next up previous contents
Next: 2.1.3 Experimental Setup and Up: 2.1 Techniques Previous: 2.1.1 Properties of

2.1.2 Production of Spin Polarized Muons and $\mu {\cal SR}$

In conventional magnetic resonance experiment spin polarization is achieved by a combination of high field and low temperature, kT can be made on the order of or less than the relevant magnetic level splitting, and thermal equilibrium will ensure some spin-polarization. Other methods for polarizing spins include nuclear reactions, tilted foil methods, and optical excitation. In $\mu {\cal SR}$, the parity violating decay of the pion is the process by which the muon spin is polarized. The two features of $\pi^+$ decay that yield the spin-polarization of the product $\mu^+$ are i) it is 2 body final state ii) it is a weak decay.

The pion is a boson, so its initial angular momentum is zero; Hence the total final angular momentum of the products must also be zero. Because the neutrino is always produced in a helicity (spin$\cdot$momentum) state of -1, and, in the rest frame of the pion, the muon and neutrino are emitted ``back-to-back'', the muon must also be in a -1 helicity eigenstate, i.e. it is spin-polarized.

This scheme is used in practice in the following way:

Once a beam of spin-polarized muons is produced in the above manner, it can be used in a $\mu {\cal SR}$ experiment. Such experiments are described in the following section.


next up previous contents
Next: 2.1.3 Experimental Setup and Up: 2.1 Techniques Previous: 2.1.1 Properties of