With the advent of a new generation of neutrino experiments which leverage high-intensity neutrino beams for precision measurements, it is timely to explore the opportunity that these experiments offer for searches of beyond the standard model (BSM) physics. The realm of BSM physics has been mostly sought at high-energy regimes at colliders, such as the LHC at CERN. Therefore, the exploration at neutrino experiments will enable complementary measurements at the energy regimes that balance that of the LHC. This, furthermore, is totally in concert with recent ideas for high-intensity beams for fixed target and beam-dump experiments world-wide, e.g., those at CERN [1].

A key feature of the long and short baseline neutrino facilities that prepare for a new generation of neutrino physics experiments is the use of advanced detector technologies. Decisive measurements are expected from the short baseline experiments on the sterile-like neutrino phenomena at the SBL program at FNAL. These experiments are by definition located close to the neutrino source, within a few hundred meters, and the driving technology choice is liquid Argon TPCs. At the same time, long baseline experiments consist of a concise near detector within a few hundred meters of the neutrino source, followed by a very large far detector, at a distance of hundreds or even a thousand kilometres away.

Neutrino beams are generated from a very intense high-energy proton drive beam with an energy typically in the range of 10 to 100 GeV, dumped on a dense target. This produces a secondary beam consisting mostly of pions and kaons, and these mesons decay to dominantly yield a muon and a neutrino. The neutrinos will pass through the absorber before continuing their way to the experiments. The near detector’s main task is to measure the un-oscillated neutrino flux, used to gauge for the observation of the neutrino interactions at the far detector. The need for more precise measurements in a near detector that allow controlling the systematic uncertainties on oscillation measurements has been demonstrated over the last years. As a result, the planned near detectors have become very sophisticated precision particle detectors, providing full acceptance and many capabilities than just monitoring the flux or providing the much needed neutrino-nucleus interaction measurements. This awareness has led recently to a number of workshops and discussions reported here.

In essence, the neutrino beam facility and near-by detectors can be considered as (non-optimized) beam dump experiments. Therefore, we have taken a closer look at the potential they offer in searches for new weakly coupled low mass long-lived particles, such as heavy neutral leptons (HNLs), low mass dark matter, dark photons and so on. Many of these search scenarios were recently also the focus of the Physics Beyond Collider study at CERN [1].

One year ago a small kick-off workshop was organized at CERN called “Near detector physics at neutrino experiments” (18-22 June) [2] with only about 20 key participants, but a clear picture and strong interest emerged from that meeting. They offer a large discovery potential, explored in depth, in searches for light exotic mediators, light dark matter, MeV range neutrinos and HNLs, the sensitivity to new physics of trident event production, and to non-standard neutrino interactions, as well as potentially very interesting  classical Standard Model type of measurements. HNLs are a very popular extensions of the Standard Model aiming to explain baryon asymmetry in the Universe, neutrino masses, and dark matter and it should be noted that they are one of the main targets of the SHiP proposal at CERN, see [3].

New physics can be probed with near detectors via the following processes (See Fig 1). Neutrinos created as described above could mix with new, perhaps right handed heavy neutrinos, if kinematically allowed. Hence some of these neutrinos may turn into HNLs that can potentially live long and decay in the near detector. BSM particles like dark photons can be produced directly in the decays of the produced light mesons and may decay in the near detector. Other new light particles than could be produced in the beam dump are for example light dark matter and milli-charged particles. The latter are foreseen in the case of a dark sector that has its own QED which kinematically mixes with the Standard Model QED. Such stable light BSM particles can interact in the near detectors via elastic or inelastic scattering processes with the target electrons and nucleons.

Figure1: Schematic view for BSM physics opportunities with neutrino near detectors

New generator tools  - such as MadDump- are being developed as well, which should facilitate experimental groups to perform such studies. In all, this first workshop was just the tip of the iceberg, showing there was a significant potential for such searches at near detectors in neutrino beamlines. This led to a larger and more open workshop organized at FNAL called “Physics opportunities in the Near DUNE detector hall (December  3-7)” [4], gathering about 80 participants. New topics were discussed in detail, such as the exciting sensitivity to light dark matter in the DUNE near detector, new ideas and sensitivity tests for discovering milli-charged particles specifically in Liquid Argon TPCs (Fig. 2), sensitivity to scalars with B-L charge that can lead to several interesting phenomena: new decays, beam-strahlung, dark matter, etc. Moreover, precision Standard Model measurements in neutrino scattering are part of the rich slew of physics topics offered by the near detector capabilities, such as measurement of sin²θ_W and electroweak physics, strange sea and charm production, measurement of strange sea contribution to the nucleon spin ∆s, and more.

Figure2: An example of BSM physics reach with different present and future possible experiments for the search of milli-charge particles related to a dark QED model. The reach is shown as function of the fractional charge and particle mass. [5]  

Finally, the community interested in searching for new physics with present and particularly future accelerator-based neutrino experiments, gathered together for a 2 day topical working meeting in Arlington, Texas on “New Opportunities at the next Generation Neutrino Experiments” (April 12-13). The purpose was in particular to prepare a first white paper. About 45 authors have compiled a paper that will be published on the arXiv in the next weeks.

The next-generation neutrino experiments definitely offer new opportunities, and a systematics mapping of the sensitivities for searches for new physics scenarios is a next goal. Obviously, the presently designed near detectors have not been optimized for such a program but are driven by the requirements of the core neutrino oscillation program. Modest detector additions, e.g., adding precision timing detectors, could strongly enhance the capabilities. Compared to the beam facility proposed for CERN in the North Hall, eg for the DUNE detector we expect to get more protons on target (1-2x10²¹ POT/year) but the beam energy is considerably lower and the position of the detector is more than 500 meters from the dump. Hence eg for HNL searches the near detector will focus more towards smaller masses, up to 2 GeV or so. There will be certainly an interesting complementarity among the different search experiments in the low mass region, all expected to start around 2027!

Further reading

[1] arXiv:1901.09966

[2] https://indico.cern.ch/event/721473/

[3] EP newsletter: https://ep-news.web.cern.ch/content/hunting-right-handed-neutrinos-new-g...

[4] https://indico.fnal.gov/event/18430/

[5] arXiv:1806.03310