Searching for sterile neutrinos at the ESSνSB
c The Authors.
850 West Campus Dr
Blacksburg, VA, 24061
106 91 Stockholm
Instituto de F sica Te orica UAM/CSIC
KTH Royal Institute of Technology, AlbaNova University Center
Center for Neutrino Physics, Physics Department
Departamento de F sica Te orica, Universidad Aut onoma de Madrid
Department of Theoretical Physics, School of Engineering Sciences
Calle Nicol as Cabrera 13-15, Cantoblanco
Calle Francisco Tomas y Valiente
Cantoblanco, Madrid, E-28049
The ESSSB project is a proposed neutrino oscillation experiment based on the European Spallation Source with the search for leptonic CP violation as its main aim. In this letter we show that a near detector at around 1 km distance from the beamline is not only very desirable for keeping the systematic errors affecting the CP search under control, but would also provide a significant sensitivity probe for sterile neutrino oscillations in the region of the parameter space favored by the long-standing LSND anomaly. We find that the effective mixing angle e can be probed down to sin2(2e) ' 2(8) 103 at 5 assuming 15% bin-to-bin (un)correlated systematics.
1 Introduction 2 3 4
Setup and simulation details
Summary and conclusions
Since the first evidence for neutrino oscillations in 1998, the experimental progress in the
field has been remarkable. Today the standard three-flavor paradigm of neutrino
oscillations is well tested and most of the parameters it contains are known to high precision .
This framework includes six parameters out of which three are mixing angles, two are
independent mass squared differences and one is a, yet unknown, CP violating phase. However,
there are a number of experimental anomalies that do not fit into this picture. The
obserintroducing one or more additional sterile neutrino states, with an additional mass squared
difference of m241 O(eV2) in order to allow for e oscillations in the observed
range of L/E. However, KARMEN did not observe any signal for similar values of L/E .
The dedicated MiniBooNE experiment  has to this day not confirmed or disproved the
seem to observe a deficit which would be compatible with sterile neutrino oscillations in
experiments [14, 15]. Overall, global analyses show strong tension between different data
sets  and further experimental input will be needed to clarify the situation.
trino oscillation experiment based upon the accelerator facilities of the future European
Spallation Source (ESS) and optimized for the goal of a high significance search for CP
atively high signal, which implies that the bottleneck of CP violation searches for the next
generation of neutrino oscillation facilities will typically be systematics errors rather than
statistics or backgrounds. The effect of such systematic uncertainties can be alleviated in
two ways. If the statistics is high enough, placing the detector close to the second
oscillation maximum enhances the relative importance of the CP-violating component of the
oscillation probability, increasing the sensitivity of the facility and reducing the impact of
for this location which was adopted as its baseline design. The second way of controlling
the negative impact of systematic uncertainties on the CP violation search is the inclusion
of a near detector. The idea is that the two detectors would observe the same flux in
absence of oscillations, thus reducing the impact of flux uncertainties. The optimal way
to do this is to place the near detector at a non-negligible distance of the neutrino source
(such as 1 or 2 km), since otherwise the geometric acceptance of the detector would make
the observed flux very different from the one expected at the far detector. However, even
of systematic errors, it could also be used to test for new physics in the neutrino sector.
a near detector placed at a baseline of 1 km would be ideal for testing oscillations of sterile
neutrinos in the parameter range indicated by the LSND anomaly. Furthermore, the high
power of the beam (5 MW) would enable the possibility to test very small values of the
oscillation probability with a very high significance.
nel. In order to do so, we will adopt a phenomenological approach where the oscillation
Here, e is an effective mixing angle, m241 m24 m21 is the active-sterile squared mass
difference, L is the distance from the source, and E is the neutrino energy. This approach
is a reasonable approximation as long as we are dealing with small enough values of L/E
such that the active-active neutrino oscillations have not developed significantly yet.
no active sterile mixing and
periments) while Case II corresponds to a situation in which the active-sterile oscillation
parameters take values which are close to the allowed regions shown in figure 7 of ref. ,
Setup and simulation details
Since the flux peak would be located around 250 MeV, a distance to the detector of 1 km
would be needed in order to match the maximum of the oscillation, assuming a squared mass
splitting m241 O(1 eV) (as LSND  and MiniBooNE  results seem to indicate).
have considered the same detector technology as for the far detector. This choice would
minimize the effect of systematic errors coming from neutrino interaction cross sections
and detector performance, which would be the primary purpose of the near detector at the
identical to the far detector in terms of efficiencies and background rejection capabilities.
The response of the detector has been simulated using migration matrices, signal and
background rejection efficiencies from ref. .
Neutrino fluxes have been explicitly simulated for a near detector placed at 1 km from
the source . A beam power of 5 MW and 1.7 107 operating seconds per year with
2.5 GeV protons, as for the long baseline experiment , is assumed. In order to respect the
configuration of the long baseline experiment (which is optimized for CP violating searches)
we keep the same ratio of neutrino (2 years) and antineutrino (8 years) running times.1
have already been accounted for.
On the other hand, we find that the expected total number of background events in
background rejection capabilities as in refs. [20, 29]. The largest contributions to the
backevents mis-identified as charged-current (CC) events. In principle, if a sterile neutrino
exists and has sizable mixing with the active sector, the background rates should also be
affected by oscillations. Nevertheless, we find the impact of this effect on the sensitivity
of the setup to be negligible, and therefore oscillations have not been considered for the
background rates in our analysis.
would be strongly limited by systematic errors, however. In fact, the results for our setup
possibility could be to combine appearance and disappearance data, since the effects in the
two channels should be partially correlated. However we have numerically checked that,
for the proposed setup and within a 3+1 oscillation framework, where the two oscillation
probabilities can be expressed in terms of one mass splitting and two mixing angles, the
addition of disappearance data does not yield an observable improvement over the results
will not be considered in this work.
range, see ref. . The energy resolution for a water Cherenkov detector would limit the
size of the energy bins to 100 MeV, implying that only a few energy bins would contain
a sizable number of events. However, we have found that the sensitivity of the setup would
1We note, however, that in order to optimize the experiment to search for a sterile neutrino, a different
ratio may be desirable, in particular taking into account the different size of neutrino and antineutrino cross
sections. See also ref.  for optimizations to standard oscillation physics.
be compromised if the analysis was done as a counting experiment only (without energy
bin separation). The main reason for this is that the signal and background spectra show
a different dependence with neutrino energy, which implies that different energy bins will
energy, with nine 100 MeV bins between 0.1 and 1 GeV.
Given the high statistics at the near detector, the performance will be limited by
systematics. Thus, we will show our results in two scenarios with a different implementation of
systematic errors. For the more optimistic option an overall 15% uncertainty, uncorrelated
between the different channels and between the signal and background components but
fully correlated among the bins of each channel, is considered. For the more conservative
option, on the other hand, we allow uncorrelated systematics among the different energy
bins in each channel, so as to account for uncertainties in the shape of the fluxes and cross
sections. However, in order to accommodate an overall normalization shift for the signal
of bins with respect to the more optimistic case. Therefore, in this conservative scenario
we also add an additional 15% normalization uncertainty which is correlated among all
bins in order to avoid this behaviour. A modified version of the GLoBES software as in
ref.  was used for this implementation of the systematics. A pull-term corresponding to
parameters to search for the minimum. The procedure is then repeated for all points in
with regard to sterile neutrino oscillations, the far detector is not considered in our
simulations. The presence of sterile neutrinos of the type considered here is also not likely to
sterile neutrino oscillations would be averaged out at the far detector and no additional
CP violating phases in the 3+1 scenario considered here can play a role, the effect of the
sterile neutrinos would be similar in the neutrino and antineutrino channels and would
expected background level could presumably be accommodated in the fit by varying the
Therefore, the CP violating discovery potential would remain unaffected, while the values
of the mixing angles may be slightly modified. We do not expect this modification to be
very significant. Indeed, we have estimated that for sin2(2e) = 1 102 this additional
background would be of the same order than the intrinsic beam contamination and, as can
the signal to background ratio is rather large and the experiment is statistics, rather than
the assumption of no active-sterile neutrino mixing (Case I). As expected from the baseline
to-bin systematics of 15%. The no-systematics limit is also shown. The shaded region corresponds
appearance experiments, taken from figure 7 of ref. . Right: the expected confidence regions for
are shown for correlated bin-to-bin systematics of 15% and for the case where the systematic
uncertainties are completely switched off in the analysis, as indicated in the legend.
length of 1 km and typical neutrino energy of 0.3 GeV, the best sensitivity is achieved
as the oscillations do not have sufficient time to develop. On the other hand, for larger
values of the mass splitting the oscillations enter the averaged regime. For comparison
with the current experimental situation, the allowed region at 99% CL from a global fit
seen in the figure, the final results will be dominated by systematic errors. Assuming
binto-bin correlated 15% systematic errors the currently allowed region could be completely
scenario of completely uncorrelated errors is somewhat more limited. However, we find
that also in this scenario most of the preferred region, including the best fit, is covered
uncertainties values of sin2(2e) down to 104 could be probed.
under the assumption of active-sterile neutrino mixing with oscillation parameters close to
no systematics and the case with 15% bin-to-bin correlated systematic errors, for which a
great precision can be achieved in most cases for both variables. In the case of bin-to-bin
uncorrelated systematic errors, we find the following results: (i) in the case of small mixing
extracted from the sensitivity contours shown in the left panel in the same figure); (ii) for
larger values of the mixing angle (sin2 2e 0.01) the confidence regions are no longer
Summary and conclusions
In this letter we have discussed the possibility of using a near detector at the recently
oscillations in the range indicated by the LSND anomaly, m241 1 eV2 and sin2 2e
102. Our study is based on the performance of a 1 kt near water Cherenkov detector at
a distance of 1 km from the source. Under the assumption of no active-sterile neutrino
Even in the more conservative scenario of fully uncorrelated bin-to-bin systematics, most
active-sterile mixing takes place with oscillation parameters in the range currently favored
extremely good accuracy.
Finally it should also be noted that, in order not to interfere with the main goal of the
searches has been performed in this work. Therefore, if a signal of the existence of sterile
neutrinos was to be found, some room for improvement over the results obtained here may
at a distance of 1 km, and the authors of ref.  for providing the curves for their global
fit allowed regions for comparison.
This work has been supported by the Goran Gustafsson Foundation [MB], and by the
U.S. Department of Energy under award number DE-SC0003915 [PC]. EFM acknowledges
financial support by the European Union through the FP7 Marie Curie Actions CIG
NeuProbes (PCIG11-GA-2012-321582) and the ITN INVISIBLES (PITN-GA-2011-289442),
and the Spanish MINECO through the Ramon y Cajal programme (RYC2011-07710)
and through the project FPA2009-09017. We also thank the Spanish MINECO (Centro
de excelencia Severo Ochoa Program) under grant SEV-2012-0249 as well as the Nordita
Scientific program News in Neutrino Physics, where this work was initiated.
This article is distributed under the terms of the Creative Commons
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any medium, provided the original author(s) and source are credited.
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