Charm Production and Cross Sections for D Mesons

New high statistics, high resolution fixed target experiments producing 10 10 fully reconstructed charm particles are allowing a detailed study of the charm sector. Recent results on charm quark production from Fermilab fixed target experiments E-791, SELEX and FOCUS are presented. INTRODUCTION This review contains recent results from three Fermilab fixed target experiments dedicated to study charm physics: E791 and E871 (SELEX) of charm hadroproduction and E831 (FOCUS) of charm photoproduction. The experiment E791 used a 500 GeV/c π beam incident on platinum and diamond target foils and took data in 1991-1992. A loose transverse energy trigger was used to record 2 interactions. Silicon microstrip detectors (6 in the beam and 17 downstream of the target) provided precision track and vertex reconstruction. The precise location of production and decay vertices of long lived charm particles allowed to reconstruct over 2 charm particles. SELEX used a 600 GeV hyperon beam of negative polarity to make a mixed beam of Σ and π in roughly equal numbers. The positive beam was composed of 92 % of protons, and 8 % of π. Interactions occurred in a segmented target of 5 foils, 2 Cu and 3 C. The experiment was designed to study charm production in the forward hemisphere, with good mass and decay vertex resolution for charm momenta in the range 100-500 GeV/c. A major innovation was the use of online selection criteria to identify events that had evidence for a secondary vertex. Data taking finished in 1997. The FOCUS experiment used a photon beam with < Eγ >= 170 GeV on a Beryllium Oxide segmented target. Sucessor of Fermilab E687, FOCUS had upgrades 1) This work is partially supported by CLAF/CNPq Brazil and CONACyT México 2) To appear, Proc. VII Mexican Workshop on Particles and Fields, Merida Yuc. Méx., Nov.1999 3) [email protected] 4) [email protected] in vertexing, Cerenkov identification, electromagnetic calorimetry and muon identification. The target segmentation and the use of additional silicon microstrip detectors after each pair of target segments were major improvements. The experiment took data in 1996-1997 and collected over 7 billion triggers of charm candidates. FOCUS has the largest charm sample available in the world, with over 10 fully reconstructed charmed particles. The data available from these three experiments will allow to make high precision charm physics, to study rare decays and to search for new physics. CHARM PRODUCTION AND CROSS SECTIONS FOR D MESONS Charm production is a combination of short and long range processes. Perturbative Quantum Chromodynamics (pQCD) can be used to calculate the parton cross section, the short range process. The long range process of parton hadronization has to be modeled from experimental data. The two processes occur at different time scales and should not affect each other, leading to factorization properties. The general expression for production of charm in hadroproductions is [1]