Serpico, Pasquale Dario

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29th International Cosmic Ray Conference Pune (2005) 5, 3336Galactic beta-beams as a tool to measure the neutrino 13-mixing angleand the CP-phase with neutrino telescopesP.D. Serpico and M. KachelriessMax-Planck-Institut f¤ur Physik (Werner-Heisenberg-Institut), F¤ohringer Ring 6, 80805 Munich, GermanyPresenter: P.D. Serpico (serpico@mppmu.mpg.de), ger-serpico-PD-abs1-og25-oralThe observed excess of high-energy cosmic rays from the Galactic plane in the energy range around 10 eVmay be naturally explained by neutron primaries generated in the photo-dissociation of nuclei. In this scenario,neutrons with lower energy decay before reaching the Earth and produce a detectable flux in a kmneutrinotelescope. The initial flavor composition of these (anti)neutrinos,	, offers theopportunity to perform a combined ﬀappearance and disappearance experiment. The observable ratioof ﬁ tracks over ﬂ and ﬃ showers depends appreciably on the 13-mixing angle    and the leptonic CP phase "!$#, thus providing a new way to measure these quantities.1. IntroductionThe determination of the mixing parameters controlling the solar and atmospheric neutrino oscillations hasalready entered the precision era, but currently it exists only an upper limit for the 13-mixing angle   % ,&('*)ﬀ+, .-ﬀ/0[1], while the CP-violating Dirac phase 1!$#(the only one entering neutrino oscillation phe-nomenology) is completely unconstrained. Both the mixing angle   % and the phase "!$#are key featuresof a genuine three-flavour mixing scenario, but are observable in solar and atmospheric neutrino oscillationexperiments only as small, sub-leading effects. Strong experimental efforts are planned to measure   % andeventually !$#, though the latter appears unlikely for the next generation of facilities [2].It it then of primary interest to find other observables allowing to explore as soon as possible this “terra incog-nita” in the neutrino sector. In the following, we discuss the possibility to use high-energy neutrinos producedby decaying galactic neutrons to perform a measurement sensitive (in particular) to   % and !$#at neutrinotelescopes. Further details can be found in [3] and Refs. therein. If realized in Nature, such a source wouldhave a similar potential for neutrino physics as a “ 2 -beam” facility [4], with the main and non-negligible dif-ference of being available for free. The other advantage is that it could be studied at observatories which willbe build anyway for astrophysical purposes. We will estimate the relevance of the signal and the feasibility ofthe measurement in IceCube, which is the largest neutrino telescope under construction [5].Neutron primaries have been invoked to explain an excess of high-energy cosmic rays (CRs) from two regionsin the Galactic plane [6, 7]. This signal, in a limited energy range around" eV, has been observed by severalexperiments with different techniques: the AGASA collaboration found a correlation of the arrival directionsof CRs with the Galactic plane at the 4 3 level [8]. This excess, which is roughly 4% of the diffuse flux, isconcentrated towards the Cygnus region, with a second hot spot towards the Galactic Center (GC) [9]. Sucha signal has been independently confirmed by the Fly’s Eye Collaboration [10] and by a re-analysis of theSUGAR data [11]. Complementary evidence for a cosmic accelerator in the Cygnus region comes from thedetection of an extended TeV 4 -ray source by the HEGRA experiment [12]. Similarly, multi-TeV 4 -rays fromthe vicinity of the GC have been recently detected by HESS [13].The excess from the Cygnus and GC region is seen at 5768 %eV, i.e. at energies where charged cosmicrays still suffer large deflections in the Galactic magnetic field so that only a neutral primary can produce adirectional signal. Another evidence for neutrons as primaries is that the signal appears just at that energywhere the neutron lifetime allows neutrons to propagate from a distance of several kpc.34 P.D. Serpico and M. KachelriessNeutrons can be generated either in collisions of high-energy protons on ambient photons and protons, or in thephoto-dissociation of nuclei. In the first case, the flux of from neutron decays would be negligible comparedto the neutrino flux from pion decays. Thus one expects a neutrino flavor composition of      9:,;before oscillations1, typical for most sources of high-energy neutrinos. In contrast, photo-dissociation ofnuclei produces a pure initial flux. Since the energy fraction transferred to the is of the order of"ﬀ<andonly neutrons with 5=" %eV can decay on galactic distances, the neutrino flux from photo-dissociation islimited to sub-PeV energies. Moreover, the threshold for photo-dissociation on UV photons implies a lowercut-off at 5?> TeV for the energies.In the following we use as our basic assumption that photo-dissociation of nuclei is the origin of the decayingneutrons. Some arguments supporting this scenario are discussed in [3]. We study the case where other neutrinosources contaminating the pure initial flux can be neglected. Later on, we will relax this assumption, whichis in any case experimentally falsifiable. To be specific, we use the model of Anchordoqui et al. in Ref. [6],who calculated the neutrino flux from the Cygnus region, which is in the field of view of the kmtelescopeIceCube. These authors estimated an integrated flux from neutron decays of > ,A@" < ( CBCD<+&< at5FETeV by normalizing the neutron flux to the 4% anisotropic component observed by AGASA. This fluxcorresponds to 6 ,events (of all flavours) per year in IceCube.2. Flavor composition after oscillations and discrimination in a neutrino telescopeEven at PeV energies, the galactic distances far exceed the experimentally known oscillation lengths. Thusthe interference terms sensitive to the mass splittings GIH + J0K0L in the usual oscillation formulas average-out.Moreover, the extremely low density of the interstellar medium allows to neglect matter effects, so the relevantprobabilities MONQPSRTM:UV NSWX PY can be written solely in terms of the vacuum mixing matrix Z . The fluxes[Parriving at the detector are then given by[P\NMN]PNMP_^ P:`,\aCbdcfehgUiZkjPaZPcZaZljcYnmpo(1)where we have insertedNU1ooY , and greek (latin) letters are flavor (mass) indices. The followingproperties also hold: M%is independent from + and !ﬀ#, M%and Mndepend on !ﬀ#only viaBCq& !ﬀ#, andMn.MnUr+W_+tsvu,Y .Let us now recall briefly the flavor-discrimination possibilities in a neutrino telescope [14]. For the TeV-PeVenergies relevant here, the charged-current interactions of and are distinguishable in principle by thedifferent muon content in electromagnetic (e.m.) and hadronic showers. Since this measurement is experi-mentally challenging, we shall conservatively consider and as indistinguishable in a neutrino telescope.By contrast, in charged-current interactions the long range of muons ensures that the muon track is alwaysvisible and allows the identification of these events. In the following, we will also neglect neutral currentevents (see the discussion in [3]) and consider the combined and flux[nw and the flux[ as our twoobservables. If no prior information on the absolute source fluxis assumed, the only measurable quantitysensitive to mixing parameters is the flux ratio xRy[z[%wf . Under these assumptions, this probe alone doesnot allow the simultaneous measurement of both   % and !$#. In Fig. 1, we show the expected ratio x as afunction of    for three representative values of + , each curve for the two extreme cases of 8!ﬀ#;{and "!$#"|}}{. In all the cases we fix the value of the solar mixing angle to the best fit value   +~,/z{. Firstnotice how the three sets of curves show a clear sensitivity to the octant of the angle + . Moreover, in the bestWe denote with z the combined flux of V and V .Galactic beta-beams . . . 350 2 4 6 8 10Θ13 H°L0.00.10.20.30.40.5ΦΜDΦe+ΤDΘ23 = 45°Θ23 = 55°Θ23 = 35°∆CP0°180°Figure 1. Flux ratio 9Łﬀ Łﬀn$ at Earth (for initial uxes Ł  Ł  Ł   p ) as a function of   for"1" 1¡%¢18  , and "8  , each one shown for the two extreme values, £¥¤"¦¨§ª©;«lv¬	 . The ratios v z­  and ]­  expectedrespectively for standard astrophysical sources and in case of no oscillations are also shown for comparison.case of !$#®;{, x varies by > 50% in the interval;{°¯ %¯9";{and differs in the extreme by a factorof three from the standard value, xQ, , expected by a typical astrophysical source. Finally, varying !$#in the interval;{±¯ !ﬀ#¯®"|}}{the ratio x can change up to 40% (in the best case   % 68}{). This effectobviously disappears when    W.3. Event rates in IceCubeThe excellent angular resolution ofﬀ/²}{expected for IceCube applies only for muon induced showers, whilefor and events the resolution is only about ,;{[14]. From the estimate in Ref. [6] one easily derivesthat, in aﬀ/²{radius around the Cygnus region, one expects roughly 6 2.3 events per year ( 5³E 1 TeV), tobe compared with about ´µyr< signal events ( +´}{,   %T). A , 3 detection of the flux is thenwithin 1 yr capability of IceCube. Rescaling this background number to a cone of ,${opening angle, oneexpects about 2900 background events and 145 background showers2. The resulting statistical fluctuationof the background shower number is ¶ ·¸6, . Thus integrating one year the 6 16 yr< rate from Cygnusone expects az/ ~3 signal hint, or equivalently a ´/,3 measurement in a decade. Here we used the fact that the atmospheric neutrino background has a flavor ratio of  t¹  µ¹  »º½¼8¾ ¼V¿»¹1À¹C¼ in the energy rangeof interest, À¼nÁnÂSÃÄÅÃÀ¼ÆeV [15].36 P.D. Serpico and M. KachelriessObviously, the poor angular resolution for and events is the most serious obstacle to improve this mea-surement. Theoretical predictions for the neutron spectrum at the source could also be used to optimize thedetection strategy: for example, for a spectrum harder than the atmospheric neutrino background, the signalto background ratio could be improved by an increase of the threshold energy.Let us now comment on the case where we add to our signal some contamination from conventional pion decaysource. A 10% (100%) flux “pollution”3 would led to an upward shift of 0.01 (0.1) in the flux ratio x . Note that,since the background from pion decay always pushes x towards1, , a detection of x -Q, would in any caseconstrain the mixing parameter space, in particular if complementary information from terrestrial experimentswill be available. If neutrons would be generated mainly in ÇzÇ or Ç$4 collisions, a much larger flux of neutrinosfrom pion decays, with x?1, , is expected. This would simplify the detection of these galactic point sourcesby neutrino telescopes, though the    and !$#searches discussed here would be probably hopeless. In thiscase, however, one could still exploit the information for astrophysical source diagnostics as well as for CRcomposition studies.In summary, it has been argued that the excess of high-energy cosmic rays from the Galactic Plane in theenergy range around" %eV is caused by neutron primaries generated in the photo-dissociation of nuclei.If this model is correct, we showed that these sources could be used as “galactic beta beams” in neutrinotelescopes, providing in particular a tool to measure   % and !ﬀ#via the observable ratio of track to showerevents. Obviously, a better theoretical modeling of sources as well as more experimental studies are highlydesirable. Especially worthwhile would be a confirmation of the anisotropy by the Auger observatory [16] andmore detailed chemical composition studies by the Kascade-Grande experiment [17].Acknowledgments M.K. acknowledges an Emmy Noether grant of the Deutsche Forschungsgemeinschaft.References[1] M. Apollonio et al., Eur. Phys. J. C 27, 331 (2003) [hep-ex/0301017].[2] P. Huber et al., Phys. Rev. D 70, 073014 (2004) [hep-ph/0403068].[3] P. D. Serpico and M. Kachelrieß, Phys. Rev. Lett. 94, 211102 (2005) [hep-ph/0502088].[4] P. Zucchelli, Phys. Lett. B 532, 166 (2002); J. Bouchez, Nucl. Phys. Proc. Suppl. B147, 93 (2005).[5] A. R. Fazely [The IceCube Collaboration], astro-ph/0406125; see also http://icecube.wisc.edu[6] L. A. Anchordoqui et al., Phys. Lett. B 593, 42 (2004) [astro-ph/0311002].[7] R. M. Crocker et al., Astrophys. J. 622, 892 (2005) [astro-ph/0408183]; astro-ph/0411471.[8] N. Hayashida et al. [AGASA Collaboration], Astropart. Phys. 10, 303 (1999) [astro-ph/9807045].[9] M. Teshima et al., in Proc. 27th ICRC, Copernicus Gesellschaft, 2001, p.341.[10] D. J. Bird et al. [HIRES Collaboration], Astrophys. J. 511, 739 (1999) [astro-ph/9806096].[11] J. A. Bellido et al.,Astropart. Phys. 15, 167 (2001) [astro-ph/0009039].[12] F. A. Aharonian et al., Astron. Astrophys. 393, L37 (2002) [astro-ph/0207528]; astro-ph/0501667[13] F. Aharonian et al. [The HESS Collaboration], astro-ph/0408145.[14] J. F. Beacom et al., Phys. Rev. D 68, 093005 (2003) [hep-ph/0307025].[15] J. F. Beacom and J. Candia, JCAP 0411, 009 (2004) [hep-ph/0409046].[16] J. W. Cronin, Nucl. Phys. Proc. Suppl. B28, 213 (1992); see also http://www.auger.org/[17] G. Navarra et al., Nucl. Instrum. Meth. A 518, 207 (2004); see alsohttp://www-ik.fzk.de/KASCADE home.htmlA contamination of È (10%) is indeed what is expected for the model of [6].

Galactic beta-beams as a tool to measure the neutrino 13-mixing angle and the CP-phase with neutrino telescopes

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