[BeamSpot] fit 1D projections of PV distribution to get initial values for 3D fit parameters

[rmanzoni]

We found that having the initial beam spot position parameters of the fit to the 3D PV distribution hardcoded can lead to instabilities and non convergence problems.

https://hypernews.cern.ch/HyperNews/CMS/get/beamspot/135.html

We dynamically set the 3D fit initial BS position to correspond to the BS position as measured by means of 1D fit to the X, Y and Z projections (these fits were run even before, but not used).

This improves the fit stability.

[cmsbuild]

A new Pull Request was created by @rmanzoni (Riccardo Manzoni) for CMSSW_7_6_X.

[BeamSpot] fit 1D projections of PV distribution to get initial values for 3D fit parameters

It involves the following packages:

RecoVertex/BeamSpotProducer

@diguida, @cvuosalo, @cerminar, @cmsbuild, @slava77, @mmusich can you please review it and eventually sign? Thanks.
@makortel, @GiacomoSguazzoni, @rovere, @VinInn, @cerati, @dgulhan this is something you requested to watch as well.
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[slava77]

@cmsbuild please test

[cmsbuild]

The tests are being triggered in jenkins.

[cmsbuild]

+1
Tested at: 345e8eb
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-10780/7322/summary.html

[cmsbuild]

Comparison is ready
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-10780/7322/summary.html

[cvuosalo]

+1

For #10780 345e8eb

Changing the initial values of the primary vertex fit.

The code changes are satisfactory. Jenkins tests against baseline CMSSW_7_6_X_2015-08-14-1100 show very small changes in PV fit results, but none of these differences are significant. An example from workflow 4.53_RunPhoton2012B is shown below.

[diguida]

+1
for 345e8eb
It fixes instabilities and non-convergence problems of the fit to the 3D PV distribution: now the 3D fit initial positions are not hardcoded, but dynamically set to correspond to the centroid position as measured by means of 1D gaussian fit to the X Y and Z projections.

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next CMSSW_7_6_X IBs (tests are also fine). This pull request requires discussion in the ORP meeting before it's merged. @davidlange6, @Degano, @smuzaffar

[davidlange6]

+1