The subatomic particle must have a mass less than 1.1 electron volts
So, not mass-less, but pretty darn unmassive.
Science News said:The maximum possible mass of a barely there particle has just gotten smaller.
Subatomic particles called neutrinos are extremely lightweight. Now, scientists with the KATRIN experiment in Karlsruhe, Germany, have shrunk the potential mass range for these runts of the particle litter. Neutrinos must have a mass of 1.1 electron volts or less, the researchers report in a paper posted September 13 at arXiv.org and in a talk on the same day at the Topics in Astroparticle and Underground Physics conference in Toyama, Japan.
That new number — about half of the previous ceiling on neutrino mass — means that it would take more than 460,000 neutrinos to reach the mass of an electron, “and possibly a lot more,” says physicist Diana Parno of Carnegie Mellon University in Pittsburgh.
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So, not mass-less, but pretty darn unmassive.