The IceCube neutrino telescope has observed for the first time a diffuse flux
of high energy neutrinos, with a possible astrophysical origin. Up to now there
are no evidence of sources and many hypothesis are still plausible in order to
explain the measured flux. In this proceeding we analyze an alternative way to
interpret the IceCube neutrinos, in terms of sum of contributions from
different sources.Comment: Prepared for the proceeding of the conference IFAE 201

Palladino, Andrea

English

arXiv

The IceCube neutrino telescope has observed for the first time a diffuse flux of high energy neutrinos, with a possible astrophysical origin. Up to now there is no evidence of sources and many hypotheses are still plausible in order to explain the measured flux. In this work we analyze an alternative way to interpret the IceCube neutrinos, in terms of sum of contributions from different sources

Palladino, A.

Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2018-18065-0Colloquia: IFAE 2017IL NUOVO CIMENTO 41 C (2018) 65A multi-component model for the IceCube neutrino eventsA. PalladinoGran Sasso Science Institute - L’Aquila, Italyreceived 21 April 2018Summary. — The IceCube neutrino telescope has observed for the first time adiffuse flux of high energy neutrinos, with a possible astrophysical origin. Up to nowthere is no evidence of sources and many hypotheses are still plausible in order toexplain the measured flux. In this work we analyze an alternative way to interpretthe IceCube neutrinos, in terms of sum of contributions from different sources.1. – IntroductionThe IceCube neutrino telescope is a km3 detector, located in the South Pole. Ithas provided the first evidence of a diffuse flux of high energy neutrinos (HEν), with apossible astrophysical origin [1]. At present the excess measured by IceCube, above theexpected atmospheric background, has a significance greater than 6σ [2, 3].The IceCube neutrino telescope observes neutrinos in two different ways:1) High energy starting events (HESE), mainly hadronic and/or electromagnetic show-ers from the southern hemisphere, with the vertex of interaction contained into thedetector, above 30 TeV. HESE are sensible to all neutrino flavors.2) Throughgoing muons. They are induced muons, produced by the charge currentinteraction of νμ outside the detector. They come from the northern hemisphere [4].Historically the diffuse flux of HEν from extragalactic sources was expected to bedistributed as E−2 [5]. At present the HESE suggest a softer spectrum E−α, with α =2.5±0.1 [3]. On the contrary, the dataset of 29 throughgoing muons [4], with a depositedenergy above 200 TeV, is in agreement with a harder spectrum with α = 2.13 ± 0.13.There is a tension of 3.6σ among them. On the other side the highest energy part ofHESE suggests an E−2 spectrum, in agreement with throughgoing muons.Here we discuss the possibility that the two hemispheres are observing different pop-ulations of HEν [6, 7].Creative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0) 12 A. PALLADINO2. – The different contributions2.1. Extragalactic. – The throughgoing muons dataset is well representative of theextragalactic component of HEν, thanks to the high energy threshold. At this energy thecontamination given by the conventional atmospheric background is negligible whereasthere can be a certain contamination due to prompt neutrinos, produced into the decayof heavy mesons. In [8] it has been estimated that 2/3 of the troughgoing muons eventscan be attributed to the astrophysical signal and 1/3 to the atmospheric background.Therefore it is reasonable to assume that the extragalactic flux of HEν is distributed asE−2 [6] or as E−2.13 [7] and not as E−2.5, as suggested by HESE [3].From the theoretical point of view, there are several sources that have the potential toproduce HEν. Nowadays, despite the low statistic of IceCube, some theoretical scenariosare already in strong tension with the observations:• Gamma ray bursts. They are disfavored by the non observation of correlation inspace and time with the IceCube neutrinos. Their contribution to the diffuse fluxcan be no more than few % [9];• Blazars. They are a subclass of AGNs with the emitting jet pointing into thedirection of the Earth. Blazars are the brightest objects in the γ-rays sky above100 GeV. Therefore it is natural to consider them as a promising sources of high en-ergy neutrinos. Nowadays it is known that i) there are correlations between HESEand blazars [10]. Anyway HESE are mainly shower-like events, with a poor angularresolution of 10◦–15◦; ii) the IceCube Collaboration has calculated that the contri-bution of blazars to the diffuse flux of HEν is not greater than 25% [11]; iii) blazarsare strongly constrained by the non observation of multiplets [12, 13]; iv) a sub-class of blazars, the BL Lacs, are strongly constrained by the non observation ofcorrelations with throughgoing muons above 200 TeV [8].• Starbust galaxies. They are galaxies in which the gas density is much higher thanwhat is observed in quiescent galaxies and for this reason the proton-proton (pp)interaction is a plausible mechanism to produce HEν. At present there is no ten-sion between the flux of HEν theoretically expected and the flux detected by Ice-Cube [14].2.2. Galactic from disk . – At present a galactic component of HEν has not beenmeasured. There is only an upper limit on this flux, provided by the ANTARES Col-laboration [15]. Anyway a diffuse flux of galactic γ-rays from disk has been observed byFermi [16] and it is plausible that also a galactic flux of HEν could exist.The flux of galactic neutrinos could be produced by the pp interaction of galacticcosmic rays with the matter contained into the galactic disk. In this type of interactionneutrinos take about 5% of the primary proton’s energy and the spectrum of primaryprotons is replicated by neutrinos. Since the galactic cosmic rays are distributed asE−2.7 before the knee (at 3 PeV for cosmic rays), we expect that galactic neutrinos aredistributed as E−2.7 and they give a contribution between 30 TeV and ∼150TeV [6].On the other side, looking at the diffuse flux of galactic γ-rays, an E−2.4 spectrum forgalactic neutrinos seems to be more plausible, because an E−2.7 spectrum is hard toreconcile with the galactic γ-rays, when it is extrapolated at TeV energy [7].2.3. Prompt neutrinos. – The atmospheric prompt neutrinos, produced in the decayof heavy mesons, are expected [17] but still not detected. Up to now there is an upperA MULTI-COMPONENT MODEL FOR THE ICECUBE NEUTRINO EVENTS 3limit provided by IceCube [4]. The flux of these neutrinos is expected to be distributedas E−2.7, i.e., the same spectral index of the observed cosmic rays in the TeV-PeV range.We do not expect a contribution of prompt neutrinos greater than few % to the HEν flux.Moreover it is important to take into account also this small effect that can contributeto soften the spectrum below 100 TeV.3. – The multi-component fluxOn the light of what was said before, the diffuse flux of HEν observed by IceCubecan be explained as the sum of different contributions, as follows:dφdEν=3∑i=1Ni ×10−18GeV cm2 sec sr(Eν100 TeV)−αi,where N1 = 2.7 ± 0.9 and α1 = 2.13 ± 0.13 are the coefficients of the extragalacticcomponent, that is isotropically distributed and it is the dominant one. It should beresponsible of 25 ± 3 of the 54 HESE observed; N2 = 1.5 ± 0.8 and α2 = 2.4 are thecoefficients of the galactic component, that gives a smaller contribution. This componentis present only in the flux that comes from the southern hemisphere, in first approxi-mation, and it produces 6.0 ± 3.5 events [6, 7]; N3 = 0.6 ± 0.3 and α3 = 2.7 are thecoefficients given by prompt neutrinos. This normalization respects the IceCube upperlimit, namely 0.5 ERS [4] and it can produce only a small of fraction of HESE, namely3.5 ± 1.2. The other events are due to the conventional atmospheric background and toatmospheric muons (see [2, 7]).4. – ConclusionThe discovery of a diffuse flux of high energy neutrinos has opened a new era forneutrino astronomy. Up to now there are several theoretical models that are able toexplain the observed flux and more data are required to clarify the situation. Most ofthe flux observed by IceCube is likely to have an extragalactic origin, but the source isunclear; GRBs and BL Lacs seem to be disfavored, starburst galaxies are still into thegame, but at present it is not possible to say much more. A part of the IceCube signalcould be given by a galactic component, produced by the pp interaction of galactic cosmicrays with the matter contained into the galactic disk. This hypothesis can reconcile thespectral tension observed in the IceCube data; moreover a null galactic component isdisfavored at 2 sigma by spectral and spatial informations. Also the small contributionof atmospheric prompt neutrinos should be taken into account below 100 TeV. Thereforea multi-component model is reasonable to explain the IceCube data and it could be agood improvement with respect to the single power law model.REFERENCES[1] IceCube Collaboration (Aartsen M. G. et al.), Science, 342 (2013) 1242856.[2] IceCube Collaboration (Aartsen M. G. et al.), Phys. Rev. Lett., 113 (2014) 101101.[3] IceCube Collaboration (Aartsen M. G. et al.), Astrophys. J., 809 (2015) 98.[4] IceCube Collaboration (Aartsen M. G. et al.), Astrophys. J., 833 (2016) 3.[5] Waxman E. and Bahcall J. N., Phys. Rev. D, 59 (1999) 023002.4 A. PALLADINO[6] Palladino A. and Vissani F., Astrophys. J., 826 (2016) 185.[7] Palladino A., Spurio M. and Vissani F., JCAP, 12 (2016) 045.[8] Palladino A. and Vissani F., Astron. Astrophys., 604 (2017) A18, arXiv:1702.08779.[9] IceCube Collaboration (Aartsen M. G. et al.), Astrophys. J., 824 (2016) 115.[10] Padovani P. et al., Mon. Not. R. Astron. Soc., 457 (2016) 3582.[11] IceCube Collaboration (Aartsen M. G. et al.), Astrophys. J., 835 (2017) 45.[12] Murase K. and Waxman E., Phys. Rev. D, 94 (2016) 103006.[13] Mertsch P., Rameez M. and Tamborra I., JCAP, 03 (2017) 011.[14] Tamborra I., Ando S. and Murase K., JCAP, 09 (2014) 043.[15] ANTARES Collaboration (Albert A. et al.), arXiv:1705.00497 [astro-ph.HE].[16] Fermi-LAT Collaboration (Zaharijas G. et al.), Nucl. Phys. Proc. Suppl., 239-240(2013) 88.[17] Enberg R., Reno M. H. and Sarcevic I., Phys. Rev. D, 78 (2008) 043005.

A multi-component model for the IceCube neutrino events

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