The IceCube observation of astrophysical neutrinos has been nicely (and quickly!) recognized.
First, Physics World magazine recognized us their 2013 "Breakthrough of the year". IceCube wont he top honors, beating out nine other stories, including the Rex-Isolde observation that some atomic nuclei are pear-shaped.
Then, Scientific American selected IceCube as one of their top 10 science stories of 2013.
All-in-all, it was a pretty darn good month for IceCube.
Friday, December 20, 2013
Wednesday, November 27, 2013
Big Bird joins Bert and Ernie
Bert and Ernie have company!
IceCube has found another PeV neutrino, event more energetic than Bert
or Ernie. The pictures below shows the IceCube standard event display, with side and top views. Unfortunately, some of the optical modules
see so much light that you can’t see what’s happening near the neutrino
interaction. In fact, the photodetectors
in the optical modules are badly saturated. The two event displays below show the side and top views of Big Bird; the enormous hit in the nearby DOMs overwhelms the image. Each colored sphere represents an optical module that observed light; the colors show the relative time of the hit, from red (earliest) through orange and yellow. The size of the spheres shows the amount of light detected.
Like Bert and Ernie, it is well contained within the detector, with no sign of early hits that might signal an entering muon; it is quite clearly a neutrino induced cascade (particle shower).
The event was found by LBNLs Lisa Gerhardt, on her last day working on IceCube, before she moved to a position focused on high-performance computing - still at LBNL, but now at the National Energy Research Supercomputer Center). Talk about going out in style!
I was privileged to be able to first show this event at the 2013 International Cosmic
Ray Conference (ICRC), in Rio de Janeiro, Jul2 2-9th. I would have loved to blog about it earlier,
but the only available reference is the writeup of my talk. Because that writeup covers the evidence for extra-terrestrial neutrinos (my last two posts), we agreed not to post it publicly until the Science paper appeared. Now that the Science paper is out, the writeup is available on the Cornell preprint server as arXiv:1311.6519.
Unfortunately, the collaboration decided not to release any
information on the event energy or direction, pending a systematic analysis,
but it is a nice little monster. As you should expect, we are working hard on these analyses, so I should be able to say more in the not too distant future.
Friday, November 22, 2013
Public Access to "Evidence for High-Energy Extraterrestrial Neutrinos at the IceCube Detector"
Our Science paper has now been posted to the Cornell preprint server (arXiv), and is available at
http://arxiv.org/abs/1311.5238
Click on the 'PDF' link on the upper right to get the complete text.
For those of you not familiar with science publishing, the preprint server contains the full text (as of today) of 892,854 scientific papers, covering most of physics and math, as uploaded by the authors. In high-energy and nuclear physics, virtually every relevant paper is uploaded to the server, usually at the same time that it is submitted to a journal or conference proceedings. Although it does not provide the benefits of peer review, it does provide quick access to the latest work - papers appear between 24 and 48 hours after they are uploaded.
The server also has a search facility, but, for particle/nuclear physics, I prefer INSPIRE . INSPIRE isn't perfect, but it has broader coverage than the arXiv, since it provides overage before 1995, when the arXiv was established. The arXiv coverage was also very spotty for the first ~ decade of operation. INSPIRE also covers most of the relevant articles that are not uploaded to the arXiv.
http://arxiv.org/abs/1311.5238
Click on the 'PDF' link on the upper right to get the complete text.
For those of you not familiar with science publishing, the preprint server contains the full text (as of today) of 892,854 scientific papers, covering most of physics and math, as uploaded by the authors. In high-energy and nuclear physics, virtually every relevant paper is uploaded to the server, usually at the same time that it is submitted to a journal or conference proceedings. Although it does not provide the benefits of peer review, it does provide quick access to the latest work - papers appear between 24 and 48 hours after they are uploaded.
The server also has a search facility, but, for particle/nuclear physics, I prefer INSPIRE . INSPIRE isn't perfect, but it has broader coverage than the arXiv, since it provides overage before 1995, when the arXiv was established. The arXiv coverage was also very spotty for the first ~ decade of operation. INSPIRE also covers most of the relevant articles that are not uploaded to the arXiv.
Thursday, November 21, 2013
"Evidence for high-energy extra-terrestrial neutrinos" on the cover of Science
This weeks issue of Science has a the IceCube paper
that we’ve all been waiting for: Evidence for high-energy extra-terrestrial neutrinos. The paper describes a follow-on analysis to
Bert and Ernie (our two 1-PeV neutrinos. The analysis was designed to find more
events like Bert and Ernie. It did
not. It did, however, find 28 events
that appeared to come from interactions within the detector, with no evidence
of an incoming muon track as expected from downward-going cosmic ray muons. One
of the events even made the cover of Science. Unfortunately, Science requires a subscription, but we will release freely-available version of the paper this afternoon; I'll post the URL when it comes out.
The
thing that makes this analysis so successful is that it brought together
multiple techniques to reject most background and estimate the reset, leading
to a convincing detection of a 4-sigma excess of events above the background
level expected from atmospheric neutrinos.
The first technique has been around since the first IceCube cascade
analysis: using the edges of the detector for a veto, with a smaller fiducial
(active) volume in the center. This
eliminates most background from downward-going muons entering the
detector. These downward-going muons
outnumber the neutrinos by 500,000 to 1, and estimating the fraction that
sneaks through the veto region is tricky, requiring voluminous simulations. The new
analysis uses a data-driven estimate instead.
The estimate uses two independent nested veto regions surrounding a
smaller fiducial volume. It counts events
tagged in the outer veto which miss the inner veto to determine the veto miss
fraction.
The other background is atmospheric neutrinos. These are, on average, less energetic than
the extra-terrestrial events. The new
analysis considers the expected energy spectrum, but it adds a new handle. Energetic downward-going atmospheric neutrinos
should be accompanied by a cosmic-ray muons which may trigger the veto
mentioned above, so they are less likely to pass the final event
selection. The new study is the first
one to search for downward-going cascades.
This atmospheric neutrino ‘self-veto’
probability is included in the background estimates. The background estimates also took advantage
of the latest IceCube measurements of the atmospheric neutrino rates.
In total, our best estimate of the background was 12.1
events (including 1.5 ‘prompt’ atmospheric neutrinos from the decay of charmed
particles), giving a significance as an extra-terrestrial signal ‘at the
4-sigma level.’ Of course, there are
some caveats, but this looks like a fairly robust detection, especially with
Bert and Ernie.
The energy spectrum of the events is shown in the figure above (the points with errors). The blue
histogram shows the atmospheric neutrino background, while the magenta and
green lines include two estimates of the prompt atmospheric neutrinos; the
shading shows the uncertainty. The red
shows the remaining downward-going muon background, while the grey line
includes these backgrounds, plus an assumed astrophysical component. The extra-terrestrial signal is significant
starting at energies above 60 TeV. The
absence of events at energies much above 1 PeV is significant, indicating that
the spectrum is cut off at very high energies (between 2 and 10 PeV); this may
be a clue about the accelerators that produced the neutrinos.
The apparent flux of extra-terrestrial neutrinos is toward the high end of current theoretical estimates, near the Waxman-Bahcall (WB) bound. The WB bound is a calculation based on the measured cosmic-ray flux, assuming that, when these particles are accelerated, they interact with background gas or photons (light) in the accelerator, producing particles (pions) which decay, producing the neutrinos that IceCube observes. Further studies, with more data, should give us clues which will help us located these accelerators.
For comparison, the only other observations of
extra-terrestrial neutrinos have been from our Sun (created by the nuclear
fusion that powers it) and a short burst of neutrinos when supernova 1987a
exploded. These neutrinos were all a
million times lower in energy than the ones that IceCube observed.
Many institutions
have issued press releases and feature stories about the paper, A few of them are
My apologies for the length and technical level of this
post, but this analysis is quite intricate, and I wanted to do it justice.
Monday, October 14, 2013
Shutting down Science
As most of you know, the U.S. federal government has largely shut down, except for services essential to preserve life or property. Federal contractors, such as the Department of Energy laboratories are, so far, mostly remaining opening, burning through any carry-over funds.
Even though many institutions have remained open, the consequences for science (and, by analogy, for many federal activities) are severe. Many scientific experiments are rather large, and it requires significant planning to do anything. These efforts cannot 'turn on a dime.'
This is particularly true for the U.S. Antarctic program. Antarctic science is cyclical - almost everything happens during the brief Austral summer. It is also incredibly dependent on an amazing logistics program, to make sure that everyone and everything needed makes it to Antarctica. There is no corner store if you forget a pair of pliers.
In a normal October, Lockheed-Martin Antarctic Support, the U.S. Air Force, and other logistics providers would be shipping supplies and people to Antarctica, opening up buildings, preparing to set up field camps, etc. This is a prelude to the brief season when most of the science is done. Now, mid-stride, they have to abandon a year of planning, stop moving people and material South, and prepare to reverse the process, while they determine what services are considered 'essential.' Even if Congress and the President reach agreement, and re-open the federal government tomorrow, major damage will have been done to the season, and many (possibly even most) of the planned scientific studies will have to be scrapped or postponed.
As a scientist, I am distressed by these abrupt cancellations. It is disheartening to have our faces rubbed in the fact that our bosses (here, I mean Congress) really do not care what we do. In this fight, science is so insignificant that it merits neither support nor opposition.
As a taxpayer, I am appalled by the enormous financial waste. The NSF spends tens of millions of dollars to hire and train people, equip them and fly them to Antarctica. Most of this money has already been spend, and many of the people are already in Antarctica. Cancelling the season now would makes absolutely no sense, and would be a huge financial waste.
And, as a husband and father, I am concerned about my own future. I am 'fortunate' to work in a division at a national laboratory that is still operating, so I am still being paid. However, if the shutdown continues, none of us know how long this will last.
Before I close, here are a two other blog posts on the shutdown:
From an anonymous woman who blogs from McMurdo Station: http://icewishes.wordpress.com/
From the users group for Brookhavens Relativistic Heavy Ion Collider: http://rhicusers.blogspot.com/
Even though many institutions have remained open, the consequences for science (and, by analogy, for many federal activities) are severe. Many scientific experiments are rather large, and it requires significant planning to do anything. These efforts cannot 'turn on a dime.'
This is particularly true for the U.S. Antarctic program. Antarctic science is cyclical - almost everything happens during the brief Austral summer. It is also incredibly dependent on an amazing logistics program, to make sure that everyone and everything needed makes it to Antarctica. There is no corner store if you forget a pair of pliers.
In a normal October, Lockheed-Martin Antarctic Support, the U.S. Air Force, and other logistics providers would be shipping supplies and people to Antarctica, opening up buildings, preparing to set up field camps, etc. This is a prelude to the brief season when most of the science is done. Now, mid-stride, they have to abandon a year of planning, stop moving people and material South, and prepare to reverse the process, while they determine what services are considered 'essential.' Even if Congress and the President reach agreement, and re-open the federal government tomorrow, major damage will have been done to the season, and many (possibly even most) of the planned scientific studies will have to be scrapped or postponed.
As a scientist, I am distressed by these abrupt cancellations. It is disheartening to have our faces rubbed in the fact that our bosses (here, I mean Congress) really do not care what we do. In this fight, science is so insignificant that it merits neither support nor opposition.
As a taxpayer, I am appalled by the enormous financial waste. The NSF spends tens of millions of dollars to hire and train people, equip them and fly them to Antarctica. Most of this money has already been spend, and many of the people are already in Antarctica. Cancelling the season now would makes absolutely no sense, and would be a huge financial waste.
And, as a husband and father, I am concerned about my own future. I am 'fortunate' to work in a division at a national laboratory that is still operating, so I am still being paid. However, if the shutdown continues, none of us know how long this will last.
Before I close, here are a two other blog posts on the shutdown:
From an anonymous woman who blogs from McMurdo Station: http://icewishes.wordpress.com/
From the users group for Brookhavens Relativistic Heavy Ion Collider: http://rhicusers.blogspot.com/
Thursday, April 25, 2013
Bert and Ernie Step Out
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We are also hard at work at follow-up studies, but we're not ready to present these yet.
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Monday, April 8, 2013
Dark Matter - after 80 years, a mystery endures
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There are many efforts to look for dark matter, using diverse techniques. High-energy physicists are working hard, looking for signs that these particles have been produced in proton-proton collisions at the LHC.
Others physicists are looking for the signature of dark matter interacting in a laboratory detector - "direct detection." In essence, a dark matter particle will bounce off a target nucleus, causing it to recoil. The details of the recoil energy spectrum depend on the type of dark matter particle and its mass. Because the cross-sections are tiny and because very little energy is transferred in these interactions, this is best done using large detectors cooled to cryogenic temperatures, and operated in deep underground laboratories, to avoid the background from cosmic rays. Many different types of dark matter detectors are being pursued, with rather different experimental strategies, involving different types of target material, and different strategies to observe the recoil energy.
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This is a lot of different approaches to dark matter. One may wonder if there is a coherent strategy here. The answer is 'sort-of.' In the absence of a single clear idea what dark matter is, no single approach is known to work. So, we are pursuing a large number of different approaches, based on what different scientists (and funding agencies) find attractive. For the current scope of experiments, the multiple approaches are technically and financially feasible. If none of the current efforts bear fruit, a larger experiment may be needed; this will require a clear strategy choice. However, to inject a note of caution, different models of dark matter predict widely varying behaviors (interaction and annihilation cross-sections, etc.), and not all of these models lead to experimentally observable consequences, with current or planned future detectors.
In short, dark matter is an enduring mystery. After 80 years of effort in diverse areas, we know a great deal about its affect on the universe. However, we still don't have any direct evidence for its existence, and we really don't know what it is.
Friday, January 25, 2013
ARIANNA - the 2012 season
Even though this blog has been quiet, ARIANNA construction has been proceeding. In November, 2012, a group from the
University of California, Irvine, led by Stuart Kleinfelder, visited the site
and deployed three radio-detection stations.
These three stations are the first half of the 7-station hexagonal
array; the remaining four stations should be deployed next season.
These stations include many improvements beyond the
prototypes that were previously deployed.
The electronics have been completely redesigned to use much less power
(less than 10 Watts, instead of 30). The
power systems are much beefier, with much larger wind generators, on much
higher towers. They should provide power
even though the winds only blow a small fraction of the time. The towers include a much larger solar panel,
which will power the station longer into the Antarctic twilight. The stations also sports new lithium batteries
which replace the old lead-acid gel batteries.
They have a much higher power:weight ratio (key for helicopter
transport) and better low-temperature performance. The stations are able to communicate directly
with the internet repeater on Mt. Discovery.
In short, this is the first half of an array that should be
able to make a physics measurement, either observing GZK neutrinos produced
when ultra-high energy cosmic rays interact with the cosmic microwave background radiation, or setting a
competitive limit on their flux.
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