Additional Informantion on the CDF Experiment
We provide here some additional intersting information on the CDF experiment and on High Energy (Particle) Physics. Please also see:
- an introduction to the CDF experiment
- an introduction to the Physics processes being studied by the CDF experiment
- an introduction to the CDF Detector along with short descriptions of the particles we measure
- an introduction to the CDF Event Displays
In this note, we provide some additional information regarding the CDF experiment and related topics, beyond what is being provided in the above introduction web pages. Here is a list of these topics
- A Short History of CDF
- A Short History of the Top Quark Discovery
- A Short History of the Bottom Quark Discovery
- Signal Vs. Background, how Physicists Convince Themselves and the World That They Discover Somthing
- What Is a GeV?
A Short History of CDF
The CDF experiment and the Fermilab Tevatron Collider was first envisaged in the mid 1970's. At that time, Fermilab's accelerator, called the "Main Ring", is the highest energy accelerator in the world, providing a proton beam with energy of 200 and eventually upgraded to 400 GeV. It was a single ring of magnets which make the protons go around in a circle with circumference of 4 miles. The Accelerator used RF cavities to "kick" the proton everytime it passes the cavities, which is about 50,000 times per second.
Professors Carlo Rubbia of Harvard and Peter McIntyre of (?? Univ. of Texas) proposed that the Main Ring be converted into a collider, with beams of protons and anti-protons going in opposite directions in the same ring !!! This is possible because the antiproton, with negative charge, would be bend by the magnets in the opposite direction as the protons, and thus will stay in the same ring AS LONG AS IT'S GOING IN THE OPPOSITE DIRECTION as the proton !!!
Of course, there are many problems with this proposal -- how to generate antiprotons, and how to make sure that the protons and antiporons stay in the ring of magnets for many hours (as a accelerator, the protons need to stay in the ring for only about 10 seconds -- so, if we loses 1% of the beam per second of circulation, it's not a big deal for an accelerator -- but that level of losses would not be tolerated for a collider).
Bob Wilson, the founding director of Fermilab, decided that Fermilab is not ready for this step at that point. There are a lot of physics to be done with the Main Ring (where the Bottom quark was discovered in 1977), and there was plan for a second ring of superconducting magnets (eventually call the Tevatron) which will be ready in the Early 80's, and which will be more suitable for colliding beams (the magnet quality would be far better than the main ring).
So, Rubbia and colleagues instead went to the CERN, Geneva Lab (the European collaboration lab for HEP physics) and propose that they convert the machine there, the SPS (which was similar to the main ring, though capable of a lower energy of 270 GeV), into a collider. This was done in the early 80's, with 2 detectors there -- UA1 (proposed and lead by Rubbia) and UA2. Within months of first data, they were able to discover the W and Z bosons -- Rubbia and Van der Meer (whose technical innovation enable the development of an intense anti-proton beam, and thus provided enough collisions for enough W's and Z's to show up) won the Nobel price for that discovery. Note that the tradition of 2 similar (though different in detail) detectors at one collider is a long one. If a major discovery is made, it is important for that to be confirmed by a different experiment (since a single experiment has been known to make a mistake a incorrectly think that it has made a major discovery). This "confirmation" process also applied to the Fermilab Tevatron, where CDF and D0 both co-discovered the Top quark.
Even before the W and Z discovery, Fermilab began construction of the Tevatron collider, and the CDF experiment detector which will study the collisions of the Tevatron Collider -- which begin initial studies in 1985, and first real physics results obtained in the 1988 run.
A second detector, D0, was subsequently approved, and constructed to take data by 1992, just in time to for both experiment to combine the discovery of the Top quark in 1994-5.
The Top Quark -- Discovered at Fermilab 1995
See Discoveries at Fermilab-The Top Quark for more info.
The Top Quark was discovered at Fermilab by two groups of experimenters-- The CDF collaboration and the D0 collaboration. The discovery was announced on March 2, 1995.
The Discovery of the Bottom Quark -- Fermilab 1977
For more info, see The Discovery of the b quark : The experiment Coordinator's story-- (Note : This is a .pdf file, so you need to have Adobe Acrobat reader installed in your computer to read this)
See also Discoveries at Fermilab-The Bottom Quark for more info.
Signal Vs. Background -- How Do We Convince Ourselves and the World That We Have Discovered Something New
One of the most serious issue is "S/B" (Or Signal vs. Background). (to be contined)
What Is a GeV? Physicist's Energy Scale and Naming Conventions
You may have noticed that I used the word "GeV" often in these introductory notes --so, "What is a GeV?"
1 GeV is a Giga-Electron-Volt. Historically, Volt is defined as a unit of electrical difference between 2 objects (one positive, one negative) which would tend to push an electron away from the negative node and towards the positive node. Consider 2 metal plates with a 1V difference between then. An electron that is initially at rest near the negative plate will be accelerated by the voltage difference and thus reach the positive plate with some knietic energy (like a moving car). The amount of energy gained by the electron when it passes over a 1 volt differential is call 1 ev (Electron-volt).
1 Kev is 1000 ev (K is kilo);
1 MeV is 1,000,000 ev (M is Mega), and
1 GeV is 1,000,000,000 ev (G is Giga --some older notes use Bev (for billion electron volt)--but by now, G is universally accepted),
I TeV is 1,000,000,000,000 ev (T for Tera, with is 10**12)
and so on. The Fermilab Tevatron is so named because the proton beam is accelerated to about 1 TeV in the machine.
So, how does 1 GeV related to the rest of the world? Well, Einstein's
E = m c squared means that a moving object will have a heavier mass than
the same object at rest. The rest mass of
an electron is .511 Mev while that of
a proton is 938 Mev--almost 1 GeV.
The J/Psi particles (composed of a charm quark and an anti-charm quark) has a 3.1 GeV mass, and
the Upsilon particle (composed of a b abd a b-bar--means b antiquark-- has a mass of 9.46 GeV, 10 times heavier than a proton). The B mesons has a mass of about 5 GeV.
The Top quark have a mass of 175 GeV, almost 200 times heavier than a proton !!!
Why do physicist build bigger and bigger machine to reach higher and higher mass? Well, Massive particle can only be produced with collisions whose center-of-mass energy exceeds the mass of the particles produced--even more so for proton collisions (In an e+e- collider, many collision produce a virtual state of the entire e+e- mass system, so a 175 GeV e+ collising with a 175 GeV e- could produce a pair of 175 GeV Top quarks--though only for an instant before they decay. By contrast, in a proton-antiproton (or proton- proton collider, like the LHC), the 2 partons that actually collide are the quark and gluon constituents of the proton--which carries only a fraction of the proton energy, and thus a 175 GeV proton colliding with an 175 GeV antiproton can never give us the 350 GeV needed for a pair of tops--and most likely only 100 or more rarely 200 GeV of center-of-mass energy.) Thus, until there are collisions with a total energy exceeding 350 GeV, Top quark pairs can not be produced (except during the big bang). Thus, Tevatron is the only current collider with sufficient energy to produce Top pairs. Hence, the need for bigger and bigger machines.
The next Holy grail of High Energy Physics is the Higgs particle-- which is unfortunately very difficult to see. So, despite the possibility that the mass may only be 120 GeV or so, and might have already been produced at the Tevatron, looking for the Higgs is like looking for the needle in a haystack--and only some rare decay modes (Golden events) of the Higgs (most likely too rare to have occured so far at the tevatron) could be used to dig the signal out of the background. Run 2b of the Fermilab Tevatron might be able to produce enough of these "Golden events" for us (CDF and D0 at Fermilab) to have a chance to "discover" the Higgs, but the odds are not too good.
The CERN LHC, due to start operation in 2007, with much higher energy (Mass of 14 TeV vs. 2 TeV for the Fermilab Tevatron) and more collisions per second, is most likely needed to get enough of the Higgs "Golden events" to firmly establish its existance--even then, this would be a difficult process--and would require all the ingenuity of very hard-working and smart physicists in order to dig the signal out of the background--and would probably take years after the first collisions at the LHC.
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