Matching matrix elements and shower evolution for top-quark production in hadronic collisions

TL;DR

Assesses the matching of LO multijet matrix elements with parton-shower evolution for ttbar production in hadronic collisions at Tevatron and LHC, comparing ALPGEN MLM matching with parton-level predictions and MC@NLO to evaluate consistency and accuracy. The study implements MLM-like matching in ALPGEN, performs detailed consistency checks across generation/matching parameters and higher jet multiplicities, and compares to MC@NLO, including an examination of spin correlations. Results show overall good agreement with MC@NLO for inclusive observables after applying K factors, and robust stability under parameter variations, but reveal a notable discrepancy in the rapidity distribution of the leading jet that invites further investigation. The work underscores the importance of spin correlations in top decays and points to areas where shower modeling (e.g., heavy-quark emissions) can impact jet observables relevant for LHC phenomenology.

Abstract

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.

Figures (13)

• Figure 1: Comparison of the ALPGEN $S_1$ results and the LO PL spectra for the inclusive transverse momentum and rapidity of top quarks, for the transverse momentum of the $t \bar{t}$ pair, and for their azimuthal correlations. All distributions are absolutely normalized. The contribution of the 0$_{{exc}}$ sample is shown by the dashed line. The plots on the left are for the Tevatron, those on the right for the LHC.
• Figure 2: Comparison between the distributions obtained from the $S_1$ event samples (0$_{{exc}}$+1$_{{inc}}$) and from the $S_3$ event samples (0$_{{exc}}$+1$_{{exc}}$+2$_{{exc}}$+3$_{{inc}}$), for various ($\le 1$)-parton observables at the Tevatron (left-hand side) and LHC (right-hand side). Cumulative contributions from the 0$_{{exc}}$, 1$_{{exc}}$ and 2$_{{exc}}$ subsamples are shown by the dashed histograms.
• Figure 3: Comparison between the distributions obtained from the $S_1$ event samples (0$_{{exc}}$+1$_{{inc}}$) and from the $S_3$ event samples (0$_{{exc}}$+1$_{{exc}}$+2$_{{exc}}$+3$_{{inc}}$), for various higher-order parton observables at the Tevatron (left-hand side) and at the LHC (right-hand side).
• Figure 4: Comparison between the three alternative sets of generation (left) and matching (right) parameters given in table \ref{['tab:cuts_syst']}, at the Tevatron.
• Figure 5: Comparison between the three alternative sets of generation and matching parameters given in table \ref{['tab:cuts_syst']}, for multijet distributions at the Tevatron.