The magnetic field at CMS is often given as 4 T. According to this picture the field outside the solenoid is just 2 T. Is this just because the field of a solenoid is not constant and decreases with distance? Or is there another effect, or perhaps a second source for the magnetic field in the muon chambers, not related to the solenoid of CMS?

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    $\begingroup$ For an infinitely long solenoid the external field would be zero, but since all the magnetic field lines are closed and in a short solenoid the field has to return close to the solenoid, the field on the outside will be strong. CMS has a steel return yoke and steel typically saturates at about 1-1.5T, so the yoke wouldn't do much if the field was much larger than 2T. If I remember correctly, the field on the outside is also used for tracking, but I would have to go back to the CMS documentation for the details. $\endgroup$ – CuriousOne Jun 20 '15 at 18:57
  • $\begingroup$ I confirm that the field at the outside (around 2T) is used for tracking the muons. A side remark, the magnetic field of CMS at the center of the detector is 3.8T (not 4 as initially foreseen). $\endgroup$ – Paganini Jun 21 '15 at 15:13

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