A new observable to measure the top-quark mass at hadron colliders
S. Alioli1, J. Fuster2, P. Fernández2, *, A. Irles2, S.Moch3, P. Uwer4, and M. Vos21LBNL & UC Berkeley, CA 94720 Berkeley, USA
2IFIC (CSIC-Universitat de València), E-46980 Paterna, Spain
3DESY, D-15738 Zeuthen, Germany
4Humboldt-Universität zu Berlin, D-12489 Berlin, Germany
Motivation
In the Standard Model (SM), the mass of each elementary particle is represented by a free parameter of the theory. These parameters must be determined experimentally with the highest accuracy possible so that precision tests can be carried out to check the validity of the model. The measurement of the top-quark mass is particularly interesting due to its value, the largest of the SM. This corresponds to the very short lifetime of the top, which does not allow it to form bound states but, instead, it quickly decays – mostly into a W-boson and a b-quark – and so, in contrast to lighter quarks, its spin information is not diluted by hadronization.
Furthermore, due to the high value of its mass, the top quark strongly couples to the Higgs boson, which suggests that it may play a special role in the EWSB mechanism, and to many other particles predicted by BSM theories. Therefore, the understanding and determination of the top-quark properties may be vital to the full verification of the SM and its extensions.
Top-quark mass measurement
The development of high-energy particle colliders and, in particular, the progress of the Large Hadron Collider - already reaching 8 TeV proton-proton collisions - has provided a high top-quark pair production rate that allows to perform more accurate studies and measurements of the top quark properties.
Previous measurements of the top-quark mass were accomplished through several methods; the most commonly used are:
1. Reconstruction of the invariant mass from the top quark decay products: This is a kinematic approach based on the quasi-free behaviour of this quark and associates the measurement to the pole mass. Currently, this method achieves the highest experimental precision (Figure 1), however, it presents several theoretical uncertainties due to the color reconnection mechanism and the intrinsic ambiguity in the pole mass definition.
[GeV]
mtop
150 160 170 180 190
1 7
± 0.8 0.6 173.2 ±
Tevatron Average July 2011
= 4.7 fb-1 CONF-2012-082, Lint
ATLAS 2011, dilepton* 175.2 ± 1.6 ± 3.0
= 2.05 fb-1 CONF-2012-030, Lint
ATLAS 2011, all jets*
3.8
± 2.1
± 174.9
= 1.04 fb-1 Eur. Phys. J. C72 (2012) 2046, Lint
ATLAS 2011, l+jets 174.5 ± 0.6 ± 2.3
= 35 pb-1 CONF-2011-033, Lint
ATLAS 2010, l+jets*
± 4.9 4.0 169.3 ±
(*Preliminary) - 4.7 fb-1
= 35 pb-1
summary - July 2012, Lint
ATLAS mtop
(syst.) (stat.) ±
±
ATLAS Preliminary
Figure 1: Summary of top-quark mass kinematic measurements updated by ATLAS in July 2012.
2. Extraction of the top mass from the top-quark pair production cross section: Through the calculation of the production cross section in perturbation theory, the renormalization scheme can be fixed (in contrast to the kinematic method) and a well-defined top mass can be determined by comparing the theoretical prediction with the experimental cross section.
However, the low sensitivity of this observable to the mass leads to large experimental errors that reduce the accuracy of the measurement.
Therefore, the need of a very precise and well-defined top-quark mass value motivates the search for new competitive methods.
A new observable
We present an alternative method to measure the top mass based on the definition of a new observable (R).
This observable is defined from the differential cross section of top-quark pairs including an additional jet, with respect to the inverse of the invariant mass of the three-jet system (Figure 2).
This observable has been cal- culated at NLO and its theore-
tical properties have been studied. PDF (Parton Distribution Functions) and theoretical scale uncertainties have been evaluated and are well defined below 1 GeV, resulting in a very stable observable with a high sensitivity to the top-quark mass in the region close to the production threshold (ρs ≈ 1). Also, experimental viability studies have been carried out and major uncertainties (backgrounds, event generator, unfolding procedure, statistics, etc.) have been estimated using a specific detector set-up, and again, the impact on the top-quark mass measurement is below 1 GeV. Furthermore, with this theoretically well-defined observable, the renormalization scheme can be fixed and the measured mass is unambiguously determined (the pole mass here). Therefore, we present an alternative method able to reach a precise top-quark mass measurement complementary to the existing approaches.
References
[1] A. Quadt, Top Quark Physics at Hadron Colliders, Eur. Phys. J. C 48, 835-1000 (2006) [2] S. Dittmaier, P. Uwer, S.Weinzierl, Hadronic top-quark pair production in association with a hard jet at next-to-leading order QCD, arXiv:0810.0452v2 [hep-ph] (2008)
[3] S Alioli, S Moch and P Uwer, J. High Energy Phys. 2012, 137 (2012)
Figure 2: R(mtop, ρs) calculated at NLO accuracy for different top-quark masses mtop = 165, 172.5 and 180 GeV.
