Announcement:

Redo problem 3 of homework 5 for extra credit. (Pair creation problem.) Due Wednesday, March 19, 2003.

Recall from last lecture:

Experimental Examples (continued)

Particle Identification

We'd like to know whether a charged track is due to a e⁺, μ⁺, π⁺, K⁺, or p. This is the task of particle identification. There are several ways to differentiate particle types:

• differences in the way they interact -- electrons readily bremstrahl and compton scatter, pions, kaons, and protons interact strongly, muons do neither.
• sort out the possibilities by the kinematics of a decay.
• differences in mass.

Energy loss formula

The Bethe-Bloch formula:

Taking a look at Fig. 26.1 in the Review of Particle Properties handout, we can see the range over which the Bethe-Bloch formula is applicable, from βγ of about 0.1 to several hundred. This plot is for μ⁺, but the conclusion about the range of applicability of the Bethe-Block formula is true for all heavy charged particles, μ, π, K, p, and d. I will ignore the region of βγ<0.1 as it is not generally important in particle physics experiments.

The region of βγ> few hundred is where radiative losses (bremstrahlung) turns on. For corresponds to muon momenta greater than about 100 GeV/c. Again, most experiments don't have muons of such high momentum, so that bremstrahlung by muons and heavier particles is basically ignored. (There are people now considering the effects of muon bremstrahlung in the next generation of experiments.) Bremstrahlung is an important effect for electrons.

The energy loss function has a minimum around βγ=3. The energy loss at this point is known as minimum ionizing, and a particle with this energy is called a minimum ionizing particle or MIP. This value is important, since it quantifies the smallest signals that a particular detector expects to see.