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.

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Figure 1: Comparisons between data (black points) and the various simulated Monte Carlo samples stacked one top of the other for both signal and background events, and showing the distribution of invariant top quark candidate masses.  The plot is from a recent ATLAS publication using data collected in 201, and from a measurement of the top quark mass made in the 'semileptonic' or 'lepton + jets' channel.  One can observe the shape of the distribution of signal events (shown in red) compared with the other various backgrounds which can 'fake' the signal and which must be taken into account.  In Carleton's top quark mass analysis in the 'all-hadronic channel', one will hope to observe a similar such peak above the background, though the actual background shape and composition will differ.

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Figure 2: A plot showing the production cross-sections for various processes at the LHC experiments, including the top-antitop production relevant to Carleton's top quark mass analysis.  The cross-section is a measure of how often one can expect to produce such a particle or pair of particles via proton-proton collisions at the centre of the ATLAS detector.  Looking at this plot from one type of particle production to another allows one to then see their expected relative production rates at the LHC experiments.  For the production of pairs of top quarks, labelled as tt in the plot, It is these top quarks (one top and one anti-top), once produced and following their decay into ligher particles, which physicists will then attempt to reconstruct in order to make a top quark mass measurement. 

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Figure 3: A plot summarizing recent measurements of the top quark mass made by the ATLAS collaboration in the three main decay channels.  At the top one can see the top quark mass measurement made by ATLAS using 2011 data, together with a statistical and systematic uncertainty corresponding to the measurement made.  It is a similar measurement that is being made with the group at Carleton, but using the much larger set of 2012 data - and at a higher energy - than that used in making the 2011 measurement.  At the bottom are recent measurements made by two other similar experiments, CMS and D0, and averaged of all decay channels, from the Tevatron collider at Fermilab near Chicago.