An example of the precision measurements still being made using LHC data from the last couple of years, as we prepare for higher energies in 2015

I spent yesterday in Paris as a member of the committee for the "Habilitation à diriger des recherches" of one of my colleagues on the ATLAS experiment at the CERN Large Hadron Collider. His successful "Habilitation" means he can now officially supervise PhD students.

Giovanni was the leader of a group in ATLAS measuring photon cross sections, and his talk was a great review of how we do this in ATLAS, right from the first signals in the experiment, that trigger us to save the result of a collision, all the way to plotting the mass of pairs of photons and finding the Higgs boson as a bump in the distribution.



(link to video)

So it was good to see this morning that another one of our papers on photons made it through the internal review process and out into the world. Here it is: The "Measurement of the inclusive isolated prompt photon cross section in pp collisions at square root of s = 7 TeV with the ATLAS detector using 4.6 fb-1".

Unpacking the snappy title a bit,

"Inclusive": means the photon is measured regardless of what else is produced in the collision.

"Isolated": means the photon doesn't have many other particles near to it in the detector.



"Prompt": There are several different ways photons can be produced in a collision at the LHC. One of the most common is in the decay of neutral pions, which are hadrons created copiously when protons are smashed up. By "prompt" though, we means photons which are produced promptly in the collision, before the quarks and gluons have had time to form hadrons, and well before those hadrons decay.



Photon: the quantum of light. Carrier of the electromagnetic force (and of sunsets).

"Cross section": This is a measure of how often the photons are seen, given how many protons we collide.

"Square root s" is a fancy way of saying the energy of the collisions.

"ATLAS detector" is the apparatus with which we measure and record the collisions.

"4.6 fb-1" is the amount of data collected, measured in inverse femtobarns, which I explained here.

So much jargon in one title.

The measurements are compared to various calculations using the Standard Model of particle physics (specifically the strong force), and they agree with them pretty well. One consequence is that we can probably get more information from these measurements about how quarks and gluons are distributed inside the proton, which is useful for improving the precision of other calculations for physics processes at the LHC and elsewhere.

Not an Earth-shaker, but a nice paper, and I felt like writing something. There'll hopefully be plenty more such papers over the next year or so, as we continue preparations for higher energy running (square root s of 13 TeV or so) starting in spring 2015.