# Detecting Quarks

Hi:

I have heard that It was found at SLAC that the nucleons are made of 3 quarks, how did they found that if quarks can not be isolated?, and by the way, how do you detect those quarks if they can not be found free in nature?

Regards

mathman
Shooting high speed electrons at protons led to the observation that there are three scattering centers in the protons.

Shooting high speed electrons at protons led to the observation that there are three scattering centers in the protons.
I have never felt at ease with this answer. There is an arbitrary large number of partons in a hadrons, provided one looks close enough. So I always thought "three scatering centers" is oversimplfying. But maybe you'd call this nitpicking.

Hi:

I have heard that It was found at SLAC that the nucleons are made of 3 quarks, how did they found that if quarks can not be isolated?, and by the way, how do you detect those quarks if they can not be found free in nature?

Regards

When you shoot two protons against each other, if protons were elementary particles, you would observed two scattered protons after the collisions (I'm simplifying).
Instead of that, we observe what we call two "jets" back to back + some remnants. Jet is a collimated flux of hadrons particles (protons, neutrons + other weird particles called pions, kaons,...). We interpret this inelastic collision as the proof that protons are not elementary particles and our models are predicting that the elementary particles constituing the protons would produce jets when escaping from each other.

I'm not sure if I was clear.... :-)

What's actually done is to look at relationships called QCD sum rules, which are related to the number of valence quarks: in particular, the Adler sum rule measures $$N(u) - N(d)$$ and experimentally this is equal to 1. Add that to the quark charge assignments and you get 3 valence quarks. More directly is the Gross & Llewellen-Smith sum rule, which measures $$N(u) + N(d)$$. Unfortunately, this is rather hard to measure and there are higher-order QCD corrections which make the measured quantity about 11-12% low. The measurement is around 2.5, so $$N(u) + N(d)$$ is measured to be close to 2.8 or so: closer to three than any other integer, but not exactly what the simple calculations lead one to expect.