Experimental Subatomic Physics

* Some subatomic physics experiments look in nature to study naturally occurring subatomic processes

* but most experiments usually bang things together to see:

* In order to extract this information from experiments,

* After analysis, the results are expressed as

* Theoretical ideas are tested by comparing predictions and observations.


The first two subatomic particles to be discovered were the electron and the atomic nucleus. The concepts used in their discoveries are still used today - just vastly scaled up in energy, size, money, and people.

Thompson's discovery of the electron was the first example of a non-accelerator particle search experiment. Thompson showed that cathode rays were electrically charged particles with a fixed value of e /m. Since e was roughly known, he showed that the cathode particles were ~2000 times lighter than a hydrogen atom. Atoms were considered to be the indivisible fundamental constituents of matter, so nothing lighter was expected to exist.

Rutherford's discovery of the atomic nucleus was the prototype for all subsequent high energy physics experiments.



Rutherford CDF
  team Geiger(RA),   Marsden(UG) cast of thousands
  beam a particles
  (Rd, ~5 MeV)
  (Tevatron, 900 GeV)
  target foil (Au,Ag) proton/antiproton beam
  detector ZnS screen + eyeball 10000 ton hermetic   detector
  data acquisition pencil networks, workstations, disks, tapes, ....
  analysis student+pencil ~102 students + computers



Starting with ordinary matter (electrons, nucleons, photons), we want to probe smaller and smaller distance scales and produce new states of matter.

We use accelerators to produce our high energy probes, and detectors to observe what happens.


electromagnetic energy (electromagnetic fields, waves)

        Þ energetic (charged) particles


energetic (charged) particles

        Þ electromagnetic energy (light, charge)



In order to do physics, an accelerator must provide us with:

(1) energy
   l=hc/E ,  E>2m
(2) luminosity
   Rate (s-1) = s (cm-2) L (cm-2 s-1)


The cross-section (s) parameterizes the probability of an interaction. e.g. billiard balls

For point particles (r ~ l) interacting by a point interaction (range ~ l):

s < 4p(l/2p)2= 4p(h/2p p)2    Unitarity Limit

    Unitarity Limit

So if we want to see the same number of interactions as we increase the energy, we must have


e.g. Consider two bunches of particles colliding head on. If the particles in each bunch are uniformly distributed over an cross sectional area A, the probability of any two particles travelling in opposite directions interacting is just s/A. The total number of collisions between particles in the two bunches is then (s/A)N1N2, where N1 and N2 are the number of particles in each bunch. If bunches of particles are colliding with a frequency, f, then the total collision rate is R = (s/A)N1N2f, so the luminosity is

Lcolliding beams = R/s = N1N2f/A



Simplest Accelerator