INTRODUCTION TO PHYSICS GOALS OF THE CDF EXPERIMENT

Draft, version 0.92, John Yoh, 1/1/05

We briefly describe some of the Physics Processes that CDF is searching for and studying. Please see
for an introduction to the CDF experiment
The Standard Model for High Energy Physics
for an introduction to the Physic processes being studied by the CDF experiment
for an introduction to the CDF Detector along with short descriptions of the particles we measure
for an introduction to the CDF Event Displays
for 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 Standard Model for High Energy Physics, a brief review of the current status of theoretical High Energy Physics, and for additional comments on the CDF experiment --the Discovery of the Top and Bottom quarks at Fermilab, A discussion of the energy scale of HEP Physics, How do we determine that we have found something new, etc. --these will clarify issues discussed in this web page.

At CDF our goals is to

SEARCHING FOR NEW PARTICLES OR PROCESSES

NEW PHYSICS --One of the most exciting thing about having a detector 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 such above the background "noise". One such example is the discovery of the Top quark 10 years ago by CDF and its sister detector D0 both at Fermilab- which could not have been produced in any accelerator or collider up to now due to the heavy mass of Top quarks (A pair of Top quarks "weight" about 350 GeV!!!--roughly 400 times the mass of a proton)

SOME POTENTIAL DISCOVERIES--CDF are looking a a host of different signatures of New particles. One example is looking for a Mass bump--such as in the distribution of Mass of 2 leptons (electrons or muons). While the Z boson, at a mass of about 90 GeV, could decay into these final states, there are other known Processes (referred to as the Drell-Yan process, or virtual photon) which will produce arapidly falling (vs. Mass) distribution of di-leptons. Figure 1 shows such an observed distribution of electron pairs.
POTENTIAL NEW OBJECTS DECAYING INTO HIGH MASS DI-LEPTONS Some theories would predict the production of heavy objects (such as .....) which would be massive--many times heavier than the Z bosons- which could show up as a bump in the mass distribution. So far, we have not seen any such bump other than the Z boson--and we have set limits on the production of these up to several hundreds of GeV for various model of such potential new particles. With the forthcoming Run 2b (2005-2009), we hope to see 20 times more "luminosity", which would made us sensitive to even higher mass---wish us luck.
SUPERSYMMETRY --Another very exciting possibility is that of Supersymmetry- a theory that predicts that every known fundamental particle (quark, leptons, gauge bosons) have a sister particle that differs from it by spin of 1/2. These Super-particles (already given such exotic names a sleptons, stop, stau, selectron, etc.) could potentially be produced at the Tevatron collider (if the mass is light enough) and could be discovered here.

HIGGS PARTICLE --One of the most important "missing piece" in the current theory is that of the Higgs particle, the particle whose coupling with quarks give them mass !!! (Leon Lederman calls it the "God particle").

MEASURE AND STUDY THE DECAYS OF KNOWN PARTICLES

Many of the known particles have only been measured briefly --for example, the W bosons discovered 20 years ago--only a total of a few 10s of thousand examples of these have been observed. CDF (and D0) will eventually study almost 10 times more such events.

DECAY DISTRIBUTIONS AND PROPERTIES OF RECENTLY FOUND PARTICLES--

Another important set of measurements are the production rate, distribution, and properties of know particles at the Tevatron collider. many of these measurements test out knowledge of physics theory, perhaps validating some models and invaldiating others. These measurements also let us understand what's inside a proton (the "structure functions" which give the probability of how often we will find a certain parton (quark or gluon) of a particular energy inside a fast-moving proton).

MEASURE AND STUDY THE PRODUCTION OF KNOWN PARTICLES AND JETS

DEVIATION FROM STANDARD MODEL--The rate and properties of the production and decays of Known particles can be calculated through 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.
For example,

STRUCTURE FUNCTIONS OF QUARK/GLUON 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 distruction 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 .2 and .25 of the momentum of the proton is xxx %. Since there are 2 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, but also old physics processes, such as a pairs of W bosons (which might be background for the Higgs search).

As a example of the importance of such measurement of both production and decays, some of the particles has very special properties that would result in asymmetries or other perhaps unusual behavior--predicted by theory. In many cases, the rate of a particular decay is predicted to a high degree of accuracy 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 processes is in play which contributed to the observed events, thus leading to the discrepancies.

[example of possible deviation for theoretical predictions]