Top physics in ATLAS

Krzysztof Sliwa
2016 Proceedings of Proceedings of the Corfu Summer Institute 2015 — PoS(CORFU2015)   unpublished
A review of top quark physics and results from the ATLAS Collaboration is presented. Most of the results[1] are based on the data from the LHC Run 1, in which the ATLAS detector recorded events from proton-proton collisions corresponding to the integrated luminosity of ∼ 5/ f b at √ s = 7 TeV and ∼ 20/ f b at √ s = 8 TeV . The Standard Model and beyond The Standard Model (SM) of particle physics [2], developed in 1961-1972, is a gauge theory based on the SU(3) C × SU(2) I × U(1) Y symmetry
more » ... (C-colour, I-weak isospin and Yhypercharge). The SU(3) C is an unbroken symmetry, it is the basis of Quantum Chromo-Dynamics (QCD), a quantum theory of strong interactions, whose carriers, massless gluons, couple to color (strong force charge). The SU(2) ×U(1) -which gives rise to the quantum theory of electroweak interactions -is spontaneously broken by the Brout-Englert-Higgs mechanism, which gives mass to the electroweak bosons (massive W + ,W − , Z o and a massless photon), and all fermions. Matter is build of fermions -quarks and leptons. There are three families of each, with corresponding antiparticles. Quarks exist in three colors, leptons are color singlets, and as such they do not couple to gluons, and they don't interact strongly. Bosons are the carriers of interactions: there are 8 massless gluons, three heavy weak bosons (W ± , Z o ) and a massless photon. In the Minimal Standard Model (MSM), the Higgs sector is the simplest possible, it contains a single weak isospin doublet of complex Higgs fields, which after giving masses to W + ,W − , Z o , leaves a single neutral scalar Higgs particle. A neutral scalar Higgs field permeates the Universe and is, in some way, responsible for masses of other particles -they originate from couplings to the Higgs field. There are 26 parameters not predicted by MSM -masses of quarks and leptons, coupling constants, Higgs mass and the vacuum expectation value, mixing angles and complex phases in the quark Cabibbo-Kobayashi-Maskawa mixing matrix and the lepton Maki-Nakagawa-Sakata mixing matrix, and the QCD phase, θ . All must be measured. The discovery of the Higgs boson, the only particle missing in the MSM, was announced on July 4th, 2012 by the ATLAS and CMS Collaborations at the Large Hadron Collider (LHC) at CERN. It was a great success for those more than 20 years-long projects. The discovery also brought many questions, some new and many old, the most important one: is the new particle the Minimal Standard Model boson? Answering this question will take time and many precision measurements. With the Higgs mass known, all MSM couplings can be calculated. There are many outstanding questions in the SM: why so many (26) free parameters: all masses, all couplings, all mixing angles and CP-violating phases; why 6 quarks and 6 leptons -is there an additional symmetry? why quarks and and leptons come in three pairs (generations)? is CP not an exact symmetry, or why are laws of physics not symmetrical between matter and antimatter -is this fact related to the questions of why is our Universe matter-dominated? there seem to be not enough sources of CP violation in the SM to explain the latter? what is the nature of Dark Matter, about 5 times more prevalent in the Universe than ordinary matter? how to include gravity ? There is still plenty to understand, and all this means that the Standard Model is perhaps just a low energy approximation. Many new theories were considered as possible beyond Standard Model physics models: Supersymmetry (SUSY); Grand Unified Theories based on some larger symmetry groups, e.g. SU(5), SO(10), E8, Monster group... There exist also "new physics" models based on the extensions of Kaluza-Klein theory, string theory, superstring theory, branes, M-theory, quantum gravity or Technicolor. Finding Higgs boson does not solve SM shortcomings. It is quite clear that new experimental data and analyses are badly needed. (Personally, I think it would be much more interesting if the Higgs boson were not there, or if the new-found particle is NOT a Minimal Standard Model 2 PoS(CORFU2015)045 Top quark physics in ATLAS Krzysztof Sliwa boson). Top Quark in the Standard Model The top quark was expected in the Standard Model (SM) of electroweak interactions as a partner of the b-quark in a SU(2) doublet of weak isospin in the third family of quarks. The first evidence for top quark was published by CDF in 1994[3], after some earlier hints, and was followed by the discovery papers by CDF and D0 Collaborations in 1995[4]. Top quark mass is about 173 GeV, which makes the top quark the most massive of the known elementary particles, and it may be playing a special role in electroweak symmetry breaking. At the LHC, achieving a good understanding of top physics is a necessary first step in almost any search for physics beyond the Standard Model at high mass scale. Most "new physics "will show up as excess of events beyond the SM expectations, including 6 quarks. Top quark production will be the most dominant background in almost any new physics searches, and has to be understood well. In addition to direct searches for "new physics ", the precision studies of tt spin correlations and asymmetries could be one of the best window to the "beyond the SM "physics. Top studies may also be the best testing ground for the higher order theoretical calculations. Recently, significant progress has been achieved with Czakon and Mitov finalizing the complete NNLO calculations [5]. Studies of top quarks are very interesting on their own. Because of very large mass of the top quark, its lifetime is very short, ∼ 5 × 10 −25 seconds, much shorter that the characteristic time of the strong interactions, which is of the order of ∼ 10 −22 seconds. Top quark thus decays before any effects due to the strong interactions (hadronization) may take place. This allows a direct access to the information about the top quark spin, which is very difficult, if not impossible, for any other quark. Krzysztof Sliwa M. Czakon / Nuclear Physics B Proceedings Supplement 00 (2014) 1-15 13 3 but for the P T,tt di↵erential asymmetry. Plot
doi:10.22323/1.263.0045 fatcat:ch27jegr4ncsjn23ycuhv2qqzu