This paper presents a new event generator, ALPGEN, dedicated to the study of multiparton hard processes in hadronic collisions. The code performs, at the leading order in QCD and EW interactions, the calculation of the exact matrix elements for a large set of parton-level processes of interest in the study of the Tevatron and LHC data. The current version of the code describes the following final states: (W → ff ′ )QQ + N jets (Q being a heavy quark, and f = ℓ, q), with N ≤ 4; (Z/γ * → ff ) QQ + N jets (f = ℓ, ν), with N ≤ 4; (W → ff ′ ) + charm + N jets (f = ℓ, q, N ≤ 5); (W → ff ′ ) + N jets (f = ℓ, q) and (Z/γ * → ff ) + N jets (f = ℓ, ν), with N ≤ 6; nW + mZ + lH + N jets, with n + m + l + N ≤ 8, N ≤ 3, including all 2-fermion decay modes of W and Z bosons, with spin correlations; QQ + N jets, with t → bff ′ decays and relative spin correlations included if Q = t, and N ≤ 6; QQQ ′ Q ′ + N jets, with Q and Q ′ heavy quarks (possibly equal) and N ≤ 4; HQQ + N jets, with t → bff ′ decays and relative spin correlations included if Q = t and N ≤ 4; N jets, with N ≤ 6. Parton-level events are generated, providing full information on their colour and flavour structure, enabling the evolution of the partons into fully hadronised final states.
We study the matching of multijet matrix elements and shower evolution in the case of top production in hadronic collisions at the Tevatron and at the LHC. We present the results of the matching algorithm implemented in the ALPGEN Monte Carlo generator, and compare them with results obtained at the parton level, and with the predictions of the MC@NLO approach. We highlight the consistency of the matching algorithm when applied to these final states, and the excellent agreement obtained with MC@NLO for most inclusive quantities. We nevertheless identify also a remarkable difference in the rapidity spectrum of the leading jet accompanying the top quark pair, and comment on the likely origin of this discrepancy.
Abstract. We compare different procedures for combining fixed-order tree-level matrix-element generators with parton showers. We use the case of W -production at the Tevatron and the LHC to compare different implementations of the so-called CKKW and MLM schemes using different matrix-element generators and different parton cascades. We find that although similar results are obtained in all cases, there are important differences.
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