In particle physics, there are all sorts of analyses. There are the biggies, like the one resulting in the discovery of the Higgs boson, or the ones that slowly but steadily rule out versions of supersymmetric theories. Those are important.
But there are other analyses that are extremely important and hard. One of those things is to understand the performance of an apparatus like the CMS detector when it is hit by hadronic particles.
Protons are of the class of particles called hadrons, but the most common particles are pions, and they are, by far, the most common particle generated in LHC collisions. In short, until you understand how hadrons interact in your detector, you will never fully understand your measurements.
So you’d think that understanding hadrons would be the first priority. But that would be wrong. It turns out that understanding how they interact with a detector is very hard. That’s at least one reason that the discovery of the Higgs boson involved photons, electrons, muons and/or neutrinos hitting the detector. None of these particles are hadrons, and that’s simply because they are all easier to characterize.
Now that doesn’t mean that experiments like CMS and ATLAS haven’t studied collisions in which pions are produced. In fact, the LHC collides protons, and the quarks and gluons inside the protons collide and make jets of particles that are predominantly pions. But, for many measurements of the hadron energy, the precision isn’t absolutely critical. Getting it reasonably accurate is sufficient. That’s because many of them require simply the identification of concentrations of hadronic energy, along with a decent measurement of the location and amount.
However, there are classes of measurements that hinge crucially on a very precise understanding of how the detector responds to hadronic energy. These measurements are ones that study the behavior of jets originating from the quarks and gluons scattered out of the protons. These are the collisions that can probe the highest energies possible at the LHC.
CMS scientists just released the result of a careful study of hadronic detector response for data recorded in 2012. The results and methodologies on this study are now in use in most of CMS analyses. In the LHC era, this analysis took a very long time because understanding hadronic energy is just that hard.
In physics, as in life, sometimes the unglamorous jobs are the most difficult. And determining the hadronic response of a detector is the Rodney Dangerfield* of particle physics: a hard and sucessful analysis that claims “I don’t get no respect.”
*Rodney Dangerfield was an accomplished American comedian who, despite his enormous success, repeatedly claimed “I don’t get no respect”.