DUNE Physics Studies

DUNE logoIntroduction

Fundamentally, DUNE is a long-baseline neutrino oscillation experiment, with L/E value optimised for θ23 and θ13.  The primary physics goal of DUNE is to study neutrino oscillations, specifically:

  • CP violation—the difference in oscillatory behaviour between neutrinos and antineutrinos, caused by the e factor in the PMNS matrix;
  • matter-enhanced oscillations, which should be significant at DUNE's 1300 km baseline (and not for Hyper-Kamiokande's 300 km)—which are sensitive to the mass hierarchy, i.e. whether m3 is the lightest or the heaviest neutrino mass eigenstate.

(It is assumed that the actual oscillation parameters θ23, θ13, Δm213 and Δm223 will be well measured by the time DUNE is operational, although reducing the systematic error budget to the levels anticipated in the DUNE and Hyper-Kamiokande design reports will also permit improvements in these.)

These are important measurements, especially the measurement of CP violation which, it could reasonably be argued, is on a par with the discovery of the Higgs boson: the Higgs boson tells us about the origin of mass, while CP violation in the neutrino sector may tell us about the origin of matter, in the sense of the matter-antimatter asymmetry in the Universe (why is our Universe almost entirely matter, when creating particles in the laboratory always yields particle-antiparticle pairs?).  However, the DUNE and Hyper-Kamiokande physics programmes both extend well beyond neutrino oscillations.  In particular, the duty cycle of neutrino beams is usually poor (which is actually a good thing, assuming that you don't want to vaporise the target—it simply means that the duration of the intense bunches of protons on target is much shorter than the time between bunches), so both experiments have a wide programme of non-accelerator physics to make use of the data that the far detector collects between bunches.  This is especially relevant for DUNE, since the first module of the DUNE far detector is scheduled to be up and running two years before the beamline.  Sheffield physicists are active in several aspects of the non-accelerator programme.

Proton decay

It is not always remembered that searching for proton decay was the original motivation for the first generation of large water Cherenkov detectors: KamiokaNDE originally stood for Kamioka Nucleon Decay Experiment not Neutrino Detection Experiment.  The motivation for looking for proton decay was, and remains, the study of theories of particle physics which go beyond the Standard Model.  Such theories include supersymmetry—which predicts an entire spectrum of new particles—and Grand Unified Theories, i.e. theories which attempt to unify the strong force with the electroweak force.  By their nature, GUTs include interactions which couple hadrons to leptons, allowing the violation of baryon number and the possibility of proton decay (albeit with an extremely long lifetime!).  At that time, the favoured decay mode was p → e+ π0: a particularly suitable signature for water Cherenkovs since the positron and the two photons from the π0 decay will produce clear Cherenkov rings. Indeed, limits on proton decay from water Cherenkov detectors sounded the death knell of the first popular Grand Unified Theory, SU(5).  SU(5) predicted this decay mode for the proton, with a lifetime of < 1032 years: Kamiokande and IMB set lower limits approaching 1033 years, and SU(5) in its original form was therefore disfavoured.  It's amusing to note that another prediction of SU(5) is massless neutrinos, so the large water Cherenkovs definitely seem to have a dislike for this particular GUT!  (A somewhat different GUT called "flipped SU(5)" is still with us: theorists are inventive people and not easily deterred.)

Feynman diagram for SUSY-mediated proton decay

Supersymmetry-based GUTs tend to predict much longer lifetimes (moving from 1028–32 years to 1035–38 years): this is because the GUT-mediated proton lifetime is of order M42m5, where M is the unification mass scale, α is the fine structure constant and m is the proton mass, and including supersymmetry pushes the unification scale up by a bit more than a factor of 10.  However, SUSY itself introduces additional mechanisms for proton decay: the most straightforward one, d + u → s̃ → e+ + u̅, predicting p →e+ π0 with a lifetime of 1 second(!), is suppressed by R-parity (decays such as this are the reason for introducing R-parity in the first place), but loop diagrams such as the one in the figure are still allowed.  This shifts the dominant decay mode from p → e+ π0 to p → K+ ν̅τ and predicts a proton lifetime of 1029–35 years depending on the SUSY mass scale.

This decay mode is much harder for water Cherenkovs: the neutrino is invisible and to reduce background you really need to identify the kaon (i.e. separate kaons from pions at kinetic energies of a hundred MeV or so).  With its good tracking of low-energy particles and potentially good particle identification by dE/dx, DUNE should be well suited to investigating decay modes like this.

The critical issue in all searches for rare events is controlling the background.  In this case, the decay signature is an isolated, fully contained kaon track.  The kaon energy for the decay of a stationary free proton would be 599 MeV (momentum 339 MeV), but some variation will be introduced by the decaying proton being bound in a nucleus.  One possible source of background would be cosmic-ray interactions just outside the detector volume: if such a decay produced a K0, the K0 could propagate into the detector volume unseen and then undergo a charge-exchange scatter, K0 + p → K+ + n (the d quark in the K0 being exchanged for one of the u quarks in the proton), which could in principle give the isolated-kaon signature.  Sheffield's Vitaly Kudryavtsev is an expert in the simulation of cosmic-ray muons and is co-convenor of the DUNE cosmogenic backgrounds working group, and has been working on assessing this background and developing cuts to suppress it.

For more information on this topic, please see Martin Richardson's PhD thesis.

Supernova neutrinos and supernova relic neutrinos

The dominant interaction channel for low-energy neutrinos in DUNE is quasi-elastic scattering off argon, νe + 40Ar → e + 40K*.  Note that this is sensitive to electron neutrinos, unlike the dominant channel in water Cherenkovs, inverse beta decay on hydrogen, which sees electron antineutrinos.  The signature is a single low-energy electron, with the possibility of tagging the gamma ray from the de-excitation of the potassium as a further identification.

Detected neutrinos from a 10 kpc SN in DUNEThe unique property of electron neutrinos from a core-collapse supernova is that the initial neutronisation spike from the formation of the neutron star produces only νe, from p + e → n + νe.  As can be seen from the figure, this feature is considerably suppressed by neutrino oscillations in the case of inverted ordering of neutrino masses (i.e. m3 < m1 < m2)—but it's totally eliminated in the case of normal ordering (m1 < m2 < m3).  Therefore, observation of the neutronisation spike in a Galactic core-collapse supernova should immediately resolve the mass hierarchy.

Recent 3D simulations of core-collapse supernovae exhibit a striking and unexpected feature known as lepton-emission self-sustained asymmetry (LESA).  This is a dipole anisotropy between νe and ν̅e fluxes, which can have an amplitude of up to 20%.  Involvement in both DUNE (sensitive mainly to νe) and Hyper-Kamiokande (mainly ν̅e) will offer the opportunity to investigate this effect: with several thousand events expected in DUNE and tens of thousands in Hyper-K, an asymmetry of a few percent should be clearly visible.

The principal problem for supernova neutrino detection with DUNE is that the detector is optimised for much higher energy neutrinos (the LBNF neutrino beam is in the several-GeV range), and so supernova neutrinos in DUNE are "crummy little stubs" (Kate Scholberg, 2016; see figure) and will present a challenge to trigger designers: although the burst itself is probably unmistakable (several thousand events in a 10-second window), it will be necessary to trigger on such events without deluging the DAQ with triggers from background.  It is likely that this makes studying supernova relic neutrinos in DUNE extremely difficult—they have the same low energies (even lower if redshifted) without the large flux.

Simulated supernova neutrino event in DUNE

In order to realise the full physics potential of a core-collapse supernova observation in DUNE, it is necessary not only to count the events but also to reconstruct them and measure the neutrino energy.  Collective neutrino oscillations, driven by neutrino-neutrino interactions in the interior of the supernova, may cause changes in the observed neutrino energy spectrum as a result of essentially complete transformation of one neutrino flavour into another above a certain threshold.  Such effects are more dramatic in the νe case than ν̅e, because the νe spectrum is more different from νμ,τ than is the case for antineutrinos, but clearly in order to investigate this it is necessary to reconstruct the neutrino energy.  (This is the advantage of both DUNE and Hyper-K over IceCube—with its much larger effective volume, IceCube will see far more events, but it will be unable to do anything other than simply count them.)  

In summary, observing supernova neutrinos in DUNE provides a unique window on the supernova—both water Cherenkovs and large liquid scintillator detectors see ν̅e rather than νe.  However, reconstructing such low-energy events is a non-trivial task, and the comparatively small size of DUNE compared to Hyper-K and IceCube will make it difficult to explore some of the features of modern supernova simulations (e.g. rapid oscillations in flux) owing to lack of statistics.  Combining data from all experiments (IceCube for statistics, Hyper-K for lower statistics but with energy measurements, DUNE for comparatively poor statistics but νe sensitivity) will be the best way to maximise the physics potential of a Galactic core-collapse supernova, should we be fortunate enough to observe one while these experiments are operational.