CMSTB Notes

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Studies Students Can Perform

  • Shower Depth
  • Lateral Size
  • Beam Purity
  • Detector Resolution
  • Other studies devised by students


Shower Depth Study

Shower Depth studies allow students to discover how deeply into the calorimeter a particular type of particle deposits its energy. This deposition is often called a shower because its spatial pattern in the detector starts small and spreads out like a shower.

In the calorimeter, particles first pass through the electromagnetic calorimeter (Ecal) and then pass through the hadronic calorimeter (Hcal). Particles that deposit most of their energy in Ecal are said to have a lesser shower depth and those that deposit most of their energy in Hcal are said to have a greater shower depth. Electrons deposit most of their energy in the Ecal. Pions deposit most of their energy in the Hcal. When CMS begins taking colliding beam data with the LHC, particles other than electrons, pions and muons will deposit energy in the calorimeters. Electrons and photons will deposit most of their energy in Ecal while hadrons will deposit most of their energy in the Hcal. Muons pass through the calorimeter depositing little energy in either Ecal or Hcal.

Students should begin by studying one kind of particle at single energy (e.g. 50GeV electrons or 100 GeV pions). Then they should look at the same particle at different energies. The ratios of Ecal energy to Total energy and of Hcal energy to Total energy may be useful.		Image:MuStations.gif



Lateral Size Study

Lateral size studies allow students to discover how spatially spread out the energy deposition of a particle's shower is in the plane perpendicular to the particle's trajectory. In the diagram the green part of the detector is Ecal and HCal is in yellow. The left shower is an electron shower and the right shower is a photon shower. The left shower is from a charged hadron, like a pion, while the right shower is from a neutral particle shower, like a neutron. It seems that the left shower is more "spread out", has more lateral size, than the one on the right.Image:hcal_shower_smaller.jpg

The broadness of the shower depends on both the type and energy of the particle. Narrower showers should have higher ratios 1x1 Total energy vs 3x3 Total energy, while in broader showers this ratio should be lower. (The 3x3 is a grid of nine detector tiles, including the targeted (1x1) tile in its center.)

Students should begin by studying one kind of particle at a single energy (e.g. 50GeV electrons or 100 GeV pions). Then they should look at the same particle at different energies. We recommend that students do the shower depth study first. With that experience and their results from this study, students should be able to correlate shower depth with lateral shower size.

Beam Purity Study

Beam purity studies allow students to discover what fraction of a beam (in a test beam) is actually the type of particle requested. No beam is 100% pure. For instance, a pion beam may only be 95% pions. (The image above shows a mixture of several types of particles at different energies.) A very pure beam can be produced, but only at the expense of reducing the number of beam particles. The number of beam particles is called 'luminosity'. Decreased luminosity decreases the rate of collisions, which in turn lengthens the required data collection time.Image:Graph_3_small.gif

To discover the types of particles present in the beam, students should do a scatter plot of Ecal vs Hcal with Ogre. Students should begin by studying one kind of particle at a single energy (e.g. 50 GeV electrons or 100 GeV pions). Then they should look at the same particle at different energies. We recommend that students do shower depth and lateral shower studies first. With that experience and the results of this study, students should be able to estimate what percentage of the beam is the particle after which the beam is named.

Detector Resolution Study

Detector Resolution studies allow students to determine how precisely the calorimeter measures a particle's energy. As in the other studies, the students should begin by choosing a particular type of beam and energy (e.g. 50 GeV electrons or 100 GeV pions). Students can use OGRE to select data and generate a histogram of the Ecal energies for electron beams and Hcal energies for pion beams. The histogram should resemble a bell-shaped curve. Students can then measure the half-width of the curve at half of the curve's maximum height. This width will be in units of energy (GeV).Image:bell-curve.png


A typical result would be (100+15) GeV: 100GeV is the mean (average) value and 15GeV is the Half-Width-at-Half-Maximum (HWHM) measurement of precision. Another valuable (and maybe more familiar) indicator of precision is the RMS (root mean square deviation) of the distribution.

We recommend that students do shower depth, lateral size and beam purity studies first. With that experience and the results of this study, students should be able to explain how precisely the detector measures the energy of a particular particle beam.

Use Root Tools Extend Studies

OGRE is an interface for ROOT--an object-oriented data analysis framework--which simplifies the analysis for students: instead of writing code, students save time by checking boxes, pulling down menus, etc. When the students have completed all four of the basic studies using OGRE, they might be interested in learning to use ROOT. It is the same analysis program that the scientists use to analyze collision data. ROOT is free software and can be downloaded with all the tutorials from http://root.cern.ch. Image:root_logo.jpg


Presently I2U2 online analysis tools allow students to analyze Test Beam data in order to study how particles interact with the CMS calorimeter. These types of studies are the precursors to the calibrations of the calorimeters. In 2009, the LHC will provide the highest energy beam-beam collisions in the world (14 TeV). Through another e-Lab students will be able to select and analyze this data at the energy frontier.

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