What’s next for OPERA’s emulsion-detection technology?

Developed in the late 1990s, the OPERA detector design was based on a hybrid technology, using both real-time detectors and nuclear emulsions. The construction of the detector at the Gran Sasso underground laboratory in Italy started in 2003 and was completed in 2007 – a giant detector of around 4000 tonnes, with 2000 m³ volume and nine million photographic films, arranged in around 150,000 target units, the so-called bricks. The emulsion films in the bricks act as tracking devices with micrometric accuracy, and are interleaved with lead plates acting as neutrino targets. The longitudinal size of a brick is around 10 radiation lengths, allowing for the detection of electron showers and the momentum measurement through the detection of multiple Coulomb scattering. The experiment took data for five years, from June 2008 until December 2012, integrating 1.8 × 10²⁰ protons on target.

The aim of the experiment was to perform the direct observation of the transition from muon to tau neutrinos in the neutrino beam from CERN. The distance from CERN to Gran Sasso and the SPS beam energy were just appropriate for tau-neutrino detection. In 1999, intense discussions took place between CERN management and Council delegations about the opportunity of building the CERN Neutrino to Gran Sasso (CNGS) beam facility and the way to fund it. The Italian National Institute for Nuclear Physics (INFN) was far-sighted in offering a sizable contribution. Many delegations supported the idea, and the CNGS beam was approved in December 1999. Commissioning was performed in 2006, when OPERA (at that time not fully equipped yet) detected the first muon-neutrino interactions.

With the CNGS programme, CERN was joining the global experimental effort to observe and study neutrino oscillations. The first experimental hints of neutrino oscillations were gathered from solar neutrinos in the 1970s. According to theory, neutrino oscillations originate from the fact that mass and weak-interaction eigenstates do not coincide and that neutrino masses are non-degenerate. Neutrino mixing and oscillations were introduced by Pontecorvo and by the Sakata group, assuming the existence of two sorts (flavours) of neutrinos. Neutrino oscillations with three flavours including CP and CPT violation were discussed by Cabibbo and by Bilenky and Pontecorvo, after the discovery of the tau lepton in 1975. The mixing of the three flavours of neutrinos can be described by the 3 × 3 Pontecorvo–Maki–Nakagawa–Sakata matrix with three angles – that have since been measured – and a CP-violating phase, which remains unknown at present. Two additional parameters (mass-squared differences) are needed to describe the oscillation probabilities.

Several experiments on solar, atmospheric, reactor and accelerator neutrinos have contributed to the understanding of neutrino oscillations. In the atmospheric sector, the strong deficit of muon neutrinos reported by the Super-Kamiokande experiment in 1998 was the first compelling observation of neutrino oscillations. Given that the deficit of muon neutrinos was not accompanied by an increase of electron neutrinos, the result was interpreted in terms of ν_μ → ν_τ oscillations, although in 1998 the tau neutrino had not yet been observed. The first direct evidence for tau neutrinos was announced by Fermilab’s DONuT experiment in 2000, with four reported events. In 2008, the DONuT collaboration presented its final results, reporting nine observed events and an expected background of 1.5. The Super-Kamiokande result was later confirmed by the K2K and MINOS experiments with terrestrial beams. However, for an unambiguous confirmation of three-flavour neutrino oscillations, the appearance of tau neutrinos in ν_μ → ν_τ oscillations was required.

OPERA comes into play

OPERA reported the observation of the first tau-neutrino candidate in 2010. The tau neutrino was detected by the production and decay of a τ^– in one of the lead targets, where τ^– → ρ^–ν_τ. A second candidate, in the τ^– → π^–π⁺π^–ν_τ channel, was found in 2012, followed in 2013 by a candidate in the fully leptonic τ^– → μ⁻ν_μν_τ decay. A fourth event was found in 2014 in the τ^– → h^–ν_τ channel (where h^– is a pion or a kaon), and a fifth one was reported a few months ago in the same channel. Given the extremely low expected background of 0.25±0.05 events, the direct transition from muon to tau neutrinos has now been measured with the 5σ statistical precision conventionally required to firmly establish its observation, confirming the oscillation mechanism.

The extremely accurate detection technique provided by OPERA relies on the micrometric resolution of its nuclear emulsions, which are capable of resolving the neutrino-interaction point and the vertex-decay location of the tau lepton, a few hundred micrometres away. The tau-neutrino identification is first topological, then kinematical cuts are applied to suppress the residual background, thus giving a signal-to-noise ratio larger than 10. In general, the detection of tau neutrinos is extremely difficult, due to two conflicting requirements: a huge, massive detector and the micrometric accuracy. The concept of the OPERA detector was developed in the late 1990s with relevant contributions from Nagoya – the emulsion group led by Kimio Niwa – and from Naples, under the leadership of Paolo Strolin, who led the initial phase of the project.

The future of nuclear emulsions

Three years after the end of the CNGS programme, the OPERA collaboration – about 150 physicists from 26 research institutions in 11 countries – is finalising the analysis of the collected data. After the discovery of the appearance of tau neutrinos from the oscillation of muon neutrinos, the collaboration now plans to further exploit the capability of the emulsion detector to observe all of the three neutrino flavours at once. This unique feature will allow OPERA to constrain the oscillation matrix by measuring tau and electron appearance together with muon-neutrino disappearance.

An extensive development of fully automated optical microscopes for the scanning of nuclear-emulsion films was carried out along with the preparation and running of the OPERA experiment. These achievements pave the way for using the emulsion technologies in forthcoming experiments, including SHiP (Search for Hidden Particles), a new facility that was recently proposed to CERN. If approved, SHiP will not only search for hidden particles in the GeV mass range, but also study tau-neutrino physics and perform the first direct observation of tau antineutrinos. The tau-neutrino detector of the SHiP apparatus is designed to use nuclear emulsions similar to those used by OPERA. The detector will be able to identify all three neutrino flavours, while the study of muon-neutrino scattering with large statistics is expected to provide additional insights into the strange-quark content of the proton, through the measurement of neutrino-induced charmed hadron production.

Currently, the R&D work on emulsions continues mainly in Italy and Japan. Teams at Nagoya University have successfully produced emulsions with AgBr crystals of about 40 nm diameter – one order of magnitude smaller than those used in OPERA. In parallel, significant developments of fully automated optical-scanning systems, carried out in Italy and Japan with innovative analysis technologies, have overcome the intrinsic optical limit and achieved the unprecedented position resolution of 10 nm. Both achievements make it possible to use emulsions for the detection of sub-micrometric tracks, such as those left by nuclear recoils induced by dark-matter particles (Weakly Interacting Massive Particles, WIMPs). This paves the way for the first large-scale dark-matter experiment with directional information. The NEWS experiment (Nuclear Emulsions for WIMP Search) plans to carry out this search at the Gran Sasso underground laboratory.

Thanks to their extreme accuracy and capability of identifying particles, nuclear emulsions are also successfully employed in fields beyond particle physics. Exploiting the cosmic-ray muon radiography technique, sandwiches of OPERA-like emulsion films and passive materials were used to image the shallow-density structure beneath the Asama Volcano in Japan and, more recently, to image the crater structure of the Stromboli volcano in Italy. Detectors based on nuclear emulsions are also used in hadron therapy to characterize the carbon-ion beams and their secondary interactions in human tissues. The high detection accuracy provided by emulsions allows experts to better understand the secondary effects of radiation, and to monitor the released dose with the aim of optimizing the planning of medical treatments.

• For more information, visit http://operaweb.lngs.infn.it/.