Arteaga-Velazquez, J C

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29th International Cosmic Ray Conference Pune (2005) 9, 151154Estimation of the detectable flux of astrophysical neutrinos at the PierreAuger observatory by means of horizontal air showersJ. C. Arteaga-Vela´zquez and A. ZepedaDepartment of Physics, Cinvestav, Apartado Postal 14-740, 07000, Mexico, DF, Mexico.Presenter: A. Zepeda (zepeda@fis.cinvestav.mx), mex-zepeda-A-abs2-he22-posterSeveral astrophysical models and extensions of the Standard Model of elementary particles predict the exis-tence of important sources of very high energy neutrinos across the universe. The detection of these energeticparticles will open a new astronomical window and will constitute a new tool to probe deeply into several thickand energetic regions of the universe which can be hardly studied by using only the electromagnetic radiation.When the Pierre Auger Observatory is completed, it will be able to detect ultrahigh energy astrophysical neu-trinos ( EeV) by observing the air showers of secondary particles induced by the neutrinos in collisionswith the atmosphere on their route to the detector. In particular, the observatory will look for those eventstraveling near the horizontal direction. In this work we calculated the detectable flux of ultrahigh energy muonneutrinos at the Pierre Auger Observatory for several particle physics and astrophysical models. Based on theseestimates, we found that, in some scenarios, the diffuse flux of active galactic nuclei and topological defectshave the greatest possibility of detection of all the spectra which we analyzed and that the muon neutrinos fromthe GZK cut off will be very difficult to observe by using the above technique and only muon neutrinos.1. IntroductionAlthough, the southern Pierre Auger observatory (PAO) has not been completed yet, it is already the largestworking surface array in the world to study ultrahigh energy cosmic rays (UHECR) by means of the air showertechnique. This observatory is now under construction in Mendoza, Argentina [1]. In its final stage, It willcover a total surface of 		 kmwith an array of 1600 water Cherenkov detectors (separated by 1.5 km) andfour stations with fluorescent telescopes [1]. The above instruments will study the properties of the secondaryair showers induced in the atmosphere by UHECR to infer the energy and direction of these particles: TheCherenkov detectors will sample the lateral distribution of the cascades at ground level and the fluorescentdetector will measure the longitudinal development of the air shower [1].In addition to cosmic ray research, the PAO will have the possibility to study ultrahigh energy neutrinos (UHE  )[2]. The information obtained in this way will be complementary to that resulting from neutrino telescopesbased on the Cherenkov technique [3]. The PAO will observed neutrinos indirectly. In particular, muon neutri-nos will be detected through the secondary air showers that they induce in the atmosphere on their way to thedetector. The search of UHE  will be focused on the horizontal direction in the PAO, since, in this way, it willbe easier to separate the showers induced by neutrinos from those of cosmic rays [2].In order to explore the potential of the PAO to muon neutrinos, we calculate the detectable flux of extraterrestrial neutrinos at ultrahigh energies under several astrophysical and exotic scenarios with the Cherenkov array.2. Detection of muon neutrinos at the PAOWhen a muon neutrino with energy interacts weakly with a nucleon (  ) of the atmosphere, it produces ahadronic shower (  ) with energy  that generates the cascade that is expected to be observed withthe PAO. At ultrahigh energies the interaction with the hadron is deeply inelastic and it is mediated by charged152 J.C. Arteaga-Vel·azquez and A. Zepeda10-1910-1810-1710-1610-1510-1410-1310-1210-1110-1010-9103 104 105 106 107 108 109 1010 1011E ν dΦν µ+ν¯ µ/dE ν (cm-2  s-1  sr-1 )Eν (GeV)AGN-Man95BAGN-Man95AAGN-Pro96AGN-Hal95GRB-WB9810-1910-1810-1710-1610-1510-1410-1310-1210-1110-1010-9103 104 105 106 107 108 109 1010 1011E ν dΦν µ+ν¯ µ/dE ν (cm-2  s-1  sr-1 )Eν (GeV)TD-Sig00BTD-Sig00ATD-Sta96GZK-Eng00BGZK-Eng00AFigure 1. Astrophysical and exotic uxes of high energy muon neutrinos. See text for description.or neutral currents. In the first case, we have the following reaction: 	ﬁﬀﬃﬂ ! , and in the second one,"ﬀ#$% .Following Gandhi et al. [3] and neglecting neutrino absorption in the atmosphere (which is valid since at theenergies considered here the  interaction length surpasses more thantimes the horizontal atmosphericdepth [3]), the rate of detectable muon neutrinos at the PAO is given by the expression&')(+*-,/.021!3!46571398;:<>=0@?13!8;:<A=B17 C CEDGFHJI KMLONI &KMLC%PRQJS&TVUI WKCW(1)where YX,/Z&#[W	\ GeV is the threshold energy for horizontal air shower observations and ]X*-^?_?W?GeV, the maximum neutrino energy. `' is the Avogadro number, while (a*-,/.cbaed g/cmis thedensity of the atmosphere [3]. Finally, the quantitiesDF6HI 	K,NI &KandC%PRQeSTVUI +WK6fCrepresent theinitial muon neutrino flux that arrives to the Earth, the acceptance of the PAO and the differential    totalcross section, respectively. For the case of muon antineutrinos we applied a similar equation.The astrophysical and exotic models that were explored in this work are shown in figure 1. We employedHalzen’s (AGN-Hal95) [4], Protheroe’s (AGN-Pro96) [5] and Mannheim’s models (AGN-Man95A, AGN-Man95B) [6] to describe the diffuse flux of muon neutrinos from active galactic nuclei (AGN). The secondAGN spectrum was multiplied by 0.25 according to reference [7]. We also worked with the Waxman andBahcall’s diffuse flux from gamma ray bursts [8], and with two spectra for 	 neutrinos from the GZK cut offdue to Engel et al. (GZK-Eng01A and GZK-Eng01B), with different cosmological evolution for the cosmicray sources [9]. Two Sigl’s models of topological defects (TD-Sig00A, TD-Sig00B) [10] and one proposed byProtheroe and Stanev et al. (TD-Sta96) [11] were also included in this paper.We estimated the total Ihg$K cross sections at leading order with the quark-parton model. The latest par-ton distribution functions from CTEQ were used in this evaluation (in particular, the set CTEQ6-L1) [12] incombination with the Double Logarithmic Approximation [13]. Here,  is an isoscalar target.The acceptance of the Cherenkov array of the PAO to horizontal air showers (with zenith angles between ijlkand m%k ) was calculated by P. Billoir in reference [14] up to shower energies of?FGeV. As it was the case in[3], we had to extrapolate Billoir’s acceptance to higher energies in order to evaluate the rates given in (1) with X*h^n?_?I?KGeV. A graph of the acceptance of the PAO,NI %K, as a function of &, is presentedin figure 2a.Estimation of the detectable ux of astrophysical . . . 153101810191020108 109 1010 1011Acceptance (cm3 sr)Esh (GeV)10-610-510-410-310-210-1100101102108 109 1010 1011E ν dNsh/dE ν (year-1 )Eν (GeV)TD-Sig00AGZK-Eng00AAGN-Pro96GRB-WB98 (a)  (b)Figure 2. (a) Acceptance of the Cherenkov array of the Pierre Auger Observatory to horizontal air showers as a functionof the energy of the cascade ( oMprq ). (b) Differential rates of detectable air showers at the PAO produced by ultrahigh energymuon neutrinos and antineutrinos from different spectra as a function of the energy ots . We used oEu)v-ws xzy-{|}|GeV andou)~p}q xy-{GeV.3. Results and discussionsThe final rates of detectable muon neutrinos at the PAO are shown in table 1. From there, we can see that thesevalues are small but not negligible, except for the GRB model. If `X,/Z	[ GeV and YX*-^?_?GeV,the higher detectable rates come from some AGN and TD models, while the lower ones belong to the GZKand the GRB fluxes. However, the detection of the diffuse   flux from TD’s and AGN’s could be also asdifficult as for the GZK models under the TD-Sta96 and the AGN-Man95A scenarios. The situation is worstwhen X,Z& is increased to	\ GeV because the detectable rates become smaller. In addition, the Halzen’sAGN model, which predicts neutrinos with a maximum energy of\ GeV (assuming that AGN’s acceleratescosmic rays up to jL?\ GeV), gives no events.In figure 2b, we show the number of detectable events in one year produced by neutrinos with different energies from some specific models and using X*-^?_? GeV and with X,Z&	[ GeV. In this graph, weobserve the different energy intervals in the primary   spectra that are relevant for the production of detectableevents in several cosmic scenarios. For the GRB spectrum and the Protheroe’s model of AGN muon neutrinos,almost 99% of the events are produced by   neutrinos with energy in the intervalL%[ŁiL\ GeV,respectively. In contrast, the relevant energy region in the Sigl’s fluxes of TD neutrinos extends froma[ up to X*h^?_? GeV.Based on the magnitude of these rates, we can realize that the Pierre Auger observatory will have to usecomplementary channels to search for cosmic neutrinos, besides that which involves   interactions in theatmosphere. One of these channels could be the additional search of + interactions, again by lookingfor horizontal air showers [3]. Astrophysical models produce only % and   neutrinos with the followingproportions %  	$	` at the source [15], but in scenarios with neutrino oscillations, there appearsalso a $ flux from astrophysical sources produced by   $ oscillations that occurs in the space whenneutrinos travel to the Earth [15]. The detection of tau neutrinos is another channel that could be explored inthe PAO [2]. When traveling to the detector,   neutrinos that arrives near the horizontal direction or skimmingthe Earth may interact by charged currents with the rock around the observatory, i.e., with the rock in themountains and inside the ground, producing taus. Thus, a   could be also detectable if the tau escapes from154 J.C. Arteaga-Vel·azquez and A. ZepedaTable 1. Detectable rate of extraterrestrial muon neutrinos and antineutrinos at the Pierre Auger Observatory with theCherenkov array at ultrahigh energies under different cosmic scenarios and with o u)v-ws xy-{ |}|J y{ |AhGeV.Model AGN-Man95B AGN-Man95A AGN-Pro96  25 % AGN-Hal95 GRB-WB98o u ~;p}q(GeV)y{4.53 0.28 1.31 3.69 1.63 y{y{1.73 0.11 0.21 0 1.68 y{ aModel TD-Sig00B TD-Sig00A TD-Sta96 GZK-Eng00B GZK-Eng00AoEu ~;p}q(GeV)y{ 2.81 (2.99) 1.44 (1.61) 0.28 (0.42) 0.27 0.10y{1.63 (1.80) 0.94 (1.10) 0.21 (0.34) 0.15 0.063the rock and decays in the atmosphere producing an observable air shower [2]. With all the above windows theprobability of detection of UHEv sources would be higher than by observing muon neutrinos alone.4. ConclusionsThe Pierre Auger observatory will be able to detect some ultrahigh energy muon neutrinos from outer spaceaccording to some AGN and TD models. However, there are also pessimistic AGN and TD scenarios whichpredicts a low rate of detectable muon neutrinos at the observatory. On the other hand, the background signalof muon neutrinos from the GZK cut off and the diffuse 	 flux from GRB’s will be very difficult to detect withthe Pierre Auger observatory. From these results, it can be concluded that It will be important to complementthe cosmic neutrino search by using other channels, for instance, looking for horizontal air showers induced by ’s and   ’s interactions in the atmosphere and by tau decays, which could be produced by   ’s interactingwith the rock in the mountains near the detector or, for neutrinos skimming the Earth, in the ground surroundingthe observatory.5. Acknowledgements. We acknowledge partial support from CONACyT.References[1] J. Abraham et al. (Pierre Auger collaboration), Nucl. Instr. and Meth. A523, 50 (2004).[2] X. Bertou et al., Astropart. Phys. 17, 183 (2002).[3] R. Gandhi et al., Phys. Rev. 58D, 093009-1 (1998).[4] F. Halzen, electronic archive: astro-ph/0301143.[5] R. J. Protheroe, electronic archive: astro-ph/979607165.[6] K. Mannheim, Astropart. Phys. 3, 295 (1995).[7] R. J. Protheroe, electronic archive: astro-ph/9809144.[8] E. Waxman and J. Bahcall, Phys. Rev. 59D, 023002 (1998).[9] R. Engel, D. Seckel and Todov Stanev, Phys.Rev. D64, 093010 (2001).[10] G. Sigl, electronic archive: astro-ph/0008364.[11] R. J. Protheroe and T. Stanev, Phys. Rev. Lett. 77, 3708 (1996).[12] J. Pumplin et al., JHEP 07, 012 (2002).[13] L. V. Gribov, E. M. Levin, M. G. Ryskin, Phys. Rep. 100, 1 (1983); D. W. McHay, J. P. Ralston,Nucl.Phys. Proc.Suppl. 18, number 3, 86 (1991).[14] Pierre Billoir, Pierre Auger technical note GAP-1997-049, http://www.auger.org.[15] S. I. Dutta, M.H. Reno, I. Sarcevic, Phys. Rev. 62D, 123001-1 (2000).

Estimation of the detectable flux of astrophysical neutrinos at the Pierre Auger observatory by means of horizontal air showers

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