Beginner's Guide Page 3 Page 5
Okay, you surely tried to find a particle in this form If you want to see, what you should have found, look into the solution...
If you had done this a couple of decades earlier, you would have received the Nobel Prize by now!

`Unstable' Particles
The particle you just found, is not directly visible in an HEP detector. but since you can detect the decay products, we still prove its existence and can measure its properties, like the charge:
Since the total charge is conserved in the decay process, and the outcoming particles af a total charge of 0 (-1 + 1 = 0), the ``mother particle'' was uncharged.

With this kind of logic you can infere a lot of information about the reactions taking place inside of your detector. Finding ``unstable particles'' is the main job of a physicist in particle physics, and to extract all possible information about it is a highly needed skill in this field.

Finding showers and the neutral Pi
Let's use the crystal calorimeter for finding photons.
This form will give you the opportunity to do this. The basic idea is, to select ``showers'' in the detector that are not caused by impacting charged particles. A shower is a set of neighbouring crystals, that measured a light flash in the same event. You already saw an event picture with lot of showers. Again, there is a CLEO software package that matches tracks projected into the CC with showers. All you have to do is to cut away all showers that have a match flag set.

A particle, called Pi, comes in three ``kinds'', charged (positive and negative) and neutral. The neutral type is decaying nearly always into two photons. As before the momentum of the Pi can be reconstructed from those two photons.
This is now your new assignment! Try to find the mass of the neutral Pi using this form. Hint: This time the ``bump'' is much broader than before!

  1. Try first a histogramm between 0 and 4 GeV for the mass.
  2. You will huge number of entries at low masses
    Switch on the cut to suppress showers from tracks, what changes?
  3. Zoom into the mass region around 0.150 GeV. Do you notice something?
  4. The minimum energy for a photon is normally set to 0.030 GeV, to suppress noise faking a photon. This number drastically changes the number of noise entries in your plot. Increase this energy cut and observe the ``bump''.
  5. Since the energies of the two photons are very likely asymmetric it is more useful to cut in the total momentum of the combined two photon vector. With this cut you ``clean up'' the picture very easily. Try a minimum momentum of about 1 GeV...

Click The Time Of Flight counters (TOF)

If you read the 4-vector explanation you saw an equation relating the speed of a particle and its energy and momentum. This can be used to measure the mass of a particle! The only thing you need, is a measurement of the momentum and a speed measurement.
Well, we already know, that the DR gives us the momentum of each charged particle. We only need the speed of each particle. This can find that by measuring, how long it took the particle to travel a known distance, since speed is defined as the ratio of the distance it traveled and the time it took.

Therefore CLEO has the drift chamber and the crystal calorimeter.

shows the very first event picture again. You can see small rectangles at the outside of the drift chamber and tracks pointing to them. These rectangles are ``scintillation counters'' that can very precisely measure time. How long does it take a typical particle to travel the distance of, say 1 meter? If it would travel with the read this.

After we know the momentum of a particle, we can predict the speed for each possible particle candidate and compare it with the TOF data. Since all measurements have a non zero resolution, a phyicists ``scales'' the importance of a deviation from the expected value by the known resolution of the data. The resolution is mostly defined by the shape of the distribution of many measurments. You can read more about that in here. This shape gives you information about how many measurements of 100 will be further away from the expected value with more than a certain difference. If outside of this region are 32 of 100 data points, this difference is called ``one sigma''. This is the scale that is used to weigh the actually measured difference for a certain track.

At CLEO three more kinds of particles besides electrons and muons are identified using TOF data: pions, kaons, and protons.
This means, there is a difference measurement for each kind of particle, expressed in units of sigma. In CLEO ``slang'' they are called SGELTF, SGMUTF, SGPITF, SGKATF, and SGPRTF, ``TF'' denoting that they belong to the TOF system.

There is a neutral particle, called ``K short'', (short means ``short lifetime''...) that decays into a charged pion pair. Try to identify pions, and search for the mass of this particle!
We prepared this form for you to do this.

  1. Plot a histogram showing the pion hypothesis value for TOF. Remember, it is normalized so that sigma is at 1.0.
  2. Cut this value, so that most of the candidate close to 0 survive. Plot the Kaon hypothesis value.
  3. Cut away the ambiguities, by removing ``good kaon'' candidates. (the number of tracks cut away should be small...)
  4. Look into the invariant mass of the two pions. What do you see at small masses?
Beginner's Guide Page 3 Page 5
CLEO WEB PAGES
Updated: 23. January 1996
 Author: Andreas H. Wolf (ahw@mps.ohio-state.edu)