In the past, studies using different jet radii have been performed on polarized proton-proton collision at a center of mass energy of 200 GeV at STAR. These studies indicated that a jet radius R=0.6 works well at this energy.  However, no study has been done at 500 GeV and it remains to  be determine if R=0.6 is appropriate at this higher energy.

Changing the jet radius ulitmately alters the amount of energy contained in a single jet. For example, if we increase (decrease) the radius of a jet the amount of energy contained within that jet also increases (decreases). However, the underlying event and hadronization effects also plays a factor in altering the jet energy scale and the goal is to determine a jet radius where the combined effects are minimal. 

Therefore, this analysis calculated the dijet cross-section at 500 GeV at the particle and parton level for 3 different radii: R=0.4, 0.6, and 0.8. Then proceed to determine the ratio of cross-sections with respect to the the standard R=0.6 cross-section valuses.

Fig 1: Ratio of the dijet cross-section at different radii with respect to R=0.6.

At the parton-level, the underlying event and hadronization effect are irrelevant. And by comparing the particle and parton levels we can deteremine how theses effects manifest in our cross-sections at different radii.  In figure 1, the ratio of the cross-sections at the particle level for R=0.8 (red) falls 2.2 at low invariant mass to 1.4 at high invariant mass. However, at the parton level (yellow) the ratio remains flat at ~1.2 for entire invariant mass range. This seems to indicate that a radius increases the number of dijets produced at lower invariant mass since it picks up more energy that was once lost by hadronization.  

                 Fig 2: The ratio of the cross sections at the parton-level and the particle-level for each radius.
 
We can see from figure 2 that at R=0.6 the cross section ratio of the parton and particle level lie close to one. This verifys that the underlying event and hadronization effects at the this radius are miminal when compared to the other examined radii.  And further indicates that the R=0.6 is also optimal for the 500GeV data.