Study of low pT D0 meson production at CDF II in pp-bar collisions at √s = 900 GeV.

Master Thesis degree (2012)

The production cross section of the D0 meson at low momenta for different energies enriches the information about the strong interaction behavior in the region where c-quark production is nearly non-perturbative and QCD calculations do not apply. In fact, the current QCD theory cannot predict the behavior of the strong interaction in the low transferred four-momentum region (low Q2) because the strong coupling constant (αs) is of the order of the unity, thus perturbative expansions are no longer permitted. Current phenomenological models are only able to describe few aspects of the observed physical quantities and fail in predicting the strong interaction behavior in its whole complexity. Experimental studies in this region of interaction are crucial to overcome the theoretical limitations and model new theories.

In my thesis, I present a study of D0 meson production in proton-antiproton collisions on the CDF II data at the Tevatron Collider of the Fermi National Accelerator Laboratory. My work is part of a specific effort by the CDF Collaboration to measure the inclusive differential cross section of prompt charmed mesons in the low pT kinematic region. My thesis concerns the analysis of the D0 meson production in the data samples collected during a low energy run at 900 GeV. At this energy, my work is the first measurement of D0 production ever performed in the low pT range. The sample analyzed is the largest ever collected by a hadron collider at these conditions.

The primary purpose of my analysis is the measurement of the D0 yields as a function of the transverse momentum; this is the main step towards the measurement of the D0 production cross section. The D0 decay channel used is D0 → K−π+
(D0-bar → K+π−), because of its simple topology, high reconstruction efficiency (all the produced particles are charged and visible by the CDF tracking system) and relatively high branching ratio (about 3.9%).
The beam position is a crucial information for this analysis: in fact,  knowing the beam position is necessary to unfold the D0 signal, whose signature is a secondary vertex displaced from the beam of several microns.  Unfortunately, the beam position had not been recorded f
or the data set used in my analysis. Thus,  I developed a procedure to identify the beam on a run-by-run basis and I successfully reconstructed its position for the CDF low energy runs. The reconstructed beam position can be further exploited by other analyses of the collaboration. 

The beam position preliminary work aside, my thesis primarily focuses on the search and analysis of the D0 signal in both data and MC. I measured the D0 yield as a function of the transverse momentum. Specifically, I evaluated the raw yield of D0 meson in the decay channel D0 → K−π+ (D0-bar → K+π−) as a function of the D0 transverse momentum in the range 0.5 ≤ pT ≤ 6.5 GeV/c.

Successfully unfolding the D0 signal from the background is the main step toward the measurement of the D0 differential production cross section as a function of transverse momentum. This is of particular importance since such measurement is likely to remain the only one of its kind for several years. My result is consistent with what is expected from a rough extrapolation of the total cc ̄ cross section measurement performed at higher energy.

Elena Gramellini

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