measure the quark content of the proton, this precision is not achievable for the gluon, especially at high x . At low x , the gluon distribution can be determined precisely using QCD scaling violation in DIS data. Studying the high- x gluon distribution functions requires data from hadron-hadron scattering. The effect of including Tevatron jet data in global fits to determine PDFs is described in Section V C. Because a parton carries the strong charge, it is not directly observable. It showers into many partons which combine together to form a large number of particles which travel in roughly the same direction as the initial parton. The kinematic properties of the initial parton can be inferred either from the shower of partons or from the jet of collimated particles. For this inference, these particles or partons must be clustered into a jet by an algorithm. In pQCD, at NLO and higher orders, a jet algorithm is needed to define physics observables which are well-defined i.e. they are soft/collinear safe. Jet algorithms are run on a few partons generated in pQCD calculations to construct such variables. Experimentally, the final state particles are observed as tracks in the tracker systems or as towers of energy in the calorimeter. These tracks or towers must also combined into a jet so that they can be compared to the parton produced in the hard interaction. In the following, we will collectively call the (a) partons in a pQCD calculation, (b) partons or (c) particles produced in Monte Carlo event generators, or (d) towers or (e) tracks, or (f) reconstructed particles observed in a detector as the objects which are input for a clustering algorithm. For a valid comparison between observations and theoretical predictions, the clustering algorithm must satisfy some basic criteria [26, 28]. The algorithm must be safe against soft (infra-red IR) and collinear radiation, invariant to boosts along the beam direction, and should be insensitive to the non-perturbative hadronization effects. In an algorithm which is not safe against soft/collinear radiation, the virtual and real contributions in pQCD cal- culations do not cancel completely and thus the predicted cross sections are ill-defined. One should be able to run the same algorithm on the detector calorimeter towers or tracks, particles or the multi-parton state from event generators and partons in fixed order pQCD calculations and get sensible results. Experimentally, jet clustering algorithms should be insensitive to the energy from additional hadron-hadron collisions in the same bunch cross- 9 III. JET CLUSTERING ALGORITHMS