Carleton The ATLAS Group

Top Analysis

The top quark, first discovered at the Tevatron in 1995, is by far the heaviest of the known fundamental particles in the so-called Standard Model of Particle Physics. A consequence of the top quark's large mass (roughly equivalent to that of an entire gold atom!) is that it has the strongest coupling of all such particles to the recently discovered Higgs Boson, the particle believed to be responsible for the mass of fundamental particles.  As such, a good understanding of the properties of the top quark is necessary in order to best interpret and further study the properties of the Higgs Boson.   Furthermore, any attempts to search for sources of disagreement between data and predictions made by the Standard Model theory could also first manifest themselves in deviations from predicted values in top quark analyses, making top physics a very exciting area of research in collider physics!

With the large center of mass energies available at the LHC, the proton-proton accelerator is often referred to as a top quark factory, producing millions of top quark pairs in a year.  In fact, during peak of the the 2012 data-taking period the LHC produced more than one top-antitop quark pair every second at the centre of the ATLAS detector!  Because the top quark decays so quickly, it cannot be directly observed; physicists must 'reconstruct' top quark candidates, by searching for and recombining what are believed to have been its decay products.  In fact, there is more than one single final 'signature' (collection of final 'pieces' into which the top quarks decay) that one can look for when reconstructing top quark candidates, and each such decay signature or 'decay channel' offers its own advantages and disadvantages.

At Carleton, where physicists work together with colleagues at the Max Planck Institute for Particle Physics in Munich, Germany, the top quark mass is being measured in the so-called 'all-hadronic channel', meaning that the final signature consists of six quarks - two bottom quarks and four lighter quarks (up, down, strange and charm quarks).  The quarks themselves manifest themselves as high-energy, collimated sprays of particles, referred to as 'jets', which are then recombined to build the top quark candidates themselves.  One of the greatest challenges in this channel is the fact that there are many other processes which can fake such a signal, and as such one must counter (and well model) the significant QCD multi-jet background before one can hope to make a precision measurement of the top quark mass, let alone observe any peak on top of the overwhelmingly large and formidable background.

It is important to note that analyses involving top quark physics rely on a very solid understanding of all aspects of the detector, in particular the detector's calorimeter system - an area in which the Carleton ATLAS group has contributed, and continues to contribute, in a very significant way.