Most quantum gravity theories insert randomness into the structure of space-time that should causes neutrino oscillations over long distances to cease to become coherent. The IceCube Neutrino observatory at the South Pole measures neutrinos from space that would be expected to reveal this effect if many quantum gravity theories are correct. But, so far, this hasn't been observed, setting strict limits on this hypothesized quantum gravity effect.
Neutrino oscillations at the highest energies and longest baselines provide a natural quantum interferometer with which to study the structure of spacetime and test the fundamental principles of quantum mechanics. If the metric of spacetime has a quantum mechanical description, there is a generic expectation that its fluctuations at the Planck scale would introduce non-unitary effects that are inconsistent with the standard unitary time evolution of quantum mechanics. Neutrinos interacting with such fluctuations would lose their quantum coherence, deviating from the expected oscillatory flavor composition at long distances and high energies.
The IceCube South Pole Neutrino Observatory is a billion-ton neutrino telescope situated in the deep ice of the Antarctic glacier. Atmospheric neutrinos detected by IceCube in the energy range 0.5-10 TeV have been used to test for coherence loss in neutrino propagation. No evidence of anomalous neutrino decoherence was observed, leading to the strongest experimental limits on neutrino-quantum gravity interactions to date, significantly surpassing expectations from natural Planck-scale models. The resulting constraint on the effective decoherence strength parameter within an energy-independent decoherence model is Γ0≤1.17×10^−15 eV, improving upon past limits by a factor of 30. For decoherence effects scaling as E^2, limits are advanced by more than six orders of magnitude beyond past measurements.
R. Abbasi, et al., "Searching for Decoherence from Quantum Gravity at the IceCube South Pole Neutrino Observatory" arXiv:2308.00105 (July 25, 2023).
4 comments:
If the metric of spacetime has a quantum mechanical description, there is a generic expectation that its
the end of LQG ?
neutrino oscellation is already outside the standard model...
@mw
This is a pedantic interpretation of what is part of the Standard Model, although it is certainly a late addition to it and is the only glaring loose end of the theoretical structure of the Standard Model.
A more mainstream and practical view is that massive SM neutrinos and the PMNS matrix are part of the Standard Model, even though the exact source of the mass of SM neutrinos has not been determined and this is a loose end for the SM to solve.
This is a loose end because the Higgs mechanism that imparts rest mass to all of the other Standard Model fundamental particles is hard to conclude is a source of neutrino mass in the Standard Model, for technical reasons related to the fact that there are no right parity neutrinos or left parity anti-neutrinos. But there is no positive evidence to support the alternative hypothesis that neutrinos are their own anti-particles and have Majorana mass, nor is there positive evidence of any non-Higgs mechanism by which neutrinos acquire their slight rest masses if those masses are Dirac masses (a hypothetical see-saw mechanism involving right handed neutrinos is often proposed but there are many competing models for this, none of which have any positive evidence to support them). Majorana mass implies neutrinoless double beta decay which has not been observed but shouldn't be detectable quite yet if the neutrino masses are as low as cosmology based estimates imply. The PMNS matrix is implicitly a Dirac mass rather than Majorana mass interpretation, since neutrino oscillation with Majorana mass has more than four parameters. But current experiments are not precise enough to rule out the existence of additional Majorana mass oscillation parameters.
Of course, the values of the CP violating PMNS matrix parameter has been measured only at a very crude level of precision (ruling out a value of zero at the two sigma level, however), and the quadrant of one of the other parameters of the PMNS matrix is not known with confidence. Likewise, while the two neutrino mass eigenstate mass differences are known with some precision, the absolute rather than relative values of the three neutrino mass parameters (including whether there is a normal or inverse neutrino mass hierarchy) still need to be pinned down and is known only very crudely. But, a lack of precision in measuring free parameters of a model is not a bar to it being a part of the Standard Model. It is just more work for experimentalists to do.
thanks..
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