Top quark mass determination from the energy peaks of b-jets and B-hadrons at NLO QCD

TL;DR

This paper proposes and analyzes a method to determine the top quark mass using the energy peaks of b-jets and B-hadrons at NLO QCD, aiming for reduced sensitivity to production details and uncertainties.

Contribution

It introduces an NLO QCD analysis of the energy-peak method for top mass measurement using b-jets and B-hadrons, including assessment of theoretical uncertainties and potential improvements.

Findings

Top quark mass can be extracted with approximately 1.4 GeV total uncertainty from b-jet energy peaks.

Using B-hadron energy peaks can reduce experimental uncertainties related to jet energy scale.

Fragmentation scale dependence is the dominant theoretical uncertainty in B-hadron based extraction.

Abstract

We analyze the energy spectra of $single$ b-jets and B-hadrons resulting from the production and decay of top quarks within the SM at the LHC at the NLO QCD. For both hadrons and jets, we calculate the correlation of the peak of the spectrum with the top quark mass, considering the "energy-peak" as an observable to determine the top quark mass. Such a method is motivated by our previous work where we argued that this approach can have reduced sensitivity to the details of the production mechanism of the top quark, whether it is higher-order QCD effects or new physics contributions. As part of the NLO improvement over the original proposal, we assess the residual sensitivity of the extracted top quark mass to perturbative effects both in top quark production and decay. For a 1% jet energy scale uncertainty (and assuming negligible statistical error), the top quark mass can then be extracted using the energy-peak of b-jets with an error +- (1.2 (exp) + 0.6(th)) GeV. We note that recently the CMS collaboration reported a top quark mass measurement based on the original proposal (with b-jets) so that our result contributes to a precise evaluation of the associated theory uncertainty. In view of the dominant jet energy scale uncertainty in the measurement using b-jets, we also investigate the extraction of the top quark mass from the energy-peak of the corresponding B-hadrons which, in principle, can be measured without this uncertainty. The calculation of the B-hadron energy spectrum is carried out using fragmentation functions at NLO. The dependence on the fragmentation scale turns out to be the largest theoretical uncertainty in this extraction of top quark mass. Future improvement of the treatment of bottom quark hadronization can reduce this uncertainty, rendering methods based on the B-hadron energy-peak competitive for the top quark mass measurement.