ANNIE at Sheffield
The Accelerator Neutrino Neutron Interaction Experiment, ANNIE, is a new detector operating in the Booster Neutrino Beam (BNB) at Fermilab. It consists of an upstream muon veto, a gadolinium-loaded 30 tonne water Cherenkov detector, and a downstream muon range detector, all located 100 metres downstream from the BNB target. ANNIE's physics goals include characterising neutron multiplicity as a function of energy in neutrino interactions, and measurements of neutrino cross-sections on oxygen.
ANNIE's technical goals include running the first Gd-loaded water Cherenkov experiment to be situated in a neutrino beam, and the first particle physics experiment to use the next-generation photon sensors known as Large Area Picosecond Photodetectors (LAPPDs). ANNIE Phase I has been running as a test experiment since Spring 2016; the goals of this initial run are to make a position-dependent determination of the neutron backgrounds in the experimental hall. A neutron capture volume (NCV) of Gd-loaded scintillator can be moved within 30 tonnes of pure water viewed by 60 eight inch photomultiplier tubes. Captures in the NCV will tag the incoming neutrons from neutrino interactions in rock, as well as the 'skyshine' neutrons reflected downwards from the atmosphere. These crucial measurements will enable Phase II, where the physics goals will be realised.
ANNIE in the Sheffield neutrino physics programme
The ANNIE project feeds directly into several aspects of the Sheffield neutrino programme:
- ANNIE will provide the first experience of running a gadolinium-loaded water Cherenkov detector in a neutrino beam. The EGADS experiment demonstrated that 0.2% Gd2(SO4)3 can be loaded into a water Cherenkov without degrading the water transparency (see right); ANNIE will investigate the effect on physics observables, and measure the neutron yield in accelerator neutrino interactions.
- ANNIE and SBND are located in the same beamline and hence have common neutrino flux and spectrum systematics. A comparison of CCQE results from ANNIE and SBND will therefore provide a very clean cross-calibration of water Cherenkov and liquid argon neutrino detectors. This is essential to fully realise the potential complementarity of the Hyper-K and DUNE long-baseline neutrino programmes.
- ANNIE will also serve as a testbed for LAPPDs as photodetectors. If successful, this work has possible applications outside the scope of high-energy physics, e.g. medical imaging. The application of particle physics technology to other fields is a long-standing focus of the Sheffield high energy physics group.
ANNIE at Sheffield
The key question that ANNIE sets out to answer is the neutron yield in neutrino interactions. To a first approximation,charged-current quasi-elastic neutrino scattering, νμ + n → μ– + p, produces a proton and no neutrons, but recent experiments have shown that this simple picture is greatly complicated by effects within the target nucleus, and as a result there can be as many as 7 or 8 neutrons emitted! Failing to account for these neutrons would badly skew the inferred characteristics of the interaction, and influence experimental results in complex ways. Characterizing how many neutrons are ejected as a function of neutrino energy and scattering angle is therefore a critical study. This is the main aim of ANNIE Phase II, and will be a principal focus of the Sheffield group.
As ANNIE is a relatively small experimental collaboration, Sheffield has a significant role in many aspects of the experiment. Sheffield has built the hardware and developed custom software for control and monitoring of the high voltage system for the detector. We are also heavily involved in the development of simulation and reconstruction software, as well as working on the physics analysis. Finally, we contribute to the daily running of the detector, and will be doing substantial hands-on work as part of refurbishments needed for 'Phase II' planned to take place this summer.