Next-to-leading order QCD effects and the top quark mass measurements at the LHC

TL;DR

The paper evaluates the precision of top quark mass measurements at the LHC by performing next-to-leading order QCD calculations for observables tied to top decays, including final states with identified $B$-mesons and dilepton channels. It develops a fully differential NLO framework for $t \to l\nu + B + X$ using dipole subtraction and connects $b\to B$ fragmentation via AP evolution with both perturbative and non-perturbative inputs. Results show that NLO corrections can shift central values and reduce or modify theoretical uncertainties compared to leading order and parton-shower predictions, underscoring the need for fixed-order control over fragmentation and decay dynamics to achieve sub-GeV–GeV precision in $m_t$. The study demonstrates that NLOQCD provides more trustworthy uncertainty estimates, which are crucial for leveraging precise $m_t$ measurements to constrain beyond-Standard-Model scenarios.

Abstract

It is anticipated that a number of techniques to measure the top quark mass at the LHC will yield m_top with uncertainties of about 0.5-1 percent. These uncertainties are mostly theoretical; they are usually estimated using parton shower Monte Carlo programs whose reliability at this level of precision is difficult to assess. The goal of this paper is to contrast those estimates with the results of NLO QCD computations for a few observables, often discussed in the context of high-precision top quark mass measurements at the LHC. In particular, we study the NLO QCD corrections to the invariant mass distribution of a charged lepton and a B-meson in lepton+jets channels. In the dilepton channel we investigate the invariant mass distribution of a charged lepton and a b-jet, the average energy of the two leptons and the average energy of the b-jets from top decays.

Figures (8)

• Figure 1: Results of the linear fit to $\langle m_{Bl} \rangle^{\rm NLO}$ are shown. Left panel -- no cut on $m_{Bl}$ is applied. Right panel -- $m_{Bl} > 50~{\rm GeV}$ cut is applied. In both cases, decays of isolated top quarks are considered.
• Figure 2: Result of the linear fit to $\langle m_{Bl} \rangle^{\rm NLO}$ is shown, with all kinematic cuts on the final state particles applied. See text for details.
• Figure 3: The invariant mass distribution of the lepton and the $b$-jet. Note that the lepton and the $b$-jet do not necessarily come from the decay of the same top quark, see text. The left panel shows the scale uncertainty bands for $\mu_R = \mu_F = [0.5m_t, 0.75m_t,m_t,1.25 m_t]$. The right panel shows two NLO normalized$m_{lb}$ distributions for $m_t = 171~{\rm GeV}$ and $m_t = 179~{\rm GeV}$.
• Figure 4: Results of a linear fit to $M_{\rm est}$, Eq. (\ref{['eq01']}), at leading and next-to-leading order in perturbative QCD.
• Figure 5: Left panel: normalized distribution of the sum of lepton energies at leading and next-to-leading order calculated for $m_t = 175~{\rm GeV}$. The renormalization and factorization scales are set to $m_t$ and the MRST (left panel) and CTEQ (right panel) parton distribution functions set is used. Note a shift in the position of the maximum of this distribution. Right panel: normalized distributions of the sum of lepton energies at next-to-leading order, for $m_t = 171~{\rm GeV}$ and $m_t = 179~{\rm GeV}$.