As I recall, the neutrino was noticed precisely because the energies of neutron decay didn't consistently add up. It became clear that something else was being produced, something that was carrying away some of the energy, even if we couldn't see it directly. Shouldn't something like that have happened already for the dark energy particle?
Well, to be fair, they didn't say that. I merely deduced it.
Even if they don't bang into other particles very readily, wouldn't the mass/energy disappearing into the production of these things have been noticed by now?
That's essentially where I was going with the "ASP" signature, but it's a tricky measurement even for neutrinos. Whether we should have seen it or not depends on the size of the coupling constant, but they should have enough data to calculate what that is.
[Geek Alert: the ASP (Anomalous Single Photon) detector was an electron-positron collision experiment at the PEP collider at SLAC. (Coincidentally, my boss at Penn, Prof. Robert Hollebeek, was the spokesman for the ASP Collaboration.) The detector was optimized for low-angle photon detection. Its purpose was to count the number of invisible particle species being produced in e+e- collisions at (IIRC) 29 GeV. It does this through a quantum process known as Bremsstrahlung radiation. As the electron and positron come into very close proximity, there is a very large chance that one of them will emit a very energetic photon. The distribution of these photons is sharply peaked along the beam axis. (Double Geek alert: this is also called "initial-state radiation". There can also be intermediate-state radiation, which is the real money winner for getting large transverse momentum.) All collisions have some probability of emitting such photons, but if charged particles, hadrons or photons are produced in the annihilation, they will be detected. If neutrinos (or other invisible particles) are produced, the photon alone will be detected. Since the backgrounds are calculable and the three neutrino cross-sections are known, the rate of single photon production (and its angular distribution) can be accurately predicted. Any unknown process that produces undetectable particles will make itself known in this experiment by producing an anomalous distribution of single photons, if the coupling is large enough for a given sample size.]
As I recall, the neutrino was noticed precisely because the energies of neutron decay didn't consistently add up. It became clear that something else was being produced, something that was carrying away some of the energy, even if we couldn't see it directly. Shouldn't something like that have happened already for the dark energy particle?
Unfortunately, there are no spontaneous decays that would produce these particles, in analogy to the decay spectra in which the existence of neutrinos was first noticed.