Factorization at the LHC: From PDFs to Initial State Jets (Ph.D. thesis)

Abstract

New physics searches at the LHC or Tevatron typically look for a specific number of hard jets, leptons and photons. To obtain an exclusive N-jet sample, one can measure the event shape "N-jettiness" \tau_N and veto additional undesired jets by requiring \tau_N <<1. However, this leads to large logarithms of \tau_N in the cross section, that need to be summed for reliable theory predictions. For N=0 in hadronic collisions, \tau_N reduces to the "beam thrust" eventshape \tau_B. We derive a factorization theorem for pp -> XL, where a central jet veto \tau_B<<1 is imposed on the hadronic final state X and L is non-hadronic. This factorization theorem enables us to sum large logarithms of \tau_B in the cross section. The initial state is described by "beam functions", which depend on the momentum fraction x and the (transverse) virtuality t of the colliding partons. The dependence on t can be calculated in perturbation theory by matching beam functions onto the parton distribution functions (PDFs) at the intermediate scale \mu_B^2 ~ t, and we have calculated all one-loop matching coefficients. Unlike the PDFs, the evolution of the beam function is in t and does not affect x or the parton type. We show results for the beam thrust cross section for Drell-Yan and Higgs production through gluon fusion at next-to-next-to-leading-logarithmic order. For N>0, we obtain a factorization formula with inclusive jet and beam functions that allows us to sum the large logarithms of \tau_N.