Measurement of the differential cross-section of highly boosted top quarks as a function of their transverse momentum in $\sqrt{s}$ = 8 TeV proton-proton collisions using the ATLAS detector

TL;DR

This ATLAS study measures the differential cross-section for highly boosted tt̄ production as a function of the hadronically decaying top quark’s pT in 8 TeV pp collisions, using a lepton+jets final state and large-R jet tagging to capture boosted top decays. The analysis performs a near-detector-level fiducial measurement and a parton-level extrapolation, employing sophisticated unfolding with migration matrices and multiple correction factors to relate detector data to theoretical quantities. Systematic uncertainties, particularly from large-R jet calibration, and MC modeling are rigorously propagated into covariance matrices used for χ² comparisons with various NLO/LO generators. The results show overall agreement with most MC predictions within uncertainties, extending the top-quark pT reach up to 1.2 TeV and providing valuable constraints on high-pT tt̄ production and PDF/ISR modeling. Alpgen+Herwig is notably disfavored, while Powheg-based predictions with appropriate tuning and PDFs describe the data reasonably well, highlighting the importance of generator choices and jet-scale calibrations in boosted-top analyses.

Abstract

The differential cross-section for pair production of top quarks with high transverse momentum is measured in 20.3 fb$^{-1}$ of proton-proton collisions at a center-of-mass energy of 8 TeV. The measurement is performed for $t\bar{t}$ events in the lepton+jets channel. The cross-section is reported as a function of the hadronically decaying top quark transverse momentum for values above 300 GeV. The hadronically decaying top quark is reconstructed as an anti-$k_t$ jet with radius parameter $R=1.0$ and identified with jet substructure techniques. The observed yield is corrected for detector effects to obtain a cross-section at particle level in a fiducial region close to the event selection. A parton-level cross-section extrapolated to the full phase space is also reported for top quarks with transverse momentum above 300 GeV. The predictions of a majority of next-to-leading-order and leading-order matrix-element Monte Carlo generators are found to agree with the measured cross-sections.

Figures (10)

• Figure 1: Distributions of (a) transverse momentum $\pt$ of the lepton candidates, (b) $\pt$ of selected small-$R$ jets, (c) missing transverse momentum $\met$, (d) and pseudorapidity $\eta$, (e) mass and (f) $\pt$ of the leading selected anti-$k_t$$R$=1.0 jets for the $\ell$+jets channel. The $\ttbar$ prediction is obtained using the nominal Powheg+Pythia sample. The ratio of the MC prediction to the data is shown in the insets below the histograms. The hashed area includes all the object-related uncertainties (on the jet, lepton, and $\met$), and the uncertainties from the background estimation, luminosity and MC statistics. The vertical lines indicate the data statistical uncertainty.
• Figure 2: (a) Distribution of the correction factor $f_{\mathrm{reco!ptcl}}$ as a function of $p_{\mathrm{T,reco}}$. It represents the ratio of the number of events that meet both the detector-level and particle-level to the number of events that satisfy the detector-level selection requirements. (b) Distribution of the correction factor $f_{\mathrm{ptcl!parton}}$ as a function of $p_{\mathrm{T,ptcl}}$. It represents the ratio of the number of events that meet both the parton-level and particle-level to the number of events that satisfy only the particle-level selection requirements.
• Figure 3: (a) Migration matrix between the particle-level $p_{\mathrm{T,ptcl}}$ and reconstructed detector-level $p_{\mathrm{T,reco}}$. (b) Migration matrix between the generated $p_{\mathrm{T,parton}}$ and the particle-level $p_{\mathrm{T,ptcl}}$. The unit of the matrix elements is the probability (expressed in percentage) for an event generated at a given value to be reconstructed at another value (each row adds up to 100%).
• Figure 4: (a) Distribution of the correction factor $f_{\mathrm{ptcl!reco}}$ as a function of $p_{\mathrm{T,ptcl}}$. It represents the ratio of events that meet both the particle-level and detector-level to those that satisfy the particle-level selection requirements. (b) Distribution of the correction factor $f_{\mathrm{parton!ptcl}}$ as a function of $p_{\mathrm{T,parton}}$. It represents the ratio of events that meet both the parton-level and particle-level to those that satisfy the parton-level selection requirements.
• Figure 5: Relative uncertainties on (a) the particle-level differential cross section ${ d}\sigma_{t\bar{t}}/dp_{\mathrm{T,ptcl}}\xspace^{i}$ and (b) the parton-level differential cross section ${ d}\sigma_{t\bar{t}}/dp_{\mathrm{T,parton}}\xspace^{i}$. The total uncertainty (band) is shown along with the effect of the dominant uncertainties. The components "Large-$R$ (JES) stat." and "Large-$R$ (JES) data vs MC" are, respectively, the statistical uncertainty and the systematic uncertainty associated with the difference in jet response between data and MC simulation when balancing $\pt$ in photon+jet events.