I have read several sources about tracking detectors used in particle accelerators like LHC, but still have not found a more detailed source that can still be understood by a layperson like myself. I am looking at CERN's article, "How a detector works". I am hoping to learn more details about this part:

Tracking devices reveal the paths of electrically charged particles as they pass through and interact with suitable substances. Most tracking devices do not make particle tracks directly visible, but record tiny electrical signals that particles trigger as they move through the device. A computer program then reconstructs the recorded patterns of tracks.

My core question is this: With the uncertainty principle and the observer effects in mind, how do these tracking/tracing devices measure both the position and momentum of particles with the kind of accuracy that they seem to get with the beautiful color pictures you see of particle traces coming out of a collision?

Do they use some kind of charged gas that emits light when a charged particle, such as an electron, passes through them? Can electrons be tracked, or just certain heavier particles?

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    $\begingroup$ The quantum mechanics of these particle states only plays a role at the interaction point of the beams where the relevant size scale is a small fraction of the size of the nucleus, everywhere else the (now) free particle states can be treated with classical physics (more precisely with special relativity). For the purposes of quantum physics a detector is a weak measurement device, i.e. we never extract the full precision of either the position or the momentum information, which means be never bump up against the uncertainty relation. $\endgroup$ – CuriousOne May 27 '15 at 16:27
  • $\begingroup$ @CuriousOne: thank you, it seems counter-intuitive that classical measurements (and also relativistic) are used in finding and measuring very small quantum particles $\endgroup$ – Brad Cooper - Purpose Nation May 28 '15 at 2:37
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    $\begingroup$ It seems counterintuitive, doesn't it? The discrepancy between the (almost) classical operation of the detectors and the fact that we are doing a fundamentally quantum physical measurement gets resolved by the fact that the detectors are far away from the actual point of measurement, which is in the interaction point where the particles beams cross. What we are interested in is the physics of the vacuum and the detectors are merely measuring the aftermath of collisions, they are not involved in the actual physics of these collisions, at all. $\endgroup$ – CuriousOne May 28 '15 at 3:20

First of all, the uncertainty principle and observer effects are completely irrelevant. The tracking devices in modern detectors are large enough to be firmly in the realm of classical physics. Any uncertainty in the detector's wavefunction is negligible compared to the size and energy of the device itself, and the effect of detected particles on the tracker is not more than the loss of a few electrons here and there. Granted, over trillions of collisions, this could become a problem, but trackers are built to resist this kind of damage. They have electrical connections to replenish lost electrons, and they are made of dense materials that will retain their structure even if the occasional atomic nucleus gets transmuted into another one due to radiation.

As for how these tracking devices actually work: there are several different types. Each of them records a particular type of information, and is sensitive to only certain particles. The trackers are arranged around the beamline (the path through the center of the detector, where the incoming particles go) in a way that allows scientists to identify the signature of a particular particle by cross-checking the outputs of different types of trackers. It looks basically like this picture, from Wikipedia:

ATLAS schematic

(that's the ATLAS detector).

A typical detector includes the following types of components, working from the inside out:

  • A silicon tracker consists of small "panels" of silicon arranged in concentric layers around the beamline. A charged particle produced in a collision will pass through one of these panels and knock a few electrons off the conduction band of the silicon (via the electromagnetic interaction), creating an electrical signal. Each panel is connected to its own dedicated wire, and the other end of that wire runs to the detector's readout circuit (an interface between the detector itself and the CERN computers), so the computer knows exactly which panels were exposed to outgoing particles, and to some extent, how much.

    Silicon trackers don't measure the momentum of a particle, but they don't change it very much either. They're more focused on accurately measuring position. Since the individual silicon panels are quite small - maybe a few centimeters on a side - the computer gets access to precise information about the location of the particle as it passed through this tracker. And with six or seven concentric layers of silicon, spaced a few centimeters apart, you can reconstruct the path of the particle pretty well. You can see a visualization of the information received from the silicon tracker in the center of this image from CMS, the red blocks in the middle:

    CMS event display

    At this stage, it's impossible to know what kind of particle the tracker is seeing, but only charged particles interact with the silicon, so anything that leaves a track has to be charged: probably an electron, muon, or light hadron.

  • Next up are the calorimeters, which are massive blocks of metal designed to absorb certain particles and measure their energies and momenta. There are usually two kinds: electromagnetic calorimeters, which absorb light particles that interact electromagnetically (electrons, and photons), and hadronic calorimeters, which absorb particles that interact via the strong force (almost everything else).

    Calorimeters are shaped into thin "wedges" that are pointed toward the interaction point, as you can kind of see from the first picture on this page (see the yellow layer). Each particle deposits its energy into one wedge of the calorimeter, corresponding to the direction in which it exited the silicon tracker. But the calorimeters don't detect individual particles; they can only identify how much energy was deposited into a particular wedge, and thereby get a distribution of the directions in which energy came out of the collision. The amount of energy deposited can be determined by measuring how hard the cooling system has to work to maintain the calorimeter at a constant temperature.

    If you were to look at the data collected by the calorimeters only, you'd get something like the yellow blocks in this image:

    calorimeter event display from ATLAS

  • Outside the calorimeters, modern detectors include a muon spectrometer, which operates a bit like the silicon tracker but on a much larger scale, using crossed strips of metal instead of silicon. The muon spectrometer records the tracks of muons by checking which strips receive electrical signals as the muons pass through them, and it can determine their momenta because the entire detector is inside a magnetic field, which makes the muons' paths curve. The radius of curvature tells you how much momentum the particle had.

At this point, everything except neutrinos has been detected, and there's nothing you can do about the neutrinos, so we just let them go.

As I mentioned before, the electrical signals from the components get fed into readout circuits, which convert them into digital signals that are then passed on to the computer. A detector sees thousands of collisions per second and collects an enormous amount of data on each one, so it can't all be stored. Instead, the signals get sent through several levels of triggering systems. The first level simply combines the readings from different parts of the detector and throws out any detections which are "boring" - for example, none of the trackers got any readings, or the readings don't exceed a certain threshold, or whatever the detector team decides is not important. (They go through a long process of analysis to decide what is not important.) After that, anything which hasn't been eliminated is sent to the CERN computer cluster for a more sophisticated analysis. What comes out at the end are sets of numbers giving the signal strength measured by each of the detector components, but only when all those signal strengths together constitute an interesting event.

If you have access to these signal strengths, you can feed them into a computer program which will produce an image of the detector and plot the corresponding signals on top of it. That's where the particle traces you've seen come from: the detector press team (or others who have access to these raw measurements) will pull out the best-looking ones and release computer-generated "pictures" that show the measurements.

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    $\begingroup$ Thank you, @David Z. This helps, esp "The tracking devices in modern detectors are large enough to be firmly in the realm of classical physics." That was sort of the root of my question, whether these detectors are somehow overcoming the limitations of quantum uncertainty... seems like they are not. It also seems as though they really do not care about the paths and momentums of low mass particles like electrons? more interest in high mass particles like Muons, Higgs, heavy quarks, etc? Perhaps I need to look elsewhere for quantum-level position/momentum accurate measurements? suggestions? $\endgroup$ – Brad Cooper - Purpose Nation May 27 '15 at 14:35
  • $\begingroup$ Are you sure that the observer effect is "completely irrelevant"? My understanding is that the particles pass through many layers of detectors (esp. the muons) and it would seem this would have quite an important effect on the speed, direction and momentum of the particles as they pass through? I assume the scientists account for these possible interactions, exclude or change their measurements accordingly, but are you sure they consider them "completely irrelevant"? $\endgroup$ – Brad Cooper - Purpose Nation May 28 '15 at 13:24
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    $\begingroup$ (2 comments up) no, the detectors measure electrons too. Actually, the only particles directly measured by the detector are electrons, muons, protons, neutrons, pions, kaons, maybe a couple other light mesons, and photons. Everything else decays inside the beam pipe. (1 comment up) These outgoing particles have such high momentum and energy that they are not appreciably slowed by a few thin layers of silicon. So yes, the observer effect is basically irrelevant. That being said, it's possible they do account for it in some detector simulations. $\endgroup$ – David Z May 28 '15 at 16:22
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    $\begingroup$ @DavidZ No, detector simulation doesn't account for observer effects. As was mentioned in a comment on the question, the macroscopic length scales on which the detector elements interact with particles are such that it is a weak measurement: the detector is not significantly entangled with the hard process at the pp interaction. Significant correlations only occur within a nuclear radius of the primary interaction. There are quantum effects in e.g. how a charged particle interacts with silicon, but again each interaction is factorized/unentangled so effective interaction models can be used. $\endgroup$ – andybuckley May 30 '15 at 12:01
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    $\begingroup$ @PurposeNation Exactly, they can measure all these things, but nowhere near the level of accuracy where the uncertainty principle is limiting. An optimistic estimate of tracker measurement accuracy would be 1 MeV/c uncertainty on the momentum measurement, and a spatial resolution of 1 micron; even that high level of precision is 5 x above the Heisenberg limit. $\endgroup$ – andybuckley May 30 '15 at 12:12

OK, I have found additional material on the CERN site which further describes the tracker detector and the silicon pixels and silcon strips within the detectors. Fascinating stuff.

The first article says that each measurement of the detector is accurate to 10 micrometers. Seems like great accuracy, esp. for their purposes. However, I calculate about 100,000 atoms or about 10,000,000 gamma ray wavelengths would fit inside that kind of spacial variation -- so it seems the uncertainty principle is fairly intact with these types of measurements... ie, not a very accurate measurement of both the position and momentum of an individual low-mass particle, like an electron, I suppose unless it was a higher mass particle (like a Higgs Boson)?

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    $\begingroup$ Higgs bosons are far to shortlived to reach the detector from the beamlines. Higgs bosons are only measured indirectly by their decay products. $\endgroup$ – Sebastian Riese May 27 '15 at 14:35
  • $\begingroup$ The granularity of these detectors has nothing to do with quantum uncertainty and everything to do with simple physical extent of the detector elements and fitting errors. We are many orders of magnitude away from caring about quantum uncertainty. $\endgroup$ – dmckee --- ex-moderator kitten Jun 7 '15 at 17:53

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