Thursday, April 26, 2012
IceCube published a paper in Nature last week, about neutrino production in gamma-ray bursts (GRBs); this marks a significant step forward for the experiment. Unfortunately, it wasn't an observation - we saw no neutrinos. It was, however, our first "interesting" upper limit, where "interesting is" defined as heavily constraining current theories of particle acceleration in GRBs.
The IceCube results disfavor GRBs as the source of ultra-high energy cosmic-rays. More precisely, "either the proton density in gamma ray burst fireballs is substantially below the level required to explain the highest energy cosmic rays or the physics in gamma ray burst shocks is significantly different from that included in current models." There was considerable collaboration discussion about the nuances in this statement; the end result was something that we could all live with. The nuances were required because current theories of how GRBs accelerate particles are quite primitive, with simplified models of the geometry of the object, the acceleration, and the neutrino production. So, to rule out a theory, we had to look at not only the central prediction of the theory (the most likely number of neutrinos), but also at how much one can reasonably adjust the parameters in the theory to reduce the predicted number of neutrinos. We concluded that it is not easy to adjust the parameters of the existing theories enough to reduce the number of neutrinos below our sensitivity level. It is, of course, likely that theorists will adjust their theories to reduce the neutrino production, but, of course, we will continue to analyze data - this result was based on two years of data with 50% and 75% of the detector complete, respectively. In the end, though, IceCube is sensitive enough that we should either see a signal, or the required adjustments to the theory will make them seem unattractive.
The paper received considerable media attention, with coverage on MSNBC, the Christian Science Monitor, the BBC, Scientific American, Science News etc. The Register gets the award for the best language: "Eggheads stumped after killer gamma rays ruled out. Probably"
Unfortunately, most of these writeups were based on IceCube press releases. You can find the press coverage on google news; the press releases are available here:
The paper is available from Nature here. A subscription is required for see this, but the paper is also freely available on the Cornell preprint server, here.
(The photo above is from the SWIFT satellite, showing X-ray emission GRB090212 (which, unfortunately, was not used in this analysis)).