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Dear all,
sorry for this "spam" mail, but several people have expressed interest in our findings about "theta oscillations", which may be relevant also for E4 and E1-DVCS.
Shortly, the problem seems to be that with some CLAS experiments (with targets upstream of the regular target position?), there is a clear pattern in the angular distribution of scattered electrons (other particles remain to be checked), in particular if one selects a tight momentum region (e.g., 2.0 GeV/c - 2.2 GeV/c). This pattern can (sometimes) be seen in theta-reconstructed as well as theta at the first DC superlayer and sometimes even for various projections of y or theta on the scintillator or EC plane.
This problem was originally encountered during the analysis of EG1a (see my message below for relevant links). Some additional plots can be found at
http://www.jlab.org/Hall-B/secure/eg1/jyun/ecyi.gifhttp://www.jlab.org/Hall-B/secure/eg1/alex/waves/oscillations.htmlhttp://www.jlab.org/Hall-B/secure/eg1/tforest/Wiggle/Wiggle.html (for explanation)
The most recent analysis on EG1b data is from Nevzat Guler - see
http://www.jlab.org/Hall-B/secure/eg1/EG2000/nevzat/ThetaWigglePage_wfid/thetaWiggle.html
Slava Tkachenko has looked at this for BoNuS - see
http://www.jlab.org/~slava/Check44/Check44_18/ThetaEL/0227/
It is not clear what exactly is happening, but the most likely explanation is that tracks are getting shifted out of the "valleys" into the "peaks", leading to wrong theta assignment by about 1-2 degrees. (It is less likely that the valleys are just inefficiencies). This may or may not affect inclusive data, but it might be useful to ascertain this for each experiment.
- Sebastian
Sebastian Kuhn wrote:
> Dear (former) EG1ers (and especially "oldtimers"),
>
> I am trying to remember what our final conclusion was regarding the
> infamous "wiggle/theta oscillation" problem with EG1a - see
> http://www.jlab.org/ccc/mail_archives/CLAS/eg1_run/msg00274.html to jog
> your memory. I went through all my accumulated notes but couldn't find
> any conclusive solution (and my memory is just not as good as it used to
> be... ;-)
> It appears possible that we have a similar problem in EG1b, see
> http://www.jlab.org/Hall-B/secure/eg1/EG2000/sharon/theta_compare/theta_compare_5_6.gif
> (and BoNuS, for that matter! ->
> http://www.jlab.org/~slava/Check44/Check44_18/ThetaEL/thetael_raw_0211_44_1_pre_el5gev.jpg
> ).
>
> Any recollections are welcome - Sebastian