Even if neutrino oscillations were discovered more than 20 years ago, our knowledge of mixing, flavor oscillation and CP violation in the neutrino sector is still far from the precision achieved for quarks. CP violation in the quark sector was established in 1964 and the CKM matrix is measured at sub-percent level both for light and heavy flavors. On the contrary, CP violation in neutrinos is not established, yet, and unitary tests of the neutrino counterpart of the CKM matrix (the Pontecorvo-Maki-NakagawaSakata – PMNS – matrix) have not even started. All in all, flavor physics in neutrinos is a discipline still in its infancy but such a gap comes as no surprise: the neutrino cross sections are the faintest among elementary fermions and the experimental study of flavor physics for the neutrinos remains a major challenge in particle physics.

Accelerator neutrino beams are the workhorse of precision neutrino physics: they produce high intensity beams of muon-neutrinos and play a role similar to b-factories in quark physics. Unlike bfactories, however, the luminosity of these facilities, i.e. the neutrino flux at source, are known with a very limited precision - typically at the level of 10%. Such limitation impacts in a direct manner on all neutrino cross section measurements which, in turn, are determined at the 10% level or worse. At the time of the discovery of neutrino oscillations, a 10% precision in the knowledge of standard model cross sections was a tolerable nuisance but nowadays it is a hindrance that jeopardizes the physics reach of the next generation neutrino facilities, including DUNE and Hyper-Kamiokande. A breakthrough in cross section physics require a neutrino source with a unprecedented control of the flux, energy and flavor. The design of such a source is the main aim of NP06/ENUBET, one of the most ambitious R&D carried on in the framework of the CERN Neutrino Platform.

The ENUBET Collaboration together with particle and accelerator physicists at CERN is developing the technology of “monitored neutrino beams”, i.e. neutrino beams where the flux of the neutrinos at source is measured at single particle level. The flux is measured in the most direct manner: detecting the charged lepton that is produced together with the neutrino after the decay of the parent meson. In general, neutrino beams are produced by the decay in flight of pions and kaons along a decay tunnel located after a target and a system of magnetic lenses (“horns”). The ENUBET “narrow band beam” is a moderate intensity muon-neutrino beam with a fully instrumented decay tunnel. The tunnel instrumentation of ENUBET monitors the production of electron neutrinos detecting the positrons produced by the K+® e + p 0 ne decays of the kaons. ENUBET is therefore a fully controlled electron neutrino source. Similarly, beam diagnostics monitor the decay of the pions after a transfer line with a narrow momentum acceptance: it results in a high precision muon neutrino source where the energy of the neutrino is known within 10%.

Figure 1: Schematics of the ENUBET neutrino beam in the static focusing option (not to scale)

The challenges of ENUBET are first of all the challenges that the LHC has to face, namely handling huge particle rates in a high radiation environment: the decay tunnel of a 100 kW power fixed target experiment. This is the reason why ENUBET resorts to technologies originally developed for collider experiments and is so closely connected to CERN. The ENUBET Collaboration has been established in 2016 and funded by the European Research Council through the 2015 Consolidator Grant Programme (PI: A. Longhin). Since 2016, the technology of monitored neutrino beams has progressed enormously in all fields: the design of the proton extraction scheme, the focusing and transfer line and the instrumentation of the decay tunnel. The final goal of NP06/ENUBET is to build a full-fledged segment of the decay tunnel and test it in realistic conditions at the Renovated East Experimental Area of the PS. The run of the “ENUBET demonstrator” will be a major step toward the validation of the monitored neutrino beam concept. The CERN Neutrino Platform is providing the ideal environment for this R&D to progress, while at this time there is no commitment for a future neutrino beam at CERN.

Even if several challenges have still to be overcome, results achieved so far have really exceeded our expectations. In summer 2018, the ENUBET Collaboration demonstrated that an intensity sufficient for high precision cross section measurement could be reached with a purely static focusing system. A static system dilutes the particle flow in the wall of the tunnel by more than an order of magnitude so that secondary leptons could be not only be monitored but also time-tagged on a particle-by-particle basis if the timing system reaches a precision of 100 ps. Timing at such level of precision is routine work at the LHC but would represent a landmark in experimental neutrino physics: the neutrinos observed at the neutrino detector could be uniquely associated with the lepton and the other decay product of the kaon on an event-by-event basis. Such a facility has been envisaged in the 60s and is customarily called a “tagged neutrino beam”.

Tagged neutrino beams are the holy grail of accelerator neutrino physics because each neutrinos is flavor tagged by the corresponding lepton at source on an event by event basis. As noted by B. Pontecorvo in 1979, “the possibility of using tagged-neutrino beams in high-energy experiments must have occurred to many people” but in 2019 this possibility is still well beyond current technologies. Time tagging of neutrinos with the associated lepton in the decay tunnel has defeated all attempts performed in the 80s because the technological challenges were insurmountable at that time and still remain at the frontier of experimental physics. In a neutrino beam, 10¹³ pions or kaons must be produced in a 50 m decay tunnel in order to observe a 1 GeV neutrino in a 500 ton detector. Their decay products must be recorded by the front-end electronics, time tagged and associated with the neutrino event at the detector. The ENUBET static focusing neutrino beam combined with the gigantic technology leaps on particle detection, timing, data acquisition and analysis fostered by collider experiments bring Pontecorvo’s dream-facility well within reach.

Figure 2: (Top Left): The prototype exposed at the CERN-PS T9 particle beams. The calorimeter is composed of an inner part (bottom part of the picture) made of seven calorimetric blocks and a 60 cm long “hadronic” block in the outer part (upper part of the picture). The system was mounted on a tunable mechanical cradle to record particles impinging at different angles. (Top Right): distribution of the tracks’ impact parameters on the front side of the calorimeter as predicted from Silicon tracking chambers using the 5 GeV beam. The layout of the calorimeter is superimposed: each square represents a 3×3 cm2 UCM. (Bottom): The lateral readout prototype tested at the PS-T9 area in September 2018.

In the next years, NP06/ENUBET will bring monitored beams to a level of readiness suitable for a Conceptual Design Report, which will ground a new generation of cross section experiments. These experiments will run in parallel to the high intensity long-baseline facilities as DUNE and HyperKamiokande providing the precision and knowledge of Standard Model interactions that currently limit their physics reach. They will also perform exquisite tests of physics beyond the Standard Model in the neutrino sector due to the unprecedented level of control of the source. Last but not least, they will serve a wide and vibrant community of neutrino physicists investigating neutrino scattering and cross sections. Their detectors (the liquid argon TPC of ProtoDUNE’s, the near detectors for T2K-II), are under development in the framework of the CERN Neutrino Platform: a programme that makes CERN  a focus for European involvement in precision neutrino physics for the decade to come.