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Profile view of the DUNE ND hall and the reference DUNE ND detectors foreseen to operate in Phase~II. The neutrino beam enters from the right.

Profile view of the DUNE ND hall and the reference DUNE ND detectors foreseen to operate in Phase~II. The neutrino beam enters from the right.

A charged current $\nu_\mu$ event with seven low energy protons (kinetic energies ranging from 7 to 51~MeV) in a 10~atm argon-based TPC, simulated and reconstructed with the GArSoft software suite. The detector has a radius of approximately 2.5 m and a width of 5 m. Charged particle trajectories for both simulated (thin lines) and reconstructed (thick lines) tracks are shown. The $\mu^{-}$ (green reconstructed track) enters the ECAL, while the protons are contained within the TPC. The reconstruction algorithm finds all eight tracks, although only six are visible by eye in this view.

A charged current $\nu_\mu$ event with seven low energy protons (kinetic energies ranging from 7 to 51~MeV) in a 10~atm argon-based TPC, simulated and reconstructed with the GArSoft software suite. The detector has a radius of approximately 2.5 m and a width of 5 m. Charged particle trajectories for both simulated (thin lines) and reconstructed (thick lines) tracks are shown. The $\mu^{-}$ (green reconstructed track) enters the ECAL, while the protons are contained within the TPC. The reconstruction algorithm finds all eight tracks, although only six are visible by eye in this view.

A charged current $\nu_\mu$ event with two pions in a 10~atm argon-based TPC, simulated and reconstructed with the GArSoft software suite. The detector has a radius of approximately 2.5 m and a width of 5 m. The annotations are from Monte Carlo truth. Charged particle trajectories for both simulated (thin lines) and reconstructed (thick lines) tracks are shown, as well as reconstructed clusters in the ECAL (shown as green polyhedra). The $\pi^{+}$ decays in flight, and the resulting muon enters the ECAL. Also seen in the ECAL are reconstructed clusters from the decay of a final state $\pi^{0}$ created in the neutrino interaction (the two $\gamma$'s from the $\pi^{0}$ decay do not produce ionization tracks in the TPC, and so are only detected by the ECAL), and a cluster from the outgoing $\mu^{-}$ in the interaction.

A charged current $\nu_\mu$ event with two pions in a 10~atm argon-based TPC, simulated and reconstructed with the GArSoft software suite. The detector has a radius of approximately 2.5 m and a width of 5 m. The annotations are from Monte Carlo truth. Charged particle trajectories for both simulated (thin lines) and reconstructed (thick lines) tracks are shown, as well as reconstructed clusters in the ECAL (shown as green polyhedra). The $\pi^{+}$ decays in flight, and the resulting muon enters the ECAL. Also seen in the ECAL are reconstructed clusters from the decay of a final state $\pi^{0}$ created in the neutrino interaction (the two $\gamma$'s from the $\pi^{0}$ decay do not produce ionization tracks in the TPC, and so are only detected by the ECAL), and a cluster from the outgoing $\mu^{-}$ in the interaction.

$dE/dx$-based particle identification in the TPC of the PEP-4 detector at SLAC~\cite{Grupen:1999by}. This TPC used a gas mixture of 80:20 Ar-CH$_4$, operated at 8.5~atm~\cite{TPCTwoGamma:1982efn}.

$dE/dx$-based particle identification in the TPC of the PEP-4 detector at SLAC~\cite{Grupen:1999by}. This TPC used a gas mixture of 80:20 Ar-CH$_4$, operated at 8.5~atm~\cite{TPCTwoGamma:1982efn}.

Impact of the Phase~I and reference near detector configurations on CP violation sensitivity, shown for maximal CP violation (left) and for 50\% coverage of $\delta_{CP}$ space.

Impact of the Phase~I and reference near detector configurations on CP violation sensitivity, shown for maximal CP violation (left) and for 50\% coverage of $\delta_{CP}$ space.

Impact of the Phase~I and reference near detector configurations on CP violation sensitivity, shown for maximal CP violation (left) and for 50\% coverage of $\delta_{CP}$ space.

Impact of the Phase~I and reference near detector configurations on CP violation sensitivity, shown for maximal CP violation (left) and for 50\% coverage of $\delta_{CP}$ space.

The background-rejection capability of the ND-GAr detector results in improved coverage of parameter space in HNL searches, as shown by the comparison of the ``no background'' sensitivity with the ``on-axis only'' and ``DUNE-PRISM'' sensitivities~\cite{koppetalpaper}.

The background-rejection capability of the ND-GAr detector results in improved coverage of parameter space in HNL searches, as shown by the comparison of the ``no background'' sensitivity with the ``on-axis only'' and ``DUNE-PRISM'' sensitivities~\cite{koppetalpaper}.

Cutaway view of the full ND-GAr detector system showing the HPgTPC, the calorimeter, the magnet, and the iron yoke. The detectors for the muon-tagging system are not shown.

Cutaway view of the full ND-GAr detector system showing the HPgTPC, the calorimeter, the magnet, and the iron yoke. The detectors for the muon-tagging system are not shown.

The left image shows the full system, with the magnet return yoke shown in red. The right image shows a cutaway view of one of the end plates, where some of the ``stays'' that support the load of the flat heads are visible.

The left image shows the full system, with the magnet return yoke shown in red. The right image shows a cutaway view of one of the end plates, where some of the ``stays'' that support the load of the flat heads are visible.

The left image shows the full system, with the magnet return yoke shown in red. The right image shows a cutaway view of one of the end plates, where some of the ``stays'' that support the load of the flat heads are visible.

The left image shows the full system, with the magnet return yoke shown in red. The right image shows a cutaway view of one of the end plates, where some of the ``stays'' that support the load of the flat heads are visible.

Diagram of a possible arrangement of readout chambers of the ND-GAr HPgTPC, based on a drawing of Ref.~\cite{Alme:2010ke}. The Central Readout Chambers would need to be designed and added to the HPgTPC.

Diagram of a possible arrangement of readout chambers of the ND-GAr HPgTPC, based on a drawing of Ref.~\cite{Alme:2010ke}. The Central Readout Chambers would need to be designed and added to the HPgTPC.