Hunting for Particles in Accelerators: Revealing Virtual Reality?

[Lapidus]

I do not intend to start yet another thread on 'virtual' particles and whether they are part of physical reality. But I still have a question, though.

When physicists are hunting for a previously unobserved, say a 70 GeV particle in Fermilab or LHC or any other particle accelerator, where is this particle hiding?

Or, what are they revealing if not 'virtual' particles?

I must assume that the particle exists 'virtually' before, for the very short time that the uncertainty relation forbids us to observe it. We build particle accelerators then to create sufficiently sudden energetic interaction to reveal them, to make them observable.

Or not?

thanks

 

[Bill_K]

Lapidus,

Even when there is sufficient energy to create a particle such as the Higgs boson, you won't be able to see it directly because such particles are short-lived, decaying into other particles (which usually in turn decay into something else). And unfortunately the decay products don't unambiguously identify what kind of particle they came from. Consequently you may very well be producing Higgs bosons all the time but not know it, because they are 'hiding' among the data. You need to do a careful statistical analysis on a very large number of collisions to be able to say whether the particle you're looking for was actually there. It is estimated that the LHC will need to collect a quadrillion collisions (yes, literally a quadrillion!) to be able to identify the presence of the Higgs with enough certainty. They currently have accumulated 4.7 trillion.

 

[Lapidus]

thanks, Bill K!

So is it ok to say there are two length-scales: one is its Compton wavelength hbar/(Mc), and the other is the lengthscale L which gives the spatial resolution of the experiment (e.g. the scale at which one can resolve the track left by the particle in a detector).
For massive particles, usually, hbar/(Mc) << L.

If the particle propagates, between emission and absorption, for a distance much larger than L, it is useful to call it a "real particle". This is the case of a particle which leaves a track into a detector, or of a cosmic ray proton coming from astrophysical distances. (One could insist that strictly speaking these should still be called virtual particles, since they are still emitted and absorbed somewhere. This is a logically possible definition, but probably not a very useful one, and definitions are something that we chose in such a way that they are useful.)

In the opposite limit, when the particle propagates over a distance much smaller than hbar/(Mc) (or when it lives for a time much smaller than hbar/(Mc^2) ) it is useful to call it a virtual particle, to stress that we cannot observe it directly as a track left in a detector. Still, it has other physical effects.

For a massive particle, there can also be an intermediate regime, when the particle propagates over distances much larger than hbar/(Mc) but still much smaller than the experimental resolution L (or lives for a time much larger than hbar/Mc^2, but much less than the experimental time resolution). This is the case, for instance, of the Z boson. Again, the Z boson has very real effects (e.g. it produces a bumb in the cross section) which however are different from the effects of what we have called a real particle or a virtual particle. To stress that we are in a yet different regime, it is useful to give it a different name, which is "resonance".

 

[Bill_K]

That's a good summary. The only thing I'd object to is the term 'resonance', which is a rather old-fashioned term, and implies the particle is a composite made up of other things. 'Short-lived' is a better word. I don't think anyone would call the Z boson or Higgs boson a resonance.

 

[Lapidus]

That's a good summary.

Thanks, a friend who studies physics told me that.

The only thing I'd object to is the term 'resonance', which is a rather old-fashioned term, and implies the particle is a composite made up of other things. 'Short-lived' is a better word. I don't think anyone would call the Z boson or Higgs boson a resonance.

So could we simply say, we are searching for 'short-lived' particles when doing particle accelerators experiments?

Searching for the Higgs particle, the questions is not where it is, but rather for how long. They are supposed to be there all the time, but each of them is there just for a very short time, each of them is only very 'short-lived'. Which makes them so hard to detect.