Constraining Sterile Neutrino Interpretations of the LSND and Miniboone Anomalies with Coherent Neutrino Scattering Experiments

PHYSICAL REVIEW D 101, 075051 (2020) Constraining sterile neutrino interpretations of the LSND and MiniBooNE anomalies with coherent neutrino scattering experiments Carlos Blanco ,1,2 Dan Hooper,2,3,4 and Pedro Machado5 1Department of Physics, University of Chicago, Chicago, Illinois 60637, USA 2Kavli Institute for Cosmological Physics, University of Chicago, Chicago, Illinois 60637, USA 3Theoretical Astrophysics Group, Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA 4Department of Astronomy and Astrophysics, University of Chicago, Chicago, Illinois 60637, USA 5Theoretical Physics Department, Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA (Received 14 February 2019; accepted 7 April 2020; published 29 April 2020) We propose to test the light sterile neutrino solution to the Liquid Scintillator Neutrino Detector (LSND) and MiniBooNE anomalies by measuring the coherent elastic scattering rate of neutrinos from a pulsed spallation source. Coherent scattering is universal across all active neutrino flavors, and thus can provide a measurement of the total Standard Model neutrino flux. By performing measurements over different baselines and making use of timing information, it is possible to significantly reduce the systematic uncertainties and to independently measure the fluxes of neutrinos that originate as νμ or as either νe or ν¯μ. We find that a 100 kg CsI detector would be sensitive to a large fraction of the sterile neutrino parameter space that could potentially account for the LSND and MiniBooNE anomalies. DOI: 10.1103/PhysRevD.101.075051 I. INTRODUCTION Together, these anomalies appear to point to the exist- A large number of neutrino oscillation experiments ence of a sterile neutrino with a mass near the electronvolt carried out over the past two decades have lead to the scale. Complicating this interpretation, however, are the ν → ν establishment of a standard framework, in which the results of several experiments which measure μ μ neutrino sector is comprised of three massive neutrinos transitions. In particular, the IceCube [14] and MINOS/ with relatively large mixing angles and subelectronvolt MINOS+ [15] experiments should be sensitive to the mass splittings. Some of the data that has been collected, presence of such a sterile neutrino, but have observed no however, cannot be explained within the context of this deviation from the standard (three-neutrino) framework. standard framework. Particularly puzzling have been the Detailed statistical analyses of the combined short baseline neutrino data have revealed a large degree of tension, at the results of the Liquid Scintillator Neutrino Detector (LSND) 4 7σ ν → ν [1] and MiniBooNE [2] experiments. In the former, an . level, between the appearance ( μ e) and disap- pearance (νμ → νμ and νe → νe) datasets, at least within the excess of ν¯e events was observed from a source of pions decaying at rest, while in the latter an excess of electronlike context of oscillations with one or more sterile neutri- events was observed in both νμ and ν¯μ beams. Although nos [3,16,17]. these experiments bear very distinct neutrino energy Motivated by this tension, a number of alternative explanations for the short baseline anomalies have recently profiles, their results are each consistent with νμ → νe oscillations occurring over a short baseline, requiring a been put forward. For example, models in which neutrinos upscatter to heavier fermions can provide a good fit to relatively large mass splitting [3–6]. Moreover, reexami- MiniBooNE’s energy spectrum and angular distribution nations [5,7–9] of the Gallium calibration experiments [18–21], but cannot account for the LSND excess. Other [10,11] and reactor antineutrino fluxes [12,13] have explanations involving “dark” particles produced in the identified a deficit of νe events relative to theoretical beam have also been considered, but are strongly con- expectations. strained by the MiniBooNE beam dump run data [22,23]. Moreover, scenarios that connect the neutrino mass gen- eration to extra dimensions, leading to an infinite Kaluza- Klein tower of sterile neutrinos, also seem to be disfavored Published by the American Physical Society under the terms of by data [24–32]. Importantly, an alternative analysis of the the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to LSND data finds that the potential signal may be at the edge the author(s) and the published article’s title, journal citation, of the detector’s sensitivity [33]. At this time, the correct and DOI. Funded by SCOAP3. interpretation of this data remains unclear, and further 2470-0010=2020=101(7)=075051(6) 075051-1 Published by the American Physical Society CARLOS BLANCO, DAN HOOPER, and PEDRO MACHADO PHYS. REV. D 101, 075051 (2020) information will be required in order to clarify this dσ G2 M ¼ F ½N − Zð1 − 4 2θ Þ2 confusing situation. dE 4π sin W T In this paper, we propose to measure or constrain the E ME E2 occurrence of short-baseline active-to-sterile neutrino oscil- 1 − T − T þ T F2ðE Þ; ð Þ × E 2E2 2E2 T 2 lations using neutrinos from a source of pions decaying at ν ν ν rest, which are then measured via neutral current coherent E E θ elastic neutrino-nucleus scattering. This interaction, which where T is the recoil energy, ν is the neutrino energy, W is M N Z has recently been observed for the first time by the the weak mixing angle, is the target nuclear mass, ( )is F COHERENT Collaboration [34], using a CsI target [35], the number of neutrons (protons) in the nucleus, and is the is universal across all three active neutrino flavors, and thus Helm nuclear form factor given by the following [46]: can provide a measurement of the total (all flavor) active 3j1ðR0QÞ Qs neutrino flux [36,37] (see also Refs. [38–40]). Furthermore, FðQÞ¼ exp − ; by taking data on two different baselines, we can sub- R0Q 2 x x stantially reduce the systematic uncertainties associated j ðxÞ¼sin − cos ; ð Þ with the detector efficiency, as well as with the overall flux 1 x2 x 3 and cross section [38,41–43]. The use of timing information provides information pertaining to neutrino flavor, where j1 is the first-order spherical-Bessel function, Q2 ¼ −p pμ ¼ 2ME allowing us to disentangle the effects of sterile neutrino pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiμ T is the momentum transfer, and mixing with νμ and νe, and thus providing a powerful probe 2 2 2 R0 ¼ R − 5s is determined by the surface thickness, of the LSND and MiniBooNE anomalies. s ¼ 0.5ðfmÞ, and the effective nuclear radius, R ¼ 1 1.2A3 ðfmÞ [47]. In this study, we will focus on the case II. COHERENT NEUTRINO SCATTERING of CsI as our target for coherent scattering. We have For our experimental setup, we have in mind a source of neglected spin-dependent contributions to the cross section neutrinos that is functionally similar to the Spallation which come from axial couplings since, relative to the vector Neutron Source at the Oak Ridge National Laboratory contributions, axial contributions should be suppressed by a 1= [44,45]. In particular, we consider a pulsed beam of 1 GeV factor of A [48]. 23 −1 The total coherent scattering cross section is given by protons with a luminosity of L0 ¼ 4 × 10 yr , of which approximately 8% (6%) produce a πþ (π−) upon striking integrating Eq. (2) up to a maximum recoil energy of E ¼ð2E2=ðM þ 2E ÞÞ the spallation target. T max ν ν . We also include a signal The high-Z nature of the liquid mercury target depletes acceptance function, which we take to interpolate linearly E ¼ 2 5 E ¼ 5 virtually all of the negatively charged pions through nuclear between zero at T . keV and 100% at T keV. capture (only ∼2.3 × 10−5 decay prior to nuclear capture), This represents an improvement of a factor of 2 in the E while the positively charged pions are rapidly slowed but midpoint T over that described in Ref. [34]. not captured [44]. These particles then decay at rest, via By utilizing timing information, it is possible to dis- þ þ þ tinguish the νμ flux from the combined flux of νe and ν¯μ.In π → μ νμ → e νeν¯μνμ, yielding the following isotropic þ particular, the muon lifetime (τμ ¼ 2.197 μs) delays the νe spectrum per π : and ν¯μ arrival times to a degree that is long compared to both the pion lifetime (0.026 μs) and the Gaussian pulse dN 2 2 width (0.16 μs), but short compared to the time between νμ mπ − mμ ¼ δ Eν − ; pulses at the Spallation Neutron Source (0.017 s). This is dE μ 2m νμ π illustrated in Fig. 1, where we show the time profile of the dN 6E2 m 2 neutrino luminosity from such a pulsed spallation source. ν¯μ ¼ ν¯μ μ − E ;E≤ m =2; 4 ν¯μ ν¯μ μ These profiles are calculated by convolving the decay dEν¯ ðmμ=2Þ 2 3 μ profile of each progenitor particle with the time profile dN 12E2 m νe νe μ of the beam pulse. ¼ − Eν ;Eν ≤ mμ=2; ð1Þ dE ðm =2Þ4 2 e e νe μ III. PROJECTED SENSITIVITY TO STERILE NEUTRINOS where mμ and mπ are the masses of the muon and pion, α respectively. The probability of a neutrino of flavor oscillating to β After traveling to the location of the detector, these flavor is given by the following: neutrinos can be observed through coherent elastic neutrino- X Δm2 L Ã Ã jk nucleus scattering. The cross section for this process is given Pα→β ¼ Uα;jUβ;jUα;kUβ;k exp −i ; ð4Þ 2Eν as follows: j;k 075051-2 CONSTRAINING STERILE NEUTRINO INTERPRETATIONS OF … PHYS.

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