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I'm not clear on how RICH (Ring Imaging Cherenkov) detectors identify particles, as much as I've tried to read up on it. So they measure the angle Cherenkov light is emitted at, which is related to the refractive index of the material, speed of light and the speed of the particle. How can that be used for particle ID? And why are these detectors particularly good at differentiating between pions and kaons?

I realise this seems like a question that should be easily solved by googling, but I'm not having any luck there!

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    $\begingroup$ Expanding the acronym: Ring Imaging Cherenkov. $\endgroup$ – Pieter Dec 6 '16 at 21:58
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A RICH alone doesn't ID a particle. The only thing it measures is velocity.

But by combining that information with either momentum (from a spectrometer) or energy (from a calorimeter) you can often deduce the species or at least constrain it to some range of possibilities.

As for why they are used to separate pions and kaons (I assuming that you have a particular detector system in mind for this, probably something at the LHC), these particles are both (relatively) long-lived, light hadrons. They are hard to reliably distinguish from one another based on decay products or shower parameters in a calorimeter, but their masses are sufficiently different that there will often be a measurable difference of velocity at interesting momenta.

In other contexts it may be more difficulty to distinguish pions from electrons and Cerenkov devices can be tuned for that as well.

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