Fermilab

Introduction to the Physics Goals of the CDF Experiment

We briefly describe some of the physics processes that CDF has searched for and studied. For more information please see
an introduction to the CDF experiment,
The Standard Model of High Energy Physics,
an introduction to the CDF Detector (along with short descriptions of the particles),
an introduction to the CDF Event Displays, and finally
additional comments on the CDF experiment.

Note : In order to better understand items discussed in this web page, it might be useful for the reader to look at:

The Wikipedia entry for The Standard Model of High Energy Physics, a review of the current status of theoretical High Energy Physics.

The goals of the CDF experiment can be summarized as follows:

  • to search for evidence of previously undiscovered particles or processes--such as the Higgs Boson, or physics beyond the Standard Model;
  • to measure and study the decays of known particles; and
  • to measure the production properties of known particles and jets.

Any deviation of the production and decay of known particles could point indirectly to new physics.

SEARCH FOR NEW PARTICLES OR PROCESSES

NEW PHYSICS--One of the most exciting things about detecting collisions in the world's highest energy collider is that new physics objects or processes could be produced at such a high rate that it might be possible to observe these above the background "noise". One such example is the 1994 discovery of the top quark by CDF and its sister detector D0, both at Fermilab. The top quark could not have been produced in any accelerator or collider up to then due to its high mass (a pair of top quarks "weigh" about 350 GeV -- roughly 400 times the mass of a proton!)

SOME POTENTIAL DISCOVERIES--CDF looked at a host of different signatures of new particles. One example is the search for for a mass bump--such as in the distribution of mass of two charged leptons (electrons or muons). The Z boson, at a mass of about 90 GeV, could decay into these final states. Or a virtual photon could be exchanged, producing a rapidly falling (vs. mass) distribution of di-leptons. Some theories predict the production of heavy objects which would be massive--many times heavier than the Z bosons--which could show up as a bump in the mass distribution. We have not seen any such bump other than the Z boson and we have set limits on the production of new particles up to a mass of order 1000 GeV for various models of potential new particles.

SUPERSYMMETRY --Another very exciting possibility is that of Supersymmetry, a theory that predicts that every known fundamental particle (quarks, leptons, gauge bosons) has a sister particle that differs from it by spin of 1/2. These Super-particles (given such exotic names as sleptons, stop, stau, selectron) could potentially be produced at the Tevatron collider (if the mass is low enough) and be discovered with the CDF detector.

MEASURE AND STUDY THE DECAYS OF KNOWN PARTICLES

Many of the known particles were measured only briefly prior to the Tevatron. For example, only a few tens of thousand W bosons (discovered in the 1980s) had been observed prior to the Tevatron. CDF and D0 studied 100 times more such events, which have been used to produce the world's most precise determination of the W boson mass.

DECAY DISTRIBUTIONS AND PROPERTIES OF RECENTLY FOUND PARTICLES--

Another important set of measurements are the decay distributions and properties of known particles at the Tevatron collider. Many of these measurements test our knowledge of physics theories, validating some models and invalidating others. For example, the decays of the top quark can be used to measure its mass and incisively test the Standard Model. The decays can also be used to search for exotic particles such as new charged Higgs bosons.

MEASURE AND STUDY THE PRODUCTION OF KNOWN PARTICLES AND JETS

DEVIATION FROM THE STANDARD MODEL--The rate and properties of the production and decay of known particles can be calculated within the Standard Model. Thus, an indirect way to find physics beyond the Standard Model is to measure these accurately, to check if they agree with the calculations.

STRUCTURE FUNCTIONS OF QUARKS AND GLUONS INSIDE A PROTON--An important measurement is the determination of "structure functions" of the proton. The proton consists of a cloud of quarks and gluons, and there is a probability distribution of these, which are called "structure functions". For example, we may be able to say that the probability of a u quark inside a proton that carries between 0.2 and 0.25 of the momentum of the proton is a certain percentage. Since there are two u quarks inside each proton, the area underneath the u "structure function" between 0.0 and 1.0 would be 2.
You might think that this is pretty esoteric stuff, of little practical use. However, these structure functions actually determine the likelihood of a "hard collision" of a certain mass, and thus would directly determine the probability of production of not only new physics objects, such as the Higgs boson, but also known physics processes, such as a pair of W bosons (a background to the Higgs boson search).

As another example of the importance of measurements of both production and decays, some particles have special properties that result in asymmetries or other unusual behavior predicted by theory. If these behaviors are not exactly as predicted, this would imply that either the theory is wrong (or mis-calculated), or that some new physics process is contributing to the observed events, thus causing the discrepancies.