Re: Spin tracking of moving muons

Help: Forums, postings and threads

Forums, postings and threads

Click on the Forum title, e.g. on the "Forums by Category" page, to read a sequence of postings to the Forum and its threads all in one page. If you are only interested in one thread or the thread following a specific posting, click the thread or the posting, which takes you to a smaller page, which contains only the part you are interested in and may be easier to navigate.

Layout of the posting thread(s)

The posting threads can be shown with/without the text of the postings ("Inline depth"). In addition the depth of the thread can be controlled ("Outline depth"). For both cases several values are provided, which can be toggled. Note that the displayed values are those, which can be chosen, they do not correspond to the actual display status of the thread(s).

Add message

Add a new posting thread to this Forum

Show subscribers

Click "Show subscribers" to get the List of all subscribers, who receive e-mail from postings to this Forum, if they have not explictly unsubscribed from a posting thread. This includes:

Subscribers to the forum
Explicit subscribers to the posting or any of its ancestors
The author of the posting (if he has not unsubscribed)
The authors of any ancestor posting (if they have not unsubscribed)

Some browsers display old pages from the cache

Some browsers (e.g. Firefox) display a page from the cache even though the page has been updated. As a result, one may not get the latest messages in the forum's outline or the logon ID of the previous session appears on the upper right corner. If one suspects that he or she does not get the latest content of a page, try clearing up the personal data from the browser's cache as the following:

Tools -> Clear Private Data -> Clear Private Data (from browser Firefox)

Tools -> Delete Private Data (from browser Opera)

then reflesh the current page to get the latest content.

Details on "Inline depth" and "Outline depth"

Threads and the follow-up messages are numbered. There is one line per posting, normally starting with the posting number. The URLs of a posting and its responses reflect the depth of the follow-up postings. Watch the URLs to see how this is done. Depending on the chosen display options, the thread-display may be incomplete; the number of messages at the next level is, however, displayed, e.g. "…3 message(s)". You may not see all contributors unless you choose "All".

Messages are "chained" if there are only replies at the first level, i.e. 1/1.html, 1/1/1.html etc. In case of "chained" messages the message number is replaced by the icon and there is no indentation.

Inline: Display the subject line only or also the text of the posting(s); for the choice "All" the "Outline" choices are switched off.

Argument Default value Alternative Comment

1 0 1 no text / full text of posting

2 1 All text for level 1 only / text for All postings

Argument	Default value	Alternative		Comment
1	0	1		no text / full text of posting
2	1	All		text for level 1 only / text for All postings

Outline: Choose the depth of the posting thread, successive toggle controls provide increasing detail.

Argument Default value Alternative Comment

1 2 1 2 levels / 1 level (original posting)

2 3 2 3 levels / 2 levels

3 3 All 3 levels / all levels (all postings)

Argument	Default value	Alternative	Comment
1	2	1	2 levels / 1 level (original posting)
2	3	2	3 levels / 2 levels
3	3	All	3 levels / all levels (all postings)

Hi Kevin,

Thanks for your detailed and constructive forum posting.

3) As for your last question, the missing third term is an oversight. I noticed it a little while ago when I went back to Jackson. I made mental note to investigate and fix it. Actually, I have no feeling as to when this term (size of E field and beta) becomes sizable relative to the others. Until now, I only thought of E fields inside drift chambers. According to Jackson: "If the particle is relativistic (beta->1), even the presence of an electric field causes the longitudinal polarization to change only very slowly, at a rate proportional to gamma**(-2) times the electric field component perpendicular to v_vector." (I assume you are dealing with 'surface muons' - longitudinal polarized muons?)

2) G4EqEMFieldWithSpin copies from G4EqMagElectricField while G4Mag_SpinEqRhs is a copy of G4Mag_UsualEqRhs. Only if the field can change energy, then the time must be integrated, else:

http://www-geant4.kek.jp/lxr/source/processes/transportation/src/G4Transportation.cc#L480

the time is incremented from the particle's velocity and step length.

1) > Practically, given the way Geant4 handles the energy loss, should I be

> worried about this?

> Do we ever end up in a condition where 1/\beta gets big enough to be a

> problem? If so, is there any good way to deal with this?

You have a very good point. I don't know the answers.... I suggest you flag large final step spin variations - should they occur - in your application. My experience has only ever dealt with muons stopping in a rather homogeneous, albeit strong magnetic field. We never observed a weired spin. We were very sensitive to any false prediction (depolarization) at the 10-3 level and consequently plotted the final spin predicition for each event history for many millions of such histories.

A group who extensively uses spin tracking in G4 is from PSI:

A general paper:
              T. Shiroka et al.,
              "Geant4 as a simulation framework in muSR"
              proceedings of the muSR 2008 conference,  Tsukuba, Japan
              (21-25th July 2008), to be published in Physica B.