<html><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; ">Dear Kirk,<div><br></div><div>It occurs to me if muons are born in a magnetic field, say from pion or K decay in a solenoid and/or a magnetic horn, then the presence of this field (its associated vector potential) may have to be taken into account. For an analogy, it is known that when electrons are born in a magnetic field (from a photocathode immersed in a magnetic field), the associated vector potential must be taken into account. Although not yet widely appreciated from this perspective, it is the effect of the vector potential, coupled with the invariance of eigen emittances, that lies behind the flat beam transformer. See the preprint</div><div><font class="Apple-style-span" face="Calibri" size="5"><span class="Apple-style-span" style="font-size: 18px;"><font class="Apple-style-span" face="Helvetica"><span class="Apple-style-span" style="font-size: medium;"><br></span></font></span></font></div><div><font class="Apple-style-span" face="Calibri" size="5"><span class="Apple-style-span" style="font-size: 18px;"><span class="Apple-style-span" style="font-family: 'Lucida Grande', helvetica, arial, verdana, sans-serif; font-size: 13px; "><table summary="Additional metadata" style="position: static; z-index: auto; "><tbody><tr><td class="tablecell arxivid" style="padding-top: 0.1em; padding-right: 0.5em; padding-bottom: 0em; padding-left: 0em; vertical-align: top; font-weight: bold; "><a href="http://arxiv.org/abs/1010.1558v2" style="text-decoration: none; ">arXiv:1010.1558v2</a> [physics.acc-ph]</td></tr></tbody></table><br></span></span></font></div><div><font class="Apple-style-span" face="Calibri" size="5"><span class="Apple-style-span" style="font-size: 18px;"><span class="Apple-style-span" style="font-family: 'Lucida Grande', helvetica, arial, verdana, sans-serif; font-size: 13px; ">Best,</span></span></font></div><div><font class="Apple-style-span" face="Calibri" size="5"><span class="Apple-style-span" style="font-size: 18px;"><span class="Apple-style-span" style="font-family: 'Lucida Grande', helvetica, arial, verdana, sans-serif; font-size: 13px; "><br></span></span></font></div><div><font class="Apple-style-span" face="Calibri" size="5"><span class="Apple-style-span" style="font-size: 18px;"><span class="Apple-style-span" style="font-family: 'Lucida Grande', helvetica, arial, verdana, sans-serif; font-size: 13px; ">Alex</span></span></font></div></body></html>