What Does “Luminosity” Mean in Particle Physics?
Sarah Charley, Brookhaven National Laboratory / Phys.org / Feb. 3, 2021

As you may have noticed,

(I haven’t, but that may just be me)

when physicists talk about particle collisions, they talk about a measurement called luminosity.

It doesn’t tell scientists exactly how many particle collisions are happening inside a collider; rather, luminosity measures how tightly packed the particles are in the beams that cross. The tighter the squeeze, the more likely it is that some of the particles will collide.

Like in cities and bars?

Protons aren’t solid orbs that bounce, break or shatter when they come into contact with each other. Rather, they are messy packages of fields and even smaller particles called quarks.

Two protons could pass right through each other, and there’s a chance all they would do is replay that scene from the movie Ghost in which actor Patrick Swayze, playing the titular phantom, sticks his ethereal head into a moving train—to no effect. You can bring the protons into a head-on collision, but you can’t make them interact.

My gramma used to say that.

Even if two protons do interact, does it count as a collision? If two protons zip past one another and the shockwave from their intersecting electromagnetic fields ejects a few photons, does that count? What if one of these stray photons plunges through the heart of another proton? What if two protons graze each other and shoot off a bunch of particles, but stay intact?

Collisions are complicated. So physicists talk about luminosity instead.

The same principle applies to physicists and girls.