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About CJ

The CJ (CTEQ-Jefferson Lab) Collaboration studies the quark and gluon structure of the nucleon by performing global QCD fits of parton distribution functions (PDFs). These parametrize a vast array of data including deep-inelastic lepton-nucleon scattering, proton-proton collisions (lepton pair creation, W-boson and jet production), and other reactions, with particular focus on the large-x region.

The latest CJ12 fits use less restrictive cuts on Q2 (> 1.69 GeV2) and W 2 (> 3 GeV2), which greatly increases the amount of data accessible at large x, thereby reducing the statistical uncertainty in large-x PDFs, Fig. 1. The effects are most dramatic on the d quark PDF and the d/u ratio, illustrated in Fig. 2 and Fig. 3. The CJ12min, CJ12mid and CJ12max PDF sets correspond to nuclear models of the deuteron with mild, medium and strong nuclear corrections, respectively.

Fig. 1. (Left) Selection of DIS data points used in the CJ12 fits The long-dashed and dot-dashed curves correspond to the W 2 > 14 GeV2 and W 2 > 3 GeV2 cuts, respectively. (Right) Relative PDF errors for u and d quarks, normalized to the relative errors in the reference fit, illustrating the improved statistical precision with the weaker kinematic cuts.

Fig. 2. (Left) d quark PDF uncertainties for the CJ12min, CJ12mid and CJ12max sets, relative to the reference CJ12mid set at Q2 = 100 GeV2. (Right) PDF uncertainties compared with the nuclear correction uncertainties, and with a fit excluding all deuteron data.

Fig. 3. (Left) d/u ratio for the CJ12min, CJ12mid and CJ12max sets. (Right) d/u ratio for the CJ12mid, CT10, MSTW08 and ABKM09 PDFs, relative to their central values.