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. 

Monday, October 13, 2014

Now you see them, now you don't - oscillating neutrinos

One of the most interesting particle physics discoveries over the past 20 years has been that neutrinos oscillate.  That is, they change from one flavor into another.  For example, an electron neutrino (produced in associate with an electron) may, upon travelling some distance, interact as if it was a muon neutrino.  If you have a detector that is only sensitive to one flavor (type) of neutrino, it may appear as if neutrinos are disappearing.

This oscillation is important for a number of reasons.  First, it shows that neutrinos can mix - that the states that propagate through space are not exactly the same states that appear when they are created or interact.  This is one of those weird quantum mechanical puzzles, in that you should use different basis (set of possible states) to describe neutrinos as they are created or interact, then when they are propagating through space. 

The second thing is that it requires that neutrinos have mass, since it is the different mass that differentiates the states that propagate through space.  Many, many papers have been written about how this can work; Harry Lipkin has written a nice article that focuses on the interesting quantum mechanics.  The presence of neutrino masses is often considered one of the first clear signs of physics beyond the 'standard model' of particle physics (dark matter and dark energy) are two other signs.

There are two important parameters in neutrino oscillations.  The first is the mixing angle (do the neutrinos mix perfectly, or only a little bit), and the second is the square of the mass difference (\delta m^2), which controls how long it takes for the oscillations to occur; the conversion probability scales as sin^2(\delta m^2*L/4E) where L is the distance traveled, and E is the energy.   Of course, since there are three neutrino flavors (electron, muon and tau), there is more than one \delta m^2 and mixing angle.

Interestingly, neutrino oscillations were discovered almost entirely by astroparticle physicists, rather than at accelerators.  The first clue came from a gold mine in Lead, South Dakota.  There, in the  late 1960's,Ray Davis and colleagues placed a 100,000 gallon tank of perchloroethylene (a dry cleaning fluid) 1500 meters underground.  Theory predicted that neutrinos produced by nuclear fusion in the sun would turn a couple of chlorine atoms a day into argon.  Through careful chemistry, these argon atoms could be collected and counted.  David found only about 1/3 as many argon atoms as expected.  Needless to say, this was a very difficult experiment, and it took years of data collection and careful checks, and further experiments (see below) before the discrepancy was accepted.  The photo at the top of this post shows Davis swimming in the 300,000 gallon water tanks that served as a shield for the perchloroethylene.

The next step forward was by the Super-Kamiokande experiment in Japan, also underground. In 1998, they published a paper comparing the rate of upward-going and downward-going muons from muon-neutrino interactions.  The found a deficit of upward-going events which they described as consistent with neutrino oscillations, with the downward-going events (which had traveled just a short distance, and so presumably were un-oscillated) serving as a reference.

The third major observation was the by SNO collaboration, who deployed 1,000 tons of heavy water (D_2O) in a mine in Sudbury, Ontario, Canada.  The deuterium in heavy water has one fairly unique property, in that it is undergoes a reaction that is sensitive to all three neutrino flavors.  SNO measured the total flux of neutrinos from the Sun, and found that it was as expected.  In contrast, and in agreement with Ray Davis experiment, the flux of electron neutrinos was only 1/2 to 1/3 of that expected (the exact number depends on the energy range).  So, the missing electron neutrinos were not disappearing; they were turning into another flavor of neutrinos.

Some years later, the Kamland collaboration performed an experiment to observe the oscillations from neutrinos that were produced in nuclear reactors, further confirming the observation.

Now, IceCube has also made measurements of neutrino oscillations, using the same atmospheric neutrinos used by Super-Kamiokande.  Although it is not easy for IceCube to reach down low enough in energy for these studies, we have the advantage of having an enormous sensitive area, so that we can quickly gather enough data for a precision result.  The plot below shows a recent IceCube result.  The top plot shows IceCube data, as a function of L/E; L is determined from the zenith angle of the event, which is used to calculate how far the neutrino propagated through the Earth.  The data is compared with two curves, on showing the expectations without oscillations (red curve), and the second a best-fit, which returns mixing angles and \delta m^2 compatible with other experiments.   Although our measurement is not yet quite as powerful as some other experiments, we are quickly collecting more data, and also working on improving our understanding of the detector; the latter is accounted for in the systematic errors.   The bottom plot is just the ratio of the data to the no-oscillation expectation; no-oscillations are clearly ruled out.