IIceCube Sterile Neutrino Searches
B. J. P.
Jones , ∗ For the IceCube Collaboration University of Texas at Arlington, 108 Science Hall, 502 Yates St, Arlington, TX
Abstract.
Anomalies in short baseline experiments have been interpreted asevidence for additional neutrino mass states with large mass splittings fromthe known, active flavors. This explanation mandates a corresponding signa-ture in the muon neutrino disappearance channel, which has yet to be observed.Searches for muon neutrino disappearance at the IceCube neutrino telescopepresently provide the strongest limits in the space of mixing angles for eV-scale sterile neutrinos. This proceeding for the Very Large Volume NeutrinoTelescopes (VLVnT) Workshop summarizes the IceCube analyses that havesearched for sterile neutrinos and describes ongoing work toward enhanced,high-statistics sterile neutrino searches.
Sterile neutrinos are hypothetical particles that have been invoked to explain anomalies inshort baseline accelerator decay-at-rest [1] and decay-in-flight experiments [2], reactor neu-trino fluxes [3, 4] and radioactive source experiments [5]. In low-energy, short baseline exper-iments that are sensitive to ν e appearance, matter e ff ects can be neglected and an oscillationof the form: P ν µ → ν e = sin θ µ e sin (cid:34) ∆ m L E (cid:35) (1)is predicted. A large ∆ m thus introduces an oscillation at a small characteristic L / E , withthe e ff ective mixing parameter, sin θ µ e governing the amplitude of oscillation. To introduceflavor-change at similar L / E as exhibited in the MiniBooNE and LSND experiments, mixingparameters sin θ µ e of 10 − or larger are required, with favored parameter space in the one-to-few eV mass splittings. [6–8].Oscillations in experiments sensitive to disappearance signatures exhibit a similar func-tional form but with a di ff erent e ff ective mixing parameters. Oscillation probabilities in theabsence of matter e ff ects take the form: P ν α → ν α = − sin θ αα sin (cid:34) ∆ m L E (cid:35) , (2)Where α is the disappearing flavor. To explain apparent anomalies in disappearance exper-iments, mixing parameters sin θ ee of O (0 .
1) and ∆ m values of larger than ∼ arerequired [9]. ∗ e-mail: [email protected] a r X i v : . [ h e p - e x ] F e b s c ill a t i o n P r o b a b ili t y Figure 1.
Left: theoretical oscillation probabilities displaying matter resonance; Right: observablesignature of this parameter point in the IceCube high-energy sterile neutrino search.
In the minimal scenario with a single heavy sterile neutrino (3 + ff ective mixingparameters in vacuum-like oscillation experiments sin θ µ e and sin θ ee can be related toelements of an extended leptonic mixing matrix via:sin θ ee = | U e | (1 − | U e | ) , sin θ µµ = | U µ | (1 − | U µ | ) , sin θ µ e = | U µ | | U e | (3)A finite ν µ → ν e appearance signature implies a finite value for both sin θ µµ and sin θ ee . Ageneric prediction of sterile neutrino models that explain short baseline ν µ → ν e appearanceanomalies is that there should be a finite disappearance signature in the channel ν µ → ν µ .Muon neutrino disappearance can be probed by atmospheric neutrino oscillation exper-iments such as SuperKamiokande [10] and IceCube [11, 12], as well as accelerator neu-trino experiments [13, 14]. Of these, IceCube probes the highest energy range, with a high-statistics sample of well-reconstructed atmospheric neutrinos spanning the range 6 GeV to20 TeV. This is a regime where matter e ff ects are not only non-negligible, but can be verylarge. For a 1 eV sterile neutrino, for example, a large matter-induced resonance would beexpected at 3 TeV, greatly amplifying the ordinarily small oscillation probability [15–19]. Anexample oscillation spectrum calculated by the NuSQUIDS software package [20] is shown inFig 1.IceCube has performed searches for sterile neutrinos in both high-energy [11] and low-energy [12] data samples. No evidence for anomalous muon neutrino disappearance wasobserved in either case. These null observations, along with disappearance constraints fromother experiments, have introduced a severe tension into sterile neutrino models. The viableparameter space for a simple sterile neutrino explanation of short baseline and ν e disappear-ance anomalies is now small according to some commentators [21] and vanishing accordingto others [22]. This has prompted consideration of more exotic scenarios to explain theanomalies, which may [23–26] or may not [27–31] include sterile neutrinos. A new high-energy sterile neutrino analysis from IceCube using seven years of data and commensuratelyimproved control of systematic uncertainties is now in preparation. This analysis will probethe region of small mixing angles to further constrain the parameter space of sterile neutrinomodels.In this proceeding we briefly describe the one-year IceCube high-energy sterile neutrinoanalysis (Sec. 2), the three-year IceCube low-energy sterile neutrino analysis (Sec. 3), andthe forth-coming seven-year IceCube sterile neutrino search (Sec. 4). igure 2.
90% CL limit from the IceCube high-energy sterile neutrino search compared with allowedregions from appearance experiments (blue) and limits at the time of publication (grey / black), as wellas subsequent ν µ disappearance results published since (purple). The high-energy sterile neutrino search at IceCube [11] used one year of atmospheric neu-trino data passing an event selection developed for a search for di ff use astrophysical muonneutrinos [32]. The up-going tracks from atmospheric neutrinos dominate below ∼
100 TeV,and have a particularly clean signature, with the Earth acting as a filter against contaminationfrom muons created in cosmic ray air showers. The sample is thus e ff ectively background-free. The dataset used to search for resonant sterile neutrino oscillations contains 20,145reconstructed up-going muon tracks in the approximate energy range 320 GeV to 20 TeV.In order to remain sub-dominant to statistical uncertainty, systematic uncertainties in theshape of the reconstructed spectrum must be controlled at the level of around 6-7% per bin.The dominant uncertainties include the properties of the South Pole ice [33], e ffi ciency ofthe digital optical modules [34], and the atmospheric neutrino flux shape, which was param-eterized by a tunable spectral index, ¯ ν/ν ratio, π/ K production ratio and a set of discrete pri-mary cosmic ray models propagated via the MCEq cascade calculation [35, 36]. Additionalsources of systematic uncertainty including neutrino cross section, Earth density model andatmospheric density profile were also incorporated, but shown to be sub-dominant.No evidence for oscillation was found within experimental sensitivity. This places astrong constraint on the mixing angle sin θ extending to 0.02 at 0.3 eV . This limit on θ is constructed with the conservative choice of θ =
0. A stronger limit in sin θ is im-plied for non-zero θ [19]. The negative IceCube result, compared to other negative resultsobtained from searches for ν µ disappearance experiments as well as and the allowed regionfrom appearance experiments at the time of the IceCube publication are shown in Fig 2. igure 3. Results from the IceCube low-energy sterile neutrino search assuming each mass ordering,compared to similar results from SuperKamiokande.
At low energies ( <
100 GeV) IceCube is sensitive to standard atmospheric neutrino oscilla-tions [37]. The inclusion of sterile neutrino mixing within an extended neutral lepton mixingmatrix impacts the oscillation probability in this region [38], with scale of e ff ect proportionalto the matter density traversed. In IceCube, the observable e ff ect is independent of ∆ m within the mass range of interest, since the oscillations are fast enough to be averaged by theenergy resolution of the detector. The most pronounced e ff ect is expected at an energy of 20GeV for upgoing muons [12].An event selection was developed to isolate muon neutrino events between 6.3 and56 GeV, and applied to three years of IceCube data to yield 5,118 total events. Becauseof their low energies, reconstruction is more challenging and background rejection more dif-ficult than in the higher energy sample. To mitigate against backgrounds, the DeepCoresub-array was used for event selection and reconstruction with the remainder of the IceCubearray serving as a veto against atmospheric muon backgrounds.At these energies the properties of the refrozen ice in the immediate vicinity of the detec-tor strings dominates the ice uncertainty budget. Along with digital optical module e ffi ciency,this represents the largest detector systematic uncertainty. low-energy neutrino interactionsrequire control of distinct cross section uncertainties to the high-energy, deep inelastic sam-ples, including the resonant and quasielastic axial masses. Flux parameter uncertainties in-cluding spectral index and an energy dependent ν/ ¯ ν flux ratio are included. Finally, sincelower energy muons are more challenging to reconstruct and select than their higher energycounterparts, ν e and atmospheric muon contamination in the sample is calculated and param-eterized with an uncertainty.No evidence of atmospheric muon neutrino disappearace was observed, leading to alimit expressed in terms of the mixing matrix elements | U µ | = sin θ and | U τ | = sin θ cos θ , shown in Fig. 3 Future Plans
The IceCube collaboration is preparing an extended, 7 year high-energy sterile neutrinosearch. The event selection has been enhanced, with increased e ffi ciency, especially at low-energy, while retaining an e ff ectively background-free selection. The number of expectedevent is approximately 280,000, which is 13 times as many as in the published one-year anal-ysis. With this enhancement of statistical precision comes a need for corresponding controlof systematic uncertainties, which have undergone significant improvements for this analysis.Bulk ice uncertainties arising from the depth-dependent dust distribution within the Ice-Cube detector have been studied using a multidimensional procedure producing covariancematrices in analysis space. These matrices encode the ice model variability allowed withinconstraints from LED calibration data, complete with all depth-dependent correlations, ex-tending beyond the e ff ective uncertainty on global absorption and scattering coe ffi cients usedin previous analyses. A continuous parameterizartion of refrozen hole ice scattering hasbeen incorporated, deriving from advances in understanding its properties from lower energyanalyses. An advanced treatment of the atmospheric flux parameters has been implementedusing the “Barr scheme” [39], applying 6 e ff ective parameters that capture the uncertainty inhadronic modelling of the air shower, constrained by collider data. This replaces the e ff ective π/ K and ν/ ¯ ν parameters with a more physically motivated and complete uncertainty param-eterization. Finally, the next generation of the sterile neutrino search will treat the e ff ects ofnon-zero θ explicitly, to provide confidence intervals in θ and ∆ m at several fixed θ points, rather than simply providing the most conservative limit at θ = References [1] C. Athanassopoulos et al. (LSND), Phys. Rev. Lett. , 3082 (1996), nucl-ex/9605003 [2] A.A. Aguilar-Arevalo et al. (MiniBooNE), Phys. Rev. Lett. , 221801 (2018), [3] A. Serebrov et al. (NEUTRINO-4) (2018), [4] G. Mention, M. Fechner, T. Lasserre, T.A. Mueller, D. Lhuillier, M. Cribier, A. Le-tourneau, Phys. Rev. D83 , 073006 (2011), [5] C. Giunti, M. Laveder, Phys. Rev.
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