Luca Stanco: The NESSIE Concept for Sterile Neutrinos
Luca started by listing the tensions in the standard model which exist between quark and lepton sectors. These are: the absence of right-handed neutrinos in the classical SM; the Majorana masses and the hierarchy problem are issues where the coherence of the SM is tested; then, neutrinos have large mixings and tiny masses; and finally, there is also a non coherent picture with discrepancies at 3-4 sigma level in various measurements. So one should talk of maximum principles. The Higgs is where one expected it, and there seems to be nothing beyond the SM in that sector. So neutrinos are the place where to commit oneself to looking for new physics.
You has to be sure that you will be able to set five-sigma discoveries. To reach this condition, your detectors need to be suited to the task.
Which measurements can be done with neutrinos: three angles, two mass differences, the sign of the mass difference, one cp phase, the source of atmospheric oscillations, the absolute mass scale, and a solution of the issue of whether neutrinos are Dirac or Majorana particles. The last question is: are there more non-standard neutrinos ? Here, on this last question, there are no well consolidated experiments.
Neutrino oscillations have established a beautiful picture, but anomalies appear in neutrino data in the region of dm^2 of the eV^2 scale. This predominantly comes from single detector experiments. Luca’s personal conclusion on the LSND effect is that MiniBooNE spent ten years, money, work, and was inconclusive. So one should be careful to avoid this kind of situation in the future.
Pontecorvo first introduced the notion of sterile neutrinos in 1967. This is the idea of a deficit of neutral currents. One can also study it with NC/CC ratios. There are two distinct classes of anomalies that have been analyzed. One comes from the excess signal of antielectron-neutrinos, and one from the disappearance signal of anti-nu_e in reactor sources. But we are always talking of electron antineutrinos. So, what to do ?
The way to go ahead is with short baseline projects. We might have a beam at CERN with which to discover something. We need to understand if we want to discover or disprove, or measure. These are different answers to different questions. Also, one wants to use new technologies.
Luca mentioned the ICARUS result on the LSND anomaly, and mentioned that there are underfluctuations in the data, so while the limit is important, to have a clear picture at a high statistical level of confidence, more inputs are necessary.
In Stanco’s opinion neither radioactive sources, reactors, or beams can provide the definitive answer, which is to measure both neutrinos and antineutrinos, both muons and electrons, in different sites (>1), and to provide a 5-sigma result. We thus need a superior class experiment for that purpose.
Fitting all together the information, one has observed that each combination of two sets of data (electron nu disappearance, muon nu disappearance, mu-> appearance) is consistent, but there is tension if one uses all three. It also has been shown that when one goes to scenarios with more sterile neutrinos the tension is still there. Anomalies are at the level of 2-3 sigma in a number of places.
CERN will create an infrastructure, and with short-baseline accelerator experiments by comparing near and far site results one can see clearly the effects at play. One year ago a proposal was submitted (1203.3432) for near and far sites with an imaging LAr TPC detector, and a magnetic spectrometer to determine muon charge and momentum.
The NESSIE concept answers the problem of investigating low dm^2 values by measuring a wide spectrum of momenta. Uses iron dipolar magnets similar to those used in Opera. One can use the magnetic spectrometers of Opera and an air-core magnet. The choice of an air-core magnet is due to the fact that this is able to keep the mis-id of muons at low energy low. The momentum measurement at 6% can be achieved with a dE/dx up to 3 GeV.
NESSIE would be coupled with the liquid argon detector. In one year, one would collect several hundred thousand events. For electron appearance, one would expect O(1000) per year. With this amount of statistics one can explore smaller and smaller mixing angles.
Then Luca discussed the status of the approval process. INFN is presently the main contributor to the experiments (not the beam). The project is under evaluation by the technical scientific committee. The time schedule is tight, because one wants to go to data taking in 2017. But they are using existing detectors, so they believe they are compatible with the schedule.
Luca concluded by saying that there is the possibility of exciting discoveries, with vast consequences or a complete clarification of the present anomalies. The time scale for these endeavours is favourable thanks to the use of existing detectors. This is an opportunity to revive neutrino physics in Europe.