Why is this detector so complicated? I often find myself asking that question. It is usually coupled with the exasperated ‘Why isn’t this code compiling?’ or ‘Why is the DAQ not configuring?’ or ‘Why has the front-end electronics stopped sending data?’
As I was meandering through interactions.org (which is a great resource for fancy physics pictures), I came across this nice picture of particle interactions in ATLAS. Which does in a simple way start to address the question of why ATLAS is as complicated as it is.
What is shown here is a pie-slice of ATLAS from the perspective of looking down the beam-pipe. The white circle at the bottom is the beam pipe, in the center of which the proton-proton collisions occur. One of ATLAS’s design goals is to detect new particles such as the Higgs or Supersymmetric particles. But that is a bit misleading because we don’t really detect these particles themselves, we detect their decay products. By measuring those decay products, we can reconstruct any new particle’s properties, such as its mass.
So the particles we actually observe in the detectors are mostly just the ordinary things like electrons, photons, muons, protons, pions and the like. Different detector types are better at measuring say, an electron, than a muon, therefore in ATLAS we use many different detector technologies so that we can be sure we don’t miss anything.
This picture nicely shows which sub-detectors within ATLAS are better at measuring what. The closest detectors to the interaction are the pixel detector, the semiconducting tracker (SCT) detector and the Transition Radiation Tracker (TRT). Collectively known as the ‘trackers’ or ‘inner detector’, these detectors aim to track the trajectory of charged particles. The charge particles are bent by the magnetic field provided by a solenoid magnet. From the direction and magnitude of the curvature, we can determine the charge and momentum of the particle.
The next layer, the calorimeters, measure the particles’ energies. The first layer of calorimetry, the electromagnetic calorimeter measures the energy from photons and electrons whereas proton and neutron energies are largely measured in the second layer of calorimetry, the hadronic calorimeter. AKA Tile Cal. Muons are hard to stop and generally exit the detector completely. Similar to the inner detector, the Muon system is a series of tracking chambers to measure the trajectory of the muons. Here there is a second magnetic field (not shown in the figure), the toroid magnetic which again is used to bend the muon’s path (and is where the `T’ in ATLAS comes from). Particles like neutrinos are completely invisible to ATLAS. We can only infer their existence by measuring the `missing energy’—the energy that the neutrino takes with it as it leaves the interaction and the detector.
In that light, if you have ever wondered, ‘are all those sub-systems really necessary’, the answer is definitely, ‘yes!’.
Hi Monica,
Your comment about “missing energy” got me wondering..
What exactly can be inferred by the amount of missing energy? If it were small for example, would that restrict the possible number of WIMP/supersymmetric/dark/other particles and their possible masses?
Also, I’m also wondering how the average amount of missing energy during ATLAS might relate to previous particle detectors with lower collision speeds?
Is it simply that “missing energy = input energy – what we have the capability to detect”? So in a theoretical detector that could perfectly measure every particle, “missing energy” would equal zero?
Sam
How are the magnetic fields on atlas orientated? Do the poles stay in the same orientation (north and south) and do they stay that way all the time? Or are they flipped periodically? Also how long, do these magnets stay on for?
Sean
Hi Sam,
Indeed if there was such an ideal detector which could detect every particle, we would expect no missing energy. So missing energy is a signature of a particle escaping ATLAS undetected not of some physical process which violates energy conservation. Missing energy as we use the term in experimental physics is a consequence of how we make the measurement.
In ATLAS as well as other previous particle detectors, the neutrino (a light, neutral, very weakly interacting particle) is a source of missing energy as is shown in the picture. But in searches for new physics, events with large amounts of missing energy are very interesting. Many models of new physics, such as supersymmetry predict a heavy neutral particle which would not be detected in ATLAS. If SUSY exists, we might therefore see events with unexplained amounts of energy missing. It is a very difficult measurement to make though because the detector is not perfect. Imagine for example that a jet of particles happen to pass through the ‘gap’ between the barrel and extended barrel calorimeters. We have no active detector in that region, so it looks like there was missing energy, when in reality the particles passed through a region of the detector where we were ‘blind’.
Hi Sean,
There are two magnetic fields in ATLAS. The Solenoid is a 2 Tesla field, in the direction along the beam pipe (which direction along the beam pipe, honestly I can never remember). The Toroid magnetic field is 4 Tesla, in a circular direction around the beam pipe (if it is clockwise or counter-clockwise, again I can never remember). As to whether or not the fields change direction, I am not sure but I think no. That would be quite a big change for the detectors as well as our simulations.
When the beam is running, that magnets will also be running. So they will stay on for quite long periods at a time. During the month of May, we have many tests of the full magnet system planned which I will certainly be reporting about here. It should be very exciting.
Hi Monica,
I am raja from India.Being my career in engineering I too interested what’s happening inside LHC searching for a God’s particle.
After the LHC started its operation is there any way to monitor the collisions happening inside the CMS through the online? or any other software available to do this monitoring online? if u know any ways please reply.
I got doubt about the higgs boson (or higgs particle) that itself heavier than so many times the mass of proton how can it give mass to all other particles?I cant understand this weird concept of giving “MASS” to a particle?can u explain please?