{"id":2925,"date":"2015-04-01T01:37:36","date_gmt":"2015-04-01T01:37:36","guid":{"rendered":"http:\/\/www.particlebites.com\/?p=2925"},"modified":"2017-02-19T01:29:50","modified_gmt":"2017-02-19T01:29:50","slug":"the-glashow-resonance-on-ice","status":"publish","type":"post","link":"https:\/\/www.particlebites.com\/?p=2925","title":{"rendered":"The Glashow Resonance on Ice"},"content":{"rendered":"

Are\u00a0cosmic neutrinos trying to tell us something, deep in the\u00a0Antarctic ice?<\/p>\n

Presenting:<\/p>\n

“Glashow resonance as a window into cosmic neutrino sources,”
\nby Barger, Lu, Learned, Marfatia, Pakvasa, and Weiler
\nPhys.Rev. D90<\/strong> (2014) 121301<\/a> [1407.3255<\/a>]<\/br><\/br> Related work:<\/b> Anchordoqui et al. [1404.0622<\/a>], Learned and Weiler [1407.0739<\/a>], Ibe and Kaneta [1407.2848<\/a>]<\/p>\n<\/div>\n
\"Is<\/a>
Is there an neutrino energy cutoff preventing Glashow resonance events in IceCube?<\/figcaption><\/figure>\n

The\u00a0IceCube<\/a>\u00a0Neutrino Observatory is a gigantic\u00a0neutrino detector\u00a0located in the Antarctic. Like an iceberg, only a small fraction of the lab is above ground: 86 strings\u00a0extend to a depth of 2.5 kilometers into the ice, with each string instrumented with\u00a060 detectors.<\/p>\n

\"2<\/a>
2 PeV event from the IceCube 3 year analysis; nicknamed “Big Bird.” From 1405.5303.<\/figcaption><\/figure>\n

These detectors search\u00a0ultra high energy neutrinos by looking for Cerenkov radiation<\/strong><\/a> as they pass through the ice. This is really the optical version of a sonic boom<\/a>. An example event is shown above, where the color\u00a0and size of the spheres\u00a0indicate the strength of the Cerenkov signal in each detector.<\/p>\n

IceCube has released data for its first three years of running (1405.5303<\/a>) and has found three\u00a0events with very large energies: 1-2\u00a0peta<\/em>-electron-volts: that’s ten thousand times the mass of the Higgs boson. In addition, there’s a spectrum of neutrinos\u00a0in the 10-1000 TeV range.<\/p>\n

\"Glashow<\/a>
Glashow resonance diagram.<\/figcaption><\/figure>\n

These ultra high energy neutrinos are believed to originate from outside our galaxy through processes\u00a0involving particle acceleration by black holes<\/a>. One expects\u00a0the flux of such neutrinos to go as a power law of the energy, $latex \\Phi \\sim E^{-\\alpha}$ where $latex \\alpha = 2$ is a estimate from certain acceleration models<\/a>. The existence of the three super\u00a0high energy events at the PeV scale has led\u00a0some people to think\u00a0about\u00a0a\u00a0known deviation from the power law spectrum: the\u00a0Glashow resonance<\/strong>. This is the sharp increase in the rate of neutrino\u00a0interactions with matter\u00a0coming from the resonant production<\/a> of\u00a0W<\/em> bosons, as shown in the Feynman diagram<\/a> to the left.<\/p>\n

The Glashow resonance sticks out like a sore thumb in the spectrum.\u00a0The position of the resonance is set by the energy required for an electron anti-neutrino to hit\u00a0an electron at rest such that the center of mass energy is the\u00a0W<\/em> boson mass.<\/p>\n

\"astro-ph\/0101216\"<\/a>
Sharp peak in the neutrino scattering rate from the Glashow resonance; image from Engel, Seckel, and Stanev in\u00a0astro-ph\/0101216<\/a>.<\/figcaption><\/figure>\n

If you work through the math on the back of an envelope, you’ll find that the resonance occurs for incident electron anti-neutrinos with an energy of \u00a06.3 PeV; see figure\u00a0to the leftt. This is “right around the corner” from the 1-2 PeV events already seen, and one might wonder\u00a0whether it’s significant that we haven’t seen anything.<\/p>\n

The authors of\u00a0[1407.3255<\/a>] have found that the absence of Glashow resonant neutrino events in IceCube is not yet a bona-fide “anomaly.”\u00a0In fact, they point out\u00a0that the future observation or non-observation of such neutrinos\u00a0can give us valuable hints\u00a0about the\u00a0hard-to-study origin of these ultra high energy neutrinos.\u00a0They present \u00a0six simple particle physics\u00a0scenarios for how high energy neutrinos can be formed from\u00a0cosmic rays that were accelerated by astrophysical accelerators\u00a0like black holes. Each of these processes predict a\u00a0ratio of neutrino and anti-neutrinos flavors at Earth (this includes neutrino oscillation effects<\/a> over long distances).\u00a0Since the Glashow resonance only occurs for\u00a0electron<\/em>\u00a0anti<\/em>-neutrinos,\u00a0the\u00a0authors point out that the\u00a0appearance or non-appearance of the Glashow resonance in future data can\u00a0constrain what types of processes may have produced these high energy neutrinos.<\/p>\n

In\u00a0more speculative work, the authors of [1404.0622<\/a>]\u00a0suggest\u00a0that the absence of\u00a0Glashow resonance events may even\u00a0suggest some kind of\u00a0new physics that impose a\u00a0“speed limit” on\u00a0neutrinos propagating through space that prevents neutrinos from ever reaching 6.3 PeV (see top figure).<\/p>\n

Further Reading:<\/p>\n