Infrared Parton Shower Dynamics and the Top Quark Mass

Funding Agency: FWF, Der Wissenschaftsfonds

Project Number:  FWF-P32383-N27

Project Start: June 1st, 2019

Project Participants: Andre H. Hoang (PI), Simon Plätzer, Daniel Samitz

Project Description:
With a weight of about the mass of a gold atom, the top quark is the heaviest of the known elementary particles. It thus has a particular relation to the Higgs boson which gives mass to all the elementary particles. A precise measurement of the top quark mass is therefore of great importance for the Standard Model of elementary particle physics as well as extensions of it.
The most precise measurements of the top quark mass are based on the so-called direct reconstruction method, which by now at the Large Hadron Collider (LHC) has reached errors of around half the proton mass. Here, properties of jets and lepton that come from decays of top quarks are compared with simulations from multipurpose Monte-Carlo event generators (MMC’s). Measurement values for the top quark mass are then obtained from a fit procedure. These MMC’s form the backbone of all experimental measurements of physical quantities at the LHC, and there are well tested methods to reliably estimate the measurement errors. When using MMC’s for measuring the top quark mass, however, a unique problem arises: the top quark is not a physical quantity, but a theoretical and scheme dependent one. This issue is related to the well-known fact that quarks exist, but are not directly observable particles. So, values of the quark masses can only be obtained from the comparison of experimental data with corresponding theoretical predictions that are calculated in a particular quark mass scheme. In MMC’s this scheme dependence is, however, not yet understood. This results in an additional uncertainty of the top mass measurements, which up to now could not be fully clarified.  
It is the aim of the project to analyse and quantify the theoretical scheme dependence of the top quark mass in MMC’s for observables that enter the direct reconstruction method. In a recent work on event shapes it was found that the infrared cutoff of the parton shower evolution, which is the perturbative component in MMC’s, is the essential property that determines this scheme dependence. In the proposed project the parton shower algorithms shall be solved analytically for the observables mentioned above and compared with theoretical factorisation computations in which the same infrared cutoff is accounted for. In this way the perturbative component of the scheme dependence of the top quark mass in MMC’s can be quantified coherently for observables that enter the direct reconstruction method. In addition, also non-perturbative corrections and effects of the finite top quark lifetime shall be analysed in a systematic manner.