High-energy (PeV) neutrinos observed!

At June's "Neutrino 2012" conference, IceCube showed two very interesting neutrino events.  Both were cascades (electromagnetic or hadronic showers), with an apparent energy of about "PeV to 10 PeV."  This is equivalent to the mass energy of over 1,000,000 protons, or about 250 times the energy of one of the protons that are accelerated at the LHC (Large Hadron Collider).   In other words, it is far beyond anything we could imagine producing on Earth.

The events, recorded on August 9, 2011 and on January 3rd, 2012, are shown below.  

The side (top/left grahic) and top (bottom/right) view of the January 3rd event.  Each dot shows one IceCube optical module; the colored spheres show the 312 optical modules that were hit in the event.  The color indicates the relative time (red = earliest, blue = latest), and the size of the sphere shows the number of observed photons. About 96,000 photons were observed from the event. This is a very small fraction of the total number that were created by the shower.

The side (top/left) and top (bottom/right) view of the  August 9th event. Each dot shows one IceCube optical module; the colored spheres show the 354 optical modules that were hit in the event.  The color indicates the relative time (red = earliest, blue = latest), and the size of the sphere shows the number of observed photons.About 70,000 photons were observed


In both events, the cascades were far from the detector edges, and there was no sign of any incoming muon, so the events are unlikely to be background.  We are also confident that they are real, and not due to a detector problem of some sort.    Analysis of these events is still on-going (for example, to better determine their energies), but these are clearly far more energetic than the events previously seen by IceCube, and the estimated background from atmospheric neutrinos is about 0.14 event.    That estimate is high because it does not include at least one mitigating factor -if these were created in a roughly downward-going cosmic-ray air shower, then we would have seen evidence of the shower in the IceTop surface detection array.   Also, the 0.14 events is for all types of neutrino interactions with a lot of produced light, while these two events look like either electron neutrinos, or neutral-current (NC)  interactions of any type.  The NC interactions deposit only a fraction of the neutrino energy in the detector, so if these were NC interactions, the neutrinos would have to have been even more energetic.

For those who want more information, Aya Ishihara's talk is posted here.  n.b.  Everything above (including the event plots) is taken directly from Aya's talk.

It is too early to say what these events will mean, but this is a very very interesting development.   Stay tuned.

No Nus is big news

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:

UW Madison
LBNL
DESY

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)).

Science in the Theater

"Science in the Theater - Extreme Science" is over. It was an interesting experience, and, although I can't say that the four presentations melded well, they were all both good and interesting, and both the speakers and the audience enjoyed it.

We spoke in the ~ 600 seat Roda theater at Berkeley Repertory Theater. We arrived at 6 pm, for a 7 pm curtain, allowing time for mike and video checks, plus a quick dinner. The sound guy was impressive - I don't see how he could switch our microphones on and off so quickly whenever one of us said anything. It was cool to see the backstage area. My wife Ruth and I had seen Moliere's "A doctor in spite of himself" there only two days earlier. It was interesting to see some of the props up-close, and this certainly contributed to the feeling of being on the 'big stage.'

I brought a prototype Digital Optical Module as a prop. It was a pain to lug around, but made a great prop. As a bonus, I got to park in the Rep loading dock, mere feet from the stage.

The audience started arriving early - a few people were already there when we arrived. It was a good crowd - the lower level of the theater was full, with a sprinkling of people upstairs. It was an interesting mix of ages, with a good number of high school kids, plus some from junior high.

Andrew Minor started off, describing how he uses electron microscopes to explore the effects of extreme environments (such as inside a nuclear reactor) on matter. Then, Caroline Ajo-Franklin talked about her studies of organisms that live in extreme environments (for example, microbes that get their energy by oxidizing metals, instead of using oxygen), and about how we might be able to use the techniques and genes in these organisms to generate energy. Tamas Torok talked about his travels in the former USSR, searching for organisms that live in extreme environments, such as in heavily acidic lakes, volcanic craters, and in Lake Baikal.

Then, I discussed my work, starting with a brief explanation of why we do neutrino astronomy, leading to a 'guided tour' of IceCube and ARIANNA. Then, we had a phone call from the South Pole, with our two IceCube winter-overs, Carlos Pobes and Sven Lidstrom on the other end. I had been a bit nervous about the logistics for this, particularly getting the timing right. In the end, the timing was almost perfect. The audience seemed blown away by the call. One woman said that she couldn't get her head around what a temperature of minus 93 degrees meant; judging by the reaction, this was a common view.

Then we answered questions for an hour. These varied widely in level and in form. A number of people expressed concern, with varying degrees of specificity, about the genetic engineering aspects of the extreme biology.

A video of the event is posted on youtube at http://www.youtube.com/watch?v=zuyaPaFbT3A.
Also, there are photos of the event here.

Many thanks to Carlos, Sven and IceCube outreach coordinator Laurel Norris for their help with the phone call to the South Pole. Also thanks to the LBNL folk who organized this, particularly Jeff Miller and and Dan Krotz, plus videographer Ivan Berry, and also to the folks at Berkeley Rep for providing great logistical support.

"Extreme Science" -- in the theater

On February 27th, I will be one of four participants in a presentation on "Extreme Science" - part of Berkeley Labs "Science in the Theater" series, to be held at Berkeley Repertory's theater. It should be an interesting mixture - two scientists who work in extreme environments (Antarctica and Siberia & other places), and two whose work is extreme in other ways. Each of us will speak for 10-12 minutes, and then answer questions. Previous "Science in the Theater" series have featured scientific quartets with similar interests; it will be interesting to see how this works with four people from diverse backgrounds.

Details (time, location, etc.) are available on the Friends of Berkeley Lab website. There is also a nice promo video on youtube.

ARIANNA 2011

I have been quite remiss about posting to this blog. One of my new years resolutions has been to do better.

Despite the lack of postings, ARIANNA has been moving forward. In November/December, three graduate students from UC Irvine visited the site, and deployed another prototype station. The setup if similar to the current system, but this station has a new waveform digitizer system, based on a new chip from Stuart Kleinfelder. It also has electronics (computer, etc.) that should handle the cold better.

They also rejuvenated the old station. Now, as long as the power holds out, we will have two stations for comparison purposes.

One of the students kept his own blog, at

http://ericcberg.blogspot.com/

It is well worth a read.

-Spencer