This week was one of the quarterly big CMS collaboration meetings. I didn’t go, as it’s the last week of classes here, but I’ve been following it as much as I can from here. What we’re seeing is some impressive work with real CMS data. ”But wait,” you are saying, “the accelerator broke three months ago, what real data are you talking about?” Ah, but I said CMS data, not LHC data. Cosmic rays are still raining down on CMS, and the collaboration spent several weeks this fall recording data from muons that came through the detector. This took a lot of operational effort, and it really seems to be paying off in our understanding of how the detector works, and how our software interprets the data that come out of it.
There are a lot of questions you can answer with these data. In any given cosmic-ray event, you expect to see one and only one muon come through. Do you see only one, or do you reconstruct additional fake charged particles? You can measure the muon momentum in either the muon system or the silicon-based tracker. Do the two measurements agree? There should be very little activity elsewhere in the detector. Is that so? If not, we’d better understand it; many searches for new physics will revolve around looking for energy imbalances in the detector, and if the energy is already imbalanced when there is nothing happening, we’re in trouble.
And then you can also do some studies related to the cosmic rays themselves. Do you observe the right ratio of positive and negative muons? Do they have the right angular distribution (predominantly from directly above, but falling off with the square of the cosine of the angle from vertical)? Can you see the shadow of the moon when it is overhead? These are harder to do (that last one in particular is tricky, but appears to be worth trying), but they could show that we really are getting a grasp on how this detector, in the making for years, actually works. We’d rather have collision data, of course, but my colleagues are really making the most of the data we have.
Lemonade, anyone?
Hah! That is awesom, The moon thing really made me laugh.
Now you got me excited on the cos^2 thing, I want to try and calculate that too and see if I get the same result.
In general, how much material does it take to stop a muon? meters? kilometers?
Thankyou for the update!
The answer is much closer to meters than kilometers. It depends on what the material is, and the energy of the muon. See
http://pdg.lbl.gov/2008/reviews/rpp2008-rev-atomic-nuclear-prop.pdf
for details. The column dE/dx|min gives the amount of energy lost by a muon of reasonable energy per unit length of material, although the lengths are given in the somewhat odd unit g^{-1} cm^2. However, if you multiply that by the density in the next column, you’ll get a perfectly sensible length. Very high energy muons (for instance at the TeV scale…quite possible at the LHC!) will lose energy more quickly, because other radiative processes become possible.
Hi Ken,
like you, I also tried to follow the CMS Collaboration meeting from the U.S. I agree that the studies conducted with cosmic rays are very encouraging – a lot of things look good, yet there are interesting discrepancies to be resolved. What pleases me the most is the eagerness of some of the younger people to try to extract some physics results from these data. They are tired of simulations and recognize that they will only be real experimentalists once they have completed an analysis of real data. All power to them!
regards,
Michael
Granted, the LHC will likely make some great discoveries. But whether it proves our theories true or false, how will the knowledge benefit us, other than by the satisfaction of having a better understanding of our universe?letting
It’s exciting that the accelerator can use Cosmic rays to help calibrate it! I can’t wait until it’s up and running so we can find out whether the data presents itself as expected or not.