Measurement of the $b\bar{b}$ di-jet cross section at CDF

Abstract

The dominant b production mechanism at the Tevatron is pair production through strong interactions. The lowest order QCD diagrams contain only b and $\bar{b}$ quarks in the final state, for which momentum conservation requires the quarks to be produced back-to-back in azimuthal opening angle. When higher order QCD processes are considered, the presence of additional light quarks and gluons in the final state allows the azimuthal angle difference, Δφ, to spread. The next to leading order QCD calculation includes diagrams up to O(α$3\atop{s}$) some of which, commonly known as flavor excitation and gluon splitting, provide a contribution of approximately the same magnitude as the lowest order diagrams. The study of b$\bar{b}$ angular correlation gives predictions on the effective b quark production mechanisms and on the different contributions of the leading order and next-to-leading order terms. The first experimental results on inclusive bottom production at the Tevatron were strongly underestimated by the exact NLO QCD prediction. Later on this disagreement had been explained and reduced by theoretical and experimental improvements: new QCD calculations that implement the Fixed Order with Next-to- Leading-Logarithms calculation (FONLL); updated parton distribution functions and fragmentation functions; and more precise measurements. Previous measurements of b$\bar{b}$ azimuthal angle correlation have, instead, reached various level of agreement with parton shower Monte Carlo and NLO predictions. Here we present a measurement of the b$\bar{b}$ jet cross section and azimuthal angle correlation performed on about 260 pb-1 of data collected by the CDF II detector at Fermilab from March 2002 to September 2004. This study extends the energy range investigated by previous analyses, measuring jet transverse energies (ET) up to values of about 220 GeV. It relies on the good tracking capabilities of the CDF detector both at the trigger level and offline. Events with heavy quarks are selected online using the Secondary Vertex Trigger (SVT), which can measure in real time the impact parameter of the tracks, in particular those originated from the decay of long-lived particles. The SVT represents the key element for all the heavy flavor measurement performed by CDF, and this analysis describes one of the first cases in which the SVT trigger is used to study high pT physics. The total cross section is mesured together with the di-jet differential cross sections as a function of the highest energy jet ET and the di-jet invariant mass. The azimuthal angular correlation (Δφ) between the two jets is also measured. As expected this distribution proves that the largest contribution to b$\bar{b}$ production is due to lowest order QCD diagrams, corresponding to a back to back configuration of the two b-jets (large Δφ values). The most interesting fact is, however, that the low Δφ region also results highly populated, suggesting an important role played by higher order production terms. To verify this conclusion, results are compared to Monte Carlo predictions at leading order and next to leading order QCD. When technical details are correctly taken into account, as the contribution of the underlying event for example, it is possible to conclude that the data are in agreement with a next to leading order model. Nevertheless the agreement is not perfect and the data present some excess with respect to theoretical predictions. This thesis describes the analysis steps in details as support to the PRL paper forseen to be published soon.