«Probing New Physics with Astrophysical Neutrinos Nicole F. Bell School of Physics, The University of Melbourne, Victoria 3010, Australia E-mail: ...»
Probing New Physics with Astrophysical Neutrinos
Nicole F. Bell
School of Physics, The University of Melbourne, Victoria 3010, Australia
Abstract. We review the prospects for probing new physics with neutrino astrophysics. High
energy neutrinos provide an important means of accessing physics beyond the electroweak scale.
Neutrinos have a number of advantages over conventional astronomy and, in particular, carry
information encoded in their ﬂavor degree of freedom which could reveal a variety of exotic neutrino properties. We also outline ways in which neutrino astrophysics can be used to constrain dark matter properties, and explain how neutrino-based limits lead to a strong general bound on the dark matter total annihilation cross-section.
1. Introduction Neutrino astronomy is in its infancy. To date, the only neutrinos we have observed from beyond our solar system are those from SN1987A. Together with solar neutrinos, and those produced by cosmic ray interactions in the atmosphere, these form the complete inventory of astrophysical neutrinos that have been detected. For distant astrophysical objects, we currently have only upper limits on the neutrino ﬂuxes. However, a plethora of exciting new experiments are now coming on line with excellent prospects of detecting a signal. The eagerly awaited era of neutrino astronomy is likely to prove extremely revealing, both in terms of the properties of astrophysical neutrino sources, and the properties of neutrinos themselves. In this article I concentrate on the later.
Studying the astrophysical neutrino ﬂux produced by sources beyond the solar system, may eventually be as revealing as the solar neutrino ﬂux has proven to be. From an astrophysics point of view, neutrinos have the advantage that (unlike cosmic ray protons) they are not deﬂected by magnetic ﬁelds and thus their arrival direction points back to the source. In addition, they are not attenuated by intervening matter. Neutrino astrophysics will thus allow us to see further in the cosmos and deeper into astrophysical sources. In addition, the ﬂavor composition of astrophysical neutrino ﬂuxes may encode important information about neutrino properties.
There are many interesting sources of high energy astrophysical neutrinos, including cosmic accelerators such as gamma ray bursts, supernovae remnants or active galactic nuclei.
Interactions of accelerated nucleons in the vicinity of these sources lead to the production of charged pions, and hence neutrinos via their decays. If the sources are optically thin, the neutrino ﬂuxes may be related to the ﬂuxes of cosmic rays and gamma rays [1, 2], while for optically thick sources these constraints do not apply . There may even be “hidden sources” for which the density of matter is such that only neutrinos escape; see, for example, Ref. [3, 4]. In addition, “cosmogenic” neutrinos are generated via the interaction of high energy cosmic rays with the cosmic microwave background. Finally, dark matter annihilation or decay may contribute a source of high energy neutrinos that are detected in neutrino telescope experiments.
2. Above the electroweak scale One of the most exciting prospects of neutrino astronomy is ability to access physics beyond the electroweak scale. For neutrinos with PeV energies, the center of mass energy in a neutrinonucleon interaction is at the TeV scale. At such high energies, the neutrino-nucleon cross sections have not been measured and must be extrapolated from lower energy data [5, 6, 7].
Cross-sections either smaller or larger than the standard model extrapolation could signal new physics contributions. Possible eﬀect that could enhance neutrino cross-sections include the exchange of towers of Kaluza-Klein gravitons [8, 9] or the production of black holes [10, 11, 12].
Event rates in neutrino telescopes obviously depend on both the neutrino ﬂuxes and crosssections. However, it is possible to disentangle ﬂux and cross-section, since event rates in the up-going, down-going, and earth-skimming directions have a diﬀerent dependence on neutrino cross-sections, due to absorption of neutrinos which traverse the Earth [13, 10]. Current Amanda data place weak ﬂux and cross-section constraints at center of mass energies of order ∼ 1 TeV , while IceCube and other experiments have potential to make a measurement of these parameters.
Many example of physics beyond the Standard Model may also show up in neutrino telescopes.
For instance, in some supersymmetric models a very distinctive process would be the production of long-lived NLSP pairs, for which the signature in IceCube would be a pair of two parallel charged tracks [15, 16, 17].
3. Flavor Composition Neutrinos from astrophysical sources are expected to arise dominantly from the decays of pions, which result in initial ﬂavor ratios of φνe : φνµ : φντ = 1 : 2 : 0. The ﬂuxes of each mass eigenstate are given by φνi = α φsource |Uαi |2, where Uαi are elements of the neutrino mixing να matrix. If we assume exact νµ –ντ symmetry (θ23 = 45◦ and θ13 = 0) this implies that neutrinos are produced in the ratios φν1 : φν2 : φν3 = 1 : 1 : 1 in the mass eigenstate basis, independent of the solar mixing angle. Oscillations do not change these ratios, only the relative phases between mass eigenstates, which will be washed out by uncertainties in the energy or distance since δm2 × L/E ≫ 1. An incoherent mixture with the ratios 1 : 1 : 1 in the mass basis implies an equal mixture in any basis (since UIU † ≡ I) and in particular in the ﬂavor basis in which the neutrinos are detected [18, 19].
Variation from the assumed νµ –ντ symmetry lead to only small deviations of the ﬂavor ratios.
However, such deviations could be used to measure neutrino mixing parameters, if suﬃciently high precision measurements of the astrophysical ﬂux were to be made [20, 21, 22]. On the other hand, the ﬂavor composition of astrophysical neutrino ﬂuxes may encode important information about exotic neutrino properties. Variations to the expected ﬂavor ratios may reveal new physics such as neutrino decay , CPT violation , oscillation to sterile neutrinos [25, 26, 27], and various other exotic scenarios [28, 29, 30].
Neutrino decay can result in particularly large deviations to the expected ﬂavor ratios .
For non-radiative decays such as νi → νj + X and νi → νj + X, where X is a massless particle ¯ (e.g. a Majoron) existing limits are quite weak. If neutrinos are unstable, the cosmic neutrinos detected may all be in the lightest mass eigenstate. The ﬂavor composition of this lightest eigenstate is φνe : φνµ : φντ = 5 : 1 : 1 in the case of the normal hierarchy, and 0 : 1 : 1 in the case of the inverted hierarchy. For either hierarchy, this represents a large and distinctive deviation to the expected ﬂavor equality.
Another feature of astrophysical neutrino experiments is the enormous distance scales at our disposal. With neutrino from distant astrophysical sources, we may do oscillation experiments with baselines comparable to the size of observable Universe. Given a neutrino oscillation length scale of ∼ 2E/δm2, cosmological scale baselines provide sensitivity to oscillations with extremely small mass splittings [25, 26, 27]. In Fig. 1 we show the δm2 sensitivity of various
Figure 1. The energy and distance ranges covered in various neutrino experiments.
The diagonal lines indicate the mass-squared diﬀerences (in eV2 ) that can be probed with vacuum oscillations; at a given L/E, larger δm2 values can be probed by averaged oscillations. The shaded regions display the sensitivity of solar, atmospheric, reactor, supernova (SN), shortbaseline (SBL), long-baseline (LBL), LSND, and extensive air shower (EAS) experiments. The KM3 region describes the parameter space that would be accessible to a 1-km3 scale neutrino telescope, given suﬃcient ﬂux. Figure taken from Ref. .
neutrino experiments. An example in which such tiny mass splittings occur is the case of pseudoDirac neutrinos, in which a Dirac neutrino is split into a pair of almost degenerate Majorana neutrinos by the presence of tiny, sub-dominant, Majorana mass terms. In this scenario the active neutrinos are each maximally mixed with a sterile partner with very tiny δm2. The deviations to the astrophysical neutrino ﬂavor ratios due to oscillations driven by these tiny mass splittings would be milder than those predicted for neutrino decay. However, it is a potential probe of tiny Majorana mass terms (and thus lepton number violation) not discernible via any other means.
If neutrinos are produced via some mechanism other than conventional pion decay, there will also be departures from the canonical ﬂavor ratios 1 : 1 : 1. One scenario is that where neutrons, produced in the Galaxy by photo-disintegration of heavy nuclei, decay to a pure νe ¯ ﬂux [31, 32]. After oscillations wash out phase information, this ﬂux is transformed to the ratios φνe : φνµ : φντ = 5 : 2 : 2. Another possibility is a muon damped source in which charged pions decay to muons and neutrinos, but the muon daughters loose energy before decaying further .
The pure νµ ﬂux produced is transformed by oscillations to φνe : φνµ : φντ = 1 : 2 : 2.
The ﬂavor degree of freedom clearly carries important information about both the astrophysical neutrino production mechanism, and exotic physics in the neutrino sector. An important question is whether a given ﬂavor signature is unique to a particular scenario.
However, a number of the scenarios discussed above have large and distinctive eﬀect on the ﬂavor ratios. For example, it is diﬃcult to see how the neutrino decay of prediction φνe : φνµ = 5 : 1 could be replicated by another mechanism.
Neutrino ﬂavor ratios will not be directly measured at neutrino telescope experiments, but can be inferred from the ratios of diﬀerent types of events. In an experiment like IceCube, the ratio of muon tracks to shower events is likely to be most useful, and would permit the νe fraction of the ﬂux to be calculated. In Ref. , it was found that a νe fraction of 1/3 (the default prediction) could be measured to a range of approximately 0.2–0.4, provided the neutrino spectrum was also measured.
Double-bang and lollipop events, which are unique to ντ, would provide important direct information on the size of the tau neutrino ﬂux [18, 35]. A double-bang event consists of a shower initiated by a charged current interaction of ντ, followed by a second shower initiated by the decay of the resulting tau lepton. (Lollipop events consists of the second of these two showers, along with a reconstructed tau lepton track.) The detection threshold for these ντ events is a few PeV, and thus expected events rates will be small. However, given that the some exotic physics scenario can lead to large deviations from the expected ﬂavor equality, even small numbers of events can provide important information.
Figure 2. Neutrino and gamma-ray limits on the dark matter total annihilation cross section in galaxy halos, selecting Br(γγ) = 10−4 as a conservative value.
The general unitarity bound is shown for comparison, while the KKT limit denotes the point at which annihilations would signiﬁcantly modify dark matter halo density proﬁles . The overall bound on the total cross section at a given mass is determined by the strongest of the various upper limits. Figure taken from Ref. .
is the DM number density) which is enhanced by the clustering of dark matter in halos, while the monochromatic neutrino energy is smeared by redshift to form a broader spectrum. A complementary approach, with comparable or slightly better sensitivity, is to consider the signal from annihilations within our Galactic halo . In order for the annihilation ﬂux to be detectable, it must be larger than the atmospheric neutrino background. We may adopt the conservative criteria that the signal be as large as the angle averaged atmospheric neutrino background, and use the Super-Kamiokande atmospheric neutrino measurements  to bound the possible neutrino ﬂux arising from dark matter annihilation.
In Fig. 2, we show the constraints on the total dark matter annihilation cross-section, obtained by conservatively setting the branching ratio to neutrinos at 100% (the ﬁgure displays the Milky Way constraints derived in Ref ). Also shown are constrains on the annihilation crosssection obtained by assuming a 10−4 branching ratio to the state γγ . The neutrino results are surprisingly strong, particularly for large dark matter mass. In particular, they are more stringent than the general unitarity bound  over a large range of masses, and strongly rule out proposals in which annihilation rates are large enough to modify dark matter halos (denoted by KKT  in Fig. 2).
The technique to constrain the dark matter total annihilation cross-section can be applied to MeV energies using the Super-Kamiokande data , and analogous bounds on the DM decay rate can also be derived . Note that although we have set the branching ratio to neutrinos at 100% (in order to derive a conservative and model independent bound) ﬁnal state neutrinos will inevitably by accompanied by gamma rays due to electroweak radiative corrections. However, these gamma ray constraints on the annihilation cross-section are weaker than or comparable to those obtained directly with neutrinos [47, 48, 49].