Atmospheric Neutrinos: Overview and Opportunities
Chris Walter gave an interesting overview of the techniques used for extracting physics measurements from neutrinos produced in the atmosphere by cosmic rays.
Atmospheric neutrinos span a huge path length and wide range of energies, include both nu_e and nu_mu, and antiparticles. So this is a one-fits-all source. For every possible chosen L and energy you can draw an oscillation probability graph, but if you put things together the curve becomes much more complicated.
The detectors we can use to look at atmospheric neutrinos are water cherenkov tanks, which have a cherenkov threshold. Then one can think of iron calorimeters, like MINOS. These have good tracking, but it is hard to see low-energy particles. THey are good for muons but not so much for electrons. Then there are liquid argon detectors, like ICARUS. These are electronic bubble chembers with excellent resolution and background rejection, can see low energy particles. And finally one can have water or ice telescopes, with enormous mass but are challenging as far as reconstruction and systematics are concerned.
If one looks at the energy spectrum of atm neutrinos, one sees that by pushing down the thresholds of the ice and ocean detectors, we might greatly increase the statistics in interesting oscillation regions.
At high energy, the upward going over downward going ratio is near one, and it is known to 1 percent or better. Also the flavour ratio is well known. But how can you measure L and E ? You need to measure the lepton direction. At low energy this becomes a critical issue. Also, if you look at the horizon one has large errors in the point of origin from small angle errors.
If we take SuperKamiokande, which produced the first observation of nu oscillations in 1998, wne may try to separate neutrinos from antineutrino events, and isolate electron appearance regions depending on the momentum measured and the type of events. Systematic uncertainties have dozens of sources in these measurements, coming from knowledge of the flux, the interaction with the detector, the reconstruction, and others. Many of thse are evaluated separately for different run periods. All are then constrained in a global fit. If one puts these in a list, it is really impressive. [This looks like a wow list for anybody who has not seen how we combine results on Higgs boson searches at the LHC – TD].
With present-day data we may draw “oscillograms” (I hope I will be able to paste one here, they are very colorful and cool -TD). One can see the regions of parameters yielding equal probabilities of oscillations for different energy and angles.
The Earth can be used to untangle the oscillations. Matter effects cause enhancements in the rate of high-energy upward-going electron neutrinos going through the Earth’s core. One needs a model for the matter distribution in the Earth of course. Using this information, a resonance may happen for neutrinos or anti-neutrinos depending on whether the hierarchy is normal or inverted. In a water detector the total cross sections are different for neutrinos and antineutrinos, and the mixture changes with energy. So by looking as a function of energy one can combine things in a likelihood and disentangle the various effects.
Chris then discussed several proposed future detector layouts. One is Hyper-Kamiokande, which has a 560kton fiducial mass, and is instrumented with 99000 photomultiplier tubes. With such a thing one can study upward-going electrons and see how many years it takes to measure the value of sin^2 theta_23 or other parameters with three-sigma significance. He showed some interesting summaries that show that a HK detector would significantly extend our knowledge with a few years of running.
Another design is INO@ICAL, a 50kton magnetized calorimeter. It uses atmospheric muon neutrinos. You can get to a three-sigma separation of the normal and inverted hierarchy in 10 years. A Liquid Argon LBNE/Glacier/LBNO detector underground could also effectively study a mass hierarchy in few years of exposure.
Finally, one could extend IceCube to a “DeepCore”. Precision oscillation measurements can be done to go to low energies, of the order of 1 GeV. With multi-mega tons of mass, one can still measure a over-3-sigma determination of the mass hierarchy within 5 years. However, it is a tough call to see if systematics in the flux at low energy can be constrained to a manageable level.
In conclusion, atmospheric neutrinos are nice because they’re always there for us. Large experiments can do precision oscillation physics and contribute to the measurements of sub-leading effects.