Mononeutrino at DUNE: New signals from neutrinophilic thermal dark matter

We introduce the mononeutrino signal at neutrino detectors as a smoking gun of sub-GeV scale dark matter candidates that mainly interact with standard model neutrinos. In a mononeutrino process, invisible particles, either dark matter themselves or the mediator particle, are radiated off a neutrino when it undergoes the charged-current weak interaction. The associated signals include a missing transverse momentum with respect to the incoming neutrino beam direction and the production of wrong-sign charged leptons. We demonstrate the potential leading role of the future DUNE experiment, using its proposed liquid and gas argon near detectors, in probing these new signals and the thermal origins of neutrinophilic dark matter.

Figure 1

Annihilation channels for dark matter to freeze out in model IA (upper left), IB (upper right), II (lower left) and III (lower right). Time flows from left to right. Arrows represent the direction of lepton number flows.

Figure 2

Expected reach of mononeutrino search at DUNE versus the theory landscape of neutrinophilic dark matter with thermal relic density: model IA: solid (dashed) red curve corresponds to $y_{IA}/m_{\varphi }={\lambda }_{\mu \mu }$ ($y_{IA}/m_{\varphi }=1.0$); model IB: solid (dashed) blue curve corresponds to $y_{IB}={\lambda }_{\mu \mu }$ ($y_{IB}=1.0$); model II: solid (dashed) yellow curve corresponds to $y_{II}={\lambda }_{\mu \mu }$ ($y_{II}=1.0$); model III: solid (dashed) green curve corresponds to $y_{III}={\lambda }_{\mu \mu }$ ($y_{III}=1.0$) and $\mathrm{\Delta }=0.2$. We assume $m_{\chi }=m_{\varphi }/10$ for all the curves. Also shown are the present and future experimental constraints. The gray shaded regions are already ruled out by the charged kaon semileptonic decays (upper-left region), the Higgs boson invisible decay (large ${\lambda }_{\mu \mu }$ region) and the CMB (small $m_{\varphi }$ region) measurements, as will be discussed in Sec. 5. The solid and dot-dashed thick black curves show the DUNE mononeutrino reach with and without taking into account of charge identification, as will be discussed in detail in Sec. 4.

Figure 3

Neutrino beamstrahlung process that appears as a mononeutrino event at DUNE. The $\varphi$ particle is radiated off the incoming neutrino when a CC process occurs, followed by its invisible decay into dark matter or neutrinos. Time flows from left to right. Arrows represent the direction of lepton number flow.

Figure 4

A schematic plot of the DUNE near detector setup, with liquid and magnetized gaseous argon detectors located downstream the neutrino beam.

Figure 5

Expected number of events per year at the DUNE near detector that are identified as signal events in neutrino mode (solid lines) and antineutrino mode (dashed lines). See text and Tables 1 and 2 for explanation of signal. We show events for fully contained muons ($E_{\mu }<1\mathrm{GeV}$), and in the right panel, we further restrict to events with reconstructed neutrino energy $E_{\nu }^{\mathrm{reco}}<1.5\mathrm{GeV}$ to highlight the difference in signal and background distributions as a function of this variable. For the mononeutrino process of interest, red (blue) lines assume $m_{\varphi }=500\mathrm{MeV}$ (1 GeV) and ${\lambda }_{\mu \mu }=1$. Distributions take into account the factors regarding Michel electron tagging discussed in the text.

Figure 6

Left: Same as Fig. 2 but assuming a mass relation $m_{\varphi }=3m_{\chi }$. Right: a reproduction of Fig. 2 for comparison.