The top quark is the heaviest of all known fundamental particles making up the universe we live in.  Through collider physics experiments such as ATLAS, we have the ability to study the fascinating properties the top quark – one of nature's most elusive particles.


A Bit of history

It is only in relatively recent history that we have conclusive evidence for the existence of the top quark.   Speculation on the existence of the top quark came about in the mid-seventies, but due to the quark's high mass (close to that of an entire tungsten atom), it was not until 1995 that the discovery of the top quark was made by the D0 and CDS collaborations – the two flagship experiments at the Tevatron accelerator at Fermilab's accelerator complex near Chicago, USA.
We are now in an era of precision top quark measurements, where systematic sources of uncertainty dominate over the statistical uncertainties.  Top quarks are now produced copiously at the centre of the ATLAS detector at CERN, thanks to the high luminosity and centre-of-mass energy delivered by the Large Hadron Collider (LHC). 

During 2011 & 2012 data-taking, roughly 15 million top quarks were produced at the centre of the ATLAS detector.  As a consequence, the LHC can sometimes be referred to as a 'top quark factory', producing large numbers of these heavy particles and consequently allowing particle physicists to reconstruct top quark candidates from the telltale signatures the true top quarks leave – via their decay products – in the various components of the enormous detector.

For a large (and growing!) list of some of the various top quark measurement results from the ATLAS collaboration, follow the link here.


The mass of the top Quark

Among the many properties of the top quark (such as its electromagnetic charge, its quantum mechanical spin, and its polarization which dictates the kinematics of its decay products and becomes directly accessible due to the fact that the top quark does not hadronize), the particle's mass is perhaps the most important. 

The top quark mass is a fundamental parameter of the Standard Model of Particle Physics, and is important to study in its own right.  Moreover, the precise value of has implications in the kinematics and production rates of other particles.  The dominant production mode for a Standard Model Higgs boson is via a so-called top-quark loop – differences in the value of the top quark mass can consequently affect the production rate of these Higgs bosons.  There are a number of direct and indirect ways in which the top quark mass shows up in the theory of the Standard Model.  There are also many theories which propose the existence of as-of-yet undiscovered particles which could preferentially decay to top quarks.

The following figure illustrates a few of the ways the top quark manifests itself in the theory behind the analyses of collision data from the LHC experiments:

A few examples highlighting just some of the ways that the top quark manifests itself in the Standard Model (SM) and non-SM production and decay diagrams.

A few examples highlighting just some of the ways that the top quark manifests itself in the Standard Model (SM) and non-SM production and decay diagrams.

Top quarks also play an important role in several background processes at the LHC, where the final-state signature of the given signal process in question (in other words what physicists for a given analysis are really interested in) is similar to that one would see if one were actually looking for top quark events.  By better understanding top quarks, we can better study the other processes we're interested in since we have a better handle on their background processes.

The top quark mass has been measured using the data collected from several collider experiments (such as CDS, D0, CMS & ATLAS), and by studying a variety of final state topologies.  My Ph.D. research focused on a making a precision measurement of the top quark mass by searching for pairs of top quarks which decayed to an all-hadronic final state.  The measurement made use of the complete 2012 ATLAS dataset at a centre-of-mass energy of √s = 8 TeV.  It was a collaborative effort on behalf of the entire ATLAS collaboration, and consisting primarily of members from Carleton University in Ottawa, Canada, and the Max Planck Institute of Physics in Munich, Germany.


Looking AHEAD

The LHC is well into its second data-collection run at a centre-of-mass energy of 13 TeV.  In this higher energy regime, top quarks will play an even more important role.  Stay tuned for some exciting results to come from both the ATLAS and CMS collaborations!