Finding the Beam Spot

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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 a set of good tracks. To reach a precision in the order of microns, it is necessary to loop over several events in a sample to collect at least 1000 good tracks.

1. Install a CMSSW release 1_2_x. Please 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 QCD jets
cd RecoVertex/BeamSpotProducer/test
cmsRun analyze_d0_phi.cfg
The relevant output is the following:
 calculating beam spot...
 we will use 13922 good tracks out of 53731
 number of tracks used: 8349
 total number of iterations = 19
BSFitter: default fit does not extract beam width, assigning a width of zero.
            Calculated Beam Spot

   X0 = -4.33631e-05 +/- 6.49699e-05 [cm]
   Y0 = -7.2154e-05 +/- 6.42539e-05 [cm]
   Z0 = -0.178506 +/- 0.66625 [cm]
 Sigma Z0 = 5.09478 +/- 0.589555 [cm]
 dxdz = 1.56378e-05 +/- 1.26866e-05 [radians]
 dydz = 7.92095e-06 +/- 1.22361e-05 [radians]
 Beam Width = 0 +/- 0 [cm]

This sample was generated at (0,0,0) with no crossing-angles, a RMS bunch length in Z of 5.5 cm and the average transverse beam width is of 15 microns. The beam width can be extracted using the log likelihood fit implemented in this package which requires more tracks and a tighten track quality criteria.

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


Beam spot data format and dummy beam spot

You can access the beam spot parameters through the data format class reco::BeamSpot

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: r9 - 2007-03-29 - FranciscoYumiceva

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