HREPHAH 0H3HKA, 2004, moM 67, № 6, c. 1200-1203
NEUTRINO PHYSICS AND ASTROPHYSICS
ASTROPHYSICAL TAU NEUTRINOS AND THEIR DETECTION BY LARGE NEUTRINO TELESCOPES
© 2004 E. V. Bugaev*, T. Montaruli1)'2), I. A. Sokalski2)
Institute for Nuclear Research, Russian Academy of Sciences, Moscow Received November 11, 2003
We present results of the detailed Monte Carlo calculation of the rates of double-bang events in 1-km3 underwater neutrino telescope with taking into account the effects of t-neutrino propagation through the Earth. As an input, the moderately optimistic theoretical predictions for diffuse neutrino spectra of AGN jets are used.
1. INTRODUCTION
During these years there have been many discussions [1, 2] on the possibilities of very high-energy t-neutrino detection by large neutrino telescopes, which would prove oscillations of muon neutrinos of astrophysical origin ("astrophysical long baseline experiment" [3]). It is known now that neutrino oscillations definitely exist. Nevertheless, the detection of astrophysical t neutrinos is still very interesting and useful, e.g., for a better understanding of neutrino properties as well as properties of astrophysical sources [4].
Intrinsic flux of t neutrinos from a typical source in which there is acceleration of protons and production of neutrinos through pp and pj reactions is very small. The dominant channel of t-neutrino birth is through the inclusive production of charmed mesons [5],
PP,PY
D+ + X,
tvt
(1)
and the smallness of vT flux follows from the relations
(t1p^dx < 10_3
rsj ")
Vyp^-nX
Br (L>+ -»■ vT) Br(vr vß)
7 x 10
-2
$
std = H f. f.o
1 2
Assuming maximal atmospheric mixing and Ue3 = 0, that is:
sin 913 = 0, sin 2023 = 1,
''Physics Department, Bari University, Italy.
2)INFN/Bari, Italy.
E-mail: bugaev@pcbai10.inr.ruhep.ru
one obtains, after averaging over many oscillations, the ideal equipartition between neutrino flavors in the astrophysical neutrino flux at Earth,
$
std
1 1 1
3' 3 ' 3
(5)
(2)
Therefore, the production of vT at source is negligible, and if all muons can decay, it can be expected for the three flavors the following proportion:
(3)
(4)
independently on the value of the solar mixing angle 612. Moreover, in the case when there is maximal atmospheric mixing and Ue3 = 0, for any proportions between flavors at the source one has, after propagation, the equality fy^ = fyT [4].
The experimental verification of the relation fy^ = = by detection of astrophysical diffuse fluxes of vT and vn with flavor identification capabilities is very important because the inequality fy^ = is predicted in some exotic scenarios, e.g., [4], (i) if CPT invariance is violated in the neutrino sector and the neutrino and antineutrino mixing matrices do not coincide, fyM, in general, is not equal to fyT, (ii) if neutrinos can decay [6] en route from the sources to the Earth, the neutrino fluxes and flavor ratios are very sensitive to uncertainties in the neutrino mixing matrix and strongly depend on the hierarchy of neutrino masses. So, the inequality fy^ = fyT would reveal unconventional neutrino physics.
Inside of some astrophysical and cosmological objects there are processes in which very energetic quarks and gluons fragmenting into jets of hadrons are expected to be produced. Even the usual inclusive spectrum of high-energy pp or py collisions always contains the jet component (described well by perturbative QCD). Jets can appear in decays of (hypothetical) supermassive particles which, in turn, are produced in decays of topological defects [7], in processes of superheavy dark matter annihilation inside of stars [8], in a process of Hawking evaporation of primordial black holes [9]. It was shown in [10]
ASTROPHYSICAL TAU NEUTRINOS
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that decays of top quarks contained in the jets lead to production of vT flux from the jets, which is of the same order as v^ and ve fluxes (at large neutrino energies, close to the value of the jet mass). This means that the intrinsic vT flux from the sources in which the jet phenomena are essential is, in general, not suppressed (in comparison with intrinsic v^ and ve fluxes).
We studied in this paper the possibility of detection of extragalactic diffuse vT fluxes by large (~1 km3) underwater neutrino telescopes. In Section 2 we discuss the general problems connected with the detection of very high-energy vT (Ev > 106 GeV). In Section 3 we present the diffuse neutrino spectra for the AGN jets chosen for the numerical calculations. Details of the calculations, results, and conclusions are given in Section 4.
2. DETECTION OF t NEUTRINOS
The main feature of charge current interactions of t neutrinos is the fact that t leptons decay long before they lose a large fraction of their energy. This leads to the absence of absorption of t neutrinos during their propagation through the Earth and, as a consequence, to the characteristic modification of the neutrino spectrum after crossing the Earth (there is a pileup of events with energies around ^100 TeV, if the incident neutrino spectrum is not too steep). At neutrino energies ~106—107 GeV t-lepton track length becomes larger than ^100 m (in water), it becomes possible to identify t-neutrino interactions in large neutrino telescopes by selection of "double-bang" or "lollipop" events [1]. A double-bang event consists of two showers and a t-lepton track connecting them. The first (hadronic) shower is initiated by the charge current interaction of t neutrino and the second one (hadronic or electromagnetic) is produced by the t-lepton decay. In lollipop events one of these showers is not detected (e.g., if the t-neutrino interaction takes place outside of the detector).
The detection of events with double-bang structure would reliably indicate the appearance of t neutrinos in the detector because this signature is unique. However, the probability of observing a double-bang event PDB decreases with neutrino energy for Ev > 108 GeV (due to t-lepton range increase). The lollipop structure is not so spectacular although the observation probability PL of lollipop events is at Ev ~ 108 GeV considerably (by a factor of 10 [11]) larger than PDB.
Diffuse astrophysical and cosmological neutrino fluxes in the ultrahigh-energy region, Ev > 109 GeV (GZK neutrinos (see, e.g., [12]), hypothetical neutrino fluxes from topological defects [13], from
Z-bursts [14], etc.) are too small and inaccessible for a study by underwater neutrino telescopes. Registration of t neutrinos of such energies can be accomplished by detecting the air showers caused by t leptons produced by neutrino—nucleon interactions far away from the detector ("Earth-skimming idea" [15]). The huge modern air shower detectors (e.g., the HiRes [16] or the Pierre Auger detector [17]) could be suitable for such experiments, but expected event rates are low.
3. DIFFUSE NEUTRINO SPECTRA FROM AGN JETS
The concrete calculations were carried out with two theoretical diffuse neutrino spectra from AGNs.
1. The curve denoted by "MPR (limit)" in the figure is the generic upper bound on the diffuse neutrino spectrum from AGN jets obtained by Mannheim, Protheroe, and Rachen [18]. This limit was found using the assumption that the AGN source is not completely optically thin for a cosmic ray flux: due to opacity effects there is a spectral break between 107 and 1011 GeV in the escaping cosmic ray spectrum (the resulting cosmic ray spectrum is a combination of several model spectra with different values of the spectral break and with the fixed Emax (Emax = = 1011 GeV), and the normalization of the resulting spectrum is such that the cosmic ray intensity does not exceed the proton spectrum estimated from observations). Neutrinos are mostly produced in decays of mesons from accelerated proton interactions with synchrotron photons emitted by high-energy electrons in the jets. It is noted in [18] that various models of AGN jets (see, e.g., [19—21]) predict diffuse neutrino spectra which are compatible with the MPR limit (at the important neutrino energy range below -107 GeV).
2. The model by Protheroe [21] (the curve denoted by "P" in the figure) is the optically thick proton blazar model, in which it is assumed that the target for the pion production is provided by the thermal UV photons emitted from the accretion disc rather than by the synchrotron photons produced in the jets. The diffuse neutrino spectrum is normalized to the experimental data on y-ray background. It is seen from the figure that the prediction of this model is, indeed, not in the contradiction with the MPR bound, if Ev < 107 GeV. The proton blazar model, developed by Mannheim [19] (in which accelerated protons interact with the synchrotron radiation of jet electrons) predicts very similar diffuse neutrino spectrum.
One should note that the prediction given by Stecker and Salamon [22] for diffuse neutrino spectrum from radio-quiet AGNs is much more optimistic
8 ftOEPHAfl OH3HKA tom 67 № 6 2004
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BUGAEV et al.
E V I(EV), GeV cm-2 s-1 sr-
10
1-6
SSQC MPR (limit)
10-
10-
10
-12
9
log (Ev/GeV)
Different theoretical predictions for diffuse neutrino (v)A + v)A) spectra from AGNs: MPR (limit ) — upper bound for v)A flux from AGN jets [18], P — vflux for proton blazar model of [21], SSQC — prediction of v/A flux from radio-quiet AGNs [22].
(SSQC curve in the figure) at neutrino energies Ev ~ ~ 106—107 GeV. Their model, however, predicts a nonthermal spectrum of X rays at energies below ^500 keV (these X rays result from reprocessing of Y rays from pion decays by electromagnetic cascades due to the high photon density near the black hole) while the observed X-ray spectra of radio-quiet AGNs (as well as the spectrum of the diffuse X-ray background) turn over steeply below ^100 keV with no sign for a nonthermal component [23]. Therefore, the existing X-ray data cannot be used for a normalization of the radio-quiet AGN neutrino sp
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