Thursday, October 23, 2014

Looking ahead - is bigger better?

Now that IceCube has a few years of data under our belt, it is natural to size up where we are, and where we want to go.  

Where we are is pretty clear.  We have observed a clear astrophysical neutrino signal, with a spectral index most likely in the 2.3-2.5 range, so the neutrino flux goes as dN_nu/dE_nu ~ E_nu^-2.3.  However, we have seen no sign of a point source signal (either continuous or transient), so we do not yet know of any specific neutrino sources.  Dedicated searches for neutrinos in coincidence with gamma-ray bursts (GRBs) have largely (but not completely) eliminated GRBs as the source of the observed neutrinos.  Although we continue to collect statistics, we have enough data now that the only way that we could observe a statistically significant point source signal in the near future would be if a powerful transient source appears.   It is likely that IceCube is too small to observe point sources.

This observation of point sources is a natural focus for a next-generation instrument.  ARIANNA (or another radio-neutrino detector) is clearly of high interest, but probably has an energy threshold too high to study the class of cosmic neutrinos seen by IceCube.  So, there growing interest in an optical Cherenkov detector like IceCube, but 10 times bigger.   It wouldn't be possible to deploy 800 strings in a reasonable time (or at a reasonable cost), so most of the preliminary designs involve of order 100 strings of optical sensors with spacings larger than the 125 m prevalent in IceCube - 254 to 360 m are  popular separations. It would have better optical sensors than were available when we built IceCube.

This detector would necessarily have a higher energy threshold than IceCube, but would be well-matched to the astrophysical neutrino signal spectrum.  Clearly, it would do a much better job of measuring the neutrino energy spectrum than IceCube did. 

But, the key question is whether it is sensitive enough to have a good chance of seeing point sources.  There are many calculations which attempt to answer this question.  Unfortunately, the results depend on the assumptions about the distribution of sources in the universe, and their strengths and energy spectra, and, at least so far, we cannot demonstrate this convincingly.  So, stay tuned.

Of course, an alternative direction is toward an infill array with even higher density than PINGU.  This is at a more advanced stage of design, and will be covered in my next post. 

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