Dijet production, collision centrality and backgrounds in high-energy   p-p collisions
  Two aspects of high-energy \pp collisions share common phenomenological elements: (a) A correlation between jet production and \pp centrality is suggested by the transverse partonic structure of hadrons inferred from deep-inelastic scattering data. (b) The {\em underlying event} (UE) is defined as the final-state particles complementary to a triggered high-energy dijet. An observable common to both topics is variation of so-called {\em transverse multiplicity} $N_\perp$ with a $p_{t,trig}$ dijet trigger. We test assumptions associated with \pp collision centrality and the UE. We determine the nature of the UE and explore the relation between jet production and \pp centrality. We use the {\em two-component model} (TCM) of spectra and correlations derived from 200 GeV \pp collisions to construct a simulated particle distribution on $(p_t,n_{ch})$ to predict the $N_\perp$ response to $p_{t,trig}$. The $p_t$ spectrum TCM combined in this analysis with measured minimum-bias \pp angular correlations suggests that the UE includes a substantial contribution from the triggered dijet in addition to the contribution from projectile fragmentation (beam-beam remnants). The jet contribution to $N_\perp$ may represent a universal large-angle base common to all dijets that extends across $2\pi$ azimuth. The analysis further suggests that \pp centrality is not controlled significantly by $p_{t,trig}$ but may be correlated to some extent with an imposed $n_{ch}$ condition, depending on the role of fluctuations. Future correlation studies may better determine the role of \pp centrality. These results may have implications for ongoing RHIC analysis and LHC searches for physics beyond the standard model. 
Thomas A. Trainor
hep-ph hep-ex nucl-ex nucl-th