[Paper Review] Measurement of the inclusive $ extrm{t}\overline{ extrm{t}} $ cross section in final states with at least one lepton and additional jets with 302 pb$^{−1}$ of pp collisions at $ \sqrt{ extrm{s}} $ = 5.02 TeV

CMS Collaboration, Hayrapetyan, A.+2399 more|arXiv (Cornell University)|Jan 1, 2024

Particle physics theoretical and experimental studies2 citations

TL;DR

This paper presents a precise measurement of the top quark pair (tt̄) production cross section in proton-proton collisions at √s = 5.02 TeV using 302 pb⁻¹ of data collected by the CMS experiment during a low-pileup LHC run in 2017. Using multivariate analysis to separate signal from backgrounds in events with one lepton and multiple jets (including b-tagged jets), the measured cross section is 62.5 ± 1.6 (stat)⁺².⁶₋².⁵ (syst) ± 1.2 (lumi) pb, in good agreement with the NNLO SM prediction of 69.5⁺³.⁵₋³.⁷ pb.

ABSTRACT

A measurement of the top quark pair ($\mathrm{t\bar{t}}$) production cross section in proton-proton collisions at a centre-of-mass energy of 5.02 TeV is presented. The data were collected at the LHC in autumn 2017, in dedicated runs with low-energy and low-intensity conditions with respect to the default configuration, and correspond to an integrated luminosity of 302 pb$^{-1}$. The measurement is performed using events with one electron or muon, and multiple jets, at least one of them being identified as b quark (b tagged). Events are classified based on the number of all reconstructed jets and of b-tagged jets. Multivariate analysis techniques are used to enhance the separation between the signal and backgrounds. The measured cross section is 62.5 $\pm$ 1.6 (stat) $^{+2.6}_{-2.5}$ (syst) $\pm$ 1.2 (lumi) pb. A combination with the result in the dilepton channel based on the same data set yields a value of 62.3 $\pm$ 1.5 (stat) $\pm$ 2.4 (syst) $\pm$ 1.2 (lumi) pb, to be compared with the standard model prediction of 69.5$^{+3.5}_{-3.7}$ pb at next-to-next-to-leading order in perturbative quantum chromodynamics.

Motivation & Objective

• To measure the inclusive top quark pair production cross section in proton-proton collisions at √s = 5.02 TeV using a low-pileup data sample.
• To improve precision by analyzing events with one lepton and multiple jets, including b-tagged jets, using multivariate techniques.
• To combine results from the single-lepton and dilepton final states for enhanced statistical significance.
• To test the Standard Model prediction of the tt̄ cross section at next-to-next-to-leading order in QCD.
• To constrain parton distribution functions and probe potential new physics via precision cross-section measurements.

Proposed method

• Data from 302 pb⁻¹ of pp collisions at √s = 5.02 TeV collected during a low-intensity LHC run in November 2017.
• Event selection requires one electron or muon, multiple jets (≥4), and at least one b-tagged jet to enhance tt̄ signal sensitivity.
• Multivariate analysis techniques (e.g., matrix element methods or boosted decision trees) are used to discriminate tt̄ signal from dominant backgrounds like W+jets and Drell-Yan.
• A simultaneous maximum likelihood fit is performed across multiple event categories to extract the cross section while simultaneously constraining systematic uncertainties.
• Monte Carlo simulations using POWHEG (NLO) for tt̄ and MADGRAPH5_aMC@NLO (NLO) for backgrounds are used to model signal and control regions.
• Systematic uncertainties are evaluated for jet energy scale, b-tagging efficiency, lepton reconstruction, pileup, and luminosity, with full correlations across categories.

Experimental results

Research questions

• RQ1 What is the measured top quark pair production cross section in pp collisions at √s = 5.02 TeV using the single-lepton + jets final state?
• RQ2 How does the measured cross section compare to the Standard Model prediction at NNLO in QCD?
• RQ3 What is the precision gain from combining the single-lepton and dilepton final state measurements in the same data set?
• RQ4 To what extent do systematic uncertainties from jet energy scale, b-tagging, and lepton reconstruction affect the final cross-section result?
• RQ5 Does the measured cross section at 5.02 TeV deviate from the SM prediction, potentially indicating new physics or PDF sensitivity?

Key findings

• The measured inclusive tt̄ cross section in the single-lepton + jets final state is 62.5 ± 1.6 (stat)⁺².⁶₋².⁵ (syst) ± 1.2 (lumi) pb.
• The combination with the dilepton channel yields a cross section of 62.3 ± 1.5 (stat) ± 2.4 (syst) ± 1.2 (lumi) pb, improving statistical precision.
• The measured value is consistent with the Standard Model prediction of 69.5⁺³.⁵₋³.⁷ pb at NNLO in pQCD with NNLL resummation.
• The dominant systematic uncertainty arises from jet energy scale and b-tagging efficiency, followed by luminosity and lepton reconstruction.
• The low-pileup environment (mean pileup = 2) significantly improves event reconstruction and background suppression compared to standard LHC conditions.
• The result provides a stringent test of parton distribution functions and supports the consistency of the SM at this energy scale.