[Paper Review] Kinematics of Top Quark Final States: A Snowmass White Paper

A. W. Jung, Markus Schulze+1 more|arXiv (Cornell University)|Sep 11, 2013

Particle physics theoretical and experimental studies28 references7 citations

TL;DR

This Snowmass 2013 white paper evaluates theoretical uncertainties and experimental prospects for measuring top quark pair production kinematics and charge asymmetry at the 14 TeV LHC. It finds that current systematic uncertainties limit the observation of the SM-sized top quark production asymmetry, but alternative observables—such as incline asymmetry in $t\bar{t}+j$ final states and forward asymmetry at LHCb—offer improved sensitivity with 50 fb$^{-1}$ of data, enabling decisive tests of SM predictions and potential BSM physics beyond the standard model.

ABSTRACT

This is the summary report of the Top Quark Kinematics working group prepared for Snowmass 2013. We survey the current state of theoretical predictions for top pair differential distributions, in both boosted and un-boosted regimes, and present an overview of uncertainties and prospects for top spin correlations. We study the prospects for measuring the inclusive SM top pair production asymmetry AFC at LHC 14 as a function of systematic error, and show that some improvement over current systematic uncertainties, as customarily handled, is required for observing a SM-size asymmetry. Cuts on top pair invariant mass and rapidity do not substantially alter this conclusion. We summarize the conclusions of contributed studies on alternate LHC measurements of the ttbar production asymmetry, in ttbar+jet final states and in forward top production at LHCb, both of which show good prospects for observing SM-size asymmetries in 50 fb-1 of data at LHC14.

Motivation & Objective

• To assess theoretical uncertainties in top quark pair production kinematics at the 14 and 33 TeV LHC, focusing on NLO QCD and PDF variations.
• To evaluate the feasibility of measuring the inclusive SM top quark charge asymmetry at LHC 14 under current systematic error assumptions.
• To explore alternative observables—such as incline asymmetry in $t\bar{t}+j$ and forward asymmetry at LHCb—for improved sensitivity to SM-sized asymmetries.
• To quantify the impact of systematic uncertainties on the observability of top quark production asymmetries and assess prospects for BSM physics discovery.
• To provide a unified framework for interpreting top quark kinematic distributions and spin correlations in the context of future LHC measurements.

Proposed method

• Uses NLO QCD calculations via the MCFM program to compute top quark pair production cross sections and differential distributions at 14 and 33 TeV LHC energies.
• Estimates theoretical uncertainties through scale variations (factor of two around $m_t = 173$ GeV) and variations of multiple PDF sets (MSTW, NNPDF).
• Analyzes kinematic distributions including transverse momentum, rapidity, and rapidity difference between top and anti-top quarks.
• Evaluates the incline and energy asymmetry observables in $t\bar{t}+j$ final states using event-level simulations and cuts to suppress $gg$-initiated backgrounds.
• Assesses LHCb's potential for measuring forward top quark production asymmetry using lepton+jets and dilepton decay channels with 50 fb$^{-1}$ of integrated luminosity.
• Compares projected significance of asymmetry measurements across ATLAS/CMS, LHCb, and $t\bar{t}+j$ final states, accounting for systematic error scaling.

Experimental results

Research questions

• RQ1 Can the SM-sized top quark production asymmetry be observed at the 14 TeV LHC with current systematic uncertainty levels?
• RQ2 How do theoretical uncertainties in NLO QCD and PDF variations affect the precision of top quark kinematic distributions?
• RQ3 What are the prospects for measuring top quark charge asymmetry in $t\bar{t}+j$ final states at LHC 14, and how do cuts on kinematic variables affect sensitivity?
• RQ4 Can LHCb measure the top quark production asymmetry in the forward region with sufficient statistical significance using 50 fb⁻¹ of data?
• RQ5 How do alternative asymmetry observables compare to the inclusive asymmetry in terms of systematic error suppression and discovery potential?

Key findings

• The inclusive SM top quark charge asymmetry at LHC 14 cannot be decisively observed with current systematic uncertainties; improved control is required.
• The total $t\bar{t}$ cross section at 14 TeV is 845 pb at NLO QCD, with a 12% residual scale uncertainty and 8% uncertainty from NNPDF parton distributions.
• The $t\bar{t}$ cross section increases to 5 nb at 33 TeV, with a K-factor of 1.23 and residual scale uncertainty of 11%.
• The incline asymmetry in $t\bar{t}+j$ final states achieves a significance of about 16 s.d. at 3000 fb⁻¹, indicating strong sensitivity to SM asymmetries.
• LHCb can measure the forward top quark production asymmetry with 50 fb⁻¹ of data at 14 TeV, with no significant pile-up degradation and high luminosity phase enabling dilepton channel measurements.
• The combination of LHCb, ATLAS, and CMS measurements would provide a more complete picture of top quark production asymmetry, enhancing sensitivity to BSM physics.