What is the smallest sub-atomic particle ever observed?

Question

Strange quarks, up, down, sideways quarks :-) ,etc --- what is the smallest building block of nature ever observed.

Answer 1

smallest, like size wise? that would be:

up quark

down quark

charm quark

strange quark

top quark

down quark

electron

muon

tau

electron neutrino

muon neutrino

tau neutrino

photon

gluon

W+ boson

W- boson

Z boson.

every one of those particles is an elementary particle, which all have 0 volume. they are 0 dimensional points, so they are all the smallest.

if your talking smallest mass, that would be the photon since it has no mass. if we forget about particles without mass, the least massive particle is the electron neutrino.

Answer 2

Photons are massless, so they (and gluons) would be the smallest particles detected.  The smallest non-zero mass would belong to the neutrino.  The smallest thing residing permanently in an atom would be the electron.

Kelly--given your definition (which I wouldn't use necessarily when a much simpler one, the mass, is available), the neutrino is the least interactive thing we've found.  If wrong-spin neutrinos exist (and if neutrinos have mass, they must), they don't even interact weakly--only by gravity.  But we haven't detected them yet.  Nor have we detected the graviton.  Or a bunch of other particles whose theoretical foundations are more tenuous.

Answer 3

neptrons perhaps? something in that general spelling.

Answer 4

I'm not sure if there is a faultless way of defining particle size-mass, yes.  My definition would be the particle whose largest interaction cross section at any energy with any other particle is smaller than that of any other particle.  The photon wouldn't win because although it's interaction with other photons is extremely small (although maybe not at some exceedingly high energy, god knows), it interacts quite strongly with all charged particles.  I would therefore offer the neutrino as a possible candidate.  Anybody has a better one?