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NERSC, PDSF, Neutrino Oscillations and the 2015 Physics Nobel Prize

October 8, 2015

Perhaps the most rewarding aspect of working at NERSC is sharing in the scientific enterprise, working day-to-day with the best scientists in the world seeking to answer the most interesting questions ever posed. How does the nanoworld work? Where did our universe come from and where is it going? How are we affecting our environment and what could we do about it?

Because large-scale computing and data capabilities have touched every area of science and transformed the way science is conducted, working in the High Performance Computing (HPC) field offers a unique opportunity to be part of it all. We can and do make a difference – sometimes big and sometimes small – on a global scale.

So it’s with great excitement and joy that NERSC is once again part of the story behind a Nobel Prize. This time it’s the 2015 Nobel Prize in Physics, awarded to the leaders of two large experiments that discovered that neutrinos transform periodically from one type to another (more about that below). This year’s prize went to Takaaki Kajita of Tokyo University, who led the Super-Kamiokande experiment, and Arthur B. McDonald of Queen’s University in Kingston, Ontario, head of the SNO collaboration.

NERSC’s connection is through SNO, short for the Sudbury Neutrino Experiment. SNO is a huge neutrino detector located in Sudbury, Ontario. It's located in an old Copper mine in 2 km underground to shield it as much as possible from the noisy background of particle interactions that take place on Earth’s surface.

The SNO detector was turned on in 1999, and from the earliest days, SNO data was transferred to NERSC and NERSC’s PDSF cluster was used for what became known as the “West Coast Analysis.” When the discovery of neutrino flavor mixing in solar neutrinos was published in 2001 in Physical Review Letters (Measurement of the Rate of νe+d→p+p+e Interactions Produced by B8 Solar Neutrinos at the Sudbury Neutrino Observatory,” PhysRevLett.87.071301) NERSC’s role was well established and recognized by scientists working on the project: they presented a signed and framed copy of the journal article to NERSC’s PDSF team.

In 2002 NERSC featured the SNO results in its annual reports along with the picture of the PDSF team superimposed on a visualization of results from the SNO experiment: (R-L) Steve Chan, Shane Canon, Cary Whitney, Iwona Sakrejda, and Tom Langley.

The experiment wound down in 2006, but its data would be invaluable forever. To assure its integrity and safekeeping, NERSC was chosen to be the trusted home to the data archive. Archiving and moving all the data to NERSC required a close collaboration between SNO, NERSC and DOE’s Energy Sciences Network (ESnet). In 2010, Alan Poon, one of the project leads from Berkeley Lab, wrote to NERSC’s Damian Hazen:

First, let me say that I have been really pleased with the help you provided. From testing the transfer speed, tuning the network and identifying packet losses, to the final archiving at HPSS, your help and expertise have saved us a lot of headache. These are technical issues that laymen like us would take a long time to solve (if at all). Thank you!

“NERSC has been providing great support to SNO for over a decade. I still recall using the old PDSF cluster to do some of the early analyses for the SNO experiment. Preserving the SNO data is very important as the experiment has made tremendous contributions to our understanding of neutrinos.

“When we looked around at different facilities and talked to colleagues that have used the center's High Performance Storage System [HPSS] extensively, we immediately concluded that one copy of our data should be stored at NERSC.”

What does it mean?

This year’s award honored research that resolved one of the greatest physics mysteries of the 20th century: the solar neutrino problem. There’s nothing that the scientists like better than chasing after something they don’t understand, and the solar neutrino problem was a doozy. Back when I was a graduate student in physics at the University of Illinois in the 80s and early 90s, this was one of the hottest topics around. Kind of the dark energy problem of its day.

The Sun is prodigious producer of a kind of neutrino, called the electron neutrino, via the nuclear reaction that fuses Hydrogen atoms to produce Helium (and releases the energy that powers the sun and thus just about everything on Earth). Subsequent reactions can build up heavier elements (like Boron 8) that then decay and produce additional electron neutrinos. The details of this process  were worked out by Princeton astrophysicist John Bahcall, whose models made specific predictions concerning neutrino production. The problem was that a detector built in a collaboration between Bahcall and Raymond Davis of Brookhaven National Lab observed only ⅓ to ½ the number of electron neutrinos that they should according to Bahcall’s theory. Many in the physics community did not think much of the discrepancy, but Bahcall stood firm in his belief that there was something interesting and fundamental going on.

So where were the missing neutrinos? The answer, we now know, is that the neutrino detectors were only observing electron neutrinos and those neutrinos were changing “flavor” as they traveled from the center of the sun to us here on Earth, thus avoiding detection. This was not something that was expected at the time; the leading theory said that neutrinos did not oscillate from one flavor to another. To do so would imply neutrinos with mass and neutrinos were thought to be massless, like the photon.

The SNO observations changed all that. They detected high-energy neutrinos of all three flavors – electron, muon, and tau – coming from the Sun. This proved that the three flavors couple to one another and oscillate from one state to another, a process that requires the neutrinos to have a finite mass. One way to think about this is to construct a mental cartoon where particles are represented as vibrations on a guitar string. When the string is at rest, there’s no particle. But when you set it moving, that’s a particle. If that vibration can “couple” (perhaps through the air or the guitar’s body) to another nearby string and get it moving, you have another kind of particle (a different “flavor” of neutrino). Picture the vibrations sloshing back and forth between these strings and you get an oscillation between neutrino flavors. For neutrinos the strength of this coupling depends on their mass. No mass, no coupling and no flavor oscillations. What the SNO results showed was that neutrinos of all flavors were coming from the Sun, and since the Sun only produces electron neutrinos, they had to be oscillating from one flavor to another. Gigantic physics mystery solved!

As a follow-on there were still lots of detailed questions to be asked and answered. The study of neutrino interactions remains one of the most active areas in physics. A more recent discovery that also involved NERSC, PDSF, and ESnet was the 2012 measurement of the ϴ13 neutrino “mixing angle” by the Daya Bay neutrino experiment team. Much of that analysis was performed at NERSC and NERSC is again home to the data. The “mixing angles” describe the strength of coupling between neutrino flavors and are related to the ratio of different neutrino masses. That Daya Bay result was listed as one of Science Magazine’s Top Ten Breakthroughs of the Year for 2012.

Other Nobel Prizes Associated with NERSC

The 2015 Physics Nobel research is just the latest to be associated with NERSC. Martin Karplus, a 2013 Nobel winner in Chemistry for developing molecular simulation techniques; Saul Perlmutter, who won the Physics prize in 2011 for discovery of the accelerating expansion of the universe; and George Smoot, the 2006 Physics winner for discovery of variations in the CMB have all been NERSC users and led projects that used NERSC resources. Some of the important climate simulations that backed the 2007 Nobel Peace Prize for the Intergovernmental Panel on Climate Change (IPCC) were performed at NERSC. NERSC resources also played a small part in world-wide computations needed to discover the Higgs Boson, an effort that was awarded the Physics Nobel in 2013.

Thanks to Debbie Bard and Lisa Gerhardt for their comments and suggestions on this blog post.