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Something I have read multiple times that I've never intuitively understood is that "heavier" particles are harder to detect than "lighter" ones... For example, I quote from Stephen Hawking's "The Grand Design" in relation to supersymmetry:

But various calculations that physicists have performed indicate that the partner particles corresponding to the particles we observe ought to be a thousand times as massive as a proton, if not even heavier. That is too heavy for such particles to have been seen in any experiments to date, but there is hope that such particles will eventually be created in the Large Hadron Collider in Geneva.

Could someone please explain, in simple terms, why heavy particles are harder to detect? Intuitively (to me, a non-physicist) it would seem that it should be the other way around, because a particle with more mass should interact more strongly with other matter.

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2 Answers 2

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They are harder to detect for mainly two reasons:

First, because they decay very quickly into lighter particles. Infact the heaviest elementary particle we know of ( top quark ) decay's so rapidly it is theoretically and technically impossible to measure it in any other way than indirectly through its decay products.

This poses another problem as to what statistical and theoretical basis one can with some certainty claim detection of such particles.

Another problem is that of production, since they have more mass, they require much more energy to produce, thus larger and more expensive accelerators.

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@kalle43, your answer is wrong. Light neutrino is harder to detect than heavier proton, light neutral pion is harder to detect than heavier omega-hyperon. –  voix Dec 18 '10 at 9:29
@voix This question has a pretty clear context. –  user1708 Dec 18 '10 at 10:18
Let's just say it is incomplete. It's indeed not that heavier particles are harder to detect than lighter ones. What makes a particle harder to detect is how weakly it interacts. But, heavy particles are indeed unstable, because they pack a lot of energy in a tiny space, and precisely because of their potential to interact they will decay into lighter particles. So, it's more the instability of heavy particles which makes them inaccessible than their possibly low interactivity. –  Raskolnikov Dec 18 '10 at 10:23
@Raskolnikov, there are many unstable light particles with a short half-life too. So-called resonances. –  voix Dec 18 '10 at 11:29
I think that part of the problem here is that the OP wrote "detect" when "create as on shell particles" might have been better. It is much harder to make on on-shell top quark than it is a on-shell charm quark because of the energy-in-one-place requirement. And it takes more energy to make on-shells tau than on-shell muons. Contrast this with neutrinos which are easy to make (in all flavors if you are willing to stand back and let oscillation happen), but a real bear to detect. Detecting heavy quark states is complicated mostly by the abundant backgrounds. –  dmckee Dec 18 '10 at 23:49

Hawking's phrase "seen in any experiments" really means the following. First you have to produce the heavy particle. Ordinary matter, consisting of electrons, protons and neutrons (the latter two themselves bound states of quarks), does not contain these new heavy particles, so they must be produced in high-energy collisions. By Einstein's famous formula $E=mc^2$, the production of very heavy particles (say with mass in the $TeV/c^2$ range) requires energies of order a $TeV$. Hence the LHC. Then there is the separate problem of detecting the particles once you have produced them. Your expectation that particles with more mass interact more strongly with other particles is true for gravity, but it is not true for the strong, weak or electromagnetic interactions which are relevant for the detection of elementary particles (the gravitational interaction is much too weak for these purposes). The strength of electromagnetic interactions depends on the electric charge and there are analogous "charges" for the weak and strong interactions. Detecting the particles is extremely complicated and depends on their mass, their lifetime, whether they have strong, weak or electromagnetic interactions or not, and what kinds of Standard Model background effects might have the same experimental signal as the particle you are trying to detect. This link from the LHC website has some information about particle detectors: http://public.web.cern.ch/public/en/LHC/LHCExperiments-en.html

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Nice overview and I guess it's hard to add anything else without going into messy details of interactions. For now I'll link to the related question of detecting the Higgs boson which has pretty neat answers (of various technical levels) that elaborate a little on the stuff you mention. physics.stackexchange.com/questions/185/… –  Marek Dec 19 '10 at 20:26

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