Measurement of the t t-bar production cross section in the all-jets final state in pp collisions at sqrt(s) = 8 TeV

TL;DR

This CMS study measures the $t\overline{t}$ production cross section in the all-jets final state at $\sqrt{s}=8$ TeV using 18.4 fb$^{-1}$, obtaining $\sigma_{t\overline{t}}=275.6\pm 6.1\,\mathrm{(stat)}\pm 37.8\,\mathrm{(syst)}\pm 7.2\,\mathrm{(lumi)}$ pb for $m_t=172.5$ GeV, with the SM prediction at NNLO+NNLL around $252.9^{+6.4}_{-8.6}$ pb. The analysis provides normalized differential cross sections as a function of top-quark $p_T$ at detector, parton, and particle levels, and finds the measured spectra to be significantly softer than theoretical predictions. Differential results are unfolded using RooUnfold, and the parton-level results carry substantial extrapolation uncertainties, while particle-level results are more stable for MC tuning. Across all levels, the data show a softer top-quark $p_T$ spectrum than predicted, highlighting areas for MC improvement and providing stringent tests of QCD in a challenging all-hadronic channel.

Abstract

The cross section for t t-bar production in the all-jets final state is measured in pp collisions at a centre-of-mass energy of 8 TeV at the LHC with the CMS detector, in data corresponding to an integrated luminosity of 18.4 inverse femtobarns. The inclusive cross section is found to be 275.6 +/- 6.1 (stat) +/- 37.8 (syst) +/- 7.2 (lumi) pb. The normalized differential cross sections are measured as a function of the top quark transverse momenta, pt, and compared to predictions from quantum chromodynamics. The results are reported at detector, parton, and particle levels. In all cases, the measured top quark pt spectra are significantly softer than theoretical predictions.

Figures (9)

• Figure 1: Distribution of the reconstructed top quark mass after the kinematic fit. The normalizations of the ${t}\overline{{t}}$ signal and the QCD multijet background are taken from the template fit to the data. The bottom panel shows the fractional difference between the data and the sum of signal and background predictions, with the shaded band representing the MC statistical uncertainty.
• Figure 2: Distribution of the kinematic fit probability (top). Distribution of the distance between the reconstructed b partons in the $\eta$--$\phi$ plane (bottom). The normalizations of the ${t}\overline{{t}}$ signal and the QCD multijet background are taken from the template fit to the data. The bottom panels show the fractional difference between the data and the sum of signal and background predictions, with the shaded band representing the MC statistical uncertainty.
• Figure 3: Distribution of the $p_{\mathrm{T}}$ of the six leading jets. The normalizations of the ${t}\overline{{t}}$ signal and the QCD multijet background are taken from the template fit to the data. The bottom panels show the fractional difference between the data and the sum of signal and background predictions, with the shaded band representing the MC statistical uncertainty.
• Figure 4: Distribution of the leading (top) and subleading (bottom) reconstructed top quark $p_{\mathrm{T}}$. The normalizations of the ${t}\overline{{t}}$ signal and the QCD multijet background are taken from the template fit to the data. The bottom panels show the fractional difference between the data and the sum of signal and background predictions, with the shaded band representing the MC statistical uncertainty.
• Figure 5: Distribution of the $p_{\mathrm{T}}$ (top) and the rapidity (bottom) of the reconstructed top quark pair. The normalizations of the ${t}\overline{{t}}$ signal and the QCD multijet background are taken from the template fit to the data. The bottom panels show the fractional difference between the data and the sum of signal and background predictions, with the shaded band representing the MC statistical uncertainty.