Combining dark matter detectors and electron-capture sources to hunt for new physics in the neutrino sector
Jonathan M. Link
c The Authors.
850 West Campus Dr
Blacksburg, VA 24061
Center for Neutrino Physics, Physics Department
 C. Broggini, C. Giunti and A. Studenikin, Electromagnetic properties of neutrinos
In this letter we point out the possibility to study new physics in the neutrino sector using dark matter detectors based on liquid xenon. These are characterized by very good spatial resolution and extremely low thresholds for electron recoil energies. When combined with a radioactive e source, both features in combination allow for a very competitive sensitivity to neutrino magnetic moments and sterile neutrino oscillations. We find that, for realistic values of detector size and source strength, the bound on the neutrino magnetic moment can be improved by an order of magnitude with respect to the present value. Regarding sterile neutrino searches, we find that most of the gallium anomaly could be explored at the 95% confidence level just using shape information.
1 Introduction 2 3 4
Constraints on the neutrino magnetic moment
Sterile neutrino searches
Results for larger exposure
Summary and conclusions
Neutrinos have long been a rich hunting ground for physics beyond the Standard Model
(BSM). In fact, neutrino mass is so far the only BSM physics that has been established
in laboratory experiments. Astrophysical evidence of dark matter suggests the existence
of BSM particles, which have nevertheless not been observed yet. Among all feasible
candidates, weakly interacting massive particles (WIMPs) are theoretically rather appealing.
These may be observable through their interactions within detectors, as the earth moves
through the sea of WIMPs. This possibility has triggered a cornucopia of experimental
efforts of direct dark matter detection .
In this letter we examine the physics potential of combining a liquid xenon (LXe)
detector, designed to search for WIMP dark matter, with an intense electron-capture neutrinos
dark matter detectors has been proposed before in the literature, see for instance refs. [2
5]. Direct dark matter detection relies on observing nuclear recoils with electron-equivalent
tion cross section, large detector masses and low background levels are also required. A
LXe time projection chamber (TPC) can provide a large volume, low detection thresholds
(sub-keV) and a very low background rate at the energies of interest. At the same time
the electron density is higher in xenon than in any other stable noble gas, thus providing
the largest possible target density in any given volume near the source. The idea of using
never developed. As a by-product we also find non-negligible sensitivity to sterile neutrino
1 eV2 range suggested by recent terrestrial experiments .
When a nucleus decays via electron-capture almost all of the available energy goes into
a mono-energetic neutrino. Among possible nuclei which decay via electron-capture, 51Cr
offers several practical advantages: it is readily produced by thermal neutron capture ,
has a mean lifetime of 39.96 days and produces two mono-energetic neutrino lines at 750 keV
(90%) and 430 keV (10%). Mega-curie-scale 51Cr sources have been produced in the past
and used to calibrate the gallium radiochemical solar neutrino detectors GALLEX [9, 10]
Constraints on the neutrino magnetic moment
increase in the number of events at low electron recoil energies. This makes two-phase
LXe TPCs , with their low-energy detection threshold, ideal detectors for such a
. 3 1012B. The best constraint from terrestrial experiments, on the other hand,
has been obtained by the GEMMA experiment, < 2.9 1011B at 90% CL . In
and references therein.
For our sensitivity estimate, we assume a data taking period of 100 days, using a
51Cr source with initial strength of 5 MCi. Our choice for the strength of the source is
based on simulations conducted for the SOX experiment  of the GALLEX enriched 50Cr
material  irradiated in the High Flux Isotope Reactor at Oak Ridge National Laboratory.
We consider a generic LXe detector, but for definiteness we chose a design similar to the
proposed LZ detector [17, 22]. We assume a cylindrical fiducial volume with equal diameter
source is placed 1 m below the fiducial volume, along the central axis of the cylinder.
Neutrinos are detected via electron elastic scattering in the detector, see eq. (2.1). Under
these assumptions, a total of 12, 518 signal events are expected for a 100 day run. Regarding
backgrounds, we have considered contributions from solar neutrino interactions, 222Rn and
is estimated to be 1.05 counts per ton and day for pp neutrinos and 0.51 counts per ton
and day for 7Be neutrinos. The 85Kr and 136Xe backgrounds have been taken directly
from figure 2 in ref.  and rescaled according to our run length and detector mass, while
reduction with respect to what has been achieved for EXO-200 .
Finally, an important source of background could come from the source itself. In 10%
of 51Cr decays there is a 320 keV gamma, which can easily be shielded with just a few
cm of tungsten. However, impurities present in the chromium prior to irradiation, can
lead to the production of MeV gamma emitters [11, 25]. These will require significant
additional shielding to be reduced down to an acceptable level. We base our calculation of
The expected background event rates are also shown for comparison (green squares). Right panel:
this background on the measured gamma activity of the GALLEX source . Our source
is assumed to be shielded by a 17 cm thick tungsten layer, which, when combined with
70 cm of LXe (present between the tungsten shield and the edge of the detector), provides
per day (in all directions), we expect about 10 to pass through the shielding and Compton
scatter in the detector fiducial volume. Most of these would deposit energy in excess of
the maximum from a 51Cr neutrino, though. Further suppression could come from a veto
on mulit-site Compton scattering events. We estimate the surviving background in the
fiducial volume from source gammas to be less than 1 event per day at the start of the data
77Ge and 24Na), this rate should rapidly decay with time. Therefore, we will neglect these
events in our analysis, but we note that care must be taken in the preparation of the 50Cr
source material to ensure the required level of purity is reached.
only. The recoil energies are smeared on an event-by-event basis according to a Gaussian
with (T ) = 0.20T . Generally, we find that the energy resolution does not have a
wide bins in T , from 2 keV to 140 keV unless otherwise stated. In this energy range a total
of 3, 656 signal events are expected, together with a total of 3, 450 background events. The
distribution of signal and background events in electron recoil energy is shown in figure 1
(left). Reducing the backgrounds does not significantly improve the sensitivity to this
background rate is already quite low. On the other hand, we find the low-energy threshold
to be the most relevant parameter in this analysis (as expected from eq. (2.1)). Figure 1
(right) shows the dependence of the sensitivity as a function of detector threshold. With
a fairly conservative threshold of 2 keV the 95% CL bound is < 4.9 1012B. Such
values would yield an improvement of a factor of 5 over the currently best terrestrial limit.
Sterile neutrino searches
A second possible application of the setup studied here would be the search for light sterile
neutrinos. Sterile neutrinos arise in most models of neutrino mass generation. Searches
for oscillations between active and sterile neutrinos have been conducted using many
combinations of neutrino sources, detectors and oscillation channels . Experimental results
are inconclusive, with strong tension between different data sets (see, e.g., refs. [7, 26, 27]).
In particular, an analysis of GALLEX and SAGE data shows an apparent deficit of events
which is consistent with oscillations involving a sterile neutrino with a squared mass
1 eV2 and a mixing angle such that sin2 2 0.1 ,
which is commonly referred to as the gallium anomaly. The smoking gun of a sterile
neutraveled by the neutrino divided by its energy. A monochromatic neutrino source reduces
the oscillating pattern to a pure function of L. Given the energy of the primary 51Cr
pattern would be observable inside a meter-scale detector. The expected spatial
resolution in LXe TPCs is at the sub-cm level, which makes them ideal candidates for such an
In our analysis, we have adopted a phenomenological approach based on a 3+1
framework, where there is only one extra sterile state at the eV scale. In this framework, the
one mass squared splitting only, as:
to sterile neutrino oscillations is computed using the distance between the source and the
using 3 cm wide bins. The detector is assumed to have a constant spatial resolution of 1 cm;
however, the largest uncertainty in L comes from the shape and size of the radioactive
source itself. In the present work, we assume that the source will be a cylinder with both
height and diameter equal to 14 cm. To achieve such a compact source will require that
achieved by GALLEX , but similar to what was reached by SAGE . A smearing
function was generated, by a Monte Carlo calculation, that simultaneously accounts for
the finite source size and the detector resolution. Two nuisance parameters are added
is performed over both nuisance parameters. Unless otherwise stated, no constraint is
Shape only, 55MCi
parameter space to the right of each line would be excluded at 95% CL (2 d.o.f.). The shaded areas
show the 95% CL allowed regions for the reactor (yellow) and gallium (pink) anomalies from a
global fit to the 3+1 scenario, while the star indicates the best fit point from a combined fit to both
anomalies . Left panel: expected sensitivity using shape information only (i.e., normalization
is left completely free). Black lines show the expected contours for the LXe experiment described
in the text. For comparison, the solid blue line shows the SOX sensitivity using shape information
alone. In both cases a 5 MCi radioactive 51Cr source is assumed. Right panel: expected sensitivity
for the LXe experiment with five source deployments. Results in this panel are shown from analyses
using shape information only (solid lines) and shape plus normalization (dotted lines).
assumed for the flux normalization and it is therefore left completely free during the fit.
For the backgrounds, an uncertainty of 0.5% is assumed. We expect this could be achieved
by using the data collected during the source-free operation of the detector corresponding
parameter space. Since the low-end threshold for the electron recoil energy is not expected
to have a great impact on the sterile neutrino analysis, it is set to 5 keV in this case.
The results for the sterile neutrino sensitivity are shown in figure 2. Since the
sensitivity mainly comes from a shape analysis, this measurement is far from being limited by
systematics. We find the main limiting factor to be the 136Xe background. The expected
number of background events (across the full energy range) is around 51, 130, from which
LXe depleted in 136Xe, though. The sensitivities for both possibilities are shown in figure 2
(left). For contrast, we also compare the LXe sensitivity to that of Borexino/SOX 
using a comparable, 5 MCi 51Cr source (the SOX proposal is based on 10 MCi). The
commarkable that, for a depleted Xe experiment, most of the region favored by the gallium
anomaly is covered with shape information alone. Therefore, if the gallium anomaly is
correct this configuration would, with high likelihood, confirm it by a clear observation of
the oscillatory pattern.
Results for larger exposure
by repeated redeployments of the 51Cr source. Repeated deployment has a precedent in
GALLEX, which irradiated and deployed the same source material twice . We will
consider five deployments, each identical in source strength and duration to our previously
considered single deployment.
At 95% CL, assuming a low-energy threshold of 2 keV, the corresponding bound is pushed
limit would be surpassed.
The sensitivity to sterile neutrino oscillations with increased statistics is shown in
figure 2 (right). In this case, we assume 90% depleted Xe, and show sensitivities from both
a shape-only analysis, and an analysis with the shape information plus a 2% uncertainty
in the normalization. As can be seen in the comparison of the two bounds, no major
improvement is expected from imposing the normalization constraint, since the information
where the oscillation is averaged out, would a normalization constraint help. According to
our results, after five deployments the full gallium anomaly as well as a sizable region of
the reactor anomaly would be covered at 95% CL.
in the 100 keV range a number of precision tests of the electro-weak sector of the SM become
possible. Assuming an absolute normalization at the 1% level and statistical errors at or
below 1%, overall accuracies for the total cross section of 12% appear feasible, providing
constraints on the weak mixing angle and its running.
Summary and conclusions
To summarize, our results indicate that the combination of a large liquid xenon detector,
designed and built to search for dark matter, with a Mega-curie scale electron capture
neutrino source would provide excellent reach in the search for the neutrino magnetic moment,
exceeding the current laboratory bounds by at least a factor of 5. With repeated source
deployments, such an experiment would even be competitive with the best astrophysical
limits. Moreover, this combination would allow a test of the reactor and gallium anomalies;
specifically, their interpretation as oscillations due to an eV-scale sterile neutrino. Its reach
would be complementary to other source proposals with sensitivity to the oscillating
patstudy is required, but so far no major technological obstacles have been identified.
This work has been supported by the U.S. Department of Energy under award numbers
DE-SC0003915 and DE-SC0009973. We would like to thank the authors of ref.  for
sharing with us their results for the global fit of the reactor and gallium anomalies.
This article is distributed under the terms of the Creative Commons
Attribution License (CC-BY 4.0), which permits any use, distribution and reproduction in
any medium, provided the original author(s) and source are credited.
 D. Bauer et al., Dark matter in the coming decade: complementary paths to discovery and
beyond, arXiv:1305.1605 [INSPIRE].
 M. Pospelov, Neutrino physics with dark matter experiments and the signature of new
baryonic neutral currents, Phys. Rev. D 84 (2011) 085008 [arXiv:1103.3261] [INSPIRE].
JCAP 07 (2012) 026 [arXiv:1202.6073] [INSPIRE].
 M. Pospelov and J. Pradler, Elastic scattering signals of solar neutrinos with enhanced
baryonic currents, Phys. Rev. D 85 (2012) 113016 [arXiv:1203.0545] [INSPIRE].
 M. Pospelov and J. Pradler, Dark matter or neutrino recoil? Interpretation of recent
experimental results, Phys. Rev. D 89 (2014) 055012 [arXiv:1311.5764] [INSPIRE].
Russian-American gallium experiment to neutrinos from a 51Cr source,
Phys. Rev. C 59 (1999) 2246 [hep-ph/9803418] [INSPIRE].
 ZEPLIN-III collaboration, V.N. Lebedenko et al., Limits on the spin-dependent
WIMP-nucleon cross-sections from the first science run of the ZEPLIN-III experiment,
Phys. Rev. Lett. 103 (2009) 151302 [arXiv:0901.4348] [INSPIRE].
 XENON100 collaboration, E. Aprile et al., The XENON100 dark matter experiment,
Astropart. Phys. 35 (2012) 573 [arXiv:1107.2155] [INSPIRE].
 LUX collaboration, D.S. Akerib et al., The Large Underground Xenon (LUX) experiment,
Nucl. Instrum. Meth. A 704 (2013) 111 [arXiv:1211.3788] [INSPIRE].
 XENON1T collaboration, E. Aprile, The XENON1T dark matter search experiment,
Springer Proc. Phys. C12-02-22 (2013) 93 [arXiv:1206.6288] [INSPIRE].
 D.C. Malling et al., After LUX: the LZ program, arXiv:1110.0103 [INSPIRE].
Phys. Rept. 320 (1999) 319 [INSPIRE].
 A. Beda et al., The results of search for the neutrino magnetic moment in GEMMA
 Borexino collaboration, G. Bellini et al., SOX: Short distance neutrino Oscillations with
 LZ collaboration, private communication.
 L. Baudis et al., Neutrino physics with multi-ton scale liquid xenon detectors,
JCAP 01 (2014) 044 [arXiv:1309.7024] [INSPIRE].