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)
