which options are most feasible and best accomplish the physics goals of the SBN program.

External links

• Official website

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