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Are those tracks?

A blog post from Quantum Diaries by Anna Phan, published on Sunday, May 1st, 2011

Today’s post is a continuation of my description of the LHCb detector. From my other post on identifying vertices using the VELO, we naturally progress to tracking charged particles. As I mentioned in my first post, the VELO along with the TT, T1, T2, and T3 stations are used to reconstruct particle tracks inside LHCb.


Particle tracking is somewhat akin to animal tracking. The first thing you need is some material where particle tracks will leave a trace. It is very hard to find animal tracks on concrete, but very easy on sand or snow…

This where the VELO along with the TT, T1, T2 and T3 come in. When charged particles pass though these detector components, they leave hits. Two different types of technology are used to measure particle interactions. The VELO, TT and the inner sections of the T stations are made of layers of silicon strips while the outer sections of the T stations consist of straw tubes filled with a mixture of argon and carbon dioxide gas. The layout of the TT and T stations is shown below. The silicon sections are coloured purple, while the drift tube sections are coloured blue.

Depending on which detector components register hits, tracks can be classified into four different groups:

  1. Long tracks which pass through all parts of the tracking system, from the VELO, through the TT to the outer T stations;
  2. Upstream tracks which only pass through the VELO and TT stations;
  3. Downstream tracks which only pass though the TT and T stations;
  4. VELO tracks which only pass through the VELO; and
  5. T tracks which only pass through the T stations.

Each of these track types is shown in the image below:

All of these types of tracks are useful for reconstructing B meson events. An example of a reconstructed event is displayed below. The average number of successfully reconstructed tracks in fully simulated B meson events is about 72, which are distributed among the track types as follows: 26 long tracks, 11 upstream tracks, 4 downstream tracks, 26 VELO tracks and 5 T tracks.

You may notice that in the images above, the tracks are curved. This is due to the LHCb dipole magnet. The experiment contains what essentially is a very large horseshoe magnet, which produces a field of 4 Tesla between its two large coils. Particles normally travel in straight lines, but in a magnetic field the paths of charged particles curve, with positive and negative particles moving in opposite directions.

So that’s how we measure particle tracks in LHCb and the types of tracks we record. Stay tuned for how we figure out what type of particle left which track…

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