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Detector in South Pole’s ice cap spots rare high-energy neutrinos

Some of the earliest and most successful neutrino detectors were based on enormous tanks of water. For example, Japan's Super Kamiokande held 3,000 tons of water, and researchers used the detector to watch for a sign that a neutrino had bumped up against one of the water molecules. A recently constructed detector takes a similar approach, observing about a cubic kilometer of water using over 5,000 optical sensors. It just relies on nature to provide the water. The detector is called IceCube, and its detectors are buried in the South Pole's ice cap.

IceCube has now scored its first big success, detecting the highest-energy neutrinos ever spotted. Odds are good that these neutrinos originated from an event distant from Earth, but remaining uncertainties mean that we can't conclude that with certainty.

The reason so much water is needed is that neutrinos don't like to interact with normal matter. Each second, a trillion neutrinos pass through your hand, but only about two will interact with an atom in your body throughout your entire lifetime. Spotting a neutrino requires a detector with a lot of material. Water has worked well, simply because it's relatively easy to get lots of it into one place and because it's transparent to much of the light that's created when high-energy neutrinos collide with an atom. Simply point enough photodetectors at a big tank of water and wait.

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First Planck results: the Universe is still weird and interesting

The cosmic microwave background—temperature fluctuations left over from 380,000 thousand years after the Big Bang. This new map is based on data from the Planck mission.
ESA/Planck Collaboration/D. Ducros

Our current model of cosmology—the origin and structure of the whole Universe—has survived another major test, with the release of the first 15 months of data from the Planck mission. Planck is a European Space Agency mission, designed to study the cosmic microwave background (CMB), which preserves information about the conditions that persisted immediately after the Big Bang.

Combined with results from prior experiments, Planck has revealed a Universe a little older than previously thought, and with a slightly different balance of ingredients. Although there were no major surprises, some of its data provided stronger hints about inflation, a popular model that explains why the modern Universe looks the way it does. Other measurements ruled out extra neutrinos, provided even stronger evidence for the existence (though not the identity) of dark matter, and indicated that there's a bit less dark energy than previous measurements had suggested.

But amid these incremental changes, there was a bit of a surprise: despite the best hopes of researchers, Planck data does not rule out the existence of anomalous temperature fluctuations at large scales. These may hint at either new physics that influenced the Universe's expansion, or previously unknown foreground sources that alter the CMB.

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What have neutrinos done for you lately?

It takes a lot of individual detectors to have a reasonable chance of spotting a neutrino interacting with other matter.

Don't call neutrinos elusive. Francis Halzen, who makes it his job to detect them, said that the particles' unusual properties, like their minuscule masses and tendency not to interact with matter, tend to make "elusive" one of the most common adjectives used to describe them. But in reality, they're all around us: each cubic centimeter of the Universe has hundreds of them, left over from the Big Bang. Every second, trillions of them flow through our bodies. In fact, there's probably as much matter in the form of neutrinos as there is visible matter. If we could just get a bit better at detecting them, they could tell us a lot about the Universe.

The World Science Festival hosted a discussion led by three people who have focused their careers on detecting neutrinos. Halzen helps run the new Ice Cube detector at the South Pole, while MIT's Janet Conrad works on the MiniBooNE detector, which picks up neutrinos made by the Fermilab accelerator chain. Conrad was joined by her fellow MITer, Joe Formaggio, who works on the Sudbury Neutrino Observatory, buried in a mine in Canada.

The panel was rounded out by theoretician Lawrence Krauss, and moderated by noted science writer John Rennie.

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Faster-than-light neutrino findings really, thoroughly dead

The OPERA detector, which started the whole kerfuffle.
Symmetry Magazine

This week, the XXV International Conference on Neutrino Physics and Astrophysics (better known as Neutrino 2012) is taking place in Japan, and a number of announcements have been made in association with the meeting. Neutrinos have some fascinating properties (which we'll discuss at length this weekend), but it's now clear there is one exceptional feature they lack: the ability to go faster than light. Even the detector that originally reported this finding now agrees that the results were an artifact.

This morning, CERN updated its press release that dates back to the original results, indicating that the four different detectors on the receiving end of its neutrino beam—Borexino, ICARUS, LVD, and OPERA, all located in Italy's Gran Sasso facility—generated timing results that were consistent with the neutrinos traveling at the speed of light. The inability to discern a difference between the speed of neutrinos and photons is the product of the neutrinos' extremely tiny mass. A proton is 10,000,000,000 times more massive, so it takes substantially less energy to get a neutrino up to speeds where the distance to the speed of light is just a rounding error.

This appears to confirm that the results were the product of an improperly seated optical cable in the OPERA experiment, which introduced a small, but significant timing delay. Those of you who are interested in the technical details of what went wrong may want to read Matt Strassler's extensive discussion of how this problem was found, and why it could produce the timing problem.

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IceCube detector puts the chill on fireball model of gamma ray bursts

The Universe contains much better particle accelerators than anything we humans can contrive. While the Large Hadron Collider (LHC) is capable of sending individual protons to energies of 7 trillion electron volts (7 TeV, or 7×1012 eV), cosmic ray protons can exceed 1018 eV—a million times more energetic. Achieving this acceleration requires a highly energetic source. The leading candidates are gamma ray bursts (GRBs), which are exceedingly bright astronomical events, often associated with supernovae. According to a commonly accepted model of GRB explosions, the proton acceleration should be accompanied by a flood of neutrinos—low-mass neutral particles.

That model is apparently in trouble. An analysis of high-energy neutrinos observed by the IceCube experiment at the South Pole has found too few neutrinos relative to what GRB models say we should see. By comparing the incidences of GRBs from satellite observations to the flux of neutrinos at the IceCube neutrino observatory, researchers were able to set an upper limit on the total number of neutrinos at the energies associated with GRBs. They determined that no current GRB model is able to match the observed flux, meaning either that GRBs are not the primary source of high-energy cosmic rays, or that the model for GRB neutrino production is incorrect.

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New detector weighs in: neutrinos don’t exceed light speed

We now have yet another indication that neutrinos cannot travel faster than the speed of light after all, provided by a neighbor of the OPERA detector that set off the fuss in the first place. OPERA's detector sits deep underground at Gran Sasso in Italy, where it receives neutrinos from a beam generated at CERN, 730km away on the French-Swiss border. Because the neutrino beam spreads out over the intervening distance, it's possible to run multiple detectors at the same site, all listening in on the same beam. The team running one of Gran Sasso's other detectors (called ICARUS) has now performed time-of-flight measurements on neutrinos and determined that they don't seem to be moving faster than light.

These results are significant because they largely took advantage of precisely the same infrastructure used to generate the OPERA results. ICARUS used the short, widely spaced bunches of neutrinos produced by CERN to help narrow down potential errors in the earlier results (read our discussion of these errors). The ICARUS team also used the same timing and position infrastructure used by OPERA, which gives them uncertainties of only nanoseconds and centimeters, respectively. WIth all that in place, the ICARUS team captured data from the arrival of seven neutrinos.

With just about everything but the detector itself identical between the two tests, the ICARUS team concluded, "The result is compatible with the simultaneous arrival of all events with equal speed, the one of light." (Neutrinos have such a small mass that it's relatively easy to accelerate them to a speed that is only marginally slower than light.)

One difference between the two detectors is the technology used to detect the arrival of neutrinos—OPERA uses a photographic emulsion, while ICARUS uses liquid argon. It's possible that this difference may provide an indication of why the results differed.

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Fast neutrinos, C-P violations, and the shrinking space for the Higgs

It has been a busy week in the world of particle physics, with attention focused on the home of the LHC: CERN. This year, the LHC generated five inverse femtobarns worth of data—nearly half the amount generated during the entire lifetime of the Tevatron—before shutting down the proton program a few weeks ago. From now until its scheduled winter shutdown, the LHC will be doing lead ion collisions to examine the quark-gluon interactions that dominated the Universe immediately after the Big Bang.

In the mean time, analysis of the data has continued, and some significant news has come out this week. A further dissection of last year's data has placed tighter limits on where the Higgs boson, which provides mass to other particles, might be hiding (assuming it exists). Meanwhile, the LHCb detector, which studies particles that contain heavy quarks, has found an anomalous behavior that might hint at physics beyond the Standard Model. And the LHC accelerator chain has sent some more neutrinos to detectors at Italy's Gran Sasso, which has helped them eliminate some potential sources of error in their faster-than-light findings. We'll take a look at each of these in turn.

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