The Short Baseline Near Detector (SBND)

SBND logoIntroduction

The Standard Model of particle physics contains three neutrinos, nominally one for each of the three charged leptons e, μ and τ.  In the 1990s, experiments at the LEP collider in CERN demonstrated that there were three and only three distinct types of light, weakly-interacting neutrino (thus indicating that the Standard Model contains only three generations).  Any further weakly-interacting neutrinos would need to have masses above 45 GeV/c2 (so that they could not be produced in Z decays, Z → νν̅) to evade this restriction.

If there are three neutrino species and three distinct mass eigenstates (which, as shown by the results of neutrino oscillation experiments, are not aligned with the flavour eigenstates νe, νμ, ντ), then there are exactly two independent mass differences (or, in neutrino oscillation theory, squared-mass differences): the third one can be expressed as the sum or difference of the other two.  If we know that m22 – m12 = 7.5×10–5 eV2 and |m32 – m12| = 2.52×10–3 eV2, then |m32 – m22| is either the difference of these (if m3 > m2 > m1, "normal ordering") or the sum (if m2 > m1 > m3, "inverted ordering").

Effect of sterile neutrino on CMB power spectrum

However, several neutrino experiments have reported results which could be interpreted as oscillation signals with a squared-mass difference of about 1 eV2, which is certainly not consistent with the experimentally determined values for the known neutrinos.  The longest-standing of these is LSND, which measured an excess of ν̅e events from a ν̅μ source (μ+ decays at rest).  Others include MiniBooNE, which observes an excess of low-energy events, and the reactor neutrino experiments, which consistently observe neutrino fluxes about 3% lower than the best available calculations (with the caveat that these calculations are challenging!).  If these results are indeed explained by neutrino oscillations, there must be a fourth species of neutrino, and the LEP results imply that it cannot be produced in Z decays—i.e., it is not weakly interacting ( a so-called sterile neutrino).  On the other hand, analyses of the cosmic microwave background tend to prefer three neutrino species, as shown in the figure.  There is also some tension between the neutrino and antineutrino results, which prefer different parameters for the fourth state.

In summary, then, the experimental picture is not as clear as we would wish, and there is no obvious theoretical motivation for a light sterile neutrino.  (Right-handed neutrino states, which would be sterile because the weak interaction is inherently left-handed, are theoretically motivated as a natural explanation for the extremely low neutrino masses, but this only works if the right-handed states are very massive.)  Therefore better data are needed to resolve the existing confusion.  The Fermilab Short Baseline Neutrino Program is intended to provide the better data, while at the same time acting as a technology development testbed for the DUNE experiment.  It consists of three LAr TPCs at different distances from the Fermilab Booster neutrino beam: ICARUS (imported from Gran Sasso) at 600 m, MicroBooNE at 470 m, and SBND at 110 m.  Sheffield's involvement is with SBND.

SBND has a number of physics goals:

  • It is the reference (unoscillated beam) detector for the short-baseline neutrino oscillation measurement intended to confirm or refute the existence of a ~1 eV sterile neutrino.
  • It is expected to record over a million neutrino interactions per year and should therefore provide accurate cross-section measurements for neutrino interactions on argon.  These are critical inputs to the DUNE physics programme.
  • The SBND detector and electronics designs are similar to (though not identical to) the DUNE designs, and SBND is therefore an important technology demonstrator for DUNE.  This is also, of course, the role played by the ProtoDUNE project at CERN, but, unlike ProtoDUNE, SBND is actually on a neutrino beamline and the events it records will therefore be very similar to DUNE events.

 SBND at Sheffield

The Sheffield group plays a leading role in SBND.  Our hardware responsibilities include:

  • design and manufacture of the anode plane assembly (APA) frames that hold the wire readouts inside the cryostat;
  • design and manufacture of wire combs, which support the wires and prevent/minimise sag in case of a drop in wire tension;
  • contributing to general TPC construction and assembly;
  • electronics testing;
  • quality control and quality assurance.

In software and physics analysis, we are

  • developing liquid argon analysis tools including track and shower reconstruction, particle identification, momentum estimation and liquid argon purity measurements;
  • planning to measure neutrino-argon cross sections with the aim of comparing CCQE and/or CC inclusive cross section results with the ANNIE water Cherenkov detector (ANNIE and SBND are in the same beamline and at a very similar baseline so many flux systematics should cancel);
  • participating in sterile neutrino searches using the full suite of SBN detectors (SBND, MicroBooNE and ICARUS).

For further information, see the items in the right-hand menu.