Top Mass Determination Research Articles

Top-mass determination from leptonic final states

The top-quark mass is a fundamental parameter of the Standard Model, as it plays a crucial role in the electroweak precision tests, stability of the vacuum and inflation. I review the method and the main results contained in a recent ATLAS analysis which measures the top mass by using the invariant mass of the leptons coming from W and B-hadron decays. The extracted top mass turns out to be the most precise single measurement by ATLAS.

Top quark mass measurement in radiative events at electron-positron colliders

In this letter, we evaluate the potential of linear e+e− colliders to measure the top quark mass in radiative events and in a suitable short-distance scheme. We present a calculation of the differential cross section for production of a top quark pair in association with an energetic photon from initial state radiation, as a function of the invariant mass of the tt¯ system. This matched calculation includes the QCD enhancement of the cross section around the tt¯ production threshold and remains valid in the continuum well above the threshold. The uncertainty in the top mass determination is evaluated in realistic operating scenarios for the Compact Linear Collider (CLIC) and the International Linear Collider (ILC), including the statistical uncertainty and the theoretical and experimental systematic uncertainties. With this method, the top quark mass can be determined with a precision of 110 MeV in the initial stage of CLIC, with 1 ab−1 at s=380 GeV, and with a precision of approximately 150 MeV at the ILC, with L=4 ab−1 at s=500 GeV. Radiative events allow measurements of the top quark mass at different renormalization scales, and we demonstrate that such a measurement can yield a statistically significant test of the evolution of the MSR mass mtMSR(R) for scales R<mt.

Towards precise top mass measurement at LHC

The top quark mass plays an important role in a variety of discussions both within and beyond the Standard Model. However, a precise determination of a theoretically well-defined top quark mass is still missing. Towards a precise determination of a theoretically well-defined top quark mass at the LHC, we propose a method which uses lepton energy distribution and has a boost-invariant nature. We investigate its experimental viability by performing a simulation analysis at the leading order. We estimate several major uncertainties in the top mass determination with this method and they amount to 1.7 GeV with an integrated luminosity of 100 fb−1 at s=14 TeV. The uncertainties should be reduced by considering the next-to-leading order corrections to the method.

Effects of color reconnection on tt¯ $$ t\overline{t} $$ final states at the LHC

The modeling of color reconnection has become one of the dominant sources of systematic uncertainty in the top mass determination at hadron colliders. The uncertainty on the top mass due to color reconnection is conventionally estimated by taking the difference in the predictions of a model with and a model without color reconnection. We show that this procedure underestimates the uncertainty when applied to the existing models in {\sc Pythia}~8. We introduce two new classes of color reconnection models, each containing several variants, which encompass a variety of scenarios that could be realized in nature and we study how they affect the reconstruction of the top mass. After tuning the new models to existing LHC data, the remaining spread of predictions is used to derive a more realistic uncertainty for the top mass, which is found to be around 500 MeV. We also propose how future LHC measurements with $t\bar{t}$ events can be used to further constrain these models and reduce the associated modeling uncertainty.

Determination of the top quark mass circa 2013: methods, subtleties, perspectives

We present an up-to-date overview of the problem of top quark mass determination. We assess the need for precision in the top mass extraction in the LHC era together with the main theoretical and experimental issues arising in precision top mass determination. We collect and document existing results on top mass determination at hadron colliders and map the prospects for future precision top mass determination at e+e- colliders. We present a collection of estimates for the ultimate precision of various methods for top quark mass extraction at the LHC.

Top mass determination, Higgs inflation, and vacuum stability

The possibility that new physics beyond the Standard Model (SM) appears only at the Planck scale $M_P$ is often considered. However, it is usually argued that new physics interactions at $M_P$ do not affect the SM stability phase diagram, so the latter is obtained neglecting these terms. According to this diagram, for the current experimental values of the top and Higgs masses, our universe lives in a metastable state (with very long lifetime), near the edge of stability. Contrary to these expectations, however, we show that the stability phase diagram strongly depends on new physics and that, despite claims to the contrary, a more precise determination of the top (as well as of the Higgs) mass will not allow to discriminate between stability, metastability or criticality of the electroweak vacuum. At the same time, we show that the conditions needed for the realization of Higgs inflation scenarios (all obtained neglecting new physics) are too sensitive to the presence of new interactions at $M_P$. Therefore, Higgs inflation scenarios require very severe fine tunings that cast serious doubts on these models.

Recent theoretical progress in top quark pair production at hadron colliders

This is a writeup of a plenary talk given at the conference {\it HCP 2012} held November 2012 in Kyoto, Japan. This writeup reviews recent theoretical developments in the following areas of top quark physics at hadron colliders: (a) the forward-backward asymmetry anomaly at the Tevatron, (b) precision top mass determination, (c) state of the art NLO calculations and (d) progress in NNLO calculations.

Jets from massive unstable particles: Top-mass determination

We construct jet observables for energetic top quarks that can be used to determine a short-distance top quark mass from reconstruction in e+e- collisions with accuracy better than LambdaQCD. Using a sequence of effective field theories we connect the production energy, mass, and top width scales, Q>>m>>Gamma, for the top jet cross section, and derive a QCD factorization theorem for the top invariant mass spectrum. Our analysis accounts for alphas corrections from the production and mass scales, corrections due to constraints in defining invariant masses, nonperturbative corrections from the cross talk between the jets, and alphas corrections to the Breit-Wigner line shape. This paper mainly focuses on deriving the factorization theorem for hemisphere invariant mass distributions and other event shapes in e+e- collisions applicable at a future linear collider. We show that the invariant mass distribution is not a simple Breit-Wigner function involving the top width. Even at leading order it is shifted and broadened by nonperturbative soft QCD effects. We predict that the invariant mass peak position increases linearly with Q/m due to these nonperturbative effects. They are encoded in terms of a universal soft function that also describes soft effects for massless dijet events. In a future paper we compute alphas corrections to the jet invariant mass spectrum, including a summation of large logarithms between the scales Q, m, and Gamma.

Top mass determination in leptonic final states with J/ ψ

Taking advantage of large top production rates at the LHC, the leptonic final states with J/ ψ are explored for a precise determination of the top quark mass. The top is partially reconstructed by combining the isolated lepton from the W with the J/ ψ from the decays of the corresponding b-quark. The method relies, to a large extent, on the proper Monte Carlo description of top production and decay. The main emphasis is put on the expected systematics uncertainties. Mass measurement accuracy is dominated by the current understanding of theoretical uncertainties which result in a systematic error of ≲ 1 GeV.

Colour correlations and multiplicities in top events

In events of the type e + e − → t t → bW + bW − , particle production could depend in a non-trivial way on the kinematics of the process. Energetic perturbative gluon radiation can be generated (when kinematically allowed) by the original t t system and by the t → bW + and t → bW − decays, with negligible interference between the production and decay between the t and t decays. Soft perturbative gluon emission and non-perturbative fragmentation does introduce a correlation, however. To illustrate the size of these effects, we study the multiplicity as a function of the angle between the b and b jets, close to the t t threshold. Also potential uncertainties in top mass determinations are briefly addressed.