This might become the title of a series of posts, much like my "say of the week" series. In fact, the Large Hadron Collider is back in operation since earlier this month, and the instantaneous luminosity at which it collides protons at a center-of-mass energy of 7 TeV is going to keep gradually increasing, as better orbit parameters are found, more protons are injected in the machine, more bunches are made to circulate, and beams are squeezed in the surroundings of the experimental halls.
Still, it feels nice to report that the value of L = 2.4E32 cm^-2 s^-1
has been now achieved, a full 20% more than the highest number recorded
in the 2010 running.
Instantaneous luminosity L is a measure of the rate of collisions that take place in the interaction points. In order to make sense of this concept, you need to know at least what is the total proton-proton cross section at 7 TeV. This number, call it "sigma", is about 80 millibarns; and a millibarn is 10^-27 cm^2. Then, you need to know that the rate of collisions is computed with the formula N = sigma L, and you can start obtaining meaningful numbers.
From sigma=80 millibarns, that is 80 x 10^-27 cm^-2, and L=2.4 x 10^32 cm^-2 s^-1, you get N= 19,200,000 collisions per second. That is the number of proton-proton interactions that take place in the core of the detectors. 19 millions per second.
If however you are more interested in knowing the number of top quarks that are produced, you need to take sigma = 160 picobarns, the production cross section of top pair production. Inserting it into the equation above gives N = 160 x 10^-36 cm^-2 x 2.4 x 10^32 cm^-2 s^-1 = 0.0384 s^-1, or one top pair every 26 seconds or so.
The rapid increase in the luminosity of LHC bodes well for the forecast of integrated luminosity that will be delivered to ATLAS and CMS this year. The "official" numbers talk of one to two inverse femtobarns, but many are confident that by Christmas we will have a larger statistics to play with.
Will LHC find new physics in 2011, with such datasets ? I bet it won't, but stay tuned for surprises!
