Study of the Atmospheric Muon and Neutrinos for the IceCube Observatory Ryan Birdsall Paolo Desiati, Patrick Berghaus, Teresa Montaruli (IceCube Collaboration) University of Wisconsin - Madison The goal of the IceCube Neutrino Telescope is to detect high-energy neutrinos of extraterrestrial origins. The flux of neutrinos produced by the impact of cosmic rays in the Earth’s atmosphere constitutes an irreducible foreground among which cosmic neutrinos are searched. Therefore the detailed measurement and knowledge of the atmospheric neutrinos is fundamental. Extensive air showers initiated by high energy cosmic ray particles have been simulated using CORSIKA generator, with Hoerandel polygonato model of cosmic ray spectrum and composition, and with three different high energy interaction models: QGSJET01, QGSJET-II, and SIBYLL. With these models, the “conventional” muon and neutrino fluxes, i.e. from the decay of pions and kaons in the atmosphere, have been generated at sea level. The resulting muon bundle energy spectrum and m+/m- ratio as a function of energy, is compared with various experimental results, such as MINOS, L3Cosmic, and other underground detectors, and with various mathematical calculations. Since muons and neutrinos are produced by the same physical processes, these direct comparisons are used to assess the dependency of neutrino flux on the different interaction models at energies above 1 TeV, i.e. relevant for IceCube. The atmospheric muons, which are easier to detect with high event statistics, have been experimentally used to benchmark the high energy hadronic interaction cross sections. For this analysis the atmospheric muons and neutrinos have been generated with CORSIKA [2] at Earth's surface, using Hoerandel polygonato model of the cosmic ray spectrum and composition [3]. Three different high energy interaction models have been used : SIBYLL [4], QGSJET01 [5] and QGSJET-II [6]. Above is the muon energy spectrum above 1 TeV compared with experimental measurements by L3+Cosmic [7], MINOS [8] and LVD [9]. SIBYLL seems to agree better with the experimental results, whereas the two QGSJET models are known to underestimate the muon intensity by about 25-30%. Benchmarking high energy interaction models with muons is very effective, but the kinematics of  ± and K ± decay is different for muons and neutrinos. The figure on the left shows the fractional contribution of p and K to m and nm (from [10]). The figure on the right shows the dependency on the different interaction models. Neutrinos are mostly produced by K decay above 100 GeV, whereas muons are still mostly generated by p decay up to higher energies. Therefore, the higher uncertainties on K production affect more significantly neutrinos than muons. SIBYLL predicts a more significant fraction of  and  from K decays than other interaction models. This is related to the fact that in SIBYLL the K mesons are produced with higher multiplicities than in the QGSJET models. On the other hand the  production has relatively less variability among the different interaction models. Therefore the K physics is the major player in the neutrino uncertainties up to about 100 TeV. Atmospheric Foreground and Its Importance in IceCube The main goal of a Neutrino Telescope such as the IceCube Observatory [1] is the detection of high energy neutrinos from extra-terrestrial sources such as Supernova Remnants, Active Galactic Nuclei (AGN), and Gamma Ray Bursts (GRB). These extra-terrestrial neutrinos, on the other hand, are concealed by the intense flux of neutrinos produced by the interaction of cosmic rays in the Earth's atmosphere. These interactions generate  and K mesons, which from their decays, produce a flux of muons and neutrinos. In order to detect extra-terrestrial neutrino sources, we first must understand the energy spectrum of the muons and neutrinos generated in the atmosphere. Specifically, SIBYLL produces a more pronounced K + /K - asymmetry than the other models. The excess in K + multiplicity produces a higher  + /  - than experimentally measured by MINOS and L3+Cosmic, as shown in the above figure. Therefore although SIBYLL seems to better describe the overall muon spectrum above 1 TeV and its intensity, it still cannot reproduce some observables which have been measured with precision. This, in turn, means that a corresponding excess of  over anti  is expected, since they are produced by K + decay. Muon from  Muon from  Neutrino from  Neutrino from  SIBYLL QGSJET01 QGSJETII Here we have all three hadronic models compared to theoretical models from Bartol model [11] and Honda 2004 [12]. The figure above includes both  and   for the CORSIKA-generated and the two predictions. SIBYLL predicts a higher flux of  than  , consistently with the higher K + multiplicity. SUMMARY We used CORSIKA to generate air shower data with three high energy hadronic interaction model SIBYLL, QGSJET01, and QGSJETII. SIBYLL predicts the muon energy spectrum better than the GQSJET models, even if its higher K + multiplicity is not compatible with the experimentally measured  + /  - ratio. (1)K production in the atmosphere is affected by higher uncertainties than  production. Moreover, neutrinos above 100 GeV are mostly generated by K; therefore, variability in K production rate has a higher impact in the neutrino flux than in the muon. (2) Above ~100 TeV, neutrinos from mesons with charm quark, whose production is highly uncertain and still being debated, might be the dominant component of atmospheric neutrinos and the most dangerous foreground for Neutrino Telescopes. (3) Uncertainties on K and charm productions at high energies produced higher discrepancies between hadronic interaction models above 10 TeV. (4) Km 3 Neutrino telescopes, such as IceCube, will measure unprecedented statistics of high energy atmospheric neutrinos. For the first time, we will be able to probe the neutrino spectrum in the high energy range and provide a different benchmark for high energy hadronic interaction models. This plot shows all hadronic interaction models compared with data recorded by the AMANDA detector for the  +   neutrino spectrum [13]. We see that SIBYLL matches with the recorded spectrum better than QGSJET01 or QGSJETII do, even if all are currently within the experimental uncertainties. We conclude that SIBYLL is, so far, the best of the interaction models for simulation up to high energy. REFERENCES [1] See A.Karle and K.Hoffman talks at this Conference [2] CORSIKA : Comput.Phys.Commun. 56 (1989) [3] J.R.Hoerandel, Astrop. Phys. 19 (2003) [4] R.Engeletal., Proc. 26th ICRC (Salt Lake City, U.S.A.) 1 (1999) 415 [5] N.N.Kalmykovetal.,Nucl. Phys. B(Proc. Suppl.) 52B (1997) 17 [6] S.Ostapchenko,Nucl. Phys. B (Proc. Suppl.) (2005), hep-ph/ ; hep-ph/ [7] P.Achardetal. (L3+C), Phys. Lett. B 598, 15 (2004), hep-ph/ [8] P.Adamson et al., Phys.Rev. D 76 (2007) , arXiv: [9] M.Aglietta et al., Phys. Rev. D 58 (1998) [10] T.K.Gaisser, Cosmic Rays and Particle Physics, Cambridge University Press, 1990 [11] G.D.Barr et al., astro-ph/ v1 [12] M.Honda et al., astro-ph/ [13] K.M.Muenich and J.Luenemann, IceCube collaboration, 30th ICRC (Merida, Mexico) (2006) [14] P.Berghaus, T.Montaruli (UW-Madison), J.Ranft (Siegen U.). Dec arXiv: Above ~100 TeV neutrinos are also produced by the decay of rare mesons containing the charm quark [14]. Charm production in the atmosphere is much more uncertain, and it is the most important (however not well known) contribution to atmospheric neutrinos. Neutrinos from charmed meson decay happen to be in the energy range where we expect the extra-terrestrial neutrino signal for Neutrino telescopes such as IceCube.  =3