Factorization at the LHC: From PDFs to Initial State Jets

Details Factorization at the LHC: From PDFs to Initial State Jets Factorization at the LHC: From PDFs to Initial State Jets

2009-10-05

arxiv.org/abs/0910.0467v3

Physics

High Energy Physics

High Energy Physics - Phenomenology

41 pages (19 for everyone + 22 for experts), 16 figures; v2: Notational typos fixed. Added sentences to emphasize that measuri

Scientific paper

We study proton-(anti)proton collisions at the LHC or Tevatron in the presence of experimental restrictions on the hadronic final state and for generic parton momentum fractions. At the scale Q of the hard interaction, factorization does not yield standard parton distribution functions (PDFs) for the initial state. The measurement restricting the hadronic final state introduces a new scale \mu_B << Q and probes the proton prior to the hard collision. This corresponds to evaluating the PDFs at the scale \mu_B. After the proton is probed, the incoming hard parton is contained in an initial-state jet, and the hard collision occurs between partons inside these jets rather than inside protons. The proper description of such initial-state jets requires "beam functions". At the scale \mu_B, the beam function factorizes into a convolution of calculable Wilson coefficients and PDFs. Below \mu_B, the initial-state evolution is described by the usual PDF evolution which changes x, while above \mu_B it is governed by a different renormalization group evolution which sums double logarithms of \mu_B/Q and leaves x fixed. As an example, we prove a factorization theorem for "isolated Drell-Yan", pp -> Xl+l- where X is restricted to have no central jets. We comment on the extension to cases where the hadronic final state contains a certain number of isolated central jets.

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