Improved limit to the diffuse flux of ultrahigh energy neutrinos from the Pierre Auger Observatory

Neutrinos in the cosmic ray flux with energies near 1 EeV and above are detectable with the Surface Detector array (SD) of the Pierre Auger Observatory. We report here on searches through Auger data from 1 January 2004 until 20 June 2013. No neutrino candidates were found, yielding a limit to the diffuse flux of ultrahigh energy neutrinos that challenges the Waxman-Bahcall bound predictions. Neutrino identification is attempted using the broad time structure of the signals expected in the SD stations, and is efficiently done for neutrinos of all flavors interacting in the atmosphere at large zenith angles, as well as for “Earth-skimming” neutrino interactions in the case of tau neutrinos. In this paper the searches for downward-going neutrinos in the zenith angle bins 60°–75° and 75°–90° as well as for upward-going neutrinos, are combined to give a single limit. The 90% C.L. single-flavor limit to the diffuse flux of ultrahigh energy neutrinos with an $E^{-2}$ spectrum in the energy range $1.0\times 10^{17}\mathrm{eV}–2.5\times 10^{19}\mathrm{eV}$ is $E_{\nu }^{2}d{N}_{\nu }/d{E}_{\nu }<6.4\times {10}^{-9}\mathrm{GeV}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}{\mathrm{sr}}^{-1}$.

Figure 1

Distributions of $⟨\text{AoP}⟩$ (the variable used to identifiy neutrinos in the ES selection for data after 1 June 2010) after applying the inclined shower selection in Table 1. Gray-filled histogram: the data in the training period. Black histogram: data in the search period. These two distributions are normalized to the same number of events for comparison purposes. Blue histogram: simulated ES ${\nu }_{\tau }$ events. The dashed vertical line represents the cut on $⟨\text{AoP}⟩>1.83$ above which a data event is regarded as a neutrino candidate. An exponential fit to the tail of the distribution of training data is also shown as a red dashed line (see text for explanation).

Figure 2

Distributions of the Fisher variable $\mathcal{F}$ in inclined events selected by the “Inclined showers” DGH criteria in Table 1, before applying the “Young showers” cuts. In particular the distribution of events with number of triggered tanks $7\le N_{\text{st}}\le 11$ is shown. Gray-filled histogram: data in the training period corresponding to $\sim 23\%$ of the whole data sample between 1 January 2004 and 20 June 2013. Black line: data in the search period. The distributions are normalized to the same number of events for comparison purposes. Blue line: simulated DGH $\nu$ events. The dashed vertical line represents the cut on $\mathcal{F}>3.28$ above which a data event is regarded as a neutrino candidate. The red dashed line represents an exponential fit to the tail of the training distribution (see text for explanation).

Figure 3

Combined exposure of the SD of the Pierre Auger Observatory (1 January 2004–20 June 2013) as a function of neutrino energy after applying the three sets of selection criteria in Table 1 to Monte Carlo simulations of UHE neutrinos (see text for explanation). Also shown are the individual exposures corresponding to each of the three selections. For the downward-going channels the exposure represents the sum over the three neutrino flavors as well as CC and NC interactions. For the Earth-skimming channel, only ${\nu }_{\tau }$ CC interactions are relevant.

Figure 4

Top panel: Upper limit (at 90% C.L.) to the normalization of the diffuse flux of UHE neutrinos as given in Eqs. (2) and (3), from the Pierre Auger Observatory. We also show the corresponding limits from ANITAII [31] and IceCube [32] experiments, along with expected fluxes for several cosmogenic neutrino models that assume pure protons as primaries [33, 34] as well as the Waxman-Bahcall bound [13]. All limits and fluxes converted to single flavor. We used $N_{\text{up}}=2.39$ in Eq. (2) to obtain the limit (see text for details). Bottom panel: Same as top panel, but showing several cosmogenic neutrino models that assume heavier nuclei as primaries, either pure iron [33] or mixed primary compositions [9].

Figure 5

Upper limit to the normalization of the diffuse flux of UHE neutrinos (at 90% C.L. and in bins of width 0.5 in ${\log }_{10}{E}_{\nu }$—see text for details) from the Pierre Auger Observatory (straight steps). We also show the corresponding limits from ANITAII [31] (dot-dashed line) and IceCube [32] (dashed line) experiments (with appropriate normalizations to take into account the energy bin width, and to convert to single flavor), along with expected fluxes for several cosmogenic neutrino models [9, 33, 34] as well as the Waxman-Bahcall bound [13] (all converted to single flavor).