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The scope of the this section addresses how the the reference NND described in Section 7.2 meets the requirements of the oscillation studies described in Section 3.8.2 of [4] and the short-baseline precision measurements described in Sections 6.1.2 and 6.1.3 of the same document. First, we present the oscillation related systematics; the systematics affecting the precision measurements and new-physics search program follows.

7.3.1

Oscillation Analyses

The Table 3.8 in Section 3.6.2 of [4] presents a conservative projection of systematic errors affecting

• Beam νe: The FGT will offer an event-by-event measurement of the beam νe via the iden-

tification of the emergent e− in STT while rejecting the π0 → γ → e background via the determination of the missing-PT vector to a high degree. In a 5-year neutrino-run (focus-

positive) the FGT will accumulate 500,000 νe with'55% efficiency and≥95% purity. The

resulting νe/νµ ratio will be determined to ≤ 1% precision. Furthermore, by constraining

the sources of νe (µ+,K+ and KL0), the FGT will predict the the ratio of νe and νµ spectra

at the far detector (FD) with respect to the near detector (ND) as a function of the neutrino energy, FD/ND (Eν).

• Beamν¯e: Although the FD does not distinguishνefrom ¯νe, the FGT will accurately measure

the beam ¯νe by identifying the emergente+ in the STT with efficiency and purity similar to

those forνe. (We point out that the dominant kaonic source of ¯νeisKL0; the neutrino spectra

from K_L0 are different from those ofK+ at the FD.) In a 5-year neutrino-run, the FGT will accumulate 40,000 ¯νe and 5×106 ν¯µ samples, providing a precise FD/ND prediction for the

antineutrino to '1%. These constraints will be even more valuable during the antineutrino run (focus-negative) where the wrong-sign backgrounds are larger.

• Cross Sections: First, the FGT will measure the absolute flux, viaν-e scattering, to'2.5% precision. Second, using radiator targets, the FGT will measure exclusive channels, such as quasi-elastic, resonance, coherent-mesons and the inclusive DIS channel with unparalleled precision. Since Argon is the far detector nuclear target, a set of various nuclear targets will allow translation of the cross-section measurements toν-Ar scattering.

• Nuclear Effects: The FGT will employ a suite of nuclear targets including Ar-gas in pres- surized tubes, a thin solid calcium target (which has the same A=40 at Ar), a C-target, etc. Specifically, the number of ν-Ar interaction will be 10 times larger than that expected in a 40−kt FD, without oscillations. Additionally, comparisons of calculations of elastic and inelastic interactions in Ar versus Ca, including FSI effects, indicate negligible differences between the two targets. Thus, the combination of the Ar and Ca targets will provide a strong constraint on the nuclear effects from both initial and final state interactions. Finally, FGT’s ability to isolate ν (¯ν) off free-hydrogen, via subtraction of hydrocarbon and carbon targets, will provide a model-independent measurement of nuclear effects.

• Hadronization: A notable strength of FGT is to identify the yield ofπ0separatelyin neutral- current (NC) and charged-current (CC) interactions. (The estimatedπ0detection efficiency is

'50%.) In addition, FGT will determine the yields ofπ−andπ+, the dominant backgrounds to the νµ and ¯νµ disappearance. Finally, the measurement of the composition, energy, and

angle of the hadronic jet will provide a tight constraint on the overall hadronization models.

• Energy Scale: The average density of about 0.1 g/cm3 (close to that of liquid hydrogen) makes the STT mostly transparent to secondary particles — the entire STT length is equiv- alent to∼1.4X₀ including the various nuclear targets. As a result we will be able to measure accurately the 4-momenta of secondary particles as well as the missing-PT vector in the CC

processes. This redundant missing-PT vector measurement provides a most important con-

straint on the neutrino and antineutrino energy scales. The capabilities of this kinematic analysis have been demonstrated by the NOMAD experiment (with similar density and B field). In addition, measurements of exclusive topologies like quasi-elastic, resonance and

coherent meson production offer additional constraints on the nuclear effects affecting the neutrino energy scale (see Section 6.1.2 of [4]). The requirements on the muon energy scale (<0.2%) and total hadron energy scale (<0.5%) uncertainties have been already achieved by the NOMAD experiment. The muon energy scale is calibrated with the mass constraint from the K0 reconstructed in STT and was statistics limited in NOMAD. The total hadron energy scale is calibrated using the yBj distribution in different energy bins. Compared to

NOMAD, FGT will have ×100 higher statistics and×10 higher granularity.

In summary the FGT will accurately quantify all four neutrino species and predict the ratio FD/ND for them. It will measure the 4-momenta of the outgoing hadrons composing the hadronic jets in a variety of nuclear targets, in essence proving a data-driven event generator which can be applied to the FD.

Since the FGT is based upon a different technology than the FD, it cannot account for effects of LAr reconstruction inefficiencies in the FD. The corresponding cancellation could be achieved only with an identical ND, which to some extent is an ill-defined concept due to a number of factors including size, beam profile and composition, rates, etc. However, given the detailed program to calibrate LAr detectors in test beams and multiple neutrino experiments employing LAr detectors, by the time DUNE becomes operational the reconstruction of particles in LAr will likely be well understood. Finally, Section 7.4 outlines the enhancement of the ND complex via the placement of complementary LAr detector(s) upstream of the FGT.

7.3.2

Short Baseline Precision Measurements and Searches

Sections 6.1.2 and 6.1.3 of [4] summarize a rich physics program at the near site providing a generational advance in precision measurements and sensitive searches. This short-baseline physics program and the long-baseline oscillation analyses share similar detector requirements and offer a deep synergy and mutual feedback. The reference FGT meets the requirements of the short- baseline studies as briefly outlined below:

• Resolution: The FGT is designed to have an order of magnitude higher granularity than NOMAD, the most precise, high statistics neutrino experiment. The corresponding im- provements include better tracking, electron/positron ID through TR, dE/dx measurement providing hadron-ID, 4πcalorimetry, 4π muon coverage and a larger transverse area for event containment.

• Statistics: The 1.2−MW neutrino source at LBNF will offer a factor of 100 enhancement in statistics compared to NOMAD. The program of measuring ν and ¯ν interactions in a set of nuclear targets, including Ar and H, will enhance the physics potential of precision measurements and searches.

7.3.3

Future Tasks to Quantify the Systematic Errors

To quantify the systematic errors in oscillation studies and precision measurement program, three tasks are still outstanding:

• Geant4 Simulation: A Geant4 simulation of the FGT is needed to confirm and correct the projected systematic errors and the salient sensitivity studies.

• Event Reconstruction: A program to reconstruct tracks in STT and to match the informa- tion from different subdetectors is needed to identify secondary particles.

• Translating ND-Measurements to FD: A concerted effort needs to be mounted to transfer the precision measurements in ND to the far detector.

The DUNE collaboration plans to pursue these issues with high priority in the coming years before CD-2.