Measurement of the top-quark production cross-section and charge asymmetry at LHCb

The first measurements of the top- and antitop-quark differential production cross-sections and the top-quark charge asymmetry in the forward region are presented, using proton-proton collision data collected by the LHCb experiment at a centre-of-mass energy of 13 TeV corresponding to an integrated luminosity of 5.4 $fb^{-1}$. The total production cross-sections of top and antitop quarks are also determined. Measurements are performed using the $μ+b\text{-jet}$ final state within a fiducial region defined by a $b\text{-jet}$ $p_{\text{T, jet}}>50$ GeV and pseudorapidity $2.2<η_{\text{jet}}<4.0$,, with the muon from the $W$-boson decay required to have $p_{\text{T},μ}>25$ GeV and pseudorapidity $2.0<η_μ<4.5$. The muon and $b$-jet system must satisfy $p_{T}(μ+\text{jet}) > 20$ GeV. The measured integrated production cross-sections for the top and antitop quarks are $σ_{t} = 0.95 \pm 0.04 \pm 0.08 \pm 0.02$ pb, $σ_{\bar{t}} = 0.81 \pm 0.03 \pm 0.07 \pm 0.02$ pb, where the first uncertainty is statistical, the second systematic, and the third accounts for the luminosity uncertainty. The top-quark charge asymmetry is measured to be $A_C^{t} = 0.08 \pm 0.03 \pm 0.01$, where the first uncertainty is statistical and the second is systematic. These results are consistent with next-to-leading-order Standard Model predictions.

💡 Research Summary

This paper presents the first measurements of the differential production cross-sections and the charge asymmetry of top and antitop quarks in the forward region, conducted by the LHCb collaboration using proton-proton collision data at a centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 5.4 fb⁻¹.

The analysis focuses on the decay channel where a top quark decays into a W boson and a b-quark, with the W boson subsequently decaying into a muon and a neutrino (t→Wb→μνb). The measurement is performed within a fiducial phase-space defined by stringent requirements: a b-tagged jet with transverse momentum pT > 50 GeV and pseudorapidity 2.2 < η < 4.0, and a muon with pT > 25 GeV and 2.0 < η < 4.5. Additional selection criteria, including muon isolation and the total transverse momentum of the muon-jet system, are applied to suppress background, primarily from QCD multijet processes.

Key to the analysis is the use of a sophisticated deep neural network (DNN) classifier for b-jet identification, achieving a purity of approximately 74%. Background contributions are meticulously estimated: QCD multijet events are evaluated using a data-driven ABCD method in a two-dimensional plane, while electroweak backgrounds from Z+jet and W+jet production are constrained using data control samples and simulation. After background subtraction, the observed signal yields are corrected for detector effects (muon and jet reconstruction efficiencies, b-tagging efficiency), selection efficiencies, acceptance effects, and bin migrations.

The measured integrated production cross-sections are: σ_t = 0.95 ± 0.04 (stat) ± 0.08 (syst) ± 0.02 (lumi) pb for top quarks, σ_ȯ = 0.81 ± 0.03 (stat) ± 0.07 (syst) ± 0.02 (lumi) pb for antitop quarks. The dominant sources of systematic uncertainty are related to b-jet tagging and the jet energy scale/resolution.

Furthermore, the differential cross-sections as a function of the muon’s pseudorapidity are extracted. From these, the top-quark charge asymmetry, a sensitive probe of next-to-leading-order QCD effects, is determined to be A_C^t = 0.08 ± 0.03 (stat) ± 0.01 (syst). All results are found to be consistent with predictions from the Standard Model at next-to-leading order.

This work marks a significant milestone, demonstrating LHCb’s unique capability to probe top-quark production in the forward region—a kinematic regime complementary to the central detectors ATLAS and CMS. The results provide valuable input for constraining the gluon parton distribution function at high momentum fractions and offer a new vantage point for studying charge asymmetry in top-quark production at the LHC.