[Paper Review] Top quark phenomenology

Rikkert Frederix|arXiv (Cornell University)|Sep 30, 2010

Particle physics theoretical and experimental studies3 citations

TL;DR

This paper examines three 2-sigma deviations from Standard Model predictions in top quark physics at the Tevatron: an excess in the tail of the $H_T$ distribution, a larger-than-expected top quark charge asymmetry, and a discrepancy in CDF's discrimination of s- and t-channel single top events. The study evaluates theoretical uncertainties in Monte Carlo modeling, particularly in parton shower matching and higher-order corrections, as potential explanations, though no definitive new physics is confirmed.

ABSTRACT

In this talk three 2-sigma deviations from the Standard Model predictions in the top quark sector are briefly discussed. These are the excess of events in the tail of the H_T distribution in ttbar events, the top-quark charge asymmetry and the discrimination of s- and t-channel events in single top. The latter has only been observed by CDF, while the other two are found by both CDF and D0.

Motivation & Objective

• To investigate three experimental measurements in top quark physics that deviate from Standard Model predictions by approximately 2 sigma.
• To evaluate whether these deviations arise from theoretical uncertainties in Monte Carlo event generation, particularly in parton shower matching and higher-order QCD corrections.
• To assess the impact of improved NLO calculations and merging procedures on the interpretation of single-top channel discrimination.
• To determine whether the observed discrepancies could be due to statistical fluctuations or signs of new physics, such as a fourth-generation top quark.

Proposed method

• Analyzes the $H_T$ distribution—defined as the scalar sum of transverse energies of jets, leptons, and missing $E_T$—in $t\bar{t}$ events to identify excesses in the tail.
• Compares experimental $H_T$ distributions from CDF and DØ with Monte Carlo predictions, evaluating the role of higher-order QCD corrections and parton shower mismodeling.
• Evaluates the top quark charge asymmetry using NLO QCD predictions, including virtual and real emission contributions, and applies resummation techniques to assess higher-order effects.
• Uses NLO calculations in the 4-flavor scheme for t-channel single top production to compute spectator $b$ quark distributions and compares them with CDF and DØ Monte Carlo samples.
• Applies merging techniques (e.g., MC@NLO, POWHEG) to match $2\to2$ and $2\to3$ matrix elements with parton showers, improving modeling of initial-state $b$ quarks.
• Compares CDF’s ZTOP-based Monte Carlo samples with recent NLO predictions to quantify discrepancies in spectator $b$ quark yields and their impact on s- vs. t-channel discrimination.

Experimental results

Research questions

• RQ1 Is the observed excess in the tail of the $H_T$ distribution due to shortcomings in Monte Carlo modeling of higher-order QCD corrections and parton shower matching?
• RQ2 Could the larger-than-predicted top quark charge asymmetry be explained by unaccounted higher-order virtual corrections or resummation effects?
• RQ3 Why does CDF observe a significant discrepancy in s- vs. t-channel single top event discrimination, while DØ does not, despite similar theoretical expectations?
• RQ4 To what extent do current Monte Carlo event generators underestimate the number of spectator $b$ quarks in t-channel single top events, and how does this affect channel separation?
• RQ5 Are the three 2-sigma deviations statistical fluctuations, or do they signal new physics such as a fourth-generation top quark?

Key findings

• The $H_T$ distribution tail shows a small but persistent excess in both CDF and DØ data, which is better described by models including a heavy $t^\prime$ quark, though the effect is not statistically significant.
• The top quark charge asymmetry is measured to be $A_{FB}(t\bar{t}) = 0.078 \pm 0.009$, exceeding the NLO QCD prediction of $0.051 \pm 0.006$, suggesting possible missing higher-order corrections.
• CDF observes a 2-sigma discrepancy in s- vs. t-channel single top cross section discrimination, with Monte Carlo predictions underestimating spectator $b$ quark yields by about 50% compared to recent NLO calculations.
• Despite the underestimation of spectator $b$ quarks in CDF’s ZTOP-based samples, the resulting shift in s-channel fraction is too small to explain the observed discrepancy, indicating an unresolved issue.
• The discrepancy in CDF’s single-top channel discrimination persists even after accounting for improved NLO predictions, suggesting potential flaws in event generator modeling or unaccounted systematic uncertainties.
• All three deviations remain within 2-sigma significance and are not yet conclusive evidence for new physics; they may stem from theoretical uncertainties in higher-order corrections and parton shower modeling.