What IceCube Neutrinos are Telling Us
Paolo Lipari (left) gave a talk on the interpretation of the signals of astrophysical sources of neutrinos. Neutrinos have two purposes: they are messengers, which we can use to study the universe. And they are also probes, to infer the fundamental properties of the standard model and of fundamental laws. As messengers, they tell us about the sun, the Earth, and the cosmos, Supernovas. They span many orders of energy. This topic can be divided in many, as the energy range is very broad. The IceCube signal is from 10^14 to 10^16 eV. So actually the topic is the high-energy Universe. You have an ensemble of astrophysical objects that generate relativistic particles. Neutrinos must be understood as messengers, and together with cosmic rays, photons, and gravitational waves they can help us uncover deep mysteries.
The mechanism of creation is understood as acceleration of hadrons, that decay to the neutrinos; but neutral pions generate photons, and there you have a relation between the two fluxes. Then also leptonic fluxes are related to that. The concept is that you generate a population of protons, they interact in the medium and produce gammas and neutrinos. The spectra of neutrinos from pions end up being very similar in spectrum.
If the target is not a gas but a radiation field, the interaction is p-gamma and not p-p. So single pion production is dominated, and you have an excess of positive pions, which changes the ratio of photons, neutrinos, antineutrinos. If the energy increases you then find more photons. The spectral index of neutrinos reflects the ones of the progenitors. If you observe a source in photons you can then relate it to neutrinos and you can distinguish the mechanisms of creation.
The gamma-ray sky is composed of three componends: the galactic plane, a uniform flux, and some sources. Eighty percent of the photons at 1 GeV is concentrated at +-5 degrees around the galactic equator. This is measured by Fermi with a spectrum with a spectral index of 2.7. The flux is also concentrated in the galactic center. Then there is an ensemble of sources, more than 3000 found by Fermi. 440 of these are galactic. In the TeV range you see the same thing.
Then there is an extragalactic gamma ray flux. It is isotropic and with a power of E^-3. This is nearly certainly an effect of absorption, due to their impossibility to travel on extragalactic distances. And then there are sources: very interesting beasts of different kinds. Active galactic nuclei emanate jets from the center, with the brightest sources of extragalactic gamma rays. Our own galactic center is also active, with a black hole at the center that we are studying.
The extragalactic flux is composed of two parts: a resolved one, from bright sources. One is 3C454, then there are others, about 2000, and very likely there is then a half of the flux is from unresolved sources. So it is not surprising that if you see 54 neutrinos you do not see a point source – the brightest in photons is 1.8% of the total.
The speaker moved then to discuss the IceCube data, where the statistical significance of the new component is largely coming from the highest-energy events. There are events where the interaction of the neutrino is within the detector, and then there is the flux from muons. The two results seem to give two different power spectra, as discussed by the previous speaker. There is a lot of discussion on this topic with hundreds of papers published. Within systematics they are not so different, but the possibility of two components is intriguing.
A critical discussion of the result should address several questions. First, is the signal of astrophysical neutrinos real ? The answer is most certainly yes. The second is, can the signal be contaminated by a non negligible contribution of atmospheric neutrinos? If you consider the three components (conventional atmo neutrinos, prompt atmo neutrinos, and astrophysical ones), the real dangerous one is from prompt atmospheric neutrinos, from the decay of charm. It is a difficult problem. The perturbative way to produce charm is through gluon fusion diagrams. The cross section is measured, but the Ice Cube result uses this calculation as a model for charm production, fixing it. There are a number of papers that discuss this issue critically. Francis Halzen himself wrote a paper discussing the charm contribution, and it is interesting that he concludes that non-perturbative mechanisms can be a dominant source of neutrino production from charm. This contribution cannot accommodate the PeV flux, and it is not important in the entire energy range. However the non-perturbative effects are the most important ones.
There is a handle to try and distinguish the two effects. The speaker commented that we cannot entirely dismiss the possibility that a charm contribution distorts the spectrum. There have been discussions on measuring these effects at the LHC, to cast more light on the issue.
Another question is, does the IceCube signal have a Galactic component? If one looks at the map, it is difficult to establish by eye. In the literature there are attempts to model the signal as entirely due to galactic origin. They could be generated by dark matter in the galaxy. Another paper that assumes that the signal is entirely galactic is one that is inspired by the observations of the “fermi bubbles”, with large halos around the galaxy.
In this conference it has been suggested that a significant fraction of the signal comes from the disk. It would be very nice if IceCube itself could give an answer to this question. Several works discuss model with both a galactic and an extragalactic component. The speaker mentioned a few of these. The problem is that the galactic emission that we see seems to be lower than what we see.
To conclude, Lipari mentioned the interest of studying the extragalactic component lies in the fact that it could be coming from an ensemble of sources, and identifying these sources is the focus of the issue.