New top cross section blessed!

Today Giorgio, the PhD student who's been working with me at a new analysis of top physics, has blessed our measurement of the production cross section for top quark pairs.

...So what is a production cross section ? It is a number proportional to the probability that a subnuclear collision will yield as a product a particular final state. In the case at hand, our collisions are between protons and antiprotons at an energy of 2 TeV (Tera-electronvolt = 1 trillion times the energy an electron gains when moving through a potential difference of a Volt, or if you prefer, about an erg=0,0000001 Joules), and the final state we're measuring is a pair of top and antitop quarks.

Blessing a physics result in CDF is a long, tedious process. You have to convince your collaborators that all you did is correct and answer all possible questions they may want to ask, plus produce any additional table or plot to fortify the evidence of your arguments. Giorgio did well all of these things, and at the top meeting today he had a long talk where he showed all the results along with the answers to all questions received.

So at the end of the day what do we have ? A new evaluation of the probability of producing top quark pairs, computed using a particular final state of top quarks decay: one which does not explicitly look for electrons or muons (the typical clean byproduct of the decay of a W boson, which in turn was produced when the top or antitop disintegrated), but rather selects events with significant missing energy. Missing energy is the detection of a unbalancing of the energy flow out of the collision point, orthogonally to the incoming proton and antiproton beams. It signals the escape of a particle which did not interact with the detector - a neutrino!

Tomorrow I will post some plots and a better explanation. For today, suffices to say that this new measurement is the first of its kind, that is the first time one measures top production by looking inclusively at missing transverse energy. The result, for those of you still reading here, is

sigma(top-antitop) = 5.9 +1.8 -1.6 picobarns (the +1.8 and -1.6 are positive and negative errors, and they represent the uncertainty in the central value 5.9).

(Theory predicts this number to be 6.1 picobarns, so we're right on the money!)

A picobarn is a trillionth of a barn, which is the area of a square whose side measures a trillionth of a centimeter! That's a tiny area! It is like you were an antiproton looking at a proton coming toward you, and were to say, "if it hits me with that tiny bit of its surface right here, we'll give rise to a pair of top quarks"! By comparison, the total cross section of a proton-antiproton collision (the total area of the proton coming towards you) is a fifteenth of a barn, something like 100 billion times more.

Comments

[Fred]

Tommaso,

Your postings are splashed with humor, intended or not.
1. Your statement, "Blessing a physics result in CDF is a long, tedious process.", evokes a vision of a Cardinal tossing holy water over some poor wretched soul, while casting a discerning eye over the entire presentation. What do you mean by the word: blessing?
2. The size of a standard barn used to take up space covering a good corner of an acre. Talk about compression!

Adios, Fred

[Tommaso]

Hi Fred,
sure, it's hilarious to use the term "blessing"... Always thought so. But my collaboration gave a real new meaning to the word, and it is by now used without alluding at cardinals and holy water - although we do have sort of "cardinals" in our collaboration, as well as sanctified datasets.

As for the barn, the name does come from the real barn. The thing is, in a subnuclear process a barn is a huge cross section! So, "as big as a barn" retains its meaning even when you compress the thing by about a ten-thousandth of a trillionth.
Cheers,
T.