The gluon density is one of the most poorly constrained PDFs. The inclusive jet measurements have been used in global PDF fits resulting in PDFs with reduced uncertainties. Such precision measurements are fundamental tests of the standard model and any deviations could be a hint of new physics. Necessary to fully map out the accessible kinematic region in order to test the universality of the PDFs Refined PDFs allow more precise estimates of backgrouds... Hadrons, such as the proton, consist of constituent quarks plus a "sea" of lower-energy quarks and gluons which is constantly changing as quark-antiquark pairs annhilate into gluons or are born from gluons. The Parton Distribution Functions (PDFs) give the probability of finding a quark or gluon inside the proton with a fraction x of the proton's momentum. Uncertainties on PDFs translate into errors on measurements at several stages in any analysis, including event selection, determination of acceptance and calculation of luminosity. The gluon PDF has a large uncertainty at large x, as it is rare that a gluon would carry most of the proton's momentum (Fig. 1). This in turn gives a large uncertainty in predictions for the Higgs cross section, for example. Figure 2 shows the range of theoretical predictions for the inclusive jet cross section compared to CDF Run I data; the variation in predictions at large transverse energy is due mainly to the uncertainty in the gluon density. (A jet is the "spray" of particles produced when the proton is broken up and the quarks and gluons recombine into hadrons.)
The modest increase in center-of-mass energy of collisions at the Tevatron from Run I to Run II increases the reach in transverse momentum (pT) of jets, as shown in Fig. 3. Measurements of the inclusive jet cross section at Run II have reached as far as 600 GeV in pT (Fig. 4) using approximately 350/pb integrated luminosity. The Run II inclusive jet cross section measurement in the forward region can be used to constrain the PDFs, as no new physics is expected in that kinematic region. (Figure 5 shows the different kinematic regions covered by the Run I CDF central measurement and the D0 measurement which extended into the forward region.) Then the high-pT cross section in the central region can be explored for new physics (such as quark substructure -- the idea that the quarks themselves may be made of something smaller).
Although the LHC will certainly produce higher pT jets, the larger center-of-mass energy requires higher-order corrections in theoretical predictions. The cross sections there will be harder to relate directly to the PDFs, making it more difficult to distinguish new physics.


Tevatron measurements that provide information about the PDFs