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Top Quark Production in Proton-Antiproton Collisions

The CDF Collaboration

January 2001

 

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 theories governing 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 measurement).

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 "Measurement of the`tt Production Cross Section in`pp Collisions at Ö s = 1.8 TeV ", published is available from hep-ex/0101036.

For further information contact Dr. Giorgio Apollinari.


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last modified 7/19/2001   email CDF
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