News of delays are always a little disheartening but it doesn’t mean that we at CERN will be twiddling our thumbs until the machine starts. For the past year we’ve been working on preparing software and understanding the detector as best we can. Mostly people are looking at Monte Carlo Simulations. These are our best guesses as to what will or could possibly be seen at the experiments of the LHC. We use them to make predictions and set up analyses. We have models of the detector and the collisions; and combined they paint a picture of what we may find. Then once the machine turns on, we can check our predictions.
In addition to Monte Carlo Simulations, others, like me, are looking at the current data we’re getting — real data. Every second Atlas gets bombarded by cosmic ray muons. Since we are a particle detector and cosmic rays just so happen to be particles, we can see these speedy critters as they travel through the different layers of our detector.
Muon showering process
Cosmic ray muons occur when particles (mostly protons) from outer space collide with our atmosphere. This interaction causes bunches of mesons – a meson shower – to occur. (Note: a meson is a particle comprised of a quark-antiquark pair, as opposed to a baryon which is a 3 quark particle – like a proton). The mesons, which are mostly pions (up and down quark pairs), then decay into muons and muon neutrinos (muons are like heavier electrons). Muons have a relatively long lifetime, depending on their energy and the material that they have to travel through, so usually they are what makes it to the detectors. (It’s also possible to see electrons if the muon decays inside the detector). For the most part muons travel through relatively unaffected. As the muons travel through the detector material, they ionize a small number of atoms and deposit a minimum amount of energy. Thus we call them Minimum Ionizing Particles (MIPs).
Muon going through the ATLAS detector as seen from beam pipe perspective
We see these ionized parts of the detector as a signal and can predict the energy deposition as it travels through. Then we can check to see if what we predict is what we actually measure. Things like the detector response, how uniform the material is, and how well we know the timing can all be observed using this type of data. Studies with cosmic rays will hopefully make the process of calibrating the detector that much easier once the LHC turns on. Until then I’ll be looking at cosmics.
(Regina Caputo, Stony Brook University)
Since I am working over the summer with two Physicists who are part of the ATLAS collaboration, this is quite interesting to me. I am a Quarknet teacher and I have a fermilab type detector in my classroom, the type which detects the exact same particle to which you are referring. I had wondered about this exact thing! I wondered, since muons typically pass through unaffected, whether or not it would influence your data in the detector, I just hadn’t really had the opportunity to ask my mentors. I’m so glad you posted this!
Here’s my question: How do you get rid of these signals once the LHC actually comes online? Do you just cut these particles when you look for a common vertex? Or are they of sufficiently low energy that they are just in the background?
Thanks for the info. You have answered a nagging question.
Hi Jody,
Excellent question. There are a couple of tell tale signs that an event is a cosmic muon. One of which you stated in your comment having to do with the vertex. The chances of getting a cosmic muon that goes through the interaction point at exactly the same time as a collision are very small. So we can “veto” most of the events by making a tracking hit requirement.
The other part of the detector that we have to think about is the calorimeter. It’s also very unusual for a muon to deposit enough energy to make it past the trigger threshold. For our cosmic studies, we’ve designed special triggers (with a much lower threshold than data triggers) for just that reason.
For well understood events (like the creation of Z’s or W’s for example), we know what kind of signal to expect and can predict the rate at which comic muons enter the detector. We can then “clean” the signal by removing them.
As far as I know, cosmic muons would potentially be a problem is in exotics studies which have predictions of long lifetime particles (long lifetime meaning they would decay in a part of the detector that isn’t close to the interaction point). There a cosmic muon which doesn’t go through the center of the detector may look like an exotic signal. All this would have to be taken into account as part of the backgrounds.
I’m sorry if I used too much particle physics jargon. If you have more questions please feel free to continue the discussion. I’m happy to answer as best as I can.
Hi Regina,
Would it be of any help to place radioactive isotopes in the interaction point for calibration of the detectors?
Why is that not done?
Hi Lac Leman,
Yes, good idea! This is done (but not at the interaction point). The Tile Calorimeter has a source that moves through its detector for calibration.
Cheers,
Adam