The CDF Collaboration
The top quark, discovered in 1995 by two experiments (CDF and D0) at Fermilab,
is the heaviest elementary object known to mankind. Although top quarks were extremely abundant in the early
Universe (a few moments after the Big Bang) nowadays they are very hard to come by. In fact the only
possibility to produce them is in high energy cosmic ray collisions or in the high energy collisions
taking place at Fermilab between protons
and antiprotons. We exploit the equivalence between mass and energy described by the famous Einstein formula,
E=mc2. Not only are top quarks very hard to produce, but they also live for an
infinitesimally short moment before disintegrating into more mundane particles.
A cross section is a quantity describing the frequency with which an elementary
phenomenon (in this case the production of the top quark) happens in a given reaction (in this case the
collisions of protons and antiprotons). In High Energy Physics, cross sections are measured in units of
area (cm2 or subunits of it, like the barn=10-24 cm2). The smaller this
area, the harder (i.e. less frequent) is the production of the phenomenon under consideration.
The importance of an accurate production cross section measurement is twofold:
in the first place it allows physicists to test and validate their understanding of the basic
the interactions between elementary particles. A measurement differing from the theoretical expectations
would indicate the presence of a yet-unknown process capable of producing top quarks. In the second place,
an accurate production cross section measurement allows physicists to have a reference point for further
progress in their search of rare processes.
In principle the procedure to measure the cross cection is relatively simple. An
experiment needs to count how many top quarks are produced, divide by the total number of collisions and
obtain the production frequency. In practice, however, there are many difficulties. Top quarks are
produced very rarely in proton-antiproton collisions. Billions of collisions take place and millions
are recorded by the CDF detector for every top quark produced. One of the major tasks facing a physicist
searching for the top quark is the rejection of background, i.e. collisions which can be explained
by known physics processes without invoking the creation of a top quark. In this paper we review and
calibrate the CDF computation of known backgrounds (calibration is the process of correcting a
A second correction is necessary to account for the number of times the decay of the
top quark leaves a clear signature in the CDF detector. One of the daughters produced in the top quark decays
is an elementary particle called the bottom quark (discovered at Fermilab in 1977). A feature of the
bottom quark is the ability to travel up to few centimeters before decaying into other particles. The detector
sometimes records this feature. Physicists quantify the word "sometimes" in the previous sentence with
the concept of efficiency (which is essentially the probability of observing a given phenomenon). In
this paper we review and calibrate the CDF measurement of the efficiency to observe top quarks. A large number
of cross-checks and calibrations are used to insure that the efficiencies are computed correctly by comparing
expectations and actual observation on known physics processes.
After all these corrections are made, CDF is left with approximately two dozen
events which look like top quark decays. We derive a production cross section of 6.5
pb (picobarn) (with an estimated error of approximately 1.6 pb). This tiny cross section (1 pb = 10-12
barn = 10-40 m2) is in agreement with measurements performed by D0 and theoretical predictions as shown in the figure below.
A copy of the paper "