The recent study on the the 6-year up-going muon neutrinos by the IceCube
Collaboration support the hypothesis of a two-component scenario explaining the
diffuse TeV-PeV neutrino flux. Once a hard astrophysical power-law is
considered, an excess in the IceCube data is shown in the energy range 10-100
TeV (low-energy excess}). By means of a statistical analysis on the neutrino
energy spectrum and on the angular distribution of neutrino arrival directions,
we characterize a two-component neutrino flux where decaying/annihilating Dark
Matter particles provide a contribution to the IceCube observations.Comment: 4 pages, 1 figure. Prepared for the proceedings of the conference
  IFAE 201

Chianese, Marco

English

arXiv

The recent study on the 6 year up-going muon neutrinos by the IceCube Collaboration support the hypothesis of a two-component scenario explaining the diffuse TeV-PeV neutrino flux. Once a hard astrophysical power-law is considered, an excess in the IceCube data is shown in the energy range 10–100 TeV (low-energy excess). By means of a statistical analysis on the neutrino energy spectrum and on the angular distribution of neutrino arrival directions, we characterize a two-component neutrino flux where decaying/annihilating Dark Matter particles provide a contribution to the IceCube observations

Chianese M.

Archivio della ricerca - Università degli studi di Napoli Federico II

The IceCube low-energy excess: A Dark Matter interpretation

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DOI 10.1393/ncc/i2018-18056-1Colloquia: IFAE 2017IL NUOVO CIMENTO 41 C (2018) 56The IceCube low-energy excess: A Dark Matter interpretationMarco Chianese(1)(2)(1) Dipartimento di Fisica, Università di Napoli “Federico II” - Complesso Univ. MonteS. Angelo, Via Cinthia, I-80126 Napoli, Italy(2) INFN, Sezione di Napoli - Complesso Univ. Monte S. Angelo, Via Cinthia, I-80126 Napoli,Italyreceived 21 April 2018Summary. — The recent study on the 6 year up-going muon neutrinos by the Ice-Cube Collaboration support the hypothesis of a two-component scenario explainingthe diffuse TeV-PeV neutrino flux. Once a hard astrophysical power-law is con-sidered, an excess in the IceCube data is shown in the energy range 10–100 TeV(low-energy excess). By means of a statistical analysis on the neutrino energy spec-trum and on the angular distribution of neutrino arrival directions, we characterizea two-component neutrino flux where decaying/annihilating Dark Matter particlesprovide a contribution to the IceCube observations.1. – IntroductionThe IceCube Neutrino Telescope has observed for the first time a diffuse extraterres-trial neutrino flux in the TeV–PeV energy range, with a deviation from the atmosphericbackground of about 7σ [1-4]. However, until now the origin of such a diffuse neutrinoflux is unclear. Under the reasonable assumption of a correlation with hadronic cosmicrays, one would expect that standard astrophysical sources give rise to a neutrino fluxparametrized in terms of a power-law behavior E−γν with γ being the spectral index [5-7].The recent IceCube observations of 6 year up-going muon neutrinos [4] are well ex-plained at high energies (Eν ≥ 100TeV) by a single hard power-law with γ = 2.13±0.13.Such a value is in a 3.3σ tension with the previous analyses that provide a combinedbest-fit spectral index of 2.50 ± 0.09 [2]. This tension suggests the presence of a secondcomponent in the 10–100 TeV energy range. Moreover, such a new component may havea galactic origin since the 6 year up-going muon neutrino data do not point towards theGalactic Center of the Milky Way.Indeed, once an astrophysical power-law flux with spectral index 2.0 (2.2) is consideredfor all neutrino flavours(1), a low-energy excess (10–100 TeV) appears in both 2 year(1) In the case of standard astrophysical sources, the flavour ratio at the Earth is (νe : νμ :ντ ) = (1 : 1 : 1) due to the neutrino oscillations.Creative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0) 12 MARCO CHIANESEMESE [1] and 4 year HESE [3] IceCube data with a local statistical significance of 2.3σ(1.9σ) [8,9]. Assuming that the low-energy excess is not just a statistical fluctuation, wehave characterized the properties of a Dark Matter (DM) signal able to explain it.2. – Two-component neutrino fluxIn addition to the atmospheric background, we have proposed the following two-component neutrino flux:(1)dφdEνdΩ= φAstro0(Eν100TeV)−γ+dφDMdEνdΩ,where the DM neutrino flux takes the form(2)dφDMdEνdΩ∣∣∣∣dec.=14π mDM τDM{fGdec.[ρh (s, θ) ,dNdEν]+ fEGdec.[dNdEν]},(2)dφDMdEνdΩ∣∣∣∣ann.=〈σv〉8π m2DM{fGann.[ρh (s, θ)2,dNdEν]+ fEGann.[dNdEν, B (z)]},(3)in the decaying (dec.) and annihilating (ann.) cases, respectively. In the above expres-sions, mDM is the DM mass, whereas τDM and 〈σv〉 are the lifetime and the thermallyaveraged cross-section, respectively. In the brackets, the Galactic component fG de-pends on the angular coordinate θ measuring the angular distance from the GalacticCenter through the DM halo density profile ρh [10](3), while the ExtraGalactic onefEG is isotropic. The behaviour of the DM neutrino flux as a function of the energyis instead described by the energy spectrum dN/dEν that depends on the particulardecaying/annihilating channel considered. Finally, in the case of annihilating DM theExtraGalactic component also depends on the so-called boost factor B (z) [10].In order to infer the properties of the DM neutrino flux explaining the low-energyexcess, we have performed two complementary studies: an angular analysis and an en-ergetic one. The angular analysis is based on comparing the distribution of the arrivaldirections of IceCube events with the angular distributions expected from a DM signal.Since decaying and annihilating DM fluxes have distinct angular distributions due to thedifferent dependence on the DM halo density profile (see eqs. (2) and (3)), such an angu-lar analysis can discern among the two DM signals. On the other hand, the analysis onthe neutrino energy spectrum is sensitive to the decaying/annihilating channel consideredsince, for instance, the energy spectrum is quite different in the case of leptons or quarksin the final states. Moreover, such an analysis also provides the allowed regions in theparameters spaces mDM-τDM and mDM-〈σv〉 compatible with the IceCube observations.3. – Results and conclusionsThe angular analysis performed on the 4-year HESE data [8] shows that only thecase of annihilating DM with a small boost factor is already ruled out, while other(2) More details about the expressions of the DM flux can be found in ref. [9].(3) We consider the Navarro-Frenk-White (NFW) distribution [11] as a benchmark.THE ICECUBE LOW-ENERGY EXCESS: A DARK MATTER INTERPRETATION 3Fig. 1. – Statistical significance of the two-component scenario with respect to the case of asingle astrophysical power-law, once a spectral index of 2.0 is considered. The left (right) panelsrefer to the case of leptonic (hadronic) final-states, whereas the top (bottom) plots correspondto the decaying (annihilating) DM scenario. The plots are taken from ref. [9].DM scenarios are still allowed by data. Indeed, in order to statistically rule out a DMinterpretation of the excess, hundreds of events in the 10–100 TeV energy range arerequired [8].The main results of the analysis on the neutrino energy spectrum are reported infig. 1 (see ref. [9] for more details). The plots show the statistical preference in standarddeviations σ (evaluated by means of a likelihood-ratio statistical test) of the IceCubedata for the two-components scenario provided in eq. (1). The maximum significance ofthe DM component (white dot) reaches about 4σ. Moreover, the red lines delimit fromabove the region in the parameter space that is excluded by the IceCube measurements.The gamma-rays constraints are shown by the black lines, which are related to differentDM contributions (1%, 10% and 100%) to the Isotropic diffuse Gamma-Ray Background(IGRB) measured by Fermi-LAT [12]. Finally, the yellow lines bound the region that isnot allowed according to the unitarity constraint on the cross-section [13].It is worth observing that, since it is reasonable to assume that standard astrophysicalsources account at least for the 90% of the IGRB spectrum, the neutrino and gamma-ray data favour a decaying DM interpretation of the IceCube low-energy excess over an4 MARCO CHIANESEannihilating one. Furthermore, leptonic final states (represented by the τ+τ− channel)are preferred with respect to hadronic ones (represented by the tt channel).∗ ∗ ∗We acknowledge support by the Istituto Nazionale di Fisica Nucleare I.S. TASP andthe PRIN 2012 Theoretical Astroparticle Physics of the Italian Ministero dell’Istruzione,Università e Ricerca.REFERENCES[1] IceCube Collaboration (Aartsen M. G. et al.), Phys. Rev. D, 91 (2015) 022001,arXiv:1410.1749.[2] IceCube Collaboration (Aartsen M. G. et al.), Astrophys. J., 809 (2015) 98,arXiv:1507.03991.[3] IceCube Collaboration (Aartsen M. G. et al.), The IceCube neutrino observatory -Contributions to ICRC 2015 Part II: Atmospheric and astrophysical diffuse neutrinosearches of all flavors, arXiv:1510.05223.[4] IceCube Collaboration (Aartsen M. G. et al.), Observation and characterization of acosmic muon neutrino flux from the northern hemisphere using six years of IceCube data,arXiv:1607.08006.[5] Loeb A. and Waxman E., J. Cosmol. Astropart. Phys., 05 (2006) 003, arXiv:astro-ph/0601695.[6] Kelner S. R., Aharonian F. A. and Bugayov V. V., Phys. Rev. D, 74 (2006) 034018;79, (2009) 039901(E), arXiv:astro-ph/0606058.[7] Winter W., Phys. Rev. D, 88 (2013) 083007, arXiv:1307.2793.[8] Chianese M., Miele G., Morisi S. and Vitagliano E., Phys. Lett. B, 757 (2016) 251,arXiv:1601.02934.[9] Chianese M., Miele G. and Morisi S., J. Cosmol. Astropart. Phys., 01 (2017) 007,arXiv:1610.04612.[10] Cirelli M. et al., J. Cosmol. Astropart. Phys., 03 (2011) 051; 10 (2012) E01E,arXiv:1012.4515.[11] Navarro J. F., Frenk C. S. and White S. D. M., The Structure of cold dark matterhalos, arXiv:astro-ph/9508025.[12] Fermi-LAT Collaboration (Ackermann M. et al.), Astrophys. J., 799 (2015) 86,arXiv:1410.3696.[13] Griest K. and Kamionkowski M., Phys. Rev. Lett., 64 (1990) 615.

The IceCube low-energy excess: a Dark Matter interpretation

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