Finding the Beam Spot

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Detailed Review status

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The beam position needs to be measured precisely because of the following reasons:

  • Important input for physics e.g. b-tagging, lifetime, etc.
  • For patter recognition (input for HLT and offline).
  • Extraction of the tracking impact parameter resolution.
  • Quick check of global alignment.
  • Provide feedback to accelerator groups.
  • Beam monitor.

The algorithm used to calculated the transverse beam position is the so called d0-phi algorithm. This is a robust and fast $\raisebox{-1.3ex}\chi^2$ fit which just needs 1000 good tracks to reach micron precision. Using a Log-Likelihood fit, the beam width can be extracted using 20000 good tracks. A more detail description can be found in the CMS note # [].

Determination of beam postion using an EDAnalyzer

In this section, we describe how to calculate the beam spot parameters using good tracks. To reach a good precision (~ microns), it is necessary to loop over several events in the sample to collect at least 1000 good tracks.

1. Install a CMSSW release 1_2_x (see Setting up your Computing Environment) 2. Additional packages needed for 1_2_x:

cvs co -r V00-00-02 DataFormats/BeamSpot
cvs co -r V00-00-02 RecoVertex/BeamSpotProducer
3. Built the libraries:
scramv1 b
4. Run over some events. In this example, we run over a release validation sample of b jets
cd RecoVertex/BeamSpotProducer/test
cmsRun analyze_d0_phi.cfg
The relevant output is the following:
 calculating beam spot...
 we will use 7151 tracks.
            Calculated Beam Spot

   X0 = -0.000654478 +/- 6.67288e-05 [cm]
   Y0 = 0.00124222 +/- 6.50948e-05 [cm]
   Z0 = -0.153607 +/- 0.795389 [cm]
 Sigma Z0 = 4.7401 +/- 0.639149 [cm]
 dxdz = -0.000108076 +/- 1.30824e-05 [radians]
 dydz = 5.43768e-05 +/- 1.26999e-05 [radians]
 Beam Width = 0 +/- 0 [cm]
Note that the beam width is zero for the default calculation. To estimate the beam width, we need to run a Log-Likelihood fit over much more tracks and apply a tighten track quality criteria.

5. Track quality cuts can be modified via the framework, see the configuration file


Review status

Reviewer/Editor and Date (copy from screen) Comments
FranciscoYumiceva - 10 Feb 2007 first version of page

Responsible: FranciscoYumiceva
Last reviewed by: FranciscoYumiceva - 10 Feb 2007
Latex rendering error!! dvi file was not created.

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Topic revision: r5 - 2007-02-12 - FranciscoYumiceva

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