Finding the Beam Spot

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Introduction

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 V00-00-03 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.
 DEFAULT:
-----------------------------------------------------
            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

data/d0_phi_analyzer.cff

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: r10 - 2007-03-29 - FranciscoYumiceva



 
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