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HPS-SOFTWARE  July 2018

HPS-SOFTWARE July 2018

Subject:

Re: Track Momentum Oddity

From:

"Nelson, Timothy Knight" <[log in to unmask]>

Reply-To:

Software for the Heavy Photon Search Experiment <[log in to unmask]>

Date:

Tue, 3 Jul 2018 23:44:58 +0000

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (1 lines)

Hi Kyle,

I think the field map is in global coordinates, so you are stepping upwards vertically, and I’m guessing the origin is at the nominal target based upon the presence of a significant z component and less than full field in y.

As to the original problem, I’m very puzzled. I’m pretty sure Norman and Miriam would be tripping over this constantly if it were a typical problem.

I’m also not sure I understand all of the features in your MCParticle momentum distribution w.r.t. what is expected for tridents, but that seems beside the point. You might get more response to this on the Slack in the software channel.

Tim

> On Jul 3, 2018, at 4:29 PM, Kyle McCarty <[log in to unmask]> wrote:
>
> Hello Matt,
>
> I couldn't find that line anywhere when I run the code (does it only output for readout? recon?), but I did put in a print statement in the "detectorChanged(Detector)" method when I run my analysis. It outputs the values for "Detector.getFieldMap().getField(new double[] { 0, y, 0 })", where I vary the value of y from y = 0 to y = 1400 in steps of 25. It gives the following values:
>
> (0, 0, 0) >> ( -0.000, -0.431, -0.159)
> (0, 25, 0) >> ( -0.000, -0.431, -0.159)
> (0, 50, 0) >> ( -0.000, -0.431, -0.159)
>
> Everything thereafter is entirely 0. I'm not sure what units the argument vector expects, so I'm not sure whether this is reasonable or not. I can also check the log you suggested if you can direct me on where to find it.
>
> Thanks,
>
> Kyle
>
> On Tue, Jul 3, 2018 at 9:15 AM, Graham, Mathew Thomas <[log in to unmask]> wrote:
> Do you know what the value of the B-field is set to in the track finding code? There should be a line in the log files from this line of code (only By matters):
>
> LOGGER.config("fieldInTracker: Bx = " + fieldInTracker.x() + "; By = " + fieldInTracker.y() + "; Bz = " + fieldInTracker.z());
>
>
>> On Jul 3, 2018, at 12:39 AM, Kyle McCarty <[log in to unmask]> wrote:
>>
>> Hello hps-software,
>>
>> While performing hodoscope studies, Rafo and and I have stumbled on some odd output when attempting to obtain track momenta. I'm not sure if this is an issue with what I'm doing to obtain it, or if it's something odd happening with the momentum calculations. Note that this is done on the iss166 branch with the hodoscope detector, so it is also possible that something here is causing an issue. To see the issue, consider the momentum distribution for the initial MC particles for all events in the data set (pure tridents). We see the following:
>>
>> <MCParticleMomentum.png>
>>
>> This is reasonable for 2.3 GeV data. Now, consider the momentum of the observed tracks (using track state "0"):
>>
>> <ActualMomentum_X.png> <ActualMomentum_Y.png> <ActualMomentum_Z.png>
>>
>> We see that virtually all of the momentum is in the x-direction. This is reasonable - we expect most of the momentum to be in the direction of the beam line. However, we can see that the track momenta are dramatically lower than what we would expect, with almost nothing above 500 MeV.
>>
>> I checked to make sure this wasn't a field map issue, and verified that all steps in the process (SLIC, readout, and recon) used the correct field map (209acm2_5kg_corrected_unfolded_scaled_1.04545.dat), so this is not the issue. Rafo suggested manually calculating the momentum from the track parameters using the following method:
>>
>> final double aa = 3.e-4;
>> final double B_PSpec = 0.5242301;
>> final double a_Bz = aa*B_PSpec;
>>
>> double phi = track.getTrackStates().get(0).getPhi();
>> double omega = track.getTrackStates().get(0).getOmega();
>> double tanLambda = track.getTrackStates().get(0).getTanLambda();
>>
>> double magP = a_Bz / Math.abs(omega);
>>
>> double px = magP * Math.cos(phi);
>> double py = magP * Math.sin(phi);
>> double pz = magP * tanLambda;
>>
>> Using this method, we obtain a more reasonable total momentum distribution.
>>
>> <ActualMomentum_X_Fixed.png>
>>
>> This suggests to me that there might be something weird going on with the momentum calculation in HPS-Java (or at least this branch of it). Another sign that this may be the case can be observed by looking at the track state at the calorimeter. I obtain this with the following method suggested by Miriam:
>>
>> TrackUtils.getTrackStateAtECal(Track)
>>
>> When calling "Track.getMomentum()", I see two classes of outcome. One where the momentum is absurdly high, and one where it is absurdly low. Some examples from actual data are included below, with the format "EcalState(px, py, pz) << >> State0(px, py, pz)":
>>
>> ( 0.004, 0.001, -0.000) << >> (0.315, 0.007, -0.005)
>> ( 0.003, -0.001, -0.000) << >> (0.208, 0.011, -0.006)
>> ( 0.004, 0.001, 0.000) << >> (0.285, 0.006, 0.006)
>>
>> (458.765, -133.711, 9.243) << >> (0.160, 0.005, 0.003)
>> (453.831, 149.604, - 9.234) << >> (0.174, 0.005, -0.003)
>> (447.438, 167.760, 11.122) << >> (0.149, 0.003, 0.004)
>> (459.146, -132.398, -18.623) << >> (0.165, 0.004, -0.006)
>>
>> Clearly, something quite odd is going on, since it is completely unreasonable for a track to have the greater part of 500 GeV momentum in a 2.3 GeV beam. Likewise, I would not really expect us to see tracks with nearly no momentum at all. This suggests to me that the track state calculations for states other than "track state 0" (the target?) may be incorrect.
>>
>> I'm hoping someone more knowledgeable on the SVT side of things can give some insight into what is going on here, and correct me if I'm doing something wrong. Also, if there are any tests I can do or files that would be useful to have access to, let me know.
>>
>> Thanks,
>>
>> Kyle
>>
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>
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