I could probably tease this information out of the Particle Data Group website, given enough time, but somebody with a background in particle physics can probably answer this in two seconds, so I appeal to the Internets:
What is the shortest lifetime of a particle that has been directly detected?
By "directly detected" I mean, well, directly detected. Something that has been identified from a track in a detector, or a click in a calorimeter, and not something whose existence has been inferred from a resonance observed in the production of other things.
My half-assed guess would be a neutral pion, with a lifetime on the order of 10-16s, but I could easily be wrong. So what's the right answer?
(This is a factoid to be dropped in a discussion of "virtual particles" in my book. I could just as easily skip it, but it ocurred to me as something to throw in, so if I can get the answer, I'll use it...)
(The "neutral pion" guess is because I know that pions were discovered relatively early, and I know that the neutral pion has a really short lifetime. I'm not sure about the detection history, though.)
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Not sure if it's the shortest, but the Z-boson is 10e-25 seconds.
http://en.wikipedia.org/wiki/Z-boson
I would just as well advise you to drop it, you can split hairs all day debating what has been "directly detected," and any answer is bound to make someone angry. You just want bubble chamber evidence? Lab frame lifetime or rest frame? My lazy guess is probably some B meson.
Sorry Chad, but Stephen is right; "directly detected" is a very ill-defined term -- especially since you chose the neutral pion as an example. How do you "directly detect" a neutral particle, which obviously doesn't leave chamber tracks? The only neutral particles that can be detected by their direct interactions are those long-lived enough to actually hit something, which basically means the neutron and neutrinos.
The neutral pion was detected by the tracks of its decay products; the ultra-short-lived particles being investigated today are found the same way. Are you going to stop counting them as being "directly detected" when the gap in the tracks (representing the neutral particle's trajectory) is too short to see? (What if we buy a bigger microscope?)
For that matter, "particle" isn't an entirely well-defined term either. For example, do the different resonance states of the same quark pair or triplet count as different particles? Ask ten particle physicists and you'll get eleven different answers*. And is there some minimum lifetime below which an object is just a temporary association of quarks and not really a particle? Our ten physicists will probably give the same answer to that one**. See also "pentaquark".
[* One yes, one no, and nine different two-hour lectures starting with "Well, it depends..."]
[** "Gee, I dunno."]
The top quark has a lifetime of 10e-23. It decays into a W boson and a b-quark on a timescale faster than QCD interaction occur.
I think your qualification for direct detection means that you want the detector to actually interact with the partile itself and not the decay products. The top quark decays long before in comes near any part of the detector, so it fails my understanding of your direct detection requirement.
That reminded me of the time at the University I was attending (something over twenty years ago) when I asked a professor in the Physics department if black holes conserved lepton number or not....
Doc Pion steps in to remind you that a neutral pion does not leave any tracks. It has a bit of trouble causing much ionization. It is only detected by its decay products (two photons) as others noted.
(Aside: This comment reminded me of a guy in grad school who wrote, on a comprehensive exam!, that the way to detect a neutron was to spray electrons on it so you could bend it with a magnet. He did not pass.)
The pi-0 might deserve mention for the shortest lived particle ever used for a nuclear physics measurement, but its outgoing energy and angle were inferred from precision measurement of the energies and angles of the two photons from its decay. Being able to resolve nuclear levels with that tool was a major accomplishment at LAMPF. The important detail is that its lifetime is long enough that it makes it out of the nucleus before it decays (and out of the atom as well), but doesn't actually get out of the target. D=v*t is about 10 nm at 50 MeV.
By "directly detected" I think that a neutral particle track should count. Sherlock Holmes and the dog that didn't bark and all that.
For some reason I thought that the highest resolution tracks are seen in nuclear emulsion experiments, mostly for cosmic rays. If so, those experiments should see the shortest tracks but not necessarily the shortest lifetimes.
J.Slaunwhite, the top quark has a lifetime of 10^-25 seconds, which is hundred times faster than 10^-23.
If it was 10^-23, that would be not faster than the timescale of QCD, and top hadrons could have a chance of forming.
Cheers,
T.