What is the smallest non-zero mass in physics? I saw a previous similar question that seemed ill-defined with wish-washy answers, so I'll just ask here. What is the smallest known mass of a particle; is there a limit as to how far we can go? Or is the answer to either of these questions non-sensical in the same way in which talking about Newtonian mechanics at a particle level is non-sensical; why?
 A: In the Standard Model, the smallest mass is zero. Photons have zero mass, as do gluons.
You might quip that this answer is based on theory, and you really want an experimental result. In this case, we can say that the photon mass is bounded to be less than $1\times 10^{-18} {\rm eV}$, using the headline result in the particle data book. Even if the photon had exactly this mass, it would be by far the lightest particle in the Standard Model.
It could be that you want to know the smallest non-zero mass. In which case, one of the neutrinos has the smallest mass. But, we don't know the mass of any of the neutrinos individually. What we know is that the neutrino masses must be non-zero, and we know that the squared difference between the neutrino masses. See, for instance, https://arxiv.org/abs/1601.07777 for more details. A recent upper limit on the neutrino mass says that neutrinos must weigh less than $1\ {\rm eV}$, https://arxiv.org/abs/1909.06048.
The smallest, non-zero mass of a particle that has actually been measured, would be the electron mass, $511\ {\rm keV}$.
There is no limit to how small a mass can be. But, a mass smaller than the Hubble scale (meaning, a frequency of oscillation that is smaller than one divided by the age of the Universe) probably cannot be distinguished from zero.
A: The lightest particle is of course the photon which is massless. Experimental tests for the photon mass (see Wiki) show that its mass is $< 10^{-27}$ eV which is as close to zero as is currently possible.
The lightest known massive particle in the universe is currently the neutrino. There are three types of neutrinos - electron, muon and tau neutrinos. Their mass is very difficult to measure (see Wiki for a survey of experimental results). The sum of masses of the three neutrinos is $\sim0.23$ eV making the average mass around $\sim0.076$ eV. In contrast, the electron mass is 0.511 MeV and the proton mass is 938 MeV.
Within the framework of quantum field theory, there is no theoretical lower bound on the mass of any particle.
Within the framework of string theory, prior to symmetry breaking, the lightest massive particle will have a mass of the order of the Planck mass (assuming $\ell_s \sim \ell_P$) which is $\sim 10^{19}$ GeV. The light particles that are observed in our universe gain a mass via symmetry breaking but a mechanism for this is yet to be understood in string theory.
Note: 1 eV = $1.7 \times 10^{-36}$ Kg
A: Since mass is not quantized (so far), there is no "less massive possible particle", nor a reason to believe there is a specific limit. Neutrinos are very low mass particles, but others could be found in the future.
There is a misconception that Planck's mass could be a lower or upper bound, but that's not actually the case.
