Re: mu+ Polarization Query

[Tom Banks <tbanks@socrates.berkeley.edu>]

> In pi- decay, as anti-neutrinos are right handed, the mu- produced
> must be right handed in the pi CMS, i.e. 100% polarized along
> their momentum direction. If one tunes the channel momentum to
> say 33 MeV/c, in the lab system we would collect 33 MeV/c muons from
>
>   - forward decay in the pion CMS from pions with very small momentum
>     <5 MeV/c.
>   - backward decay in the pion CMS from pions with 85 MeV/c.
>
> My understanding is that the backward decay muons dominate, giving
> a mu- polarization opposite to the beam direction.


Dear Peter,

your explanation seems to be consistent with Bernhard's opinion that the
mu+ are polarized in the direction of the beam momentum (i.e. pointed
downstream).  His argument was predicated largely on the polarization flip
which occurs around 30 MeV, as illustrated in Figure 7 of the PiE3
document you referenced:

   http://aea.web.psi.ch/beam2lines/pie3.pdf

Bernhard says that you (Peter) and he observed this delicate spin flip in
2004 before Run8 production began.  When below the 30 MeV threshold, mu+
are polarized opposite the beam momentum; above the 30 MeV threshold, mu+
are polarized in the direction of the beam momentum.  Our operational
momentum was safely above the ~30 MeV threshold, so the mu+ were pointed
downstream.

So, the Run8 muon polarizations were:

   mu- : upstream   (-z-direction)
   mu+ : downstream (+z-direction)

Since the "upper" (+y-direction) gondolas are the first to see a peak in
the muSR precession, the mu+ must precess from

   +z  ->  +y  ->  -z  ->  -y

This means that our saddle-coil magnet's field lines pointed in the
+x-direction.

Thanks for your input,
Tom