Home

Rutherford scattering

Inside the Nucleus

Wave or Particle?

Deep inelastic scattering

SLAC

HERA

ZEUS

Event pictures

More Examples

Identifying events

Useful links 

Credits

 

If you want to know more

Fundamental particles

Scattering

Cross section

Energy units

Virtual photons

Rutherford's Notebook

The Stanford Linear Accelerator Center (SLAC)

slac_sm.jpg (14594 bytes)

SLAC is situated near Stanford University in California.   Its centrepiece is a 2 mile long linear accelerator which took 4 years to build (construction began in July 1963 and the accelerator was dedicated in September 1967). It was here in 1968 that the first experimental evidence for quarks was found.   Originally the particles detected by SLAC were called partons - a name coined by Richard Feynman -  as the theory of quarks was a disputed topic at the time.  Murray Gell-Mann and George Zweig had both developed theories that explained various particles found in high energy experiments in terms of smaller objects with charges of +2/3 and -1/3 of the proton's charge. Gell-Mann called these objects quarks.  But in contrast to the earlier experiments of Rutherford and Chadwick, no one had suceeded in knocking out a single quark from a proton.

The linear accelerator   (click to find out more) at SLAC was from the start able to accelerate electrons up to 20 GeV and so could probe deep into the nucleus.  This is a far higher energy than the 7 MeV alpha-particles used by Rutherford or the 400 MeV of Hofstadter's experiments.

end-A_sm.jpg (27603 bytes)  
The detectors used at SLAC. The nearest structure of the 8 GeV spectrometer, also shown in the diagrams below. 

Probing the proton: The experiments at SLAC were very similar to that of Rutherford's scattering experiment in that a projectile hit a stationary target and was scattered off in various directions.  In this case the projectile was a high energy electron and the target a proton.  Due to the high energy and momentum of the electron (and so its very short wavelength) it was able to probe inside the proton.

endaside.gif (3314 bytes)  
Side elevation of the 8 GeV spectrometer. Magnets (B1, B2) bend scattered particles for momentum measurement.

Hofstadter's experiments had shown that the proton is not a point charge, and he had been able to measure its size.  But this did not mean that the proton was not fundamental; it could be indivisible with its charge spread over its size.  If the proton could not be divided up further, electrons with wavelengths smaller than the size of the proton would tend to pass through the proton and be deviated by only a small amount (they are able to pass through because the proton is not a solid sphere). This is as Rutherford had expected for alpha particle scattering if the proton had been like a "plum pudding". It was therefore thought that the high energy electrons at SLAC would be deflected by small amounts.

At low energies at SLAC this was found to be the case, but as they increased the energy of the electron (and so reduced its wavelength) the results showed far more scattering at larger angles than expected.   In fact the results suggested that the proton was not an elementary (fundamental) particle but was made of smaller, point-like particles which could deflect the electron by a large amount.  Just as Rutherford had found a point like nucleus within the atom, so the team at SLAC had found points of charge within the proton.  These fundamental particles were given the name partons, by Richard Feynman. We now know they are quarks, but at the time they were given the name partons as people believed that they could be something other than quarks. Later experiments, at SLAC and elsewhere, confirmed the fractional charges of the partons, as 2/3 and -1/3, just as Gell-Mann had hypothesised.

endaplan.gif (3596 bytes)  
Plan view of the 8-GeV spectrometer, which could be positioned at various angles to the electron beam direction.

fwd_btn_.gif (616 bytes)Click here for modern experiments at HERA