1/31/2024 0 Comments Xsection 8 applicationParticles like neutrons are actually quantum clouds that pass through each other or interact with an energy-dependent probability - the likelihood of collision has little to do with a solid, cross-sectional area. It is as though the neutrons change the area of their cross section mid-flight. But soon, disparities arose: Neutrons are more likely to collide with certain nuclei when they are moving slowly than when they are fast. Rutherford’s experiment, which collided alpha particles and gold nuclei in 1911, revealed that nuclei are much smaller than previously supposed. When Xerxes darkened the sky with arrows at the Battle of Thermopylae, the probability of getting hit by an arrow was very high.Įarly collision experiments were intended to measure the size of particles from their collision rate. The collision probability for a cloud of projectiles is simply the ratio of area covered by them to the total area of the cloud. Subatomic particles are so small that aiming individual particles at each other is out of the question - the best anyone can do is to shoot a lot of them in the same general area. When marbles or billiard balls are rolled at each other, the probability that they will collide is proportional to the size of the balls, unless they are precisely aimed. In the early days of particle physics, particles were thought to be tiny indestructible balls. How did particle physicists come to use “cross section” in such a strange way? It’s a long story. For instance, when CMS physicists measure the “proton-proton to top-antitop” cross section, they are counting how many top-antitop pairs were created when a given number of protons were fired at each other. A particle physicist might use the word this way, but more often it is used to mean the probability that two particles will collide and react in a certain way. In everyday speech, “cross section” refers to a slice of an object. The term “cross section” is a berry of particle physics - its technical meaning is very different from the common usage. Talking to a botanist friend of mine, I learned that tomatoes are berries, but strawberries are not - the scientific meaning of a berry has more to do with the reproductive structures of the plant than the way it tastes. Sometimes, everyday words are co-opted by scientists and used as technical terms.
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