Protons have parts that, as mentioned in previous columns, have the not terribly imaginative name of “partons.” When a proton collides with another proton, a parton from one proton can collide directly with a parton from the other proton. Cases in which the collision is nearly head-on are interesting for discovering new forms of energy and matter; that is the basic reason for the Tevatron and LHC science programs.
Occasionally, two partons from the first proton can collide individually with two partons from the other proton. This double-parton scattering process does not involve the creation of new forms of energy or matter, but it can look that way; it can form a “background.” So it is important to measure the rate of double-parton processes to separate them from new physics.
The natural thing to want to measure is the ratio of single-parton to double-parton collisions. But what exactly is it that one measures? What are the numbers that go into that ratio? This is where the barns come in.
If two objects that go whizzing by each other are very likely to collide, they are in some sense fat, wide objects; if they are likely to pass without colliding, they are narrow objects. We say that the wide object has a large cross section; the narrow object, a small cross section. In particle physics, the nucleus of a uranium atom is huge; to hit it with another particle is no harder than to “hit the broad side of a barn,” as the saying goes. And so the unit for measuring cross sections, the barn, is an area corresponding to roughly the area that a uranium nucleus would cover on the top of your desk (assuming you could get away with having a uranium atom on top your desk, which is pretty unlikely).
The symbol for a cross section is σ, sigma. A single-parton collision that creates some set of particles, say A, has a cross section σA; a single-parton collision that creates set B has cross section σB and the double-parton collision, σAB. That interesting ratio, the ratio of single-parton to double-parton collisions, is (σAσB)/σAB. The smaller this ratio is, the more double-parton collisions occur and the more background one has to new physics.
DZero has recently measured this ratio (called σeff) in the case where one parton collision created a pair of photons and another parton collision created a pair of jets (sprays of particles all moving in the same direction). Then, having measured this ratio for this kind of collision, we compared it with the same ratio in other processes. The new result shown in the figure agrees well with previous studies and gives us confidence that the double-parton scattering backgrounds to new production are understood, so that we can allow for their contributions when looking for new physics.
This is my last Frontier Science Result article for DZero. I’d like to thank my DZero colleagues and my Fermilab Today editor Leah Hesla for all their help in producing them.