[G8b_run] Tagger Sag
Franz Klein
fklein at jlab.org
Sat Aug 22 11:43:02 EDT 2009
Chuck and Volker,
I am very glad that Chuck did this study, I assume using a 3C fit (i.e.
reaction p pi +pi- and no constraint on photon energy). Mike Williams got
his fitter 'tweaked' for *real* CLAS data by simple factors to the
existing tracking covariance matrix (which misses variation of energy loss
and a correct statistical estimate of multiple scattering: I only added
averaged, momentum dependent values from simulation studies because the
code is not using Runge-Kutta but helix propagation (with calulation of
all associated errors).
"In a non-perfect world like CLAS" the kin.fitter is only as good as
*average* errors are tweaked, which should not vary strongly between run
periods (except different field settings, DC alignment and calibration,
energy loss in target area).
In most kin.fitters I've seen photon energy has not been correctly taken
into consideration in the sense that (for finite width of tagger
counters, for CLAS ~0.001*Ebeam*fac(Eg), where fac(Eg) depends on beam
position, emittance and divergence) the energy is not Gaussian but square
distribution (its error for the assumed normal distribution would be
width/sqrt(12)). Differences ("Etrue"-Emeas) smaller than half the width
of the counters are not meaningful ("Etrue" is the fit estimate of the
*unknown* correct energy).
Sorry for the long introduction!
Now to Mike Dugger's questions/suggestions:
How I understand it, he wants you to do some cuts on M(x) in gp->p pi+pi-X
before doing the kinematic fit (at some point similar to the CMU approach:
do some (loose) cuts on a topology and let the fitter do the rest). It
would be nice to see not only the variation of the mean value but also
the width (PAR[2]) for the slices.
For an assumed 'good' knowledge of the errrors the cut on prob() (or CL)
shouldn't be 10% but more of order 1% (or 0.1% but I guess the prob()
distribution isn't flat below 1-2%(?)).
Besides, what is cut for the photon pull 'Sit04'? Is it 0.001=0.1% or
0.001%=10^-5? Can you give us the 2nd (variance) and 3rd moment
(asymmetry) of the distribution?
Thanks
Franz
On Fri, 21 Aug 2009, Volker Crede wrote:
> Barry, Mike,
>
> I think Chuck has shared his corrections already with the run group, but
> probably did not communicate things properly.
>
> It's not easy to tweak the pulls such that they all have mean zero and
> width one, but I agree it should be the goal. As a matter of fact, the
> whole pull and covariance matrix business is very difficult. Chuck used
> the proton pulls (all three of them) to determine the momentum corrections
> and this also improved the photon energy pull substantially. We never
> touched the energy directly, but you can see the improvement in the plots
> he sent out. We have discovered that the pulls are extremely sensitive to
> changes in the initial 4-vecs. A proton momentum change of less than 1%
> can sometimes move the pull mean from positive to negative. On the other
> hand, mean and width can be studied and adjusted fairly independently. The
> width is mainly based on the error of a variable (and thus determined by
> the covariance matrix), while the mean is dominated by the value of the
> variable itself, more or less independent of its error.
>
> Mike, a cut on the tails of the pull distribution is indeed equivalent to
> a confidence level cut. The difference we are looking at here is just a
> pull that is not properly normalized. However, I agree we should study the
> dependence of CL cuts on the tagger sag. My prediction is that the result
> remains more or less unchanged.
>
> With regard to the four suggestions:
>
> i) Forming "mass X^2" from the reaction "gamma p -> p pi+ pi- X" will not
> work when you use the kinematic fitter. In fact, you need constraints. The
> hypothesis "g p -> p pi+ pi-" is nothing else but energy and momentum
> conservation and requires "X = 0" for all events. All 4-vecs are tweaked
> such that "X = 0" and then we check what CL value we get. If the "X = 0"
> requirement cannot be met, then the fit does not converge and the event is
> not kinematically fitted. To cut a long story short, the "mass X^2" plot
> is a spike.
>
> ii) Well, X = 0 ...
>
> iii) This is a good point, but we need to apply the CL cut; otherwise the
> events may not be fitted. But, we can study the Gaussian 'mean' dependence
> on the CL cut.
>
> Best wishes
>
> Volker
>
>
>
> On Fri, 21 Aug 2009, Barry Ritchie wrote:
>
>> Chuck, these are all good suggestions from Mike. Would you please:
>>
>> 1. Share your momentum corrections with Mike ASAP?
>> 2. Try the four mini-tests he suggests to firm up your results.
>> 3. Things need to be tweaked until the pulls have mean zero and standard
>> deviation 1. Until then, there's still work to do.
>>
>> ---BGR
>>
>> Professor Barry G. Ritchie
>> Department of Physics
>> Arizona State University
>> Tempe, AZ 85287-1504
>>
>> Telephone: (480) 965-4707
>> Fax: (480) 965-7954
>>
>> -----Original Message-----
>> From: g8b_run-bounces at jlab.org [mailto:g8b_run-bounces at jlab.org] On
>> Behalf Of Michael Dugger
>> Sent: Friday, August 21, 2009 3:20 PM
>> To: Charles Hanretty
>> Cc: g8b_run at jlab.org
>> Subject: Re: [G8b_run] Tagger Sag
>>
>>
>> Chuck and g8b,
>>
>> Thanks for all your work.
>>
>> The problem we face is that there has been no real agreement between
>> Chuck, Stuart, or me :(
>>
>> We need some sort of convergence and then everyone needs to use the same
>>
>> momentum and energy corrections.
>>
>> My feeling is that a kinematic fitter is most likely to give acceptable
>> results for determination of the corrections needed.
>>
>> I would like to have Chucks momentum correction and see if that helps me
>>
>> get a better agreement with him on the energy correction.
>>
>> One comment:
>> I do not think you can make this statement->
>> "The projection of this plot onto the y-axis is indeed a Gaussian as the
>>
>> confidence level cut only cuts off the tails of the distribution and
>> does
>> not affect the peak position."
>> If this were true, wouldn't a cut on the tails of the distribution be
>> equivalent to a confidence level cut? Did you test to see if the center
>> changed with change in confidence level cut?
>>
>> Some questions:
>>
>> * The title on the plot
>> http://hadron.physics.fsu.edu/~hanretty/Files/TaggerSag/photonPull_sit04
>> _WithMomCorrs.gif
>> says conf cut at 0.001%. Is this true?
>>
>> * If you correct the photon energies do you get a pull at zero? What is
>> the standard deviation of the pull? It looks different than 1. Don't you
>>
>> need to have all of the pulls close to having a standard deviation = 1.
>> Isn't the kinematic fitter very sensitive to having the correct
>> covariance matrix?
>>
>> Chuck, in the name of comparing apples to apples:
>>
>> * Can you do the same analysis using only amorphous data? It should not
>> change the results, but then we would have the same data in common.
>>
>> * Can you
>> i) form massX^2 from the reaction
>>
>> gamma p -> p pi+ pi- X
>>
>> ii) restrict events to be
>>
>> -.01 GeV^2 < massX^2 < 0.00385 GeV^2
>>
>> iii) turn off confidence level cut.
>>
>> iv) see if your results remain unchanged
>>
>> If the result remains the same (and it should), than wouldn't this prove
>>
>> that the confidence level cut is not forcing the energy correction to be
>>
>> small?
>>
>> Please correct me if I made any stupid comments or silly questions. We
>> need to get this resolved so that we move forward.
>>
>> Thanks for your time.
>>
>> Sincerely,
>> Michael
>>
>>
>>
>> On Fri, 21 Aug 2009, Charles Hanretty wrote:
>>
>>> Hello All,
>>> During our last g8b meeting Mike showed some plots regarding the
>> tagger
>>> sag. I was asked to recreate this plot to verify what Mike saw. I've
>> been
>>> working on this for the past few days and have some plots to share. I
>> have a
>>> few more plots than what Mike showed I did this to verify my
>> verification. :)
>>>
>>> My process:
>>>
>>> STEP 1) Make 2 plots by running over one full run for each coherent
>> edge energy
>>> (a total of 121 data files).
>>>
>>>
>> http://hadron.physics.fsu.edu/~hanretty/Files/TaggerSag/EtrueDivEmeas_v_
>> Emeas_ForSlicing.gif
>>>
>> http://hadron.physics.fsu.edu/~hanretty/Files/TaggerSag/EtrueMinusEmeas_
>> v_Emeas_ForSlicing.gif
>>>
>>> Both of these histos are TH2F's, produced by running over the data
>> files. I
>>> defined Etrue as the photon energy coming out of the kinematic fitter
>> and Emeas
>>> as the photon energy as taken from either the GPID or TAGR bank. For
>> the plot
>>> to be filled with these values, the event must be a ppippim final
>> state
>>> and pass a fit to no missing particle (energy & momentum conservation)
>>> with a confidence level of >10%.
>>> EtrueDivEmeas_v_Emeas_ForSlicing: x-axis-> 0.8-5 GeV, 168 bins
>>> y-axis-> 0.998-1.002, 40 bins
>>> EtrueMinusEmeas_v_Emeas_ForSlicing: x-axis->0.8-5 GeV, 168 bins
>>> y-axis->-0.0024-0.0024, 40 bins
>>>
>>> STEP 2) Use the FitSlicesY() function to slice the *_ForSlicing histos
>>> (above) along the x-axis, one slice per bin, and fit each slice to a
>> gaussian. This
>>> function also makes a histogram containing the mean values of the
>> gaussian
>>> fits, bin by bin:
>>>
>>>
>> http://hadron.physics.fsu.edu/~hanretty/Files/TaggerSag/EtrueDivEmeas_v_
>> Emeas_SlicedFitMeanValues.gif
>>>
>> http://hadron.physics.fsu.edu/~hanretty/Files/TaggerSag/EtrueMinusEmeas_
>> v_Emeas_SlicedFitMeanValues.gif
>>>
>>> The structure that Mike showed in the meeting is clearly seen
>> again
>>> in the *_SlicedMeanFitValues.gif plots (good!). However, as you all
>> have
>>> probably already noticed, my y-scale is much much smaller. This goes
>> back
>>> to the original histograms (the ones made in STEP 1->
>> *_ForSlicing.gif).
>>> I first tried using the exact same scale and binning as Mike and I got
>> this:
>>>
>> http://hadron.physics.fsu.edu/~hanretty/Files/TaggerSag/EtrueMinusEmeas_
>> v_Emeas_MikeRange.gif
>>>
>>> You'll notice that the plot looks like a strip with sharp edges.
>> These
>>> edges are a result of my use of a confidence level cut, if I were to
>> not
>>> use a fitter, I would have a larger spread like what Mike has (this
>>> confidence level cut only allows for events where Etrue and Emeas are
>>> close). Therefore I had to "zoom in" on the y-axis in order carry this
>> process
>>> out and the result of this "zooming" is the
>> EtrueMinusEmeas_v_Emeas_ForSlicing.gif
>>> plot. The projection of this plot onto the y-axis is indeed a
>> Gaussian as the
>>> confidence level cut only cuts off the tails of the distribution and
>> does not
>>> affect the peak position.
>>>
>>> I am assuming that Mike is using ELoss and Stuart's momentum
>> corrections
>>> when he generates these plots. Since Eloss is the same for everyone
>> and I have
>>> my own momentum corrections (that I made using the kinematic fitter,
>> fitting to
>>> a ppippim() final state), I decided to produce the
>>> *_SlicedMeanFitValues.gif plots with the inclusion and exclusion of
>> (my)
>>> momentum corrections to see the effect these corrections have (also to
>> cover
>>> all my bases). The two distributions you see on these plots are with
>> and
>>> w/o momentum corrections (the upper is without, the lower is with).
>> The use of
>>> momentum corrections does not affect the scale of the y-axis, but only
>>> moves the distribution closer to zero (for subtractions) or closer to
>> one
>>> (for the division).
>>>
>>> Photon Pulls for a fit to gamma p -> p pip pim ()
>>> with MomCorrs:
>>>
>> http://hadron.physics.fsu.edu/~hanretty/Files/TaggerSag/photonPull_sit04
>> _WithMomCorrs.gif
>>> without MomCorrs:
>>>
>> http://hadron.physics.fsu.edu/~hanretty/Files/TaggerSag/photonPull_sit04
>> _NoMomCorrs.gif
>>>
>>> Although these pulls are not at zero, they are symmetric. The
>> shifts from
>>> zero are indicative of a systematic error and improve once I include
>> my
>>> momentum corrections. If you look at the effect my momCorrs have on
>> the
>>> *_SlicedMeanFitValues plots, you see that they only shift the
>> distribution and
>>> have no real effect on the y-axis. I would argue that if I were to
>> get all of
>>> my pulls to exactly zero (an "in a perfect world" senario), then the
>>> distribution would only shift down some more but the general shape
>> would
>>> be unaffected.
>>> Reminder: A pull is the difference between the true value (the value
>> from the
>>> fitter) and the measured value (the value found in the data)
>> normalized
>>> to the error of that particular measurement. A pull centered at zero
>>> indicates that the systematic errors for that particular variable are
>>> negligible.
>>>
>>> To summarize: I have seen the same structure in my plots that
>> Mike showed
>>> at our last g8b meeting using two appraoches. Where he and I differ
>> is in the
>>> scale of the y-axis. It seems that this difference in scale
>>> arises from my use of a kinematic fitter and also a cut on the
>> confidence level
>>> for the fit whereas Mike used some iterative routine (not saying that
>> Mike's
>>> routine is garbage). If I were to not use a fitter then I would
>>> have a spread in my *_ForSlicing plots similar to that seen in Mike's
>> plots.
>>> My use of a fitter forces me to "zoom in" on the y-axis if I ever hope
>> to slice
>>> the histogram and fit the slices. This shrinking of scale carries
>> through to
>>> the *_SlicedFitMeanValues plots and forces the amplitude of the
>> distribution
>>> shape to be much smaller. The important thing is I see the same
>> shape.
>>>
>>> -Chuck
>>> _______________________________________________
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>>>
>>
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>
> --
> --------------------------------
>
> Volker Crede
> Assistant Professor
> Florida State University
> Department of Physics
> 206 Keen Building
> Tallahassee, FL 32306 USA
>
> Phone: 001 850 644-2423
> Fax: 001 850 644-4478
> crede at fsu.edu
>
> _______________________________________________
> G8b_run mailing list
> G8b_run at jlab.org
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>
===============================================================
Franz J. Klein, Associate Professor
CUA, Department of Physics
Washington, DC 20064
office: Hannan Hall 206 phone: 202-319-6190
or: Jefferson Lab,CC F-243 phone: 757-269-6671
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