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Category Archives: neutrinos

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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