CDF beats theory on the top pair cross section August 18, 2008Posted by dorigo in news, physics, science.
Tags: CDF, QCD, top quark
Among the huge amount of beautiful new measurements produced at the Tevatron by the CDF and D0 experiments last month, just in time for showing at ICHEP 2008, the international conference in High-Energy Physics, there is one which does not make headlines, but it deserves one. It is the measurement of the top pair production cross section, a number which is by itself not terribly informative – it is basically only a check that perturbative calculations with Quantum Chromodynamics work well when they deal with an energy scale where the strong coupling constant is small enough. That is: the above is the only thing one gets from a precise measurement of the top cross section provided one is convinced that there is no other process, so far undiscovered, hiding in top production or top decay.
It is absolutely fair to ask oneself whether top pairs are produced at the Tevatron energy solely by quark-antiquark annihilation and gluon-gluon fusion, the two leading order QCD processes, or whether there is a heavy object X which decays to top quarks, thus enhancing the observed rate of top quarks over what QCD predicts. It is also perfectly legitimate to investigate whether the cross section is in line with predictions regardless of the final state in which one searches for top quarks: some non-standard decays of top could modify the mix. Further, one could hypothesize that the top quark dataset -the data enriched with top events which are used by the experiments to measure cross sections- contains some other process which messes up some of the measurements.
The above ideas are to me the most important reason for being interested, 14 years after I first got to know that the top quark existed, in the very precise new determinations of top quark pair cross section obtained by CDF. So let us look at the graph on the right, which details some of the recent determinations, which have been averaged into a result which carries a 8% total uncertainty, beating by 1% the most precise theoretical estimates (9% relative error).
One interesting thing to note is that the cross sections measured with SLT are higher than the average. SLT is the soft-lepton tagging algorithm, which tags b-quark jets coming from top decay through the identification of a muon or an electron embedded in the jet. In Run I, CDF measured a top cross section which was 9 picobarns when using SLT, while about 6 picobarns when using SVX tags -secondary vertices in the jets. Back then, the disagreement was the source of a huge controversy on the hypothesized presence of new physics in the sample of events containing SLT tags. The data did lend itself to some exotic interpretations, but things petered out after years of review and internal diatribas. Now, it does not look like there will be a reprise of that controversy, but the fact remains that SLT cross sections are still there: higher than they should be!
In any case, I salute this new, important result by the CDF top group, and by dozens of dedicated physicists who put their time and efforts into obtaining a very precise measurement. Now the ball is in the theorists’ court, to improve the precision on the theoretical estimate.
UPDATE – ok, a moment after posting the above piece, I looked back at the picture, and I realized that it is not true that the CDF determination is more accurate than theory. It is the theory band which has an 8% uncertainty if I am not mistaken, while CDF has the 9% measurement. That does not change much of the discussion, however, since once the result found by D0 is added to the above one, experiment does get the better hand.
UPDATE II: I also forgot to point interested readers to the public note describing the result!