Friday, 13 September 2013

Storm in Ice Cube


IceCube is a neutrino detector located right at the South Pole.  Most of what they see are atmospheric neutrinos produced when cosmic rays hit Earth's atmosphere. Obviously, nobody would be freezing his off in the Antarctica for that. The goal of IceCube was always to catch neutrinos of astrophysical origin, thus inaugurating neutrino astronomy.  Finally, last year they announced the detection of 2 neutrino cascade events, both with the same (within experimental uncertainty) 1 PeV = 1000 TeV energy. This is interesting not only because these are the highest energy neutrinos ever detected. The main point is that  the atmospheric neutrino spectrum is expected to drop off quickly at large energies, and is very unlikely (at about 3 sigma or so) to produce as much as two PeV events.  The more recent development is that there is in fact more than just two events.  There appears to be a broad excess (quantified at 4.3 sigma) of 28 events  above 30 TeV  where only about 10 are expected from atmospheric neutrinos alone. The flavor composition in that range is consistent with 1:1:1 ratio of the 3 neutrino species, as expected for faraway sources because of neutrino oscillations along the way.

All in all, there is little doubt that the neutrino spectrum measured in IceCube has a component which is harder than the one predicted for the atmospheric neutrino and therefore is most likely of astrophysical origin. One can dig out a few more interesting bits of information. The observed angular distribution of the ultra-high-energy events is compatible with an isotropic incoming flux. There seems to be a dip in the spectrum just below 1 PeV - no events are observed in the two bins between 0.4 PeV and 1 PeV. Finally, the spectrum probably has a sharp cut-off just above 1 PeV - otherwise several multi-PeV events should have been observed.

All these more detailed statements are at this point not very significant statistically, but if they persist after future updates things will get even more interesting. Indeed, sharp spectral features are more tricky to explain in terms of known astrophysical phenomena, which are rather expected to produce a smooth power-law spectrum (typically assumed as 1/Energy^2). For particle physicists the natural question is whether these neutrinos could be the signal of annihilation or decay of dark matter in our galaxy. Dark matter would have to be made of PeV mass particles - more than is typically considered. But why not: any mass between sub-eV and Planck scale is equally plausible at this point.    

Of course, theorists have models for every occasion up their sleeves.  On interesting proposal is that the two PeV events can be interpreted as a monochromatic neutrino line. This  could arise if dark matter decays into a 2-body final state containing at least one neutrino (dark matter annihilation with a large enough cross section is difficult to realize for such a heavy mass). To explain the observation of 2 events in IceCube,  the life-time of dark matter should be about 10^28 seconds or 10^21 years - much longer than the age of the Universe and compatible with all existing constraints. The presence of the continuum excess around 100 TeV makes the picture more complicated... but not necessarily unrealistic. It is perfectly conceivable that dark matter has more than one decay channels, for example 10% branching fraction into 2 neutrinos, and the remaining 90% into quarks. The former would be responsible for the PeV events, and the latter would produce neutrinos with smaller energies from the hadronic cascades, which would explain the broad feature near 100 TeV. This paper shows that a general scenario of this type can fit the data very well, somewhat better than the smooth 1/E^2  spectrum.

So,  as usual, the possibilities are endless, and we badly need more experimental input. The dataset analyzed so far corresponds to the 2010-2012 period, so there should be 50% more events already waiting in the pipeline. Most likely, it will turn out that these ultra-high energy neutrinos are produced by some boring astrophysical processes (although I admit that the boring astrophysics producing PeV monster neutrinos must be less boring than typical boring astrophysics). But, well, who knows, maybe this time they will hit the jackpot?  

For more details and naked photos of Bert and Ernie, see this talk.

16 comments:

Anonymous said...

Well it almost has to be something cool. Even just an excuse to use the lonely "peta-" prefix is nice in and of itself.

Dan D. said...

Thanks for the post about the latest IceCube results. Let's say for the sake of argument that these results are the first hints of a PeV-scale decaying DM particle. What sort of models would be most likely to accomodate such a particle? SUSY models? If so, does this suggest that the rest of the SUSY spectrum lies at least at the PeV scale and above, in which case we are hosed for any foreseeable near-future collider that could produce such particles? Are there any models that predict or accommodate PeV dark matter that also allow for other particles at lower energies? Apologies if these are dumb questions. (I'm a scientist in another field who has recently taken a layman's interest in particle physics). I just wonder if we could get into a situation where we finally discover something BSM, only to be left high and dry afterwards.

Nick Waters said...

Why isn't the detector flooding with events?

Anonymous said...

The thing is, if these events were the result of DM decays in the galaxies, you would expect a larger anisotropy than what is observed.

Anonymous said...

@Dan D.

I think a PeV scale hadron collider is the highest energy feasible with foreseeable technology. It would be an enormous undertaking, but if we had clear data suggesting that there is something going on in the ~PeV range, we could get there. So, we wouldn't be left completely without hope of better understanding.

Jester said...

Dan, the only concrete things you could say about the PeV dark matter is that it could never be in a thermal equilibrium with the rest of matter (otherwise there would be too much of it around). For this reasons typical MSSM scenarios wouldn't work. But there are many other SUSY or non-SUSY models that could accommodate 1 PeV dark matter (FIMPS, WIMPzillas,...). Obviously, at this point we have almost no hints about specific models.

Jester said...

Well, by eye the signal is largest around the galactic center... At least according 1308.1105 the angular distribution is so far compatible with decaying dark matter.

Anonymous said...

Nit-Pick..

There seems to be an extraneous sentence (market with >> <<)in the 4th paragraph...

...the life-time of dark matter should be about 10^28 >>This paper shows that a general scenario of this type can fit the data very well, somewhat better than the smooth 1/E^2 spectrum.<<
seconds or 10^21 years - much longer than the age of the Universe.

This should read...

... the life-time of dark matter should be about 10^28 seconds or 10^21 years - much longer than the age of the Universe.

Sorry 'bout that :-)

Jester said...

thanks, corrected. Lately I have had a lot of problems with Blogger, one thing being random rearranging of the text after pasting a picture....

SimonApers said...

Hey Jester,
I will be writing my masters thesis on some new photo detectors for Ice Cube. For that I would like to learn a little more about neutrino physics. Any reference works you could recommend me? (Graduate level)
Thanks!
Simon

Jester said...

I like the review of Strumia and Vissani hep-ph/0606054
Of course their constraints on the masses are mixing angle are outdated, for the newer ones see e.g. arXiv:1209.3023

Dan D. said...

Thanks for the replies, Jester and Anonymous. That would be quite something if we ever built a PeV-scale collider!

Anonymous said...

Hmm, a PeV collider. Muons, I assume. Call it 2400 km circumference. About 4 km/G$, so 600 G$. Less than some recent wars. Finding a place one can keep the ring flat within 200 m of the surface at the circumference would be challenging. And what would the neutrino flux be from the decays? So maybe protons again, even though they're messy.

Anonymous said...

Hmm, a PeV collider. Muons, I assume. Call it 2400 km circumference. About 4 km/G$, so 600 G$. Less than some recent wars. Finding a place one can keep the ring flat within 200 m of the surface at the circumference would be challenging. And what would the neutrino flux be from the decays? So maybe protons again, even though they're messy.

Askaboutphysics said...

Frist of all, Jesper thanks for once again clearly explaining new results! I just wanted to comment on some of the above comments. There is as I understand a difference between actual centre of mass energy of a particle collider like lhc and a fixed target setup like the ice cube can be thought of. To my knowledge at least atmospheric collisions of 1pev = 1000 tev roughly corresponds to a 14 tev collision in a collider like lhc! If somebody knows more of this please comment.
/A

Anonymous said...

Wouldn't the signature of a very heavy particle show up in the CMB? In general, why do people even do direct detection experiments when CMB requires an extremely small cross-section?