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Michael DuVernois | Wisconsin IceCube Particle Astrophysics Center | USA

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News: Daya Bay reports on Theta13

Thursday, March 8th, 2012

From the Press Release: sin2 2 θ13, is equal to 0.092 plus or minus 0.017.

More info can be found on the public web pages of the experiment.

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IceCube looking at the solar event

Thursday, March 8th, 2012

The largest solar storm in years is approaching the Earth. IceCube is interesting in that, even though the primary goals are at much higher energies, there is a subgroup within the project utilizing the surface detector component of the IceCube Experiment for solar physics. It gives some nice tools for watching the solar energetic particle fireworks. The IceTop summary. This is usually a day behind so the big effects have yet to appear as of this writing. The neutron monitor data are usually in near real time (a few minutes). The main page for the Bartol Neutron Monitors.

In more detail you can see the event really setting in at this link. (The shock is really sweeping the cosmic rays out of the inner solar system.) This link gives a more complete picture of what the cosmic ray anisotropy is doing. The process is applicable to particles up to perhaps 100 GeV. At present we do not think we are seeing with IceCube any freshly accelerated solar particles at GeV energy. But stay tuned…

Thanks to Paul Evenson for the links. This is quite a distance from what most people consider particle physics, but it is an interesting, real-time phenomena.

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What can we learn from “faster-than-light” neutrinos?

Wednesday, March 7th, 2012

I’ve let the news aspect of this story die back a little before writing about it. It now appears that the OPERA results were due to a mistake in the end. Rumors have it that it was a bad connector on a fiber optic link between a GPS and a computer that gave a 60ns time shift. New data taking with the tightened connector will be required to verify that this was, in fact, the cause of the problem. See also the Nature page.

Of course this was not a grand surprise, the vast majority of physicists felt that a mundane explanation would be found in the experiment rather than a rewrite of much of fundamental physics. But what I want to explore here is instead, what would you do? And, how does this illuminate the differences between theory and experiment?

The first question was fairly explicitly asked to me by colleagues at a meeting just days after the announcement of the preliminary OPERA “faster-than-light” neutrinos. I stumbled over an answer that I can abstract as “if you make a measurement, you can think about it, and even not believe it, but eventually you publish it” presumably with enough caveats that you aren’t misleading the readers into a different level of confidence in the results that you yourself hold. But I’m not sure if this answer (though probably close to what any official answer would be) is truly correct.

Extraordinary claims do demand extraordinary proof. And at first look the OPERA folks seemed to be extraordinarily careful in their review of their own work. Since the velocity measurement  in the experiment fundamentally comes just from the distance and the time of flight of the particles, a lot of effort went into the metrology and surveying for the distance measurement and a careful evaluation of the clocks involved. But a loose connection seems to have been missed before public announcements and the wild theorist party (see below) that emerged from the smoke at the initial CERN lecture cum press conference.

Connectors are the bane of an experiment. From the horrid Lemo 00 connectors still found all too often in particle and nuclear physics, to the stiff cadmium-plated circular military connectors beloved of the aerospace concerns, down to the simple is-it-really-connected-securely screw terminals on the back of an old power supply, this is where so much debugging time and effort goes. So it seems plausible that the error could be there. But when should it have been found?

I’d want to tear the experiment down and build it back up, re-cable, re-connect, tear everything apart before I’d be willing to claim a major discovery. At the time, the word was that the OPERA folks had put lots of time and effort into trying to find the problem, the mistake, but couldn’t locate it, so the news was released and the world started talking (and writing papers for arxiv) about faster-than-light neutrinos. I think we still don’t have a good enough picture as to the level of due diligence at the experiment. Did folks rebuild all of the timing system multiple times? Did the full signal chain get carefully looked at?

We tend to not be too critical of other physicists, and without knowing what happened within the OPERA collaboration, it’s easy for me to ask these questions without a real response. What experimentalists, in my opinion, need to take away from this is a real understanding of responsibility for being self-critical especially, but not exclusively, if there is a lot at stake. We well know the “solid four sigma” results which fade away in a few months, and yet it happens again and again. We know what will play in the popular press, and we’re careless about how we explain ourselves. (But enough about quantum teleportation illustrated with Star Trek visuals.)

I suspect that more than a few folks within the experiment, as well as outside, got terribly excited by the slim possibility of a major discovery. Within the group, this hopefully did not affect the critical thinking and tear-down of the experiment. Outside, in the larger community, certainly every neutrino experiment discussed very seriously what could be done to make such a measurement, and the theorists started producing papers. Why the results were wrong. Why the results were right, and agreed with their favored ideas. What it means for the rest of physics.

At times like this the cultural divide between theory and experiment never seems larger! A flood of papers since even a slight touch on a big discovery is worth something it seems. And now what? After the experimental error seems to be, well, an experimental error that wasn’t caught for a very long time, what do we think of all of theory papers? Presumably they just fade away, a light bright (?) spot of activity in late 2011 that someone will write a book about in five years, “The Faster-than-Light Neutrino Craze of ’11?” Some people got a little bit of publicity for misunderstanding GPS or for boldly extrapolating the neutrino velocity to higher energies. Is there regret over the waste of time? Or just a little exclamation, “ahhh…those experimentalists not checking their cables.”

More on this as the story develops, and as the water-cooler arguments continue.

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The largest neutrino detector?

Sunday, January 22nd, 2012

So what is the largest neutrino detector in the world? This discussion came up in regards to a very nice little educational video on YouTube that mentions the ANITA experiment:

(these minutephysics pieces are quite good!)

So, ANITA is the balloon-borne experiment mentioned in the video and of which I am a collaborator. But folks at IceCube claim that’s the world’s largest neutrino detector. And that’s a project I also work on. Furthermore, I was just at the South Pole working on a new neutrino detector called ARA (the Askaryan Radio Array) which has been mentioned as the largest neutrino detector in the world, even when only partially constructed. (See arxiv for a good ARA summary.)

So what’s the truth? Well, as in so many different endeavors, it comes down to the definition of largest. Or largest in what sense.

IceCube: This is an instrumented volume of a full cubic kilometer. Made up of over five thousand individual digital optical modules (DOMs) it is certainly the largest instrumented volume in the world. It uses the Cherenkov effect of neutrino-induced shower particles in the optically clear ice to image the shower and hence the neutrino.

ANITA: During an ANITA balloon flight, the payload observes a simply astonishing, more than a million square kilometers at a time. Only for certain narrow angular ranges can events form in the ice, refract through the surface and reach the balloon floating at 120,000 feet, but it is the largest observed area. This uses the Askaryan Effect which is a Cherenkov-like radio pulse emission from showers in dense materials.

ARA: The full ARA will cover hundreds of cubic kilometers of ice, but will have just 37 stations, each with four strings of four antennas. A much larger volume than IceCube, but much more sparsely instrumented due to the better attenuation length of radio than optical photons in the cold polar ice. The engineering test station that has been running since January 2011 has the largest volumetric acceptance of any neutrino detector in the world, several cubic kilometers. This also uses the radio technique.

So, largest neutrino detector in the world? Depends on your definition.

Read more about them: ANITA, IceCube on Facebook, Ice Cube on Facebook, ARA homepage, other radio neutrino efforts…RICE, ARIANNA, SalSA

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IceCube at the Rose Bowl

Monday, January 2nd, 2012

Well, it isn’t often that a large physics experiment is featured in an advertisement during a football game.

Youtube link. IceCube is one of the three great things coming out of the University of Wisconsin.

Publicity!

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Greetings from the South Pole

Saturday, December 31st, 2011

As a new Quantum Diarist, I wanted to introduce myself and my research work. And say hello from the bottom of the world. I am at the South Pole Station working on a new neutrino experiment called ARA, the Askaryan Radio Array.

This project is a second generation effort to look at the the highest-energy (GZK) neutrinos using radio detection of the coherent (Askaryan) emission from showers in dense materials. We’re building at the South Pole to take advantage of the largest block of dense, radio-transparent media on Earth, the 3km thick ice sheet that covers the continent. This is my second summer season at Pole working on ARA, last year we installed an engineering detector that has operated quite successfully throughout the year and now we’re installing the first production detector station. Ultimately we’re aiming for a detector array covering about 100 square kilometers with the antennas capable of detecting signals down to the bedrock below. A truly large detector. It’s much larger, but optimized for higher-energy events, than the IceCube detector completed last summer season at the Pole.

ARA is built on the experiences of the ANITA balloon-borne radio neutrino detector and the ice-drilling and radio spinoff efforts (RICE, AURA, & SATRA) of the IceCube experiment. It’s a small collaboration but with most of the world experience in radio neutrino detection and a significant block of the experience in hot-water drilling down into the ice. The main goal are the so-called GZK neutrinos, neutrinos produced by the interaction of the highest energy (charged particle) cosmic rays with the 3K microwave background radiation. More on all these physics topics in future postings…right now mostly wanting to say hello.

It’s New Year’s Eve at the South Pole. There are about 240 people here at the US South Pole Base, Amundsen-Scott Base, living and working in the new elevated station, or in the many small smaller building around the area. Some folks here are fairly traditional astronomers, working on the 10m South Pole Telescope, others have magnetometers, aurora cameras, seismometers, air sampling gear, and other scientific pursuits. The majority of people at Pole are here in support roles, driving heavy equipment, cooking dinner, washing the dishes, managing the cargo flow, and staffing the communications facility. Tonight there will be four bands performing, followed by a DJ set. We get satellite network a few hours per day and I’ll post this in the morning, in the new year for us.

Physics has gotten me to a lot of interesting places, and this is certainly way up there on that list, though it’s not the first project that had gotten me to Antarctica. I had worked on the CREAM and ANITA balloon experiments, both of which have flown from McMurdo Base on Ross Island just off the coast of Antarctica. (It’s probably close enough to count as Antarctica.) My background is in cosmic rays, I worked on spacecraft isotopic measurements of the cosmic rays while I was a graduate student at the University of Chicago. As a postdoc at Penn State, I worked on the HEAT balloon experiment measuring cosmic ray antimatter and the Pierre Auger Observatory in the Argentine grasslands. Later as a professor at the University of Minnesota, I added CREAM (a cosmic ray elemental abundance balloon experiment) and ANITA (the balloon neutrino experiment parent of the current ARA work). Now I am at the University of Wisconsin at Madison working on ARA, IceCube, and the HAWC TeV gamma-ray observatory currently under construction in Mexico. Detectors and their associated hardware are as important to me intellectually as the physics now.

So, greetings from the South Pole, a good place to do physics. And I’m looking forward to sharing my corner of the world of astroparticle physics with you, my dear readers.

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