[d2n-analysis-talk] Fwd: Couple of BigBite questions

[Diana Parno]

What follows is an email from Jin explaining some things about  
BigBite, its optics, its classes, etc, as well as Transversity's  
general approach to the BigBite optics. I think it's of fairly general  
interest. Enjoy!

Best,
Diana

Begin forwarded message:

> From: "Jin Huang" <jinhuang at jlab.org>
> Date: September 2, 2010 12:51:45 PM PDT
> To: "'Diana Parno'" <dparno at cmu.edu>
> Subject: RE: Couple of BigBite questions
> Reply-To: <jinhuang at jlab.org>
>
> Hi, Diana,
>
> There is no convention on definition of central ray on Bigbite.  
> Therefore, each people are free to define his/her own version. But,  
> once defined, the replay code and its interpretation have to follow  
> it.
>
> THaGoldenTrack is an obsolete code which select the first track of a  
> general spectrometer. Therefore, it will work with both Bigbite (no  
> matter which optics module mounted) and HRS. It's only useful for  
> online analysis to simplify multitrack drawing. However, in sense of  
> off line analysis, just forget about it, otherwise analysis could be  
> biased to first track.
>
> BTW, THaOpticsE06010 database is fit at 30Degree Transversity  
> setting. You might want to recalibrate/make sure it works for d2n too.
>
> Similar thing for THaPrimaryKine. It usually take track information  
> from THaGoldenTrack. Then it only process the first track. During  
> Transversity analysis, we built a code to work on replayed root file  
> and calculate kinematics based on raw spectrometer output, ex,  
> BB.tr.px[]. Then the kinematics variables, which are calculated for  
> each track, are merged back into root files. I do suggest you do  
> same for d2n. You can cross check the calculation code by comparing  
> first track kinematics to those generated by THaPrimaryKine.
>
> Spectrometer angle are supposed to be read into THaBigbite through  
> run database file, ie, db_run.dat. The code trace back to
> http://hallaweb.jlab.org/root/doc/html_v15/src/THaSpectrometer.cxx.html#gcCzfE
>
> However THaOpticsE06010 is special. It is not designed by Xin for  
> general purpose Hall A spectrometer but for Transversity setup only.  
> It overloaded the spectrometer angle through sieve_angle variable in  
> db_BB.optics.dat. As I understand, the 3rd variable is the angle in  
> rad. You might want talk to Xin for details. He used some of his own  
> definition in the code, for ex. th_tg and ph_tg is inversed relative  
> to HRS convension. But, again, since we interpreted the variables  
> accordingly, THaOpticsE06010 worked well for Transversity.
>
> Glad to know you’ve settled down at Seattle. As you can see, I have  
> been traveled a lot this summer. But I enjoyed it too  :-P
>
> Cheers,
>
> Jin
>
>
>
>
> -----Original Message-----
> From: Diana Parno [mailto:dparno at cmu.edu]
> Sent: Tuesday, August 31, 2010 2:40 PM
> To: jinhuang at jlab.org
> Subject: Couple of BigBite questions
>
> Hi Jin,
>
> I'm working through the BigBite kinematics and wondering if you can  
> help me understand a few things. I've attached a copy of a replay  
> file in case it's a help. Here are my questions:
>
> - Do you know how the BigBite central ray is defined? That is, is it  
> the angle between the center of the magnet; the center of the first  
> chamber; or what?
>
> - In the attached file, I mount the THaOpticsE06010 class. Do you  
> know if the THaGoldenTrack class uses this class, since it's the  
> mounted optics? Matt is under the impression the THaGoldenTrack  
> variables use some outdated optics class, but I'm not sure how that  
> could happen unless I'm missing something obvious in the code. If  
> THaGoldenTrack does use some other class, how can it be induced to  
> use THaOpticsE06010?
>
> - The THaPrimaryKine class seems to generate one number per entry in  
> the tree. Which track does it choose when there is more than one  
> track? I am under the impression, from some comments in the code,  
> that it uses the golden track, but I want to double check.
>
> - Do you know where the spectrometer angle is encoded for BigBite?
> (This is the angle of the central ray; it was 30 degrees during  
> Transversity and 45 degrees during d2n.) Somewhere in the optics or  
> spectrometer classes, this must come into play so that the momentum  
> vectors can be transformed into Hall A coordinates. I found some  
> (probably outdated) documentation implying that it's in db_run.dat;  
> I found a few lines about the chamber position in the  
> db_BB.optics.dat file, from which a spectrometer angle could be  
> extracted.
>
> Other than that, I've moved to Seattle and am mostly settled in,  
> although there are still way too many boxes! Today's the first real  
> rainy day in a while. I hope you're settled in back in Boston and  
> doing well there.
>
> Best,
> Diana

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