Hello Holly,
 Here is what I did to create the field map.

I used the corrected field map calculations that I received
from Greg Kalicy. In order to scale them to the required beam 
energy one should use the scaling factor 0.22727  as explained in 
the note that you can  find here:

https://wiki.jlab.org/hps-run/images/c/c6/Chicane_settings.pdf

We were running at 1.056GeV so the nominal field was:

.22727*1.056  = 0.24

The lowest settings simulated were 3kG, so I had to scale down that 
file by:

.22727*1.056/.3003 = 0.7992

I also filled in the remaining three quadrants symmetrically, leading to the
final file:

125acm2_3kg_corrected_unfolded_scaled_0.7992.dat

Note that the values in this file are in the magnet coordinate system.
We translate into HPS coordinates in the compact.xml file, viz.

<field name="HPSDipoleFieldMap3D" type="FieldMap3D" zoffset="45.72*cm" yoffset="0.0*cm" xoffset="2.117*cm" filename="fieldmap/125acm2_3kg_corrected_unfolded_scaled_0.7992.dat"/>

I realize I should write this up formally, but I wanted to get you the information.
Let me know if you need any additional information.
Norman


-----Original Message-----
From: [log in to unmask] [mailto:[log in to unmask]] On Behalf Of Sho Uemura
Sent: Thursday, January 28, 2016 1:48 PM
To: Holly Vance
Cc: hps-software
Subject: Re: B-field for GBL re-fit

Yes, that code is meant to return the B-field at a point near the middle.

THe value of the field there is whatever is in the fieldmap - I know Norman generated the fieldmap by scaling the fieldmap he was given, but I don't know where he got the scale factor and it may not agree with the number you're using. Looking at the fieldmap file, the peak field there appears to be more like 0.240T.

On Wed, 27 Jan 2016, Holly Vance wrote:

> Hello,
>
> I've been calculating the momentum parameters separately offline and 
> comparing them to what we get with just the invariant mass from recon.
>
> Does anyone know exactly what B-field value is being used? I see the 
> call in TrackUtils:
> public static Hep3Vector getBField(Detector detector) {
>        return detector.getFieldMap().getField(new BasicHep3Vector(0., 
> 0., 500.0));
>    }
>
> So I assume this is the field value at the middle, but I wanted to check.
> Also do we expect this value to be the same as the one we obtained in mya?
> Does anyone know anything about this? I've been using 0.239T, but I 
> get something slightly different (~1 MeV on invariant mass).
>
> Thanks,
> Holly
>
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