In the mid 1990s, we were studying electron-proton collisions at the HERA collider, in Hamburg. I was a postdoc, just finished with my doctoral research, and I was interested in the quarks and gluons produced in the collisions at HERA. We were trying to measure them.
Quarks and gluons are “confined”, which means they don’t directly appear in particle detectors. What we do see is a spray, or “jet” of other particles, called hadrons, which contain quarks and gluons. We were using sophisticated computer programs called Monte Carlo generators to “correct” the measured jets, so we could produce measurements of quarks and gluons.
The problem was that the measured quark and gluon distributions depended strongly on abstruse details of the code. A small bug could make a huge difference; approximations which should be equivalently good gave totally contradictory results; and depending on which version of which author’s code you used, the measurement changed again, even though all of them were supposed to be doing the same thing. This was just horrible.
The idea, the moment of revelation came (I think) late one night. All these problems were nothing to do with our detector; in fact they were nothing to do with our data at all. They were to do with interpretation of the data through various theoretical models, implemented in software. A real measurement of something should not care about this stuff.
The problem was that individual quarks and gluons are not physical observables. They influence the observable results of a collision – the jets of hadrons – but they themselves only appear in the quantum mechanical calculations used to predict these jets.
So, we went ahead and measured the jets themselves, as precisely as we could.
They contain information about quarks and gluons, and that information can be (and is) extracted using whatever theoretical calculation one prefers. Much more precise calculations are available now than were back then, and because our measurement didn’t have the theory “built in”, that part of the interpretation can be upgraded, and our data remain useful.
It’s an application of a principle, an idea, that has continued to be very important for us at the Large Hadron Collider, as we explore even further into the heart of matter.
This article was prompted by a request from “The Birth of an Idea”. See their main site for more contributions (and this one will hopefully appear there soon. ... 12/1/2015 it’s there now.).
While I’m here, here’s a cool little status report from the LHC:
Jon Butterworth has written a book about being involved in the discovery of the Higgs boson, Smashing Physics, available here (and soon to appear in the US and Canada as “Most wanted particle”. Some interesting events where you might be able to hear him talk about it etc are listed here. Also, Twitter.
