and carbon-scattering [73,74] experiments can be nontrivial,
but we do not consider these.
A global fit of the 3 þ 1 neutrino oscillation framework
to short-baseline oscillation data was recently performed in
Ref. [12]. The authors find that the best-fit point to global
¯
ν
e
disappearance data—including reactor, gallium, solar,
and β-decay experiments—with unfixed reactor fluxes
fΔm
2
41
; sin
2
2θ
ee
g¼f1.3 eV
2
; 4.04 × 10
−2
g. This point
is indicated by the black star in Fig. 2.
C. Low-threshold experiments
The possibility of observing active-sterile oscillations
through coherent elastic neutrino-nucleus scattering
(CEνNS) has previously been discussed in Refs. [27–31];
these provide a complementary probe of the sin
2
2θ
ee
-Δm
2
41
space to reactor antineutrino experiments. In fact, the
experiments that we will consider are also based at nuclear
reactors, but we separate these from those of the previous
subsection because the underlying signal process—CEνNS—
is distinct from inv erse beta decay (IBD).
Reference [30] reports a constraint in the sin
2
2θ
ee
-Δm
2
41
plane from a combined analysis of
¯
νe scattering data from
the Krasnoyarsk [75], Rovno [76], MUNU [77], and
TEXONO [78] experiments.
4
We reproduce the resulting
exclusion in purple in Fig. 3. This constraint is quite weak
relative to the other experiments in the figure, but it is the
only such analysis that can currently exclude any portion of
this parameter space.
We focus on a subset the many (existing and proposed)
CEνNS experiments [79–86], starting with the RED100
[87] and MINER [88] proposals. The sensitivities of these
experiments to sterile neutrinos were studied in Ref. [30].
Several benchmark scenarios were considered for each of
these experiments; we consider the most aggressive sce-
narios to assess how these experiments fare under the most
optimistic assumptions. We take baselines of 15 m for
RED100 and 1 m for MINER and assume a 100% efficiency
for each. The 90% C.L. sensitivities for RED100 and
MINER are reproduced in dot-dashed red and double-
dot–dashed green curves, respectively, in Fig. 3.
The sensitivity of the COHERENT experiment [26] to
sterile neutrinos has been studied in Refs. [31,86].However,
COHERENT is more sensitive to a sterile neutrino mixing
with ν
μ
=
¯
ν
μ
than it is to mixing with ν
e
=
¯
ν
e
. Consequently, we
do not include COHERENT in our analysis. We remark,
however, that near-term expansions to the neutrino program
at the Spallation Neutron Source could yield meaningful new
constraints in this parameter space [89].
Last, we consider the CONUS experiment [90],for
which an official exclusion result does not yet exist. We
estimate the sensitivity of the CONUS experiment to
oscillations involving a sterile neutrino following the
procedures employed in Refs. [9 1,92] ; we present this
analysis fully in Appendix B,butprovidesomerelevant
details here. We consider two benchmark configurations
for CONUS. The first is the nominal CONUS c onfigu-
ration, consisting of 4 k g of natural germanium and a
recoil threshold of 1.2 keV taking data over one year;
we call this configuration “CONUS.” The second is a
more aggressive configuration , consisting of 100 k g of
88% enriched germanium with a threshold of 0.1 keV,
taking data for five years; we call this configuration
“CONUS100.” We also assume that systematic uncertain-
ties will improve from Oð1%Þ down to Oð0.1%Þ.Other
relevant details are summarized in Appendix B.
The resulting sensitivities for the CONUS and
CONUS100 scenarios are shown in long-dashed blue and
short-dashed light blue curves, respectively, in Fig. 3.The
sensitivity of the default CONUS configuration is relati v ely
weak—it is comparable to existing bounds from
¯
ν
e
e
scattering. The CONUS Collaboration has completed their
data taking and is expected to be releasing their results in the
FIG. 3. Constraints in the sin
2
2θ
ee
-Δm
2
41
plane from low-
threshold experiments. The purple, shaded region is excluded
at 90% C.L. by
¯
νe scattering [30]. Also shown are the expected
sensitivities of RED100 (dot-dashed red curve, 90% C.L.),
MINER (double-dot–dashed green curve, 90% C.L.), CONUS
(long-dashed blue curve, 95% C.L.), and CONUS100 (short-
dashed light blue curve, 95% C.L.). The black, five-pointed start
represents the best-fit point from the global analysis of Ref. [12].
4
This is, of course, not a CEνNS process, which is why we
have opted to call this class of bounds “low threshold.”
JEFFREY M. BERRYMAN PHYS. REV. D 100, 023540 (2019)
023540-4