Short Baseline Neutrino

MicroBooNE and SBND

One outstanding physics goal in neutrino physics is to address the still unsolved puzzle originated from past short-baseline neutrino oscillation experiments and neutrino reactor data. The LSND experiment measured an oscillation rate of muon antineutrinos to electron antineutrinos neither compatible with the solar nor with the atmospheric neutrino oscillation parameters. Similarly, the MiniBooNE experiment detected a significant deviation from the expected number of electron neutrino events at low energy. More results come from reactor experiments that have analyzed the spectrum of reactor electron antineutrinos obtaining an intriguing, although not conclusive hint for the possible existence of sterile neutrino states, as well. The excess of events in MiniBooNE could be due to electrons or single photons, since these are indistinguishable in their Cherenkov imaging detector. The size of the observed signals indicates a difference of the neutrino mass squared of the order of 0.5-1 eV2, which can only be accommodated with one or more additional neutrino mass eigenstates. Since the weak interaction only knows about three neutrino flavors, the existence of sterile neutrinos, i.e. not coupling to other fermions except via the oscillation mechanism, must be advocated.

The Bern group has worked in the last few years on the establishment of a comprehensive experimental program able to explore these interesting findings by a high-sensitivity short baseline neutrino program that has been approved at Fermilab, based on a multi-detector approach. The SBN program (Short Baseline Neutrinos) is based on three liquid argon time projection chamber (LAr TPC) detectors, respectively realized by the international collaborations SBND, MicroBooNE and ICARUS, to perform a high-sensitivity search for muon- to electron-neutrino oscillation appearance and muon-neutrino disappearance, both pointing to the oscillation parameter region indicated by the anomaly.

Bern is an active member of the first two collaborations. The detectors are placed in the Booster Neutrino Beam (BNB); given the L/E ratio of (110, 470 and 600 m)/~1 GeV of the SBN detectors we expect that: SBND will accurately measure the unoscillated neutrino flux, while MicroBooNE and ICARUS should detect the possible effect of oscillations due to sterile neutrinos. The first SBN detector to start running in 2015 has been MicroBooNE. Its goal is to assess if the source of the LSND/MiniBooNE result is due to an unknown or underestimated source of background, to deficiencies in our understanding of neutrino-nucleus interactions, or to a signal, possibly due to an excess of photon-like or electron-like events. After that, the whole 3-detector program will provide conclusive evidence, with a detailed study of the signal, of the possible evidence for new physics, namely the existence of additional sterile neutrino states. Discovery of these unknown states would obviously be a major science result, but also the clarification of the persistent anomalies constitutes, with no doubts, a very compelling objective.

The LAr TPC technique has been adopted for the three SBN detectors thanks to its unsurpassed capability in e/g separation and neutrino event imaging, required to improve the sensitivity of previous explorations. Needless to say, our more than 10-year long experience with this type of detector has placed the Bern group in a unique position within the USA based collaborations at Fermilab. Our competences in the technology were established with the ARGONTUBE program in Bern and with the small size ArgoNeuT LAr TPC exposed to the NuMI beam, originally meant as a test device but eventually able to provide interesting science results.

MicroBooNE has started data taking in summer 2015, with excellent performance already from day-1, with only about 10% of the TPC readout channels (wires) exhibiting problems at the start. The full three-detector setup will run from 2018. Our group played an important role in the commissioning and first operation of MicroBooNE, in particular, after many years of R&D studies in house, with our original UV-laser calibration system capable of generating tracks simulating those from ionizing particles. The knowledge of the electric field inside the drift volume of a TPC is a key aspect for accurate event reconstruction. Since distortions of particle tracks due to field non-uniformities are indistinguishable from particle multiple scattering, they affect the accuracy of the momentum reconstruction. The deviation of the field map may arise from the accumulation of positive argon ions in the drift volume (e.g. induced by the cosmic muon flux). While electrons produced by ionizing particles are quickly swept towards the readout system, ions have significantly lower mobility. A pulsed UV-laser (l=266 nm) can be used to ionize argon via multi-photon absorption. The resulting ionization track is straight, characterized by low electron density, free of delta-electrons and practically not subjected to charge recombination losses, unlike cosmic muon tracks. Our laser system is integrated in the data taking system of MicroBooNE and our people are responsible for the operation of the device and for the related data analysis.

A further hardware contribution from Bern concerns a major upgrade of the detector driven by our idea of realizing, installing and operating a cosmic-muon detector to reduce the influence of cosmogenic background. Cosmic-ray muons are the most abundant background particles in TPCs placed at surface or at shallow depth. Although muons are not a direct background to the sterile neutrino search, they produce d-rays that are able to mimic electron-neutrino interactions. The system we have started to build will increase the rejection of the electromagnetic background and the detection efficiency of genuine ne CC interactions, by comparing the time distribution of the muon-related signal from an external scintillating tracker to that of the TPC light detection system and to the beam gate. Muon tracks detected in the TPC are extrapolated to the scintillator planes that constitute our so-called Cosmic Muon Tagger (CRT). The time distribution of the signals of the tracker units that are crossed by the extrapolated muon trajectories is used to build a correspondence with the internal TPC light collection system. Those tracks that are seen by the CRT and the light system outside of the beam gate are unambiguously identified as cosmic muons, and the electromagnetic activity within the chosen radius around the track is then discarded. In other cases, a significant number of neutrinos will interact outside of the detector in the ground and concrete (so-called "dirt events").  Beam-correlated photons produced in such events may convert inside the TPC and, once again, fake ne CC interactions. In the case of high-multiplicity events, there is the probability that some of these photons convert outside or within the scintillating tracker and produce a signal in it. By using coordinate and time information from the tracker one can define a region in the TPC to reject such events, too.

The CRT is build by assembling planes of scintillator strips readout by silicon photomultipliers (SiPMs) according to the original design we developed in Bern. The modules include a novel front-end electronics embedded in the modules. We profited from a dedicated FLARE grant for the detector construction. The realization of the CRT is taking place in our laboratory in Bern and 20 of the the 50 detector planes have already been shipped to Fermilab for installation. The design and R&D work for SBND progressed very fast in 2015. For SBND we proposed to realize an UV-laser calibration system based on an advanced design compared to that of MicroBooNE, and a cosmic muon tagger similar to that being built for MicroBooNE.

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