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I got a little puzzled with the concept of hadronic/electromagnetic calorimeter. Do they measure the energy of the particle e.g. an electron, like the first entry of the four-momentum, or the momentum, so the "kinetic" energy?

If I search for it in the internet, I just find, that the particles deposit their energy in the calorimeters, so it can be measured. I think, the electron e.g. cannot deposit all its energy (so the momentum and the 0.5MeV mass) because then its not there anymore and then the charge cannot be conserved.

So how can this fit together?

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  • $\begingroup$ Technically neither (detectors like these can only measure coarse approximations for these quantities) and usually both, in that most calorimeters these days are actually spatially resolved detectors. Part of the art of high energy physics is having to live with large compromises in resolution and precision. Even the case that a particle can not deposit all of its energy in the detector is quite common, although it's not about measuring rest mass, which is known and physically of no importance at the usual energy ranges these days. $\endgroup$ – CuriousOne Sep 12 '15 at 16:10
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What they actually measure is the amount of charge produced as these particles interact with the material of the detector.

From that and a understanding of how charged particles interact with the detector (PDF link) they deduce it's kinetic energy (with some degree of ambiguity at times). To get momentum you pair the energy measurement with either some kind of particle ID or with tracking through a magnetic field (with segmented calorimeters some of this information can come directly from the calorimeter itself). Sometimes the pairing can clear up the ambiguity from the calorimeter.


As you noted it is usual in writing about these devices to simply say "energy", not "kinetic energy" because the reader is assumed to know enough physics to understand what is available to be deposited. Also, in the high energy world, it is common for the kinetic energy to be many time the particles rest energy (mass), so only a small error would be made by conflating the two in any case.

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